malloc
malloc
malloc
malloc
malloc-Related Functions
printf
getopt
argp_parse Function
argp_parse
argp_help Function
argp_help Function
sysconf
pathconf
The C language provides no built-in facilities for performing such common operations as input/output, memory management, string manipulation, and the like. Instead, these facilities are defined in a standard library, which you compile and link with your programs.
The GNU C library, described in this document, defines all of the library functions that are specified by the ISO C standard, as well as additional features specific to POSIX and other derivatives of the Unix operating system, and extensions specific to the GNU system.
The purpose of this manual is to tell you how to use the facilities of the GNU library. We have mentioned which features belong to which standards to help you identify things that are potentially non-portable to other systems. But the emphasis in this manual is not on strict portability.
This manual is written with the assumption that you are at least somewhat familiar with the C programming language and basic programming concepts. Specifically, familiarity with ISO standard C (see section ISO C), rather than "traditional" pre-ISO C dialects, is assumed.
The GNU C library includes several header files, each of which provides definitions and declarations for a group of related facilities; this information is used by the C compiler when processing your program. For example, the header file `stdio.h' declares facilities for performing input and output, and the header file `string.h' declares string processing utilities. The organization of this manual generally follows the same division as the header files.
If you are reading this manual for the first time, you should read all of the introductory material and skim the remaining chapters. There are a lot of functions in the GNU C library and it's not realistic to expect that you will be able to remember exactly how to use each and every one of them. It's more important to become generally familiar with the kinds of facilities that the library provides, so that when you are writing your programs you can recognize when to make use of library functions, and where in this manual you can find more specific information about them.
This section discusses the various standards and other sources that the GNU C library is based upon. These sources include the ISO C and POSIX standards, and the System V and Berkeley Unix implementations.
The primary focus of this manual is to tell you how to make effective use of the GNU library facilities. But if you are concerned about making your programs compatible with these standards, or portable to operating systems other than GNU, this can affect how you use the library. This section gives you an overview of these standards, so that you will know what they are when they are mentioned in other parts of the manual.
See section Summary of Library Facilities, for an alphabetical list of the functions and other symbols provided by the library. This list also states which standards each function or symbol comes from.
The GNU C library is compatible with the C standard adopted by the American National Standards Institute (ANSI): American National Standard X3.159-1989---"ANSI C" and later by the International Standardization Organization (ISO): ISO/IEC 9899:1990, "Programming languages--C". We here refer to the standard as ISO C since this is the more general standard in respect of ratification. The header files and library facilities that make up the GNU library are a superset of those specified by the ISO C standard.
If you are concerned about strict adherence to the ISO C standard, you should use the `-ansi' option when you compile your programs with the GNU C compiler. This tells the compiler to define only ISO standard features from the library header files, unless you explicitly ask for additional features. See section Feature Test Macros, for information on how to do this.
Being able to restrict the library to include only ISO C features is important because ISO C puts limitations on what names can be defined by the library implementation, and the GNU extensions don't fit these limitations. See section Reserved Names, for more information about these restrictions.
This manual does not attempt to give you complete details on the differences between ISO C and older dialects. It gives advice on how to write programs to work portably under multiple C dialects, but does not aim for completeness.
The GNU library is also compatible with the ISO POSIX family of standards, known more formally as the Portable Operating System Interface for Computer Environments (ISO/IEC 9945). They were also published as ANSI/IEEE Std 1003. POSIX is derived mostly from various versions of the Unix operating system.
The library facilities specified by the POSIX standards are a superset of those required by ISO C; POSIX specifies additional features for ISO C functions, as well as specifying new additional functions. In general, the additional requirements and functionality defined by the POSIX standards are aimed at providing lower-level support for a particular kind of operating system environment, rather than general programming language support which can run in many diverse operating system environments.
The GNU C library implements all of the functions specified in ISO/IEC 9945-1:1996, the POSIX System Application Program Interface, commonly referred to as POSIX.1. The primary extensions to the ISO C facilities specified by this standard include file system interface primitives (see section File System Interface), device-specific terminal control functions (see section Low-Level Terminal Interface), and process control functions (see section Processes).
Some facilities from ISO/IEC 9945-2:1993, the POSIX Shell and Utilities standard (POSIX.2) are also implemented in the GNU library. These include utilities for dealing with regular expressions and other pattern matching facilities (see section Pattern Matching).
The GNU C library defines facilities from some versions of Unix which are not formally standardized, specifically from the 4.2 BSD, 4.3 BSD, and 4.4 BSD Unix systems (also known as Berkeley Unix) and from SunOS (a popular 4.2 BSD derivative that includes some Unix System V functionality). These systems support most of the ISO C and POSIX facilities, and 4.4 BSD and newer releases of SunOS in fact support them all.
The BSD facilities include symbolic links (see section Symbolic Links), the
select function (see section Waiting for Input or Output), the BSD signal
functions (see section BSD Signal Handling), and sockets (see section Sockets).
The System V Interface Description (SVID) is a document describing the AT&T Unix System V operating system. It is to some extent a superset of the POSIX standard (see section POSIX (The Portable Operating System Interface)).
The GNU C library defines most of the facilities required by the SVID that are not also required by the ISO C or POSIX standards, for compatibility with System V Unix and other Unix systems (such as SunOS) which include these facilities. However, many of the more obscure and less generally useful facilities required by the SVID are not included. (In fact, Unix System V itself does not provide them all.)
The supported facilities from System V include the methods for
inter-process communication and shared memory, the hsearch and
drand48 families of functions, fmtmsg and several of the
mathematical functions.
The X/Open Portability Guide, published by the X/Open Company, Ltd., is a more general standard than POSIX. X/Open owns the Unix copyright and the XPG specifies the requirements for systems which are intended to be a Unix system.
The GNU C library complies to the X/Open Portability Guide, Issue 4.2, with all extensions common to XSI (X/Open System Interface) compliant systems and also all X/Open UNIX extensions.
The additions on top of POSIX are mainly derived from functionality available in System V and BSD systems. Some of the really bad mistakes in System V systems were corrected, though. Since fulfilling the XPG standard with the Unix extensions is a precondition for getting the Unix brand chances are good that the functionality is available on commercial systems.
This section describes some of the practical issues involved in using the GNU C library.
Libraries for use by C programs really consist of two parts: header files that define types and macros and declare variables and functions; and the actual library or archive that contains the definitions of the variables and functions.
(Recall that in C, a declaration merely provides information that a function or variable exists and gives its type. For a function declaration, information about the types of its arguments might be provided as well. The purpose of declarations is to allow the compiler to correctly process references to the declared variables and functions. A definition, on the other hand, actually allocates storage for a variable or says what a function does.)
In order to use the facilities in the GNU C library, you should be sure that your program source files include the appropriate header files. This is so that the compiler has declarations of these facilities available and can correctly process references to them. Once your program has been compiled, the linker resolves these references to the actual definitions provided in the archive file.
Header files are included into a program source file by the `#include' preprocessor directive. The C language supports two forms of this directive; the first,
#include "header"
is typically used to include a header file header that you write yourself; this would contain definitions and declarations describing the interfaces between the different parts of your particular application. By contrast,
#include <file.h>
is typically used to include a header file `file.h' that contains definitions and declarations for a standard library. This file would normally be installed in a standard place by your system administrator. You should use this second form for the C library header files.
Typically, `#include' directives are placed at the top of the C source file, before any other code. If you begin your source files with some comments explaining what the code in the file does (a good idea), put the `#include' directives immediately afterwards, following the feature test macro definition (see section Feature Test Macros).
For more information about the use of header files and `#include' directives, see section `Header Files' in The GNU C Preprocessor Manual.
The GNU C library provides several header files, each of which contains the type and macro definitions and variable and function declarations for a group of related facilities. This means that your programs may need to include several header files, depending on exactly which facilities you are using.
Some library header files include other library header files automatically. However, as a matter of programming style, you should not rely on this; it is better to explicitly include all the header files required for the library facilities you are using. The GNU C library header files have been written in such a way that it doesn't matter if a header file is accidentally included more than once; including a header file a second time has no effect. Likewise, if your program needs to include multiple header files, the order in which they are included doesn't matter.
Compatibility Note: Inclusion of standard header files in any order and any number of times works in any ISO C implementation. However, this has traditionally not been the case in many older C implementations.
Strictly speaking, you don't have to include a header file to use a function it declares; you could declare the function explicitly yourself, according to the specifications in this manual. But it is usually better to include the header file because it may define types and macros that are not otherwise available and because it may define more efficient macro replacements for some functions. It is also a sure way to have the correct declaration.
If we describe something as a function in this manual, it may have a macro definition as well. This normally has no effect on how your program runs--the macro definition does the same thing as the function would. In particular, macro equivalents for library functions evaluate arguments exactly once, in the same way that a function call would. The main reason for these macro definitions is that sometimes they can produce an inline expansion that is considerably faster than an actual function call.
Taking the address of a library function works even if it is also defined as a macro. This is because, in this context, the name of the function isn't followed by the left parenthesis that is syntactically necessary to recognize a macro call.
You might occasionally want to avoid using the macro definition of a function--perhaps to make your program easier to debug. There are two ways you can do this:
For example, suppose the header file `stdlib.h' declares a function
named abs with
extern int abs (int);
and also provides a macro definition for abs. Then, in:
#include <stdlib.h>
int f (int *i) { return abs (++*i); }
the reference to abs might refer to either a macro or a function.
On the other hand, in each of the following examples the reference is
to a function and not a macro.
#include <stdlib.h>
int g (int *i) { return (abs) (++*i); }
#undef abs
int h (int *i) { return abs (++*i); }
Since macro definitions that double for a function behave in exactly the same way as the actual function version, there is usually no need for any of these methods. In fact, removing macro definitions usually just makes your program slower.
The names of all library types, macros, variables and functions that come from the ISO C standard are reserved unconditionally; your program may not redefine these names. All other library names are reserved if your program explicitly includes the header file that defines or declares them. There are several reasons for these restrictions:
exit to do something completely different from
what the standard exit function does, for example. Preventing
this situation helps to make your programs easier to understand and
contributes to modularity and maintainability.
In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore (`_') and all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter are reserved names. This is so that the library and header files can define functions, variables, and macros for internal purposes without risk of conflict with names in user programs.
Some additional classes of identifier names are reserved for future extensions to the C language or the POSIX.1 environment. While using these names for your own purposes right now might not cause a problem, they do raise the possibility of conflict with future versions of the C or POSIX standards, so you should avoid these names.
float and long double arguments,
respectively.
In addition, some individual header files reserve names beyond those that they actually define. You only need to worry about these restrictions if your program includes that particular header file.
The exact set of features available when you compile a source file is controlled by which feature test macros you define.
If you compile your programs using `gcc -ansi', you get only the ISO C library features, unless you explicitly request additional features by defining one or more of the feature macros. See section `GNU CC Command Options' in The GNU CC Manual, for more information about GCC options.
You should define these macros by using `#define' preprocessor
directives at the top of your source code files. These directives
must come before any #include of a system header file. It
is best to make them the very first thing in the file, preceded only by
comments. You could also use the `-D' option to GCC, but it's
better if you make the source files indicate their own meaning in a
self-contained way.
This system exists to allow the library to conform to multiple standards.
Although the different standards are often described as supersets of each
other, they are usually incompatible because larger standards require
functions with names that smaller ones reserve to the user program. This
is not mere pedantry -- it has been a problem in practice. For instance,
some non-GNU programs define functions named getline that have
nothing to do with this library's getline. They would not be
compilable if all features were enabled indiscriminately.
This should not be used to verify that a program conforms to a limited standard. It is insufficient for this purpose, as it will not protect you from including header files outside the standard, or relying on semantics undefined within the standard.
The state of _POSIX_SOURCE is irrelevant if you define the
macro _POSIX_C_SOURCE to a positive integer.
If you define this macro to a value greater than or equal to 1,
then the functionality from the 1990 edition of the POSIX.1 standard
(IEEE Standard 1003.1-1990) is made available.
If you define this macro to a value greater than or equal to 2,
then the functionality from the 1992 edition of the POSIX.2 standard
(IEEE Standard 1003.2-1992) is made available.
If you define this macro to a value greater than or equal to 199309L,
then the functionality from the 1993 edition of the POSIX.1b standard
(IEEE Standard 1003.1b-1993) is made available.
Greater values for _POSIX_C_SOURCE will enable future extensions.
The POSIX standards process will define these values as necessary, and
the GNU C Library should support them some time after they become standardized.
The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that
if you define _POSIX_C_SOURCE to a value greater than
or equal to 199506L, then the functionality from the 1996
edition is made available.
Some of the features derived from 4.3 BSD Unix conflict with the corresponding features specified by the POSIX.1 standard. If this macro is defined, the 4.3 BSD definitions take precedence over the POSIX definitions.
Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1,
you need to use a special BSD compatibility library when linking
programs compiled for BSD compatibility. This is because some functions
must be defined in two different ways, one of them in the normal C
library, and one of them in the compatibility library. If your program
defines _BSD_SOURCE, you must give the option `-lbsd-compat'
to the compiler or linker when linking the program, to tell it to find
functions in this special compatibility library before looking for them in
the normal C library.
_POSIX_SOURCE and
_POSIX_C_SOURCE are automatically defined.
As the unification of all Unices, functionality only available in BSD and SVID is also included.
If the macro _XOPEN_SOURCE_EXTENDED is also defined, even more
functionality is available. The extra functions will make all functions
available which are necessary for the X/Open Unix brand.
If the macro _XOPEN_SOURCE has the value @math{500} this includes
all functionality described so far plus some new definitions from the
Single Unix Specification, version 2.
fseeko and ftello are available. Without
these functions the difference between the ISO C interface
(fseek, ftell) and the low-level POSIX interface
(lseek) would lead to problems.
This macro was introduced as part of the Large File Support extension (LFS).
The new functionality is made available by a new set of types and
functions which replace the existing ones. The names of these new objects
contain 64 to indicate the intention, e.g., off_t
vs. off64_t and fseeko vs. fseeko64.
This macro was introduced as part of the Large File Support extension
(LFS). It is a transition interface for the period when 64 bit
offsets are not generally used (see _FILE_OFFSET_BITS).
_LARGEFILE64_SOURCE makes the 64
bit interface available as an additional interface,
_FILE_OFFSET_BITS allows the 64 bit interface to
replace the old interface.
If _FILE_OFFSET_BITS is undefined, or if it is defined to the
value 32, nothing changes. The 32 bit interface is used and
types like off_t have a size of 32 bits on 32 bit
systems.
If the macro is defined to the value 64, the large file interface
replaces the old interface. I.e., the functions are not made available
under different names (as they are with _LARGEFILE64_SOURCE).
Instead the old function names now reference the new functions, e.g., a
call to fseeko now indeed calls fseeko64.
This macro should only be selected if the system provides mechanisms for
handling large files. On 64 bit systems this macro has no effect
since the *64 functions are identical to the normal functions.
This macro was introduced as part of the Large File Support extension (LFS).
_ISOC99_SOURCE should be defined.
If you want to get the full effect of _GNU_SOURCE but make the
BSD definitions take precedence over the POSIX definitions, use this
sequence of definitions:
#define _GNU_SOURCE #define _BSD_SOURCE #define _SVID_SOURCE
Note that if you do this, you must link your program with the BSD compatibility library by passing the `-lbsd-compat' option to the compiler or linker. Note: If you forget to do this, you may get very strange errors at run time.
Unlike on some other systems, no special version of the C library must be used for linking. There is only one version but while compiling this it must have been specified to compile as thread safe.
We recommend you use _GNU_SOURCE in new programs. If you don't
specify the `-ansi' option to GCC and don't define any of these
macros explicitly, the effect is the same as defining
_POSIX_C_SOURCE to 2 and _POSIX_SOURCE,
_SVID_SOURCE, and _BSD_SOURCE to 1.
When you define a feature test macro to request a larger class of features,
it is harmless to define in addition a feature test macro for a subset of
those features. For example, if you define _POSIX_C_SOURCE, then
defining _POSIX_SOURCE as well has no effect. Likewise, if you
define _GNU_SOURCE, then defining either _POSIX_SOURCE or
_POSIX_C_SOURCE or _SVID_SOURCE as well has no effect.
Note, however, that the features of _BSD_SOURCE are not a subset of
any of the other feature test macros supported. This is because it defines
BSD features that take precedence over the POSIX features that are
requested by the other macros. For this reason, defining
_BSD_SOURCE in addition to the other feature test macros does have
an effect: it causes the BSD features to take priority over the conflicting
POSIX features.
Here is an overview of the contents of the remaining chapters of this manual.
sizeof
operator and the symbolic constant NULL, how to write functions
accepting variable numbers of arguments, and constants describing the
ranges and other properties of the numerical types. There is also a simple
debugging mechanism which allows you to put assertions in your code, and
have diagnostic messages printed if the tests fail.
isspace) and functions for
performing case conversion.
FILE * objects). These are the normal C library functions
from `stdio.h'.
char data type.
setjmp and
longjmp functions. These functions provide a facility for
goto-like jumps which can jump from one function to another.
If you already know the name of the facility you are interested in, you can look it up in section Summary of Library Facilities. This gives you a summary of its syntax and a pointer to where you can find a more detailed description. This appendix is particularly useful if you just want to verify the order and type of arguments to a function, for example. It also tells you what standard or system each function, variable, or macro is derived from.
Many functions in the GNU C library detect and report error conditions, and sometimes your programs need to check for these error conditions. For example, when you open an input file, you should verify that the file was actually opened correctly, and print an error message or take other appropriate action if the call to the library function failed.
This chapter describes how the error reporting facility works. Your program should include the header file `errno.h' to use this facility.
Most library functions return a special value to indicate that they have
failed. The special value is typically -1, a null pointer, or a
constant such as EOF that is defined for that purpose. But this
return value tells you only that an error has occurred. To find out
what kind of error it was, you need to look at the error code stored in the
variable errno. This variable is declared in the header file
`errno.h'.
errno contains the system error number. You can
change the value of errno.
Since errno is declared volatile, it might be changed
asynchronously by a signal handler; see section Defining Signal Handlers.
However, a properly written signal handler saves and restores the value
of errno, so you generally do not need to worry about this
possibility except when writing signal handlers.
The initial value of errno at program startup is zero. Many
library functions are guaranteed to set it to certain nonzero values
when they encounter certain kinds of errors. These error conditions are
listed for each function. These functions do not change errno
when they succeed; thus, the value of errno after a successful
call is not necessarily zero, and you should not use errno to
determine whether a call failed. The proper way to do that is
documented for each function. If the call failed, you can
examine errno.
Many library functions can set errno to a nonzero value as a
result of calling other library functions which might fail. You should
assume that any library function might alter errno when the
function returns an error.
Portability Note: ISO C specifies errno as a
"modifiable lvalue" rather than as a variable, permitting it to be
implemented as a macro. For example, its expansion might involve a
function call, like *_errno (). In fact, that is what it is
on the GNU system itself. The GNU library, on non-GNU systems, does
whatever is right for the particular system.
There are a few library functions, like sqrt and atan,
that return a perfectly legitimate value in case of an error, but also
set errno. For these functions, if you want to check to see
whether an error occurred, the recommended method is to set errno
to zero before calling the function, and then check its value afterward.
All the error codes have symbolic names; they are macros defined in `errno.h'. The names start with `E' and an upper-case letter or digit; you should consider names of this form to be reserved names. See section Reserved Names.
The error code values are all positive integers and are all distinct,
with one exception: EWOULDBLOCK and EAGAIN are the same.
Since the values are distinct, you can use them as labels in a
switch statement; just don't use both EWOULDBLOCK and
EAGAIN. Your program should not make any other assumptions about
the specific values of these symbolic constants.
The value of errno doesn't necessarily have to correspond to any
of these macros, since some library functions might return other error
codes of their own for other situations. The only values that are
guaranteed to be meaningful for a particular library function are the
ones that this manual lists for that function.
On non-GNU systems, almost any system call can return EFAULT if
it is given an invalid pointer as an argument. Since this could only
happen as a result of a bug in your program, and since it will not
happen on the GNU system, we have saved space by not mentioning
EFAULT in the descriptions of individual functions.
In some Unix systems, many system calls can also return EFAULT if
given as an argument a pointer into the stack, and the kernel for some
obscure reason fails in its attempt to extend the stack. If this ever
happens, you should probably try using statically or dynamically
allocated memory instead of stack memory on that system.
The error code macros are defined in the header file `errno.h'. All of them expand into integer constant values. Some of these error codes can't occur on the GNU system, but they can occur using the GNU library on other systems.
You can choose to have functions resume after a signal that is handled,
rather than failing with EINTR; see section Primitives Interrupted by Signals.
exec functions (see section Executing a File) occupy too much memory space. This condition never arises in the
GNU system.
exec functions; see section Executing a File.
link (see section Hard Links) but
also when you rename a file with rename (see section Renaming Files).
In BSD and GNU, the number of open files is controlled by a resource
limit that can usually be increased. If you get this error, you might
want to increase the RLIMIT_NOFILE limit or make it unlimited;
see section Limiting Resource Usage.
rename can cause this error if the file being renamed already has
as many links as it can take (see section Renaming Files).
SIGPIPE signal; this signal terminates the program if not handled
or blocked. Thus, your program will never actually see EPIPE
unless it has handled or blocked SIGPIPE.
EWOULDBLOCK is another name for EAGAIN;
they are always the same in the GNU C library.
This error can happen in a few different situations:
select to find out
when the operation will be possible; see section Waiting for Input or Output.
Portability Note: In many older Unix systems, this condition
was indicated by EWOULDBLOCK, which was a distinct error code
different from EAGAIN. To make your program portable, you should
check for both codes and treat them the same.
fork
can return this error. It indicates that the shortage is expected to
pass, so your program can try the call again later and it may succeed.
It is probably a good idea to delay for a few seconds before trying it
again, to allow time for other processes to release scarce resources.
Such shortages are usually fairly serious and affect the whole system,
so usually an interactive program should report the error to the user
and return to its command loop.
EAGAIN (above).
The values are always the same, on every operating system.
C libraries in many older Unix systems have EWOULDBLOCK as a
separate error code.
connect; see section Making a Connection) never return
EAGAIN. Instead, they return EINPROGRESS to indicate that
the operation has begun and will take some time. Attempts to manipulate
the object before the call completes return EALREADY. You can
use the select function to find out when the pending operation
has completed; see section Waiting for Input or Output.
ENOMEM; you may get one or the
other from network operations.
EDESTADDRREQ instead.
connect.
PATH_MAX; see section Limits on File System Capacity) or host name too long (in gethostname or
sethostname; see section Host Identification).
fork. See section Limiting Resource Usage, for details on
the RLIMIT_NPROC limit.
On some systems chmod returns this error if you try to set the
sticky bit on a non-directory file; see section Assigning File Permissions.
ENOSYS unless you
install a new version of the C library or the operating system.
If the entire function is not available at all in the implementation,
it returns ENOSYS instead.
term protocol return
this error for certain operations when the caller is not in the
foreground process group of the terminal. Users do not usually see this
error because functions such as read and write translate
it into a SIGTTIN or SIGTTOU signal. See section Job Control,
for information on process groups and these signals.
The following error codes are defined by the Linux/i386 kernel. They are not yet documented.
The library has functions and variables designed to make it easy for
your program to report informative error messages in the customary
format about the failure of a library call. The functions
strerror and perror give you the standard error message
for a given error code; the variable
program_invocation_short_name gives you convenient access to the
name of the program that encountered the error.
strerror function maps the error code (see section Checking for Errors) specified by the errnum argument to a descriptive error
message string. The return value is a pointer to this string.
The value errnum normally comes from the variable errno.
You should not modify the string returned by strerror. Also, if
you make subsequent calls to strerror, the string might be
overwritten. (But it's guaranteed that no library function ever calls
strerror behind your back.)
The function strerror is declared in `string.h'.
strerror_r function works like strerror but instead of
returning the error message in a statically allocated buffer shared by
all threads in the process, it returns a private copy for the
thread. This might be either some permanent global data or a message
string in the user supplied buffer starting at buf with the
length of n bytes.
At most n characters are written (including the NUL byte) so it is up to the user to select the buffer large enough.
This function should always be used in multi-threaded programs since
there is no way to guarantee the string returned by strerror
really belongs to the last call of the current thread.
This function strerror_r is a GNU extension and it is declared in
`string.h'.
stderr;
see section Standard Streams.
If you call perror with a message that is either a null
pointer or an empty string, perror just prints the error message
corresponding to errno, adding a trailing newline.
If you supply a non-null message argument, then perror
prefixes its output with this string. It adds a colon and a space
character to separate the message from the error string corresponding
to errno.
The function perror is declared in `stdio.h'.
strerror and perror produce the exact same message for any
given error code; the precise text varies from system to system. On the
GNU system, the messages are fairly short; there are no multi-line
messages or embedded newlines. Each error message begins with a capital
letter and does not include any terminating punctuation.
Compatibility Note: The strerror function is a new
feature of ISO C. Many older C systems do not support this function
yet.
Many programs that don't read input from the terminal are designed to
exit if any system call fails. By convention, the error message from
such a program should start with the program's name, sans directories.
You can find that name in the variable
program_invocation_short_name; the full file name is stored the
variable program_invocation_name.
argv[0]. Note
that this is not necessarily a useful file name; often it contains no
directory names. See section Program Arguments.
program_invocation_name minus
everything up to the last slash, if any.)
The library initialization code sets up both of these variables before
calling main.
Portability Note: These two variables are GNU extensions. If
you want your program to work with non-GNU libraries, you must save the
value of argv[0] in main, and then strip off the directory
names yourself. We added these extensions to make it possible to write
self-contained error-reporting subroutines that require no explicit
cooperation from main.
Here is an example showing how to handle failure to open a file
correctly. The function open_sesame tries to open the named file
for reading and returns a stream if successful. The fopen
library function returns a null pointer if it couldn't open the file for
some reason. In that situation, open_sesame constructs an
appropriate error message using the strerror function, and
terminates the program. If we were going to make some other library
calls before passing the error code to strerror, we'd have to
save it in a local variable instead, because those other library
functions might overwrite errno in the meantime.
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
FILE *
open_sesame (char *name)
{
FILE *stream;
errno = 0;
stream = fopen (name, "r");
if (stream == NULL)
{
fprintf (stderr, "%s: Couldn't open file %s; %s\n",
program_invocation_short_name, name, strerror (errno));
exit (EXIT_FAILURE);
}
else
return stream;
}
This chapter describes how processes manage and use memory in a system that uses the GNU C library.
The GNU C Library has several functions for dynamically allocating virtual memory in various ways. They vary in generality and in efficiency. The library also provides functions for controlling paging and allocation of real memory.
Memory mapped I/O is not discussed in this chapter. See section Memory-mapped I/O.
One of the most basic resources a process has available to it is memory. There are a lot of different ways systems organize memory, but in a typical one, each process has one linear virtual address space, with addresses running from zero to some huge maximum. It need not be contiguous; i.e. not all of these addresses actually can be used to store data.
The virtual memory is divided into pages (4 kilobytes is typical). Backing each page of virtual memory is a page of real memory (called a frame) or some secondary storage, usually disk space. The disk space might be swap space or just some ordinary disk file. Actually, a page of all zeroes sometimes has nothing at all backing it -- there's just a flag saying it is all zeroes.
The same frame of real memory or backing store can back multiple virtual
pages belonging to multiple processes. This is normally the case, for
example, with virtual memory occupied by GNU C library code. The same
real memory frame containing the printf function backs a virtual
memory page in each of the existing processes that has a printf
call in its program.
In order for a program to access any part of a virtual page, the page must at that moment be backed by ("connected to") a real frame. But because there is usually a lot more virtual memory than real memory, the pages must move back and forth between real memory and backing store regularly, coming into real memory when a process needs to access them and then retreating to backing store when not needed anymore. This movement is called paging.
When a program attempts to access a page which is not at that moment backed by real memory, this is known as a page fault. When a page fault occurs, the kernel suspends the process, places the page into a real page frame (this is called "paging in" or "faulting in"), then resumes the process so that from the process' point of view, the page was in real memory all along. In fact, to the process, all pages always seem to be in real memory. Except for one thing: the elapsed execution time of an instruction that would normally be a few nanoseconds is suddenly much, much, longer (because the kernel normally has to do I/O to complete the page-in). For programs sensitive to that, the functions described in section Locking Pages can control it.
Within each virtual address space, a process has to keep track of what is at which addresses, and that process is called memory allocation. Allocation usually brings to mind meting out scarce resources, but in the case of virtual memory, that's not a major goal, because there is generally much more of it than anyone needs. Memory allocation within a process is mainly just a matter of making sure that the same byte of memory isn't used to store two different things.
Processes allocate memory in two major ways: by exec and programmatically. Actually, forking is a third way, but it's not very interesting. See section Creating a Process.
Exec is the operation of creating a virtual address space for a process,
loading its basic program into it, and executing the program. It is
done by the "exec" family of functions (e.g. execl). The
operation takes a program file (an executable), it allocates space to
load all the data in the executable, loads it, and transfers control to
it. That data is most notably the instructions of the program (the
text), but also literals and constants in the program and even
some variables: C variables with the static storage class (see section Memory Allocation in C Programs).
Once that program begins to execute, it uses programmatic allocation to gain additional memory. In a C program with the GNU C library, there are two kinds of programmatic allocation: automatic and dynamic. See section Memory Allocation in C Programs.
Memory-mapped I/O is another form of dynamic virtual memory allocation. Mapping memory to a file means declaring that the contents of certain range of a process' addresses shall be identical to the contents of a specified regular file. The system makes the virtual memory initially contain the contents of the file, and if you modify the memory, the system writes the same modification to the file. Note that due to the magic of virtual memory and page faults, there is no reason for the system to do I/O to read the file, or allocate real memory for its contents, until the program accesses the virtual memory. See section Memory-mapped I/O.
Just as it programmatically allocates memory, the program can programmatically deallocate (free) it. You can't free the memory that was allocated by exec. When the program exits or execs, you might say that all its memory gets freed, but since in both cases the address space ceases to exist, the point is really moot. See section Program Termination.
A process' virtual address space is divided into segments. A segment is a contiguous range of virtual addresses. Three important segments are:
This section covers how ordinary programs manage storage for their data,
including the famous malloc function and some fancier facilities
special the GNU C library and GNU Compiler.
The C language supports two kinds of memory allocation through the variables in C programs:
A third important kind of memory allocation, dynamic allocation, is not supported by C variables but is available via GNU C library functions.
Dynamic memory allocation is a technique in which programs determine as they are running where to store some information. You need dynamic allocation when the amount of memory you need, or how long you continue to need it, depends on factors that are not known before the program runs.
For example, you may need a block to store a line read from an input file; since there is no limit to how long a line can be, you must allocate the memory dynamically and make it dynamically larger as you read more of the line.
Or, you may need a block for each record or each definition in the input data; since you can't know in advance how many there will be, you must allocate a new block for each record or definition as you read it.
When you use dynamic allocation, the allocation of a block of memory is an action that the program requests explicitly. You call a function or macro when you want to allocate space, and specify the size with an argument. If you want to free the space, you do so by calling another function or macro. You can do these things whenever you want, as often as you want.
Dynamic allocation is not supported by C variables; there is no storage class "dynamic", and there can never be a C variable whose value is stored in dynamically allocated space. The only way to get dynamically allocated memory is via a system call (which is generally via a GNU C library function call), and the only way to refer to dynamically allocated space is through a pointer. Because it is less convenient, and because the actual process of dynamic allocation requires more computation time, programmers generally use dynamic allocation only when neither static nor automatic allocation will serve.
For example, if you want to allocate dynamically some space to hold a
struct foobar, you cannot declare a variable of type struct
foobar whose contents are the dynamically allocated space. But you can
declare a variable of pointer type struct foobar * and assign it the
address of the space. Then you can use the operators `*' and
`->' on this pointer variable to refer to the contents of the space:
{
struct foobar *ptr
= (struct foobar *) malloc (sizeof (struct foobar));
ptr->name = x;
ptr->next = current_foobar;
current_foobar = ptr;
}
The most general dynamic allocation facility is malloc. It
allows you to allocate blocks of memory of any size at any time, make
them bigger or smaller at any time, and free the blocks individually at
any time (or never).
To allocate a block of memory, call malloc. The prototype for
this function is in `stdlib.h'.
The contents of the block are undefined; you must initialize it yourself
(or use calloc instead; see section Allocating Cleared Space).
Normally you would cast the value as a pointer to the kind of object
that you want to store in the block. Here we show an example of doing
so, and of initializing the space with zeros using the library function
memset (see section Copying and Concatenation):
struct foo *ptr; ... ptr = (struct foo *) malloc (sizeof (struct foo)); if (ptr == 0) abort (); memset (ptr, 0, sizeof (struct foo));
You can store the result of malloc into any pointer variable
without a cast, because ISO C automatically converts the type
void * to another type of pointer when necessary. But the cast
is necessary in contexts other than assignment operators or if you might
want your code to run in traditional C.
Remember that when allocating space for a string, the argument to
malloc must be one plus the length of the string. This is
because a string is terminated with a null character that doesn't count
in the "length" of the string but does need space. For example:
char *ptr; ... ptr = (char *) malloc (length + 1);
See section Representation of Strings, for more information about this.
malloc
If no more space is available, malloc returns a null pointer.
You should check the value of every call to malloc. It is
useful to write a subroutine that calls malloc and reports an
error if the value is a null pointer, returning only if the value is
nonzero. This function is conventionally called xmalloc. Here
it is:
void *
xmalloc (size_t size)
{
register void *value = malloc (size);
if (value == 0)
fatal ("virtual memory exhausted");
return value;
}
Here is a real example of using malloc (by way of xmalloc).
The function savestring will copy a sequence of characters into
a newly allocated null-terminated string:
char *
savestring (const char *ptr, size_t len)
{
register char *value = (char *) xmalloc (len + 1);
value[len] = '\0';
return (char *) memcpy (value, ptr, len);
}
The block that malloc gives you is guaranteed to be aligned so
that it can hold any type of data. In the GNU system, the address is
always a multiple of eight on most systems, and a multiple of 16 on
64-bit systems. Only rarely is any higher boundary (such as a page
boundary) necessary; for those cases, use memalign,
posix_memalign or valloc (see section Allocating Aligned Memory Blocks).
Note that the memory located after the end of the block is likely to be
in use for something else; perhaps a block already allocated by another
call to malloc. If you attempt to treat the block as longer than
you asked for it to be, you are liable to destroy the data that
malloc uses to keep track of its blocks, or you may destroy the
contents of another block. If you have already allocated a block and
discover you want it to be bigger, use realloc (see section Changing the Size of a Block).
malloc
When you no longer need a block that you got with malloc, use the
function free to make the block available to be allocated again.
The prototype for this function is in `stdlib.h'.
free function deallocates the block of memory pointed at
by ptr.
free. It's provided for
backward compatibility with SunOS; you should use free instead.
Freeing a block alters the contents of the block. Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it. Copy whatever you need out of the block before freeing it! Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to:
struct chain
{
struct chain *next;
char *name;
}
void
free_chain (struct chain *chain)
{
while (chain != 0)
{
struct chain *next = chain->next;
free (chain->name);
free (chain);
chain = next;
}
}
Occasionally, free can actually return memory to the operating
system and make the process smaller. Usually, all it can do is allow a
later call to malloc to reuse the space. In the meantime, the
space remains in your program as part of a free-list used internally by
malloc.
There is no point in freeing blocks at the end of a program, because all of the program's space is given back to the system when the process terminates.
Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block. For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer.
You can make the block longer by calling realloc. This function
is declared in `stdlib.h'.
realloc function changes the size of the block whose address is
ptr to be newsize.
Since the space after the end of the block may be in use, realloc
may find it necessary to copy the block to a new address where more free
space is available. The value of realloc is the new address of the
block. If the block needs to be moved, realloc copies the old
contents.
If you pass a null pointer for ptr, realloc behaves just
like `malloc (newsize)'. This can be convenient, but beware
that older implementations (before ISO C) may not support this
behavior, and will probably crash when realloc is passed a null
pointer.
Like malloc, realloc may return a null pointer if no
memory space is available to make the block bigger. When this happens,
the original block is untouched; it has not been modified or relocated.
In most cases it makes no difference what happens to the original block
when realloc fails, because the application program cannot continue
when it is out of memory, and the only thing to do is to give a fatal error
message. Often it is convenient to write and use a subroutine,
conventionally called xrealloc, that takes care of the error message
as xmalloc does for malloc:
void *
xrealloc (void *ptr, size_t size)
{
register void *value = realloc (ptr, size);
if (value == 0)
fatal ("Virtual memory exhausted");
return value;
}
You can also use realloc to make a block smaller. The reason you
would do this is to avoid tying up a lot of memory space when only a little
is needed.
In several allocation implementations, making a block smaller sometimes
necessitates copying it, so it can fail if no other space is available.
If the new size you specify is the same as the old size, realloc
is guaranteed to change nothing and return the same address that you gave.
The function calloc allocates memory and clears it to zero. It
is declared in `stdlib.h'.
calloc returns.
You could define calloc as follows:
void *
calloc (size_t count, size_t eltsize)
{
size_t size = count * eltsize;
void *value = malloc (size);
if (value != 0)
memset (value, 0, size);
return value;
}
But in general, it is not guaranteed that calloc calls
malloc internally. Therefore, if an application provides its own
malloc/realloc/free outside the C library, it
should always define calloc, too.
malloc
As opposed to other versions, the malloc in the GNU C Library
does not round up block sizes to powers of two, neither for large nor
for small sizes. Neighboring chunks can be coalesced on a free
no matter what their size is. This makes the implementation suitable
for all kinds of allocation patterns without generally incurring high
memory waste through fragmentation.
Very large blocks (much larger than a page) are allocated with
mmap (anonymous or via /dev/zero) by this implementation.
This has the great advantage that these chunks are returned to the
system immediately when they are freed. Therefore, it cannot happen
that a large chunk becomes "locked" in between smaller ones and even
after calling free wastes memory. The size threshold for
mmap to be used can be adjusted with mallopt. The use of
mmap can also be disabled completely.
The address of a block returned by malloc or realloc in
the GNU system is always a multiple of eight (or sixteen on 64-bit
systems). If you need a block whose address is a multiple of a higher
power of two than that, use memalign, posix_memalign, or
valloc. These functions are declared in `stdlib.h'.
With the GNU library, you can use free to free the blocks that
memalign, posix_memalign, and valloc return. That
does not work in BSD, however--BSD does not provide any way to free
such blocks.
memalign function allocates a block of size bytes whose
address is a multiple of boundary. The boundary must be a
power of two! The function memalign works by allocating a
somewhat larger block, and then returning an address within the block
that is on the specified boundary.
posix_memalign function is similar to the memalign
function in that it returns a buffer of size bytes aligned to a
multiple of alignment. But it adds one requirement to the
parameter alignment: the value must be a power of two multiple of
sizeof (void *).
If the function succeeds in allocation memory a pointer to the allocated
memory is returned in *memptr and the return value is zero.
Otherwise the function returns an error value indicating the problem.
This function was introduced in POSIX 1003.1d.
valloc is like using memalign and passing the page size
as the value of the second argument. It is implemented like this:
void *
valloc (size_t size)
{
return memalign (getpagesize (), size);
}
section How to get information about the memory subsystem? for more information about the memory subsystem.
You can adjust some parameters for dynamic memory allocation with the
mallopt function. This function is the general SVID/XPG
interface, defined in `malloc.h'.
mallopt, the param argument specifies the
parameter to be set, and value the new value to be set. Possible
choices for param, as defined in `malloc.h', are:
M_TRIM_THRESHOLD
sbrk to be called with a negative argument in
order to return memory to the system.
M_TOP_PAD
sbrk is required. It also specifies the
number of bytes to retain when shrinking the heap by calling sbrk
with a negative argument. This provides the necessary hysteresis in
heap size such that excessive amounts of system calls can be avoided.
M_MMAP_THRESHOLD
mmap system call. This way it is guaranteed
that the memory for these chunks can be returned to the system on
free.
M_MMAP_MAX
mmap. Setting this
to zero disables all use of mmap.
You can ask malloc to check the consistency of dynamic memory by
using the mcheck function. This function is a GNU extension,
declared in `mcheck.h'.
mcheck tells malloc to perform occasional
consistency checks. These will catch things such as writing
past the end of a block that was allocated with malloc.
The abortfn argument is the function to call when an inconsistency
is found. If you supply a null pointer, then mcheck uses a
default function which prints a message and calls abort
(see section Aborting a Program). The function you supply is called with
one argument, which says what sort of inconsistency was detected; its
type is described below.
It is too late to begin allocation checking once you have allocated
anything with malloc. So mcheck does nothing in that
case. The function returns -1 if you call it too late, and
0 otherwise (when it is successful).
The easiest way to arrange to call mcheck early enough is to use
the option `-lmcheck' when you link your program; then you don't
need to modify your program source at all. Alternatively you might use
a debugger to insert a call to mcheck whenever the program is
started, for example these gdb commands will automatically call mcheck
whenever the program starts:
(gdb) break main Breakpoint 1, main (argc=2, argv=0xbffff964) at whatever.c:10 (gdb) command 1 Type commands for when breakpoint 1 is hit, one per line. End with a line saying just "end". >call mcheck(0) >continue >end (gdb) ...
This will however only work if no initialization function of any object
involved calls any of the malloc functions since mcheck
must be called before the first such function.
mprobe function lets you explicitly check for inconsistencies
in a particular allocated block. You must have already called
mcheck at the beginning of the program, to do its occasional
checks; calling mprobe requests an additional consistency check
to be done at the time of the call.
The argument pointer must be a pointer returned by malloc
or realloc. mprobe returns a value that says what
inconsistency, if any, was found. The values are described below.
MCHECK_DISABLED
mcheck was not called before the first allocation.
No consistency checking can be done.
MCHECK_OK
MCHECK_HEAD
MCHECK_TAIL
MCHECK_FREE
Another possibility to check for and guard against bugs in the use of
malloc, realloc and free is to set the environment
variable MALLOC_CHECK_. When MALLOC_CHECK_ is set, a
special (less efficient) implementation is used which is designed to be
tolerant against simple errors, such as double calls of free with
the same argument, or overruns of a single byte (off-by-one bugs). Not
all such errors can be protected against, however, and memory leaks can
result. If MALLOC_CHECK_ is set to 0, any detected heap
corruption is silently ignored; if set to 1, a diagnostic is
printed on stderr; if set to 2, abort is called
immediately. This can be useful because otherwise a crash may happen
much later, and the true cause for the problem is then very hard to
track down.
There is one problem with MALLOC_CHECK_: in SUID or SGID binaries
it could possibly be exploited since diverging from the normal programs
behaviour it now writes something to the standard error desriptor.
Therefore the use of MALLOC_CHECK_ is disabled by default for
SUID and SGID binaries. It can be enabled again by the system
administrator by adding a file `/etc/suid-debug' (the content is
not important it could be empty).
So, what's the difference between using MALLOC_CHECK_ and linking
with `-lmcheck'? MALLOC_CHECK_ is orthogonal with respect to
`-lmcheck'. `-lmcheck' has been added for backward
compatibility. Both MALLOC_CHECK_ and `-lmcheck' should
uncover the same bugs - but using MALLOC_CHECK_ you don't need to
recompile your application.
The GNU C library lets you modify the behavior of malloc,
realloc, and free by specifying appropriate hook
functions. You can use these hooks to help you debug programs that use
dynamic memory allocation, for example.
The hook variables are declared in `malloc.h'.
malloc uses whenever it is called. You should define this
function to look like malloc; that is, like:
void *function (size_t size, const void *caller)
The value of caller is the return address found on the stack when
the malloc function was called. This value allows you to trace
the memory consumption of the program.
realloc
uses whenever it is called. You should define this function to look
like realloc; that is, like:
void *function (void *ptr, size_t size, const void *caller)
The value of caller is the return address found on the stack when
the realloc function was called. This value allows you to trace the
memory consumption of the program.
free
uses whenever it is called. You should define this function to look
like free; that is, like:
void function (void *ptr, const void *caller)
The value of caller is the return address found on the stack when
the free function was called. This value allows you to trace the
memory consumption of the program.
memalign
uses whenever it is called. You should define this function to look
like memalign; that is, like:
void *function (size_t size, size_t alignment, const void *caller)
The value of caller is the return address found on the stack when
the memalign function was called. This value allows you to trace the
memory consumption of the program.
You must make sure that the function you install as a hook for one of these functions does not call that function recursively without restoring the old value of the hook first! Otherwise, your program will get stuck in an infinite recursion. Before calling the function recursively, one should make sure to restore all the hooks to their previous value. When coming back from the recursive call, all the hooks should be resaved since a hook might modify itself.
void (*__malloc_initialize_hook) (void) = my_init_hook;
An issue to look out for is the time at which the malloc hook functions
can be safely installed. If the hook functions call the malloc-related
functions recursively, it is necessary that malloc has already properly
initialized itself at the time when __malloc_hook etc. is
assigned to. On the other hand, if the hook functions provide a
complete malloc implementation of their own, it is vital that the hooks
are assigned to before the very first malloc call has
completed, because otherwise a chunk obtained from the ordinary,
un-hooked malloc may later be handed to __free_hook, for example.
In both cases, the problem can be solved by setting up the hooks from
within a user-defined function pointed to by
__malloc_initialize_hook---then the hooks will be set up safely
at the right time.
Here is an example showing how to use __malloc_hook and
__free_hook properly. It installs a function that prints out
information every time malloc or free is called. We just
assume here that realloc and memalign are not used in our
program.
/* Prototypes for __malloc_hook, __free_hook */
#include <malloc.h>
/* Prototypes for our hooks. */
static void *my_init_hook (void);
static void *my_malloc_hook (size_t, const void *);
static void my_free_hook (void*, const void *);
/* Override initializing hook from the C library. */
void (*__malloc_initialize_hook) (void) = my_init_hook;
static void
my_init_hook (void)
{
old_malloc_hook = __malloc_hook;
old_free_hook = __free_hook;
__malloc_hook = my_malloc_hook;
__free_hook = my_free_hook;
}
static void *
my_malloc_hook (size_t size, const void *caller)
{
void *result;
/* Restore all old hooks */
__malloc_hook = old_malloc_hook;
__free_hook = old_free_hook;
/* Call recursively */
result = malloc (size);
/* Save underlaying hooks */
old_malloc_hook = __malloc_hook;
old_free_hook = __free_hook;
/* printf might call malloc, so protect it too. */
printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
/* Restore our own hooks */
__malloc_hook = my_malloc_hook;
__free_hook = my_free_hook;
return result;
}
static void *
my_free_hook (void *ptr, const void *caller)
{
/* Restore all old hooks */
__malloc_hook = old_malloc_hook;
__free_hook = old_free_hook;
/* Call recursively */
free (ptr);
/* Save underlaying hooks */
old_malloc_hook = __malloc_hook;
old_free_hook = __free_hook;
/* printf might call free, so protect it too. */
printf ("freed pointer %p\n", ptr);
/* Restore our own hooks */
__malloc_hook = my_malloc_hook;
__free_hook = my_free_hook;
}
main ()
{
...
}
The mcheck function (see section Heap Consistency Checking) works by
installing such hooks.
malloc
You can get information about dynamic memory allocation by calling the
mallinfo function. This function and its associated data type
are declared in `malloc.h'; they are an extension of the standard
SVID/XPG version.
int arena
sbrk by
malloc, in bytes.
int ordblks
malloc requests; see
section Efficiency Considerations for malloc.)
int smblks
int hblks
mmap.
int hblkhd
mmap, in bytes.
int usmblks
int fsmblks
int uordblks
malloc.
int fordblks
int keepcost
struct mallinfo.
malloc-Related Functions
Here is a summary of the functions that work with malloc:
void *malloc (size_t size)
void free (void *addr)
malloc. See section Freeing Memory Allocated with malloc.
void *realloc (void *addr, size_t size)
malloc larger or smaller,
possibly by copying it to a new location. See section Changing the Size of a Block.
void *calloc (size_t count, size_t eltsize)
malloc, and set its contents to zero. See section Allocating Cleared Space.
void *valloc (size_t size)
void *memalign (size_t size, size_t boundary)
int mallopt (int param, int value)
int mcheck (void (*abortfn) (void))
malloc to perform occasional consistency checks on
dynamically allocated memory, and to call abortfn when an
inconsistency is found. See section Heap Consistency Checking.
void *(*__malloc_hook) (size_t size, const void *caller)
malloc uses whenever it is called.
void *(*__realloc_hook) (void *ptr, size_t size, const void *caller)
realloc uses whenever it is called.
void (*__free_hook) (void *ptr, const void *caller)
free uses whenever it is called.
void (*__memalign_hook) (size_t size, size_t alignment, const void *caller)
memalign uses whenever it is called.
struct mallinfo mallinfo (void)
malloc.
A complicated task when programming with languages which do not use garbage collected dynamic memory allocation is to find memory leaks. Long running programs must assure that dynamically allocated objects are freed at the end of their lifetime. If this does not happen the system runs out of memory, sooner or later.
The malloc implementation in the GNU C library provides some
simple means to detect such leaks and obtain some information to find
the location. To do this the application must be started in a special
mode which is enabled by an environment variable. There are no speed
penalties for the program if the debugging mode is not enabled.
mtrace function is called it looks for an environment
variable named MALLOC_TRACE. This variable is supposed to
contain a valid file name. The user must have write access. If the
file already exists it is truncated. If the environment variable is not
set or it does not name a valid file which can be opened for writing
nothing is done. The behaviour of malloc etc. is not changed.
For obvious reasons this also happens if the application is installed
with the SUID or SGID bit set.
If the named file is successfully opened, mtrace installs special
handlers for the functions malloc, realloc, and
free (see section Memory Allocation Hooks). From then on, all uses of these
functions are traced and protocolled into the file. There is now of
course a speed penalty for all calls to the traced functions so tracing
should not be enabled during normal use.
This function is a GNU extension and generally not available on other systems. The prototype can be found in `mcheck.h'.
muntrace function can be called after mtrace was used
to enable tracing the malloc calls. If no (succesful) call of
mtrace was made muntrace does nothing.
Otherwise it deinstalls the handlers for malloc, realloc,
and free and then closes the protocol file. No calls are
protocolled anymore and the program runs again at full speed.
This function is a GNU extension and generally not available on other systems. The prototype can be found in `mcheck.h'.
Even though the tracing functionality does not influence the runtime
behaviour of the program it is not a good idea to call mtrace in
all programs. Just imagine that you debug a program using mtrace
and all other programs used in the debugging session also trace their
malloc calls. The output file would be the same for all programs
and thus is unusable. Therefore one should call mtrace only if
compiled for debugging. A program could therefore start like this:
#include <mcheck.h>
int
main (int argc, char *argv[])
{
#ifdef DEBUGGING
mtrace ();
#endif
...
}
This is all what is needed if you want to trace the calls during the
whole runtime of the program. Alternatively you can stop the tracing at
any time with a call to muntrace. It is even possible to restart
the tracing again with a new call to mtrace. But this can cause
unreliable results since there may be calls of the functions which are
not called. Please note that not only the application uses the traced
functions, also libraries (including the C library itself) use these
functions.
This last point is also why it is no good idea to call muntrace
before the program terminated. The libraries are informed about the
termination of the program only after the program returns from
main or calls exit and so cannot free the memory they use
before this time.
So the best thing one can do is to call mtrace as the very first
function in the program and never call muntrace. So the program
traces almost all uses of the malloc functions (except those
calls which are executed by constructors of the program or used
libraries).
You know the situation. The program is prepared for debugging and in all debugging sessions it runs well. But once it is started without debugging the error shows up. A typical example is a memory leak that becomes visible only when we turn off the debugging. If you foresee such situations you can still win. Simply use something equivalent to the following little program:
#include <mcheck.h>
#include <signal.h>
static void
enable (int sig)
{
mtrace ();
signal (SIGUSR1, enable);
}
static void
disable (int sig)
{
muntrace ();
signal (SIGUSR2, disable);
}
int
main (int argc, char *argv[])
{
...
signal (SIGUSR1, enable);
signal (SIGUSR2, disable);
...
}
I.e., the user can start the memory debugger any time s/he wants if the
program was started with MALLOC_TRACE set in the environment.
The output will of course not show the allocations which happened before
the first signal but if there is a memory leak this will show up
nevertheless.
If you take a look at the output it will look similar to this:
= Start [0x8048209] - 0x8064cc8 [0x8048209] - 0x8064ce0 [0x8048209] - 0x8064cf8 [0x80481eb] + 0x8064c48 0x14 [0x80481eb] + 0x8064c60 0x14 [0x80481eb] + 0x8064c78 0x14 [0x80481eb] + 0x8064c90 0x14 = End
What this all means is not really important since the trace file is not
meant to be read by a human. Therefore no attention is given to
readability. Instead there is a program which comes with the GNU C
library which interprets the traces and outputs a summary in an
user-friendly way. The program is called mtrace (it is in fact a
Perl script) and it takes one or two arguments. In any case the name of
the file with the trace output must be specified. If an optional
argument precedes the name of the trace file this must be the name of
the program which generated the trace.
drepper$ mtrace tst-mtrace log No memory leaks.
In this case the program tst-mtrace was run and it produced a
trace file `log'. The message printed by mtrace shows there
are no problems with the code, all allocated memory was freed
afterwards.
If we call mtrace on the example trace given above we would get a
different outout:
drepper$ mtrace errlog - 0x08064cc8 Free 2 was never alloc'd 0x8048209 - 0x08064ce0 Free 3 was never alloc'd 0x8048209 - 0x08064cf8 Free 4 was never alloc'd 0x8048209 Memory not freed: ----------------- Address Size Caller 0x08064c48 0x14 at 0x80481eb 0x08064c60 0x14 at 0x80481eb 0x08064c78 0x14 at 0x80481eb 0x08064c90 0x14 at 0x80481eb
We have called mtrace with only one argument and so the script
has no chance to find out what is meant with the addresses given in the
trace. We can do better:
drepper$ mtrace tst errlog - 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst.c:39 - 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst.c:39 - 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst.c:39 Memory not freed: ----------------- Address Size Caller 0x08064c48 0x14 at /home/drepper/tst.c:33 0x08064c60 0x14 at /home/drepper/tst.c:33 0x08064c78 0x14 at /home/drepper/tst.c:33 0x08064c90 0x14 at /home/drepper/tst.c:33
Suddenly the output makes much more sense and the user can see immediately where the function calls causing the trouble can be found.
Interpreting this output is not complicated. There are at most two
different situations being detected. First, free was called for
pointers which were never returned by one of the allocation functions.
This is usually a very bad problem and what this looks like is shown in
the first three lines of the output. Situations like this are quite
rare and if they appear they show up very drastically: the program
normally crashes.
The other situation which is much harder to detect are memory leaks. As
you can see in the output the mtrace function collects all this
information and so can say that the program calls an allocation function
from line 33 in the source file `/home/drepper/tst-mtrace.c' four
times without freeing this memory before the program terminates.
Whether this is a real problem remains to be investigated.
An obstack is a pool of memory containing a stack of objects. You can create any number of separate obstacks, and then allocate objects in specified obstacks. Within each obstack, the last object allocated must always be the first one freed, but distinct obstacks are independent of each other.
Aside from this one constraint of order of freeing, obstacks are totally general: an obstack can contain any number of objects of any size. They are implemented with macros, so allocation is usually very fast as long as the objects are usually small. And the only space overhead per object is the padding needed to start each object on a suitable boundary.
The utilities for manipulating obstacks are declared in the header file `obstack.h'.
struct
obstack. This structure has a small fixed size; it records the status
of the obstack and how to find the space in which objects are allocated.
It does not contain any of the objects themselves. You should not try
to access the contents of the structure directly; use only the functions
described in this chapter.
You can declare variables of type struct obstack and use them as
obstacks, or you can allocate obstacks dynamically like any other kind
of object. Dynamic allocation of obstacks allows your program to have a
variable number of different stacks. (You can even allocate an
obstack structure in another obstack, but this is rarely useful.)
All the functions that work with obstacks require you to specify which
obstack to use. You do this with a pointer of type struct obstack
*. In the following, we often say "an obstack" when strictly
speaking the object at hand is such a pointer.
The objects in the obstack are packed into large blocks called
chunks. The struct obstack structure points to a chain of
the chunks currently in use.
The obstack library obtains a new chunk whenever you allocate an object
that won't fit in the previous chunk. Since the obstack library manages
chunks automatically, you don't need to pay much attention to them, but
you do need to supply a function which the obstack library should use to
get a chunk. Usually you supply a function which uses malloc
directly or indirectly. You must also supply a function to free a chunk.
These matters are described in the following section.
Each source file in which you plan to use the obstack functions must include the header file `obstack.h', like this:
#include <obstack.h>
Also, if the source file uses the macro obstack_init, it must
declare or define two functions or macros that will be called by the
obstack library. One, obstack_chunk_alloc, is used to allocate
the chunks of memory into which objects are packed. The other,
obstack_chunk_free, is used to return chunks when the objects in
them are freed. These macros should appear before any use of obstacks
in the source file.
Usually these are defined to use malloc via the intermediary
xmalloc (see section Unconstrained Allocation). This is done with
the following pair of macro definitions:
#define obstack_chunk_alloc xmalloc #define obstack_chunk_free free
Though the memory you get using obstacks really comes from malloc,
using obstacks is faster because malloc is called less often, for
larger blocks of memory. See section Obstack Chunks, for full details.
At run time, before the program can use a struct obstack object
as an obstack, it must initialize the obstack by calling
obstack_init.
obstack_chunk_alloc function. If
allocation of memory fails, the function pointed to by
obstack_alloc_failed_handler is called. The obstack_init
function always returns 1 (Compatibility notice: Former versions of
obstack returned 0 if allocation failed).
Here are two examples of how to allocate the space for an obstack and initialize it. First, an obstack that is a static variable:
static struct obstack myobstack; ... obstack_init (&myobstack);
Second, an obstack that is itself dynamically allocated:
struct obstack *myobstack_ptr = (struct obstack *) xmalloc (sizeof (struct obstack)); obstack_init (myobstack_ptr);
obstack uses when obstack_chunk_alloc fails to allocate
memory. The default action is to print a message and abort.
You should supply a function that either calls exit
(see section Program Termination) or longjmp (see section Non-Local Exits) and doesn't return.
void my_obstack_alloc_failed (void) ... obstack_alloc_failed_handler = &my_obstack_alloc_failed;
The most direct way to allocate an object in an obstack is with
obstack_alloc, which is invoked almost like malloc.
struct obstack
object which represents the obstack. Each obstack function or macro
requires you to specify an obstack-ptr as the first argument.
This function calls the obstack's obstack_chunk_alloc function if
it needs to allocate a new chunk of memory; it calls
obstack_alloc_failed_handler if allocation of memory by
obstack_chunk_alloc failed.
For example, here is a function that allocates a copy of a string str
in a specific obstack, which is in the variable string_obstack:
struct obstack string_obstack;
char *
copystring (char *string)
{
size_t len = strlen (string) + 1;
char *s = (char *) obstack_alloc (&string_obstack, len);
memcpy (s, string, len);
return s;
}
To allocate a block with specified contents, use the function
obstack_copy, declared like this:
obstack_alloc_failed_handler if allocation of memory by
obstack_chunk_alloc failed.
obstack_copy, but appends an extra byte containing a null
character. This extra byte is not counted in the argument size.
The obstack_copy0 function is convenient for copying a sequence
of characters into an obstack as a null-terminated string. Here is an
example of its use:
char *
obstack_savestring (char *addr, int size)
{
return obstack_copy0 (&myobstack, addr, size);
}
Contrast this with the previous example of savestring using
malloc (see section Basic Memory Allocation).
To free an object allocated in an obstack, use the function
obstack_free. Since the obstack is a stack of objects, freeing
one object automatically frees all other objects allocated more recently
in the same obstack.
Note that if object is a null pointer, the result is an
uninitialized obstack. To free all memory in an obstack but leave it
valid for further allocation, call obstack_free with the address
of the first object allocated on the obstack:
obstack_free (obstack_ptr, first_object_allocated_ptr);
Recall that the objects in an obstack are grouped into chunks. When all the objects in a chunk become free, the obstack library automatically frees the chunk (see section Preparing for Using Obstacks). Then other obstacks, or non-obstack allocation, can reuse the space of the chunk.
The interfaces for using obstacks may be defined either as functions or as macros, depending on the compiler. The obstack facility works with all C compilers, including both ISO C and traditional C, but there are precautions you must take if you plan to use compilers other than GNU C.
If you are using an old-fashioned non-ISO C compiler, all the obstack "functions" are actually defined only as macros. You can call these macros like functions, but you cannot use them in any other way (for example, you cannot take their address).
Calling the macros requires a special precaution: namely, the first operand (the obstack pointer) may not contain any side effects, because it may be computed more than once. For example, if you write this:
obstack_alloc (get_obstack (), 4);
you will find that get_obstack may be called several times.
If you use *obstack_list_ptr++ as the obstack pointer argument,
you will get very strange results since the incrementation may occur
several times.
In ISO C, each function has both a macro definition and a function definition. The function definition is used if you take the address of the function without calling it. An ordinary call uses the macro definition by default, but you can request the function definition instead by writing the function name in parentheses, as shown here:
char *x; void *(*funcp) (); /* Use the macro. */ x = (char *) obstack_alloc (obptr, size); /* Call the function. */ x = (char *) (obstack_alloc) (obptr, size); /* Take the address of the function. */ funcp = obstack_alloc;
This is the same situation that exists in ISO C for the standard library functions. See section Macro Definitions of Functions.
Warning: When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in ISO C.
If you use the GNU C compiler, this precaution is not necessary, because various language extensions in GNU C permit defining the macros so as to compute each argument only once.
Because memory in obstack chunks is used sequentially, it is possible to build up an object step by step, adding one or more bytes at a time to the end of the object. With this technique, you do not need to know how much data you will put in the object until you come to the end of it. We call this the technique of growing objects. The special functions for adding data to the growing object are described in this section.
You don't need to do anything special when you start to grow an object.
Using one of the functions to add data to the object automatically
starts it. However, it is necessary to say explicitly when the object is
finished. This is done with the function obstack_finish.
The actual address of the object thus built up is not known until the object is finished. Until then, it always remains possible that you will add so much data that the object must be copied into a new chunk.
While the obstack is in use for a growing object, you cannot use it for ordinary allocation of another object. If you try to do so, the space already added to the growing object will become part of the other object.
obstack_blank, which adds space without initializing it.
obstack_grow, which is
the growing-object analogue of obstack_copy. It adds size
bytes of data to the growing object, copying the contents from
data.
obstack_copy0. It adds
size bytes copied from data, followed by an additional null
character.
obstack_1grow.
It adds a single byte containing c to the growing object.
obstack_ptr_grow. It adds sizeof (void *) bytes
containing the value of data.
int can be added by using the
obstack_int_grow function. It adds sizeof (int) bytes to
the growing object and initializes them with the value of data.
obstack_finish to close it off and return its final address.
Once you have finished the object, the obstack is available for ordinary allocation or for growing another object.
This function can return a null pointer under the same conditions as
obstack_alloc (see section Allocation in an Obstack).
When you build an object by growing it, you will probably need to know
afterward how long it became. You need not keep track of this as you grow
the object, because you can find out the length from the obstack just
before finishing the object with the function obstack_object_size,
declared as follows:
obstack_object_size will return zero.
If you have started growing an object and wish to cancel it, you should finish it and then free it, like this:
obstack_free (obstack_ptr, obstack_finish (obstack_ptr));
This has no effect if no object was growing.
You can use obstack_blank with a negative size argument to make
the current object smaller. Just don't try to shrink it beyond zero
length--there's no telling what will happen if you do that.
The usual functions for growing objects incur overhead for checking whether there is room for the new growth in the current chunk. If you are frequently constructing objects in small steps of growth, this overhead can be significant.
You can reduce the overhead by using special "fast growth" functions that grow the object without checking. In order to have a robust program, you must do the checking yourself. If you do this checking in the simplest way each time you are about to add data to the object, you have not saved anything, because that is what the ordinary growth functions do. But if you can arrange to check less often, or check more efficiently, then you make the program faster.
The function obstack_room returns the amount of room available
in the current chunk. It is declared as follows:
While you know there is room, you can use these fast growth functions for adding data to a growing object:
obstack_1grow_fast adds one byte containing the
character c to the growing object in obstack obstack-ptr.
obstack_ptr_grow_fast adds sizeof (void *)
bytes containing the value of data to the growing object in
obstack obstack-ptr.
obstack_int_grow_fast adds sizeof (int) bytes
containing the value of data to the growing object in obstack
obstack-ptr.
obstack_blank_fast adds size bytes to the
growing object in obstack obstack-ptr without initializing them.
When you check for space using obstack_room and there is not
enough room for what you want to add, the fast growth functions
are not safe. In this case, simply use the corresponding ordinary
growth function instead. Very soon this will copy the object to a
new chunk; then there will be lots of room available again.
So, each time you use an ordinary growth function, check afterward for
sufficient space using obstack_room. Once the object is copied
to a new chunk, there will be plenty of space again, so the program will
start using the fast growth functions again.
Here is an example:
void
add_string (struct obstack *obstack, const char *ptr, int len)
{
while (len > 0)
{
int room = obstack_room (obstack);
if (room == 0)
{
/* Not enough room. Add one character slowly,
which may copy to a new chunk and make room. */
obstack_1grow (obstack, *ptr++);
len--;
}
else
{
if (room > len)
room = len;
/* Add fast as much as we have room for. */
len -= room;
while (room-- > 0)
obstack_1grow_fast (obstack, *ptr++);
}
}
}
Here are functions that provide information on the current status of allocation in an obstack. You can use them to learn about an object while still growing it.
If no object is growing, this value says where the next object you allocate will start (once again assuming it fits in the current chunk).
obstack_next_free
returns the same value as obstack_base.
obstack_next_free (obstack-ptr) - obstack_base (obstack-ptr)
Each obstack has an alignment boundary; each object allocated in the obstack automatically starts on an address that is a multiple of the specified boundary. By default, this boundary is 4 bytes.
To access an obstack's alignment boundary, use the macro
obstack_alignment_mask, whose function prototype looks like
this:
The expansion of the macro obstack_alignment_mask is an lvalue,
so you can alter the mask by assignment. For example, this statement:
obstack_alignment_mask (obstack_ptr) = 0;
has the effect of turning off alignment processing in the specified obstack.
Note that a change in alignment mask does not take effect until
after the next time an object is allocated or finished in the
obstack. If you are not growing an object, you can make the new
alignment mask take effect immediately by calling obstack_finish.
This will finish a zero-length object and then do proper alignment for
the next object.
Obstacks work by allocating space for themselves in large chunks, and then parceling out space in the chunks to satisfy your requests. Chunks are normally 4096 bytes long unless you specify a different chunk size. The chunk size includes 8 bytes of overhead that are not actually used for storing objects. Regardless of the specified size, longer chunks will be allocated when necessary for long objects.
The obstack library allocates chunks by calling the function
obstack_chunk_alloc, which you must define. When a chunk is no
longer needed because you have freed all the objects in it, the obstack
library frees the chunk by calling obstack_chunk_free, which you
must also define.
These two must be defined (as macros) or declared (as functions) in each
source file that uses obstack_init (see section Creating Obstacks).
Most often they are defined as macros like this:
#define obstack_chunk_alloc malloc #define obstack_chunk_free free
Note that these are simple macros (no arguments). Macro definitions with
arguments will not work! It is necessary that obstack_chunk_alloc
or obstack_chunk_free, alone, expand into a function name if it is
not itself a function name.
If you allocate chunks with malloc, the chunk size should be a
power of 2. The default chunk size, 4096, was chosen because it is long
enough to satisfy many typical requests on the obstack yet short enough
not to waste too much memory in the portion of the last chunk not yet used.
Since this macro expands to an lvalue, you can specify a new chunk size by assigning it a new value. Doing so does not affect the chunks already allocated, but will change the size of chunks allocated for that particular obstack in the future. It is unlikely to be useful to make the chunk size smaller, but making it larger might improve efficiency if you are allocating many objects whose size is comparable to the chunk size. Here is how to do so cleanly:
if (obstack_chunk_size (obstack_ptr) < new-chunk-size) obstack_chunk_size (obstack_ptr) = new-chunk-size;
Here is a summary of all the functions associated with obstacks. Each
takes the address of an obstack (struct obstack *) as its first
argument.
void obstack_init (struct obstack *obstack-ptr)
void *obstack_alloc (struct obstack *obstack-ptr, int size)
void *obstack_copy (struct obstack *obstack-ptr, void *address, int size)
void *obstack_copy0 (struct obstack *obstack-ptr, void *address, int size)
void obstack_free (struct obstack *obstack-ptr, void *object)
void obstack_blank (struct obstack *obstack-ptr, int size)
void obstack_grow (struct obstack *obstack-ptr, void *address, int size)
void obstack_grow0 (struct obstack *obstack-ptr, void *address, int size)
void obstack_1grow (struct obstack *obstack-ptr, char data-char)
void *obstack_finish (struct obstack *obstack-ptr)
int obstack_object_size (struct obstack *obstack-ptr)
void obstack_blank_fast (struct obstack *obstack-ptr, int size)
void obstack_1grow_fast (struct obstack *obstack-ptr, char data-char)
int obstack_room (struct obstack *obstack-ptr)
int obstack_alignment_mask (struct obstack *obstack-ptr)
int obstack_chunk_size (struct obstack *obstack-ptr)
void *obstack_base (struct obstack *obstack-ptr)
void *obstack_next_free (struct obstack *obstack-ptr)
The function alloca supports a kind of half-dynamic allocation in
which blocks are allocated dynamically but freed automatically.
Allocating a block with alloca is an explicit action; you can
allocate as many blocks as you wish, and compute the size at run time. But
all the blocks are freed when you exit the function that alloca was
called from, just as if they were automatic variables declared in that
function. There is no way to free the space explicitly.
The prototype for alloca is in `stdlib.h'. This function is
a BSD extension.
alloca is the address of a block of size
bytes of memory, allocated in the stack frame of the calling function.
Do not use alloca inside the arguments of a function call--you
will get unpredictable results, because the stack space for the
alloca would appear on the stack in the middle of the space for
the function arguments. An example of what to avoid is foo (x,
alloca (4), y).
alloca Example
As an example of the use of alloca, here is a function that opens
a file name made from concatenating two argument strings, and returns a
file descriptor or minus one signifying failure:
int
open2 (char *str1, char *str2, int flags, int mode)
{
char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
stpcpy (stpcpy (name, str1), str2);
return open (name, flags, mode);
}
Here is how you would get the same results with malloc and
free:
int
open2 (char *str1, char *str2, int flags, int mode)
{
char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
int desc;
if (name == 0)
fatal ("virtual memory exceeded");
stpcpy (stpcpy (name, str1), str2);
desc = open (name, flags, mode);
free (name);
return desc;
}
As you can see, it is simpler with alloca. But alloca has
other, more important advantages, and some disadvantages.
alloca
Here are the reasons why alloca may be preferable to malloc:
alloca wastes very little space and is very fast. (It is
open-coded by the GNU C compiler.)
alloca does not have separate pools for different sizes of
block, space used for any size block can be reused for any other size.
alloca does not cause memory fragmentation.
longjmp (see section Non-Local Exits)
automatically free the space allocated with alloca when they exit
through the function that called alloca. This is the most
important reason to use alloca.
To illustrate this, suppose you have a function
open_or_report_error which returns a descriptor, like
open, if it succeeds, but does not return to its caller if it
fails. If the file cannot be opened, it prints an error message and
jumps out to the command level of your program using longjmp.
Let's change open2 (see section alloca Example) to use this
subroutine:
int
open2 (char *str1, char *str2, int flags, int mode)
{
char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
stpcpy (stpcpy (name, str1), str2);
return open_or_report_error (name, flags, mode);
}
Because of the way alloca works, the memory it allocates is
freed even when an error occurs, with no special effort required.
By contrast, the previous definition of open2 (which uses
malloc and free) would develop a memory leak if it were
changed in this way. Even if you are willing to make more changes to
fix it, there is no easy way to do so.
alloca
These are the disadvantages of alloca in comparison with
malloc:
alloca, so it is less
portable. However, a slower emulation of alloca written in C
is available for use on systems with this deficiency.
In GNU C, you can replace most uses of alloca with an array of
variable size. Here is how open2 would look then:
int open2 (char *str1, char *str2, int flags, int mode)
{
char name[strlen (str1) + strlen (str2) + 1];
stpcpy (stpcpy (name, str1), str2);
return open (name, flags, mode);
}
But alloca is not always equivalent to a variable-sized array, for
several reasons:
alloca
remains until the end of the function.
alloca within a loop, allocating an
additional block on each iteration. This is impossible with
variable-sized arrays.
Note: If you mix use of alloca and variable-sized arrays
within one function, exiting a scope in which a variable-sized array was
declared frees all blocks allocated with alloca during the
execution of that scope.
The symbols in this section are declared in `unistd.h'.
You will not normally use the functions in this section, because the functions described in section Allocating Storage For Program Data are easier to use. Those are interfaces to a GNU C Library memory allocator that uses the functions below itself. The functions below are simple interfaces to system calls.
brk sets the high end of the calling process' data segment to
addr.
The address of the end of a segment is defined to be the address of the last byte in the segment plus 1.
The function has no effect if addr is lower than the low end of the data segment. (This is considered success, by the way).
The function fails if it would cause the data segment to overlap another segment or exceed the process' data storage limit (see section Limiting Resource Usage).
The function is named for a common historical case where data storage and the stack are in the same segment. Data storage allocation grows upward from the bottom of the segment while the stack grows downward toward it from the top of the segment and the curtain between them is called the break.
The return value is zero on success. On failure, the return value is
-1 and errno is set accordingly. The following errno
values are specific to this function:
ENOMEM
brk except that you specify the new
end of the data segment as an offset delta from the current end
and on success the return value is the address of the resulting end of
the data segment instead of zero.
This means you can use `sbrk(0)' to find out what the current end of the data segment is.
You can tell the system to associate a particular virtual memory page with a real page frame and keep it that way -- i.e. cause the page to be paged in if it isn't already and mark it so it will never be paged out and consequently will never cause a page fault. This is called locking a page.
The functions in this chapter lock and unlock the calling process' pages.
Because page faults cause paged out pages to be paged in transparently, a process rarely needs to be concerned about locking pages. However, there are two reasons people sometimes are:
Be aware that when you lock a page, that's one fewer page frame that can be used to back other virtual memory (by the same or other processes), which can mean more page faults, which means the system runs more slowly. In fact, if you lock enough memory, some programs may not be able to run at all for lack of real memory.
A memory lock is associated with a virtual page, not a real frame. The paging rule is: If a frame backs at least one locked page, don't page it out.
Memory locks do not stack. I.e. you can't lock a particular page twice so that it has to be unlocked twice before it is truly unlocked. It is either locked or it isn't.
A memory lock persists until the process that owns the memory explicitly unlocks it. (But process termination and exec cause the virtual memory to cease to exist, which you might say means it isn't locked any more).
Memory locks are not inherited by child processes. (But note that on a modern Unix system, immediately after a fork, the parent's and the child's virtual address space are backed by the same real page frames, so the child enjoys the parent's locks). See section Creating a Process.
Because of its ability to impact other processes, only the superuser can lock a page. Any process can unlock its own page.
The system sets limits on the amount of memory a process can have locked and the amount of real memory it can have dedicated to it. See section Limiting Resource Usage.
In Linux, locked pages aren't as locked as you might think. Two virtual pages that are not shared memory can nonetheless be backed by the same real frame. The kernel does this in the name of efficiency when it knows both virtual pages contain identical data, and does it even if one or both of the virtual pages are locked.
But when a process modifies one of those pages, the kernel must get it a separate frame and fill it with the page's data. This is known as a copy-on-write page fault. It takes a small amount of time and in a pathological case, getting that frame may require I/O.
To make sure this doesn't happen to your program, don't just lock the pages. Write to them as well, unless you know you won't write to them ever. And to make sure you have pre-allocated frames for your stack, enter a scope that declares a C automatic variable larger than the maximum stack size you will need, set it to something, then return from its scope.
The symbols in this section are declared in `sys/mman.h'. These functions are defined by POSIX.1b, but their availability depends on your kernel. If your kernel doesn't allow these functions, they exist but always fail. They are available with a Linux kernel.
Portability Note: POSIX.1b requires that when the mlock
and munlock functions are available, the file `unistd.h'
define the macro _POSIX_MEMLOCK_RANGE and the file
limits.h define the macro PAGESIZE to be the size of a
memory page in bytes. It requires that when the mlockall and
munlockall functions are available, the `unistd.h' file
define the macro _POSIX_MEMLOCK. The GNU C library conforms to
this requirement.
mlock locks a range of the calling process' virtual pages.
The range of memory starts at address addr and is len bytes long. Actually, since you must lock whole pages, it is the range of pages that include any part of the specified range.
When the function returns successfully, each of those pages is backed by (connected to) a real frame (is resident) and is marked to stay that way. This means the function may cause page-ins and have to wait for them.
When the function fails, it does not affect the lock status of any pages.
The return value is zero if the function succeeds. Otherwise, it is
-1 and errno is set accordingly. errno values
specific to this function are:
ENOMEM
EPERM
EINVAL
ENOSYS
mlock capability.
You can lock all a process' memory with mlockall. You
unlock memory with munlock or munlockall.
To avoid all page faults in a C program, you have to use
mlockall, because some of the memory a program uses is hidden
from the C code, e.g. the stack and automatic variables, and you
wouldn't know what address to tell mlock.
mlock unlocks a range of the calling process' virtual pages.
munlock is the inverse of mlock and functions completely
analogously to mlock, except that there is no EPERM
failure.
mlockall locks all the pages in a process' virtual memory address
space, and/or any that are added to it in the future. This includes the
pages of the code, data and stack segment, as well as shared libraries,
user space kernel data, shared memory, and memory mapped files.
flags is a string of single bit flags represented by the following
macros. They tell mlockall which of its functions you want. All
other bits must be zero.
MCL_CURRENT
MCL_FUTURE
MCL_FUTURE.
See section Executing a File.
When the function returns successfully, and you specified
MCL_CURRENT, all of the process' pages are backed by (connected
to) real frames (they are resident) and are marked to stay that way.
This means the function may cause page-ins and have to wait for them.
When the process is in MCL_FUTURE mode because it successfully
executed this function and specified MCL_CURRENT, any system call
by the process that requires space be added to its virtual address space
fails with errno = ENOMEM if locking the additional space
would cause the process to exceed its locked page limit. In the case
that the address space addition that can't be accomodated is stack
expansion, the stack expansion fails and the kernel sends a
SIGSEGV signal to the process.
When the function fails, it does not affect the lock status of any pages or the future locking mode.
The return value is zero if the function succeeds. Otherwise, it is
-1 and errno is set accordingly. errno values
specific to this function are:
ENOMEM
EPERM
EINVAL
ENOSYS
mlockall capability.
You can lock just specific pages with mlock. You unlock pages
with munlockall and munlock.
munlockall unlocks every page in the calling process' virtual
address space and turn off MCL_FUTURE future locking mode.
The return value is zero if the function succeeds. Otherwise, it is
-1 and errno is set accordingly. The only way this
function can fail is for generic reasons that all functions and system
calls can fail, so there are no specific errno values.
Programs that work with characters and strings often need to classify a character--is it alphabetic, is it a digit, is it whitespace, and so on--and perform case conversion operations on characters. The functions in the header file `ctype.h' are provided for this purpose.
Since the choice of locale and character set can alter the
classifications of particular character codes, all of these functions
are affected by the current locale. (More precisely, they are affected
by the locale currently selected for character classification--the
LC_CTYPE category; see section Categories of Activities that Locales Affect.)
The ISO C standard specifies two different sets of functions. The
one set works on char type characters, the other one on
wchar_t wide characters (see section Introduction to Extended Characters).
This section explains the library functions for classifying characters.
For example, isalpha is the function to test for an alphabetic
character. It takes one argument, the character to test, and returns a
nonzero integer if the character is alphabetic, and zero otherwise. You
would use it like this:
if (isalpha (c))
printf ("The character `%c' is alphabetic.\n", c);
Each of the functions in this section tests for membership in a
particular class of characters; each has a name starting with `is'.
Each of them takes one argument, which is a character to test, and
returns an int which is treated as a boolean value. The
character argument is passed as an int, and it may be the
constant value EOF instead of a real character.
The attributes of any given character can vary between locales. See section Locales and Internationalization, for more information on locales.
These functions are declared in the header file `ctype.h'.
islower or isupper is true of a character, then
isalpha is also true.
In some locales, there may be additional characters for which
isalpha is true--letters which are neither upper case nor lower
case. But in the standard "C" locale, there are no such
additional characters.
isalpha or isdigit is
true of a character, then isalnum is also true.
"C" locale, isspace returns true for only the standard
whitespace characters:
' '
'\f'
'\n'
'\r'
'\t'
'\v'
unsigned char value that fits
into the US/UK ASCII character set. This function is a BSD extension
and is also an SVID extension.
This section explains the library functions for performing conversions
such as case mappings on characters. For example, toupper
converts any character to upper case if possible. If the character
can't be converted, toupper returns it unchanged.
These functions take one argument of type int, which is the
character to convert, and return the converted character as an
int. If the conversion is not applicable to the argument given,
the argument is returned unchanged.
Compatibility Note: In pre-ISO C dialects, instead of
returning the argument unchanged, these functions may fail when the
argument is not suitable for the conversion. Thus for portability, you
may need to write islower(c) ? toupper(c) : c rather than just
toupper(c).
These functions are declared in the header file `ctype.h'.
tolower returns the corresponding
lower-case letter. If c is not an upper-case letter,
c is returned unchanged.
toupper returns the corresponding
upper-case letter. Otherwise c is returned unchanged.
unsigned char value
that fits into the US/UK ASCII character set, by clearing the high-order
bits. This function is a BSD extension and is also an SVID extension.
tolower, and is provided for compatibility
with the SVID. See section SVID (The System V Interface Description).
toupper, and is provided for compatibility
with the SVID.
Amendment 1 to ISO C90 defines functions to classify wide
characters. Although the original ISO C90 standard already defined
the type wchar_t, no functions operating on them were defined.
The general design of the classification functions for wide characters
is more general. It allows extensions to the set of available
classifications, beyond those which are always available. The POSIX
standard specifies how extensions can be made, and this is already
implemented in the GNU C library implementation of the localedef
program.
The character class functions are normally implemented with bitsets, with a bitset per character. For a given character, the appropriate bitset is read from a table and a test is performed as to whether a certain bit is set. Which bit is tested for is determined by the class.
For the wide character classification functions this is made visible.
There is a type classification type defined, a function to retrieve this
value for a given class, and a function to test whether a given
character is in this class, using the classification value. On top of
this the normal character classification functions as used for
char objects can be defined.
wctype_t can hold a value which represents a character class.
The only defined way to generate such a value is by using the
wctype function.
wctype returns a value representing a class of wide
characters which is identified by the string property. Beside
some standard properties each locale can define its own ones. In case
no property with the given name is known for the current locale
selected for the LC_CTYPE category, the function returns zero.
The properties known in every locale are:
@multitable @columnfractions .25 .25 .25 .25
"alnum" @tab "alpha" @tab "cntrl" @tab "digit"
"graph" @tab "lower" @tab "print" @tab "punct"
"space" @tab "upper" @tab "xdigit"
This function is declared in `wctype.h'.
wctype.
This function is declared in `wctype.h'.
wctype if the property string is one of the known character
classes. In some situations it is desirable to construct the property
strings, and then it is important that wctype can also handle the
standard classes.
iswalpha
or iswdigit is true of a character, then iswalnum is also
true.
This function can be implemented using
iswctype (wc, wctype ("alnum"))
It is declared in `wctype.h'.
iswlower or iswupper is true of a character, then
iswalpha is also true.
In some locales, there may be additional characters for which
iswalpha is true--letters which are neither upper case nor lower
case. But in the standard "C" locale, there are no such
additional characters.
This function can be implemented using
iswctype (wc, wctype ("alpha"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("cntrl"))
It is declared in `wctype.h'.
n = 0;
while (iswdigit (*wc))
{
n *= 10;
n += *wc++ - L'0';
}
This function can be implemented using
iswctype (wc, wctype ("digit"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("graph"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("lower"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("print"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("punct"))
It is declared in `wctype.h'.
"C" locale, iswspace returns true for only the standard
whitespace characters:
L' '
L'\f'
L'\n'
L'\r'
L'\t'
L'\v'
iswctype (wc, wctype ("space"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("upper"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("xdigit"))
It is declared in `wctype.h'.
The first note is probably not astonishing but still occasionally a
cause of problems. The iswXXX functions can be implemented
using macros and in fact, the GNU C library does this. They are still
available as real functions but when the `wctype.h' header is
included the macros will be used. This is the same as the
char type versions of these functions.
The second note covers something new. It can be best illustrated by a (real-world) example. The first piece of code is an excerpt from the original code. It is truncated a bit but the intention should be clear.
int
is_in_class (int c, const char *class)
{
if (strcmp (class, "alnum") == 0)
return isalnum (c);
if (strcmp (class, "alpha") == 0)
return isalpha (c);
if (strcmp (class, "cntrl") == 0)
return iscntrl (c);
...
return 0;
}
Now, with the wctype and iswctype you can avoid the
if cascades, but rewriting the code as follows is wrong:
int
is_in_class (int c, const char *class)
{
wctype_t desc = wctype (class);
return desc ? iswctype ((wint_t) c, desc) : 0;
}
The problem is that it is not guaranteed that the wide character representation of a single-byte character can be found using casting. In fact, usually this fails miserably. The correct solution to this problem is to write the code as follows:
int
is_in_class (int c, const char *class)
{
wctype_t desc = wctype (class);
return desc ? iswctype (btowc (c), desc) : 0;
}
See section Converting Single Characters, for more information on btowc.
Note that this change probably does not improve the performance
of the program a lot since the wctype function still has to make
the string comparisons. It gets really interesting if the
is_in_class function is called more than once for the
same class name. In this case the variable desc could be computed
once and reused for all the calls. Therefore the above form of the
function is probably not the final one.
The classification functions are also generalized by the ISO C
standard. Instead of just allowing the two standard mappings, a
locale can contain others. Again, the localedef program
already supports generating such locale data files.
wctrans function.
wctrans function has to be used to find out whether a named
mapping is defined in the current locale selected for the
LC_CTYPE category. If the returned value is non-zero, you can use
it afterwards in calls to towctrans. If the return value is
zero no such mapping is known in the current locale.
Beside locale-specific mappings there are two mappings which are guaranteed to be available in every locale:
@multitable @columnfractions .5 .5
"tolower" @tab "toupper"
These functions are declared in `wctype.h'.
towctrans maps the input character wc
according to the rules of the mapping for which desc is a
descriptor, and returns the value it finds. desc must be
obtained by a successful call to wctrans.
This function is declared in `wctype.h'.
wctrans
for them.
towlower returns the corresponding
lower-case letter. If wc is not an upper-case letter,
wc is returned unchanged.
towlower can be implemented using
towctrans (wc, wctrans ("tolower"))
This function is declared in `wctype.h'.
towupper returns the corresponding
upper-case letter. Otherwise wc is returned unchanged.
towupper can be implemented using
towctrans (wc, wctrans ("toupper"))
This function is declared in `wctype.h'.
char type value to a wint_t and use it as an
argument to towctrans calls.
Operations on strings (or arrays of characters) are an important part of
many programs. The GNU C library provides an extensive set of string
utility functions, including functions for copying, concatenating,
comparing, and searching strings. Many of these functions can also
operate on arbitrary regions of storage; for example, the memcpy
function can be used to copy the contents of any kind of array.
It's fairly common for beginning C programmers to "reinvent the wheel" by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability.
For instance, you could easily compare one string to another in two
lines of C code, but if you use the built-in strcmp function,
you're less likely to make a mistake. And, since these library
functions are typically highly optimized, your program may run faster
too.
This section is a quick summary of string concepts for beginning C programmers. It describes how character strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section.
A string is an array of char objects. But string-valued
variables are usually declared to be pointers of type char *.
Such variables do not include space for the text of a string; that has
to be stored somewhere else--in an array variable, a string constant,
or dynamically allocated memory (see section Allocating Storage For Program Data). It's up to
you to store the address of the chosen memory space into the pointer
variable. Alternatively you can store a null pointer in the
pointer variable. The null pointer does not point anywhere, so
attempting to reference the string it points to gets an error.
"string" normally refers to multibyte character strings as opposed to
wide character strings. Wide character strings are arrays of type
wchar_t and as for multibyte character strings usually pointers
of type wchar_t * are used.
By convention, a null character, '\0', marks the end of a
multibyte character string and the null wide character,
L'\0', marks the end of a wide character string. For example, in
testing to see whether the char * variable p points to a
null character marking the end of a string, you can write
!*p or *p == '\0'.
A null character is quite different conceptually from a null pointer,
although both are represented by the integer 0.
String literals appear in C program source as strings of
characters between double-quote characters (`"') where the initial
double-quote character is immediately preceded by a capital `L'
(ell) character (as in L"foo"). In ISO C, string literals
can also be formed by string concatenation: "a" "b" is the
same as "ab". For wide character strings one can either use
L"a" L"b" or L"a" "b". Modification of string literals is
not allowed by the GNU C compiler, because literals are placed in
read-only storage.
Character arrays that are declared const cannot be modified
either. It's generally good style to declare non-modifiable string
pointers to be of type const char *, since this often allows the
C compiler to detect accidental modifications as well as providing some
amount of documentation about what your program intends to do with the
string.
The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string. In this document, the term allocated size is always used to refer to the total amount of memory allocated for the string, while the term length refers to the number of characters up to (but not including) the terminating null character.
A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions do not do this for you! Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string.
Originally strings were sequences of bytes where each byte represents a single character. This is still true today if the strings are encoded using a single-byte character encoding. Things are different if the strings are encoded using a multibyte encoding (for more information on encodings see section Introduction to Extended Characters). There is no difference in the programming interface for these two kind of strings; the programmer has to be aware of this and interpret the byte sequences accordingly.
But since there is no separate interface taking care of these
differences the byte-based string functions are sometimes hard to use.
Since the count parameters of these functions specify bytes a call to
strncpy could cut a multibyte character in the middle and put an
incomplete (and therefore unusable) byte sequence in the target buffer.
To avoid these problems later versions of the ISO C standard introduce a second set of functions which are operating on wide characters (see section Introduction to Extended Characters). These functions don't have the problems the single-byte versions have since every wide character is a legal, interpretable value. This does not mean that cutting wide character strings at arbitrary points is without problems. It normally is for alphabet-based languages (except for non-normalized text) but languages based on syllables still have the problem that more than one wide character is necessary to complete a logical unit. This is a higher level problem which the C library functions are not designed to solve. But it is at least good that no invalid byte sequences can be created. Also, the higher level functions can also much easier operate on wide character than on multibyte characters so that a general advise is to use wide characters internally whenever text is more than simply copied.
The remaining of this chapter will discuss the functions for handling wide character strings in parallel with the discussion of the multibyte character strings since there is almost always an exact equivalent available.
This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters and wide characters.
Functions that operate on arbitrary blocks of memory have names
beginning with `mem' and `wmem' (such as memcpy and
wmemcpy) and invariably take an argument which specifies the size
(in bytes and wide characters respectively) of the block of memory to
operate on. The array arguments and return values for these functions
have type void * or wchar_t. As a matter of style, the
elements of the arrays used with the `mem' functions are referred
to as "bytes". You can pass any kind of pointer to these functions,
and the sizeof operator is useful in computing the value for the
size argument. Parameters to the `wmem' functions must be of type
wchar_t *. These functions are not really usable with anything
but arrays of this type.
In contrast, functions that operate specifically on strings and wide
character strings have names beginning with `str' and `wcs'
respectively (such as strcpy and wcscpy) and look for a
null character to terminate the string instead of requiring an explicit
size argument to be passed. (Some of these functions accept a specified
maximum length, but they also check for premature termination with a
null character.) The array arguments and return values for these
functions have type char * and wchar_t * respectively, and
the array elements are referred to as "characters" and "wide
characters".
In many cases, there are both `mem' and `str'/`wcs' versions of a function. The one that is more appropriate to use depends on the exact situation. When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the `mem' functions. On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the `str'/`wcs' functions, unless you already know the length of the string in advance. The `wmem' functions should be used for wide character arrays with known size.
Some of the memory and string functions take single characters as
arguments. Since a value of type char is automatically promoted
into an value of type int when used as a parameter, the functions
are declared with int as the type of the parameter in question.
In case of the wide character function the situation is similarly: the
parameter type for a single wide character is wint_t and not
wchar_t. This would for many implementations not be necessary
since the wchar_t is large enough to not be automatically
promoted, but since the ISO C standard does not require such a
choice of types the wint_t type is used.
You can get the length of a string using the strlen function.
This function is declared in the header file `string.h'.
strlen function returns the length of the null-terminated
string s in bytes. (In other words, it returns the offset of the
terminating null character within the array.)
For example,
strlen ("hello, world")
=> 12
When applied to a character array, the strlen function returns
the length of the string stored there, not its allocated size. You can
get the allocated size of the character array that holds a string using
the sizeof operator:
char string[32] = "hello, world";
sizeof (string)
=> 32
strlen (string)
=> 12
But beware, this will not work unless string is the character array itself, not a pointer to it. For example:
char string[32] = "hello, world";
char *ptr = string;
sizeof (string)
=> 32
sizeof (ptr)
=> 4 /* (on a machine with 4 byte pointers) */
This is an easy mistake to make when you are working with functions that take string arguments; those arguments are always pointers, not arrays.
It must also be noted that for multibyte encoded strings the return
value does not have to correspond to the number of characters in the
string. To get this value the string can be converted to wide
characters and wcslen can be used or something like the following
code can be used:
/* The input is instring. The length is expected inn. */ { mbstate_t t; char *scopy = string; /* In initial state. */ memset (&t, '\0', sizeof (t)); /* Determine number of characters. */ n = mbsrtowcs (NULL, &scopy, strlen (scopy), &t); }
This is cumbersome to do so if the number of characters (as opposed to bytes) is needed often it is better to work with wide characters.
The wide character equivalent is declared in `wchar.h'.
wcslen function is the wide character equivalent to
strlen. The return value is the number of wide characters in the
wide character string pointed to by ws (this is also the offset of
the terminating null wide character of ws).
Since there are no multi wide character sequences making up one character the return value is not only the offset in the array, it is also the number of wide characters.
This function was introduced in Amendment 1 to ISO C90.
strnlen function returns the length of the string s in
bytes if this length is smaller than maxlen bytes. Otherwise it
returns maxlen. Therefore this function is equivalent to
(strlen (s) < n ? strlen (s) : maxlen) but it
is more efficient and works even if the string s is not
null-terminated.
char string[32] = "hello, world";
strnlen (string, 32)
=> 12
strnlen (string, 5)
=> 5
This function is a GNU extension and is declared in `string.h'.
wcsnlen is the wide character equivalent to strnlen. The
maxlen parameter specifies the maximum number of wide characters.
This function is a GNU extension and is declared in `wchar.h'.
You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another. The `str' and `mem' functions are declared in the header file `string.h' while the `wstr' and `wmem' functions are declared in the file `wchar.h'.
A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array. All of these functions return the address of the destination array.
Most of these functions do not work properly if the source and destination arrays overlap. For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied. Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program.
All functions that have problems copying between overlapping arrays are
explicitly identified in this manual. In addition to functions in this
section, there are a few others like sprintf (see section Formatted Output Functions) and scanf (see section Formatted Input Functions).
memcpy function copies size bytes from the object
beginning at from into the object beginning at to. The
behavior of this function is undefined if the two arrays to and
from overlap; use memmove instead if overlapping is possible.
The value returned by memcpy is the value of to.
Here is an example of how you might use memcpy to copy the
contents of an array:
struct foo *oldarray, *newarray; int arraysize; ... memcpy (new, old, arraysize * sizeof (struct foo));
wmemcpy function copies size wide characters from the object
beginning at wfrom into the object beginning at wto. The
behavior of this function is undefined if the two arrays wto and
wfrom overlap; use wmemmove instead if overlapping is possible.
The following is a possible implementation of wmemcpy but there
are more optimizations possible.
wchar_t *
wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t));
}
The value returned by wmemcpy is the value of wto.
This function was introduced in Amendment 1 to ISO C90.
mempcpy function is nearly identical to the memcpy
function. It copies size bytes from the object beginning at
from into the object pointed to by to. But instead of
returning the value of to it returns a pointer to the byte
following the last written byte in the object beginning at to.
I.e., the value is ((void *) ((char *) to + size)).
This function is useful in situations where a number of objects shall be copied to consecutive memory positions.
void *
combine (void *o1, size_t s1, void *o2, size_t s2)
{
void *result = malloc (s1 + s2);
if (result != NULL)
mempcpy (mempcpy (result, o1, s1), o2, s2);
return result;
}
This function is a GNU extension.
wmempcpy function is nearly identical to the wmemcpy
function. It copies size wide characters from the object
beginning at wfrom into the object pointed to by wto. But
instead of returning the value of wto it returns a pointer to the
wide character following the last written wide character in the object
beginning at wto. I.e., the value is wto + size.
This function is useful in situations where a number of objects shall be copied to consecutive memory positions.
The following is a possible implementation of wmemcpy but there
are more optimizations possible.
wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}
This function is a GNU extension.
memmove copies the size bytes at from into the
size bytes at to, even if those two blocks of space
overlap. In the case of overlap, memmove is careful to copy the
original values of the bytes in the block at from, including those
bytes which also belong to the block at to.
The value returned by memmove is the value of to.
wmemmove copies the size wide characters at wfrom
into the size wide characters at wto, even if those two
blocks of space overlap. In the case of overlap, memmove is
careful to copy the original values of the wide characters in the block
at wfrom, including those wide characters which also belong to the
block at wto.
The following is a possible implementation of wmemcpy but there
are more optimizations possible.
wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}
The value returned by wmemmove is the value of wto.
This function is a GNU extension.
unsigned char) into each of the first size bytes of the
object beginning at block. It returns the value of block.
memcpy, this function has undefined results if the strings
overlap. The return value is the value of to.
wmemcpy, this function has undefined results if
the strings overlap. The return value is the value of wto.
strcpy but always copies exactly
size characters into to.
If the length of from is more than size, then strncpy
copies just the first size characters. Note that in this case
there is no null terminator written into to.
If the length of from is less than size, then strncpy
copies all of from, followed by enough null characters to add up
to size characters in all. This behavior is rarely useful, but it
is specified by the ISO C standard.
The behavior of strncpy is undefined if the strings overlap.
Using strncpy as opposed to strcpy is a way to avoid bugs
relating to writing past the end of the allocated space for to.
However, it can also make your program much slower in one common case:
copying a string which is probably small into a potentially large buffer.
In this case, size may be large, and when it is, strncpy will
waste a considerable amount of time copying null characters.
wcscpy but always copies exactly
size wide characters into wto.
If the length of wfrom is more than size, then
wcsncpy copies just the first size wide characters. Note
that in this case there is no null terminator written into wto.
If the length of wfrom is less than size, then
wcsncpy copies all of wfrom, followed by enough null wide
characters to add up to size wide characters in all. This
behavior is rarely useful, but it is specified by the ISO C
standard.
The behavior of wcsncpy is undefined if the strings overlap.
Using wcsncpy as opposed to wcscpy is a way to avoid bugs
relating to writing past the end of the allocated space for wto.
However, it can also make your program much slower in one common case:
copying a string which is probably small into a potentially large buffer.
In this case, size may be large, and when it is, wcsncpy will
waste a considerable amount of time copying null wide characters.
malloc; see
section Unconstrained Allocation. If malloc cannot allocate space
for the new string, strdup returns a null pointer. Otherwise it
returns a pointer to the new string.
malloc; see section Unconstrained Allocation. If malloc
cannot allocate space for the new string, wcsdup returns a null
pointer. Otherwise it returns a pointer to the new wide character
string.
This function is a GNU extension.
strdup but always copies at most
size characters into the newly allocated string.
If the length of s is more than size, then strndup
copies just the first size characters and adds a closing null
terminator. Otherwise all characters are copied and the string is
terminated.
This function is different to strncpy in that it always
terminates the destination string.
strndup is a GNU extension.
strcpy, except that it returns a pointer to
the end of the string to (that is, the address of the terminating
null character to + strlen (from)) rather than the beginning.
For example, this program uses stpcpy to concatenate `foo'
and `bar' to produce `foobar', which it then prints.
#include <string.h>
#include <stdio.h>
int
main (void)
{
char buffer[10];
char *to = buffer;
to = stpcpy (to, "foo");
to = stpcpy (to, "bar");
puts (buffer);
return 0;
}
This function is not part of the ISO or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOG.
Its behavior is undefined if the strings overlap. The function is declared in `string.h'.
wcscpy, except that it returns a pointer to
the end of the string wto (that is, the address of the terminating
null character wto + strlen (wfrom)) rather than the beginning.
This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.
The behavior of wcpcpy is undefined if the strings overlap.
wcpcpy is a GNU extension and is declared in `wchar.h'.
stpcpy but copies always exactly
size characters into to.
If the length of from is more then size, then stpncpy
copies just the first size characters and returns a pointer to the
character directly following the one which was copied last. Note that in
this case there is no null terminator written into to.
If the length of from is less than size, then stpncpy
copies all of from, followed by enough null characters to add up
to size characters in all. This behaviour is rarely useful, but it
is implemented to be useful in contexts where this behaviour of the
strncpy is used. stpncpy returns a pointer to the
first written null character.
This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.
Its behaviour is undefined if the strings overlap. The function is declared in `string.h'.
wcpcpy but copies always exactly
wsize characters into wto.
If the length of wfrom is more then size, then
wcpncpy copies just the first size wide characters and
returns a pointer to the wide character directly following the one which
was copied last. Note that in this case there is no null terminator
written into wto.
If the length of wfrom is less than size, then wcpncpy
copies all of wfrom, followed by enough null characters to add up
to size characters in all. This behaviour is rarely useful, but it
is implemented to be useful in contexts where this behaviour of the
wcsncpy is used. wcpncpy returns a pointer to the
first written null character.
This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.
Its behaviour is undefined if the strings overlap.
wcpncpy is a GNU extension and is declared in `wchar.h'.
strdup but allocates the new string
using alloca instead of malloc (see section Automatic Storage with Variable Size). This means of course the returned string has the same
limitations as any block of memory allocated using alloca.
For obvious reasons strdupa is implemented only as a macro;
you cannot get the address of this function. Despite this limitation
it is a useful function. The following code shows a situation where
using malloc would be a lot more expensive.
#include <paths.h>
#include <string.h>
#include <stdio.h>
const char path[] = _PATH_STDPATH;
int
main (void)
{
char *wr_path = strdupa (path);
char *cp = strtok (wr_path, ":");
while (cp != NULL)
{
puts (cp);
cp = strtok (NULL, ":");
}
return 0;
}
Please note that calling strtok using path directly is
invalid. It is also not allowed to call strdupa in the argument
list of strtok since strdupa uses alloca
(see section Automatic Storage with Variable Size) can interfere with the parameter
passing.
This function is only available if GNU CC is used.
strndup but like strdupa it
allocates the new string using alloca
see section Automatic Storage with Variable Size. The same advantages and limitations
of strdupa are valid for strndupa, too.
This function is implemented only as a macro, just like strdupa.
Just as strdupa this macro also must not be used inside the
parameter list in a function call.
strndupa is only available if GNU CC is used.
strcat function is similar to strcpy, except that the
characters from from are concatenated or appended to the end of
to, instead of overwriting it. That is, the first character from
from overwrites the null character marking the end of to.
An equivalent definition for strcat would be:
char *
strcat (char *restrict to, const char *restrict from)
{
strcpy (to + strlen (to), from);
return to;
}
This function has undefined results if the strings overlap.
wcscat function is similar to wcscpy, except that the
characters from wfrom are concatenated or appended to the end of
wto, instead of overwriting it. That is, the first character from
wfrom overwrites the null character marking the end of wto.
An equivalent definition for wcscat would be:
wchar_t *
wcscat (wchar_t *wto, const wchar_t *wfrom)
{
wcscpy (wto + wcslen (wto), wfrom);
return wto;
}
This function has undefined results if the strings overlap.
Programmers using the strcat or wcscat function (or the
following strncat or wcsncar functions for that matter)
can easily be recognized as lazy and reckless. In almost all situations
the lengths of the participating strings are known (it better should be
since how can one otherwise ensure the allocated size of the buffer is
sufficient?) Or at least, one could know them if one keeps track of the
results of the various function calls. But then it is very inefficient
to use strcat/wcscat. A lot of time is wasted finding the
end of the destination string so that the actual copying can start.
This is a common example:
/* This function concatenates arbitrarily many strings. The last parameter must beNULL. */ char * concat (const char *str, ...) { va_list ap, ap2; size_t total = 1; const char *s; char *result; va_start (ap, str); /* Actuallyva_copy, but this is the name more gcc versions understand. */ __va_copy (ap2, ap); /* Determine how much space we need. */ for (s = str; s != NULL; s = va_arg (ap, const char *)) total += strlen (s); va_end (ap); result = (char *) malloc (total); if (result != NULL) { result[0] = '\0'; /* Copy the strings. */ for (s = str; s != NULL; s = va_arg (ap2, const char *)) strcat (result, s); } va_end (ap2); return result; }
This looks quite simple, especially the second loop where the strings are actually copied. But these innocent lines hide a major performance penalty. Just imagine that ten strings of 100 bytes each have to be concatenated. For the second string we search the already stored 100 bytes for the end of the string so that we can append the next string. For all strings in total the comparisons necessary to find the end of the intermediate results sums up to 5500! If we combine the copying with the search for the allocation we can write this function more efficient:
char *
concat (const char *str, ...)
{
va_list ap;
size_t allocated = 100;
char *result = (char *) malloc (allocated);
char *wp;
if (allocated != NULL)
{
char *newp;
va_start (ap, atr);
wp = result;
for (s = str; s != NULL; s = va_arg (ap, const char *))
{
size_t len = strlen (s);
/* Resize the allocated memory if necessary. */
if (wp + len + 1 > result + allocated)
{
allocated = (allocated + len) * 2;
newp = (char *) realloc (result, allocated);
if (newp == NULL)
{
free (result);
return NULL;
}
wp = newp + (wp - result);
result = newp;
}
wp = mempcpy (wp, s, len);
}
/* Terminate the result string. */
*wp++ = '\0';
/* Resize memory to the optimal size. */
newp = realloc (result, wp - result);
if (newp != NULL)
result = newp;
va_end (ap);
}
return result;
}
With a bit more knowledge about the input strings one could fine-tune
the memory allocation. The difference we are pointing to here is that
we don't use strcat anymore. We always keep track of the length
of the current intermediate result so we can safe us the search for the
end of the string and use mempcpy. Please note that we also
don't use stpcpy which might seem more natural since we handle
with strings. But this is not necessary since we already know the
length of the string and therefore can use the faster memory copying
function. The example would work for wide characters the same way.
Whenever a programmer feels the need to use strcat she or he
should think twice and look through the program whether the code cannot
be rewritten to take advantage of already calculated results. Again: it
is almost always unnecessary to use strcat.
strcat except that not more than size
characters from from are appended to the end of to. A
single null character is also always appended to to, so the total
allocated size of to must be at least size + 1 bytes
longer than its initial length.
The strncat function could be implemented like this:
char *
strncat (char *to, const char *from, size_t size)
{
to[strlen (to) + size] = '\0';
strncpy (to + strlen (to), from, size);
return to;
}
The behavior of strncat is undefined if the strings overlap.
wcscat except that not more than size
characters from from are appended to the end of to. A
single null character is also always appended to to, so the total
allocated size of to must be at least size + 1 bytes
longer than its initial length.
The wcsncat function could be implemented like this:
wchar_t *
wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
wto[wcslen (to) + size] = L'\0';
wcsncpy (wto + wcslen (wto), wfrom, size);
return wto;
}
The behavior of wcsncat is undefined if the strings overlap.
Here is an example showing the use of strncpy and strncat
(the wide character version is equivalent). Notice how, in the call to
strncat, the size parameter is computed to avoid
overflowing the character array buffer.
#include <string.h>
#include <stdio.h>
#define SIZE 10
static char buffer[SIZE];
main ()
{
strncpy (buffer, "hello", SIZE);
puts (buffer);
strncat (buffer, ", world", SIZE - strlen (buffer) - 1);
puts (buffer);
}
The output produced by this program looks like:
hello hello, wo
memmove, derived from
BSD. Note that it is not quite equivalent to memmove, because the
arguments are not in the same order and there is no return value.
memset, derived from
BSD. Note that it is not as general as memset, because the only
value it can store is zero.
You can use the functions in this section to perform comparisons on the contents of strings and arrays. As well as checking for equality, these functions can also be used as the ordering functions for sorting operations. See section Searching and Sorting, for an example of this.
Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are not equivalent rather than if they are. The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent: a negative value indicates that the first string is "less" than the second, while a positive value indicates that the first string is "greater".
The most common use of these functions is to check only for equality. This is canonically done with an expression like `! strcmp (s1, s2)'.
All of these functions are declared in the header file `string.h'.
memcmp compares the size bytes of memory
beginning at a1 against the size bytes of memory beginning
at a2. The value returned has the same sign as the difference
between the first differing pair of bytes (interpreted as unsigned
char objects, then promoted to int).
If the contents of the two blocks are equal, memcmp returns
0.
wmemcmp compares the size wide characters
beginning at a1 against the size wide characters beginning
at a2. The value returned is smaller than or larger than zero
depending on whether the first differing wide character is a1 is
smaller or larger than the corresponding character in a2.
If the contents of the two blocks are equal, wmemcmp returns
0.
On arbitrary arrays, the memcmp function is mostly useful for
testing equality. It usually isn't meaningful to do byte-wise ordering
comparisons on arrays of things other than bytes. For example, a
byte-wise comparison on the bytes that make up floating-point numbers
isn't likely to tell you anything about the relationship between the
values of the floating-point numbers.
wmemcmp is really only useful to compare arrays of type
wchar_t since the function looks at sizeof (wchar_t) bytes
at a time and this number of bytes is system dependent.
You should also be careful about using memcmp to compare objects
that can contain "holes", such as the padding inserted into structure
objects to enforce alignment requirements, extra space at the end of
unions, and extra characters at the ends of strings whose length is less
than their allocated size. The contents of these "holes" are
indeterminate and may cause strange behavior when performing byte-wise
comparisons. For more predictable results, perform an explicit
component-wise comparison.
For example, given a structure type definition like:
struct foo
{
unsigned char tag;
union
{
double f;
long i;
char *p;
} value;
};
you are better off writing a specialized comparison function to compare
struct foo objects instead of comparing them with memcmp.
strcmp function compares the string s1 against
s2, returning a value that has the same sign as the difference
between the first differing pair of characters (interpreted as
unsigned char objects, then promoted to int).
If the two strings are equal, strcmp returns 0.
A consequence of the ordering used by strcmp is that if s1
is an initial substring of s2, then s1 is considered to be
"less than" s2.
strcmp does not take sorting conventions of the language the
strings are written in into account. To get that one has to use
strcoll.
The wcscmp function compares the wide character string ws1
against ws2. The value returned is smaller than or larger than zero
depending on whether the first differing wide character is ws1 is
smaller or larger than the corresponding character in ws2.
If the two strings are equal, wcscmp returns 0.
A consequence of the ordering used by wcscmp is that if ws1
is an initial substring of ws2, then ws1 is considered to be
"less than" ws2.
wcscmp does not take sorting conventions of the language the
strings are written in into account. To get that one has to use
wcscoll.
strcmp, except that differences in case are
ignored. How uppercase and lowercase characters are related is
determined by the currently selected locale. In the standard "C"
locale the characters @"A and @"a do not match but in a locale which
regards these characters as parts of the alphabet they do match.
strcasecmp is derived from BSD.
wcscmp, except that differences in case are
ignored. How uppercase and lowercase characters are related is
determined by the currently selected locale. In the standard "C"
locale the characters @"A and @"a do not match but in a locale which
regards these characters as parts of the alphabet they do match.
wcscasecmp is a GNU extension.
strcmp, except that no more than
size wide characters are compared. In other words, if the two
strings are the same in their first size wide characters, the
return value is zero.
wcscmp, except that no more than
size wide characters are compared. In other words, if the two
strings are the same in their first size wide characters, the
return value is zero.
strncmp, except that differences in case
are ignored. Like strcasecmp, it is locale dependent how
uppercase and lowercase characters are related.
strncasecmp is a GNU extension.
wcsncmp, except that differences in case
are ignored. Like wcscasecmp, it is locale dependent how
uppercase and lowercase characters are related.
wcsncasecmp is a GNU extension.
Here are some examples showing the use of strcmp and
strncmp (equivalent examples can be constructed for the wide
character functions). These examples assume the use of the ASCII
character set. (If some other character set--say, EBCDIC--is used
instead, then the glyphs are associated with different numeric codes,
and the return values and ordering may differ.)
strcmp ("hello", "hello")
=> 0 /* These two strings are the same. */
strcmp ("hello", "Hello")
=> 32 /* Comparisons are case-sensitive. */
strcmp ("hello", "world")
=> -15 /* The character 'h' comes before 'w'. */
strcmp ("hello", "hello, world")
=> -44 /* Comparing a null character against a comma. */
strncmp ("hello", "hello, world", 5)
=> 0 /* The initial 5 characters are the same. */
strncmp ("hello, world", "hello, stupid world!!!", 5)
=> 0 /* The initial 5 characters are the same. */
strverscmp function compares the string s1 against
s2, considering them as holding indices/version numbers. Return
value follows the same conventions as found in the strverscmp
function. In fact, if s1 and s2 contain no digits,
strverscmp behaves like strcmp.
Basically, we compare strings normally (character by character), until we find a digit in each string - then we enter a special comparison mode, where each sequence of digits is taken as a whole. If we reach the end of these two parts without noticing a difference, we return to the standard comparison mode. There are two types of numeric parts: "integral" and "fractional" (those begin with a '0'). The types of the numeric parts affect the way we sort them:
strverscmp ("no digit", "no digit")
=> 0 /* same behaviour as strcmp. */
strverscmp ("item#99", "item#100")
=> <0 /* same prefix, but 99 < 100. */
strverscmp ("alpha1", "alpha001")
=> >0 /* fractional part inferior to integral one. */
strverscmp ("part1_f012", "part1_f01")
=> >0 /* two fractional parts. */
strverscmp ("foo.009", "foo.0")
=> <0 /* idem, but with leading zeroes only. */
This function is especially useful when dealing with filename sorting, because filenames frequently hold indices/version numbers.
strverscmp is a GNU extension.
memcmp, derived from BSD.
In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence `ll' is treated as a single letter that is collated immediately after `l'.
You can use the functions strcoll and strxfrm (declared in
the headers file `string.h') and wcscoll and wcsxfrm
(declared in the headers file `wchar') to compare strings using a
collation ordering appropriate for the current locale. The locale used
by these functions in particular can be specified by setting the locale
for the LC_COLLATE category; see section Locales and Internationalization.
In the standard C locale, the collation sequence for strcoll is
the same as that for strcmp. Similarly, wcscoll and
wcscmp are the same in this situation.
Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence.
The functions strcoll and wcscoll perform this translation
implicitly, in order to do one comparison. By contrast, strxfrm
and wcsxfrm perform the mapping explicitly. If you are making
multiple comparisons using the same string or set of strings, it is
likely to be more efficient to use strxfrm or wcsxfrm to
transform all the strings just once, and subsequently compare the
transformed strings with strcmp or wcscmp.
strcoll function is similar to strcmp but uses the
collating sequence of the current locale for collation (the
LC_COLLATE locale).
wcscoll function is similar to wcscmp but uses the
collating sequence of the current locale for collation (the
LC_COLLATE locale).
Here is an example of sorting an array of strings, using strcoll
to compare them. The actual sort algorithm is not written here; it
comes from qsort (see section Array Sort Function). The job of the
code shown here is to say how to compare the strings while sorting them.
(Later on in this section, we will show a way to do this more
efficiently using strxfrm.)
/* This is the comparison function used withqsort. */ int compare_elements (char **p1, char **p2) { return strcoll (*p1, *p2); } /* This is the entry point--the function to sort strings using the locale's collating sequence. */ void sort_strings (char **array, int nstrings) { /* Sorttemp_arrayby comparing the strings. */ qsort (array, nstrings, sizeof (char *), compare_elements); }
strxfrm transforms the string from using the
collation transformation determined by the locale currently selected for
collation, and stores the transformed string in the array to. Up
to size characters (including a terminating null character) are
stored.
The behavior is undefined if the strings to and from overlap; see section Copying and Concatenation.
The return value is the length of the entire transformed string. This
value is not affected by the value of size, but if it is greater
or equal than size, it means that the transformed string did not
entirely fit in the array to. In this case, only as much of the
string as actually fits was stored. To get the whole transformed
string, call strxfrm again with a bigger output array.
The transformed string may be longer than the original string, and it may also be shorter.
If size is zero, no characters are stored in to. In this
case, strxfrm simply returns the number of characters that would
be the length of the transformed string. This is useful for determining
what size the allocated array should be. It does not matter what
to is if size is zero; to may even be a null pointer.
wcsxfrm transforms wide character string wfrom
using the collation transformation determined by the locale currently
selected for collation, and stores the transformed string in the array
wto. Up to size wide characters (including a terminating null
character) are stored.
The behavior is undefined if the strings wto and wfrom overlap; see section Copying and Concatenation.
The return value is the length of the entire transformed wide character
string. This value is not affected by the value of size, but if
it is greater or equal than size, it means that the transformed
wide character string did not entirely fit in the array wto. In
this case, only as much of the wide character string as actually fits
was stored. To get the whole transformed wide character string, call
wcsxfrm again with a bigger output array.
The transformed wide character string may be longer than the original wide character string, and it may also be shorter.
If size is zero, no characters are stored in to. In this
case, wcsxfrm simply returns the number of wide characters that
would be the length of the transformed wide character string. This is
useful for determining what size the allocated array should be (remember
to multiply with sizeof (wchar_t)). It does not matter what
wto is if size is zero; wto may even be a null pointer.
Here is an example of how you can use strxfrm when
you plan to do many comparisons. It does the same thing as the previous
example, but much faster, because it has to transform each string only
once, no matter how many times it is compared with other strings. Even
the time needed to allocate and free storage is much less than the time
we save, when there are many strings.
struct sorter { char *input; char *transformed; };
/* This is the comparison function used with qsort
to sort an array of struct sorter. */
int
compare_elements (struct sorter *p1, struct sorter *p2)
{
return strcmp (p1->transformed, p2->transformed);
}
/* This is the entry point--the function to sort
strings using the locale's collating sequence. */
void
sort_strings_fast (char **array, int nstrings)
{
struct sorter temp_array[nstrings];
int i;
/* Set up temp_array. Each element contains
one input string and its transformed string. */
for (i = 0; i < nstrings; i++)
{
size_t length = strlen (array[i]) * 2;
char *transformed;
size_t transformed_length;
temp_array[i].input = array[i];
/* First try a buffer perhaps big enough. */
transformed = (char *) xmalloc (length);
/* Transform array[i]. */
transformed_length = strxfrm (transformed, array[i], length);
/* If the buffer was not large enough, resize it
and try again. */
if (transformed_length >= length)
{
/* Allocate the needed space. +1 for terminating
NUL character. */
transformed = (char *) xrealloc (transformed,
transformed_length + 1);
/* The return value is not interesting because we know
how long the transformed string is. */
(void) strxfrm (transformed, array[i],
transformed_length + 1);
}
temp_array[i].transformed = transformed;
}
/* Sort temp_array by comparing transformed strings. */
qsort (temp_array, sizeof (struct sorter),
nstrings, compare_elements);
/* Put the elements back in the permanent array
in their sorted order. */
for (i = 0; i < nstrings; i++)
array[i] = temp_array[i].input;
/* Free the strings we allocated. */
for (i = 0; i < nstrings; i++)
free (temp_array[i].transformed);
}
The interesting part of this code for the wide character version would look like this:
void
sort_strings_fast (wchar_t **array, int nstrings)
{
...
/* Transform array[i]. */
transformed_length = wcsxfrm (transformed, array[i], length);
/* If the buffer was not large enough, resize it
and try again. */
if (transformed_length >= length)
{
/* Allocate the needed space. +1 for terminating
NUL character. */
transformed = (wchar_t *) xrealloc (transformed,
(transformed_length + 1)
* sizeof (wchar_t));
/* The return value is not interesting because we know
how long the transformed string is. */
(void) wcsxfrm (transformed, array[i],
transformed_length + 1);
}
...
Note the additional multiplication with sizeof (wchar_t) in the
realloc call.
Compatibility Note: The string collation functions are a new feature of ISO C90. Older C dialects have no equivalent feature. The wide character versions were introduced in Amendment 1 to ISO C90.
This section describes library functions which perform various kinds of searching operations on strings and arrays. These functions are declared in the header file `string.h'.
unsigned char) in the initial size bytes of the
object beginning at block. The return value is a pointer to the
located byte, or a null pointer if no match was found.
memchr function is used with the knowledge that the
byte c is available in the memory block specified by the
parameters. But this means that the size parameter is not really
needed and that the tests performed with it at runtime (to check whether
the end of the block is reached) are not needed.
The rawmemchr function exists for just this situation which is
surprisingly frequent. The interface is similar to memchr except
that the size parameter is missing. The function will look beyond
the end of the block pointed to by block in case the programmer
made an error in assuming that the byte c is present in the block.
In this case the result is unspecified. Otherwise the return value is a
pointer to the located byte.
This function is of special interest when looking for the end of a string. Since all strings are terminated by a null byte a call like
rawmemchr (str, '\0')
will never go beyond the end of the string.
This function is a GNU extension.
memrchr is like memchr, except that it searches
backwards from the end of the block defined by block and size
(instead of forwards from the front).
strchr function finds the first occurrence of the character
c (converted to a char) in the null-terminated string
beginning at string. The return value is a pointer to the located
character, or a null pointer if no match was found.
For example,
strchr ("hello, world", 'l')
=> "llo, world"
strchr ("hello, world", '?')
=> NULL
The terminating null character is considered to be part of the string,
so you can use this function get a pointer to the end of a string by
specifying a null character as the value of the c argument. It
would be better (but less portable) to use strchrnul in this
case, though.
wcschr function finds the first occurrence of the wide
character wc in the null-terminated wide character string
beginning at wstring. The return value is a pointer to the
located wide character, or a null pointer if no match was found.
The terminating null character is considered to be part of the wide
character string, so you can use this function get a pointer to the end
of a wide character string by specifying a null wude character as the
value of the wc argument. It would be better (but less portable)
to use wcschrnul in this case, though.
strchrnul is the same as strchr except that if it does
not find the character, it returns a pointer to string's terminating
null character rather than a null pointer.
This function is a GNU extension.
wcschrnul is the same as wcschr except that if it does not
find the wide character, it returns a pointer to wide character string's
terminating null wide character rather than a null pointer.
This function is a GNU extension.
One useful, but unusual, use of the strchr
function is when one wants to have a pointer pointing to the NUL byte
terminating a string. This is often written in this way:
s += strlen (s);
This is almost optimal but the addition operation duplicated a bit of
the work already done in the strlen function. A better solution
is this:
s = strchr (s, '\0');
There is no restriction on the second parameter of strchr so it
could very well also be the NUL character. Those readers thinking very
hard about this might now point out that the strchr function is
more expensive than the strlen function since we have two abort
criteria. This is right. But in the GNU C library the implementation of
strchr is optimized in a special way so that strchr
actually is faster.
strrchr is like strchr, except that it searches
backwards from the end of the string string (instead of forwards
from the front).
For example,
strrchr ("hello, world", 'l')
=> "ld"
wcsrchr is like wcschr, except that it searches
backwards from the end of the string wstring (instead of forwards
from the front).
strchr, except that it searches haystack for a
substring needle rather than just a single character. It
returns a pointer into the string haystack that is the first
character of the substring, or a null pointer if no match was found. If
needle is an empty string, the function returns haystack.
For example,
strstr ("hello, world", "l")
=> "llo, world"
strstr ("hello, world", "wo")
=> "world"
wcschr, except that it searches haystack for a
substring needle rather than just a single wide character. It
returns a pointer into the string haystack that is the first wide
character of the substring, or a null pointer if no match was found. If
needle is an empty string, the function returns haystack.
wcsstr is an depricated alias for wcsstr. This is the
name originally used in the X/Open Portability Guide before the
Amendment 1 to ISO C90 was published.
strstr, except that it ignores case in searching for
the substring. Like strcasecmp, it is locale dependent how
uppercase and lowercase characters are related.
For example,
strstr ("hello, world", "L")
=> "llo, world"
strstr ("hello, World", "wo")
=> "World"
strstr, but needle and haystack are byte
arrays rather than null-terminated strings. needle-len is the
length of needle and haystack-len is the length of
haystack.
This function is a GNU extension.
strspn ("string span") function returns the length of the
initial substring of string that consists entirely of characters that
are members of the set specified by the string skipset. The order
of the characters in skipset is not important.
For example,
strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
=> 5
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
wcsspn ("wide character string span") function returns the
length of the initial substring of wstring that consists entirely
of wide characters that are members of the set specified by the string
skipset. The order of the wide characters in skipset is not
important.
strcspn ("string complement span") function returns the length
of the initial substring of string that consists entirely of characters
that are not members of the set specified by the string stopset.
(In other words, it returns the offset of the first character in string
that is a member of the set stopset.)
For example,
strcspn ("hello, world", " \t\n,.;!?")
=> 5
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
wcscspn ("wide character string complement span") function
returns the length of the initial substring of wstring that
consists entirely of wide characters that are not members of the
set specified by the string stopset. (In other words, it returns
the offset of the first character in string that is a member of
the set stopset.)
strpbrk ("string pointer break") function is related to
strcspn, except that it returns a pointer to the first character
in string that is a member of the set stopset instead of the
length of the initial substring. It returns a null pointer if no such
character from stopset is found.
For example,
strpbrk ("hello, world", " \t\n,.;!?")
=> ", world"
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
wcspbrk ("wide character string pointer break") function is
related to wcscspn, except that it returns a pointer to the first
wide character in wstring that is a member of the set
stopset instead of the length of the initial substring. It
returns a null pointer if no such character from stopset is found.
index is another name for strchr; they are exactly the same.
New code should always use strchr since this name is defined in
ISO C while index is a BSD invention which never was available
on System V derived systems.
rindex is another name for strrchr; they are exactly the same.
New code should always use strrchr since this name is defined in
ISO C while rindex is a BSD invention which never was available
on System V derived systems.
It's fairly common for programs to have a need to do some simple kinds
of lexical analysis and parsing, such as splitting a command string up
into tokens. You can do this with the strtok function, declared
in the header file `string.h'.
strtok.
The string to be split up is passed as the newstring argument on
the first call only. The strtok function uses this to set up
some internal state information. Subsequent calls to get additional
tokens from the same string are indicated by passing a null pointer as
the newstring argument. Calling strtok with another
non-null newstring argument reinitializes the state information.
It is guaranteed that no other library function ever calls strtok
behind your back (which would mess up this internal state information).
The delimiters argument is a string that specifies a set of delimiters that may surround the token being extracted. All the initial characters that are members of this set are discarded. The first character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next character that is a member of the delimiter set. This character in the original string newstring is overwritten by a null character, and the pointer to the beginning of the token in newstring is returned.
On the next call to strtok, the searching begins at the next
character beyond the one that marked the end of the previous token.
Note that the set of delimiters delimiters do not have to be the
same on every call in a series of calls to strtok.
If the end of the string newstring is reached, or if the remainder of
string consists only of delimiter characters, strtok returns
a null pointer.
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
wcstok.
The string to be split up is passed as the newstring argument on
the first call only. The wcstok function uses this to set up
some internal state information. Subsequent calls to get additional
tokens from the same wide character string are indicated by passing a
null pointer as the newstring argument. Calling wcstok
with another non-null newstring argument reinitializes the state
information. It is guaranteed that no other library function ever calls
wcstok behind your back (which would mess up this internal state
information).
The delimiters argument is a wide character string that specifies a set of delimiters that may surround the token being extracted. All the initial wide characters that are members of this set are discarded. The first wide character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next wide character that is a member of the delimiter set. This wide character in the original wide character string newstring is overwritten by a null wide character, and the pointer to the beginning of the token in newstring is returned.
On the next call to wcstok, the searching begins at the next
wide character beyond the one that marked the end of the previous token.
Note that the set of delimiters delimiters do not have to be the
same on every call in a series of calls to wcstok.
If the end of the wide character string newstring is reached, or
if the remainder of string consists only of delimiter wide characters,
wcstok returns a null pointer.
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
Warning: Since strtok and wcstok alter the string
they is parsing, you should always copy the string to a temporary buffer
before parsing it with strtok/wcstok (see section Copying and Concatenation). If you allow strtok or wcstok to modify
a string that came from another part of your program, you are asking for
trouble; that string might be used for other purposes after
strtok or wcstok has modified it, and it would not have
the expected value.
The string that you are operating on might even be a constant. Then
when strtok or wcstok tries to modify it, your program
will get a fatal signal for writing in read-only memory. See section Program Error Signals. Even if the operation of strtok or wcstok
would not require a modification of the string (e.g., if there is
exactly one token) the string can (and in the GNU libc case will) be
modified.
This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily.
The functions strtok and wcstok are not reentrant.
See section Signal Handling and Nonreentrant Functions, for a discussion of where and why reentrancy is
important.
Here is a simple example showing the use of strtok.
#include <string.h> #include <stddef.h> ... const char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *token, *cp; ... cp = strdupa (string); /* Make writable copy. */ token = strtok (cp, delimiters); /* token => "words" */ token = strtok (NULL, delimiters); /* token => "separated" */ token = strtok (NULL, delimiters); /* token => "by" */ token = strtok (NULL, delimiters); /* token => "spaces" */ token = strtok (NULL, delimiters); /* token => "and" */ token = strtok (NULL, delimiters); /* token => "punctuation" */ token = strtok (NULL, delimiters); /* token => NULL */
The GNU C library contains two more functions for tokenizing a string which overcome the limitation of non-reentrancy. They are only available for multibyte character strings.
strtok, this function splits the string into several
tokens which can be accessed by successive calls to strtok_r.
The difference is that the information about the next token is stored in
the space pointed to by the third argument, save_ptr, which is a
pointer to a string pointer. Calling strtok_r with a null
pointer for newstring and leaving save_ptr between the calls
unchanged does the job without hindering reentrancy.
This function is defined in POSIX.1 and can be found on many systems which support multi-threading.
strtok_r with the
newstring argument replaced by the save_ptr argument. The
initialization of the moving pointer has to be done by the user.
Successive calls to strsep move the pointer along the tokens
separated by delimiter, returning the address of the next token
and updating string_ptr to point to the beginning of the next
token.
One difference between strsep and strtok_r is that if the
input string contains more than one character from delimiter in a
row strsep returns an empty string for each pair of characters
from delimiter. This means that a program normally should test
for strsep returning an empty string before processing it.
This function was introduced in 4.3BSD and therefore is widely available.
Here is how the above example looks like when strsep is used.
#include <string.h> #include <stddef.h> ... const char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *running; char *token; ... running = strdupa (string); token = strsep (&running, delimiters); /* token => "words" */ token = strsep (&running, delimiters); /* token => "separated" */ token = strsep (&running, delimiters); /* token => "by" */ token = strsep (&running, delimiters); /* token => "spaces" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "and" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "punctuation" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => NULL */
basename function returns the last
component of the path in filename. This function is the prefered
usage, since it does not modify the argument, filename, and
respects trailing slashes. The prototype for basename can be
found in `string.h'. Note, this function is overriden by the XPG
version, if `libgen.h' is included.
Example of using GNU basename:
#include <string.h>
int
main (int argc, char *argv[])
{
char *prog = basename (argv[0]);
if (argc < 2)
{
fprintf (stderr, "Usage %s <arg>\n", prog);
exit (1);
}
...
}
Portability Note: This function may produce different results on different systems.
basename. It is similar in
spirit to the GNU version, but may modify the path by removing
trailing '/' characters. If the path is made up entirely of '/'
characters, then "/" will be returned. Also, if path is
NULL or an empty string, then "." is returned. The prototype for
the XPG version can be found in `libgen.h'.
Example of using XPG basename:
#include <libgen.h>
int
main (int argc, char *argv[])
{
char *prog;
char *path = strdupa (argv[0]);
prog = basename (path);
if (argc < 2)
{
fprintf (stderr, "Usage %s <arg>\n", prog);
exit (1);
}
...
}
dirname function is the compliment to the XPG version of
basename. It returns the parent directory of the file specified
by path. If path is NULL, an empty string, or
contains no '/' characters, then "." is returned. The prototype for this
function can be found in `libgen.h'.
The function below addresses the perennial programming quandary: "How do
I take good data in string form and painlessly turn it into garbage?"
This is actually a fairly simple task for C programmers who do not use
the GNU C library string functions, but for programs based on the GNU C
library, the strfry function is the preferred method for
destroying string data.
The prototype for this function is in `string.h'.
strfry creates a pseudorandom anagram of a string, replacing the
input with the anagram in place. For each position in the string,
strfry swaps it with a position in the string selected at random
(from a uniform distribution). The two positions may be the same.
The return value of strfry is always string.
Portability Note: This function is unique to the GNU C library.
The memfrob function converts an array of data to something
unrecognizable and back again. It is not encryption in its usual sense
since it is easy for someone to convert the encrypted data back to clear
text. The transformation is analogous to Usenet's "Rot13" encryption
method for obscuring offensive jokes from sensitive eyes and such.
Unlike Rot13, memfrob works on arbitrary binary data, not just
text.
For true encryption, See section DES Encryption and Password Handling.
This function is declared in `string.h'.
memfrob transforms (frobnicates) each byte of the data structure
at mem, which is length bytes long, by bitwise exclusive
oring it with binary 00101010. It does the transformation in place and
its return value is always mem.
Note that memfrob a second time on the same data structure
returns it to its original state.
This is a good function for hiding information from someone who doesn't want to see it or doesn't want to see it very much. To really prevent people from retrieving the information, use stronger encryption such as that described in See section DES Encryption and Password Handling.
Portability Note: This function is unique to the GNU C library.
To store or transfer binary data in environments which only support text one has to encode the binary data by mapping the input bytes to characters in the range allowed for storing or transfering. SVID systems (and nowadays XPG compliant systems) provide minimal support for this task.
l64a is undefined if n is negative. In the GNU
implementation, l64a treats its argument as unsigned, so it will
return a sensible encoding for any nonzero n; however, portable
programs should not rely on this.
To encode a large buffer l64a must be called in a loop, once for
each 32-bit word of the buffer. For example, one could do something
like this:
char *
encode (const void *buf, size_t len)
{
/* We know in advance how long the buffer has to be. */
unsigned char *in = (unsigned char *) buf;
char *out = malloc (6 + ((len + 3) / 4) * 6 + 1);
char *cp = out;
/* Encode the length. */
/* Using `htonl' is necessary so that the data can be
decoded even on machines with different byte order. */
cp = mempcpy (cp, l64a (htonl (len)), 6);
while (len > 3)
{
unsigned long int n = *in++;
n = (n << 8) | *in++;
n = (n << 8) | *in++;
n = (n << 8) | *in++;
len -= 4;
if (n)
cp = mempcpy (cp, l64a (htonl (n)), 6);
else
/* `l64a' returns the empty string for n==0, so we
must generate its encoding ("......") by hand. */
cp = stpcpy (cp, "......");
}
if (len > 0)
{
unsigned long int n = *in++;
if (--len > 0)
{
n = (n << 8) | *in++;
if (--len > 0)
n = (n << 8) | *in;
}
memcpy (cp, l64a (htonl (n)), 6);
cp += 6;
}
*cp = '\0';
return out;
}
It is strange that the library does not provide the complete functionality needed but so be it.
To decode data produced with l64a the following function should be
used.
l64a. The function processes at least 6 characters of
this string, and decodes the characters it finds according to the table
below. It stops decoding when it finds a character not in the table,
rather like atoi; if you have a buffer which has been broken into
lines, you must be careful to skip over the end-of-line characters.
The decoded number is returned as a long int value.
The l64a and a64l functions use a base 64 encoding, in
which each character of an encoded string represents six bits of an
input word. These symbols are used for the base 64 digits:
@multitable {xxxxx} {xxx} {xxx} {xxx} {xxx} {xxx} {xxx} {xxx} {xxx}
. @tab / @tab 0 @tab 1
@tab 2 @tab 3 @tab 4 @tab 5
6 @tab 7 @tab 8 @tab 9
@tab A @tab B @tab C @tab D
E @tab F @tab G @tab H
@tab I @tab J @tab K @tab L
M @tab N @tab O @tab P
@tab Q @tab R @tab S @tab T
U @tab V @tab W @tab X
@tab Y @tab Z @tab a @tab b
c @tab d @tab e @tab f
@tab g @tab h @tab i @tab j
k @tab l @tab m @tab n
@tab o @tab p @tab q @tab r
s @tab t @tab u @tab v
@tab w @tab x @tab y @tab z
This encoding scheme is not standard. There are some other encoding
methods which are much more widely used (UU encoding, MIME encoding).
Generally, it is better to use one of these encodings.
argz vectors are vectors of strings in a contiguous block of
memory, each element separated from its neighbors by null-characters
('\0').
Envz vectors are an extension of argz vectors where each element is a
name-value pair, separated by a '=' character (as in a Unix
environment).
Each argz vector is represented by a pointer to the first element, of
type char *, and a size, of type size_t, both of which can
be initialized to 0 to represent an empty argz vector. All argz
functions accept either a pointer and a size argument, or pointers to
them, if they will be modified.
The argz functions use malloc/realloc to allocate/grow
argz vectors, and so any argz vector creating using these functions may
be freed by using free; conversely, any argz function that may
grow a string expects that string to have been allocated using
malloc (those argz functions that only examine their arguments or
modify them in place will work on any sort of memory).
See section Unconstrained Allocation.
All argz functions that do memory allocation have a return type of
error_t, and return 0 for success, and ENOMEM if an
allocation error occurs.
These functions are declared in the standard include file `argz.h'.
argz_create function converts the Unix-style argument vector
argv (a vector of pointers to normal C strings, terminated by
(char *)0; see section Program Arguments) into an argz vector with
the same elements, which is returned in argz and argz_len.
argz_create_sep function converts the null-terminated string
string into an argz vector (returned in argz and
argz_len) by splitting it into elements at every occurrence of the
character sep.
argz_extract function converts the argz vector argz and
argz_len into a Unix-style argument vector stored in argv,
by putting pointers to every element in argz into successive
positions in argv, followed by a terminator of 0.
Argv must be pre-allocated with enough space to hold all the
elements in argz plus the terminating (char *)0
((argz_count (argz, argz_len) + 1) * sizeof (char *)
bytes should be enough). Note that the string pointers stored into
argv point into argz---they are not copies--and so
argz must be copied if it will be changed while argv is
still active. This function is useful for passing the elements in
argz to an exec function (see section Executing a File).
argz_stringify converts argz into a normal string with
the elements separated by the character sep, by replacing each
'\0' inside argz (except the last one, which terminates the
string) with sep. This is handy for printing argz in a
readable manner.
argz_add function adds the string str to the end of the
argz vector *argz, and updates *argz and
*argz_len accordingly.
argz_add_sep function is similar to argz_add, but
str is split into separate elements in the result at occurrences of
the character delim. This is useful, for instance, for
adding the components of a Unix search path to an argz vector, by using
a value of ':' for delim.
argz_append function appends buf_len bytes starting at
buf to the argz vector *argz, reallocating
*argz to accommodate it, and adding buf_len to
*argz_len.
*argz, the argz_delete function will
remove this entry and reallocate *argz, modifying
*argz and *argz_len accordingly. Note that as
destructive argz functions usually reallocate their argz argument,
pointers into argz vectors such as entry will then become invalid.
argz_insert function inserts the string entry into the
argz vector *argz at a point just before the existing
element pointed to by before, reallocating *argz and
updating *argz and *argz_len. If before
is 0, entry is added to the end instead (as if by
argz_add). Since the first element is in fact the same as
*argz, passing in *argz as the value of
before will result in entry being inserted at the beginning.
argz_next function provides a convenient way of iterating
over the elements in the argz vector argz. It returns a pointer
to the next element in argz after the element entry, or
0 if there are no elements following entry. If entry
is 0, the first element of argz is returned.
This behavior suggests two styles of iteration:
char *entry = 0;
while ((entry = argz_next (argz, argz_len, entry)))
action;
(the double parentheses are necessary to make some C compilers shut up
about what they consider a questionable while-test) and:
char *entry;
for (entry = argz;
entry;
entry = argz_next (argz, argz_len, entry))
action;
Note that the latter depends on argz having a value of 0 if
it is empty (rather than a pointer to an empty block of memory); this
invariant is maintained for argz vectors created by the functions here.
*replace_count will be
incremented by number of replacements performed.
Envz vectors are just argz vectors with additional constraints on the form of each element; as such, argz functions can also be used on them, where it makes sense.
Each element in an envz vector is a name-value pair, separated by a '='
character; if multiple '=' characters are present in an element, those
after the first are considered part of the value, and treated like all other
non-'\0' characters.
If no '=' characters are present in an element, that element is
considered the name of a "null" entry, as distinct from an entry with an
empty value: envz_get will return 0 if given the name of null
entry, whereas an entry with an empty value would result in a value of
""; envz_entry will still find such entries, however. Null
entries can be removed with envz_strip function.
As with argz functions, envz functions that may allocate memory (and thus
fail) have a return type of error_t, and return either 0 or
ENOMEM.
These functions are declared in the standard include file `envz.h'.
envz_entry function finds the entry in envz with the name
name, and returns a pointer to the whole entry--that is, the argz
element which begins with name followed by a '=' character. If
there is no entry with that name, 0 is returned.
envz_get function finds the entry in envz with the name
name (like envz_entry), and returns a pointer to the value
portion of that entry (following the '='). If there is no entry with
that name (or only a null entry), 0 is returned.
envz_add function adds an entry to *envz
(updating *envz and *envz_len) with the name
name, and value value. If an entry with the same name
already exists in envz, it is removed first. If value is
0, then the new entry will the special null type of entry
(mentioned above).
envz_merge function adds each entry in envz2 to envz,
as if with envz_add, updating *envz and
*envz_len. If override is true, then values in envz2
will supersede those with the same name in envz, otherwise not.
Null entries are treated just like other entries in this respect, so a null entry in envz can prevent an entry of the same name in envz2 from being added to envz, if override is false.
envz_strip function removes any null entries from envz,
updating *envz and *envz_len.
@ifnottex @macro cal{text} \text\
Character sets used in the early days of computing had only six, seven, or eight bits for each character: there was never a case where more than eight bits (one byte) were used to represent a single character. The limitations of this approach became more apparent as more people grappled with non-Roman character sets, where not all the characters that make up a language's character set can be represented by @math{2^8} choices. This chapter shows the functionality which was added to the C library to support multiple character sets.
A variety of solutions to overcome the differences between character sets with a 1:1 relation between bytes and characters and character sets with ratios of 2:1 or 4:1 exist. The remainder of this section gives a few examples to help understand the design decisions made while developing the functionality of the C library.
A distinction we have to make right away is between internal and external representation. Internal representation means the representation used by a program while keeping the text in memory. External representations are used when text is stored or transmitted through whatever communication channel. Examples of external representations include files lying in a directory that are going to be read and parsed.
Traditionally there has been no difference between the two representations. It was equally comfortable and useful to use the same single-byte representation internally and externally. This changes with more and larger character sets.
One of the problems to overcome with the internal representation is handling text that is externally encoded using different character sets. Assume a program which reads two texts and compares them using some metric. The comparison can be usefully done only if the texts are internally kept in a common format.
For such a common format (@math{=} character set) eight bits are certainly no longer enough. So the smallest entity will have to grow: wide characters will now be used. Instead of one byte, two or four will be used instead. (Three are not good to address in memory and more than four bytes seem not to be necessary).
As shown in some other part of this manual,
there exists a completely new family of functions which can handle texts
of this kind in memory. The most commonly used character sets for such
internal wide character representations are Unicode and ISO 10646
(also known as UCS for Universal Character Set). Unicode was originally
planned as a 16-bit character set, whereas ISO 10646 was designed to
be a 31-bit large code space. The two standards are practically identical.
They have the same character repertoire and code table, but Unicode specifies
added semantics. At the moment, only characters in the first 0x10000
code positions (the so-called Basic Multilingual Plane, BMP) have been
assigned, but the assignment of more specialized characters outside this
16-bit space is already in progress. A number of encodings have been
defined for Unicode and ISO 10646 characters:
UCS-2 is a 16-bit word that can only represent characters
from the BMP, UCS-4 is a 32-bit word than can represent any Unicode
and ISO 10646 character, UTF-8 is an ASCII compatible encoding where
ASCII characters are represented by ASCII bytes and non-ASCII characters
by sequences of 2-6 non-ASCII bytes, and finally UTF-16 is an extension
of UCS-2 in which pairs of certain UCS-2 words can be used to encode
non-BMP characters up to 0x10ffff.
To represent wide characters the char type is not suitable. For
this reason the ISO C standard introduces a new type which is
designed to keep one character of a wide character string. To maintain
the similarity there is also a type corresponding to int for
those functions which take a single wide character.
char[]
for multibyte character strings. The type is defined in `stddef.h'.
The ISO C90 standard, where this type was introduced, does not say
anything specific about the representation. It only requires that this
type is capable of storing all elements of the basic character set.
Therefore it would be legitimate to define wchar_t as
char. This might make sense for embedded systems.
But for GNU systems this type is always 32 bits wide. It is therefore
capable of representing all UCS-4 values and therefore covering all of
ISO 10646. Some Unix systems define wchar_t as a 16-bit type and
thereby follow Unicode very strictly. This is perfectly fine with the
standard but it also means that to represent all characters from Unicode
and ISO 10646 one has to use UTF-16 surrogate characters which is in
fact a multi-wide-character encoding. But this contradicts the purpose
of the wchar_t type.
wint_t is a data type used for parameters and variables which
contain a single wide character. As the name already suggests it is the
equivalent to int when using the normal char strings. The
types wchar_t and wint_t have often the same
representation if their size if 32 bits wide but if wchar_t is
defined as char the type wint_t must be defined as
int due to the parameter promotion.
This type is defined in `wchar.h' and got introduced in Amendment 1 to ISO C90.
As there are for the char data type there also exist macros
specifying the minimum and maximum value representable in an object of
type wchar_t.
WCHAR_MIN evaluates to the minimum value representable
by an object of type wint_t.
This macro got introduced in Amendment 1 to ISO C90.
WCHAR_MAX evaluates to the maximum value representable
by an object of type wint_t.
This macro got introduced in Amendment 1 to ISO C90.
Another special wide character value is the equivalent to EOF.
WEOF evaluates to a constant expression of type
wint_t whose value is different from any member of the extended
character set.
WEOF need not be the same value as EOF and unlike
EOF it also need not be negative. I.e., sloppy code like
{
int c;
...
while ((c = getc (fp)) < 0)
...
}
has to be rewritten to explicitly use WEOF when wide characters
are used.
{
wint_t c;
...
while ((c = wgetc (fp)) != WEOF)
...
}
This macro was introduced in Amendment 1 to ISO C90 and is defined in `wchar.h'.
These internal representations present problems when it comes to storing and transmittal, since a single wide character consists of more than one byte they are effected by byte-ordering. I.e., machines with different endianesses would see different value accessing the same data. This also applies for communication protocols which are all byte-based and therefore the sender has to decide about splitting the wide character in bytes. A last (but not least important) point is that wide characters often require more storage space than an customized byte oriented character set.
For all the above reasons, an external encoding which is different
from the internal encoding is often used if the latter is UCS-2 or UCS-4.
The external encoding is byte-based and can be chosen appropriately for
the environment and for the texts to be handled. There exist a variety
of different character sets which can be used for this external
encoding. Information which will not be exhaustively presented
here--instead, a description of the major groups will suffice. All of
the ASCII-based character sets [_bkoz_: do you mean Roman character
sets? If not, what do you mean here?] fulfill one requirement: they are
"filesystem safe". This means that the character '/' is used in
the encoding only to represent itself. Things are a bit
different for character sets like EBCDIC (Extended Binary Coded Decimal
Interchange Code, a character set family used by IBM) but if the
operation system does not understand EBCDIC directly the parameters to
system calls have to be converted first anyhow.
0xc2 0x61 (non-spacing
acute accent, following by lower-case `a') to get the "small a with
acute" character. To get the acute accent character on its own, one has
to write 0xc2 0x20 (the non-spacing acute followed by a space).
This type of character set is used in some embedded systems such as
teletex.
The question remaining is: how to select the character set or encoding to use. The answer: you cannot decide about it yourself, it is decided by the developers of the system or the majority of the users. Since the goal is interoperability one has to use whatever the other people one works with use. If there are no constraints the selection is based on the requirements the expected circle of users will have. I.e., if a project is expected to only be used in, say, Russia it is fine to use KOI8-R or a similar character set. But if at the same time people from, say, Greece are participating one should use a character set which allows all people to collaborate.
The most widely useful solution seems to be: go with the most general character set, namely ISO 10646. Use UTF-8 as the external encoding and problems about users not being able to use their own language adequately are a thing of the past.
One final comment about the choice of the wide character representation
is necessary at this point. We have said above that the natural choice
is using Unicode or ISO 10646. This is not required, but at least
encouraged, by the ISO C standard. The standard defines at least a
macro __STDC_ISO_10646__ that is only defined on systems where
the wchar_t type encodes ISO 10646 characters. If this
symbol is not defined one should as much as possible avoid making
assumption about the wide character representation. If the programmer
uses only the functions provided by the C library to handle wide
character strings there should not be any compatibility problems with
other systems.
A Unix C library contains three different sets of functions in two families to handle character set conversion. The one function family is specified in the ISO C standard and therefore is portable even beyond the Unix world.
The most commonly known set of functions, coming from the ISO C90 standard, is unfortunately the least useful one. In fact, these functions should be avoided whenever possible, especially when developing libraries (as opposed to applications).
The second family of functions got introduced in the early Unix standards (XPG2) and is still part of the latest and greatest Unix standard: Unix 98. It is also the most powerful and useful set of functions. But we will start with the functions defined in Amendment 1 to ISO C90.
The ISO C standard defines functions to convert strings from a multibyte representation to wide character strings. There are a number of peculiarities:
LC_CTYPE category of the current locale is used; see
section Categories of Activities that Locales Affect.
Despite these limitations the ISO C functions can very well be used
in many contexts. In graphical user interfaces, for instance, it is not
uncommon to have functions which require text to be displayed in a wide
character string if it is not simple ASCII. The text itself might come
from a file with translations and the user should decide about the
current locale which determines the translation and therefore also the
external encoding used. In such a situation (and many others) the
functions described here are perfect. If more freedom while performing
the conversion is necessary take a look at the iconv functions
(see section Generic Charset Conversion).
We already said above that the currently selected locale for the
LC_CTYPE category decides about the conversion which is performed
by the functions we are about to describe. Each locale uses its own
character set (given as an argument to localedef) and this is the
one assumed as the external multibyte encoding. The wide character
character set always is UCS-4, at least on GNU systems.
A characteristic of each multibyte character set is the maximum number of bytes which can be necessary to represent one character. This information is quite important when writing code which uses the conversion functions. In the examples below we will see some examples. The ISO C standard defines two macros which provide this information.
MB_CUR_MAX expands into a positive integer expression that is the
maximum number of bytes in a multibyte character in the current locale.
The value is never greater than MB_LEN_MAX. Unlike
MB_LEN_MAX this macro need not be a compile-time constant and in
fact, in the GNU C library it is not.
Two different macros are necessary since strictly ISO C90 compilers do not allow variable length array definitions but still it is desirable to avoid dynamic allocation. This incomplete piece of code shows the problem:
{
char buf[MB_LEN_MAX];
ssize_t len = 0;
while (! feof (fp))
{
fread (&buf[len], 1, MB_CUR_MAX - len, fp);
/* ... process buf */
len -= used;
}
}
The code in the inner loop is expected to have always enough bytes in
the array buf to convert one multibyte character. The array
buf has to be sized statically since many compilers do not allow a
variable size. The fread call makes sure that always
MB_CUR_MAX bytes are available in buf. Note that it isn't
a problem if MB_CUR_MAX is not a compile-time constant.
In the introduction of this chapter it was said that certain character sets use a stateful encoding. I.e., the encoded values depend in some way on the previous bytes in the text.
Since the conversion functions allow converting a text in more than one step we must have a way to pass this information from one call of the functions to another.
mbstate_t can contain all the information
about the shift state needed from one call to a conversion
function to another.
This type is defined in `wchar.h'. It got introduced in Amendment 1 to ISO C90.
To use objects of this type the programmer has to define such objects (normally as local variables on the stack) and pass a pointer to the object to the conversion functions. This way the conversion function can update the object if the current multibyte character set is stateful.
There is no specific function or initializer to put the state object in any specific state. The rules are that the object should always represent the initial state before the first use and this is achieved by clearing the whole variable with code such as follows:
{
mbstate_t state;
memset (&state, '\0', sizeof (state));
/* from now on state can be used. */
...
}
When using the conversion functions to generate output it is often necessary to test whether the current state corresponds to the initial state. This is necessary, for example, to decide whether or not to emit escape sequences to set the state to the initial state at certain sequence points. Communication protocols often require this.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
Code using this function often looks similar to this:
{
mbstate_t state;
memset (&state, '\0', sizeof (state));
/* Use state. */
...
if (! mbsinit (&state))
{
/* Emit code to return to initial state. */
const wchar_t empty[] = L"";
const wchar_t *srcp = empty;
wcsrtombs (outbuf, &srcp, outbuflen, &state);
}
...
}
The code to emit the escape sequence to get back to the initial state is
interesting. The wcsrtombs function can be used to determine the
necessary output code (see section Converting Multibyte and Wide Character Strings). Please note that on
GNU systems it is not necessary to perform this extra action for the
conversion from multibyte text to wide character text since the wide
character encoding is not stateful. But there is nothing mentioned in
any standard which prohibits making wchar_t using a stateful
encoding.
The most fundamental of the conversion functions are those dealing with single characters. Please note that this does not always mean single bytes. But since there is very often a subset of the multibyte character set which consists of single byte sequences there are functions to help with converting bytes. One very important and often applicable scenario is where ASCII is a subpart of the multibyte character set. I.e., all ASCII characters stand for itself and all other characters have at least a first byte which is beyond the range @math{0} to @math{127}.
btowc function ("byte to wide character") converts a valid
single byte character c in the initial shift state into the wide
character equivalent using the conversion rules from the currently
selected locale of the LC_CTYPE category.
If (unsigned char) c is no valid single byte multibyte
character or if c is EOF the function returns WEOF.
Please note the restriction of c being tested for validity only in
the initial shift state. There is no mbstate_t object used from
which the state information is taken and the function also does not use
any static state.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
Despite the limitation that the single byte value always is interpreted in the initial state this function is actually useful most of the time. Most characters are either entirely single-byte character sets or they are extension to ASCII. But then it is possible to write code like this (not that this specific example is very useful):
wchar_t *
itow (unsigned long int val)
{
static wchar_t buf[30];
wchar_t *wcp = &buf[29];
*wcp = L'\0';
while (val != 0)
{
*--wcp = btowc ('0' + val % 10);
val /= 10;
}
if (wcp == &buf[29])
*--wcp = L'0';
return wcp;
}
Why is it necessary to use such a complicated implementation and not
simply cast '0' + val % 10 to a wide character? The answer is
that there is no guarantee that one can perform this kind of arithmetic
on the character of the character set used for wchar_t
representation. In other situations the bytes are not constant at
compile time and so the compiler cannot do the work. In situations like
this it is necessary btowc.
There also is a function for the conversion in the other direction.
wctob function ("wide character to byte") takes as the
parameter a valid wide character. If the multibyte representation for
this character in the initial state is exactly one byte long the return
value of this function is this character. Otherwise the return value is
EOF.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
There are more general functions to convert single character from multibyte representation to wide characters and vice versa. These functions pose no limit on the length of the multibyte representation and they also do not require it to be in the initial state.
mbrtowc function ("multibyte restartable to wide
character") converts the next multibyte character in the string pointed
to by s into a wide character and stores it in the wide character
string pointed to by pwc. The conversion is performed according
to the locale currently selected for the LC_CTYPE category. If
the conversion for the character set used in the locale requires a state
the multibyte string is interpreted in the state represented by the
object pointed to by ps. If ps is a null pointer, a static,
internal state variable used only by the mbrtowc function is
used.
If the next multibyte character corresponds to the NUL wide character
the return value of the function is @math{0} and the state object is
afterwards in the initial state. If the next n or fewer bytes
form a correct multibyte character the return value is the number of
bytes starting from s which form the multibyte character. The
conversion state is updated according to the bytes consumed in the
conversion. In both cases the wide character (either the L'\0'
or the one found in the conversion) is stored in the string pointer to
by pwc iff pwc is not null.
If the first n bytes of the multibyte string possibly form a valid
multibyte character but there are more than n bytes needed to
complete it the return value of the function is (size_t) -2 and
no value is stored. Please note that this can happen even if n
has a value greater or equal to MB_CUR_MAX since the input might
contain redundant shift sequences.
If the first n bytes of the multibyte string cannot possibly form
a valid multibyte character also no value is stored, the global variable
errno is set to the value EILSEQ and the function returns
(size_t) -1. The conversion state is afterwards undefined.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
Using this function is straight forward. A function which copies a multibyte string into a wide character string while at the same time converting all lowercase character into uppercase could look like this (this is not the final version, just an example; it has no error checking, and leaks sometimes memory):
wchar_t *
mbstouwcs (const char *s)
{
size_t len = strlen (s);
wchar_t *result = malloc ((len + 1) * sizeof (wchar_t));
wchar_t *wcp = result;
wchar_t tmp[1];
mbstate_t state;
size_t nbytes;
memset (&state, '\0', sizeof (state));
while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0)
{
if (nbytes >= (size_t) -2)
/* Invalid input string. */
return NULL;
*result++ = towupper (tmp[0]);
len -= nbytes;
s += nbytes;
}
return result;
}
The use of mbrtowc should be clear. A single wide character is
stored in tmp[0] and the number of consumed bytes is stored
in the variable nbytes. In case the the conversion was successful
the uppercase variant of the wide character is stored in the
result array and the pointer to the input string and the number of
available bytes is adjusted.
The only non-obvious thing about the function might be the way memory is allocated for the result. The above code uses the fact that there can never be more wide characters in the converted results than there are bytes in the multibyte input string. This method yields to a pessimistic guess about the size of the result and if many wide character strings have to be constructed this way or the strings are long, the extra memory required allocated because the input string contains multibyte characters might be significant. It would be possible to resize the allocated memory block to the correct size before returning it. A better solution might be to allocate just the right amount of space for the result right away. Unfortunately there is no function to compute the length of the wide character string directly from the multibyte string. But there is a function which does part of the work.
mbrlen function ("multibyte restartable length") computes
the number of at most n bytes starting at s which form the
next valid and complete multibyte character.
If the next multibyte character corresponds to the NUL wide character the return value is @math{0}. If the next n bytes form a valid multibyte character the number of bytes belonging to this multibyte character byte sequence is returned.
If the the first n bytes possibly form a valid multibyte
character but it is incomplete the return value is (size_t) -2.
Otherwise the multibyte character sequence is invalid and the return
value is (size_t) -1.
The multibyte sequence is interpreted in the state represented by the
object pointed to by ps. If ps is a null pointer, a state
object local to mbrlen is used.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
The tentative reader now will of course note that mbrlen can be
implemented as
mbrtowc (NULL, s, n, ps != NULL ? ps : &internal)
This is true and in fact is mentioned in the official specification.
Now, how can this function be used to determine the length of the wide
character string created from a multibyte character string? It is not
directly usable but we can define a function mbslen using it:
size_t
mbslen (const char *s)
{
mbstate_t state;
size_t result = 0;
size_t nbytes;
memset (&state, '\0', sizeof (state));
while ((nbytes = mbrlen (s, MB_LEN_MAX, &state)) > 0)
{
if (nbytes >= (size_t) -2)
/* Something is wrong. */
return (size_t) -1;
s += nbytes;
++result;
}
return result;
}
This function simply calls mbrlen for each multibyte character
in the string and counts the number of function calls. Please note that
we here use MB_LEN_MAX as the size argument in the mbrlen
call. This is OK since a) this value is larger then the length of the
longest multibyte character sequence and b) because we know that the
string s ends with a NUL byte which cannot be part of any other
multibyte character sequence but the one representing the NUL wide
character. Therefore the mbrlen function will never read invalid
memory.
Now that this function is available (just to make this clear, this function is not part of the GNU C library) we can compute the number of wide character required to store the converted multibyte character string s using
wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t);
Please note that the mbslen function is quite inefficient. The
implementation of mbstouwcs implemented using mbslen would
have to perform the conversion of the multibyte character input string
twice and this conversion might be quite expensive. So it is necessary
to think about the consequences of using the easier but imprecise method
before doing the work twice.
wcrtomb function ("wide character restartable to
multibyte") converts a single wide character into a multibyte string
corresponding to that wide character.
If s is a null pointer the function resets the the state stored in
the objects pointer to by ps (or the internal mbstate_t
object) to the initial state. This can also be achieved by a call like
this:
wcrtombs (temp_buf, L'\0', ps)
since if s is a null pointer wcrtomb performs as if it
writes into an internal buffer which is guaranteed to be large enough.
If wc is the NUL wide character wcrtomb emits, if
necessary, a shift sequence to get the state ps into the initial
state followed by a single NUL byte is stored in the string s.
Otherwise a byte sequence (possibly including shift sequences) is
written into the string s. This of only happens if wc is a
valid wide character, i.e., it has a multibyte representation in the
character set selected by locale of the LC_CTYPE category. If
wc is no valid wide character nothing is stored in the strings
s, errno is set to EILSEQ, the conversion state in
ps is undefined and the return value is (size_t) -1.
If no error occurred the function returns the number of bytes stored in the string s. This includes all byte representing shift sequences.
One word about the interface of the function: there is no parameter
specifying the length of the array s. Instead the function
assumes that there are at least MB_CUR_MAX bytes available since
this is the maximum length of any byte sequence representing a single
character. So the caller has to make sure that there is enough space
available, otherwise buffer overruns can occur.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
Using this function is as easy as using mbrtowc. The following
example appends a wide character string to a multibyte character string.
Again, the code is not really useful (and correct), it is simply here to
demonstrate the use and some problems.
char *
mbscatwcs (char *s, size_t len, const wchar_t *ws)
{
mbstate_t state;
/* Find the end of the existing string. */
char *wp = strchr (s, '\0');
len -= wp - s;
memset (&state, '\0', sizeof (state));
do
{
size_t nbytes;
if (len < MB_CUR_LEN)
{
/* We cannot guarantee that the next
character fits into the buffer, so
return an error. */
errno = E2BIG;
return NULL;
}
nbytes = wcrtomb (wp, *ws, &state);
if (nbytes == (size_t) -1)
/* Error in the conversion. */
return NULL;
len -= nbytes;
wp += nbytes;
}
while (*ws++ != L'\0');
return s;
}
First the function has to find the end of the string currently in the
array s. The strchr call does this very efficiently since a
requirement for multibyte character representations is that the NUL byte
never is used except to represent itself (and in this context, the end
of the string).
After initializing the state object the loop is entered where the first
task is to make sure there is enough room in the array s. We
abort if there are not at least MB_CUR_LEN bytes available. This
is not always optimal but we have no other choice. We might have less
than MB_CUR_LEN bytes available but the next multibyte character
might also be only one byte long. At the time the wcrtomb call
returns it is too late to decide whether the buffer was large enough or
not. If this solution is really unsuitable there is a very slow but
more accurate solution.
...
if (len < MB_CUR_LEN)
{
mbstate_t temp_state;
memcpy (&temp_state, &state, sizeof (state));
if (wcrtomb (NULL, *ws, &temp_state) > len)
{
/* We cannot guarantee that the next
character fits into the buffer, so
return an error. */
errno = E2BIG;
return NULL;
}
}
...
Here we do perform the conversion which might overflow the buffer so
that we are afterwards in the position to make an exact decision about
the buffer size. Please note the NULL argument for the
destination buffer in the new wcrtomb call; since we are not
interested in the converted text at this point this is a nice way to
express this. The most unusual thing about this piece of code certainly
is the duplication of the conversion state object. But think about
this: if a change of the state is necessary to emit the next multibyte
character we want to have the same shift state change performed in the
real conversion. Therefore we have to preserve the initial shift state
information.
There are certainly many more and even better solutions to this problem. This example is only meant for educational purposes.
The functions described in the previous section only convert a single character at a time. Most operations to be performed in real-world programs include strings and therefore the ISO C standard also defines conversions on entire strings. However, the defined set of functions is quite limited, thus the GNU C library contains a few extensions which can help in some important situations.
mbsrtowcs function ("multibyte string restartable to wide
character string") converts an NUL terminated multibyte character
string at *src into an equivalent wide character string,
including the NUL wide character at the end. The conversion is started
using the state information from the object pointed to by ps or
from an internal object of mbsrtowcs if ps is a null
pointer. Before returning the state object to match the state after the
last converted character. The state is the initial state if the
terminating NUL byte is reached and converted.
If dst is not a null pointer the result is stored in the array pointed to by dst, otherwise the conversion result is not available since it is stored in an internal buffer.
If len wide characters are stored in the array dst before reaching the end of the input string the conversion stops and len is returned. If dst is a null pointer len is never checked.
Another reason for a premature return from the function call is if the
input string contains an invalid multibyte sequence. In this case the
global variable errno is set to EILSEQ and the function
returns (size_t) -1.
In all other cases the function returns the number of wide characters
converted during this call. If dst is not null mbsrtowcs
stores in the pointer pointed to by src a null pointer (if the NUL
byte in the input string was reached) or the address of the byte
following the last converted multibyte character.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
The definition of this function has one limitation which has to be
understood. The requirement that dst has to be a NUL terminated
string provides problems if one wants to convert buffers with text. A
buffer is normally no collection of NUL terminated strings but instead a
continuous collection of lines, separated by newline characters. Now
assume a function to convert one line from a buffer is needed. Since
the line is not NUL terminated the source pointer cannot directly point
into the unmodified text buffer. This means, either one inserts the NUL
byte at the appropriate place for the time of the mbsrtowcs
function call (which is not doable for a read-only buffer or in a
multi-threaded application) or one copies the line in an extra buffer
where it can be terminated by a NUL byte. Note that it is not in
general possible to limit the number of characters to convert by setting
the parameter len to any specific value. Since it is not known
how many bytes each multibyte character sequence is in length one always
could do only a guess.
There is still a problem with the method of NUL-terminating a line right after the newline character which could lead to very strange results. As said in the description of the mbsrtowcs function above the conversion state is guaranteed to be in the initial shift state after processing the NUL byte at the end of the input string. But this NUL byte is not really part of the text. I.e., the conversion state after the newline in the original text could be something different than the initial shift state and therefore the first character of the next line is encoded using this state. But the state in question is never accessible to the user since the conversion stops after the NUL byte (which resets the state). Most stateful character sets in use today require that the shift state after a newline is the initial state--but this is not a strict guarantee. Therefore simply NUL terminating a piece of a running text is not always an adequate solution and therefore never should be used in generally used code.
The generic conversion interface (see section Generic Charset Conversion)
does not have this limitation (it simply works on buffers, not
strings), and the GNU C library contains a set of functions which take
additional parameters specifying the maximal number of bytes which are
consumed from the input string. This way the problem of
mbsrtowcs's example above could be solved by determining the line
length and passing this length to the function.
wcsrtombs function ("wide character string restartable to
multibyte string") converts the NUL terminated wide character string at
*src into an equivalent multibyte character string and
stores the result in the array pointed to by dst. The NUL wide
character is also converted. The conversion starts in the state
described in the object pointed to by ps or by a state object
locally to wcsrtombs in case ps is a null pointer. If
dst is a null pointer the conversion is performed as usual but the
result is not available. If all characters of the input string were
successfully converted and if dst is not a null pointer the
pointer pointed to by src gets assigned a null pointer.
If one of the wide characters in the input string has no valid multibyte
character equivalent the conversion stops early, sets the global
variable errno to EILSEQ, and returns (size_t) -1.
Another reason for a premature stop is if dst is not a null pointer and the next converted character would require more than len bytes in total to the array dst. In this case (and if dest is not a null pointer) the pointer pointed to by src is assigned a value pointing to the wide character right after the last one successfully converted.
Except in the case of an encoding error the return value of the function is the number of bytes in all the multibyte character sequences stored in dst. Before returning the state in the object pointed to by ps (or the internal object in case ps is a null pointer) is updated to reflect the state after the last conversion. The state is the initial shift state in case the terminating NUL wide character was converted.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
The restriction mentions above for the mbsrtowcs function applies
also here. There is no possibility to directly control the number of
input characters. One has to place the NUL wide character at the
correct place or control the consumed input indirectly via the available
output array size (the len parameter).
mbsnrtowcs function is very similar to the mbsrtowcs
function. All the parameters are the same except for nmc which is
new. The return value is the same as for mbsrtowcs.
This new parameter specifies how many bytes at most can be used from the
multibyte character string. I.e., the multibyte character string
*src need not be NUL terminated. But if a NUL byte is
found within the nmc first bytes of the string the conversion
stops here.
This function is a GNU extensions. It is meant to work around the problems mentioned above. Now it is possible to convert buffer with multibyte character text piece for piece without having to care about inserting NUL bytes and the effect of NUL bytes on the conversion state.
A function to convert a multibyte string into a wide character string and display it could be written like this (this is not a really useful example):
void
showmbs (const char *src, FILE *fp)
{
mbstate_t state;
int cnt = 0;
memset (&state, '\0', sizeof (state));
while (1)
{
wchar_t linebuf[100];
const char *endp = strchr (src, '\n');
size_t n;
/* Exit if there is no more line. */
if (endp == NULL)
break;
n = mbsnrtowcs (linebuf, &src, endp - src, 99, &state);
linebuf[n] = L'\0';
fprintf (fp, "line %d: \"%S\"\n", linebuf);
}
}
There is no problem with the state after a call to mbsnrtowcs.
Since we don't insert characters in the strings which were not in there
right from the beginning and we use state only for the conversion
of the given buffer there is no problem with altering the state.
wcsnrtombs function implements the conversion from wide
character strings to multibyte character strings. It is similar to
wcsrtombs but it takes, just like mbsnrtowcs, an extra
parameter which specifies the length of the input string.
No more than nwc wide characters from the input string
*src are converted. If the input string contains a NUL
wide character in the first nwc character to conversion stops at
this place.
This function is a GNU extension and just like mbsnrtowcs is
helps in situations where no NUL terminated input strings are available.
The example programs given in the last sections are only brief and do
not contain all the error checking etc. Presented here is a complete
and documented example. It features the mbrtowc function but it
should be easy to derive versions using the other functions.
int
file_mbsrtowcs (int input, int output)
{
/* Note the use of MB_LEN_MAX.
MB_CUR_MAX cannot portably be used here. */
char buffer[BUFSIZ + MB_LEN_MAX];
mbstate_t state;
int filled = 0;
int eof = 0;
/* Initialize the state. */
memset (&state, '\0', sizeof (state));
while (!eof)
{
ssize_t nread;
ssize_t nwrite;
char *inp = buffer;
wchar_t outbuf[BUFSIZ];
wchar_t *outp = outbuf;
/* Fill up the buffer from the input file. */
nread = read (input, buffer + filled, BUFSIZ);
if (nread < 0)
{
perror ("read");
return 0;
}
/* If we reach end of file, make a note to read no more. */
if (nread == 0)
eof = 1;
/* filled is now the number of bytes in buffer. */
filled += nread;
/* Convert those bytes to wide characters--as many as we can. */
while (1)
{
size_t thislen = mbrtowc (outp, inp, filled, &state);
/* Stop converting at invalid character;
this can mean we have read just the first part
of a valid character. */
if (thislen == (size_t) -1)
break;
/* We want to handle embedded NUL bytes
but the return value is 0. Correct this. */
if (thislen == 0)
thislen = 1;
/* Advance past this character. */
inp += thislen;
filled -= thislen;
++outp;
}
/* Write the wide characters we just made. */
nwrite = write (output, outbuf,
(outp - outbuf) * sizeof (wchar_t));
if (nwrite < 0)
{
perror ("write");
return 0;
}
/* See if we have a real invalid character. */
if ((eof && filled > 0) || filled >= MB_CUR_MAX)
{
error (0, 0, "invalid multibyte character");
return 0;
}
/* If any characters must be carried forward,
put them at the beginning of buffer. */
if (filled > 0)
memmove (inp, buffer, filled);
}
return 1;
}
The functions described in the last chapter are defined in Amendment 1 to ISO C90. But the original ISO C90 standard also contained functions for character set conversion. The reason that they are not described in the first place is that they are almost entirely useless.
The problem is that all the functions for conversion defined in ISO C90 use a local state. This implies that multiple conversions at the same time (not only when using threads) cannot be done, and that you cannot first convert single characters and then strings since you cannot tell the conversion functions which state to use.
These functions are therefore usable only in a very limited set of situations. One must complete converting the entire string before starting a new one and each string/text must be converted with the same function (there is no problem with the library itself; it is guaranteed that no library function changes the state of any of these functions). For the above reasons it is highly requested that the functions from the last section are used in place of non-reentrant conversion functions.
mbtowc ("multibyte to wide character") function when called
with non-null string converts the first multibyte character
beginning at string to its corresponding wide character code. It
stores the result in *result.
mbtowc never examines more than size bytes. (The idea is
to supply for size the number of bytes of data you have in hand.)
mbtowc with non-null string distinguishes three
possibilities: the first size bytes at string start with
valid multibyte character, they start with an invalid byte sequence or
just part of a character, or string points to an empty string (a
null character).
For a valid multibyte character, mbtowc converts it to a wide
character and stores that in *result, and returns the
number of bytes in that character (always at least @math{1}, and never
more than size).
For an invalid byte sequence, mbtowc returns @math{-1}. For an
empty string, it returns @math{0}, also storing '\0' in
*result.
If the multibyte character code uses shift characters, then
mbtowc maintains and updates a shift state as it scans. If you
call mbtowc with a null pointer for string, that
initializes the shift state to its standard initial value. It also
returns nonzero if the multibyte character code in use actually has a
shift state. See section States in Non-reentrant Functions.
wctomb ("wide character to multibyte") function converts
the wide character code wchar to its corresponding multibyte
character sequence, and stores the result in bytes starting at
string. At most MB_CUR_MAX characters are stored.
wctomb with non-null string distinguishes three
possibilities for wchar: a valid wide character code (one that can
be translated to a multibyte character), an invalid code, and L'\0'.
Given a valid code, wctomb converts it to a multibyte character,
storing the bytes starting at string. Then it returns the number
of bytes in that character (always at least @math{1}, and never more
than MB_CUR_MAX).
If wchar is an invalid wide character code, wctomb returns
@math{-1}. If wchar is L'\0', it returns 0, also
storing '\0' in *string.
If the multibyte character code uses shift characters, then
wctomb maintains and updates a shift state as it scans. If you
call wctomb with a null pointer for string, that
initializes the shift state to its standard initial value. It also
returns nonzero if the multibyte character code in use actually has a
shift state. See section States in Non-reentrant Functions.
Calling this function with a wchar argument of zero when
string is not null has the side-effect of reinitializing the
stored shift state as well as storing the multibyte character
'\0' and returning @math{0}.
Similar to mbrlen there is also a non-reentrant function which
computes the length of a multibyte character. It can be defined in
terms of mbtowc.
mblen function with a non-null string argument returns
the number of bytes that make up the multibyte character beginning at
string, never examining more than size bytes. (The idea is
to supply for size the number of bytes of data you have in hand.)
The return value of mblen distinguishes three possibilities: the
first size bytes at string start with valid multibyte
character, they start with an invalid byte sequence or just part of a
character, or string points to an empty string (a null character).
For a valid multibyte character, mblen returns the number of
bytes in that character (always at least 1, and never more than
size). For an invalid byte sequence, mblen returns
@math{-1}. For an empty string, it returns @math{0}.
If the multibyte character code uses shift characters, then mblen
maintains and updates a shift state as it scans. If you call
mblen with a null pointer for string, that initializes the
shift state to its standard initial value. It also returns a nonzero
value if the multibyte character code in use actually has a shift state.
See section States in Non-reentrant Functions.
For convenience reasons the ISO C90 standard defines also functions to convert entire strings instead of single characters. These functions suffer from the same problems as their reentrant counterparts from Amendment 1 to ISO C90; see section Converting Multibyte and Wide Character Strings.
mbstowcs ("multibyte string to wide character string")
function converts the null-terminated string of multibyte characters
string to an array of wide character codes, storing not more than
size wide characters into the array beginning at wstring.
The terminating null character counts towards the size, so if size
is less than the actual number of wide characters resulting from
string, no terminating null character is stored.
The conversion of characters from string begins in the initial shift state.
If an invalid multibyte character sequence is found, this function returns a value of @math{-1}. Otherwise, it returns the number of wide characters stored in the array wstring. This number does not include the terminating null character, which is present if the number is less than size.
Here is an example showing how to convert a string of multibyte characters, allocating enough space for the result.
wchar_t *
mbstowcs_alloc (const char *string)
{
size_t size = strlen (string) + 1;
wchar_t *buf = xmalloc (size * sizeof (wchar_t));
size = mbstowcs (buf, string, size);
if (size == (size_t) -1)
return NULL;
buf = xrealloc (buf, (size + 1) * sizeof (wchar_t));
return buf;
}
wcstombs ("wide character string to multibyte string")
function converts the null-terminated wide character array wstring
into a string containing multibyte characters, storing not more than
size bytes starting at string, followed by a terminating
null character if there is room. The conversion of characters begins in
the initial shift state.
The terminating null character counts towards the size, so if size is less than or equal to the number of bytes needed in wstring, no terminating null character is stored.
If a code that does not correspond to a valid multibyte character is found, this function returns a value of @math{-1}. Otherwise, the return value is the number of bytes stored in the array string. This number does not include the terminating null character, which is present if the number is less than size.
In some multibyte character codes, the meaning of any particular byte sequence is not fixed; it depends on what other sequences have come earlier in the same string. Typically there are just a few sequences that can change the meaning of other sequences; these few are called shift sequences and we say that they set the shift state for other sequences that follow.
To illustrate shift state and shift sequences, suppose we decide that
the sequence 0200 (just one byte) enters Japanese mode, in which
pairs of bytes in the range from 0240 to 0377 are single
characters, while 0201 enters Latin-1 mode, in which single bytes
in the range from 0240 to 0377 are characters, and
interpreted according to the ISO Latin-1 character set. This is a
multibyte code which has two alternative shift states ("Japanese mode"
and "Latin-1 mode"), and two shift sequences that specify particular
shift states.
When the multibyte character code in use has shift states, then
mblen, mbtowc and wctomb must maintain and update
the current shift state as they scan the string. To make this work
properly, you must follow these rules:
mblen (NULL,
0). This initializes the shift state to its standard initial value.
Here is an example of using mblen following these rules:
void
scan_string (char *s)
{
int length = strlen (s);
/* Initialize shift state. */
mblen (NULL, 0);
while (1)
{
int thischar = mblen (s, length);
/* Deal with end of string and invalid characters. */
if (thischar == 0)
break;
if (thischar == -1)
{
error ("invalid multibyte character");
break;
}
/* Advance past this character. */
s += thischar;
length -= thischar;
}
}
The functions mblen, mbtowc and wctomb are not
reentrant when using a multibyte code that uses a shift state. However,
no other library functions call these functions, so you don't have to
worry that the shift state will be changed mysteriously.
The conversion functions mentioned so far in this chapter all had in
common that they operate on character sets which are not directly
specified by the functions. The multibyte encoding used is specified by
the currently selected locale for the LC_CTYPE category. The
wide character set is fixed by the implementation (in the case of GNU C
library it always is UCS-4 encoded ISO 10646.
This has of course several problems when it comes to general character conversion:
LC_CTYPE category,
one has to change the LC_CTYPE locale using setlocale.
This introduces major problems for the rest of the programs since
several more functions (e.g., the character classification functions,
see section Classification of Characters) use the LC_CTYPE category.
LC_CTYPE selection is global and shared by all
threads.
wchar_t representation there is at least a two-step
process necessary to convert a text using the functions above. One
would have to select the source character set as the multibyte encoding,
convert the text into a wchar_t text, select the destination
character set as the multibyte encoding and convert the wide character
text to the multibyte (@math{=} destination) character set.
Even if this is possible (which is not guaranteed) it is a very tiring
work. Plus it suffers from the other two raised points even more due to
the steady changing of the locale.
The XPG2 standard defines a completely new set of functions which has none of these limitations. They are not at all coupled to the selected locales and they but no constraints on the character sets selected for source and destination. Only the set of available conversions is limiting them. The standard does not specify that any conversion at all must be available. It is a measure of the quality of the implementation.
In the following text first the interface to iconv, the
conversion function, will be described. Comparisons with other
implementations will show what pitfalls lie on the way of portable
applications. At last, the implementation is described as far as
interesting to the advanced user who wants to extend the conversion
capabilities.
This set of functions follows the traditional cycle of using a resource: open--use--close. The interface consists of three functions, each of which implement one step.
Before the interfaces are described it is necessary to introduce a datatype. Just like other open--use--close interface the functions introduced here work using a handles and the `iconv.h' header defines a special type for the handles used.
Objects of this type can get assigned handles for the conversions using
the iconv functions. The objects themselves need not be freed but
the conversions for which the handles stand for have to.
The first step is the function to create a handle.
iconv_open function has to be used before starting a
conversion. The two parameters this function takes determine the
source and destination character set for the conversion and if the
implementation has the possibility to perform such a conversion the
function returns a handle.
If the wanted conversion is not available the function returns
(iconv_t) -1. In this case the global variable errno can
have the following values:
EMFILE
OPEN_MAX file descriptors open.
ENFILE
ENOMEM
EINVAL
It is not possible to use the same descriptor in different threads to perform independent conversions. Within the data structures associated with the descriptor there is information about the conversion state. This must not be messed up by using it in different conversions.
An iconv descriptor is like a file descriptor as for every use a
new descriptor must be created. The descriptor does not stand for all
of the conversions from fromset to toset.
The GNU C library implementation of iconv_open has one
significant extension to other implementations. To ease the extension
of the set of available conversions the implementation allows storing
the necessary files with data and code in arbitrarily many directories.
How this extension has to be written will be explained below
(see section The iconv Implementation in the GNU C library). Here it is only important to say
that all directories mentioned in the GCONV_PATH environment
variable are considered if they contain a file `gconv-modules'.
These directories need not necessarily be created by the system
administrator. In fact, this extension is introduced to help users
writing and using their own, new conversions. Of course this does not work
for security reasons in SUID binaries; in this case only the system
directory is considered and this normally is
`prefix/lib/gconv'. The GCONV_PATH environment
variable is examined exactly once at the first call of the
iconv_open function. Later modifications of the variable have no
effect.
This function got introduced early in the X/Open Portability Guide, version 2. It is supported by all commercial Unices as it is required for the Unix branding. However, the quality and completeness of the implementation varies widely. The function is declared in `iconv.h'.
The iconv implementation can associate large data structure with
the handle returned by iconv_open. Therefore it is crucial to
free all the resources once all conversions are carried out and the
conversion is not needed anymore.
iconv_close function frees all resources associated with the
handle cd which must have been returned by a successful call to
the iconv_open function.
If the function call was successful the return value is @math{0}.
Otherwise it is @math{-1} and errno is set appropriately.
Defined error are:
EBADF
This function was introduced together with the rest of the iconv
functions in XPG2 and it is declared in `iconv.h'.
The standard defines only one actual conversion function. This has therefore the most general interface: it allows conversion from one buffer to another. Conversion from a file to a buffer, vice versa, or even file to file can be implemented on top of it.
iconv function converts the text in the input buffer
according to the rules associated with the descriptor cd and
stores the result in the output buffer. It is possible to call the
function for the same text several times in a row since for stateful
character sets the necessary state information is kept in the data
structures associated with the descriptor.
The input buffer is specified by *inbuf and it contains
*inbytesleft bytes. The extra indirection is necessary for
communicating the used input back to the caller (see below). It is
important to note that the buffer pointer is of type char and the
length is measured in bytes even if the input text is encoded in wide
characters.
The output buffer is specified in a similar way. *outbuf
points to the beginning of the buffer with at least
*outbytesleft bytes room for the result. The buffer
pointer again is of type char and the length is measured in
bytes. If outbuf or *outbuf is a null pointer the
conversion is performed but no output is available.
If inbuf is a null pointer the iconv function performs the
necessary action to put the state of the conversion into the initial
state. This is obviously a no-op for non-stateful encodings, but if the
encoding has a state such a function call might put some byte sequences
in the output buffer which perform the necessary state changes. The
next call with inbuf not being a null pointer then simply goes on
from the initial state. It is important that the programmer never makes
any assumption on whether the conversion has to deal with states or not.
Even if the input and output character sets are not stateful the
implementation might still have to keep states. This is due to the
implementation chosen for the GNU C library as it is described below.
Therefore an iconv call to reset the state should always be
performed if some protocol requires this for the output text.
The conversion stops for three reasons. The first is that all characters from the input buffer are converted. This actually can mean two things: really all bytes from the input buffer are consumed or there are some bytes at the end of the buffer which possibly can form a complete character but the input is incomplete. The second reason for a stop is when the output buffer is full. And the third reason is that the input contains invalid characters.
In all these cases the buffer pointers after the last successful conversion, for input and output buffer, are stored in inbuf and outbuf and the available room in each buffer is stored in inbytesleft and outbytesleft.
Since the character sets selected in the iconv_open call can be
almost arbitrary there can be situations where the input buffer contains
valid characters which have no identical representation in the output
character set. The behavior in this situation is undefined. The
current behavior of the GNU C library in this situation is to
return with an error immediately. This certainly is not the most
desirable solution. Therefore future versions will provide better ones
but they are not yet finished.
If all input from the input buffer is successfully converted and stored
in the output buffer the function returns the number of non-reversible
conversions performed. In all other cases the return value is
(size_t) -1 and errno is set appropriately. In this case
the value pointed to by inbytesleft is nonzero.
EILSEQ
*inbuf points at the first byte of the
invalid byte sequence.
E2BIG
EINVAL
EBADF
This function was introduced in the XPG2 standard and is declared in the `iconv.h' header.
The definition of the iconv function is quite good overall. It
provides quite flexible functionality. The only problems lie in the
boundary cases which are incomplete byte sequences at the end of the
input buffer and invalid input. A third problem, which is not really
a design problem, is the way conversions are selected. The standard
does not say anything about the legitimate names, a minimal set of
available conversions. We will see how this negatively impacts other
implementations, as is demonstrated below.
iconv example
The example below features a solution for a common problem. Given that
one knows the internal encoding used by the system for wchar_t
strings one often is in the position to read text from a file and store
it in wide character buffers. One can do this using mbsrtowcs
but then we run into the problems discussed above.
int
file2wcs (int fd, const char *charset, wchar_t *outbuf, size_t avail)
{
char inbuf[BUFSIZ];
size_t insize = 0;
char *wrptr = (char *) outbuf;
int result = 0;
iconv_t cd;
cd = iconv_open ("WCHAR_T", charset);
if (cd == (iconv_t) -1)
{
/* Something went wrong. */
if (errno == EINVAL)
error (0, 0, "conversion from '%s' to wchar_t not available",
charset);
else
perror ("iconv_open");
/* Terminate the output string. */
*outbuf = L'\0';
return -1;
}
while (avail > 0)
{
size_t nread;
size_t nconv;
char *inptr = inbuf;
/* Read more input. */
nread = read (fd, inbuf + insize, sizeof (inbuf) - insize);
if (nread == 0)
{
/* When we come here the file is completely read.
This still could mean there are some unused
characters in the inbuf. Put them back. */
if (lseek (fd, -insize, SEEK_CUR) == -1)
result = -1;
/* Now write out the byte sequence to get into the
initial state if this is necessary. */
iconv (cd, NULL, NULL, &wrptr, &avail);
break;
}
insize += nread;
/* Do the conversion. */
nconv = iconv (cd, &inptr, &insize, &wrptr, &avail);
if (nconv == (size_t) -1)
{
/* Not everything went right. It might only be
an unfinished byte sequence at the end of the
buffer. Or it is a real problem. */
if (errno == EINVAL)
/* This is harmless. Simply move the unused
bytes to the beginning of the buffer so that
they can be used in the next round. */
memmove (inbuf, inptr, insize);
else
{
/* It is a real problem. Maybe we ran out of
space in the output buffer or we have invalid
input. In any case back the file pointer to
the position of the last processed byte. */
lseek (fd, -insize, SEEK_CUR);
result = -1;
break;
}
}
}
/* Terminate the output string. */
if (avail >= sizeof (wchar_t))
*((wchar_t *) wrptr) = L'\0';
if (iconv_close (cd) != 0)
perror ("iconv_close");
return (wchar_t *) wrptr - outbuf;
}
This example shows the most important aspects of using the iconv
functions. It shows how successive calls to iconv can be used to
convert large amounts of text. The user does not have to care about
stateful encodings as the functions take care of everything.
An interesting point is the case where iconv return an error and
errno is set to EINVAL. This is not really an error in
the transformation. It can happen whenever the input character set
contains byte sequences of more than one byte for some character and
texts are not processed in one piece. In this case there is a chance
that a multibyte sequence is cut. The caller than can simply read the
remainder of the takes and feed the offending bytes together with new
character from the input to iconv and continue the work. The
internal state kept in the descriptor is not unspecified after
such an event as it is the case with the conversion functions from the
ISO C standard.
The example also shows the problem of using wide character strings with
iconv. As explained in the description of the iconv
function above the function always takes a pointer to a char
array and the available space is measured in bytes. In the example the
output buffer is a wide character buffer. Therefore we use a local
variable wrptr of type char * which is used in the
iconv calls.
This looks rather innocent but can lead to problems on platforms which
have tight restriction on alignment. Therefore the caller of
iconv has to make sure that the pointers passed are suitable for
access of characters from the appropriate character set. Since in the
above case the input parameter to the function is a wchar_t
pointer this is the case (unless the user violates alignment when
computing the parameter). But in other situations, especially when
writing generic functions where one does not know what type of character
set one uses and therefore treats text as a sequence of bytes, it might
become tricky.
iconv Implementations
This is not really the place to discuss the iconv implementation
of other systems but it is necessary to know a bit about them to write
portable programs. The above mentioned problems with the specification
of the iconv functions can lead to portability issues.
The first thing to notice is that due to the large number of character sets in use it is certainly not practical to encode the conversions directly in the C library. Therefore the conversion information must come from files outside the C library. This is usually done in one or both of the following ways:
Some implementations in commercial Unices implement a mixture of these these possibilities, the majority only the second solution. Using loadable modules moves the code out of the library itself and keeps the door open for extensions and improvements. But this design is also limiting on some platforms since not many platforms support dynamic loading in statically linked programs. On platforms without his capability it is therefore not possible to use this interface in statically linked programs. The GNU C library has on ELF platforms no problems with dynamic loading in in these situations and therefore this point is moot. The danger is that one gets acquainted with this and forgets about the restrictions on other systems.
A second thing to know about other iconv implementations is that
the number of available conversions is often very limited. Some
implementations provide in the standard release (not special
international or developer releases) at most 100 to 200 conversion
possibilities. This does not mean 200 different character sets are
supported. E.g., conversions from one character set to a set of, say,
10 others counts as 10 conversion. Together with the other direction
this makes already 20. One can imagine the thin coverage these platform
provide. Some Unix vendors even provide only a handful of conversions
which renders them useless for almost all uses.
This directly leads to a third and probably the most problematic point.
The way the iconv conversion functions are implemented on all
known Unix system and the availability of the conversion functions from
character set @math{@cal{A}} to @math{@cal{B}} and the conversion from
@math{@cal{B}} to @math{@cal{C}} does not imply that the
conversion from @math{@cal{A}} to @math{@cal{C}} is available.
This might not seem unreasonable and problematic at first but it is a quite big problem as one will notice shortly after hitting it. To show the problem we assume to write a program which has to convert from @math{@cal{A}} to @math{@cal{C}}. A call like
cd = iconv_open ("@math{@cal{C}}", "@math{@cal{A}}");
does fail according to the assumption above. But what does the program do now? The conversion is really necessary and therefore simply giving up is no possibility.
This is a nuisance. The iconv function should take care of this.
But how should the program proceed from here on? If it would try to
convert to character set @math{@cal{B}} first the two iconv_open
calls
cd1 = iconv_open ("@math{@cal{B}}", "@math{@cal{A}}");
and
cd2 = iconv_open ("@math{@cal{C}}", "@math{@cal{B}}");
will succeed but how to find @math{@cal{B}}?
Unfortunately, the answer is: there is no general solution. On some systems guessing might help. On those systems most character sets can convert to and from UTF-8 encoded ISO 10646 or Unicode text. Beside this only some very system-specific methods can help. Since the conversion functions come from loadable modules and these modules must be stored somewhere in the filesystem, one could try to find them and determine from the available file which conversions are available and whether there is an indirect route from @math{@cal{A}} to @math{@cal{C}}.
This shows one of the design errors of iconv mentioned above. It
should at least be possible to determine the list of available
conversion programmatically so that if iconv_open says there is
no such conversion, one could make sure this also is true for indirect
routes.
iconv Implementation in the GNU C library
After reading about the problems of iconv implementations in the
last section it is certainly good to note that the implementation in
the GNU C library has none of the problems mentioned above. What
follows is a step-by-step analysis of the points raised above. The
evaluation is based on the current state of the development (as of
January 1999). The development of the iconv functions is not
complete, but basic functionality has solidified.
The GNU C library's iconv implementation uses shared loadable
modules to implement the conversions. A very small number of
conversions are built into the library itself but these are only rather
trivial conversions.
All the benefits of loadable modules are available in the GNU C library
implementation. This is especially appealing since the interface is
well documented (see below) and it therefore is easy to write new
conversion modules. The drawback of using loadable objects is not a
problem in the GNU C library, at least on ELF systems. Since the
library is able to load shared objects even in statically linked
binaries this means that static linking needs not to be forbidden in
case one wants to use iconv.
The second mentioned problem is the number of supported conversions. Currently, the GNU C library supports more than 150 character sets. The way the implementation is designed the number of supported conversions is greater than 22350 (@math{150} times @math{149}). If any conversion from or to a character set is missing it can easily be added.
Particularly impressive as it may be, this high number is due to the
fact that the GNU C library implementation of iconv does not have
the third problem mentioned above. I.e., whenever there is a conversion
from a character set @math{@cal{A}} to @math{@cal{B}} and from
@math{@cal{B}} to @math{@cal{C}} it is always possible to convert from
@math{@cal{A}} to @math{@cal{C}} directly. If the iconv_open
returns an error and sets errno to EINVAL this really
means there is no known way, directly or indirectly, to perform the
wanted conversion.
This is achieved by providing for each character set a conversion from and to UCS-4 encoded ISO 10646. Using ISO 10646 as an intermediate representation it is possible to triangulate, i.e., converting with an intermediate representation.
There is no inherent requirement to provide a conversion to ISO 10646 for a new character set and it is also possible to provide other conversions where neither source nor destination character set is ISO 10646. The currently existing set of conversions is simply meant to cover all conversions which might be of interest.
All currently available conversions use the triangulation method above, making conversion run unnecessarily slow. If, e.g., somebody often needs the conversion from ISO-2022-JP to EUC-JP, a quicker solution would involve direct conversion between the two character sets, skipping the input to ISO 10646 first. The two character sets of interest are much more similar to each other than to ISO 10646.
In such a situation one can easy write a new conversion and provide it
as a better alternative. The GNU C library iconv implementation
would automatically use the module implementing the conversion if it is
specified to be more efficient.
All information about the available conversions comes from a file named
`gconv-modules' which can be found in any of the directories along
the GCONV_PATH. The `gconv-modules' files are line-oriented
text files, where each of the lines has one of the following formats:
alias define an alias name for a character
set. There are two more words expected on the line. The first one
defines the alias name and the second defines the original name of the
character set. The effect is that it is possible to use the alias name
in the fromset or toset parameters of iconv_open and
achieve the same result as when using the real character set name.
This is quite important as a character set has often many different
names. There is normally always an official name but this need not
correspond to the most popular name. Beside this many character sets
have special names which are somehow constructed. E.g., all character
sets specified by the ISO have an alias of the form
ISO-IR-nnn where nnn is the registration number.
This allows programs which know about the registration number to
construct character set names and use them in iconv_open calls.
More on the available names and aliases follows below.
module introduce an available conversion
module. These lines must contain three or four more words.
The first word specifies the source character set, the second word the
destination character set of conversion implemented in this module. The
third word is the name of the loadable module. The filename is
constructed by appending the usual shared object suffix (normally
`.so') and this file is then supposed to be found in the same
directory the `gconv-modules' file is in. The last word on the
line, which is optional, is a numeric value representing the cost of the
conversion. If this word is missing a cost of @math{1} is assumed. The
numeric value itself does not matter that much; what counts are the
relative values of the sums of costs for all possible conversion paths.
Below is a more precise description of the use of the cost value.
Returning to the example above where one has written a module to directly convert from ISO-2022-JP to EUC-JP and back. All what has to be done is to put the new module, be its name ISO2022JP-EUCJP.so, in a directory and add a file `gconv-modules' with the following content in the same directory:
module ISO-2022-JP// EUC-JP// ISO2022JP-EUCJP 1 module EUC-JP// ISO-2022-JP// ISO2022JP-EUCJP 1
To see why this is sufficient, it is necessary to understand how the
conversion used by iconv (and described in the descriptor) is
selected. The approach to this problem is quite simple.
At the first call of the iconv_open function the program reads
all available `gconv-modules' files and builds up two tables: one
containing all the known aliases and another which contains the
information about the conversions and which shared object implements
them.
iconv
The set of available conversions form a directed graph with weighted
edges. The weights on the edges are the costs specified in the
`gconv-modules' files. The iconv_open function uses an
algorithm suitable for search for the best path in such a graph and so
constructs a list of conversions which must be performed in succession
to get the transformation from the source to the destination character
set.
Explaining why the above `gconv-modules' files allows the
iconv implementation to resolve the specific ISO-2022-JP to
EUC-JP conversion module instead of the conversion coming with the
library itself is straightforward. Since the latter conversion takes two
steps (from ISO-2022-JP to ISO 10646 and then from ISO 10646 to
EUC-JP) the cost is @math{1+1 = 2}. But the above `gconv-modules'
file specifies that the new conversion modules can perform this
conversion with only the cost of @math{1}.
A mysterious piece about the `gconv-modules' file above (and also
the file coming with the GNU C library) are the names of the character
sets specified in the module lines. Why do almost all the names
end in //? And this is not all: the names can actually be
regular expressions. At this point of time this mystery should not be
revealed, unless you have the relevant spell-casting materials: ashes
from an original DOS 6.2 boot disk burnt in effigy, a crucifix
blessed by St. Emacs, assorted herbal roots from Central America, sand
from Cebu, etc. Sorry! The part of the implementation where
this is used is not yet finished. For now please simply follow the
existing examples. It'll become clearer once it is. --drepper
A last remark about the `gconv-modules' is about the names not
ending with //. There often is a character set named
INTERNAL mentioned. From the discussion above and the chosen
name it should have become clear that this is the name for the
representation used in the intermediate step of the triangulation. We
have said that this is UCS-4 but actually it is not quite right. The
UCS-4 specification also includes the specification of the byte ordering
used. Since a UCS-4 value consists of four bytes a stored value is
effected by byte ordering. The internal representation is not
the same as UCS-4 in case the byte ordering of the processor (or at least
the running process) is not the same as the one required for UCS-4. This
is done for performance reasons as one does not want to perform
unnecessary byte-swapping operations if one is not interested in actually
seeing the result in UCS-4. To avoid trouble with endianess the internal
representation consistently is named INTERNAL even on big-endian
systems where the representations are identical.
iconv module data structuresSo far this section described how modules are located and considered to be used. What remains to be described is the interface of the modules so that one can write new ones. This section describes the interface as it is in use in January 1999. The interface will change in future a bit but hopefully only in an upward compatible way.
The definitions necessary to write new modules are publicly available in the non-standard header `gconv.h'. The following text will therefore describe the definitions from this header file. But first it is necessary to get an overview.
From the perspective of the user of iconv the interface is quite
simple: the iconv_open function returns a handle which can be
used in calls to iconv and finally the handle is freed with a call
to iconv_close. The problem is: the handle has to be able to
represent the possibly long sequences of conversion steps and also the
state of each conversion since the handle is all which is passed to the
iconv function. Therefore the data structures are really the
elements to understanding the implementation.
We need two different kinds of data structures. The first describes the conversion and the second describes the state etc. There are really two type definitions like this in `gconv.h'.
struct __gconv_loaded_object *__shlib_handle
const char *__modname
int __counter
const char *__from_name
const char *__to_name
__from_name and __to_name contain the names of the source and
destination character sets. They can be used to identify the actual
conversion to be carried out since one module might implement
conversions for more than one character set and/or direction.
gconv_fct __fct
gconv_init_fct __init_fct
gconv_end_fct __end_fct
int __min_needed_from
int __max_needed_from
int __min_needed_to
int __max_needed_to;
__min_needed_from value specifies how many bytes a character of
the source character set at least needs. The __max_needed_from
specifies the maximum value which also includes possible shift
sequences.
The __min_needed_to and __max_needed_to values serve the
same purpose but this time for the destination character set.
It is crucial that these values are accurate since otherwise the
conversion functions will have problems or not work at all.
int __stateful
void *__data
__data
element must not contain data specific to one specific use of the
conversion function.
char *__outbuf
char *__outbufend
__outbuf element points to the beginning of the buffer and
__outbufend points to the byte following the last byte in the
buffer. The conversion function must not assume anything about the size
of the buffer but it can be safely assumed the there is room for at
least one complete character in the output buffer.
Once the conversion is finished and the conversion is the last step the
__outbuf element must be modified to point after last last byte
written into the buffer to signal how much output is available. If this
conversion step is not the last one the element must not be modified.
The __outbufend element must not be modified.
int __is_last
int __invocation_counter
int __internal_use
mbsrtowcs et.al. I.e., the
function is not used directly through the iconv interface.
This sometimes makes a difference as it is expected that the
iconv functions are used to translate entire texts while the
mbsrtowcs functions are normally only used to convert single
strings and might be used multiple times to convert entire texts.
But in this situation we would have problem complying with some rules of
the character set specification. Some character sets require a prolog
which must appear exactly once for an entire text. If a number of
mbsrtowcs calls are used to convert the text only the first call
must add the prolog. But since there is no communication between the
different calls of mbsrtowcs the conversion functions have no
possibility to find this out. The situation is different for sequences
of iconv calls since the handle allows access to the needed
information.
This element is mostly used together with __invocation_counter in
a way like this:
if (!data->__internal_use
&& data->__invocation_counter == 0)
/* Emit prolog. */
...
This element must never be modified.
mbstate_t *__statep
__statep element points to an object of type mbstate_t
(see section Representing the state of the conversion). The conversion of an stateful character
set must use the object pointed to by this element to store information
about the conversion state. The __statep element itself must
never be modified.
mbstate_t __state
iconv module interfacesWith the knowledge about the data structures we now can describe the conversion functions itself. To understand the interface a bit of knowledge about the functionality in the C library which loads the objects with the conversions is necessary.
It is often the case that one conversion is used more than once. I.e.,
there are several iconv_open calls for the same set of character
sets during one program run. The mbsrtowcs et.al. functions in
the GNU C library also use the iconv functionality which
increases the number of uses of the same functions even more.
For this reason the modules do not get loaded exclusively for one
conversion. Instead a module once loaded can be used by arbitrarily many
iconv or mbsrtowcs calls at the same time. The splitting
of the information between conversion function specific information and
conversion data makes this possible. The last section showed the two
data structures used to do this.
This is of course also reflected in the interface and semantics of the functions the modules must provide. There are three functions which must have the following names:
gconv_init
gconv_init function initializes the conversion function
specific data structure. This very same object is shared by all
conversion which use this conversion and therefore no state information
about the conversion itself must be stored in here. If a module
implements more than one conversion the gconv_init function will be
called multiple times.
gconv_end
gconv_end function is responsible to free all resources
allocated by the gconv_init function. If there is nothing to do
this function can be missing. Special care must be taken if the module
implements more than one conversion and the gconv_init function
does not allocate the same resources for all conversions.
gconv
gconv_init and the conversion data, specific to
this use of the conversion functions.
There are three data types defined for the three module interface function and these define the interface.
As explained int the description of the struct __gconv_step data
structure above the initialization function has to initialize parts of
it.
__min_needed_from
__max_needed_from
__min_needed_to
__max_needed_to
__stateful
If the initialization function needs to communication some information
to the conversion function this can happen using the __data
element of the __gconv_step structure. But since this data is
shared by all the conversion is must not be modified by the conversion
function. How this can be used is shown in the example below.
#define MIN_NEEDED_FROM 1
#define MAX_NEEDED_FROM 4
#define MIN_NEEDED_TO 4
#define MAX_NEEDED_TO 4
int
gconv_init (struct __gconv_step *step)
{
/* Determine which direction. */
struct iso2022jp_data *new_data;
enum direction dir = illegal_dir;
enum variant var = illegal_var;
int result;
if (__strcasecmp (step->__from_name, "ISO-2022-JP//") == 0)
{
dir = from_iso2022jp;
var = iso2022jp;
}
else if (__strcasecmp (step->__to_name, "ISO-2022-JP//") == 0)
{
dir = to_iso2022jp;
var = iso2022jp;
}
else if (__strcasecmp (step->__from_name, "ISO-2022-JP-2//") == 0)
{
dir = from_iso2022jp;
var = iso2022jp2;
}
else if (__strcasecmp (step->__to_name, "ISO-2022-JP-2//") == 0)
{
dir = to_iso2022jp;
var = iso2022jp2;
}
result = __GCONV_NOCONV;
if (dir != illegal_dir)
{
new_data = (struct iso2022jp_data *)
malloc (sizeof (struct iso2022jp_data));
result = __GCONV_NOMEM;
if (new_data != NULL)
{
new_data->dir = dir;
new_data->var = var;
step->__data = new_data;
if (dir == from_iso2022jp)
{
step->__min_needed_from = MIN_NEEDED_FROM;
step->__max_needed_from = MAX_NEEDED_FROM;
step->__min_needed_to = MIN_NEEDED_TO;
step->__max_needed_to = MAX_NEEDED_TO;
}
else
{
step->__min_needed_from = MIN_NEEDED_TO;
step->__max_needed_from = MAX_NEEDED_TO;
step->__min_needed_to = MIN_NEEDED_FROM;
step->__max_needed_to = MAX_NEEDED_FROM + 2;
}
/* Yes, this is a stateful encoding. */
step->__stateful = 1;
result = __GCONV_OK;
}
}
return result;
}
The function first checks which conversion is wanted. The module from which this function is taken implements four different conversion and which one is selected can be determined by comparing the names. The comparison should always be done without paying attention to the case.
Then a data structure is allocated which contains the necessary
information about which conversion is selected. The data structure
struct iso2022jp_data is locally defined since outside the module
this data is not used at all. Please note that if all four conversions
this modules supports are requested there are four data blocks.
One interesting thing is the initialization of the __min_ and
__max_ elements of the step data object. A single ISO-2022-JP
character can consist of one to four bytes. Therefore the
MIN_NEEDED_FROM and MAX_NEEDED_FROM macros are defined
this way. The output is always the INTERNAL character set (aka
UCS-4) and therefore each character consists of exactly four bytes. For
the conversion from INTERNAL to ISO-2022-JP we have to take into
account that escape sequences might be necessary to switch the character
sets. Therefore the __max_needed_to element for this direction
gets assigned MAX_NEEDED_FROM + 2. This takes into account the
two bytes needed for the escape sequences to single the switching. The
asymmetry in the maximum values for the two directions can be explained
easily: when reading ISO-2022-JP text escape sequences can be handled
alone. I.e., it is not necessary to process a real character since the
effect of the escape sequence can be recorded in the state information.
The situation is different for the other direction. Since it is in
general not known which character comes next one cannot emit escape
sequences to change the state in advance. This means the escape
sequences which have to be emitted together with the next character.
Therefore one needs more room then only for the character itself.
The possible return values of the initialization function are:
__GCONV_OK
__GCONV_NOCONV
__GCONV_NOMEM
The functions called before the module is unloaded is significantly easier. It often has nothing at all to do in which case it can be left out completely.
__data element of
the object pointed to by the argument is of interest. Continuing the
example from the initialization function, the finalization function
looks like this:
void
gconv_end (struct __gconv_step *data)
{
free (data->__data);
}
The most important function is the conversion function itself. It can get quite complicated for complex character sets. But since this is not of interest here we will only describe a possible skeleton for the conversion function.
iconv
function it can be seen why the flushing mode is necessary. What mode
is selected is determined by the sixth argument, an integer. If it is
nonzero it means that flushing is selected.
Common to both mode is where the output buffer can be found. The
information about this buffer is stored in the conversion step data. A
pointer to this is passed as the second argument to this function. The
description of the struct __gconv_step_data structure has more
information on this.
What has to be done for flushing depends on the source character set.
If it is not stateful nothing has to be done. Otherwise the function
has to emit a byte sequence to bring the state object in the initial
state. Once this all happened the other conversion modules in the chain
of conversions have to get the same chance. Whether another step
follows can be determined from the __is_last element of the step
data structure to which the first parameter points.
The more interesting mode is when actually text has to be converted. The first step in this case is to convert as much text as possible from the input buffer and store the result in the output buffer. The start of the input buffer is determined by the third argument which is a pointer to a pointer variable referencing the beginning of the buffer. The fourth argument is a pointer to the byte right after the last byte in the buffer.
The conversion has to be performed according to the current state if the
character set is stateful. The state is stored in an object pointed to
by the __statep element of the step data (second argument). Once
either the input buffer is empty or the output buffer is full the
conversion stops. At this point the pointer variable referenced by the
third parameter must point to the byte following the last processed
byte. I.e., if all of the input is consumed this pointer and the fourth
parameter have the same value.
What now happens depends on whether this step is the last one or not.
If it is the last step the only thing which has to be done is to update
the __outbuf element of the step data structure to point after the
last written byte. This gives the caller the information on how much
text is available in the output buffer. Beside this the variable
pointed to by the fifth parameter, which is of type size_t, must
be incremented by the number of characters (not bytes) which were
converted in a non-reversible way. Then the function can return.
In case the step is not the last one the later conversion functions have to get a chance to do their work. Therefore the appropriate conversion function has to be called. The information about the functions is stored in the conversion data structures, passed as the first parameter. This information and the step data are stored in arrays so the next element in both cases can be found by simple pointer arithmetic:
int
gconv (struct __gconv_step *step, struct __gconv_step_data *data,
const char **inbuf, const char *inbufend, size_t *written,
int do_flush)
{
struct __gconv_step *next_step = step + 1;
struct __gconv_step_data *next_data = data + 1;
...
The next_step pointer references the next step information and
next_data the next data record. The call of the next function
therefore will look similar to this:
next_step->__fct (next_step, next_data, &outerr, outbuf,
written, 0)
But this is not yet all. Once the function call returns the conversion
function might have some more to do. If the return value of the
function is __GCONV_EMPTY_INPUT this means there is more room in
the output buffer. Unless the input buffer is empty the conversion
functions start all over again and processes the rest of the input
buffer. If the return value is not __GCONV_EMPTY_INPUT something
went wrong and we have to recover from this.
A requirement for the conversion function is that the input buffer pointer (the third argument) always points to the last character which was put in the converted form in the output buffer. This is trivially true after the conversion performed in the current step. But if the conversion functions deeper down the stream stop prematurely not all characters from the output buffer are consumed and therefore the input buffer pointers must be backed of to the right position.
This is easy to do if the input and output character sets have a fixed width for all characters. In this situation we can compute how many characters are left in the output buffer and therefore can correct the input buffer pointer appropriate with a similar computation. Things are getting tricky if either character set has character represented with variable length byte sequences and it gets even more complicated if the conversion has to take care of the state. In these cases the conversion has to be performed once again, from the known state before the initial conversion. I.e., if necessary the state of the conversion has to be reset and the conversion loop has to be executed again. The difference now is that it is known how much input must be created and the conversion can stop before converting the first unused character. Once this is done the input buffer pointers must be updated again and the function can return.
One final thing should be mentioned. If it is necessary for the
conversion to know whether it is the first invocation (in case a prolog
has to be emitted) the conversion function should just before returning
to the caller increment the __invocation_counter element of the
step data structure. See the description of the struct
__gconv_step_data structure above for more information on how this can
be used.
The return value must be one of the following values:
__GCONV_EMPTY_INPUT
__GCONV_FULL_OUTPUT
__GCONV_INCOMPLETE_INPUT
The following example provides a framework for a conversion function. In case a new conversion has to be written the holes in this implementation have to be filled and that is it.
int
gconv (struct __gconv_step *step, struct __gconv_step_data *data,
const char **inbuf, const char *inbufend, size_t *written,
int do_flush)
{
struct __gconv_step *next_step = step + 1;
struct __gconv_step_data *next_data = data + 1;
gconv_fct fct = next_step->__fct;
int status;
/* If the function is called with no input this means we have
to reset to the initial state. The possibly partly
converted input is dropped. */
if (do_flush)
{
status = __GCONV_OK;
/* Possible emit a byte sequence which put the state object
into the initial state. */
/* Call the steps down the chain if there are any but only
if we successfully emitted the escape sequence. */
if (status == __GCONV_OK && ! data->__is_last)
status = fct (next_step, next_data, NULL, NULL,
written, 1);
}
else
{
/* We preserve the initial values of the pointer variables. */
const char *inptr = *inbuf;
char *outbuf = data->__outbuf;
char *outend = data->__outbufend;
char *outptr;
do
{
/* Remember the start value for this round. */
inptr = *inbuf;
/* The outbuf buffer is empty. */
outptr = outbuf;
/* For stateful encodings the state must be safe here. */
/* Run the conversion loop. status is set
appropriately afterwards. */
/* If this is the last step leave the loop, there is
nothing we can do. */
if (data->__is_last)
{
/* Store information about how many bytes are
available. */
data->__outbuf = outbuf;
/* If any non-reversible conversions were performed,
add the number to *written. */
break;
}
/* Write out all output which was produced. */
if (outbuf > outptr)
{
const char *outerr = data->__outbuf;
int result;
result = fct (next_step, next_data, &outerr,
outbuf, written, 0);
if (result != __GCONV_EMPTY_INPUT)
{
if (outerr != outbuf)
{
/* Reset the input buffer pointer. We
document here the complex case. */
size_t nstatus;
/* Reload the pointers. */
*inbuf = inptr;
outbuf = outptr;
/* Possibly reset the state. */
/* Redo the conversion, but this time
the end of the output buffer is at
outerr. */
}
/* Change the status. */
status = result;
}
else
/* All the output is consumed, we can make
another run if everything was ok. */
if (status == __GCONV_FULL_OUTPUT)
status = __GCONV_OK;
}
}
while (status == __GCONV_OK);
/* We finished one use of this step. */
++data->__invocation_counter;
}
return status;
}
This information should be sufficient to write new modules. Anybody doing so should also take a look at the available source code in the GNU C library sources. It contains many examples of working and optimized modules.
Different countries and cultures have varying conventions for how to communicate. These conventions range from very simple ones, such as the format for representing dates and times, to very complex ones, such as the language spoken.
Internationalization of software means programming it to be able to adapt to the user's favorite conventions. In ISO C, internationalization works by means of locales. Each locale specifies a collection of conventions, one convention for each purpose. The user chooses a set of conventions by specifying a locale (via environment variables).
All programs inherit the chosen locale as part of their environment. Provided the programs are written to obey the choice of locale, they will follow the conventions preferred by the user.
Each locale specifies conventions for several purposes, including the following:
Some aspects of adapting to the specified locale are handled
automatically by the library subroutines. For example, all your program
needs to do in order to use the collating sequence of the chosen locale
is to use strcoll or strxfrm to compare strings.
Other aspects of locales are beyond the comprehension of the library. For example, the library can't automatically translate your program's output messages into other languages. The only way you can support output in the user's favorite language is to program this more or less by hand. The C library provides functions to handle translations for multiple languages easily.
This chapter discusses the mechanism by which you can modify the current locale. The effects of the current locale on specific library functions are discussed in more detail in the descriptions of those functions.
The simplest way for the user to choose a locale is to set the
environment variable LANG. This specifies a single locale to use
for all purposes. For example, a user could specify a hypothetical
locale named `espana-castellano' to use the standard conventions of
most of Spain.
The set of locales supported depends on the operating system you are using, and so do their names. We can't make any promises about what locales will exist, except for one standard locale called `C' or `POSIX'. Later we will describe how to construct locales.
A user also has the option of specifying different locales for different purposes--in effect, choosing a mixture of multiple locales.
For example, the user might specify the locale `espana-castellano' for most purposes, but specify the locale `usa-english' for currency formatting. This might make sense if the user is a Spanish-speaking American, working in Spanish, but representing monetary amounts in US dollars.
Note that both locales `espana-castellano' and `usa-english', like all locales, would include conventions for all of the purposes to which locales apply. However, the user can choose to use each locale for a particular subset of those purposes.
The purposes that locales serve are grouped into categories, so
that a user or a program can choose the locale for each category
independently. Here is a table of categories; each name is both an
environment variable that a user can set, and a macro name that you can
use as an argument to setlocale.
LC_COLLATE
strcoll
and strxfrm); see section Collation Functions.
LC_CTYPE
LC_MONETARY
LC_NUMERIC
LC_TIME
LC_MESSAGES
LC_ALL
setlocale to set a single locale for all purposes. Setting
this environment variable overwrites all selections by the other
LC_* variables or LANG.
LANG
When developing the message translation functions it was felt that the
functionality provided by the variables above is not sufficient. For
example, it should be possible to specify more than one locale name.
Take a Swedish user who better speaks German than English, and a program
whose messages are output in English by default. It should be possible
to specify that the first choice of language is Swedish, the second
German, and if this also fails to use English. This is
possible with the variable LANGUAGE. For further description of
this GNU extension see section User influence on gettext.
A C program inherits its locale environment variables when it starts up.
This happens automatically. However, these variables do not
automatically control the locale used by the library functions, because
ISO C says that all programs start by default in the standard `C'
locale. To use the locales specified by the environment, you must call
setlocale. Call it as follows:
setlocale (LC_ALL, "");
to select a locale based on the user choice of the appropriate environment variables.
You can also use setlocale to specify a particular locale, for
general use or for a specific category.
The symbols in this section are defined in the header file `locale.h'.
setlocale sets the current locale for category
category to locale. A list of all the locales the system
provides can be created by running
locale -a
If category is LC_ALL, this specifies the locale for all
purposes. The other possible values of category specify an
single purpose (see section Categories of Activities that Locales Affect).
You can also use this function to find out the current locale by passing
a null pointer as the locale argument. In this case,
setlocale returns a string that is the name of the locale
currently selected for category category.
The string returned by setlocale can be overwritten by subsequent
calls, so you should make a copy of the string (see section Copying and Concatenation) if you want to save it past any further calls to
setlocale. (The standard library is guaranteed never to call
setlocale itself.)
You should not modify the string returned by setlocale. It might
be the same string that was passed as an argument in a previous call to
setlocale. One requirement is that the category must be
the same in the call the string was returned and the one when the string
is passed in as locale parameter.
When you read the current locale for category LC_ALL, the value
encodes the entire combination of selected locales for all categories.
In this case, the value is not just a single locale name. In fact, we
don't make any promises about what it looks like. But if you specify
the same "locale name" with LC_ALL in a subsequent call to
setlocale, it restores the same combination of locale selections.
To be sure you can use the returned string encoding the currently selected locale at a later time, you must make a copy of the string. It is not guaranteed that the returned pointer remains valid over time.
When the locale argument is not a null pointer, the string returned
by setlocale reflects the newly-modified locale.
If you specify an empty string for locale, this means to read the appropriate environment variable and use its value to select the locale for category.
If a nonempty string is given for locale, then the locale of that name is used if possible.
If you specify an invalid locale name, setlocale returns a null
pointer and leaves the current locale unchanged.
Here is an example showing how you might use setlocale to
temporarily switch to a new locale.
#include <stddef.h>
#include <locale.h>
#include <stdlib.h>
#include <string.h>
void
with_other_locale (char *new_locale,
void (*subroutine) (int),
int argument)
{
char *old_locale, *saved_locale;
/* Get the name of the current locale. */
old_locale = setlocale (LC_ALL, NULL);
/* Copy the name so it won't be clobbered by setlocale. */
saved_locale = strdup (old_locale);
if (saved_locale == NULL)
fatal ("Out of memory");
/* Now change the locale and do some stuff with it. */
setlocale (LC_ALL, new_locale);
(*subroutine) (argument);
/* Restore the original locale. */
setlocale (LC_ALL, saved_locale);
free (saved_locale);
}
Portability Note: Some ISO C systems may define additional locale categories, and future versions of the library will do so. For portability, assume that any symbol beginning with `LC_' might be defined in `locale.h'.
The only locale names you can count on finding on all operating systems are these three standard ones:
"C"
"POSIX"
""
Defining and installing named locales is normally a responsibility of the system administrator at your site (or the person who installed the GNU C library). It is also possible for the user to create private locales. All this will be discussed later when describing the tool to do so.
If your program needs to use something other than the `C' locale, it will be more portable if you use whatever locale the user specifies with the environment, rather than trying to specify some non-standard locale explicitly by name. Remember, different machines might have different sets of locales installed.
There are several ways to access locale information. The simplest way is to let the C library itself do the work. Several of the functions in this library implicitly access the locale data, and use what information is provided by the currently selected locale. This is how the locale model is meant to work normally.
As an example take the strftime function, which is meant to nicely
format date and time information (see section Formatting Calendar Time).
Part of the standard information contained in the LC_TIME
category is the names of the months. Instead of requiring the
programmer to take care of providing the translations the
strftime function does this all by itself. %A
in the format string is replaced by the appropriate weekday
name of the locale currently selected by LC_TIME. This is an
easy example, and wherever possible functions do things automatically
in this way.
But there are quite often situations when there is simply no function
to perform the task, or it is simply not possible to do the work
automatically. For these cases it is necessary to access the
information in the locale directly. To do this the C library provides
two functions: localeconv and nl_langinfo. The former is
part of ISO C and therefore portable, but has a brain-damaged
interface. The second is part of the Unix interface and is portable in
as far as the system follows the Unix standards.
localeconv: It is portable but ...
Together with the setlocale function the ISO C people
invented the localeconv function. It is a masterpiece of poor
design. It is expensive to use, not extendable, and not generally
usable as it provides access to only LC_MONETARY and
LC_NUMERIC related information. Nevertheless, if it is
applicable to a given situation it should be used since it is very
portable. The function strfmon formats monetary amounts
according to the selected locale using this information.
localeconv function returns a pointer to a structure whose
components contain information about how numeric and monetary values
should be formatted in the current locale.
You should not modify the structure or its contents. The structure might
be overwritten by subsequent calls to localeconv, or by calls to
setlocale, but no other function in the library overwrites this
value.
localeconv's return value is of this data type. Its elements are
described in the following subsections.
If a member of the structure struct lconv has type char,
and the value is CHAR_MAX, it means that the current locale has
no value for that parameter.
These are the standard members of struct lconv; there may be
others.
char *decimal_point
char *mon_decimal_point
decimal_point is ".", and the value of
mon_decimal_point is "".
char *thousands_sep
char *mon_thousands_sep
"" (the empty string).
char *grouping
char *mon_grouping
grouping applies to non-monetary quantities
and mon_grouping applies to monetary quantities. Use either
thousands_sep or mon_thousands_sep to separate the digit
groups.
Each member of these strings is to be interpreted as an integer value of
type char. Successive numbers (from left to right) give the
sizes of successive groups (from right to left, starting at the decimal
point.) The last member is either 0, in which case the previous
member is used over and over again for all the remaining groups, or
CHAR_MAX, in which case there is no more grouping--or, put
another way, any remaining digits form one large group without
separators.
For example, if grouping is "\04\03\02", the correct
grouping for the number 123456787654321 is `12', `34',
`56', `78', `765', `4321'. This uses a group of 4
digits at the end, preceded by a group of 3 digits, preceded by groups
of 2 digits (as many as needed). With a separator of `,', the
number would be printed as `12,34,56,78,765,4321'.
A value of "\03" indicates repeated groups of three digits, as
normally used in the U.S.
In the standard `C' locale, both grouping and
mon_grouping have a value of "". This value specifies no
grouping at all.
char int_frac_digits
char frac_digits
CHAR_MAX, meaning "unspecified". The ISO standard doesn't say
what to do when you find this value; we recommend printing no
fractional digits. (This locale also specifies the empty string for
mon_decimal_point, so printing any fractional digits would be
confusing!)
These members of the struct lconv structure specify how to print
the symbol to identify a monetary value--the international analog of
`$' for US dollars.
Each country has two standard currency symbols. The local currency symbol is used commonly within the country, while the international currency symbol is used internationally to refer to that country's currency when it is necessary to indicate the country unambiguously.
For example, many countries use the dollar as their monetary unit, and when dealing with international currencies it's important to specify that one is dealing with (say) Canadian dollars instead of U.S. dollars or Australian dollars. But when the context is known to be Canada, there is no need to make this explicit--dollar amounts are implicitly assumed to be in Canadian dollars.
char *currency_symbol
""
(the empty string), meaning "unspecified". The ISO standard doesn't
say what to do when you find this value; we recommend you simply print
the empty string as you would print any other string pointed to by this
variable.
char *int_curr_symbol
int_curr_symbol should normally consist of a
three-letter abbreviation determined by the international standard
ISO 4217 Codes for the Representation of Currency and Funds,
followed by a one-character separator (often a space).
In the standard `C' locale, this member has a value of ""
(the empty string), meaning "unspecified". We recommend you simply print
the empty string as you would print any other string pointed to by this
variable.
char p_cs_precedes
char n_cs_precedes
char int_p_cs_precedes
char int_n_cs_precedes
1 if the currency_symbol or
int_curr_symbol strings should precede the value of a monetary
amount, or 0 if the strings should follow the value. The
p_cs_precedes and int_p_cs_precedes members apply to
positive amounts (or zero), and the n_cs_precedes and
int_n_cs_precedes members apply to negative amounts.
In the standard `C' locale, all of these members have a value of
CHAR_MAX, meaning "unspecified". The ISO standard doesn't say
what to do when you find this value. We recommend printing the
currency symbol before the amount, which is right for most countries.
In other words, treat all nonzero values alike in these members.
The members with the int_ prefix apply to the
int_curr_symbol while the other two apply to
currency_symbol.
char p_sep_by_space
char n_sep_by_space
char int_p_sep_by_space
char int_n_sep_by_space
1 if a space should appear between the
currency_symbol or int_curr_symbol strings and the
amount, or 0 if no space should appear. The
p_sep_by_space and int_p_sep_by_space members apply to
positive amounts (or zero), and the n_sep_by_space and
int_n_sep_by_space members apply to negative amounts.
In the standard `C' locale, all of these members have a value of
CHAR_MAX, meaning "unspecified". The ISO standard doesn't say
what you should do when you find this value; we suggest you treat it as
1 (print a space). In other words, treat all nonzero values alike in
these members.
The members with the int_ prefix apply to the
int_curr_symbol while the other two apply to
currency_symbol. There is one specialty with the
int_curr_symbol, though. Since all legal values contain a space
at the end the string one either printf this space (if the currency
symbol must appear in front and must be separated) or one has to avoid
printing this character at all (especially when at the end of the
string).
These members of the struct lconv structure specify how to print
the sign (if any) of a monetary value.
char *positive_sign
char *negative_sign
"" (the empty string), meaning "unspecified".
The ISO standard doesn't say what to do when you find this value; we
recommend printing positive_sign as you find it, even if it is
empty. For a negative value, print negative_sign as you find it
unless both it and positive_sign are empty, in which case print
`-' instead. (Failing to indicate the sign at all seems rather
unreasonable.)
char p_sign_posn
char n_sign_posn
char int_p_sign_posn
char int_n_sign_posn
positive_sign or negative_sign.) The possible values are
as follows:
0
1
2
3
4
CHAR_MAX
CHAR_MAX. We recommend you print the sign after the currency
symbol.
The members with the int_ prefix apply to the
int_curr_symbol while the other two apply to
currency_symbol.
When writing the X/Open Portability Guide the authors realized that the
localeconv function is not enough to provide reasonable access to
locale information. The information which was meant to be available
in the locale (as later specified in the POSIX.1 standard) requires more
ways to access it. Therefore the nl_langinfo function
was introduced.
nl_langinfo function can be used to access individual
elements of the locale categories. Unlike the localeconv
function, which returns all the information, nl_langinfo
lets the caller select what information it requires. This is very
fast and it is not a problem to call this function multiple times.
A second advantage is that in addition to the numeric and monetary
formatting information, information from the
LC_TIME and LC_MESSAGES categories is available.
The type nl_type is defined in `nl_types.h'. The argument
item is a numeric value defined in the header `langinfo.h'.
The X/Open standard defines the following values:
CODESET
nl_langinfo returns a string with the name of the coded character
set used in the selected locale.
ABDAY_1
ABDAY_2
ABDAY_3
ABDAY_4
ABDAY_5
ABDAY_6
ABDAY_7
nl_langinfo returns the abbreviated weekday name. ABDAY_1
corresponds to Sunday.
DAY_1
DAY_2
DAY_3
DAY_4
DAY_5
DAY_6
DAY_7
ABDAY_1 etc., but here the return value is the
unabbreviated weekday name.
ABMON_1
ABMON_2
ABMON_3
ABMON_4
ABMON_5
ABMON_6
ABMON_7
ABMON_8
ABMON_9
ABMON_10
ABMON_11
ABMON_12
ABMON_1
corresponds to January.
MON_1
MON_2
MON_3
MON_4
MON_5
MON_6
MON_7
MON_8
MON_9
MON_10
MON_11
MON_12
ABMON_1 etc., but here the month names are not abbreviated.
Here the first value MON_1 also corresponds to January.
AM_STR
PM_STR
D_T_FMT
strftime to
represent time and date in a locale-specific way.
D_FMT
strftime to
represent a date in a locale-specific way.
T_FMT
strftime to
represent time in a locale-specific way.
T_FMT_AMPM
strftime to
represent time in the am/pm format.
Note that if the am/pm format does not make any sense for the
selected locale, the return value might be the same as the one for
T_FMT.
ERA
E modifier in their format strings causes the
strftime functions to use this information. The format of the
returned string is not specified, and therefore you should not assume
knowledge of it on different systems.
ERA_YEAR
ERA it should not be necessary to use this value directly.
ERA_D_T_FMT
strftime to
represent dates and times in a locale-specific era-based way.
ERA_D_FMT
strftime to
represent a date in a locale-specific era-based way.
ERA_T_FMT
strftime to
represent time in a locale-specific era-based way.
ALT_DIGITS
ERA this
value is not intended to be used directly, but instead indirectly
through the strftime function. When the modifier O is
used in a format which would otherwise use numerals to represent hours,
minutes, seconds, weekdays, months, or weeks, the appropriate value for
the locale is used instead.
INT_CURR_SYMBOL
localeconv in the
int_curr_symbol element of the struct lconv.
CURRENCY_SYMBOL
CRNCYSTR
localeconv in the
currency_symbol element of the struct lconv.
CRNCYSTR is a deprecated alias still required by Unix98.
MON_DECIMAL_POINT
localeconv in the
mon_decimal_point element of the struct lconv.
MON_THOUSANDS_SEP
localeconv in the
mon_thousands_sep element of the struct lconv.
MON_GROUPING
localeconv in the
mon_grouping element of the struct lconv.
POSITIVE_SIGN
localeconv in the
positive_sign element of the struct lconv.
NEGATIVE_SIGN
localeconv in the
negative_sign element of the struct lconv.
INT_FRAC_DIGITS
localeconv in the
int_frac_digits element of the struct lconv.
FRAC_DIGITS
localeconv in the
frac_digits element of the struct lconv.
P_CS_PRECEDES
localeconv in the
p_cs_precedes element of the struct lconv.
P_SEP_BY_SPACE
localeconv in the
p_sep_by_space element of the struct lconv.
N_CS_PRECEDES
localeconv in the
n_cs_precedes element of the struct lconv.
N_SEP_BY_SPACE
localeconv in the
n_sep_by_space element of the struct lconv.
P_SIGN_POSN
localeconv in the
p_sign_posn element of the struct lconv.
N_SIGN_POSN
localeconv in the
n_sign_posn element of the struct lconv.
INT_P_CS_PRECEDES
localeconv in the
int_p_cs_precedes element of the struct lconv.
INT_P_SEP_BY_SPACE
localeconv in the
int_p_sep_by_space element of the struct lconv.
INT_N_CS_PRECEDES
localeconv in the
int_n_cs_precedes element of the struct lconv.
INT_N_SEP_BY_SPACE
localeconv in the
int_n_sep_by_space element of the struct lconv.
INT_P_SIGN_POSN
localeconv in the
int_p_sign_posn element of the struct lconv.
INT_N_SIGN_POSN
localeconv in the
int_n_sign_posn element of the struct lconv.
DECIMAL_POINT
RADIXCHAR
localeconv in the
decimal_point element of the struct lconv.
The name RADIXCHAR is a deprecated alias still used in Unix98.
THOUSANDS_SEP
THOUSEP
localeconv in the
thousands_sep element of the struct lconv.
The name THOUSEP is a deprecated alias still used in Unix98.
GROUPING
localeconv in the
grouping element of the struct lconv.
YESEXPR
regex function to recognize a positive response to a yes/no
question.
NOEXPR
regex function to recognize a negative response to a yes/no
question.
YESSTR
NOSTR
YESSTR is also true here.
The use of this symbol is deprecated. Instead message translation
should be used.
The file `langinfo.h' defines a lot more symbols but none of them is official. Using them is not portable, and the format of the return values might change. Therefore we recommended you not use them.
Note that the return value for any valid argument can be used for
in all situations (with the possible exception of the am/pm time formatting
codes). If the user has not selected any locale for the
appropriate category, nl_langinfo returns the information from the
"C" locale. It is therefore possible to use this function as
shown in the example below.
If the argument item is not valid, a pointer to an empty string is returned.
An example of nl_langinfo usage is a function which has to
print a given date and time in a locale-specific way. At first one
might think that, since strftime internally uses the locale
information, writing something like the following is enough:
size_t
i18n_time_n_data (char *s, size_t len, const struct tm *tp)
{
return strftime (s, len, "%X %D", tp);
}
The format contains no weekday or month names and therefore is
internationally usable. Wrong! The output produced is something like
"hh:mm:ss MM/DD/YY". This format is only recognizable in the
USA. Other countries use different formats. Therefore the function
should be rewritten like this:
size_t
i18n_time_n_data (char *s, size_t len, const struct tm *tp)
{
return strftime (s, len, nl_langinfo (D_T_FMT), tp);
}
Now it uses the date and time format of the locale selected when the program runs. If the user selects the locale correctly there should never be a misunderstanding over the time and date format.
We have seen that the structure returned by localeconv as well as
the values given to nl_langinfo allow you to retrieve the various
pieces of locale-specific information to format numbers and monetary
amounts. We have also seen that the underlying rules are quite complex.
Therefore the X/Open standards introduce a function which uses such locale information, making it easier for the user to format numbers according to these rules.
strfmon function is similar to the strftime function
in that it takes a buffer, its size, a format string,
and values to write into the buffer as text in a form specified
by the format string. Like strftime, the function
also returns the number of bytes written into the buffer.
There are two differences: strfmon can take more than one
argument, and, of course, the format specification is different. Like
strftime, the format string consists of normal text, which is
output as is, and format specifiers, which are indicated by a `%'.
Immediately after the `%', you can optionally specify various flags
and formatting information before the main formatting character, in a
similar way to printf:
LC_MONETARY
category of the locale selected at program runtime.
The next part of a specification is an optional field width. If no width is specified @math{0} is taken. During output, the function first determines how much space is required. If it requires at least as many characters as given by the field width, it is output using as much space as necessary. Otherwise, it is extended to use the full width by filling with the space character. The presence or absence of the `-' flag determines the side at which such padding occurs. If present, the spaces are added at the right making the output left-justified, and vice versa.
So far the format looks familiar, being similar to the printf and
strftime formats. However, the next two optional fields
introduce something new. The first one is a `#' character followed
by a decimal digit string. The value of the digit string specifies the
number of digit positions to the left of the decimal point (or
equivalent). This does not include the grouping character when
the `^' flag is not given. If the space needed to print the number
does not fill the whole width, the field is padded at the left side with
the fill character, which can be selected using the `=' flag and by
default is a space. For example, if the field width is selected as 6
and the number is @math{123}, the fill character is `*' the result
will be `***123'.
The second optional field starts with a `.' (period) and consists
of another decimal digit string. Its value describes the number of
characters printed after the decimal point. The default is selected
from the current locale (frac_digits, int_frac_digits, see
see section Generic Numeric Formatting Parameters). If the exact representation needs more digits
than given by the field width, the displayed value is rounded. If the
number of fractional digits is selected to be zero, no decimal point is
printed.
As a GNU extension, the strfmon implementation in the GNU libc
allows an optional `L' next as a format modifier. If this modifier
is given, the argument is expected to be a long double instead of
a double value.
Finally, the last component is a format specifier. There are three specifiers defined:
As for printf, the function reads the format string
from left to right and uses the values passed to the function following
the format string. The values are expected to be either of type
double or long double, depending on the presence of the
modifier `L'. The result is stored in the buffer pointed to by
s. At most maxsize characters are stored.
The return value of the function is the number of characters stored in
s, including the terminating NULL byte. If the number of
characters stored would exceed maxsize, the function returns
@math{-1} and the content of the buffer s is unspecified. In this
case errno is set to E2BIG.
A few examples should make clear how the function works. It is
assumed that all the following pieces of code are executed in a program
which uses the USA locale (en_US). The simplest
form of the format is this:
strfmon (buf, 100, "@%n@%n@%n@", 123.45, -567.89, 12345.678);
The output produced is
"@$123.45@-$567.89@$12,345.68@"
We can notice several things here. First, the widths of the output
numbers are different. We have not specified a width in the format
string, and so this is no wonder. Second, the third number is printed
using thousands separators. The thousands separator for the
en_US locale is a comma. The number is also rounded.
@math{.678} is rounded to @math{.68} since the format does not specify a
precision and the default value in the locale is @math{2}. Finally,
note that the national currency symbol is printed since `%n' was
used, not `i'. The next example shows how we can align the output.
strfmon (buf, 100, "@%=*11n@%=*11n@%=*11n@", 123.45, -567.89, 12345.678);
The output this time is:
"@ $123.45@ -$567.89@ $12,345.68@"
Two things stand out. Firstly, all fields have the same width (eleven characters) since this is the width given in the format and since no number required more characters to be printed. The second important point is that the fill character is not used. This is correct since the white space was not used to achieve a precision given by a `#' modifier, but instead to fill to the given width. The difference becomes obvious if we now add a width specification.
strfmon (buf, 100, "@%=*11#5n@%=*11#5n@%=*11#5n@",
123.45, -567.89, 12345.678);
The output is
"@ $***123.45@-$***567.89@ $12,456.68@"
Here we can see that all the currency symbols are now aligned, and that the space between the currency sign and the number is filled with the selected fill character. Note that although the width is selected to be @math{5} and @math{123.45} has three digits left of the decimal point, the space is filled with three asterisks. This is correct since, as explained above, the width does not include the positions used to store thousands separators. One last example should explain the remaining functionality.
strfmon (buf, 100, "@%=0(16#5.3i@%=0(16#5.3i@%=0(16#5.3i@",
123.45, -567.89, 12345.678);
This rather complex format string produces the following output:
"@ USD 000123,450 @(USD 000567.890)@ USD 12,345.678 @"
The most noticeable change is the alternative way of representing
negative numbers. In financial circles this is often done using
parentheses, and this is what the `(' flag selected. The fill
character is now `0'. Note that this `0' character is not
regarded as a numeric zero, and therefore the first and second numbers
are not printed using a thousands separator. Since we used the format
specifier `i' instead of `n', the international form of the
currency symbol is used. This is a four letter string, in this case
"USD ". The last point is that since the precision right of the
decimal point is selected to be three, the first and second numbers are
printed with an extra zero at the end and the third number is printed
without rounding.
The program's interface with the human should be designed in a way to ease the human the task. One of the possibilities is to use messages in whatever language the user prefers.
Printing messages in different languages can be implemented in different ways. One could add all the different languages in the source code and add among the variants every time a message has to be printed. This is certainly no good solution since extending the set of languages is difficult (the code must be changed) and the code itself can become really big with dozens of message sets.
A better solution is to keep the message sets for each language are kept in separate files which are loaded at runtime depending on the language selection of the user.
The GNU C Library provides two different sets of functions to support
message translation. The problem is that neither of the interfaces is
officially defined by the POSIX standard. The catgets family of
functions is defined in the X/Open standard but this is derived from
industry decisions and therefore not necessarily based on reasonable
decisions.
As mentioned above the message catalog handling provides easy extendibility by using external data files which contain the message translations. I.e., these files contain for each of the messages used in the program a translation for the appropriate language. So the tasks of the message handling functions are
The two approaches mainly differ in the implementation of this last step. The design decisions made for this influences the whole rest.
The catgets functions are based on the simple scheme:
Associate every message to translate in the source code with a unique identifier. To retrieve a message from a catalog file solely the identifier is used.
This means for the author of the program that s/he will have to make sure the meaning of the identifier in the program code and in the message catalogs are always the same.
Before a message can be translated the catalog file must be located. The user of the program must be able to guide the responsible function to find whatever catalog the user wants. This is separated from what the programmer had in mind.
All the types, constants and functions for the catgets functions
are defined/declared in the `nl_types.h' header file.
catgets function family
catgets function tries to locate the message data file names
cat_name and loads it when found. The return value is of an
opaque type and can be used in calls to the other functions to refer to
this loaded catalog.
The return value is (nl_catd) -1 in case the function failed and
no catalog was loaded. The global variable errno contains a code
for the error causing the failure. But even if the function call
succeeded this does not mean that all messages can be translated.
Locating the catalog file must happen in a way which lets the user of the program influence the decision. It is up to the user to decide about the language to use and sometimes it is useful to use alternate catalog files. All this can be specified by the user by setting some environment variables.
The first problem is to find out where all the message catalogs are stored. Every program could have its own place to keep all the different files but usually the catalog files are grouped by languages and the catalogs for all programs are kept in the same place.
To tell the catopen function where the catalog for the program
can be found the user can set the environment variable NLSPATH to
a value which describes her/his choice. Since this value must be usable
for different languages and locales it cannot be a simple string.
Instead it is a format string (similar to printf's). An example
is
/usr/share/locale/%L/%N:/usr/share/locale/%L/LC_MESSAGES/%N
First one can see that more than one directory can be specified (with
the usual syntax of separating them by colons). The next things to
observe are the format string, %L and %N in this case.
The catopen function knows about several of them and the
replacement for all of them is of course different.
%N
catgets.
%L
%l
lang[_terr[.codeset]] and this format uses the
first part lang.
%t
%c
%%
% is used in a meta character there must be a way to
express the % character in the result itself. Using %%
does this just like it works for printf.
Using NLSPATH allows arbitrary directories to be searched for
message catalogs while still allowing different languages to be used.
If the NLSPATH environment variable is not set, the default value
is
prefix/share/locale/%L/%N:prefix/share/locale/%L/LC_MESSAGES/%N
where prefix is given to configure while installing the GNU
C Library (this value is in many cases /usr or the empty string).
The remaining problem is to decide which must be used. The value
decides about the substitution of the format elements mentioned above.
First of all the user can specify a path in the message catalog name
(i.e., the name contains a slash character). In this situation the
NLSPATH environment variable is not used. The catalog must exist
as specified in the program, perhaps relative to the current working
directory. This situation in not desirable and catalogs names never
should be written this way. Beside this, this behavior is not portable
to all other platforms providing the catgets interface.
Otherwise the values of environment variables from the standard
environment are examined (see section Standard Environment Variables). Which
variables are examined is decided by the flag parameter of
catopen. If the value is NL_CAT_LOCALE (which is defined
in `nl_types.h') then the catopen function use the name of
the locale currently selected for the LC_MESSAGES category.
If flag is zero the LANG environment variable is examined.
This is a left-over from the early days where the concept of the locales
had not even reached the level of POSIX locales.
The environment variable and the locale name should have a value of the
form lang[_terr[.codeset]] as explained above.
If no environment variable is set the "C" locale is used which
prevents any translation.
The return value of the function is in any case a valid string. Either it is a translation from a message catalog or it is the same as the string parameter. So a piece of code to decide whether a translation actually happened must look like this:
{
char *trans = catgets (desc, set, msg, input_string);
if (trans == input_string)
{
/* Something went wrong. */
}
}
When an error occurred the global variable errno is set to
While it sometimes can be useful to test for errors programs normally will avoid any test. If the translation is not available it is no big problem if the original, untranslated message is printed. Either the user understands this as well or s/he will look for the reason why the messages are not translated.
Please note that the currently selected locale does not depend on a call
to the setlocale function. It is not necessary that the locale
data files for this locale exist and calling setlocale succeeds.
The catopen function directly reads the values of the environment
variables.
catgets has to be used to access the massage catalog
previously opened using the catopen function. The
catalog_desc parameter must be a value previously returned by
catopen.
The next two parameters, set and message, reflect the internal organization of the message catalog files. This will be explained in detail below. For now it is interesting to know that a catalog can consists of several set and the messages in each thread are individually numbered using numbers. Neither the set number nor the message number must be consecutive. They can be arbitrarily chosen. But each message (unless equal to another one) must have its own unique pair of set and message number.
Since it is not guaranteed that the message catalog for the language selected by the user exists the last parameter string helps to handle this case gracefully. If no matching string can be found string is returned. This means for the programmer that
It is somewhat uncomfortable to write a program using the catgets
functions if no supporting functionality is available. Since each
set/message number tuple must be unique the programmer must keep lists
of the messages at the same time the code is written. And the work
between several people working on the same project must be coordinated.
We will see some how these problems can be relaxed a bit (see section How to use the catgets interface).
catclose function can be used to free the resources
associated with a message catalog which previously was opened by a call
to catopen. If the resources can be successfully freed the
function returns 0. Otherwise it return -1 and the
global variable errno is set. Errors can occur if the catalog
descriptor catalog_desc is not valid in which case errno is
set to EBADF.
The only reasonable way the translate all the messages of a function and
store the result in a message catalog file which can be read by the
catopen function is to write all the message text to the
translator and let her/him translate them all. I.e., we must have a
file with entries which associate the set/message tuple with a specific
translation. This file format is specified in the X/Open standard and
is as follows:
$
followed by a whitespace character are comment and are also ignored.
$set followed by a whitespace character an additional argument
is required to follow. This argument can either be:
catgets interface.
It is an error if a symbol name appears more than once. All following
messages are placed in a set with this number.
$delset followed by a whitespace character an additional argument
is required to follow. This argument can either be:
$set command again messages could be added and these
messages will appear in the output.
$quote, the quoting character used for this input file is
changed to the first non-whitespace character following the
$quote. If no non-whitespace character is present before the
line ends quoting is disable.
By default no quoting character is used. In this mode strings are
terminated with the first unescaped line break. If there is a
$quote sequence present newline need not be escaped. Instead a
string is terminated with the first unescaped appearance of the quote
character.
A common usage of this feature would be to set the quote character to
". Then any appearance of the " in the strings must
be escaped using the backslash (i.e., \" must be written).
catgets interface). There is
one limitation with the identifier: it must not be Set. The
reason will be explained below.
The text of the messages can contain escape characters. The usual bunch
of characters known from the ISO C language are recognized
(\n, \t, \v, \b, \r, \f,
\\, and \nnn, where nnn is the octal coding of
a character code).
Important: The handling of identifiers instead of numbers for the set and messages is a GNU extension. Systems strictly following the X/Open specification do not have this feature. An example for a message catalog file is this:
$ This is a leading comment. $quote " $set SetOne 1 Message with ID 1. two " Message with ID \"two\", which gets the value 2 assigned" $set SetTwo $ Since the last set got the number 1 assigned this set has number 2. 4000 "The numbers can be arbitrary, they need not start at one."
This small example shows various aspects:
$ followed by
a whitespace.
". Otherwise the quotes in the
message definition would have to be left away and in this case the
message with the identifier two would loose its leading whitespace.
While this file format is pretty easy it is not the best possible for
use in a running program. The catopen function would have to
parser the file and handle syntactic errors gracefully. This is not so
easy and the whole process is pretty slow. Therefore the catgets
functions expect the data in another more compact and ready-to-use file
format. There is a special program gencat which is explained in
detail in the next section.
Files in this other format are not human readable. To be easy to use by programs it is a binary file. But the format is byte order independent so translation files can be shared by systems of arbitrary architecture (as long as they use the GNU C Library).
Details about the binary file format are not important to know since
these files are always created by the gencat program. The
sources of the GNU C Library also provide the sources for the
gencat program and so the interested reader can look through
these source files to learn about the file format.
The gencat program is specified in the X/Open standard and the
GNU implementation follows this specification and so processes
all correctly formed input files. Additionally some extension are
implemented which help to work in a more reasonable way with the
catgets functions.
The gencat program can be invoked in two ways:
`gencat [Option]... [Output-File [Input-File]...]`
This is the interface defined in the X/Open standard. If no Input-File parameter is given input will be read from standard input. Multiple input files will be read as if they are concatenated. If Output-File is also missing, the output will be written to standard output. To provide the interface one is used to from other programs a second interface is provided.
`gencat [Option]... -o Output-File [Input-File]...`
The option `-o' is used to specify the output file and all file arguments are used as input files.
Beside this one can use `-' or `/dev/stdin' for Input-File to denote the standard input. Corresponding one can use `-' and `/dev/stdout' for Output-File to denote standard output. Using `-' as a file name is allowed in X/Open while using the device names is a GNU extension.
The gencat program works by concatenating all input files and
then merge the resulting collection of message sets with a
possibly existing output file. This is done by removing all messages
with set/message number tuples matching any of the generated messages
from the output file and then adding all the new messages. To
regenerate a catalog file while ignoring the old contents therefore
requires to remove the output file if it exists. If the output is
written to standard output no merging takes place.
The following table shows the options understood by the gencat
program. The X/Open standard does not specify any option for the
program so all of these are GNU extensions.
#defines to associate a name with a
number.
Please note that the generated file only contains the symbols from the
input files. If the output is merged with the previous content of the
output file the possibly existing symbols from the file(s) which
generated the old output files are not in the generated header file.
catgets interface
The catgets functions can be used in two different ways. By
following slavishly the X/Open specs and not relying on the extension
and by using the GNU extensions. We will take a look at the former
method first to understand the benefits of extensions.
Since the X/Open format of the message catalog files does not allow symbol names we have to work with numbers all the time. When we start writing a program we have to replace all appearances of translatable strings with something like
catgets (catdesc, set, msg, "string")
catgets is retrieved from a call to catopen which is
normally done once at the program start. The "string" is the
string we want to translate. The problems start with the set and
message numbers.
In a bigger program several programmers usually work at the same time on the program and so coordinating the number allocation is crucial. Though no two different strings must be indexed by the same tuple of numbers it is highly desirable to reuse the numbers for equal strings with equal translations (please note that there might be strings which are equal in one language but have different translations due to difference contexts).
The allocation process can be relaxed a bit by different set numbers for
different parts of the program. So the number of developers who have to
coordinate the allocation can be reduced. But still lists must be keep
track of the allocation and errors can easily happen. These errors
cannot be discovered by the compiler or the catgets functions.
Only the user of the program might see wrong messages printed. In the
worst cases the messages are so irritating that they cannot be
recognized as wrong. Think about the translations for "true" and
"false" being exchanged. This could result in a disaster.
The problems mentioned in the last section derive from the fact that:
By constantly using symbolic names and by providing a method which maps the string content to a symbolic name (however this will happen) one can prevent both problems above. The cost of this is that the programmer has to write a complete message catalog file while s/he is writing the program itself.
This is necessary since the symbolic names must be mapped to numbers
before the program sources can be compiled. In the last section it was
described how to generate a header containing the mapping of the names.
E.g., for the example message file given in the last section we could
call the gencat program as follow (assume `ex.msg' contains
the sources).
gencat -H ex.h -o ex.cat ex.msg
This generates a header file with the following content:
#define SetTwoSet 0x2 /* ex.msg:8 */ #define SetOneSet 0x1 /* ex.msg:4 */ #define SetOnetwo 0x2 /* ex.msg:6 */
As can be seen the various symbols given in the source file are mangled
to generate unique identifiers and these identifiers get numbers
assigned. Reading the source file and knowing about the rules will
allow to predict the content of the header file (it is deterministic)
but this is not necessary. The gencat program can take care for
everything. All the programmer has to do is to put the generated header
file in the dependency list of the source files of her/his project and
to add a rules to regenerate the header of any of the input files
change.
One word about the symbol mangling. Every symbol consists of two parts:
the name of the message set plus the name of the message or the special
string Set. So SetOnetwo means this macro can be used to
access the translation with identifier two in the message set
SetOne.
The other names denote the names of the message sets. The special
string Set is used in the place of the message identifier.
If in the code the second string of the set SetOne is used the C
code should look like this:
catgets (catdesc, SetOneSet, SetOnetwo,
" Message with ID \"two\", which gets the value 2 assigned")
Writing the function this way will allow to change the message number and even the set number without requiring any change in the C source code. (The text of the string is normally not the same; this is only for this example.)
To illustrate the usual way to work with the symbolic version numbers here is a little example. Assume we want to write the very complex and famous greeting program. We start by writing the code as usual:
#include <stdio.h>
int
main (void)
{
printf ("Hello, world!\n");
return 0;
}
Now we want to internationalize the message and therefore replace the message with whatever the user wants.
#include <nl_types.h>
#include <stdio.h>
#include "msgnrs.h"
int
main (void)
{
nl_catd catdesc = catopen ("hello.cat", NL_CAT_LOCALE);
printf (catgets (catdesc, SetMainSet, SetMainHello,
"Hello, world!\n"));
catclose (catdesc);
return 0;
}
We see how the catalog object is opened and the returned descriptor used in the other function calls. It is not really necessary to check for failure of any of the functions since even in these situations the functions will behave reasonable. They simply will be return a translation.
What remains unspecified here are the constants SetMainSet and
SetMainHello. These are the symbolic names describing the
message. To get the actual definitions which match the information in
the catalog file we have to create the message catalog source file and
process it using the gencat program.
$ Messages for the famous greeting program. $quote " $set Main Hello "Hallo, Welt!\n"
Now we can start building the program (assume the message catalog source file is named `hello.msg' and the program source file `hello.c'):
% gencat -H msgnrs.h -o hello.cat hello.msg % cat msgnrs.h #define MainSet 0x1 /* hello.msg:4 */ #define MainHello 0x1 /* hello.msg:5 */ % gcc -o hello hello.c -I. % cp hello.cat /usr/share/locale/de/LC_MESSAGES % echo $LC_ALL de % ./hello Hallo, Welt! %
The call of the gencat program creates the missing header file
`msgnrs.h' as well as the message catalog binary. The former is
used in the compilation of `hello.c' while the later is placed in a
directory in which the catopen function will try to locate it.
Please check the LC_ALL environment variable and the default path
for catopen presented in the description above.
Sun Microsystems tried to standardize a different approach to message translation in the Uniforum group. There never was a real standard defined but still the interface was used in Sun's operation systems. Since this approach fits better in the development process of free software it is also used throughout the GNU project and the GNU `gettext' package provides support for this outside the GNU C Library.
The code of the `libintl' from GNU `gettext' is the same as the code in the GNU C Library. So the documentation in the GNU `gettext' manual is also valid for the functionality here. The following text will describe the library functions in detail. But the numerous helper programs are not described in this manual. Instead people should read the GNU `gettext' manual (see section `GNU gettext utilities' in Native Language Support Library and Tools). We will only give a short overview.
Though the catgets functions are available by default on more
systems the gettext interface is at least as portable as the
former. The GNU `gettext' package can be used wherever the
functions are not available.
gettext family of functions
The paradigms underlying the gettext approach to message
translations is different from that of the catgets functions the
basic functionally is equivalent. There are functions of the following
categories:
The gettext functions have a very simple interface. The most
basic function just takes the string which shall be translated as the
argument and it returns the translation. This is fundamentally
different from the catgets approach where an extra key is
necessary and the original string is only used for the error case.
If the string which has to be translated is the only argument this of
course means the string itself is the key. I.e., the translation will
be selected based on the original string. The message catalogs must
therefore contain the original strings plus one translation for any such
string. The task of the gettext function is it to compare the
argument string with the available strings in the catalog and return the
appropriate translation. Of course this process is optimized so that
this process is not more expensive than an access using an atomic key
like in catgets.
The gettext approach has some advantages but also some
disadvantages. Please see the GNU `gettext' manual for a detailed
discussion of the pros and cons.
All the definitions and declarations for gettext can be found in
the `libintl.h' header file. On systems where these functions are
not part of the C library they can be found in a separate library named
`libintl.a' (or accordingly different for shared libraries).
gettext function searches the currently selected message
catalogs for a string which is equal to msgid. If there is such a
string available it is returned. Otherwise the argument string
msgid is returned.
Please note that all though the return value is char * the
returned string must not be changed. This broken type results from the
history of the function and does not reflect the way the function should
be used.
Please note that above we wrote "message catalogs" (plural). This is a specialty of the GNU implementation of these functions and we will say more about this when we talk about the ways message catalogs are selected (see section How to determine which catalog to be used).
The gettext function does not modify the value of the global
errno variable. This is necessary to make it possible to write
something like
printf (gettext ("Operation failed: %m\n"));
Here the errno value is used in the printf function while
processing the %m format element and if the gettext
function would change this value (it is called before printf is
called) we would get a wrong message.
So there is no easy way to detect a missing message catalog beside comparing the argument string with the result. But it is normally the task of the user to react on missing catalogs. The program cannot guess when a message catalog is really necessary since for a user who speaks the language the program was developed in does not need any translation.
The remaining two functions to access the message catalog add some
functionality to select a message catalog which is not the default one.
This is important if parts of the program are developed independently.
Every part can have its own message catalog and all of them can be used
at the same time. The C library itself is an example: internally it
uses the gettext functions but since it must not depend on a
currently selected default message catalog it must specify all ambiguous
information.
dgettext functions acts just like the gettext
function. It only takes an additional first argument domainname
which guides the selection of the message catalogs which are searched
for the translation. If the domainname parameter is the null
pointer the dgettext function is exactly equivalent to
gettext since the default value for the domain name is used.
As for gettext the return value type is char * which is an
anachronism. The returned string must never be modified.
dcgettext adds another argument to those which
dgettext takes. This argument category specifies the last
piece of information needed to localize the message catalog. I.e., the
domain name and the locale category exactly specify which message
catalog has to be used (relative to a given directory, see below).
The dgettext function can be expressed in terms of
dcgettext by using
dcgettext (domain, string, LC_MESSAGES)
instead of
dgettext (domain, string)
This also shows which values are expected for the third parameter. One
has to use the available selectors for the categories available in
`locale.h'. Normally the available values are LC_CTYPE,
LC_COLLATE, LC_MESSAGES, LC_MONETARY,
LC_NUMERIC, and LC_TIME. Please note that LC_ALL
must not be used and even though the names might suggest this, there is
no relation to the environments variables of this name.
The dcgettext function is only implemented for compatibility with
other systems which have gettext functions. There is not really
any situation where it is necessary (or useful) to use a different value
but LC_MESSAGES in for the category parameter. We are
dealing with messages here and any other choice can only be irritating.
As for gettext the return value type is char * which is an
anachronism. The returned string must never be modified.
When using the three functions above in a program it is a frequent case
that the msgid argument is a constant string. So it is worth to
optimize this case. Thinking shortly about this one will realize that
as long as no new message catalog is loaded the translation of a message
will not change. This optimization is actually implemented by the
gettext, dgettext and dcgettext functions.
The functions to retrieve the translations for a given message have a remarkable simple interface. But to provide the user of the program still the opportunity to select exactly the translation s/he wants and also to provide the programmer the possibility to influence the way to locate the search for catalogs files there is a quite complicated underlying mechanism which controls all this. The code is complicated the use is easy.
Basically we have two different tasks to perform which can also be
performed by the catgets functions:
This is the functionality required by the specifications for
gettext and this is also what the catgets functions are
able to do. But there are some problems unresolved:
de, german, or
deutsch and the program should always react the same.
de_DE.ISO-8859-1 which means German, spoken in Germany,
coded using the ISO 8859-1 character set there is the possibility
that a message catalog matching this exactly is not available. But
there could be a catalog matching de and if the character set
used on the machine is always ISO 8859-1 there is no reason why this
later message catalog should not be used. (We call this message
inheritance.)
We can divide the configuration actions in two parts: the one is performed by the programmer, the other by the user. We will start with the functions the programmer can use since the user configuration will be based on this.
As the functions described in the last sections already mention separate
sets of messages can be selected by a domain name. This is a
simple string which should be unique for each program part with uses a
separate domain. It is possible to use in one program arbitrary many
domains at the same time. E.g., the GNU C Library itself uses a domain
named libc while the program using the C Library could use a
domain named foo. The important point is that at any time
exactly one domain is active. This is controlled with the following
function.
textdomain function sets the default domain, which is used in
all future gettext calls, to domainname. Please note that
dgettext and dcgettext calls are not influenced if the
domainname parameter of these functions is not the null pointer.
Before the first call to textdomain the default domain is
messages. This is the name specified in the specification of
the gettext API. This name is as good as any other name. No
program should ever really use a domain with this name since this can
only lead to problems.
The function returns the value which is from now on taken as the default
domain. If the system went out of memory the returned value is
NULL and the global variable errno is set to ENOMEM.
Despite the return value type being char * the return string must
not be changed. It is allocated internally by the textdomain
function.
If the domainname parameter is the null pointer no new default domain is set. Instead the currently selected default domain is returned.
If the domainname parameter is the empty string the default domain
is reset to its initial value, the domain with the name messages.
This possibility is questionable to use since the domain messages
really never should be used.
bindtextdomain function can be used to specify the directory
which contains the message catalogs for domain domainname for the
different languages. To be correct, this is the directory where the
hierarchy of directories is expected. Details are explained below.
For the programmer it is important to note that the translations which
come with the program have be placed in a directory hierarchy starting
at, say, `/foo/bar'. Then the program should make a
bindtextdomain call to bind the domain for the current program to
this directory. So it is made sure the catalogs are found. A correctly
running program does not depend on the user setting an environment
variable.
The bindtextdomain function can be used several times and if the
domainname argument is different the previously bound domains
will not be overwritten.
If the program which wish to use bindtextdomain at some point of
time use the chdir function to change the current working
directory it is important that the dirname strings ought to be an
absolute pathname. Otherwise the addressed directory might vary with
the time.
If the dirname parameter is the null pointer bindtextdomain
returns the currently selected directory for the domain with the name
domainname.
The bindtextdomain function returns a pointer to a string
containing the name of the selected directory name. The string is
allocated internally in the function and must not be changed by the
user. If the system went out of core during the execution of
bindtextdomain the return value is NULL and the global
variable errno is set accordingly.
The functions of the gettext family described so far (and all the
catgets functions as well) have one problem in the real world
which have been neglected completely in all existing approaches. What
is meant here is the handling of plural forms.
Looking through Unix source code before the time anybody thought about internationalization (and, sadly, even afterwards) one can often find code similar to the following:
printf ("%d file%s deleted", n, n == 1 ? "" : "s");
After the first complaints from people internationalizing the code people
either completely avoided formulations like this or used strings like
"file(s)". Both look unnatural and should be avoided. First
tries to solve the problem correctly looked like this:
if (n == 1)
printf ("%d file deleted", n);
else
printf ("%d files deleted", n);
But this does not solve the problem. It helps languages where the plural form of a noun is not simply constructed by adding an `s' but that is all. Once again people fell into the trap of believing the rules their language is using are universal. But the handling of plural forms differs widely between the language families. There are two things we can differ between (and even inside language families);
The consequence of this is that application writers should not try to
solve the problem in their code. This would be localization since it is
only usable for certain, hardcoded language environments. Instead the
extended gettext interface should be used.
These extra functions are taking instead of the one key string two
strings and an numerical argument. The idea behind this is that using
the numerical argument and the first string as a key, the implementation
can select using rules specified by the translator the right plural
form. The two string arguments then will be used to provide a return
value in case no message catalog is found (similar to the normal
gettext behavior). In this case the rules for Germanic language
is used and it is assumed that the first string argument is the singular
form, the second the plural form.
This has the consequence that programs without language catalogs can
display the correct strings only if the program itself is written using
a Germanic language. This is a limitation but since the GNU C library
(as well as the GNU gettext package) are written as part of the
GNU package and the coding standards for the GNU project require program
being written in English, this solution nevertheless fulfills its
purpose.
ngettext function is similar to the gettext function
as it finds the message catalogs in the same way. But it takes two
extra arguments. The msgid1 parameter must contain the singular
form of the string to be converted. It is also used as the key for the
search in the catalog. The msgid2 parameter is the plural form.
The parameter n is used to determine the plural form. If no
message catalog is found msgid1 is returned if n == 1,
otherwise msgid2.
An example for the us of this function is:
printf (ngettext ("%d file removed", "%d files removed", n), n);
Please note that the numeric value n has to be passed to the
printf function as well. It is not sufficient to pass it only to
ngettext.
dngettext is similar to the dgettext function in the
way the message catalog is selected. The difference is that it takes
two extra parameter to provide the correct plural form. These two
parameters are handled in the same way ngettext handles them.
dcngettext is similar to the dcgettext function in the
way the message catalog is selected. The difference is that it takes
two extra parameter to provide the correct plural form. These two
parameters are handled in the same way ngettext handles them.
A description of the problem can be found at the beginning of the last section. Now there is the question how to solve it. Without the input of linguists (which was not available) it was not possible to determine whether there are only a few different forms in which plural forms are formed or whether the number can increase with every new supported language.
Therefore the solution implemented is to allow the translator to specify
the rules of how to select the plural form. Since the formula varies
with every language this is the only viable solution except for
hardcoding the information in the code (which still would require the
possibility of extensions to not prevent the use of new languages). The
details are explained in the GNU gettext manual. Here only a a
bit of information is provided.
The information about the plural form selection has to be stored in the
header entry (the one with the empty (msgid string). It looks
like this:
Plural-Forms: nplurals=2; plural=n == 1 ? 0 : 1;
The nplurals value must be a decimal number which specifies how
many different plural forms exist for this language. The string
following plural is an expression which is using the C language
syntax. Exceptions are that no negative number are allowed, numbers
must be decimal, and the only variable allowed is n. This
expression will be evaluated whenever one of the functions
ngettext, dngettext, or dcngettext is called. The
numeric value passed to these functions is then substituted for all uses
of the variable n in the expression. The resulting value then
must be greater or equal to zero and smaller than the value given as the
value of nplurals.
The following rules are known at this point. The language with families are listed. But this does not necessarily mean the information can be generalized for the whole family (as can be easily seen in the table below).(1).}
Plural-Forms: nplurals=1; plural=0;Languages with this property include:
Plural-Forms: nplurals=2; plural=n != 1;(Note: this uses the feature of C expressions that boolean expressions have to value zero or one.) Languages with this property include:
Plural-Forms: nplurals=2; plural=n>1;Languages with this property include:
Plural-Forms: nplurals=3; plural=n==1 ? 0 : n==2 ? 1 : 2;Languages with this property include:
Plural-Forms: nplurals=3; \
plural=n%100/10==1 ? 2 : n%10==1 ? 0 : (n+9)%10>3 ? 2 : 1;
Languages with this property include:
Plural-Forms: nplurals=3; \
plural=n==1 ? 0 : \
n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;
(Continuation in the next line is possible.)
Languages with this property include:
Plural-Forms: nplurals=4; \
plural=n==1 ? 0 : n%10==2 ? 1 : n%10==3 || n%10==4 ? 2 : 3;
Languages with this property include:
gettext uses
gettext not only looks up a translation in a message catalog. It
also converts the translation on the fly to the desired output character
set. This is useful if the user is working in a different character set
than the translator who created the message catalog, because it avoids
distributing variants of message catalogs which differ only in the
character set.
The output character set is, by default, the value of nl_langinfo
(CODESET), which depends on the LC_CTYPE part of the current
locale. But programs which store strings in a locale independent way
(e.g. UTF-8) can request that gettext and related functions
return the translations in that encoding, by use of the
bind_textdomain_codeset function.
Note that the msgid argument to gettext is not subject to
character set conversion. Also, when gettext does not find a
translation for msgid, it returns msgid unchanged --
independently of the current output character set. It is therefore
recommended that all msgids be US-ASCII strings.
bind_textdomain_codeset function can be used to specify the
output character set for message catalogs for domain domainname.
The codeset argument must be a valid codeset name which can be used
for the iconv_open function, or a null pointer.
If the codeset parameter is the null pointer,
bind_textdomain_codeset returns the currently selected codeset
for the domain with the name domainname. It returns NULL if
no codeset has yet been selected.
The bind_textdomain_codeset function can be used several times.
If used multiple times with the same domainname argument, the
later call overrides the settings made by the earlier one.
The bind_textdomain_codeset function returns a pointer to a
string containing the name of the selected codeset. The string is
allocated internally in the function and must not be changed by the
user. If the system went out of core during the execution of
bind_textdomain_codeset, the return value is NULL and the
global variable errno is set accordingly. @end deftypefun
gettext in GUI programs
One place where the gettext functions, if used normally, have big
problems is within programs with graphical user interfaces (GUIs). The
problem is that many of the strings which have to be translated are very
short. They have to appear in pull-down menus which restricts the
length. But strings which are not containing entire sentences or at
least large fragments of a sentence may appear in more than one
situation in the program but might have different translations. This is
especially true for the one-word strings which are frequently used in
GUI programs.
As a consequence many people say that the gettext approach is
wrong and instead catgets should be used which indeed does not
have this problem. But there is a very simple and powerful method to
handle these kind of problems with the gettext functions.
As as example consider the following fictional situation. A GUI program has a menu bar with the following entries:
+------------+------------+--------------------------------------+
| File | Printer | |
+------------+------------+--------------------------------------+
| Open | | Select |
| New | | Open |
+----------+ | Connect |
+----------+
To have the strings File, Printer, Open,
New, Select, and Connect translated there has to be
at some point in the code a call to a function of the gettext
family. But in two places the string passed into the function would be
Open. The translations might not be the same and therefore we
are in the dilemma described above.
One solution to this problem is to artificially enlengthen the strings to make them unambiguous. But what would the program do if no translation is available? The enlengthened string is not what should be printed. So we should use a little bit modified version of the functions.
To enlengthen the strings a uniform method should be used. E.g., in the example above the strings could be chosen as
Menu|File Menu|Printer Menu|File|Open Menu|File|New Menu|Printer|Select Menu|Printer|Open Menu|Printer|Connect
Now all the strings are different and if now instead of gettext
the following little wrapper function is used, everything works just
fine:
char *
sgettext (const char *msgid)
{
char *msgval = gettext (msgid);
if (msgval == msgid)
msgval = strrchr (msgid, '|') + 1;
return msgval;
}
What this little function does is to recognize the case when no
translation is available. This can be done very efficiently by a
pointer comparison since the return value is the input value. If there
is no translation we know that the input string is in the format we used
for the Menu entries and therefore contains a | character. We
simply search for the last occurrence of this character and return a
pointer to the character following it. That's it!
If one now consistently uses the enlengthened string form and replaces
the gettext calls with calls to sgettext (this is normally
limited to very few places in the GUI implementation) then it is
possible to produce a program which can be internationalized.
With advanced compilers (such as GNU C) one can write the
sgettext functions as an inline function or as a macro like this:
#define sgettext(msgid) \
({ const char *__msgid = (msgid); \
char *__msgstr = gettext (__msgid); \
if (__msgval == __msgid) \
__msgval = strrchr (__msgid, '|') + 1; \
__msgval; })
The other gettext functions (dgettext, dcgettext
and the ngettext equivalents) can and should have corresponding
functions as well which look almost identical, except for the parameters
and the call to the underlying function.
Now there is of course the question why such functions do not exist in the GNU C library? There are two parts of the answer to this question.
| which is a quite good choice because it
resembles a notation frequently used in this context and it also is a
character not often used in message strings.
But what if the character is used in message strings. Or if the chose
character is not available in the character set on the machine one
compiles (e.g., | is not required to exist for ISO C; this is
why the `iso646.h' file exists in ISO C programming environments).
There is only one more comment to make left. The wrapper function above require that the translations strings are not enlengthened themselves. This is only logical. There is no need to disambiguate the strings (since they are never used as keys for a search) and one also saves quite some memory and disk space by doing this.
gettextThe last sections described what the programmer can do to internationalize the messages of the program. But it is finally up to the user to select the message s/he wants to see. S/He must understand them.
The POSIX locale model uses the environment variables LC_COLLATE,
LC_CTYPE, LC_MESSAGES, LC_MONETARY, NUMERIC,
and LC_TIME to select the locale which is to be used. This way
the user can influence lots of functions. As we mentioned above the
gettext functions also take advantage of this.
To understand how this happens it is necessary to take a look at the various components of the filename which gets computed to locate a message catalog. It is composed as follows:
dir_name/locale/LC_category/domain_name.mo
The default value for dir_name is system specific. It is computed from the value given as the prefix while configuring the C library. This value normally is `/usr' or `/'. For the former the complete dir_name is:
/usr/share/locale
We can use `/usr/share' since the `.mo' files containing the
message catalogs are system independent, so all systems can use the same
files. If the program executed the bindtextdomain function for
the message domain that is currently handled, the dir_name
component is exactly the value which was given to the function as
the second parameter. I.e., bindtextdomain allows overwriting
the only system dependent and fixed value to make it possible to
address files anywhere in the filesystem.
The category is the name of the locale category which was selected
in the program code. For gettext and dgettext this is
always LC_MESSAGES, for dcgettext this is selected by the
value of the third parameter. As said above it should be avoided to
ever use a category other than LC_MESSAGES.
The locale component is computed based on the category used. Just
like for the setlocale function here comes the user selection
into the play. Some environment variables are examined in a fixed order
and the first environment variable set determines the return value of
the lookup process. In detail, for the category LC_xxx the
following variables in this order are examined:
LANGUAGE
LC_ALL
LC_xxx
LANG
This looks very familiar. With the exception of the LANGUAGE
environment variable this is exactly the lookup order the
setlocale function uses. But why introducing the LANGUAGE
variable?
The reason is that the syntax of the values these variables can have is
different to what is expected by the setlocale function. If we
would set LC_ALL to a value following the extended syntax that
would mean the setlocale function will never be able to use the
value of this variable as well. An additional variable removes this
problem plus we can select the language independently of the locale
setting which sometimes is useful.
While for the LC_xxx variables the value should consist of
exactly one specification of a locale the LANGUAGE variable's
value can consist of a colon separated list of locale names. The
attentive reader will realize that this is the way we manage to
implement one of our additional demands above: we want to be able to
specify an ordered list of language.
Back to the constructed filename we have only one component missing.
The domain_name part is the name which was either registered using
the textdomain function or which was given to dgettext or
dcgettext as the first parameter. Now it becomes obvious that a
good choice for the domain name in the program code is a string which is
closely related to the program/package name. E.g., for the GNU C
Library the domain name is libc.
A limit piece of example code should show how the programmer is supposed to work:
{
setlocale (LC_ALL, "");
textdomain ("test-package");
bindtextdomain ("test-package", "/usr/local/share/locale");
puts (gettext ("Hello, world!"));
}
At the program start the default domain is messages, and the
default locale is "C". The setlocale call sets the locale
according to the user's environment variables; remember that correct
functioning of gettext relies on the correct setting of the
LC_MESSAGES locale (for looking up the message catalog) and
of the LC_CTYPE locale (for the character set conversion).
The textdomain call changes the default domain to
test-package. The bindtextdomain call specifies that
the message catalogs for the domain test-package can be found
below the directory `/usr/local/share/locale'.
If now the user set in her/his environment the variable LANGUAGE
to de the gettext function will try to use the
translations from the file
/usr/local/share/locale/de/LC_MESSAGES/test-package.mo
From the above descriptions it should be clear which component of this filename is determined by which source.
In the above example we assumed that the LANGUAGE environment
variable to de. This might be an appropriate selection but what
happens if the user wants to use LC_ALL because of the wider
usability and here the required value is de_DE.ISO-8859-1? We
already mentioned above that a situation like this is not infrequent.
E.g., a person might prefer reading a dialect and if this is not
available fall back on the standard language.
The gettext functions know about situations like this and can
handle them gracefully. The functions recognize the format of the value
of the environment variable. It can split the value is different pieces
and by leaving out the only or the other part it can construct new
values. This happens of course in a predictable way. To understand
this one must know the format of the environment variable value. There
are two more or less standardized forms:
language[_territory[.codeset]][@modifier]
language[_territory][+audience][+special][,[sponsor][_revision]]
The functions will automatically recognize which format is used. Less specific locale names will be stripped of in the order of the following list:
revision
sponsor
special
codeset
normalized codeset
territory
audience/modifier
From the last entry one can see that the meaning of the modifier
field in the X/Open format and the audience format have the same
meaning. Beside one can see that the language field for obvious
reasons never will be dropped.
The only new thing is the normalized codeset entry. This is
another goodie which is introduced to help reducing the chaos which
derives from the inability of the people to standardize the names of
character sets. Instead of ISO-8859-1 one can often see 8859-1,
88591, iso8859-1, or iso_8859-1. The normalized
codeset value is generated from the user-provided character set name by
applying the following rules:
"iso".
So all of the above name will be normalized to iso88591. This
allows the program user much more freely choosing the locale name.
Even this extended functionality still does not help to solve the
problem that completely different names can be used to denote the same
locale (e.g., de and german). To be of help in this
situation the locale implementation and also the gettext
functions know about aliases.
The file `/usr/share/locale/locale.alias' (replace `/usr' with whatever prefix you used for configuring the C library) contains a mapping of alternative names to more regular names. The system manager is free to add new entries to fill her/his own needs. The selected locale from the environment is compared with the entries in the first column of this file ignoring the case. If they match the value of the second column is used instead for the further handling.
In the description of the format of the environment variables we already mentioned the character set as a factor in the selection of the message catalog. In fact, only catalogs which contain text written using the character set of the system/program can be used (directly; there will come a solution for this some day). This means for the user that s/he will always have to take care for this. If in the collection of the message catalogs there are files for the same language but coded using different character sets the user has to be careful.
gettext
The GNU C Library does not contain the source code for the programs to
handle message catalogs for the gettext functions. As part of
the GNU project the GNU gettext package contains everything the
developer needs. The functionality provided by the tools in this
package by far exceeds the abilities of the gencat program
described above for the catgets functions.
There is a program msgfmt which is the equivalent program to the
gencat program. It generates from the human-readable and
-editable form of the message catalog a binary file which can be used by
the gettext functions. But there are several more programs
available.
The xgettext program can be used to automatically extract the
translatable messages from a source file. I.e., the programmer need not
take care for the translations and the list of messages which have to be
translated. S/He will simply wrap the translatable string in calls to
gettext et.al and the rest will be done by xgettext. This
program has a lot of option which help to customize the output or do
help to understand the input better.
Other programs help to manage development cycle when new messages appear in the source files or when a new translation of the messages appear. here it should only be noted that using all the tools in GNU gettext it is possible to completely automize the handling of message catalog. Beside marking the translatable string in the source code and generating the translations the developers do not have anything to do themselves.
This chapter describes functions for searching and sorting arrays of arbitrary objects. You pass the appropriate comparison function to be applied as an argument, along with the size of the objects in the array and the total number of elements.
In order to use the sorted array library functions, you have to describe how to compare the elements of the array.
To do this, you supply a comparison function to compare two elements of
the array. The library will call this function, passing as arguments
pointers to two array elements to be compared. Your comparison function
should return a value the way strcmp (see section String/Array Comparison) does: negative if the first argument is "less" than the
second, zero if they are "equal", and positive if the first argument
is "greater".
Here is an example of a comparison function which works with an array of
numbers of type double:
int
compare_doubles (const void *a, const void *b)
{
const double *da = (const double *) a;
const double *db = (const double *) b;
return (*da > *db) - (*da < *db);
}
The header file `stdlib.h' defines a name for the data type of comparison functions. This type is a GNU extension.
int comparison_fn_t (const void *, const void *);
Generally searching for a specific element in an array means that potentially all elements must be checked. The GNU C library contains functions to perform linear search. The prototypes for the following two functions can be found in `search.h'.
lfind function searches in the array with *nmemb
elements of size bytes pointed to by base for an element
which matches the one pointed to by key. The function pointed to
by compar is used decide whether two elements match.
The return value is a pointer to the matching element in the array
starting at base if it is found. If no matching element is
available NULL is returned.
The mean runtime of this function is *nmemb/2. This
function should only be used elements often get added to or deleted from
the array in which case it might not be useful to sort the array before
searching.
lsearch function is similar to the lfind function. It
searches the given array for an element and returns it if found. The
difference is that if no matching element is found the lsearch
function adds the object pointed to by key (with a size of
size bytes) at the end of the array and it increments the value of
*nmemb to reflect this addition.
This means for the caller that if it is not sure that the array contains
the element one is searching for the memory allocated for the array
starting at base must have room for at least size more
bytes. If one is sure the element is in the array it is better to use
lfind so having more room in the array is always necessary when
calling lsearch.
To search a sorted array for an element matching the key, use the
bsearch function. The prototype for this function is in
the header file `stdlib.h'.
bsearch function searches the sorted array array for an object
that is equivalent to key. The array contains count elements,
each of which is of size size bytes.
The compare function is used to perform the comparison. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument. The elements of the array must already be sorted in ascending order according to this comparison function.
The return value is a pointer to the matching array element, or a null pointer if no match is found. If the array contains more than one element that matches, the one that is returned is unspecified.
This function derives its name from the fact that it is implemented using the binary search algorithm.
To sort an array using an arbitrary comparison function, use the
qsort function. The prototype for this function is in
`stdlib.h'.
The compare function is used to perform the comparison on the array elements. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument.
Warning: If two objects compare as equal, their order after sorting is unpredictable. That is to say, the sorting is not stable. This can make a difference when the comparison considers only part of the elements. Two elements with the same sort key may differ in other respects.
If you want the effect of a stable sort, you can get this result by writing the comparison function so that, lacking other reason distinguish between two elements, it compares them by their addresses. Note that doing this may make the sorting algorithm less efficient, so do it only if necessary.
Here is a simple example of sorting an array of doubles in numerical order, using the comparison function defined above (see section Defining the Comparison Function):
{
double *array;
int size;
...
qsort (array, size, sizeof (double), compare_doubles);
}
The qsort function derives its name from the fact that it was
originally implemented using the "quick sort" algorithm.
The implementation of qsort in this library might not be an
in-place sort and might thereby use an extra amount of memory to store
the array.
Here is an example showing the use of qsort and bsearch
with an array of structures. The objects in the array are sorted
by comparing their name fields with the strcmp function.
Then, we can look up individual objects based on their names.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
/* Define an array of critters to sort. */
struct critter
{
const char *name;
const char *species;
};
struct critter muppets[] =
{
{"Kermit", "frog"},
{"Piggy", "pig"},
{"Gonzo", "whatever"},
{"Fozzie", "bear"},
{"Sam", "eagle"},
{"Robin", "frog"},
{"Animal", "animal"},
{"Camilla", "chicken"},
{"Sweetums", "monster"},
{"Dr. Strangepork", "pig"},
{"Link Hogthrob", "pig"},
{"Zoot", "human"},
{"Dr. Bunsen Honeydew", "human"},
{"Beaker", "human"},
{"Swedish Chef", "human"}
};
int count = sizeof (muppets) / sizeof (struct critter);
/* This is the comparison function used for sorting and searching. */
int
critter_cmp (const struct critter *c1, const struct critter *c2)
{
return strcmp (c1->name, c2->name);
}
/* Print information about a critter. */
void
print_critter (const struct critter *c)
{
printf ("%s, the %s\n", c->name, c->species);
}
/* Do the lookup into the sorted array. */
void
find_critter (const char *name)
{
struct critter target, *result;
target.name = name;
result = bsearch (&target, muppets, count, sizeof (struct critter),
critter_cmp);
if (result)
print_critter (result);
else
printf ("Couldn't find %s.\n", name);
}
/* Main program. */
int
main (void)
{
int i;
for (i = 0; i < count; i++)
print_critter (&muppets[i]);
printf ("\n");
qsort (muppets, count, sizeof (struct critter), critter_cmp);
for (i = 0; i < count; i++)
print_critter (&muppets[i]);
printf ("\n");
find_critter ("Kermit");
find_critter ("Gonzo");
find_critter ("Janice");
return 0;
}
The output from this program looks like:
Kermit, the frog Piggy, the pig Gonzo, the whatever Fozzie, the bear Sam, the eagle Robin, the frog Animal, the animal Camilla, the chicken Sweetums, the monster Dr. Strangepork, the pig Link Hogthrob, the pig Zoot, the human Dr. Bunsen Honeydew, the human Beaker, the human Swedish Chef, the human Animal, the animal Beaker, the human Camilla, the chicken Dr. Bunsen Honeydew, the human Dr. Strangepork, the pig Fozzie, the bear Gonzo, the whatever Kermit, the frog Link Hogthrob, the pig Piggy, the pig Robin, the frog Sam, the eagle Swedish Chef, the human Sweetums, the monster Zoot, the human Kermit, the frog Gonzo, the whatever Couldn't find Janice.
hsearch function.The functions mentioned so far in this chapter are searching in a sorted or unsorted array. There are other methods to organize information which later should be searched. The costs of insert, delete and search differ. One possible implementation is using hashing tables.
hcreate function creates a hashing table which can contain at
least nel elements. There is no possibility to grow this table so
it is necessary to choose the value for nel wisely. The used
methods to implement this function might make it necessary to make the
number of elements in the hashing table larger than the expected maximal
number of elements. Hashing tables usually work inefficient if they are
filled 80% or more. The constant access time guaranteed by hashing can
only be achieved if few collisions exist. See Knuth's "The Art of
Computer Programming, Part 3: Searching and Sorting" for more
information.
The weakest aspect of this function is that there can be at most one hashing table used through the whole program. The table is allocated in local memory out of control of the programmer. As an extension the GNU C library provides an additional set of functions with an reentrant interface which provide a similar interface but which allow to keep arbitrarily many hashing tables.
It is possible to use more than one hashing table in the program run if
the former table is first destroyed by a call to hdestroy.
The function returns a non-zero value if successful. If it return zero something went wrong. This could either mean there is already a hashing table in use or the program runs out of memory.
hdestroy function can be used to free all the resources
allocated in a previous call of hcreate. After a call to this
function it is again possible to call hcreate and allocate a new
table with possibly different size.
It is important to remember that the elements contained in the hashing
table at the time hdestroy is called are not freed by this
function. It is the responsibility of the program code to free those
strings (if necessary at all). Freeing all the element memory is not
possible without extra, separately kept information since there is no
function to iterate through all available elements in the hashing table.
If it is really necessary to free a table and all elements the
programmer has to keep a list of all table elements and before calling
hdestroy s/he has to free all element's data using this list.
This is a very unpleasant mechanism and it also shows that this kind of
hashing tables is mainly meant for tables which are created once and
used until the end of the program run.
Entries of the hashing table and keys for the search are defined using this type:
hsearch functions. They can only be used for data sets which use
the NUL character always and solely to terminate the records. It is not
possible to handle general binary data.
char *key
char *data
hcreate the
hsearch function must be used. This function can perform simple
search for an element (if action has the FIND) or it can
alternatively insert the key element into the hashing table, possibly
replacing a previous value (if action is ENTER).
The key is denoted by a pointer to an object of type ENTRY. For
locating the corresponding position in the hashing table only the
key element of the structure is used.
The return value depends on the action parameter value. If it is
FIND the value is a pointer to the matching element in the
hashing table or NULL if no matching element exists. If
action is ENTER the return value is only NULL if the
programs runs out of memory while adding the new element to the table.
Otherwise the return value is a pointer to the element in the hashing
table which contains newly added element based on the data in key.
As mentioned before the hashing table used by the functions described so
far is global and there can be at any time at most one hashing table in
the program. A solution is to use the following functions which are a
GNU extension. All have in common that they operate on a hashing table
which is described by the content of an object of the type struct
hsearch_data. This type should be treated as opaque, none of its
members should be changed directly.
hcreate_r function initializes the object pointed to by
htab to contain a hashing table with at least nel elements.
So this function is equivalent to the hcreate function except
that the initialized data structure is controlled by the user.
This allows having more than one hashing table at one time. The
memory necessary for the struct hsearch_data object can be
allocated dynamically.
The return value is non-zero if the operation were successful. if the return value is zero something went wrong which probably means the programs runs out of memory.
hdestroy_r function frees all resources allocated by the
hcreate_r function for this very same object htab. As for
hdestroy it is the programs responsibility to free the strings
for the elements of the table.
hsearch_r function is equivalent to hsearch. The
meaning of the first two arguments is identical. But instead of
operating on a single global hashing table the function works on the
table described by the object pointed to by htab (which is
initialized by a call to hcreate_r).
Another difference to hcreate is that the pointer to the found
entry in the table is not the return value of the functions. It is
returned by storing it in a pointer variables pointed to by the
retval parameter. The return value of the function is an integer
value indicating success if it is non-zero and failure if it is zero.
In the latter case the global variable errno signals the reason for
the failure.
ENOMEM
hsearch_r was called with an so far
unknown key and action set to ENTER.
ESRCH
FIND and no corresponding element
is found in the table.
tsearch function.
Another common form to organize data for efficient search is to use
trees. The tsearch function family provides a nice interface to
functions to organize possibly large amounts of data by providing a mean
access time proportional to the logarithm of the number of elements.
The GNU C library implementation even guarantees that this bound is
never exceeded even for input data which cause problems for simple
binary tree implementations.
The functions described in the chapter are all described in the System V and X/Open specifications and are therefore quite portable.
In contrast to the hsearch functions the tsearch functions
can be used with arbitrary data and not only zero-terminated strings.
The tsearch functions have the advantage that no function to
initialize data structures is necessary. A simple pointer of type
void * initialized to NULL is a valid tree and can be
extended or searched.
tsearch function searches in the tree pointed to by
*rootp for an element matching key. The function
pointed to by compar is used to determine whether two elements
match. See section Defining the Comparison Function, for a specification of the functions
which can be used for the compar parameter.
If the tree does not contain a matching entry the key value will
be added to the tree. tsearch does not make a copy of the object
pointed to by key (how could it since the size is unknown).
Instead it adds a reference to this object which means the object must
be available as long as the tree data structure is used.
The tree is represented by a pointer to a pointer since it is sometimes
necessary to change the root node of the tree. So it must not be
assumed that the variable pointed to by rootp has the same value
after the call. This also shows that it is not safe to call the
tsearch function more than once at the same time using the same
tree. It is no problem to run it more than once at a time on different
trees.
The return value is a pointer to the matching element in the tree. If a
new element was created the pointer points to the new data (which is in
fact key). If an entry had to be created and the program ran out
of space NULL is returned.
tfind function is similar to the tsearch function. It
locates an element matching the one pointed to by key and returns
a pointer to this element. But if no matching element is available no
new element is entered (note that the rootp parameter points to a
constant pointer). Instead the function returns NULL.
Another advantage of the tsearch function in contrast to the
hsearch functions is that there is an easy way to remove
elements.
tdelete can be used. It locates the matching element using the
same method as tfind. The corresponding element is then removed
and a pointer to the parent of the deleted node is returned by the
function. If there is no matching entry in the tree nothing can be
deleted and the function returns NULL. If the root of the tree
is deleted tdelete returns some unspecified value not equal to
NULL.
tdestroy. It frees all resources allocated by the tsearch
function to generate the tree pointed to by vroot.
For the data in each tree node the function freefct is called. The pointer to the data is passed as the argument to the function. If no such work is necessary freefct must point to a function doing nothing. It is called in any case.
This function is a GNU extension and not covered by the System V or X/Open specifications.
In addition to the function to create and destroy the tree data structure, there is another function which allows you to apply a function to all elements of the tree. The function must have this type:
void __action_fn_t (const void *nodep, VISIT value, int level);
The nodep is the data value of the current node (once given as the
key argument to tsearch). level is a numeric value
which corresponds to the depth of the current node in the tree. The
root node has the depth @math{0} and its children have a depth of
@math{1} and so on. The VISIT type is an enumeration type.
VISIT value indicates the status of the current node in the
tree and how the function is called. The status of a node is either
`leaf' or `internal node'. For each leaf node the function is called
exactly once, for each internal node it is called three times: before
the first child is processed, after the first child is processed and
after both children are processed. This makes it possible to handle all
three methods of tree traversal (or even a combination of them).
preorder
postorder
endorder
leaf
twalk function calls the function provided by the parameter
action. For leaf nodes the function is called exactly once with
value set to leaf. For internal nodes the function is
called three times, setting the value parameter or action to
the appropriate value. The level argument for the action
function is computed while descending the tree with increasing the value
by one for the descend to a child, starting with the value @math{0} for
the root node.
Since the functions used for the action parameter to twalk
must not modify the tree data, it is safe to run twalk in more
than one thread at the same time, working on the same tree. It is also
safe to call tfind in parallel. Functions which modify the tree
must not be used, otherwise the behaviour is undefined.
The GNU C Library provides pattern matching facilities for two kinds of patterns: regular expressions and file-name wildcards. The library also provides a facility for expanding variable and command references and parsing text into words in the way the shell does.
This section describes how to match a wildcard pattern against a particular string. The result is a yes or no answer: does the string fit the pattern or not. The symbols described here are all declared in `fnmatch.h'.
0 if they do match; otherwise, it
returns the nonzero value FNM_NOMATCH. The arguments
pattern and string are both strings.
The argument flags is a combination of flag bits that alter the details of matching. See below for a list of the defined flags.
In the GNU C Library, fnmatch cannot experience an "error"---it
always returns an answer for whether the match succeeds. However, other
implementations of fnmatch might sometimes report "errors".
They would do so by returning nonzero values that are not equal to
FNM_NOMATCH.
These are the available flags for the flags argument:
FNM_FILE_NAME
FNM_PATHNAME
FNM_FILE_NAME; it comes from POSIX.2. We
don't recommend this name because we don't use the term "pathname" for
file names.
FNM_PERIOD
FNM_PERIOD and FNM_FILE_NAME, then the
special treatment applies to `.' following `/' as well as to
`.' at the beginning of string. (The shell uses the
FNM_PERIOD and FNM_FILE_NAME flags together for matching
file names.)
FNM_NOESCAPE
FNM_NOESCAPE, then `\' is an ordinary character.
FNM_LEADING_DIR
FNM_CASEFOLD
FNM_EXTMATCH
|
separated list of patterns.
?(pattern-list)
*(pattern-list)
+(pattern-list)
@(pattern-list)
!(pattern-list)
The archetypal use of wildcards is for matching against the files in a directory, and making a list of all the matches. This is called globbing.
You could do this using fnmatch, by reading the directory entries
one by one and testing each one with fnmatch. But that would be
slow (and complex, since you would have to handle subdirectories by
hand).
The library provides a function glob to make this particular use
of wildcards convenient. glob and the other symbols in this
section are declared in `glob.h'.
glob
The result of globbing is a vector of file names (strings). To return
this vector, glob uses a special data type, glob_t, which
is a structure. You pass glob the address of the structure, and
it fills in the structure's fields to tell you about the results.
gl_pathc
gl_pathv
char **.
gl_offs
gl_pathv field. Unlike the other fields, this
is always an input to glob, rather than an output from it.
If you use a nonzero offset, then that many elements at the beginning of
the vector are left empty. (The glob function fills them with
null pointers.)
The gl_offs field is meaningful only if you use the
GLOB_DOOFFS flag. Otherwise, the offset is always zero
regardless of what is in this field, and the first real element comes at
the beginning of the vector.
gl_closedir
closedir
function. It is used if the GLOB_ALTDIRFUNC bit is set in
the flag parameter. The type of this field is
void (*) (void *).
This is a GNU extension.
gl_readdir
readdir
function used to read the contents of a directory. It is used if the
GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of
this field is struct dirent *(*) (void *).
This is a GNU extension.
gl_opendir
opendir
function. It is used if the GLOB_ALTDIRFUNC bit is set in
the flag parameter. The type of this field is
void *(*) (const char *).
This is a GNU extension.
gl_stat
stat function
to get information about an object in the filesystem. It is used if the
GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of
this field is int (*) (const char *, struct stat *).
This is a GNU extension.
gl_lstat
lstat
function to get information about an object in the filesystems, not
following symbolic links. It is used if the GLOB_ALTDIRFUNC bit
is set in the flag parameter. The type of this field is int
(*) (const char *, struct stat *).
This is a GNU extension.
For use in the glob64 function `glob.h' contains another
definition for a very similar type. glob64_t differs from
glob_t only in the types of the members gl_readdir,
gl_stat, and gl_lstat.
gl_pathc
gl_pathv
char **.
gl_offs
gl_pathv field. Unlike the other fields, this
is always an input to glob, rather than an output from it.
If you use a nonzero offset, then that many elements at the beginning of
the vector are left empty. (The glob function fills them with
null pointers.)
The gl_offs field is meaningful only if you use the
GLOB_DOOFFS flag. Otherwise, the offset is always zero
regardless of what is in this field, and the first real element comes at
the beginning of the vector.
gl_closedir
closedir
function. It is used if the GLOB_ALTDIRFUNC bit is set in
the flag parameter. The type of this field is
void (*) (void *).
This is a GNU extension.
gl_readdir
readdir64
function used to read the contents of a directory. It is used if the
GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of
this field is struct dirent64 *(*) (void *).
This is a GNU extension.
gl_opendir
opendir
function. It is used if the GLOB_ALTDIRFUNC bit is set in
the flag parameter. The type of this field is
void *(*) (const char *).
This is a GNU extension.
gl_stat
stat64 function
to get information about an object in the filesystem. It is used if the
GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of
this field is int (*) (const char *, struct stat64 *).
This is a GNU extension.
gl_lstat
lstat64
function to get information about an object in the filesystems, not
following symbolic links. It is used if the GLOB_ALTDIRFUNC bit
is set in the flag parameter. The type of this field is int
(*) (const char *, struct stat64 *).
This is a GNU extension.
glob does globbing using the pattern pattern
in the current directory. It puts the result in a newly allocated
vector, and stores the size and address of this vector into
*vector-ptr. The argument flags is a combination of
bit flags; see section Flags for Globbing, for details of the flags.
The result of globbing is a sequence of file names. The function
glob allocates a string for each resulting word, then
allocates a vector of type char ** to store the addresses of
these strings. The last element of the vector is a null pointer.
This vector is called the word vector.
To return this vector, glob stores both its address and its
length (number of elements, not counting the terminating null pointer)
into *vector-ptr.
Normally, glob sorts the file names alphabetically before
returning them. You can turn this off with the flag GLOB_NOSORT
if you want to get the information as fast as possible. Usually it's
a good idea to let glob sort them--if you process the files in
alphabetical order, the users will have a feel for the rate of progress
that your application is making.
If glob succeeds, it returns 0. Otherwise, it returns one
of these error codes:
GLOB_ABORTED
GLOB_ERR or your specified errfunc returned a nonzero
value.
See below
for an explanation of the GLOB_ERR flag and errfunc.
GLOB_NOMATCH
GLOB_NOCHECK flag, then you never get this error code, because
that flag tells glob to pretend that the pattern matched
at least one file.
GLOB_NOSPACE
In the event of an error, glob stores information in
*vector-ptr about all the matches it has found so far.
It is important to notive that the glob function will not fail if
it encounters directories or files which cannot be handled without the
LFS interfaces. The implementation of glob is supposed to use
these functions internally. This at least is the assumptions made by
the Unix standard. The GNU extension of allowing the user to provide
own directory handling and stat functions complicates things a
bit. If these callback functions are used and a large file or directory
is encountered glob can fail.
glob64 function was added as part of the Large File Summit
extensions but is not part of the original LFS proposal. The reason for
this is simple: it is not necessary. The necessity for a glob64
function is added by the extensions of the GNU glob
implementation which allows the user to provide own directory handling
and stat functions. The readdir and stat functions
do depend on the choice of _FILE_OFFSET_BITS since the definition
of the types struct dirent and struct stat will change
depending on the choice.
Beside this difference the glob64 works just like glob in
all aspects.
This function is a GNU extension.
This section describes the flags that you can specify in the
flags argument to glob. Choose the flags you want,
and combine them with the C bitwise OR operator |.
GLOB_APPEND
glob. This way you can effectively expand
several words as if they were concatenated with spaces between them.
In order for appending to work, you must not modify the contents of the
word vector structure between calls to glob. And, if you set
GLOB_DOOFFS in the first call to glob, you must also
set it when you append to the results.
Note that the pointer stored in gl_pathv may no longer be valid
after you call glob the second time, because glob might
have relocated the vector. So always fetch gl_pathv from the
glob_t structure after each glob call; never save
the pointer across calls.
GLOB_DOOFFS
gl_offs field says how many slots to leave.
The blank slots contain null pointers.
GLOB_ERR
glob tries its best to keep
on going despite any errors, reading whatever directories it can.
You can exercise even more control than this by specifying an
error-handler function errfunc when you call glob. If
errfunc is not a null pointer, then glob doesn't give up
right away when it can't read a directory; instead, it calls
errfunc with two arguments, like this:
(*errfunc) (filename, error-code)The argument filename is the name of the directory that
glob couldn't open or couldn't read, and error-code is the
errno value that was reported to glob.
If the error handler function returns nonzero, then glob gives up
right away. Otherwise, it continues.
GLOB_MARK
GLOB_NOCHECK
glob returns that there were no
matches.)
GLOB_NOSORT
GLOB_NOESCAPE
GLOB_NOESCAPE, then `\' is an ordinary character.
glob does its work by calling the function fnmatch
repeatedly. It handles the flag GLOB_NOESCAPE by turning on the
FNM_NOESCAPE flag in calls to fnmatch.
Beside the flags described in the last section, the GNU implementation of
glob allows a few more flags which are also defined in the
`glob.h' file. Some of the extensions implement functionality
which is available in modern shell implementations.
GLOB_PERIOD
. character (period) is treated special. It cannot be
matched by wildcards. See section Wildcard Matching, FNM_PERIOD.
GLOB_MAGCHAR
GLOB_MAGCHAR value is not to be given to glob in the
flags parameter. Instead, glob sets this bit in the
gl_flags element of the glob_t structure provided as the
result if the pattern used for matching contains any wildcard character.
GLOB_ALTDIRFUNC
glob implementation uses the user-supplied
functions specified in the structure pointed to by pglob
parameter. For more information about the functions refer to the
sections about directory handling see section Accessing Directories, and
section Reading the Attributes of a File.
GLOB_BRACE
, (comma) characters. The commas
themself are discarded. Please note what we said above about recursive
brace expressions. The commas used to separate the subexpressions must
be at the same level. Commas in brace subexpressions are not matched.
They are used during expansion of the brace expression of the deeper
level. The example below shows this
glob ("{foo/{,bar,biz},baz}", GLOB_BRACE, NULL, &result)
is equivalent to the sequence
glob ("foo/", GLOB_BRACE, NULL, &result)
glob ("foo/bar", GLOB_BRACE|GLOB_APPEND, NULL, &result)
glob ("foo/biz", GLOB_BRACE|GLOB_APPEND, NULL, &result)
glob ("baz", GLOB_BRACE|GLOB_APPEND, NULL, &result)
if we leave aside error handling.
GLOB_NOMAGIC
GLOB_TILDE
~ (tilde) is handled special
if it appears at the beginning of the pattern. Instead of being taken
verbatim it is used to represent the home directory of a known user.
If ~ is the only character in pattern or it is followed by a
/ (slash), the home directory of the process owner is
substituted. Using getlogin and getpwnam the information
is read from the system databases. As an example take user bart
with his home directory at `/home/bart'. For him a call like
glob ("~/bin/*", GLOB_TILDE, NULL, &result)
would return the contents of the directory `/home/bart/bin'.
Instead of referring to the own home directory it is also possible to
name the home directory of other users. To do so one has to append the
user name after the tilde character. So the contents of user
homer's `bin' directory can be retrieved by
glob ("~homer/bin/*", GLOB_TILDE, NULL, &result)
If the user name is not valid or the home directory cannot be determined
for some reason the pattern is left untouched and itself used as the
result. I.e., if in the last example home is not available the
tilde expansion yields to "~homer/bin/*" and glob is not
looking for a directory named ~homer.
This functionality is equivalent to what is available in C-shells if the
nonomatch flag is set.
GLOB_TILDE_CHECK
glob behaves like as if GLOB_TILDE is
given. The only difference is that if the user name is not available or
the home directory cannot be determined for other reasons this leads to
an error. glob will return GLOB_NOMATCH instead of using
the pattern itself as the name.
This functionality is equivalent to what is available in C-shells if
nonomatch flag is not set.
GLOB_ONLYDIR
glob
implementation. It is mainly used internally to increase the
performance but might be useful for a user as well and therefore is
documented here.
Calling glob will in most cases allocate resources which are used
to represent the result of the function call. If the same object of
type glob_t is used in multiple call to glob the resources
are freed or reused so that no leaks appear. But this does not include
the time when all glob calls are done.
globfree function frees all resources allocated by previous
calls to glob associated with the object pointed to by
pglob. This function should be called whenever the currently used
glob_t typed object isn't used anymore.
globfree but it frees records of
type glob64_t which were allocated by glob64.
The GNU C library supports two interfaces for matching regular expressions. One is the standard POSIX.2 interface, and the other is what the GNU system has had for many years.
Both interfaces are declared in the header file `regex.h'.
If you define _POSIX_C_SOURCE, then only the POSIX.2
functions, structures, and constants are declared.
Before you can actually match a regular expression, you must compile it. This is not true compilation--it produces a special data structure, not machine instructions. But it is like ordinary compilation in that its purpose is to enable you to "execute" the pattern fast. (See section Matching a Compiled POSIX Regular Expression, for how to use the compiled regular expression for matching.)
There is a special data type for compiled regular expressions:
re_nsub
There are several other fields, but we don't describe them here, because only the functions in the library should use them.
After you create a regex_t object, you can compile a regular
expression into it by calling regcomp.
regcomp "compiles" a regular expression into a
data structure that you can use with regexec to match against a
string. The compiled regular expression format is designed for
efficient matching. regcomp stores it into *compiled.
It's up to you to allocate an object of type regex_t and pass its
address to regcomp.
The argument cflags lets you specify various options that control the syntax and semantics of regular expressions. See section Flags for POSIX Regular Expressions.
If you use the flag REG_NOSUB, then regcomp omits from
the compiled regular expression the information necessary to record
how subexpressions actually match. In this case, you might as well
pass 0 for the matchptr and nmatch arguments when
you call regexec.
If you don't use REG_NOSUB, then the compiled regular expression
does have the capacity to record how subexpressions match. Also,
regcomp tells you how many subexpressions pattern has, by
storing the number in compiled->re_nsub. You can use that
value to decide how long an array to allocate to hold information about
subexpression matches.
regcomp returns 0 if it succeeds in compiling the regular
expression; otherwise, it returns a nonzero error code (see the table
below). You can use regerror to produce an error message string
describing the reason for a nonzero value; see section POSIX Regexp Matching Cleanup.
Here are the possible nonzero values that regcomp can return:
REG_BADBR
REG_BADPAT
REG_BADRPT
REG_ECOLLATE
REG_ECTYPE
REG_EESCAPE
REG_ESUBREG
REG_EBRACK
REG_EPAREN
REG_EBRACE
REG_ERANGE
REG_ESPACE
regcomp ran out of memory.
These are the bit flags that you can use in the cflags operand when
compiling a regular expression with regcomp.
REG_EXTENDED
REG_ICASE
REG_NOSUB
REG_NEWLINE
Once you have compiled a regular expression, as described in section POSIX Regular Expression Compilation, you can match it against strings using
regexec. A match anywhere inside the string counts as success,
unless the regular expression contains anchor characters (`^' or
`$').
*compiled against string.
regexec returns 0 if the regular expression matches;
otherwise, it returns a nonzero value. See the table below for
what nonzero values mean. You can use regerror to produce an
error message string describing the reason for a nonzero value;
see section POSIX Regexp Matching Cleanup.
The argument eflags is a word of bit flags that enable various options.
If you want to get information about what part of string actually
matched the regular expression or its subexpressions, use the arguments
matchptr and nmatch. Otherwise, pass 0 for
nmatch, and NULL for matchptr. See section Match Results with Subexpressions.
You must match the regular expression with the same set of current locales that were in effect when you compiled the regular expression.
The function regexec accepts the following flags in the
eflags argument:
REG_NOTBOL
REG_NOTEOL
Here are the possible nonzero values that regexec can return:
REG_NOMATCH
REG_ESPACE
regexec ran out of memory.
When regexec matches parenthetical subexpressions of
pattern, it records which parts of string they match. It
returns that information by storing the offsets into an array whose
elements are structures of type regmatch_t. The first element of
the array (index 0) records the part of the string that matched
the entire regular expression. Each other element of the array records
the beginning and end of the part that matched a single parenthetical
subexpression.
regexec. It contains two structure fields, as follows:
rm_so
rm_eo
regoff_t is an alias for another signed integer type.
The fields of regmatch_t have type regoff_t.
The regmatch_t elements correspond to subexpressions
positionally; the first element (index 1) records where the first
subexpression matched, the second element records the second
subexpression, and so on. The order of the subexpressions is the order
in which they begin.
When you call regexec, you specify how long the matchptr
array is, with the nmatch argument. This tells regexec how
many elements to store. If the actual regular expression has more than
nmatch subexpressions, then you won't get offset information about
the rest of them. But this doesn't alter whether the pattern matches a
particular string or not.
If you don't want regexec to return any information about where
the subexpressions matched, you can either supply 0 for
nmatch, or use the flag REG_NOSUB when you compile the
pattern with regcomp.
Sometimes a subexpression matches a substring of no characters. This
happens when `f\(o*\)' matches the string `fum'. (It really
matches just the `f'.) In this case, both of the offsets identify
the point in the string where the null substring was found. In this
example, the offsets are both 1.
Sometimes the entire regular expression can match without using some of
its subexpressions at all--for example, when `ba\(na\)*' matches the
string `ba', the parenthetical subexpression is not used. When
this happens, regexec stores -1 in both fields of the
element for that subexpression.
Sometimes matching the entire regular expression can match a particular
subexpression more than once--for example, when `ba\(na\)*'
matches the string `bananana', the parenthetical subexpression
matches three times. When this happens, regexec usually stores
the offsets of the last part of the string that matched the
subexpression. In the case of `bananana', these offsets are
6 and 8.
But the last match is not always the one that is chosen. It's more
accurate to say that the last opportunity to match is the one
that takes precedence. What this means is that when one subexpression
appears within another, then the results reported for the inner
subexpression reflect whatever happened on the last match of the outer
subexpression. For an example, consider `\(ba\(na\)*s \)*' matching
the string `bananas bas '. The last time the inner expression
actually matches is near the end of the first word. But it is
considered again in the second word, and fails to match there.
regexec reports nonuse of the "na" subexpression.
Another place where this rule applies is when the regular expression
\(ba\(na\)*s \|nefer\(ti\)* \)*
matches `bananas nefertiti'. The "na" subexpression does match
in the first word, but it doesn't match in the second word because the
other alternative is used there. Once again, the second repetition of
the outer subexpression overrides the first, and within that second
repetition, the "na" subexpression is not used. So regexec
reports nonuse of the "na" subexpression.
When you are finished using a compiled regular expression, you can
free the storage it uses by calling regfree.
regfree frees all the storage that *compiled
points to. This includes various internal fields of the regex_t
structure that aren't documented in this manual.
regfree does not free the object *compiled itself.
You should always free the space in a regex_t structure with
regfree before using the structure to compile another regular
expression.
When regcomp or regexec reports an error, you can use
the function regerror to turn it into an error message string.
regcomp or
regexec was working with when it got the error. Alternatively,
you can supply NULL for compiled; you will still get a
meaningful error message, but it might not be as detailed.
If the error message can't fit in length bytes (including a
terminating null character), then regerror truncates it.
The string that regerror stores is always null-terminated
even if it has been truncated.
The return value of regerror is the minimum length needed to
store the entire error message. If this is less than length, then
the error message was not truncated, and you can use it. Otherwise, you
should call regerror again with a larger buffer.
Here is a function which uses regerror, but always dynamically
allocates a buffer for the error message:
char *get_regerror (int errcode, regex_t *compiled)
{
size_t length = regerror (errcode, compiled, NULL, 0);
char *buffer = xmalloc (length);
(void) regerror (errcode, compiled, buffer, length);
return buffer;
}
Word expansion means the process of splitting a string into words and substituting for variables, commands, and wildcards just as the shell does.
For example, when you write `ls -l foo.c', this string is split into three separate words---`ls', `-l' and `foo.c'. This is the most basic function of word expansion.
When you write `ls *.c', this can become many words, because the word `*.c' can be replaced with any number of file names. This is called wildcard expansion, and it is also a part of word expansion.
When you use `echo $PATH' to print your path, you are taking advantage of variable substitution, which is also part of word expansion.
Ordinary programs can perform word expansion just like the shell by
calling the library function wordexp.
When word expansion is applied to a sequence of words, it performs the following transformations in the order shown here:
For the details of these transformations, and how to write the constructs that use them, see The BASH Manual (to appear).
wordexpAll the functions, constants and data types for word expansion are declared in the header file `wordexp.h'.
Word expansion produces a vector of words (strings). To return this
vector, wordexp uses a special data type, wordexp_t, which
is a structure. You pass wordexp the address of the structure,
and it fills in the structure's fields to tell you about the results.
we_wordc
we_wordv
char **.
we_offs
we_wordv field. Unlike the other fields, this
is always an input to wordexp, rather than an output from it.
If you use a nonzero offset, then that many elements at the beginning of
the vector are left empty. (The wordexp function fills them with
null pointers.)
The we_offs field is meaningful only if you use the
WRDE_DOOFFS flag. Otherwise, the offset is always zero
regardless of what is in this field, and the first real element comes at
the beginning of the vector.
*word-vector-ptr. The argument flags is a
combination of bit flags; see section Flags for Word Expansion, for details of
the flags.
You shouldn't use any of the characters `|&;<>' in the string
words unless they are quoted; likewise for newline. If you use
these characters unquoted, you will get the WRDE_BADCHAR error
code. Don't use parentheses or braces unless they are quoted or part of
a word expansion construct. If you use quotation characters `'"`',
they should come in pairs that balance.
The results of word expansion are a sequence of words. The function
wordexp allocates a string for each resulting word, then
allocates a vector of type char ** to store the addresses of
these strings. The last element of the vector is a null pointer.
This vector is called the word vector.
To return this vector, wordexp stores both its address and its
length (number of elements, not counting the terminating null pointer)
into *word-vector-ptr.
If wordexp succeeds, it returns 0. Otherwise, it returns one
of these error codes:
WRDE_BADCHAR
WRDE_BADVAL
WRDE_UNDEF to forbid such references.
WRDE_CMDSUB
WRDE_NOCMD to forbid command substitution.
WRDE_NOSPACE
wordexp can store part of the results--as much as it could
allocate room for.
WRDE_SYNTAX
*word-vector-ptr points to. This does not free the
structure *word-vector-ptr itself--only the other
data it points to.
This section describes the flags that you can specify in the
flags argument to wordexp. Choose the flags you want,
and combine them with the C operator |.
WRDE_APPEND
wordexp. This way you can effectively expand
several words as if they were concatenated with spaces between them.
In order for appending to work, you must not modify the contents of the
word vector structure between calls to wordexp. And, if you set
WRDE_DOOFFS in the first call to wordexp, you must also
set it when you append to the results.
WRDE_DOOFFS
we_offs field says how many slots to leave.
The blank slots contain null pointers.
WRDE_NOCMD
WRDE_REUSE
wordexp.
Instead of allocating a new vector of words, this call to wordexp
will use the vector that already exists (making it larger if necessary).
Note that the vector may move, so it is not safe to save an old pointer
and use it again after calling wordexp. You must fetch
we_pathv anew after each call.
WRDE_SHOWERR
wordexp gives these
commands a standard error stream that discards all output.
WRDE_UNDEF
wordexp Example
Here is an example of using wordexp to expand several strings
and use the results to run a shell command. It also shows the use of
WRDE_APPEND to concatenate the expansions and of wordfree
to free the space allocated by wordexp.
int
expand_and_execute (const char *program, const char *options)
{
wordexp_t result;
pid_t pid
int status, i;
/* Expand the string for the program to run. */
switch (wordexp (program, &result, 0))
{
case 0: /* Successful. */
break;
case WRDE_NOSPACE:
/* If the error was WRDE_NOSPACE,
then perhaps part of the result was allocated. */
wordfree (&result);
default: /* Some other error. */
return -1;
}
/* Expand the strings specified for the arguments. */
for (i = 0; args[i]; i++)
{
if (wordexp (options, &result, WRDE_APPEND))
{
wordfree (&result);
return -1;
}
}
pid = fork ();
if (pid == 0)
{
/* This is the child process. Execute the command. */
execv (result.we_wordv[0], result.we_wordv);
exit (EXIT_FAILURE);
}
else if (pid < 0)
/* The fork failed. Report failure. */
status = -1;
else
/* This is the parent process. Wait for the child to complete. */
if (waitpid (pid, &status, 0) != pid)
status = -1;
wordfree (&result);
return status;
}
It's a standard part of shell syntax that you can use `~' at the beginning of a file name to stand for your own home directory. You can use `~user' to stand for user's home directory.
Tilde expansion is the process of converting these abbreviations to the directory names that they stand for.
Tilde expansion applies to the `~' plus all following characters up to whitespace or a slash. It takes place only at the beginning of a word, and only if none of the characters to be transformed is quoted in any way.
Plain `~' uses the value of the environment variable HOME
as the proper home directory name. `~' followed by a user name
uses getpwname to look up that user in the user database, and
uses whatever directory is recorded there. Thus, `~' followed
by your own name can give different results from plain `~', if
the value of HOME is not really your home directory.
Part of ordinary shell syntax is the use of `$variable' to substitute the value of a shell variable into a command. This is called variable substitution, and it is one part of doing word expansion.
There are two basic ways you can write a variable reference for substitution:
${variable}
$variable
foo and expands
into `tractor-bar'.
When you use braces, you can also use various constructs to modify the value that is substituted, or test it in various ways.
${variable:-default}
${variable:=default}
${variable:?message}
${variable:+replacement}
${#variable}
These variants of variable substitution let you remove part of the variable's value before substituting it. The prefix and suffix are not mere strings; they are wildcard patterns, just like the patterns that you use to match multiple file names. But in this context, they match against parts of the variable value rather than against file names.
${variable%%suffix}
${variable%suffix}
${variable##prefix}
${variable#prefix}
Most programs need to do either input (reading data) or output (writing data), or most frequently both, in order to do anything useful. The GNU C library provides such a large selection of input and output functions that the hardest part is often deciding which function is most appropriate!
This chapter introduces concepts and terminology relating to input and output. Other chapters relating to the GNU I/O facilities are:
Before you can read or write the contents of a file, you must establish a connection or communications channel to the file. This process is called opening the file. You can open a file for reading, writing, or both.
The connection to an open file is represented either as a stream or as a file descriptor. You pass this as an argument to the functions that do the actual read or write operations, to tell them which file to operate on. Certain functions expect streams, and others are designed to operate on file descriptors.
When you have finished reading to or writing from the file, you can terminate the connection by closing the file. Once you have closed a stream or file descriptor, you cannot do any more input or output operations on it.
When you want to do input or output to a file, you have a choice of two
basic mechanisms for representing the connection between your program
and the file: file descriptors and streams. File descriptors are
represented as objects of type int, while streams are represented
as FILE * objects.
File descriptors provide a primitive, low-level interface to input and output operations. Both file descriptors and streams can represent a connection to a device (such as a terminal), or a pipe or socket for communicating with another process, as well as a normal file. But, if you want to do control operations that are specific to a particular kind of device, you must use a file descriptor; there are no facilities to use streams in this way. You must also use file descriptors if your program needs to do input or output in special modes, such as nonblocking (or polled) input (see section File Status Flags).
Streams provide a higher-level interface, layered on top of the primitive file descriptor facilities. The stream interface treats all kinds of files pretty much alike--the sole exception being the three styles of buffering that you can choose (see section Stream Buffering).
The main advantage of using the stream interface is that the set of
functions for performing actual input and output operations (as opposed
to control operations) on streams is much richer and more powerful than
the corresponding facilities for file descriptors. The file descriptor
interface provides only simple functions for transferring blocks of
characters, but the stream interface also provides powerful formatted
input and output functions (printf and scanf) as well as
functions for character- and line-oriented input and output.
Since streams are implemented in terms of file descriptors, you can extract the file descriptor from a stream and perform low-level operations directly on the file descriptor. You can also initially open a connection as a file descriptor and then make a stream associated with that file descriptor.
In general, you should stick with using streams rather than file descriptors, unless there is some specific operation you want to do that can only be done on a file descriptor. If you are a beginning programmer and aren't sure what functions to use, we suggest that you concentrate on the formatted input functions (see section Formatted Input) and formatted output functions (see section Formatted Output).
If you are concerned about portability of your programs to systems other than GNU, you should also be aware that file descriptors are not as portable as streams. You can expect any system running ISO C to support streams, but non-GNU systems may not support file descriptors at all, or may only implement a subset of the GNU functions that operate on file descriptors. Most of the file descriptor functions in the GNU library are included in the POSIX.1 standard, however.
One of the attributes of an open file is its file position that keeps track of where in the file the next character is to be read or written. In the GNU system, and all POSIX.1 systems, the file position is simply an integer representing the number of bytes from the beginning of the file.
The file position is normally set to the beginning of the file when it is opened, and each time a character is read or written, the file position is incremented. In other words, access to the file is normally sequential.
Ordinary files permit read or write operations at any position within
the file. Some other kinds of files may also permit this. Files which
do permit this are sometimes referred to as random-access files.
You can change the file position using the fseek function on a
stream (see section File Positioning) or the lseek function on a file
descriptor (see section Input and Output Primitives). If you try to change the file
position on a file that doesn't support random access, you get the
ESPIPE error.
Streams and descriptors that are opened for append access are treated specially for output: output to such files is always appended sequentially to the end of the file, regardless of the file position. However, the file position is still used to control where in the file reading is done.
If you think about it, you'll realize that several programs can read a given file at the same time. In order for each program to be able to read the file at its own pace, each program must have its own file pointer, which is not affected by anything the other programs do.
In fact, each opening of a file creates a separate file position. Thus, if you open a file twice even in the same program, you get two streams or descriptors with independent file positions.
By contrast, if you open a descriptor and then duplicate it to get another descriptor, these two descriptors share the same file position: changing the file position of one descriptor will affect the other.
In order to open a connection to a file, or to perform other operations such as deleting a file, you need some way to refer to the file. Nearly all files have names that are strings--even files which are actually devices such as tape drives or terminals. These strings are called file names. You specify the file name to say which file you want to open or operate on.
This section describes the conventions for file names and how the operating system works with them.
In order to understand the syntax of file names, you need to understand how the file system is organized into a hierarchy of directories.
A directory is a file that contains information to associate other files with names; these associations are called links or directory entries. Sometimes, people speak of "files in a directory", but in reality, a directory only contains pointers to files, not the files themselves.
The name of a file contained in a directory entry is called a file name component. In general, a file name consists of a sequence of one or more such components, separated by the slash character (`/'). A file name which is just one component names a file with respect to its directory. A file name with multiple components names a directory, and then a file in that directory, and so on.
Some other documents, such as the POSIX standard, use the term
pathname for what we call a file name, and either filename
or pathname component for what this manual calls a file name
component. We don't use this terminology because a "path" is
something completely different (a list of directories to search), and we
think that "pathname" used for something else will confuse users. We
always use "file name" and "file name component" (or sometimes just
"component", where the context is obvious) in GNU documentation. Some
macros use the POSIX terminology in their names, such as
PATH_MAX. These macros are defined by the POSIX standard, so we
cannot change their names.
You can find more detailed information about operations on directories in section File System Interface.
A file name consists of file name components separated by slash (`/') characters. On the systems that the GNU C library supports, multiple successive `/' characters are equivalent to a single `/' character.
The process of determining what file a file name refers to is called file name resolution. This is performed by examining the components that make up a file name in left-to-right order, and locating each successive component in the directory named by the previous component. Of course, each of the files that are referenced as directories must actually exist, be directories instead of regular files, and have the appropriate permissions to be accessible by the process; otherwise the file name resolution fails.
If a file name begins with a `/', the first component in the file name is located in the root directory of the process (usually all processes on the system have the same root directory). Such a file name is called an absolute file name.
Otherwise, the first component in the file name is located in the current working directory (see section Working Directory). This kind of file name is called a relative file name.
The file name components `.' ("dot") and `..' ("dot-dot") have special meanings. Every directory has entries for these file name components. The file name component `.' refers to the directory itself, while the file name component `..' refers to its parent directory (the directory that contains the link for the directory in question). As a special case, `..' in the root directory refers to the root directory itself, since it has no parent; thus `/..' is the same as `/'.
Here are some examples of file names:
A file name that names a directory may optionally end in a `/'. You can specify a file name of `/' to refer to the root directory, but the empty string is not a meaningful file name. If you want to refer to the current working directory, use a file name of `.' or `./'.
Unlike some other operating systems, the GNU system doesn't have any built-in support for file types (or extensions) or file versions as part of its file name syntax. Many programs and utilities use conventions for file names--for example, files containing C source code usually have names suffixed with `.c'---but there is nothing in the file system itself that enforces this kind of convention.
Functions that accept file name arguments usually detect these
errno error conditions relating to the file name syntax or
trouble finding the named file. These errors are referred to throughout
this manual as the usual file name errors.
EACCES
ENAMETOOLONG
PATH_MAX, or when an individual file name component
has a length greater than NAME_MAX. See section Limits on File System Capacity.
In the GNU system, there is no imposed limit on overall file name
length, but some file systems may place limits on the length of a
component.
ENOENT
ENOTDIR
ELOOP
The rules for the syntax of file names discussed in section File Names, are the rules normally used by the GNU system and by other POSIX systems. However, other operating systems may use other conventions.
There are two reasons why it can be important for you to be aware of file name portability issues:
The ISO C standard says very little about file name syntax, only that file names are strings. In addition to varying restrictions on the length of file names and what characters can validly appear in a file name, different operating systems use different conventions and syntax for concepts such as structured directories and file types or extensions. Some concepts such as file versions might be supported in some operating systems and not by others.
The POSIX.1 standard allows implementations to put additional restrictions on file name syntax, concerning what characters are permitted in file names and on the length of file name and file name component strings. However, in the GNU system, you do not need to worry about these restrictions; any character except the null character is permitted in a file name string, and there are no limits on the length of file name strings.
This chapter describes the functions for creating streams and performing input and output operations on them. As discussed in section Input/Output Overview, a stream is a fairly abstract, high-level concept representing a communications channel to a file, device, or process.
For historical reasons, the type of the C data structure that represents
a stream is called FILE rather than "stream". Since most of
the library functions deal with objects of type FILE *, sometimes
the term file pointer is also used to mean "stream". This leads
to unfortunate confusion over terminology in many books on C. This
manual, however, is careful to use the terms "file" and "stream"
only in the technical sense.
The FILE type is declared in the header file `stdio.h'.
FILE
object holds all of the internal state information about the connection
to the associated file, including such things as the file position
indicator and buffering information. Each stream also has error and
end-of-file status indicators that can be tested with the ferror
and feof functions; see section End-Of-File and Errors.
FILE objects are allocated and managed internally by the
input/output library functions. Don't try to create your own objects of
type FILE; let the library do it. Your programs should
deal only with pointers to these objects (that is, FILE * values)
rather than the objects themselves.
When the main function of your program is invoked, it already has
three predefined streams open and available for use. These represent
the "standard" input and output channels that have been established
for the process.
These streams are declared in the header file `stdio.h'.
In the GNU system, you can specify what files or processes correspond to these streams using the pipe and redirection facilities provided by the shell. (The primitives shells use to implement these facilities are described in section File System Interface.) Most other operating systems provide similar mechanisms, but the details of how to use them can vary.
In the GNU C library, stdin, stdout, and stderr are
normal variables which you can set just like any others. For example,
to redirect the standard output to a file, you could do:
fclose (stdout);
stdout = fopen ("standard-output-file", "w");
Note however, that in other systems stdin, stdout, and
stderr are macros that you cannot assign to in the normal way.
But you can use freopen to get the effect of closing one and
reopening it. See section Opening Streams.
The three streams stdin, stdout, and stderr are not
unoriented at program start (see section Streams in Internationalized Applications).
Opening a file with the fopen function creates a new stream and
establishes a connection between the stream and a file. This may
involve creating a new file.
Everything described in this section is declared in the header file `stdio.h'.
fopen function opens a stream for I/O to the file
filename, and returns a pointer to the stream.
The opentype argument is a string that controls how the file is opened and specifies attributes of the resulting stream. It must begin with one of the following sequences of characters:
As you can see, `+' requests a stream that can do both input and
output. The ISO standard says that when using such a stream, you must
call fflush (see section Stream Buffering) or a file positioning
function such as fseek (see section File Positioning) when switching
from reading to writing or vice versa. Otherwise, internal buffers
might not be emptied properly. The GNU C library does not have this
limitation; you can do arbitrary reading and writing operations on a
stream in whatever order.
Additional characters may appear after these to specify flags for the call. Always put the mode (`r', `w+', etc.) first; that is the only part you are guaranteed will be understood by all systems.
The GNU C library defines one additional character for use in
opentype: the character `x' insists on creating a new
file--if a file filename already exists, fopen fails
rather than opening it. If you use `x' you are guaranteed that
you will not clobber an existing file. This is equivalent to the
O_EXCL option to the open function (see section Opening and Closing Files).
The character `b' in opentype has a standard meaning; it requests a binary stream rather than a text stream. But this makes no difference in POSIX systems (including the GNU system). If both `+' and `b' are specified, they can appear in either order. See section Text and Binary Streams.
If the opentype string contains the sequence
,ccs=STRING then STRING is taken as the name of a
coded character set and fopen will mark the stream as
wide-oriented which appropriate conversion functions in place to convert
from and to the character set STRING is place. Any other stream
is opened initially unoriented and the orientation is decided with the
first file operation. If the first operation is a wide character
operation, the stream is not only marked as wide-oriented, also the
conversion functions to convert to the coded character set used for the
current locale are loaded. This will not change anymore from this point
on even if the locale selected for the LC_CTYPE category is
changed.
Any other characters in opentype are simply ignored. They may be meaningful in other systems.
If the open fails, fopen returns a null pointer.
When the sources are compiling with _FILE_OFFSET_BITS == 64 on a
32 bit machine this function is in fact fopen64 since the LFS
interface replaces transparently the old interface.
You can have multiple streams (or file descriptors) pointing to the same file open at the same time. If you do only input, this works straightforwardly, but you must be careful if any output streams are included. See section Dangers of Mixing Streams and Descriptors. This is equally true whether the streams are in one program (not usual) or in several programs (which can easily happen). It may be advantageous to use the file locking facilities to avoid simultaneous access. See section File Locks.
fopen but the stream it returns a
pointer for is opened using open64. Therefore this stream can be
used even on files larger then @math{2^31} bytes on 32 bit machines.
Please note that the return type is still FILE *. There is no
special FILE type for the LFS interface.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name fopen
and so transparently replaces the old interface.
stdin, stdout, and stderr. In POSIX.1 systems this
value is determined by the OPEN_MAX parameter; see section General Capacity Limits. In BSD and GNU, it is controlled by the RLIMIT_NOFILE
resource limit; see section Limiting Resource Usage.
fclose and fopen.
It first closes the stream referred to by stream, ignoring any
errors that are detected in the process. (Because errors are ignored,
you should not use freopen on an output stream if you have
actually done any output using the stream.) Then the file named by
filename is opened with mode opentype as for fopen,
and associated with the same stream object stream.
If the operation fails, a null pointer is returned; otherwise,
freopen returns stream.
freopen has traditionally been used to connect a standard stream
such as stdin with a file of your own choice. This is useful in
programs in which use of a standard stream for certain purposes is
hard-coded. In the GNU C library, you can simply close the standard
streams and open new ones with fopen. But other systems lack
this ability, so using freopen is more portable.
When the sources are compiling with _FILE_OFFSET_BITS == 64 on a
32 bit machine this function is in fact freopen64 since the LFS
interface replaces transparently the old interface.
freopen. The only difference is that
on 32 bit machine the stream returned is able to read beyond the
@math{2^31} bytes limits imposed by the normal interface. It should be
noted that the stream pointed to by stream need not be opened
using fopen64 or freopen64 since its mode is not important
for this function.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name freopen
and so transparently replaces the old interface.
In some situations it is useful to know whether a given stream is available for reading or writing. This information is normally not available and would have to be remembered separately. Solaris introduced a few functions to get this information from the stream descriptor and these functions are also available in the GNU C library.
__freadable function determines whether the stream
stream was opened to allow reading. In this case the return value
is nonzero. For write-only streams the function returns zero.
This function is declared in `stdio_ext.h'.
__fwritable function determines whether the stream
stream was opened to allow writing. In this case the return value
is nonzero. For read-only streams the function returns zero.
This function is declared in `stdio_ext.h'.
For slightly different kind of problems there are two more functions. They provide even finer-grained information.
__freading function determines whether the stream
stream was last read from or whether it is opened read-only. In
this case the return value is nonzero, otherwise it is zero.
Determining whether a stream opened for reading and writing was last
used for writing allows to draw conclusions about the content about the
buffer, among other things.
This function is declared in `stdio_ext.h'.
__fwriting function determines whether the stream
stream was last written to or whether it is opened write-only. In
this case the return value is nonzero, otherwise it is zero.
This function is declared in `stdio_ext.h'.
When a stream is closed with fclose, the connection between the
stream and the file is cancelled. After you have closed a stream, you
cannot perform any additional operations on it.
fclose function returns
a value of 0 if the file was closed successfully, and EOF
if an error was detected.
It is important to check for errors when you call fclose to close
an output stream, because real, everyday errors can be detected at this
time. For example, when fclose writes the remaining buffered
output, it might get an error because the disk is full. Even if you
know the buffer is empty, errors can still occur when closing a file if
you are using NFS.
The function fclose is declared in `stdio.h'.
To close all streams currently available the GNU C Library provides another function.
fcloseall
function returns a value of 0 if all the files were closed
successfully, and EOF if an error was detected.
This function should be used only in special situations, e.g., when an error occurred and the program must be aborted. Normally each single stream should be closed separately so that problems with individual streams can be identified. It is also problematic since the standard streams (see section Standard Streams) will also be closed.
The function fcloseall is declared in `stdio.h'.
If the main function to your program returns, or if you call the
exit function (see section Normal Termination), all open streams are
automatically closed properly. If your program terminates in any other
manner, such as by calling the abort function (see section Aborting a Program) or from a fatal signal (see section Signal Handling), open streams
might not be closed properly. Buffered output might not be flushed and
files may be incomplete. For more information on buffering of streams,
see section Stream Buffering.
Streams can be used in multi-threaded applications in the same way they are used in single-threaded applications. But the programmer must be aware of a the possible complications. It is important to know about these also if the program one writes never use threads since the design and implementation of many stream functions is heavily influenced by the requirements added by multi-threaded programming.
The POSIX standard requires that by default the stream operations are atomic. I.e., issueing two stream operations for the same stream in two threads at the same time will cause the operations to be executed as if they were issued sequentially. The buffer operations performed while reading or writing are protected from other uses of the same stream. To do this each stream has an internal lock object which has to be (implicitly) acquired before any work can be done.
But there are situations where this is not enough and there are also situations where this is not wanted. The implicit locking is not enough if the program requires more than one stream function call to happen atomically. One example would be if an output line a program wants to generate is created by several function calls. The functions by themselves would ensure only atomicity of their own operation, but not atomicity over all the function calls. For this it is necessary to perform the stream locking in the application code.
flockfile function acquires the internal locking object
associated with the stream stream. This ensures that no other
thread can explicitly through flockfile/ftrylockfile or
implicit through a call of a stream function lock the stream. The
thread will block until the lock is acquired. An explicit call to
funlockfile has to be used to release the lock.
ftrylockfile function tries to acquire the internal locking
object associated with the stream stream just like
flockfile. But unlike flockfile this function does not
block if the lock is not available. ftrylockfile returns zero if
the lock was successfully acquired. Otherwise the stream is locked by
another thread.
funlockfile function releases the internal locking object of
the stream stream. The stream must have been locked before by a
call to flockfile or a successful call of ftrylockfile.
The implicit locking performed by the stream operations do not count.
The funlockfile function does not return an error status and the
behavior of a call for a stream which is not locked by the current
thread is undefined.
The following example shows how the functions above can be used to
generate an output line atomically even in multi-threaded applications
(yes, the same job could be done with one fprintf call but it is
sometimes not possible):
FILE *fp;
{
...
flockfile (fp);
fputs ("This is test number ", fp);
fprintf (fp, "%d\n", test);
funlockfile (fp)
}
Without the explicit locking it would be possible for another thread to
use the stream fp after the fputs call return and before
fprintf was called with the result that the number does not
follow the word `number'.
From this description it might already be clear that the locking objects
in streams are no simple mutexes. Since locking the same stream twice
in the same thread is allowed the locking objects must be equivalent to
recursive mutexes. These mutexes keep track of the owner and the number
of times the lock is acquired. The same number of funlockfile
calls by the same threads is necessary to unlock the stream completely.
For instance:
void
foo (FILE *fp)
{
ftrylockfile (fp);
fputs ("in foo\n", fp);
/* This is very wrong!!! */
funlockfile (fp);
}
It is important here that the funlockfile function is only called
if the ftrylockfile function succeeded in locking the stream. It
is therefore always wrong to ignore the result of ftrylockfile.
And it makes no sense since otherwise one would use flockfile.
The result of code like that above is that either funlockfile
tries to free a stream that hasn't been locked by the current thread or it
frees the stream prematurely. The code should look like this:
void
foo (FILE *fp)
{
if (ftrylockfile (fp) == 0)
{
fputs ("in foo\n", fp);
funlockfile (fp);
}
}
Now that we covered why it is necessary to have these locking it is necessary to talk about situations when locking is unwanted and what can be done. The locking operations (explicit or implicit) don't come for free. Even if a lock is not taken the cost is not zero. The operations which have to be performed require memory operations which are save in multi-processor environments. With the many local caches involved in such systems this is quite costly. So it is best to avoid the locking completely if it is known that the code using the stream is never used in a context where more than one thread can use the stream at one time. This can be determined most of the time for application code; for library code which can be used in many contexts one should default to be conservative and use locking.
There are two basic mechanisms to avoid locking. The first is to use
the _unlocked variants of the stream operations. The POSIX
standard defines quite a few of those and the GNU library adds a few
more. These variants of the functions behave just like the functions
with the name without the suffix except that they are not locking the
stream. Using these functions is very desirable since they are
potentially much faster. This is not only because the locking
operation itself is avoided. More importantly, functions like
putc and getc are very simple and tradionally (before the
introduction of threads) were implemented as macros which are very fast
if the buffer is not empty. With locking required these functions are
now no macros anymore (the code generated would be too much). But these
macros are still available with the same functionality under the new
names putc_unlocked and getc_unlocked. This possibly huge
difference of speed also suggests the use of the _unlocked
functions even if locking is required. The difference is that the
locking then has to be performed in the program:
void
foo (FILE *fp, char *buf)
{
flockfile (fp);
while (*buf != '/')
putc_unlocked (*buf++, fp);
funlockfile (fp);
}
If in this example the putc function would be used and the
explicit locking would be missing the putc function would have to
acquire the lock in every call, potentially many times depending on when
the loop terminates. Writing it the way illustrated above allows the
putc_unlocked macro to be used which means no locking and direct
manipulation of the buffer of the stream.
A second way to avoid locking is by using a non-standard function which was introduced in Solaris and is available in the GNU C library as well.
The __fsetlocking function can be used to select whether the
stream operations will implicitly acquire the locking object of the
stream stream. By default this is done but it can be disabled and
reinstated using this function. There are three values defined for the
type parameter.
FSETLOCKING_INTERNAL
stream will from now on use the default internal
locking. Every stream operation with exception of the _unlocked
variants will implicitly lock the stream.
FSETLOCKING_BYCALLER
__fsetlocking function returns the user is responsible
for locking the stream. None of the stream operations will implicitly
do this anymore until the state is set back to
FSETLOCKING_INTERNAL.
FSETLOCKING_QUERY
__fsetlocking only queries the current locking state of the
stream. The return value will be FSETLOCKING_INTERNAL or
FSETLOCKING_BYCALLER depending on the state.
The return value of __fsetlocking is either
FSETLOCKING_INTERNAL or FSETLOCKING_BYCALLER depending on
the state of the stream before the call.
This function and the values for the type parameter are declared in `stdio_ext.h'.
This function is especially useful when program code has to be used
which is written without knowledge about the _unlocked functions
(or if the programmer was to lazy to use them).
ISO C90 introduced the new type wchar_t to allow handling
larger character sets. What was missing was a possibility to output
strings of wchar_t directly. One had to convert them into
multibyte strings using mbstowcs (there was no mbsrtowcs
yet) and then use the normal stream functions. While this is doable it
is very cumbersome since performing the conversions is not trivial and
greatly increases program complexity and size.
The Unix standard early on (I think in XPG4.2) introduced two additional
format specifiers for the printf and scanf families of
functions. Printing and reading of single wide characters was made
possible using the %C specifier and wide character strings can be
handled with %S. These modifiers behave just like %c and
%s only that they expect the corresponding argument to have the
wide character type and that the wide character and string are
transformed into/from multibyte strings before being used.
This was a beginning but it is still not good enough. Not always is it
desirable to use printf and scanf. The other, smaller and
faster functions cannot handle wide characters. Second, it is not
possible to have a format string for printf and scanf
consisting of wide characters. The result is that format strings would
have to be generated if they have to contain non-basic characters.
In the Amendment 1 to ISO C90 a whole new set of functions was
added to solve the problem. Most of the stream functions got a
counterpart which take a wide character or wide character string instead
of a character or string respectively. The new functions operate on the
same streams (like stdout). This is different from the model of
the C++ runtime library where separate streams for wide and normal I/O
are used.
Being able to use the same stream for wide and normal operations comes
with a restriction: a stream can be used either for wide operations or
for normal operations. Once it is decided there is no way back. Only a
call to freopen or freopen64 can reset the
orientation. The orientation can be decided in three ways:
fread and fwrite functions) the stream is marked as not
wide oriented.
fwide function can be used to set the orientation either way.
It is important to never mix the use of wide and not wide operations on
a stream. There are no diagnostics issued. The application behavior
will simply be strange or the application will simply crash. The
fwide function can help avoiding this.
The fwide function can be used to set and query the state of the
orientation of the stream stream. If the mode parameter has
a positive value the streams get wide oriented, for negative values
narrow oriented. It is not possible to overwrite previous orientations
with fwide. I.e., if the stream stream was already
oriented before the call nothing is done.
If mode is zero the current orientation state is queried and nothing is changed.
The fwide function returns a negative value, zero, or a positive
value if the stream is narrow, not at all, or wide oriented
respectively.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
It is generally a good idea to orient a stream as early as possible.
This can prevent surprise especially for the standard streams
stdin, stdout, and stderr. If some library
function in some situations uses one of these streams and this use
orients the stream in a different way the rest of the application
expects it one might end up with hard to reproduce errors. Remember
that no errors are signal if the streams are used incorrectly. Leaving
a stream unoriented after creation is normally only necessary for
library functions which create streams which can be used in different
contexts.
When writing code which uses streams and which can be used in different contexts it is important to query the orientation of the stream before using it (unless the rules of the library interface demand a specific orientation). The following little, silly function illustrates this.
void
print_f (FILE *fp)
{
if (fwide (fp, 0) > 0)
/* Positive return value means wide orientation. */
fputwc (L'f', fp);
else
fputc ('f', fp);
}
Note that in this case the function print_f decides about the
orientation of the stream if it was unoriented before (will not happen
if the advise above is followed).
The encoding used for the wchar_t values is unspecified and the
user must not make any assumptions about it. For I/O of wchar_t
values this means that it is impossible to write these values directly
to the stream. This is not what follows from the ISO C locale model
either. What happens instead is that the bytes read from or written to
the underlying media are first converted into the internal encoding
chosen by the implementation for wchar_t. The external encoding
is determined by the LC_CTYPE category of the current locale or
by the `ccs' part of the mode specification given to fopen,
fopen64, freopen, or freopen64. How and when the
conversion happens is unspecified and it happens invisible to the user.
Since a stream is created in the unoriented state it has at that point
no conversion associated with it. The conversion which will be used is
determined by the LC_CTYPE category selected at the time the
stream is oriented. If the locales are changed at the runtime this
might produce surprising results unless one pays attention. This is
just another good reason to orient the stream explicitly as soon as
possible, perhaps with a call to fwide.
This section describes functions for performing character- and line-oriented output.
These narrow streams functions are declared in the header file `stdio.h' and the wide stream functions in `wchar.h'.
fputc function converts the character c to type
unsigned char, and writes it to the stream stream.
EOF is returned if a write error occurs; otherwise the
character c is returned.
fputwc function writes the wide character wc to the
stream stream. WEOF is returned if a write error occurs;
otherwise the character wc is returned.
fputc_unlocked function is equivalent to the fputc
function except that it does not implicitly lock the stream.
fputwc_unlocked function is equivalent to the fputwc
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fputc, except that most systems implement it as
a macro, making it faster. One consequence is that it may evaluate the
stream argument more than once, which is an exception to the
general rule for macros. putc is usually the best function to
use for writing a single character.
fputwc, except that it can be implement as
a macro, making it faster. One consequence is that it may evaluate the
stream argument more than once, which is an exception to the
general rule for macros. putwc is usually the best function to
use for writing a single wide character.
putc_unlocked function is equivalent to the putc
function except that it does not implicitly lock the stream.
putwc_unlocked function is equivalent to the putwc
function except that it does not implicitly lock the stream.
This function is a GNU extension.
putchar function is equivalent to putc with
stdout as the value of the stream argument.
putwchar function is equivalent to putwc with
stdout as the value of the stream argument.
putchar_unlocked function is equivalent to the putchar
function except that it does not implicitly lock the stream.
putwchar_unlocked function is equivalent to the putwchar
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fputs writes the string s to the stream
stream. The terminating null character is not written.
This function does not add a newline character, either.
It outputs only the characters in the string.
This function returns EOF if a write error occurs, and otherwise
a non-negative value.
For example:
fputs ("Are ", stdout);
fputs ("you ", stdout);
fputs ("hungry?\n", stdout);
outputs the text `Are you hungry?' followed by a newline.
fputws writes the wide character string ws to
the stream stream. The terminating null character is not written.
This function does not add a newline character, either. It
outputs only the characters in the string.
This function returns WEOF if a write error occurs, and otherwise
a non-negative value.
fputs_unlocked function is equivalent to the fputs
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fputws_unlocked function is equivalent to the fputws
function except that it does not implicitly lock the stream.
This function is a GNU extension.
puts function writes the string s to the stream
stdout followed by a newline. The terminating null character of
the string is not written. (Note that fputs does not
write a newline as this function does.)
puts is the most convenient function for printing simple
messages. For example:
puts ("This is a message.");
outputs the text `This is a message.' followed by a newline.
int) to
stream. It is provided for compatibility with SVID, but we
recommend you use fwrite instead (see section Block Input/Output).
This section describes functions for performing character-oriented input. These narrow streams functions are declared in the header file `stdio.h' and the wide character functions are declared in `wchar.h'.
These functions return an int or wint_t value (for narrow
and wide stream functions respectively) that is either a character of
input, or the special value EOF/WEOF (usually -1). For
the narrow stream functions it is important to store the result of these
functions in a variable of type int instead of char, even
when you plan to use it only as a character. Storing EOF in a
char variable truncates its value to the size of a character, so
that it is no longer distinguishable from the valid character
`(char) -1'. So always use an int for the result of
getc and friends, and check for EOF after the call; once
you've verified that the result is not EOF, you can be sure that
it will fit in a `char' variable without loss of information.
unsigned char from
the stream stream and returns its value, converted to an
int. If an end-of-file condition or read error occurs,
EOF is returned instead.
WEOF is returned instead.
fgetc_unlocked function is equivalent to the fgetc
function except that it does not implicitly lock the stream.
fgetwc_unlocked function is equivalent to the fgetwc
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fgetc, except that it is permissible (and
typical) for it to be implemented as a macro that evaluates the
stream argument more than once. getc is often highly
optimized, so it is usually the best function to use to read a single
character.
fgetwc, except that it is permissible for it to
be implemented as a macro that evaluates the stream argument more
than once. getwc can be highly optimized, so it is usually the
best function to use to read a single wide character.
getc_unlocked function is equivalent to the getc
function except that it does not implicitly lock the stream.
getwc_unlocked function is equivalent to the getwc
function except that it does not implicitly lock the stream.
This function is a GNU extension.
getchar function is equivalent to getc with stdin
as the value of the stream argument.
getwchar function is equivalent to getwc with stdin
as the value of the stream argument.
getchar_unlocked function is equivalent to the getchar
function except that it does not implicitly lock the stream.
getwchar_unlocked function is equivalent to the getwchar
function except that it does not implicitly lock the stream.
This function is a GNU extension.
Here is an example of a function that does input using fgetc. It
would work just as well using getc instead, or using
getchar () instead of fgetc (stdin). The code would
also work the same for the wide character stream functions.
int
y_or_n_p (const char *question)
{
fputs (question, stdout);
while (1)
{
int c, answer;
/* Write a space to separate answer from question. */
fputc (' ', stdout);
/* Read the first character of the line.
This should be the answer character, but might not be. */
c = tolower (fgetc (stdin));
answer = c;
/* Discard rest of input line. */
while (c != '\n' && c != EOF)
c = fgetc (stdin);
/* Obey the answer if it was valid. */
if (answer == 'y')
return 1;
if (answer == 'n')
return 0;
/* Answer was invalid: ask for valid answer. */
fputs ("Please answer y or n:", stdout);
}
}
int) from stream.
It's provided for compatibility with SVID. We recommend you use
fread instead (see section Block Input/Output). Unlike getc,
any int value could be a valid result. getw returns
EOF when it encounters end-of-file or an error, but there is no
way to distinguish this from an input word with value -1.
Since many programs interpret input on the basis of lines, it is convenient to have functions to read a line of text from a stream.
Standard C has functions to do this, but they aren't very safe: null
characters and even (for gets) long lines can confuse them. So
the GNU library provides the nonstandard getline function that
makes it easy to read lines reliably.
Another GNU extension, getdelim, generalizes getline. It
reads a delimited record, defined as everything through the next
occurrence of a specified delimiter character.
All these functions are declared in `stdio.h'.
*lineptr.
Before calling getline, you should place in *lineptr
the address of a buffer *n bytes long, allocated with
malloc. If this buffer is long enough to hold the line,
getline stores the line in this buffer. Otherwise,
getline makes the buffer bigger using realloc, storing the
new buffer address back in *lineptr and the increased size
back in *n.
See section Unconstrained Allocation.
If you set *lineptr to a null pointer, and *n
to zero, before the call, then getline allocates the initial
buffer for you by calling malloc.
In either case, when getline returns, *lineptr is
a char * which points to the text of the line.
When getline is successful, it returns the number of characters
read (including the newline, but not including the terminating null).
This value enables you to distinguish null characters that are part of
the line from the null character inserted as a terminator.
This function is a GNU extension, but it is the recommended way to read lines from a stream. The alternative standard functions are unreliable.
If an error occurs or end of file is reached without any bytes read,
getline returns -1.
getline except that the character which
tells it to stop reading is not necessarily newline. The argument
delimiter specifies the delimiter character; getdelim keeps
reading until it sees that character (or end of file).
The text is stored in lineptr, including the delimiter character
and a terminating null. Like getline, getdelim makes
lineptr bigger if it isn't big enough.
getline is in fact implemented in terms of getdelim, just
like this:
ssize_t
getline (char **lineptr, size_t *n, FILE *stream)
{
return getdelim (lineptr, n, '\n', stream);
}
fgets function reads characters from the stream stream
up to and including a newline character and stores them in the string
s, adding a null character to mark the end of the string. You
must supply count characters worth of space in s, but the
number of characters read is at most count - 1. The extra
character space is used to hold the null character at the end of the
string.
If the system is already at end of file when you call fgets, then
the contents of the array s are unchanged and a null pointer is
returned. A null pointer is also returned if a read error occurs.
Otherwise, the return value is the pointer s.
Warning: If the input data has a null character, you can't tell.
So don't use fgets unless you know the data cannot contain a null.
Don't use it to read files edited by the user because, if the user inserts
a null character, you should either handle it properly or print a clear
error message. We recommend using getline instead of fgets.
fgetws function reads wide characters from the stream
stream up to and including a newline character and stores them in
the string ws, adding a null wide character to mark the end of the
string. You must supply count wide characters worth of space in
ws, but the number of characters read is at most count
- 1. The extra character space is used to hold the null wide
character at the end of the string.
If the system is already at end of file when you call fgetws, then
the contents of the array ws are unchanged and a null pointer is
returned. A null pointer is also returned if a read error occurs.
Otherwise, the return value is the pointer ws.
Warning: If the input data has a null wide character (which are
null bytes in the input stream), you can't tell. So don't use
fgetws unless you know the data cannot contain a null. Don't use
it to read files edited by the user because, if the user inserts a null
character, you should either handle it properly or print a clear error
message.
fgets_unlocked function is equivalent to the fgets
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fgetws_unlocked function is equivalent to the fgetws
function except that it does not implicitly lock the stream.
This function is a GNU extension.
gets reads characters from the stream stdin
up to the next newline character, and stores them in the string s.
The newline character is discarded (note that this differs from the
behavior of fgets, which copies the newline character into the
string). If gets encounters a read error or end-of-file, it
returns a null pointer; otherwise it returns s.
Warning: The gets function is very dangerous
because it provides no protection against overflowing the string
s. The GNU library includes it for compatibility only. You
should always use fgets or getline instead. To
remind you of this, the linker (if using GNU ld) will issue a
warning whenever you use gets.
In parser programs it is often useful to examine the next character in the input stream without removing it from the stream. This is called "peeking ahead" at the input because your program gets a glimpse of the input it will read next.
Using stream I/O, you can peek ahead at input by first reading it and
then unreading it (also called pushing it back on the stream).
Unreading a character makes it available to be input again from the stream,
by the next call to fgetc or other input function on that stream.
Here is a pictorial explanation of unreading. Suppose you have a stream reading a file that contains just six characters, the letters `foobar'. Suppose you have read three characters so far. The situation looks like this:
f o o b a r
^
so the next input character will be `b'.
If instead of reading `b' you unread the letter `o', you get a situation like this:
f o o b a r
|
o--
^
so that the next input characters will be `o' and `b'.
If you unread `9' instead of `o', you get this situation:
f o o b a r
|
9--
^
so that the next input characters will be `9' and `b'.
ungetc To Do Unreading
The function to unread a character is called ungetc, because it
reverses the action of getc.
ungetc function pushes back the character c onto the
input stream stream. So the next input from stream will
read c before anything else.
If c is EOF, ungetc does nothing and just returns
EOF. This lets you call ungetc with the return value of
getc without needing to check for an error from getc.
The character that you push back doesn't have to be the same as the last
character that was actually read from the stream. In fact, it isn't
necessary to actually read any characters from the stream before
unreading them with ungetc! But that is a strange way to write
a program; usually ungetc is used only to unread a character
that was just read from the same stream.
The GNU C library only supports one character of pushback--in other
words, it does not work to call ungetc twice without doing input
in between. Other systems might let you push back multiple characters;
then reading from the stream retrieves the characters in the reverse
order that they were pushed.
Pushing back characters doesn't alter the file; only the internal
buffering for the stream is affected. If a file positioning function
(such as fseek, fseeko or rewind; see section File Positioning) is called, any pending pushed-back characters are
discarded.
Unreading a character on a stream that is at end of file clears the end-of-file indicator for the stream, because it makes the character of input available. After you read that character, trying to read again will encounter end of file.
ungetwc function behaves just like ungetc just that it
pushes back a wide character.
Here is an example showing the use of getc and ungetc to
skip over whitespace characters. When this function reaches a
non-whitespace character, it unreads that character to be seen again on
the next read operation on the stream.
#include <stdio.h>
#include <ctype.h>
void
skip_whitespace (FILE *stream)
{
int c;
do
/* No need to check for EOF because it is not
isspace, and ungetc ignores EOF. */
c = getc (stream);
while (isspace (c));
ungetc (c, stream);
}
This section describes how to do input and output operations on blocks of data. You can use these functions to read and write binary data, as well as to read and write text in fixed-size blocks instead of by characters or lines.
Binary files are typically used to read and write blocks of data in the same format as is used to represent the data in a running program. In other words, arbitrary blocks of memory--not just character or string objects--can be written to a binary file, and meaningfully read in again by the same program.
Storing data in binary form is often considerably more efficient than using the formatted I/O functions. Also, for floating-point numbers, the binary form avoids possible loss of precision in the conversion process. On the other hand, binary files can't be examined or modified easily using many standard file utilities (such as text editors), and are not portable between different implementations of the language, or different kinds of computers.
These functions are declared in `stdio.h'.
If fread encounters end of file in the middle of an object, it
returns the number of complete objects read, and discards the partial
object. Therefore, the stream remains at the actual end of the file.
fread_unlocked function is equivalent to the fread
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fwrite_unlocked function is equivalent to the fwrite
function except that it does not implicitly lock the stream.
This function is a GNU extension.
The functions described in this section (printf and related
functions) provide a convenient way to perform formatted output. You
call printf with a format string or template string
that specifies how to format the values of the remaining arguments.
Unless your program is a filter that specifically performs line- or
character-oriented processing, using printf or one of the other
related functions described in this section is usually the easiest and
most concise way to perform output. These functions are especially
useful for printing error messages, tables of data, and the like.
The printf function can be used to print any number of arguments.
The template string argument you supply in a call provides
information not only about the number of additional arguments, but also
about their types and what style should be used for printing them.
Ordinary characters in the template string are simply written to the output stream as-is, while conversion specifications introduced by a `%' character in the template cause subsequent arguments to be formatted and written to the output stream. For example,
int pct = 37;
char filename[] = "foo.txt";
printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n",
filename, pct);
produces output like
Processing of `foo.txt' is 37% finished. Please be patient.
This example shows the use of the `%d' conversion to specify that
an int argument should be printed in decimal notation, the
`%s' conversion to specify printing of a string argument, and
the `%%' conversion to print a literal `%' character.
There are also conversions for printing an integer argument as an unsigned value in octal, decimal, or hexadecimal radix (`%o', `%u', or `%x', respectively); or as a character value (`%c').
Floating-point numbers can be printed in normal, fixed-point notation using the `%f' conversion or in exponential notation using the `%e' conversion. The `%g' conversion uses either `%e' or `%f' format, depending on what is more appropriate for the magnitude of the particular number.
You can control formatting more precisely by writing modifiers between the `%' and the character that indicates which conversion to apply. These slightly alter the ordinary behavior of the conversion. For example, most conversion specifications permit you to specify a minimum field width and a flag indicating whether you want the result left- or right-justified within the field.
The specific flags and modifiers that are permitted and their interpretation vary depending on the particular conversion. They're all described in more detail in the following sections. Don't worry if this all seems excessively complicated at first; you can almost always get reasonable free-format output without using any of the modifiers at all. The modifiers are mostly used to make the output look "prettier" in tables.
This section provides details about the precise syntax of conversion
specifications that can appear in a printf template
string.
Characters in the template string that are not part of a conversion specification are printed as-is to the output stream. Multibyte character sequences (see section Character Set Handling) are permitted in a template string.
The conversion specifications in a printf template string have
the general form:
% [ param-no $] flags width [ . precision ] type conversion
For example, in the conversion specifier `%-10.8ld', the `-'
is a flag, `10' specifies the field width, the precision is
`8', the letter `l' is a type modifier, and `d' specifies
the conversion style. (This particular type specifier says to
print a long int argument in decimal notation, with a minimum of
8 digits left-justified in a field at least 10 characters wide.)
In more detail, output conversion specifications consist of an initial `%' character followed in sequence by:
printf function are assigned to the
formats in the order of appearance in the format string. But in some
situations (such as message translation) this is not desirable and this
extension allows an explicit parameter to be specified.
The param-no part of the format must be an integer in the range of
1 to the maximum number of arguments present to the function call. Some
implementations limit this number to a certainly upper bound. The exact
limit can be retrieved by the following constant.
ARGMAX is the maximum value allowed for the
specification of an positional parameter in a printf call. The
actual value in effect at runtime can be retrieved by using
sysconf using the _SC_NL_ARGMAX parameter see section Definition of sysconf.
Some system have a quite low limit such as @math{9} for System V
systems. The GNU C library has no real limit.
int.
If the value is negative, this means to set the `-' flag (see
below) and to use the absolute value as the field width.
int, and is ignored
if it is negative. If you specify `*' for both the field width and
precision, the field width argument precedes the precision argument.
Other C library versions may not recognize this syntax.
int,
but you can specify `h', `l', or `L' for other integer
types.)
The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they use.
With the `-Wformat' option, the GNU C compiler checks calls to
printf and related functions. It examines the format string and
verifies that the correct number and types of arguments are supplied.
There is also a GNU C syntax to tell the compiler that a function you
write uses a printf-style format string.
See section `Declaring Attributes of Functions' in Using GNU CC, for more information.
Here is a table summarizing what all the different conversions do:
scanf for input
(see section Table of Input Conversions).
errno.
(This is a GNU extension.)
See section Other Output Conversions.
If the syntax of a conversion specification is invalid, unpredictable things will happen, so don't do this. If there aren't enough function arguments provided to supply values for all the conversion specifications in the template string, or if the arguments are not of the correct types, the results are unpredictable. If you supply more arguments than conversion specifications, the extra argument values are simply ignored; this is sometimes useful.
This section describes the options for the `%d', `%i', `%o', `%u', `%x', and `%X' conversion specifications. These conversions print integers in various formats.
The `%d' and `%i' conversion specifications both print an
int argument as a signed decimal number; while `%o',
`%u', and `%x' print the argument as an unsigned octal,
decimal, or hexadecimal number (respectively). The `%X' conversion
specification is just like `%x' except that it uses the characters
`ABCDEF' as digits instead of `abcdef'.
The following flags are meaningful:
strtoul function (see section Parsing of Integers) and scanf with the `%i' conversion
(see section Numeric Input Conversions).
LC_NUMERIC category; see section Generic Numeric Formatting Parameters. This flag is a
GNU extension.
If a precision is supplied, it specifies the minimum number of digits to appear; leading zeros are produced if necessary. If you don't specify a precision, the number is printed with as many digits as it needs. If you convert a value of zero with an explicit precision of zero, then no characters at all are produced.
Without a type modifier, the corresponding argument is treated as an
int (for the signed conversions `%i' and `%d') or
unsigned int (for the unsigned conversions `%o', `%u',
`%x', and `%X'). Recall that since printf and friends
are variadic, any char and short arguments are
automatically converted to int by the default argument
promotions. For arguments of other integer types, you can use these
modifiers:
signed char or unsigned
char, as appropriate. A char argument is converted to an
int or unsigned int by the default argument promotions
anyway, but the `h' modifier says to convert it back to a
char again.
This modifier was introduced in ISO C99.
short int or unsigned
short int, as appropriate. A short argument is converted to an
int or unsigned int by the default argument promotions
anyway, but the `h' modifier says to convert it back to a
short again.
intmax_t or uintmax_t, as
appropriate.
This modifier was introduced in ISO C99.
long int or unsigned long
int, as appropriate. Two `l' characters is like the `L'
modifier, below.
If used with `%c' or `%s' the corresponding parameter is
considered as a wide character or wide character string respectively.
This use of `l' was introduced in Amendment 1 to ISO C90.
long long int. (This type is
an extension supported by the GNU C compiler. On systems that don't
support extra-long integers, this is the same as long int.)
The `q' modifier is another name for the same thing, which comes
from 4.4 BSD; a long long int is sometimes called a "quad"
int.
ptrdiff_t.
This modifier was introduced in ISO C99.
size_t.
`z' was introduced in ISO C99. `Z' is a GNU extension
predating this addition and should not be used in new code.
Here is an example. Using the template string:
"|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n"
to print numbers using the different options for the `%d' conversion gives results like:
| 0|0 | +0|+0 | 0|00000| | 00|0| | 1|1 | +1|+1 | 1|00001| 1| 01|1| | -1|-1 | -1|-1 | -1|-0001| -1| -01|-1| |100000|100000|+100000| 100000|100000|100000|100000|100000|
In particular, notice what happens in the last case where the number is too large to fit in the minimum field width specified.
Here are some more examples showing how unsigned integers print under various format options, using the template string:
"|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n"
| 0| 0| 0| 0| 0| 0x0| 0X0|0x00000000| | 1| 1| 1| 1| 01| 0x1| 0X1|0x00000001| |100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|
This section discusses the conversion specifications for floating-point numbers: the `%f', `%e', `%E', `%g', and `%G' conversions.
The `%f' conversion prints its argument in fixed-point notation,
producing output of the form
[-]ddd.ddd,
where the number of digits following the decimal point is controlled
by the precision you specify.
The `%e' conversion prints its argument in exponential notation,
producing output of the form
[-]d.ddde[+|-]dd.
Again, the number of digits following the decimal point is controlled by
the precision. The exponent always contains at least two digits. The
`%E' conversion is similar but the exponent is marked with the letter
`E' instead of `e'.
The `%g' and `%G' conversions print the argument in the style of `%e' or `%E' (respectively) if the exponent would be less than -4 or greater than or equal to the precision; otherwise they use the `%f' style. Trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit.
The `%a' and `%A' conversions are meant for representing
floating-point numbers exactly in textual form so that they can be
exchanged as texts between different programs and/or machines. The
numbers are represented is the form
[-]0xh.hhhp[+|-]dd.
At the left of the decimal-point character exactly one digit is print.
This character is only 0 if the number is denormalized.
Otherwise the value is unspecified; it is implementation dependent how many
bits are used. The number of hexadecimal digits on the right side of
the decimal-point character is equal to the precision. If the precision
is zero it is determined to be large enough to provide an exact
representation of the number (or it is large enough to distinguish two
adjacent values if the FLT_RADIX is not a power of 2,
see section Floating Point Parameters). For the `%a' conversion
lower-case characters are used to represent the hexadecimal number and
the prefix and exponent sign are printed as 0x and p
respectively. Otherwise upper-case characters are used and 0X
and P are used for the representation of prefix and exponent
string. The exponent to the base of two is printed as a decimal number
using at least one digit but at most as many digits as necessary to
represent the value exactly.
If the value to be printed represents infinity or a NaN, the output is
[-]inf or nan respectively if the conversion
specifier is `%a', `%e', `%f', or `%g' and it is
[-]INF or NAN respectively if the conversion is
`%A', `%E', or `%G'.
The following flags can be used to modify the behavior:
LC_NUMERIC category;
see section Generic Numeric Formatting Parameters. This flag is a GNU extension.
The precision specifies how many digits follow the decimal-point
character for the `%f', `%e', and `%E' conversions. For
these conversions, the default precision is 6. If the precision
is explicitly 0, this suppresses the decimal point character
entirely. For the `%g' and `%G' conversions, the precision
specifies how many significant digits to print. Significant digits are
the first digit before the decimal point, and all the digits after it.
If the precision is 0 or not specified for `%g' or `%G',
it is treated like a value of 1. If the value being printed
cannot be expressed accurately in the specified number of digits, the
value is rounded to the nearest number that fits.
Without a type modifier, the floating-point conversions use an argument
of type double. (By the default argument promotions, any
float arguments are automatically converted to double.)
The following type modifier is supported:
long
double.
Here are some examples showing how numbers print using the various floating-point conversions. All of the numbers were printed using this template string:
"|%13.4a|%13.4f|%13.4e|%13.4g|\n"
Here is the output:
| 0x0.0000p+0| 0.0000| 0.0000e+00| 0| | 0x1.0000p-1| 0.5000| 5.0000e-01| 0.5| | 0x1.0000p+0| 1.0000| 1.0000e+00| 1| | -0x1.0000p+0| -1.0000| -1.0000e+00| -1| | 0x1.9000p+6| 100.0000| 1.0000e+02| 100| | 0x1.f400p+9| 1000.0000| 1.0000e+03| 1000| | 0x1.3880p+13| 10000.0000| 1.0000e+04| 1e+04| | 0x1.81c8p+13| 12345.0000| 1.2345e+04| 1.234e+04| | 0x1.86a0p+16| 100000.0000| 1.0000e+05| 1e+05| | 0x1.e240p+16| 123456.0000| 1.2346e+05| 1.235e+05|
Notice how the `%g' conversion drops trailing zeros.
This section describes miscellaneous conversions for printf.
The `%c' conversion prints a single character. In case there is no
`l' modifier the int argument is first converted to an
unsigned char. Then, if used in a wide stream function, the
character is converted into the corresponding wide character. The
`-' flag can be used to specify left-justification in the field,
but no other flags are defined, and no precision or type modifier can be
given. For example:
printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o');
prints `hello'.
If there is a `l' modifier present the argument is expected to be
of type wint_t. If used in a multibyte function the wide
character is converted into a multibyte character before being added to
the output. In this case more than one output byte can be produced.
The `%s' conversion prints a string. If no `l' modifier is
present the corresponding argument must be of type char * (or
const char *). If used in a wide stream function the string is
first converted in a wide character string. A precision can be
specified to indicate the maximum number of characters to write;
otherwise characters in the string up to but not including the
terminating null character are written to the output stream. The
`-' flag can be used to specify left-justification in the field,
but no other flags or type modifiers are defined for this conversion.
For example:
printf ("%3s%-6s", "no", "where");
prints ` nowhere '.
If there is a `l' modifier present the argument is expected to be of type wchar_t (or const wchar_t *).
If you accidentally pass a null pointer as the argument for a `%s' conversion, the GNU library prints it as `(null)'. We think this is more useful than crashing. But it's not good practice to pass a null argument intentionally.
The `%m' conversion prints the string corresponding to the error
code in errno. See section Error Messages. Thus:
fprintf (stderr, "can't open `%s': %m\n", filename);
is equivalent to:
fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));
The `%m' conversion is a GNU C library extension.
The `%p' conversion prints a pointer value. The corresponding
argument must be of type void *. In practice, you can use any
type of pointer.
In the GNU system, non-null pointers are printed as unsigned integers, as if a `%#x' conversion were used. Null pointers print as `(nil)'. (Pointers might print differently in other systems.)
For example:
printf ("%p", "testing");
prints `0x' followed by a hexadecimal number--the address of the
string constant "testing". It does not print the word
`testing'.
You can supply the `-' flag with the `%p' conversion to specify left-justification, but no other flags, precision, or type modifiers are defined.
The `%n' conversion is unlike any of the other output conversions.
It uses an argument which must be a pointer to an int, but
instead of printing anything it stores the number of characters printed
so far by this call at that location. The `h' and `l' type
modifiers are permitted to specify that the argument is of type
short int * or long int * instead of int *, but no
flags, field width, or precision are permitted.
For example,
int nchar;
printf ("%d %s%n\n", 3, "bears", &nchar);
prints:
3 bears
and sets nchar to 7, because `3 bears' is seven
characters.
The `%%' conversion prints a literal `%' character. This conversion doesn't use an argument, and no flags, field width, precision, or type modifiers are permitted.
This section describes how to call printf and related functions.
Prototypes for these functions are in the header file `stdio.h'.
Because these functions take a variable number of arguments, you
must declare prototypes for them before using them. Of course,
the easiest way to make sure you have all the right prototypes is to
just include `stdio.h'.
printf function prints the optional arguments under the
control of the template string template to the stream
stdout. It returns the number of characters printed, or a
negative value if there was an output error.
wprintf function prints the optional arguments under the
control of the wide template string template to the stream
stdout. It returns the number of wide characters printed, or a
negative value if there was an output error.
printf, except that the output is
written to the stream stream instead of stdout.
wprintf, except that the output is
written to the stream stream instead of stdout.
printf, except that the output is stored in the character
array s instead of written to a stream. A null character is written
to mark the end of the string.
The sprintf function returns the number of characters stored in
the array s, not including the terminating null character.
The behavior of this function is undefined if copying takes place between objects that overlap--for example, if s is also given as an argument to be printed under control of the `%s' conversion. See section Copying and Concatenation.
Warning: The sprintf function can be dangerous
because it can potentially output more characters than can fit in the
allocation size of the string s. Remember that the field width
given in a conversion specification is only a minimum value.
To avoid this problem, you can use snprintf or asprintf,
described below.
wprintf, except that the output is stored in the
wide character array ws instead of written to a stream. A null
wide character is written to mark the end of the string. The size
argument specifies the maximum number of characters to produce. The
trailing null character is counted towards this limit, so you should
allocate at least size wide characters for the string ws.
The return value is the number of characters which would be generated for the given input, excluding the trailing null. If this value is greater or equal to size, not all characters from the result have been stored in ws. You should try again with a bigger output string.
Note that the corresponding narrow stream function takes fewer
parameters. swprintf in fact corresponds to the snprintf
function. Since the sprintf function can be dangerous and should
be avoided the ISO C committee refused to make the same mistake
again and decided to not define an function exactly corresponding to
sprintf.
snprintf function is similar to sprintf, except that
the size argument specifies the maximum number of characters to
produce. The trailing null character is counted towards this limit, so
you should allocate at least size characters for the string s.
The return value is the number of characters which would be generated for the given input, excluding the trailing null. If this value is greater or equal to size, not all characters from the result have been stored in s. You should try again with a bigger output string. Here is an example of doing this:
/* Construct a message describing the value of a variable
whose name is name and whose value is value. */
char *
make_message (char *name, char *value)
{
/* Guess we need no more than 100 chars of space. */
int size = 100;
char *buffer = (char *) xmalloc (size);
int nchars;
if (buffer == NULL)
return NULL;
/* Try to print in the allocated space. */
nchars = snprintf (buffer, size, "value of %s is %s",
name, value);
if (nchars >= size)
{
/* Reallocate buffer now that we know
how much space is needed. */
buffer = (char *) xrealloc (buffer, nchars + 1);
if (buffer != NULL)
/* Try again. */
snprintf (buffer, size, "value of %s is %s",
name, value);
}
/* The last call worked, return the string. */
return buffer;
}
In practice, it is often easier just to use asprintf, below.
Attention: In versions of the GNU C library prior to 2.1 the
return value is the number of characters stored, not including the
terminating null; unless there was not enough space in s to
store the result in which case -1 is returned. This was
changed in order to comply with the ISO C99 standard.
The functions in this section do formatted output and place the results in dynamically allocated memory.
sprintf, except that it dynamically
allocates a string (as with malloc; see section Unconstrained Allocation) to hold the output, instead of putting the output in a
buffer you allocate in advance. The ptr argument should be the
address of a char * object, and asprintf stores a pointer
to the newly allocated string at that location.
The return value is the number of characters allocated for the buffer, or less than zero if an error occured. Usually this means that the buffer could not be allocated.
Here is how to use asprintf to get the same result as the
snprintf example, but more easily:
/* Construct a message describing the value of a variable
whose name is name and whose value is value. */
char *
make_message (char *name, char *value)
{
char *result;
if (asprintf (&result, "value of %s is %s", name, value) < 0)
return NULL;
return result;
}
asprintf, except that it uses the
obstack obstack to allocate the space. See section Obstacks.
The characters are written onto the end of the current object.
To get at them, you must finish the object with obstack_finish
(see section Growing Objects).
The functions vprintf and friends are provided so that you can
define your own variadic printf-like functions that make use of
the same internals as the built-in formatted output functions.
The most natural way to define such functions would be to use a language
construct to say, "Call printf and pass this template plus all
of my arguments after the first five." But there is no way to do this
in C, and it would be hard to provide a way, since at the C language
level there is no way to tell how many arguments your function received.
Since that method is impossible, we provide alternative functions, the
vprintf series, which lets you pass a va_list to describe
"all of my arguments after the first five."
When it is sufficient to define a macro rather than a real function, the GNU C compiler provides a way to do this much more easily with macros. For example:
#define myprintf(a, b, c, d, e, rest...) \
printf (mytemplate , ## rest...)
See section `Macros with Variable Numbers of Arguments' in Using GNU CC, for details. But this is limited to macros, and does not apply to real functions at all.
Before calling vprintf or the other functions listed in this
section, you must call va_start (see section Variadic Functions) to initialize a pointer to the variable arguments. Then you
can call va_arg to fetch the arguments that you want to handle
yourself. This advances the pointer past those arguments.
Once your va_list pointer is pointing at the argument of your
choice, you are ready to call vprintf. That argument and all
subsequent arguments that were passed to your function are used by
vprintf along with the template that you specified separately.
In some other systems, the va_list pointer may become invalid
after the call to vprintf, so you must not use va_arg
after you call vprintf. Instead, you should call va_end
to retire the pointer from service. However, you can safely call
va_start on another pointer variable and begin fetching the
arguments again through that pointer. Calling vprintf does not
destroy the argument list of your function, merely the particular
pointer that you passed to it.
GNU C does not have such restrictions. You can safely continue to fetch
arguments from a va_list pointer after passing it to
vprintf, and va_end is a no-op. (Note, however, that
subsequent va_arg calls will fetch the same arguments which
vprintf previously used.)
Prototypes for these functions are declared in `stdio.h'.
printf except that, instead of taking
a variable number of arguments directly, it takes an argument list
pointer ap.
wprintf except that, instead of taking
a variable number of arguments directly, it takes an argument list
pointer ap.
fprintf with the variable argument list
specified directly as for vprintf.
fwprintf with the variable argument list
specified directly as for vwprintf.
sprintf with the variable argument list
specified directly as for vprintf.
swprintf with the variable argument list
specified directly as for vwprintf.
snprintf with the variable argument list
specified directly as for vprintf.
vasprintf function is the equivalent of asprintf with the
variable argument list specified directly as for vprintf.
obstack_vprintf function is the equivalent of
obstack_printf with the variable argument list specified directly
as for vprintf.
Here's an example showing how you might use vfprintf. This is a
function that prints error messages to the stream stderr, along
with a prefix indicating the name of the program
(see section Error Messages, for a description of
program_invocation_short_name).
#include <stdio.h>
#include <stdarg.h>
void
eprintf (const char *template, ...)
{
va_list ap;
extern char *program_invocation_short_name;
fprintf (stderr, "%s: ", program_invocation_short_name);
va_start (ap, template);
vfprintf (stderr, template, ap);
va_end (ap);
}
You could call eprintf like this:
eprintf ("file `%s' does not exist\n", filename);
In GNU C, there is a special construct you can use to let the compiler
know that a function uses a printf-style format string. Then it
can check the number and types of arguments in each call to the
function, and warn you when they do not match the format string.
For example, take this declaration of eprintf:
void eprintf (const char *template, ...)
__attribute__ ((format (printf, 1, 2)));
This tells the compiler that eprintf uses a format string like
printf (as opposed to scanf; see section Formatted Input);
the format string appears as the first argument;
and the arguments to satisfy the format begin with the second.
See section `Declaring Attributes of Functions' in Using GNU CC, for more information.
You can use the function parse_printf_format to obtain
information about the number and types of arguments that are expected by
a given template string. This function permits interpreters that
provide interfaces to printf to avoid passing along invalid
arguments from the user's program, which could cause a crash.
All the symbols described in this section are declared in the header file `printf.h'.
printf template string template.
The information is stored in the array argtypes; each element of
this array describes one argument. This information is encoded using
the various `PA_' macros, listed below.
The argument n specifies the number of elements in the array
argtypes. This is the maximum number of elements that
parse_printf_format will try to write.
parse_printf_format returns the total number of arguments required
by template. If this number is greater than n, then the
information returned describes only the first n arguments. If you
want information about additional arguments, allocate a bigger
array and call parse_printf_format again.
The argument types are encoded as a combination of a basic type and modifier flag bits.
(argtypes[i] & PA_FLAG_MASK) to extract just the
flag bits for an argument, or (argtypes[i] & ~PA_FLAG_MASK) to
extract just the basic type code.
Here are symbolic constants that represent the basic types; they stand for integer values.
PA_INT
int.
PA_CHAR
int, cast to char.
PA_STRING
char *, a null-terminated string.
PA_POINTER
void *, an arbitrary pointer.
PA_FLOAT
float.
PA_DOUBLE
double.
PA_LAST
PA_LAST. For example, if you have data types `foo'
and `bar' with their own specialized printf conversions,
you could define encodings for these types as:
#define PA_FOO PA_LAST #define PA_BAR (PA_LAST + 1)
Here are the flag bits that modify a basic type. They are combined with the code for the basic type using inclusive-or.
PA_FLAG_PTR
PA_FLAG_SHORT
short. (This corresponds to the `h' type modifier.)
PA_FLAG_LONG
long. (This corresponds to the `l' type modifier.)
PA_FLAG_LONG_LONG
long long. (This corresponds to the `L' type modifier.)
PA_FLAG_LONG_DOUBLE
PA_FLAG_LONG_LONG, used by convention with
a base type of PA_DOUBLE to indicate a type of long double.
Here is an example of decoding argument types for a format string. We
assume this is part of an interpreter which contains arguments of type
NUMBER, CHAR, STRING and STRUCTURE (and
perhaps others which are not valid here).
/* Test whether the nargs specified objects
in the vector args are valid
for the format string format:
if so, return 1.
If not, return 0 after printing an error message. */
int
validate_args (char *format, int nargs, OBJECT *args)
{
int *argtypes;
int nwanted;
/* Get the information about the arguments.
Each conversion specification must be at least two characters
long, so there cannot be more specifications than half the
length of the string. */
argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int));
nwanted = parse_printf_format (string, nelts, argtypes);
/* Check the number of arguments. */
if (nwanted > nargs)
{
error ("too few arguments (at least %d required)", nwanted);
return 0;
}
/* Check the C type wanted for each argument
and see if the object given is suitable. */
for (i = 0; i < nwanted; i++)
{
int wanted;
if (argtypes[i] & PA_FLAG_PTR)
wanted = STRUCTURE;
else
switch (argtypes[i] & ~PA_FLAG_MASK)
{
case PA_INT:
case PA_FLOAT:
case PA_DOUBLE:
wanted = NUMBER;
break;
case PA_CHAR:
wanted = CHAR;
break;
case PA_STRING:
wanted = STRING;
break;
case PA_POINTER:
wanted = STRUCTURE;
break;
}
if (TYPE (args[i]) != wanted)
{
error ("type mismatch for arg number %d", i);
return 0;
}
}
return 1;
}
printf
The GNU C library lets you define your own custom conversion specifiers
for printf template strings, to teach printf clever ways
to print the important data structures of your program.
The way you do this is by registering the conversion with the function
register_printf_function; see section Registering New Conversions.
One of the arguments you pass to this function is a pointer to a handler
function that produces the actual output; see section Defining the Output Handler, for information on how to write this function.
You can also install a function that just returns information about the number and type of arguments expected by the conversion specifier. See section Parsing a Template String, for information about this.
The facilities of this section are declared in the header file `printf.h'.
Portability Note: The ability to extend the syntax of
printf template strings is a GNU extension. ISO standard C has
nothing similar.
The function to register a new output conversion is
register_printf_function, declared in `printf.h'.
'Y', it defines the conversion `%Y'.
You can redefine the built-in conversions like `%s', but flag
characters like `#' and type modifiers like `l' can never be
used as conversions; calling register_printf_function for those
characters has no effect. It is advisable not to use lowercase letters,
since the ISO C standard warns that additional lowercase letters may be
standardized in future editions of the standard.
The handler-function is the function called by printf and
friends when this conversion appears in a template string.
See section Defining the Output Handler, for information about how to define
a function to pass as this argument. If you specify a null pointer, any
existing handler function for spec is removed.
The arginfo-function is the function called by
parse_printf_format when this conversion appears in a
template string. See section Parsing a Template String, for information
about this.
Attention: In the GNU C library versions before 2.0 the
arginfo-function function did not need to be installed unless
the user used the parse_printf_format function. This has changed.
Now a call to any of the printf functions will call this
function when this format specifier appears in the format string.
The return value is 0 on success, and -1 on failure
(which occurs if spec is out of range).
You can redefine the standard output conversions, but this is probably not a good idea because of the potential for confusion. Library routines written by other people could break if you do this.
If you define a meaning for `%A', what if the template contains `%+23A' or `%-#A'? To implement a sensible meaning for these, the handler when called needs to be able to get the options specified in the template.
Both the handler-function and arginfo-function accept an
argument that points to a struct printf_info, which contains
information about the options appearing in an instance of the conversion
specifier. This data type is declared in the header file
`printf.h'.
printf template
string to the handler and arginfo functions for that specifier. It
contains the following members:
int prec
-1 if no precision
was specified. If the precision was given as `*', the
printf_info structure passed to the handler function contains the
actual value retrieved from the argument list. But the structure passed
to the arginfo function contains a value of INT_MIN, since the
actual value is not known.
int width
0 if no
width was specified. If the field width was given as `*', the
printf_info structure passed to the handler function contains the
actual value retrieved from the argument list. But the structure passed
to the arginfo function contains a value of INT_MIN, since the
actual value is not known.
wchar_t spec
unsigned int is_long_double
long long int, as opposed to long double for floating
point conversions.
unsigned int is_char
unsigned int is_short
unsigned int is_long
unsigned int alt
unsigned int space
unsigned int left
unsigned int showsign
unsigned int group
unsigned int extra
printf function this variable always contains the value
0.
unsigned int wide
wchar_t pad
'0' if the `0' flag was specified, and
' ' otherwise.
Now let's look at how to define the handler and arginfo functions
which are passed as arguments to register_printf_function.
Compatibility Note: The interface changed in GNU libc
version 2.0. Previously the third argument was of type
va_list *.
You should define your handler functions with a prototype like:
int function (FILE *stream, const struct printf_info *info,
const void *const *args)
The stream argument passed to the handler function is the stream to which it should write output.
The info argument is a pointer to a structure that contains information about the various options that were included with the conversion in the template string. You should not modify this structure inside your handler function. See section Conversion Specifier Options, for a description of this data structure.
The args is a vector of pointers to the arguments data. The number of arguments was determined by calling the argument information function provided by the user.
Your handler function should return a value just like printf
does: it should return the number of characters it has written, or a
negative value to indicate an error.
If you are going to use parse_printf_format in your
application, you must also define a function to pass as the
arginfo-function argument for each new conversion you install with
register_printf_function.
You have to define these functions with a prototype like:
int function (const struct printf_info *info,
size_t n, int *argtypes)
The return value from the function should be the number of arguments the
conversion expects. The function should also fill in no more than
n elements of the argtypes array with information about the
types of each of these arguments. This information is encoded using the
various `PA_' macros. (You will notice that this is the same
calling convention parse_printf_format itself uses.)
printf Extension Example
Here is an example showing how to define a printf handler function.
This program defines a data structure called a Widget and
defines the `%W' conversion to print information about Widget *
arguments, including the pointer value and the name stored in the data
structure. The `%W' conversion supports the minimum field width and
left-justification options, but ignores everything else.
#include <stdio.h>
#include <stdlib.h>
#include <printf.h>
typedef struct
{
char *name;
}
Widget;
int
print_widget (FILE *stream,
const struct printf_info *info,
const void *const *args)
{
const Widget *w;
char *buffer;
int len;
/* Format the output into a string. */
w = *((const Widget **) (args[0]));
len = asprintf (&buffer, "<Widget %p: %s>", w, w->name);
if (len == -1)
return -1;
/* Pad to the minimum field width and print to the stream. */
len = fprintf (stream, "%*s",
(info->left ? -info->width : info->width),
buffer);
/* Clean up and return. */
free (buffer);
return len;
}
int
print_widget_arginfo (const struct printf_info *info, size_t n,
int *argtypes)
{
/* We always take exactly one argument and this is a pointer to the
structure.. */
if (n > 0)
argtypes[0] = PA_POINTER;
return 1;
}
int
main (void)
{
/* Make a widget to print. */
Widget mywidget;
mywidget.name = "mywidget";
/* Register the print function for widgets. */
register_printf_function ('W', print_widget, print_widget_arginfo);
/* Now print the widget. */
printf ("|%W|\n", &mywidget);
printf ("|%35W|\n", &mywidget);
printf ("|%-35W|\n", &mywidget);
return 0;
}
The output produced by this program looks like:
|<Widget 0xffeffb7c: mywidget>| | <Widget 0xffeffb7c: mywidget>| |<Widget 0xffeffb7c: mywidget> |
printf Handlers
The GNU libc also contains a concrete and useful application of the
printf handler extension. There are two functions available
which implement a special way to print floating-point numbers.
%f except
that there is a postfix character indicating the divisor for the
number to make this less than 1000. There are two possible divisors:
powers of 1024 or powers of 1000. Which one is used depends on the
format character specified while registered this handler. If the
character is of lower case, 1024 is used. For upper case characters,
1000 is used.
The postfix tag corresponds to bytes, kilobytes, megabytes, gigabytes, etc. The full table is:
The default precision is 3, i.e., 1024 is printed with a lower-case
format character as if it were %.3fk and will yield 1.000k.
Due to the requirements of register_printf_function we must also
provide the function which returns information about the arguments.
vfprintf implementation expects
it. The format always takes one argument.
To use these functions both functions must be registered with a call like
register_printf_function ('B', printf_size, printf_size_info);
Here we register the functions to print numbers as powers of 1000 since
the format character 'B' is an upper-case character. If we
would additionally use 'b' in a line like
register_printf_function ('b', printf_size, printf_size_info);
we could also print using a power of 1024. Please note that all that is
different in these two lines is the format specifier. The
printf_size function knows about the difference between lower and upper
case format specifiers.
The use of 'B' and 'b' is no coincidence. Rather it is
the preferred way to use this functionality since it is available on
some other systems which also use format specifiers.
The functions described in this section (scanf and related
functions) provide facilities for formatted input analogous to the
formatted output facilities. These functions provide a mechanism for
reading arbitrary values under the control of a format string or
template string.
Calls to scanf are superficially similar to calls to
printf in that arbitrary arguments are read under the control of
a template string. While the syntax of the conversion specifications in
the template is very similar to that for printf, the
interpretation of the template is oriented more towards free-format
input and simple pattern matching, rather than fixed-field formatting.
For example, most scanf conversions skip over any amount of
"white space" (including spaces, tabs, and newlines) in the input
file, and there is no concept of precision for the numeric input
conversions as there is for the corresponding output conversions.
Ordinarily, non-whitespace characters in the template are expected to
match characters in the input stream exactly, but a matching failure is
distinct from an input error on the stream.
Another area of difference between scanf and printf is
that you must remember to supply pointers rather than immediate values
as the optional arguments to scanf; the values that are read are
stored in the objects that the pointers point to. Even experienced
programmers tend to forget this occasionally, so if your program is
getting strange errors that seem to be related to scanf, you
might want to double-check this.
When a matching failure occurs, scanf returns immediately,
leaving the first non-matching character as the next character to be
read from the stream. The normal return value from scanf is the
number of values that were assigned, so you can use this to determine if
a matching error happened before all the expected values were read.
The scanf function is typically used for things like reading in
the contents of tables. For example, here is a function that uses
scanf to initialize an array of double:
void
readarray (double *array, int n)
{
int i;
for (i=0; i<n; i++)
if (scanf (" %lf", &(array[i])) != 1)
invalid_input_error ();
}
The formatted input functions are not used as frequently as the formatted output functions. Partly, this is because it takes some care to use them properly. Another reason is that it is difficult to recover from a matching error.
If you are trying to read input that doesn't match a single, fixed
pattern, you may be better off using a tool such as Flex to generate a
lexical scanner, or Bison to generate a parser, rather than using
scanf. For more information about these tools, see section `' in Flex: The Lexical Scanner Generator, and section `' in The Bison Reference Manual.
A scanf template string is a string that contains ordinary
multibyte characters interspersed with conversion specifications that
start with `%'.
Any whitespace character (as defined by the isspace function;
see section Classification of Characters) in the template causes any number
of whitespace characters in the input stream to be read and discarded.
The whitespace characters that are matched need not be exactly the same
whitespace characters that appear in the template string. For example,
write ` , ' in the template to recognize a comma with optional
whitespace before and after.
Other characters in the template string that are not part of conversion specifications must match characters in the input stream exactly; if this is not the case, a matching failure occurs.
The conversion specifications in a scanf template string
have the general form:
% flags width type conversion
In more detail, an input conversion specification consists of an initial `%' character followed in sequence by:
scanf finds a conversion
specification that uses this flag, it reads input as directed by the
rest of the conversion specification, but it discards this input, does
not use a pointer argument, and does not increment the count of
successful assignments.
long int
rather than a pointer to an int.
The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they allow.
With the `-Wformat' option, the GNU C compiler checks calls to
scanf and related functions. It examines the format string and
verifies that the correct number and types of arguments are supplied.
There is also a GNU C syntax to tell the compiler that a function you
write uses a scanf-style format string.
See section `Declaring Attributes of Functions' in Using GNU CC, for more information.
Here is a table that summarizes the various conversion specifications:
wcrtomb into a
multibyte string. This means that the buffer must provide room for
MB_CUR_MAX bytes for each wide character read. In case
`%ls' is used in a multibyte function the result is converted into
wide characters as with multiple calls of mbrtowc before being
stored in the user provided buffer.
wcrtomb into a
multibyte string. This means that the buffer must provide room for
MB_CUR_MAX bytes for each wide character read. In case
`%l[' is used in a multibyte function the result is converted into
wide characters as with multiple calls of mbrtowc before being
stored in the user provided buffer.
MB_CUR_MAX bytes for each character. If `%lc' is used in
a multibyte function the input is treated as a multibyte sequence (and
not bytes) and the result is converted as with calls to mbrtowc.
printf. See section Other Input Conversions.
If the syntax of a conversion specification is invalid, the behavior is undefined. If there aren't enough function arguments provided to supply addresses for all the conversion specifications in the template strings that perform assignments, or if the arguments are not of the correct types, the behavior is also undefined. On the other hand, extra arguments are simply ignored.
This section describes the scanf conversions for reading numeric
values.
The `%d' conversion matches an optionally signed integer in decimal
radix. The syntax that is recognized is the same as that for the
strtol function (see section Parsing of Integers) with the value
10 for the base argument.
The `%i' conversion matches an optionally signed integer in any of
the formats that the C language defines for specifying an integer
constant. The syntax that is recognized is the same as that for the
strtol function (see section Parsing of Integers) with the value
0 for the base argument. (You can print integers in this
syntax with printf by using the `#' flag character with the
`%x', `%o', or `%d' conversion. See section Integer Conversions.)
For example, any of the strings `10', `0xa', or `012'
could be read in as integers under the `%i' conversion. Each of
these specifies a number with decimal value 10.
The `%o', `%u', and `%x' conversions match unsigned
integers in octal, decimal, and hexadecimal radices, respectively. The
syntax that is recognized is the same as that for the strtoul
function (see section Parsing of Integers) with the appropriate value
(8, 10, or 16) for the base argument.
The `%X' conversion is identical to the `%x' conversion. They both permit either uppercase or lowercase letters to be used as digits.
The default type of the corresponding argument for the %d and
%i conversions is int *, and unsigned int * for the
other integer conversions. You can use the following type modifiers to
specify other sizes of integer:
signed char * or unsigned
char *.
This modifier was introduced in ISO C99.
short int * or unsigned
short int *.
intmax_t * or uintmax_t *.
This modifier was introduced in ISO C99.
long int * or unsigned
long int *. Two `l' characters is like the `L' modifier, below.
If used with `%c' or `%s' the corresponding parameter is
considered as a pointer to a wide character or wide character string
respectively. This use of `l' was introduced in Amendment 1 to
ISO C90.
long long int * or unsigned long long int *. (The long long type is an extension supported by the
GNU C compiler. For systems that don't provide extra-long integers, this
is the same as long int.)
The `q' modifier is another name for the same thing, which comes
from 4.4 BSD; a long long int is sometimes called a "quad"
int.
ptrdiff_t *.
This modifier was introduced in ISO C99.
size_t *.
This modifier was introduced in ISO C99.
All of the `%e', `%f', `%g', `%E', and `%G'
input conversions are interchangeable. They all match an optionally
signed floating point number, in the same syntax as for the
strtod function (see section Parsing of Floats).
For the floating-point input conversions, the default argument type is
float *. (This is different from the corresponding output
conversions, where the default type is double; remember that
float arguments to printf are converted to double
by the default argument promotions, but float * arguments are
not promoted to double *.) You can specify other sizes of float
using these type modifiers:
double *.
long double *.
For all the above number parsing formats there is an additional optional
flag `''. When this flag is given the scanf function
expects the number represented in the input string to be formatted
according to the grouping rules of the currently selected locale
(see section Generic Numeric Formatting Parameters).
If the "C" or "POSIX" locale is selected there is no
difference. But for a locale which specifies values for the appropriate
fields in the locale the input must have the correct form in the input.
Otherwise the longest prefix with a correct form is processed.
This section describes the scanf input conversions for reading
string and character values: `%s', `%S', `%[', `%c',
and `%C'.
You have two options for how to receive the input from these conversions:
char * or wchar_t * (the
latter of the `l' modifier is present).
Warning: To make a robust program, you must make sure that the
input (plus its terminating null) cannot possibly exceed the size of the
buffer you provide. In general, the only way to do this is to specify a
maximum field width one less than the buffer size. If you
provide the buffer, always specify a maximum field width to prevent
overflow.
scanf to allocate a big enough buffer, by specifying the
`a' flag character. This is a GNU extension. You should provide
an argument of type char ** for the buffer address to be stored
in. See section Dynamically Allocating String Conversions.
The `%c' conversion is the simplest: it matches a fixed number of characters, always. The maximum field width says how many characters to read; if you don't specify the maximum, the default is 1. This conversion doesn't append a null character to the end of the text it reads. It also does not skip over initial whitespace characters. It reads precisely the next n characters, and fails if it cannot get that many. Since there is always a maximum field width with `%c' (whether specified, or 1 by default), you can always prevent overflow by making the buffer long enough.
If the format is `%lc' or `%C' the function stores wide
characters which are converted using the conversion determined at the
time the stream was opened from the external byte stream. The number of
bytes read from the medium is limited by MB_CUR_LEN * n but
at most n wide character get stored in the output string.
The `%s' conversion matches a string of non-whitespace characters. It skips and discards initial whitespace, but stops when it encounters more whitespace after having read something. It stores a null character at the end of the text that it reads.
For example, reading the input:
hello, world
with the conversion `%10c' produces " hello, wo", but
reading the same input with the conversion `%10s' produces
"hello,".
Warning: If you do not specify a field width for `%s', then the number of characters read is limited only by where the next whitespace character appears. This almost certainly means that invalid input can make your program crash--which is a bug.
The `%ls' and `%S' format are handled just like `%s'
except that the external byte sequence is converted using the conversion
associated with the stream to wide characters with their own encoding.
A width or precision specified with the format do not directly determine
how many bytes are read from the stream since they measure wide
characters. But an upper limit can be computed by multiplying the value
of the width or precision by MB_CUR_MAX.
To read in characters that belong to an arbitrary set of your choice, use the `%[' conversion. You specify the set between the `[' character and a following `]' character, using the same syntax used in regular expressions. As special cases:
The `%[' conversion does not skip over initial whitespace characters.
Here are some examples of `%[' conversions and what they mean:
As for `%c' and `%s' the `%[' format is also modified to produce wide characters if the `l' modifier is present. All what is said about `%ls' above is true for `%l['.
One more reminder: the `%s' and `%[' conversions are dangerous if you don't specify a maximum width or use the `a' flag, because input too long would overflow whatever buffer you have provided for it. No matter how long your buffer is, a user could supply input that is longer. A well-written program reports invalid input with a comprehensible error message, not with a crash.
A GNU extension to formatted input lets you safely read a string with no
maximum size. Using this feature, you don't supply a buffer; instead,
scanf allocates a buffer big enough to hold the data and gives
you its address. To use this feature, write `a' as a flag
character, as in `%as' or `%a[0-9a-z]'.
The pointer argument you supply for where to store the input should have
type char **. The scanf function allocates a buffer and
stores its address in the word that the argument points to. You should
free the buffer with free when you no longer need it.
Here is an example of using the `a' flag with the `%[...]' conversion specification to read a "variable assignment" of the form `variable = value'.
{
char *variable, *value;
if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n",
&variable, &value))
{
invalid_input_error ();
return 0;
}
...
}
This section describes the miscellaneous input conversions.
The `%p' conversion is used to read a pointer value. It recognizes
the same syntax used by the `%p' output conversion for
printf (see section Other Output Conversions); that is, a hexadecimal
number just as the `%x' conversion accepts. The corresponding
argument should be of type void **; that is, the address of a
place to store a pointer.
The resulting pointer value is not guaranteed to be valid if it was not originally written during the same program execution that reads it in.
The `%n' conversion produces the number of characters read so far
by this call. The corresponding argument should be of type int *.
This conversion works in the same way as the `%n' conversion for
printf; see section Other Output Conversions, for an example.
The `%n' conversion is the only mechanism for determining the
success of literal matches or conversions with suppressed assignments.
If the `%n' follows the locus of a matching failure, then no value
is stored for it since scanf returns before processing the
`%n'. If you store -1 in that argument slot before calling
scanf, the presence of -1 after scanf indicates an
error occurred before the `%n' was reached.
Finally, the `%%' conversion matches a literal `%' character in the input stream, without using an argument. This conversion does not permit any flags, field width, or type modifier to be specified.
Here are the descriptions of the functions for performing formatted input. Prototypes for these functions are in the header file `stdio.h'.
scanf function reads formatted input from the stream
stdin under the control of the template string template.
The optional arguments are pointers to the places which receive the
resulting values.
The return value is normally the number of successful assignments. If
an end-of-file condition is detected before any matches are performed,
including matches against whitespace and literal characters in the
template, then EOF is returned.
wscanf function reads formatted input from the stream
stdin under the control of the template string template.
The optional arguments are pointers to the places which receive the
resulting values.
The return value is normally the number of successful assignments. If
an end-of-file condition is detected before any matches are performed,
including matches against whitespace and literal characters in the
template, then WEOF is returned.
scanf, except that the input is read
from the stream stream instead of stdin.
wscanf, except that the input is read
from the stream stream instead of stdin.
scanf, except that the characters are taken from the
null-terminated string s instead of from a stream. Reaching the
end of the string is treated as an end-of-file condition.
The behavior of this function is undefined if copying takes place between objects that overlap--for example, if s is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion.
wscanf, except that the characters are taken from the
null-terminated string ws instead of from a stream. Reaching the
end of the string is treated as an end-of-file condition.
The behavior of this function is undefined if copying takes place between objects that overlap--for example, if ws is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion.
The functions vscanf and friends are provided so that you can
define your own variadic scanf-like functions that make use of
the same internals as the built-in formatted output functions.
These functions are analogous to the vprintf series of output
functions. See section Variable Arguments Output Functions, for important
information on how to use them.
Portability Note: The functions listed in this section were introduced in ISO C99 and were before available as GNU extensions.
scanf, but instead of taking
a variable number of arguments directly, it takes an argument list
pointer ap of type va_list (see section Variadic Functions).
wscanf, but instead of taking
a variable number of arguments directly, it takes an argument list
pointer ap of type va_list (see section Variadic Functions).
fscanf with the variable argument list
specified directly as for vscanf.
fwscanf with the variable argument list
specified directly as for vwscanf.
sscanf with the variable argument list
specified directly as for vscanf.
swscanf with the variable argument list
specified directly as for vwscanf.
In GNU C, there is a special construct you can use to let the compiler
know that a function uses a scanf-style format string. Then it
can check the number and types of arguments in each call to the
function, and warn you when they do not match the format string.
For details, See section `Declaring Attributes of Functions' in Using GNU CC.
Many of the functions described in this chapter return the value of the
macro EOF to indicate unsuccessful completion of the operation.
Since EOF is used to report both end of file and random errors,
it's often better to use the feof function to check explicitly
for end of file and ferror to check for errors. These functions
check indicators that are part of the internal state of the stream
object, indicators set if the appropriate condition was detected by a
previous I/O operation on that stream.
EOF is -1. In
other libraries, its value may be some other negative number.
This symbol is declared in `stdio.h'.
WEOF is -1. In
other libraries, its value may be some other negative number.
This symbol is declared in `wchar.h'.
feof function returns nonzero if and only if the end-of-file
indicator for the stream stream is set.
This symbol is declared in `stdio.h'.
feof_unlocked function is equivalent to the feof
function except that it does not implicitly lock the stream.
This function is a GNU extension.
This symbol is declared in `stdio.h'.
ferror function returns nonzero if and only if the error
indicator for the stream stream is set, indicating that an error
has occurred on a previous operation on the stream.
This symbol is declared in `stdio.h'.
ferror_unlocked function is equivalent to the ferror
function except that it does not implicitly lock the stream.
This function is a GNU extension.
This symbol is declared in `stdio.h'.
In addition to setting the error indicator associated with the stream,
the functions that operate on streams also set errno in the same
way as the corresponding low-level functions that operate on file
descriptors. For example, all of the functions that perform output to a
stream--such as fputc, printf, and fflush---are
implemented in terms of write, and all of the errno error
conditions defined for write are meaningful for these functions.
For more information about the descriptor-level I/O functions, see
section Low-Level Input/Output.
You may explicitly clear the error and EOF flags with the clearerr
function.
The file positioning functions (see section File Positioning) also clear the end-of-file indicator for the stream.
clearerr_unlocked function is equivalent to the clearerr
function except that it does not implicitly lock the stream.
This function is a GNU extension.
Note that it is not correct to just clear the error flag and retry a failed stream operation. After a failed write, any number of characters since the last buffer flush may have been committed to the file, while some buffered data may have been discarded. Merely retrying can thus cause lost or repeated data.
A failed read may leave the file pointer in an inappropriate position for a second try. In both cases, you should seek to a known position before retrying.
Most errors that can happen are not recoverable -- a second try will always fail again in the same way. So usually it is best to give up and report the error to the user, rather than install complicated recovery logic.
One important exception is EINTR (see section Primitives Interrupted by Signals).
Many stream I/O implementations will treat it as an ordinary error, which
can be quite inconvenient. You can avoid this hassle by installing all
signals with the SA_RESTART flag.
For similar reasons, setting nonblocking I/O on a stream's file descriptor is not usually advisable.
The GNU system and other POSIX-compatible operating systems organize all files as uniform sequences of characters. However, some other systems make a distinction between files containing text and files containing binary data, and the input and output facilities of ISO C provide for this distinction. This section tells you how to write programs portable to such systems.
When you open a stream, you can specify either a text stream or a
binary stream. You indicate that you want a binary stream by
specifying the `b' modifier in the opentype argument to
fopen; see section Opening Streams. Without this
option, fopen opens the file as a text stream.
Text and binary streams differ in several ways:
'\n') characters, while a binary stream is
simply a long series of characters. A text stream might on some systems
fail to handle lines more than 254 characters long (including the
terminating newline character).
Since a binary stream is always more capable and more predictable than a text stream, you might wonder what purpose text streams serve. Why not simply always use binary streams? The answer is that on these operating systems, text and binary streams use different file formats, and the only way to read or write "an ordinary file of text" that can work with other text-oriented programs is through a text stream.
In the GNU library, and on all POSIX systems, there is no difference between text streams and binary streams. When you open a stream, you get the same kind of stream regardless of whether you ask for binary. This stream can handle any file content, and has none of the restrictions that text streams sometimes have.
The file position of a stream describes where in the file the stream is currently reading or writing. I/O on the stream advances the file position through the file. In the GNU system, the file position is represented as an integer, which counts the number of bytes from the beginning of the file. See section File Position.
During I/O to an ordinary disk file, you can change the file position whenever you wish, so as to read or write any portion of the file. Some other kinds of files may also permit this. Files which support changing the file position are sometimes referred to as random-access files.
You can use the functions in this section to examine or modify the file position indicator associated with a stream. The symbols listed below are declared in the header file `stdio.h'.
This function can fail if the stream doesn't support file positioning,
or if the file position can't be represented in a long int, and
possibly for other reasons as well. If a failure occurs, a value of
-1 is returned.
ftello function is similar to ftell, except that it
returns a value of type off_t. Systems which support this type
use it to describe all file positions, unlike the POSIX specification
which uses a long int. The two are not necessarily the same size.
Therefore, using ftell can lead to problems if the implementation is
written on top of a POSIX compliant low-level I/O implementation, and using
ftello is preferable whenever it is available.
If this function fails it returns (off_t) -1. This can happen due
to missing support for file positioning or internal errors. Otherwise
the return value is the current file position.
The function is an extension defined in the Unix Single Specification version 2.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32 bit system this function is in fact ftello64. I.e., the
LFS interface transparently replaces the old interface.
ftello with the only difference that
the return value is of type off64_t. This also requires that the
stream stream was opened using either fopen64,
freopen64, or tmpfile64 since otherwise the underlying
file operations to position the file pointer beyond the @math{2^31}
bytes limit might fail.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name ftello
and so transparently replaces the old interface.
fseek function is used to change the file position of the
stream stream. The value of whence must be one of the
constants SEEK_SET, SEEK_CUR, or SEEK_END, to
indicate whether the offset is relative to the beginning of the
file, the current file position, or the end of the file, respectively.
This function returns a value of zero if the operation was successful,
and a nonzero value to indicate failure. A successful call also clears
the end-of-file indicator of stream and discards any characters
that were "pushed back" by the use of ungetc.
fseek either flushes any buffered output before setting the file
position or else remembers it so it will be written later in its proper
place in the file.
fseek but it corrects a problem with
fseek in a system with POSIX types. Using a value of type
long int for the offset is not compatible with POSIX.
fseeko uses the correct type off_t for the offset
parameter.
For this reason it is a good idea to prefer ftello whenever it is
available since its functionality is (if different at all) closer the
underlying definition.
The functionality and return value is the same as for fseek.
The function is an extension defined in the Unix Single Specification version 2.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32 bit system this function is in fact fseeko64. I.e., the
LFS interface transparently replaces the old interface.
fseeko with the only difference that
the offset parameter is of type off64_t. This also
requires that the stream stream was opened using either
fopen64, freopen64, or tmpfile64 since otherwise
the underlying file operations to position the file pointer beyond the
@math{2^31} bytes limit might fail.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name fseeko
and so transparently replaces the old interface.
Portability Note: In non-POSIX systems, ftell,
ftello, fseek and fseeko might work reliably only
on binary streams. See section Text and Binary Streams.
The following symbolic constants are defined for use as the whence
argument to fseek. They are also used with the lseek
function (see section Input and Output Primitives) and to specify offsets for file locks
(see section Control Operations on Files).
fseek or fseeko function, specifies that
the offset provided is relative to the beginning of the file.
fseek or fseeko function, specifies that
the offset provided is relative to the current file position.
fseek or fseeko function, specifies that
the offset provided is relative to the end of the file.
rewind function positions the stream stream at the
beginning of the file. It is equivalent to calling fseek or
fseeko on the stream with an offset argument of
0L and a whence argument of SEEK_SET, except that
the return value is discarded and the error indicator for the stream is
reset.
These three aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'.
On the GNU system, the file position is truly a character count. You
can specify any character count value as an argument to fseek or
fseeko and get reliable results for any random access file.
However, some ISO C systems do not represent file positions in this
way.
On some systems where text streams truly differ from binary streams, it is impossible to represent the file position of a text stream as a count of characters from the beginning of the file. For example, the file position on some systems must encode both a record offset within the file, and a character offset within the record.
As a consequence, if you want your programs to be portable to these systems, you must observe certain rules:
ftell on a text stream has no predictable
relationship to the number of characters you have read so far. The only
thing you can rely on is that you can use it subsequently as the
offset argument to fseek or fseeko to move back to
the same file position.
fseek or fseeko on a text stream, either the
offset must be zero, or whence must be SEEK_SET and
and the offset must be the result of an earlier call to ftell
on the same stream.
ungetc
that haven't been read or discarded. See section Unreading.
But even if you observe these rules, you may still have trouble for long
files, because ftell and fseek use a long int value
to represent the file position. This type may not have room to encode
all the file positions in a large file. Using the ftello and
fseeko functions might help here since the off_t type is
expected to be able to hold all file position values but this still does
not help to handle additional information which must be associated with
a file position.
So if you do want to support systems with peculiar encodings for the
file positions, it is better to use the functions fgetpos and
fsetpos instead. These functions represent the file position
using the data type fpos_t, whose internal representation varies
from system to system.
These symbols are declared in the header file `stdio.h'.
fgetpos and
fsetpos.
In the GNU system, fpos_t is equivalent to off_t or
long int. In other systems, it might have a different internal
representation.
When compiling with _FILE_OFFSET_BITS == 64 on a 32 bit machine
this type is in fact equivalent to off64_t since the LFS
interface transparently replaced the old interface.
fgetpos64 and
fsetpos64.
In the GNU system, fpos64_t is equivalent to off64_t or
long long int. In other systems, it might have a different internal
representation.
fpos_t object pointed to by
position. If successful, fgetpos returns zero; otherwise
it returns a nonzero value and stores an implementation-defined positive
value in errno.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32 bit system the function is in fact fgetpos64. I.e., the LFS
interface transparently replaced the old interface.
fgetpos but the file position is
returned in a variable of type fpos64_t to which position
points.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name fgetpos
and so transparently replaces the old interface.
fgetpos on the same stream. If successful, fsetpos
clears the end-of-file indicator on the stream, discards any characters
that were "pushed back" by the use of ungetc, and returns a value
of zero. Otherwise, fsetpos returns a nonzero value and stores
an implementation-defined positive value in errno.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32 bit system the function is in fact fsetpos64. I.e., the LFS
interface transparently replaced the old interface.
fsetpos but the file position used
for positioning is provided in a variable of type fpos64_t to
which position points.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name fsetpos
and so transparently replaces the old interface.
Characters that are written to a stream are normally accumulated and transmitted asynchronously to the file in a block, instead of appearing as soon as they are output by the application program. Similarly, streams often retrieve input from the host environment in blocks rather than on a character-by-character basis. This is called buffering.
If you are writing programs that do interactive input and output using streams, you need to understand how buffering works when you design the user interface to your program. Otherwise, you might find that output (such as progress or prompt messages) doesn't appear when you intended it to, or displays some other unexpected behavior.
This section deals only with controlling when characters are transmitted between the stream and the file or device, and not with how things like echoing, flow control, and the like are handled on specific classes of devices. For information on common control operations on terminal devices, see section Low-Level Terminal Interface.
You can bypass the stream buffering facilities altogether by using the low-level input and output functions that operate on file descriptors instead. See section Low-Level Input/Output.
There are three different kinds of buffering strategies:
Newly opened streams are normally fully buffered, with one exception: a stream connected to an interactive device such as a terminal is initially line buffered. See section Controlling Which Kind of Buffering, for information on how to select a different kind of buffering. Usually the automatic selection gives you the most convenient kind of buffering for the file or device you open.
The use of line buffering for interactive devices implies that output
messages ending in a newline will appear immediately--which is usually
what you want. Output that doesn't end in a newline might or might not
show up immediately, so if you want them to appear immediately, you
should flush buffered output explicitly with fflush, as described
in section Flushing Buffers.
Flushing output on a buffered stream means transmitting all accumulated characters to the file. There are many circumstances when buffered output on a stream is flushed automatically:
exit.
See section Normal Termination.
If you want to flush the buffered output at another time, call
fflush, which is declared in the header file `stdio.h'.
fflush causes buffered output on all open output streams
to be flushed.
This function returns EOF if a write error occurs, or zero
otherwise.
fflush_unlocked function is equivalent to the fflush
function except that it does not implicitly lock the stream.
The fflush function can be used to flush all streams currently
opened. While this is useful in some situations it does often more than
necessary since it might be done in situations when terminal input is
required and the program wants to be sure that all output is visible on
the terminal. But this means that only line buffered streams have to be
flushed. Solaris introduced a function especially for this. It was
always available in the GNU C library in some form but never officially
exported.
_flushlbf function flushes all line buffered streams
currently opened.
This function is declared in the `stdio_ext.h' header.
Compatibility Note: Some brain-damaged operating systems have been known to be so thoroughly fixated on line-oriented input and output that flushing a line buffered stream causes a newline to be written! Fortunately, this "feature" seems to be becoming less common. You do not need to worry about this in the GNU system.
In some situations it might be useful to not flush the output pending for a stream but instead simply forget it. If transmission is costly and the output is not needed anymore this is valid reasoning. In this situation a non-standard function introduced in Solaris and available in the GNU C library can be used.
__fpurge function causes the buffer of the stream
stream to be emptied. If the stream is currently in read mode all
input in the buffer is lost. If the stream is in output mode the
buffered output is not written to the device (or whatever other
underlying storage) and the buffer the cleared.
This function is declared in `stdio_ext.h'.
After opening a stream (but before any other operations have been
performed on it), you can explicitly specify what kind of buffering you
want it to have using the setvbuf function.
The facilities listed in this section are declared in the header file `stdio.h'.
_IOFBF
(for full buffering), _IOLBF (for line buffering), or
_IONBF (for unbuffered input/output).
If you specify a null pointer as the buf argument, then setvbuf
allocates a buffer itself using malloc. This buffer will be freed
when you close the stream.
Otherwise, buf should be a character array that can hold at least
size characters. You should not free the space for this array as
long as the stream remains open and this array remains its buffer. You
should usually either allocate it statically, or malloc
(see section Unconstrained Allocation) the buffer. Using an automatic array
is not a good idea unless you close the file before exiting the block
that declares the array.
While the array remains a stream buffer, the stream I/O functions will use the buffer for their internal purposes. You shouldn't try to access the values in the array directly while the stream is using it for buffering.
The setvbuf function returns zero on success, or a nonzero value
if the value of mode is not valid or if the request could not
be honored.
setvbuf function to
specify that the stream should be fully buffered.
setvbuf function to
specify that the stream should be line buffered.
setvbuf function to
specify that the stream should be unbuffered.
setvbuf. This value is
guaranteed to be at least 256.
The value of BUFSIZ is chosen on each system so as to make stream
I/O efficient. So it is a good idea to use BUFSIZ as the size
for the buffer when you call setvbuf.
Actually, you can get an even better value to use for the buffer size
by means of the fstat system call: it is found in the
st_blksize field of the file attributes. See section The meaning of the File Attributes.
Sometimes people also use BUFSIZ as the allocation size of
buffers used for related purposes, such as strings used to receive a
line of input with fgets (see section Character Input). There is no
particular reason to use BUFSIZ for this instead of any other
integer, except that it might lead to doing I/O in chunks of an
efficient size.
setvbuf with a mode argument of
_IONBF. Otherwise, it is equivalent to calling setvbuf
with buf, and a mode of _IOFBF and a size
argument of BUFSIZ.
The setbuf function is provided for compatibility with old code;
use setvbuf in all new programs.
This function is provided for compatibility with old BSD code. Use
setvbuf instead.
This function is provided for compatibility with old BSD code. Use
setvbuf instead.
It is possible to query whether a given stream is line buffered or not using a non-standard function introduced in Solaris and available in the GNU C library.
__flbf function will return a nonzero value in case the
stream stream is line buffered. Otherwise the return value is
zero.
This function is declared in the `stdio_ext.h' header.
Two more extensions allow to determine the size of the buffer and how much of it is used. These functions were also introduced in Solaris.
__fbufsize function return the size of the buffer in the
stream stream. This value can be used to optimize the use of the
stream.
This function is declared in the `stdio_ext.h' header.
__fpending
This function is declared in the `stdio_ext.h' header.
The GNU library provides ways for you to define additional kinds of streams that do not necessarily correspond to an open file.
One such type of stream takes input from or writes output to a string.
These kinds of streams are used internally to implement the
sprintf and sscanf functions. You can also create such a
stream explicitly, using the functions described in section String Streams.
More generally, you can define streams that do input/output to arbitrary objects using functions supplied by your program. This protocol is discussed in section Programming Your Own Custom Streams.
Portability Note: The facilities described in this section are specific to GNU. Other systems or C implementations might or might not provide equivalent functionality.
The fmemopen and open_memstream functions allow you to do
I/O to a string or memory buffer. These facilities are declared in
`stdio.h'.
If you specify a null pointer as the buf argument, fmemopen
dynamically allocates an array size bytes long (as with malloc;
see section Unconstrained Allocation). This is really only useful
if you are going to write things to the buffer and then read them back
in again, because you have no way of actually getting a pointer to the
buffer (for this, try open_memstream, below). The buffer is
freed when the stream is open.
The argument opentype is the same as in fopen
(see section Opening Streams). If the opentype specifies
append mode, then the initial file position is set to the first null
character in the buffer. Otherwise the initial file position is at the
beginning of the buffer.
When a stream open for writing is flushed or closed, a null character (zero byte) is written at the end of the buffer if it fits. You should add an extra byte to the size argument to account for this. Attempts to write more than size bytes to the buffer result in an error.
For a stream open for reading, null characters (zero bytes) in the buffer do not count as "end of file". Read operations indicate end of file only when the file position advances past size bytes. So, if you want to read characters from a null-terminated string, you should supply the length of the string as the size argument.
Here is an example of using fmemopen to create a stream for
reading from a string:
#include <stdio.h>
static char buffer[] = "foobar";
int
main (void)
{
int ch;
FILE *stream;
stream = fmemopen (buffer, strlen (buffer), "r");
while ((ch = fgetc (stream)) != EOF)
printf ("Got %c\n", ch);
fclose (stream);
return 0;
}
This program produces the following output:
Got f Got o Got o Got b Got a Got r
malloc; see section Unconstrained Allocation) and grown as necessary.
When the stream is closed with fclose or flushed with
fflush, the locations ptr and sizeloc are updated to
contain the pointer to the buffer and its size. The values thus stored
remain valid only as long as no further output on the stream takes
place. If you do more output, you must flush the stream again to store
new values before you use them again.
A null character is written at the end of the buffer. This null character is not included in the size value stored at sizeloc.
You can move the stream's file position with fseek or
fseeko (see section File Positioning). Moving the file position past
the end of the data already written fills the intervening space with
zeroes.
Here is an example of using open_memstream:
#include <stdio.h>
int
main (void)
{
char *bp;
size_t size;
FILE *stream;
stream = open_memstream (&bp, &size);
fprintf (stream, "hello");
fflush (stream);
printf ("buf = `%s', size = %d\n", bp, size);
fprintf (stream, ", world");
fclose (stream);
printf ("buf = `%s', size = %d\n", bp, size);
return 0;
}
This program produces the following output:
buf = `hello', size = 5 buf = `hello, world', size = 12
You can open an output stream that puts it data in an obstack. See section Obstacks.
Calling fflush on this stream updates the current size of the
object to match the amount of data that has been written. After a call
to fflush, you can examine the object temporarily.
You can move the file position of an obstack stream with fseek or
fseeko (see section File Positioning). Moving the file position past
the end of the data written fills the intervening space with zeros.
To make the object permanent, update the obstack with fflush, and
then use obstack_finish to finalize the object and get its address.
The following write to the stream starts a new object in the obstack,
and later writes add to that object until you do another fflush
and obstack_finish.
But how do you find out how long the object is? You can get the length
in bytes by calling obstack_object_size (see section Status of an Obstack), or you can null-terminate the object like this:
obstack_1grow (obstack, 0);
Whichever one you do, you must do it before calling
obstack_finish. (You can do both if you wish.)
Here is a sample function that uses open_obstack_stream:
char *
make_message_string (const char *a, int b)
{
FILE *stream = open_obstack_stream (&message_obstack);
output_task (stream);
fprintf (stream, ": ");
fprintf (stream, a, b);
fprintf (stream, "\n");
fclose (stream);
obstack_1grow (&message_obstack, 0);
return obstack_finish (&message_obstack);
}
This section describes how you can make a stream that gets input from an arbitrary data source or writes output to an arbitrary data sink programmed by you. We call these custom streams. The functions and types described here are all GNU extensions.
Inside every custom stream is a special object called the cookie.
This is an object supplied by you which records where to fetch or store
the data read or written. It is up to you to define a data type to use
for the cookie. The stream functions in the library never refer
directly to its contents, and they don't even know what the type is;
they record its address with type void *.
To implement a custom stream, you must specify how to fetch or store the data in the specified place. You do this by defining hook functions to read, write, change "file position", and close the stream. All four of these functions will be passed the stream's cookie so they can tell where to fetch or store the data. The library functions don't know what's inside the cookie, but your functions will know.
When you create a custom stream, you must specify the cookie pointer,
and also the four hook functions stored in a structure of type
cookie_io_functions_t.
These facilities are declared in `stdio.h'.
cookie_read_function_t *read
EOF.
cookie_write_function_t *write
cookie_seek_function_t *seek
fseek or fseeko on this stream can only seek to
locations within the buffer; any attempt to seek outside the buffer will
return an ESPIPE error.
cookie_close_function_t *close
fopen;
see section Opening Streams. (But note that the "truncate on
open" option is ignored.) The new stream is fully buffered.
The fopencookie function returns the newly created stream, or a null
pointer in case of an error.
Here are more details on how you should define the four hook functions that a custom stream needs.
You should define the function to read data from the cookie as:
ssize_t reader (void *cookie, char *buffer, size_t size)
This is very similar to the read function; see section Input and Output Primitives. Your function should transfer up to size bytes into
the buffer, and return the number of bytes read, or zero to
indicate end-of-file. You can return a value of -1 to indicate
an error.
You should define the function to write data to the cookie as:
ssize_t writer (void *cookie, const char *buffer, size_t size)
This is very similar to the write function; see section Input and Output Primitives. Your function should transfer up to size bytes from
the buffer, and return the number of bytes written. You can return a
value of -1 to indicate an error.
You should define the function to perform seek operations on the cookie as:
int seeker (void *cookie, fpos_t *position, int whence)
For this function, the position and whence arguments are
interpreted as for fgetpos; see section Portable File-Position Functions. In
the GNU library, fpos_t is equivalent to off_t or
long int, and simply represents the number of bytes from the
beginning of the file.
After doing the seek operation, your function should store the resulting
file position relative to the beginning of the file in position.
Your function should return a value of 0 on success and -1
to indicate an error.
You should define the function to do cleanup operations on the cookie appropriate for closing the stream as:
int cleaner (void *cookie)
Your function should return -1 to indicate an error, and 0
otherwise.
On systems which are based on System V messages of programs (especially
the system tools) are printed in a strict form using the fmtmsg
function. The uniformity sometimes helps the user to interpret messages
and the strictness tests of the fmtmsg function ensure that the
programmer follows some minimal requirements.
Messages can be printed to standard error and/or to the console. To
select the destination the programmer can use the following two values,
bitwise OR combined if wanted, for the classification parameter of
fmtmsg:
MM_PRINT
MM_CONSOLE
The erroneous piece of the system can be signalled by exactly one of the
following values which also is bitwise ORed with the
classification parameter to fmtmsg:
MM_HARD
MM_SOFT
MM_FIRM
A third component of the classification parameter to fmtmsg
can describe the part of the system which detects the problem. This is
done by using exactly one of the following values:
MM_APPL
MM_UTIL
MM_OPSYS
A last component of classification can signal the results of this message. Exactly one of the following values can be used:
Each of the parameters can be a special value which means this value is to be omitted. The symbolic names for these values are:
MM_NULLLBL
MM_NULLSEV
MM_NULLMC
MM_NULLTXT
MM_NULLACT
MM_NULLTAG
There is another way certain fields can be omitted from the output to standard error. This is described below in the description of environment variables influencing the behaviour.
The severity parameter can have one of the values in the following table:
MM_NOSEV
MM_NULLSEV.
MM_HALT
HALT.
MM_ERROR
ERROR.
MM_WARNING
WARNING.
MM_INFO
INFO.
The numeric value of these five macros are between 0 and
4. Using the environment variable SEV_LEVEL or using the
addseverity function one can add more severity levels with their
corresponding string to print. This is described below
(see section Adding Severity Classes).
If no parameter is ignored the output looks like this:
label: severity-string: text TO FIX: action tag
The colons, new line characters and the TO FIX string are
inserted if necessary, i.e., if the corresponding parameter is not
ignored.
This function is specified in the X/Open Portability Guide. It is also available on all systems derived from System V.
The function returns the value MM_OK if no error occurred. If
only the printing to standard error failed, it returns MM_NOMSG.
If printing to the console fails, it returns MM_NOCON. If
nothing is printed MM_NOTOK is returned. Among situations where
all outputs fail this last value is also returned if a parameter value
is incorrect.
There are two environment variables which influence the behaviour of
fmtmsg. The first is MSGVERB. It is used to control the
output actually happening on standard error (not the console
output). Each of the five fields can explicitly be enabled. To do
this the user has to put the MSGVERB variable with a format like
the following in the environment before calling the fmtmsg function
the first time:
MSGVERB=keyword[:keyword[:...]]
Valid keywords are label, severity, text,
action, and tag. If the environment variable is not given
or is the empty string, a not supported keyword is given or the value is
somehow else invalid, no part of the message is masked out.
The second environment variable which influences the behaviour of
fmtmsg is SEV_LEVEL. This variable and the change in the
behaviour of fmtmsg is not specified in the X/Open Portability
Guide. It is available in System V systems, though. It can be used to
introduce new severity levels. By default, only the five severity levels
described above are available. Any other numeric value would make
fmtmsg print nothing.
If the user puts SEV_LEVEL with a format like
SEV_LEVEL=[description[:description[:...]]]
in the environment of the process before the first call to
fmtmsg, where description has a value of the form
severity-keyword,level,printstring
The severity-keyword part is not used by fmtmsg but it has
to be present. The level part is a string representation of a
number. The numeric value must be a number greater than 4. This value
must be used in the severity parameter of fmtmsg to select
this class. It is not possible to overwrite any of the predefined
classes. The printstring is the string printed when a message of
this class is processed by fmtmsg (see above, fmtsmg does
not print the numeric value but instead the string representation).
There is another possibility to introduce severity classes besides using
the environment variable SEV_LEVEL. This simplifies the task of
introducing new classes in a running program. One could use the
setenv or putenv function to set the environment variable,
but this is toilsome.
fmtmsg function.
The severity parameter of addseverity must match the value
for the parameter with the same name of fmtmsg, and string
is the string printed in the actual messages instead of the numeric
value.
If string is NULL the severity class with the numeric value
according to severity is removed.
It is not possible to overwrite or remove one of the default severity
classes. All calls to addseverity with severity set to one
of the values for the default classes will fail.
The return value is MM_OK if the task was successfully performed.
If the return value is MM_NOTOK something went wrong. This could
mean that no more memory is available or a class is not available when
it has to be removed.
This function is not specified in the X/Open Portability Guide although
the fmtsmg function is. It is available on System V systems.
fmtmsg and addseverityHere is a simple example program to illustrate the use of the both functions described in this section.
#include <fmtmsg.h>
int
main (void)
{
addseverity (5, "NOTE2");
fmtmsg (MM_PRINT, "only1field", MM_INFO, "text2", "action2", "tag2");
fmtmsg (MM_PRINT, "UX:cat", 5, "invalid syntax", "refer to manual",
"UX:cat:001");
fmtmsg (MM_PRINT, "label:foo", 6, "text", "action", "tag");
return 0;
}
The second call to fmtmsg illustrates a use of this function as
it usually occurs on System V systems, which heavily use this function.
It seems worthwhile to give a short explanation here of how this system
works on System V. The value of the
label field (UX:cat) says that the error occured in the
Unix program cat. The explanation of the error follows and the
value for the action parameter is "refer to manual". One
could be more specific here, if necessary. The tag field contains,
as proposed above, the value of the string given for the label
parameter, and additionally a unique ID (001 in this case). For
a GNU environment this string could contain a reference to the
corresponding node in the Info page for the program.
Running this program without specifying the MSGVERB and
SEV_LEVEL function produces the following output:
UX:cat: NOTE2: invalid syntax TO FIX: refer to manual UX:cat:001
We see the different fields of the message and how the extra glue (the
colons and the TO FIX string) are printed. But only one of the
three calls to fmtmsg produced output. The first call does not
print anything because the label parameter is not in the correct
form. The string must contain two fields, separated by a colon
(see section Printing Formatted Messages). The third fmtmsg call
produced no output since the class with the numeric value 6 is
not defined. Although a class with numeric value 5 is also not
defined by default, the call to addseverity introduces it and
the second call to fmtmsg produces the above output.
When we change the environment of the program to contain
SEV_LEVEL=XXX,6,NOTE when running it we get a different result:
UX:cat: NOTE2: invalid syntax TO FIX: refer to manual UX:cat:001 label:foo: NOTE: text TO FIX: action tag
Now the third call to fmtmsg produced some output and we see how
the string NOTE from the environment variable appears in the
message.
Now we can reduce the output by specifying which fields we are
interested in. If we additionally set the environment variable
MSGVERB to the value severity:label:action we get the
following output:
UX:cat: NOTE2 TO FIX: refer to manual label:foo: NOTE TO FIX: action
I.e., the output produced by the text and the tag parameters
to fmtmsg vanished. Please also note that now there is no colon
after the NOTE and NOTE2 strings in the output. This is
not necessary since there is no more output on this line because the text
is missing.
This chapter describes functions for performing low-level input/output operations on file descriptors. These functions include the primitives for the higher-level I/O functions described in section Input/Output on Streams, as well as functions for performing low-level control operations for which there are no equivalents on streams.
Stream-level I/O is more flexible and usually more convenient; therefore, programmers generally use the descriptor-level functions only when necessary. These are some of the usual reasons:
fileno to get the descriptor
corresponding to a stream.)
This section describes the primitives for opening and closing files
using file descriptors. The open and creat functions are
declared in the header file `fcntl.h', while close is
declared in `unistd.h'.
open function creates and returns a new file descriptor
for the file named by filename. Initially, the file position
indicator for the file is at the beginning of the file. The argument
mode is used only when a file is created, but it doesn't hurt
to supply the argument in any case.
The flags argument controls how the file is to be opened. This is a bit mask; you create the value by the bitwise OR of the appropriate parameters (using the `|' operator in C). See section File Status Flags, for the parameters available.
The normal return value from open is a non-negative integer file
descriptor. In the case of an error, a value of @math{-1} is returned
instead. In addition to the usual file name errors (see section File Name Errors), the following errno error conditions are defined
for this function:
EACCES
EEXIST
O_CREAT and O_EXCL are set, and the named file already
exists.
EINTR
open operation was interrupted by a signal.
See section Primitives Interrupted by Signals.
EISDIR
EMFILE
RLIMIT_NOFILE resource limit; see section Limiting Resource Usage.
ENFILE
ENOENT
O_CREAT is not specified.
ENOSPC
ENXIO
O_NONBLOCK and O_WRONLY are both set in the flags
argument, the file named by filename is a FIFO (see section Pipes and FIFOs), and no process has the file open for reading.
EROFS
O_WRONLY,
O_RDWR, and O_TRUNC are set in the flags argument,
or O_CREAT is set and the file does not already exist.
If on a 32 bit machine the sources are translated with
_FILE_OFFSET_BITS == 64 the function open returns a file
descriptor opened in the large file mode which enables the file handling
functions to use files up to @math{2^63} bytes in size and offset from
@math{-2^63} to @math{2^63}. This happens transparently for the user
since all of the lowlevel file handling functions are equally replaced.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time open is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this calls to open should be
protected using cancellation handlers.
The open function is the underlying primitive for the fopen
and freopen functions, that create streams.
open. It returns a file descriptor
which can be used to access the file named by filename. The only
difference is that on 32 bit systems the file is opened in the
large file mode. I.e., file length and file offsets can exceed 31 bits.
When the sources are translated with _FILE_OFFSET_BITS == 64 this
function is actually available under the name open. I.e., the
new, extended API using 64 bit file sizes and offsets transparently
replaces the old API.
creat (filename, mode)
is equivalent to:
open (filename, O_WRONLY | O_CREAT | O_TRUNC, mode)
If on a 32 bit machine the sources are translated with
_FILE_OFFSET_BITS == 64 the function creat returns a file
descriptor opened in the large file mode which enables the file handling
functions to use files up to @math{2^63} in size and offset from
@math{-2^63} to @math{2^63}. This happens transparently for the user
since all of the lowlevel file handling functions are equally replaced.
creat. It returns a file descriptor
which can be used to access the file named by filename. The only
the difference is that on 32 bit systems the file is opened in the
large file mode. I.e., file length and file offsets can exceed 31 bits.
To use this file descriptor one must not use the normal operations but
instead the counterparts named *64, e.g., read64.
When the sources are translated with _FILE_OFFSET_BITS == 64 this
function is actually available under the name open. I.e., the
new, extended API using 64 bit file sizes and offsets transparently
replaces the old API.
close closes the file descriptor filedes.
Closing a file has the following consequences:
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time close is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this, calls to close should be
protected using cancellation handlers.
The normal return value from close is @math{0}; a value of @math{-1}
is returned in case of failure. The following errno error
conditions are defined for this function:
EBADF
EINTR
close call was interrupted by a signal.
See section Primitives Interrupted by Signals.
Here is an example of how to handle EINTR properly:
TEMP_FAILURE_RETRY (close (desc));
ENOSPC
EIO
EDQUOT
write can sometimes
not be detected until close. See section Input and Output Primitives, for details
on their meaning.
Please note that there is no separate close64 function.
This is not necessary since this function does not determine nor depend
on the mode of the file. The kernel which performs the close
operation knows which mode the descriptor is used for and can handle
this situation.
To close a stream, call fclose (see section Closing Streams) instead
of trying to close its underlying file descriptor with close.
This flushes any buffered output and updates the stream object to
indicate that it is closed.
This section describes the functions for performing primitive input and
output operations on file descriptors: read, write, and
lseek. These functions are declared in the header file
`unistd.h'.
size_t,
but must be a signed type.
read function reads up to size bytes from the file
with descriptor filedes, storing the results in the buffer.
(This is not necessarily a character string, and no terminating null
character is added.)
The return value is the number of bytes actually read. This might be less than size; for example, if there aren't that many bytes left in the file or if there aren't that many bytes immediately available. The exact behavior depends on what kind of file it is. Note that reading less than size bytes is not an error.
A value of zero indicates end-of-file (except if the value of the
size argument is also zero). This is not considered an error.
If you keep calling read while at end-of-file, it will keep
returning zero and doing nothing else.
If read returns at least one character, there is no way you can
tell whether end-of-file was reached. But if you did reach the end, the
next read will return zero.
In case of an error, read returns @math{-1}. The following
errno error conditions are defined for this function:
EAGAIN
read waits for
some input. But if the O_NONBLOCK flag is set for the file
(see section File Status Flags), read returns immediately without
reading any data, and reports this error.
Compatibility Note: Most versions of BSD Unix use a different
error code for this: EWOULDBLOCK. In the GNU library,
EWOULDBLOCK is an alias for EAGAIN, so it doesn't matter
which name you use.
On some systems, reading a large amount of data from a character special
file can also fail with EAGAIN if the kernel cannot find enough
physical memory to lock down the user's pages. This is limited to
devices that transfer with direct memory access into the user's memory,
which means it does not include terminals, since they always use
separate buffers inside the kernel. This problem never happens in the
GNU system.
Any condition that could result in EAGAIN can instead result in a
successful read which returns fewer bytes than requested.
Calling read again immediately would result in EAGAIN.
EBADF
EINTR
read was interrupted by a signal while it was waiting for input.
See section Primitives Interrupted by Signals. A signal will not necessary cause
read to return EINTR; it may instead result in a
successful read which returns fewer bytes than requested.
EIO
EIO also occurs when a background process tries to read from the
controlling terminal, and the normal action of stopping the process by
sending it a SIGTTIN signal isn't working. This might happen if
the signal is being blocked or ignored, or because the process group is
orphaned. See section Job Control, for more information about job control,
and section Signal Handling, for information about signals.
Please note that there is no function named read64. This is not
necessary since this function does not directly modify or handle the
possibly wide file offset. Since the kernel handles this state
internally, the read function can be used for all cases.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time read is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this, calls to read should be
protected using cancellation handlers.
The read function is the underlying primitive for all of the
functions that read from streams, such as fgetc.
pread function is similar to the read function. The
first three arguments are identical, and the return values and error
codes also correspond.
The difference is the fourth argument and its handling. The data block
is not read from the current position of the file descriptor
filedes. Instead the data is read from the file starting at
position offset. The position of the file descriptor itself is
not affected by the operation. The value is the same as before the call.
When the source file is compiled with _FILE_OFFSET_BITS == 64 the
pread function is in fact pread64 and the type
off_t has 64 bits, which makes it possible to handle files up to
@math{2^63} bytes in length.
The return value of pread describes the number of bytes read.
In the error case it returns @math{-1} like read does and the
error codes are also the same, with these additions:
EINVAL
ESPIPE
The function is an extension defined in the Unix Single Specification version 2.
pread function. The difference
is that the offset parameter is of type off64_t instead of
off_t which makes it possible on 32 bit machines to address
files larger than @math{2^31} bytes and up to @math{2^63} bytes. The
file descriptor filedes must be opened using open64 since
otherwise the large offsets possible with off64_t will lead to
errors with a descriptor in small file mode.
When the source file is compiled with _FILE_OFFSET_BITS == 64 on a
32 bit machine this function is actually available under the name
pread and so transparently replaces the 32 bit interface.
write function writes up to size bytes from
buffer to the file with descriptor filedes. The data in
buffer is not necessarily a character string and a null character is
output like any other character.
The return value is the number of bytes actually written. This may be
size, but can always be smaller. Your program should always call
write in a loop, iterating until all the data is written.
Once write returns, the data is enqueued to be written and can be
read back right away, but it is not necessarily written out to permanent
storage immediately. You can use fsync when you need to be sure
your data has been permanently stored before continuing. (It is more
efficient for the system to batch up consecutive writes and do them all
at once when convenient. Normally they will always be written to disk
within a minute or less.) Modern systems provide another function
fdatasync which guarantees integrity only for the file data and
is therefore faster.
You can use the O_FSYNC open mode to make write always
store the data to disk before returning; see section I/O Operating Modes.
In the case of an error, write returns @math{-1}. The following
errno error conditions are defined for this function:
EAGAIN
write blocks until the write operation is complete.
But if the O_NONBLOCK flag is set for the file (see section Control Operations on Files), it returns immediately without writing any data and
reports this error. An example of a situation that might cause the
process to block on output is writing to a terminal device that supports
flow control, where output has been suspended by receipt of a STOP
character.
Compatibility Note: Most versions of BSD Unix use a different
error code for this: EWOULDBLOCK. In the GNU library,
EWOULDBLOCK is an alias for EAGAIN, so it doesn't matter
which name you use.
On some systems, writing a large amount of data from a character special
file can also fail with EAGAIN if the kernel cannot find enough
physical memory to lock down the user's pages. This is limited to
devices that transfer with direct memory access into the user's memory,
which means it does not include terminals, since they always use
separate buffers inside the kernel. This problem does not arise in the
GNU system.
EBADF
EFBIG
EINTR
write operation was interrupted by a signal while it was
blocked waiting for completion. A signal will not necessarily cause
write to return EINTR; it may instead result in a
successful write which writes fewer bytes than requested.
See section Primitives Interrupted by Signals.
EIO
ENOSPC
EPIPE
SIGPIPE
signal is also sent to the process; see section Signal Handling.
Unless you have arranged to prevent EINTR failures, you should
check errno after each failing call to write, and if the
error was EINTR, you should simply repeat the call.
See section Primitives Interrupted by Signals. The easy way to do this is with the
macro TEMP_FAILURE_RETRY, as follows:
nbytes = TEMP_FAILURE_RETRY (write (desc, buffer, count));
Please note that there is no function named write64. This is not
necessary since this function does not directly modify or handle the
possibly wide file offset. Since the kernel handles this state
internally the write function can be used for all cases.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time write is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this, calls to write should be
protected using cancellation handlers.
The write function is the underlying primitive for all of the
functions that write to streams, such as fputc.
pwrite function is similar to the write function. The
first three arguments are identical, and the return values and error codes
also correspond.
The difference is the fourth argument and its handling. The data block
is not written to the current position of the file descriptor
filedes. Instead the data is written to the file starting at
position offset. The position of the file descriptor itself is
not affected by the operation. The value is the same as before the call.
When the source file is compiled with _FILE_OFFSET_BITS == 64 the
pwrite function is in fact pwrite64 and the type
off_t has 64 bits, which makes it possible to handle files up to
@math{2^63} bytes in length.
The return value of pwrite describes the number of written bytes.
In the error case it returns @math{-1} like write does and the
error codes are also the same, with these additions:
EINVAL
ESPIPE
The function is an extension defined in the Unix Single Specification version 2.
pwrite function. The difference
is that the offset parameter is of type off64_t instead of
off_t which makes it possible on 32 bit machines to address
files larger than @math{2^31} bytes and up to @math{2^63} bytes. The
file descriptor filedes must be opened using open64 since
otherwise the large offsets possible with off64_t will lead to
errors with a descriptor in small file mode.
When the source file is compiled using _FILE_OFFSET_BITS == 64 on a
32 bit machine this function is actually available under the name
pwrite and so transparently replaces the 32 bit interface.
Just as you can set the file position of a stream with fseek, you
can set the file position of a descriptor with lseek. This
specifies the position in the file for the next read or
write operation. See section File Positioning, for more information
on the file position and what it means.
To read the current file position value from a descriptor, use
lseek (desc, 0, SEEK_CUR).
lseek function is used to change the file position of the
file with descriptor filedes.
The whence argument specifies how the offset should be
interpreted, in the same way as for the fseek function, and it must
be one of the symbolic constants SEEK_SET, SEEK_CUR, or
SEEK_END.
SEEK_SET
SEEK_CUR
SEEK_END
The return value from lseek is normally the resulting file
position, measured in bytes from the beginning of the file.
You can use this feature together with SEEK_CUR to read the
current file position.
If you want to append to the file, setting the file position to the
current end of file with SEEK_END is not sufficient. Another
process may write more data after you seek but before you write,
extending the file so the position you write onto clobbers their data.
Instead, use the O_APPEND operating mode; see section I/O Operating Modes.
You can set the file position past the current end of the file. This
does not by itself make the file longer; lseek never changes the
file. But subsequent output at that position will extend the file.
Characters between the previous end of file and the new position are
filled with zeros. Extending the file in this way can create a
"hole": the blocks of zeros are not actually allocated on disk, so the
file takes up less space than it appears to; it is then called a
"sparse file".
If the file position cannot be changed, or the operation is in some way
invalid, lseek returns a value of @math{-1}. The following
errno error conditions are defined for this function:
EBADF
EINVAL
ESPIPE
ESPIPE if the object is not seekable.)
When the source file is compiled with _FILE_OFFSET_BITS == 64 the
lseek function is in fact lseek64 and the type
off_t has 64 bits which makes it possible to handle files up to
@math{2^63} bytes in length.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time lseek is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this calls to lseek should be
protected using cancellation handlers.
The lseek function is the underlying primitive for the
fseek, fseeko, ftell, ftello and
rewind functions, which operate on streams instead of file
descriptors.
lseek function. The difference
is that the offset parameter is of type off64_t instead of
off_t which makes it possible on 32 bit machines to address
files larger than @math{2^31} bytes and up to @math{2^63} bytes. The
file descriptor filedes must be opened using open64 since
otherwise the large offsets possible with off64_t will lead to
errors with a descriptor in small file mode.
When the source file is compiled with _FILE_OFFSET_BITS == 64 on a
32 bits machine this function is actually available under the name
lseek and so transparently replaces the 32 bit interface.
You can have multiple descriptors for the same file if you open the file
more than once, or if you duplicate a descriptor with dup.
Descriptors that come from separate calls to open have independent
file positions; using lseek on one descriptor has no effect on the
other. For example,
{
int d1, d2;
char buf[4];
d1 = open ("foo", O_RDONLY);
d2 = open ("foo", O_RDONLY);
lseek (d1, 1024, SEEK_SET);
read (d2, buf, 4);
}
will read the first four characters of the file `foo'. (The error-checking code necessary for a real program has been omitted here for brevity.)
By contrast, descriptors made by duplication share a common file position with the original descriptor that was duplicated. Anything which alters the file position of one of the duplicates, including reading or writing data, affects all of them alike. Thus, for example,
{
int d1, d2, d3;
char buf1[4], buf2[4];
d1 = open ("foo", O_RDONLY);
d2 = dup (d1);
d3 = dup (d2);
lseek (d3, 1024, SEEK_SET);
read (d1, buf1, 4);
read (d2, buf2, 4);
}
will read four characters starting with the 1024'th character of `foo', and then four more characters starting with the 1028'th character.
fpos_t or long int.
If the source is compiled with _FILE_OFFSET_BITS == 64 this type
is transparently replaced by off64_t.
off_t. The difference is that even
on 32 bit machines, where the off_t type would have 32 bits,
off64_t has 64 bits and so is able to address files up to
@math{2^63} bytes in length.
When compiling with _FILE_OFFSET_BITS == 64 this type is
available under the name off_t.
These aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'.
L_SET
SEEK_SET.
L_INCR
SEEK_CUR.
L_XTND
SEEK_END.
Given an open file descriptor, you can create a stream for it with the
fdopen function. You can get the underlying file descriptor for
an existing stream with the fileno function. These functions are
declared in the header file `stdio.h'.
fdopen function returns a new stream for the file descriptor
filedes.
The opentype argument is interpreted in the same way as for the
fopen function (see section Opening Streams), except that
the `b' option is not permitted; this is because GNU makes no
distinction between text and binary files. Also, "w" and
"w+" do not cause truncation of the file; these have an effect only
when opening a file, and in this case the file has already been opened.
You must make sure that the opentype argument matches the actual
mode of the open file descriptor.
The return value is the new stream. If the stream cannot be created (for example, if the modes for the file indicated by the file descriptor do not permit the access specified by the opentype argument), a null pointer is returned instead.
In some other systems, fdopen may fail to detect that the modes
for file descriptor do not permit the access specified by
opentype. The GNU C library always checks for this.
For an example showing the use of the fdopen function,
see section Creating a Pipe.
fileno returns @math{-1}.
fileno_unlocked function is equivalent to the fileno
function except that it does not implicitly lock the stream if the state
is FSETLOCKING_INTERNAL.
This function is a GNU extension.
There are also symbolic constants defined in `unistd.h' for the
file descriptors belonging to the standard streams stdin,
stdout, and stderr; see section Standard Streams.
STDIN_FILENO
0, which is the file descriptor for
standard input.
STDOUT_FILENO
1, which is the file descriptor for
standard output.
STDERR_FILENO
2, which is the file descriptor for
standard error output.
You can have multiple file descriptors and streams (let's call both streams and descriptors "channels" for short) connected to the same file, but you must take care to avoid confusion between channels. There are two cases to consider: linked channels that share a single file position value, and independent channels that have their own file positions.
It's best to use just one channel in your program for actual data
transfer to any given file, except when all the access is for input.
For example, if you open a pipe (something you can only do at the file
descriptor level), either do all I/O with the descriptor, or construct a
stream from the descriptor with fdopen and then do all I/O with
the stream.
Channels that come from a single opening share the same file position;
we call them linked channels. Linked channels result when you
make a stream from a descriptor using fdopen, when you get a
descriptor from a stream with fileno, when you copy a descriptor
with dup or dup2, and when descriptors are inherited
during fork. For files that don't support random access, such as
terminals and pipes, all channels are effectively linked. On
random-access files, all append-type output streams are effectively
linked to each other.
If you have been using a stream for I/O, and you want to do I/O using another channel (either a stream or a descriptor) that is linked to it, you must first clean up the stream that you have been using. See section Cleaning Streams.
Terminating a process, or executing a new program in the process, destroys all the streams in the process. If descriptors linked to these streams persist in other processes, their file positions become undefined as a result. To prevent this, you must clean up the streams before destroying them.
When you open channels (streams or descriptors) separately on a seekable file, each channel has its own file position. These are called independent channels.
The system handles each channel independently. Most of the time, this is quite predictable and natural (especially for input): each channel can read or write sequentially at its own place in the file. However, if some of the channels are streams, you must take these precautions:
If you do output to one channel at the end of the file, this will certainly leave the other independent channels positioned somewhere before the new end. You cannot reliably set their file positions to the new end of file before writing, because the file can always be extended by another process between when you set the file position and when you write the data. Instead, use an append-type descriptor or stream; they always output at the current end of the file. In order to make the end-of-file position accurate, you must clean the output channel you were using, if it is a stream.
It's impossible for two channels to have separate file pointers for a file that doesn't support random access. Thus, channels for reading or writing such files are always linked, never independent. Append-type channels are also always linked. For these channels, follow the rules for linked channels; see section Linked Channels.
On the GNU system, you can clean up any stream with fclean:
On other systems, you can use fflush to clean a stream in most
cases.
You can skip the fclean or fflush if you know the stream
is already clean. A stream is clean whenever its buffer is empty. For
example, an unbuffered stream is always clean. An input stream that is
at end-of-file is clean. A line-buffered stream is clean when the last
character output was a newline.
There is one case in which cleaning a stream is impossible on most
systems. This is when the stream is doing input from a file that is not
random-access. Such streams typically read ahead, and when the file is
not random access, there is no way to give back the excess data already
read. When an input stream reads from a random-access file,
fflush does clean the stream, but leaves the file pointer at an
unpredictable place; you must set the file pointer before doing any
further I/O. On the GNU system, using fclean avoids both of
these problems.
Closing an output-only stream also does fflush, so this is a
valid way of cleaning an output stream. On the GNU system, closing an
input stream does fclean.
You need not clean a stream before using its descriptor for control operations such as setting terminal modes; these operations don't affect the file position and are not affected by it. You can use any descriptor for these operations, and all channels are affected simultaneously. However, text already "output" to a stream but still buffered by the stream will be subject to the new terminal modes when subsequently flushed. To make sure "past" output is covered by the terminal settings that were in effect at the time, flush the output streams for that terminal before setting the modes. See section Terminal Modes.
Some applications may need to read or write data to multiple buffers,
which are separated in memory. Although this can be done easily enough
with multiple calls to read and write, it is inefficent
because there is overhead associated with each kernel call.
Instead, many platforms provide special high-speed primitives to perform
these scatter-gather operations in a single kernel call. The GNU C
library will provide an emulation on any system that lacks these
primitives, so they are not a portability threat. They are defined in
sys/uio.h.
These functions are controlled with arrays of iovec structures,
which describe the location and size of each buffer.
The iovec structure describes a buffer. It contains two fields:
void *iov_base
size_t iov_len
The readv function reads data from filedes and scatters it
into the buffers described in vector, which is taken to be
count structures long. As each buffer is filled, data is sent to the
next.
Note that readv is not guaranteed to fill all the buffers.
It may stop at any point, for the same reasons read would.
The return value is a count of bytes (not buffers) read, @math{0}
indicating end-of-file, or @math{-1} indicating an error. The possible
errors are the same as in read.
The writev function gathers data from the buffers described in
vector, which is taken to be count structures long, and writes
them to filedes. As each buffer is written, it moves on to the
next.
Like readv, writev may stop midstream under the same
conditions write would.
The return value is a count of bytes written, or @math{-1} indicating an
error. The possible errors are the same as in write.
Note that if the buffers are small (under about 1kB), high-level streams
may be easier to use than these functions. However, readv and
writev are more efficient when the individual buffers themselves
(as opposed to the total output), are large. In that case, a high-level
stream would not be able to cache the data effectively.
On modern operating systems, it is possible to mmap (pronounced "em-map") a file to a region of memory. When this is done, the file can be accessed just like an array in the program.
This is more efficent than read or write, as only the regions
of the file that a program actually accesses are loaded. Accesses to
not-yet-loaded parts of the mmapped region are handled in the same way as
swapped out pages.
Since mmapped pages can be stored back to their file when physical memory is low, it is possible to mmap files orders of magnitude larger than both the physical memory and swap space. The only limit is address space. The theoretical limit is 4GB on a 32-bit machine - however, the actual limit will be smaller since some areas will be reserved for other purposes. If the LFS interface is used the file size on 32-bit systems is not limited to 2GB (offsets are signed which reduces the addressable area of 4GB by half); the full 64-bit are available.
Memory mapping only works on entire pages of memory. Thus, addresses for mapping must be page-aligned, and length values will be rounded up. To determine the size of a page the machine uses one should use
size_t page_size = (size_t) sysconf (_SC_PAGESIZE);
These functions are declared in `sys/mman.h'.
The mmap function creates a new mapping, connected to bytes
(offset) to (offset + length) in the file open on
filedes.
address gives a preferred starting address for the mapping.
NULL expresses no preference. Any previous mapping at that
address is automatically removed. The address you give may still be
changed, unless you use the MAP_FIXED flag.
protect contains flags that control what kind of access is
permitted. They include PROT_READ, PROT_WRITE, and
PROT_EXEC, which permit reading, writing, and execution,
respectively. Inappropriate access will cause a segfault (see section Program Error Signals).
Note that most hardware designs cannot support write permission without
read permission, and many do not distinguish read and execute permission.
Thus, you may receive wider permissions than you ask for, and mappings of
write-only files may be denied even if you do not use PROT_READ.
flags contains flags that control the nature of the map.
One of MAP_SHARED or MAP_PRIVATE must be specified.
They include:
MAP_PRIVATE
PROT_WRITE.
MAP_SHARED
msync, described below, if it is important that other processes
using conventional I/O get a consistent view of the file.
MAP_FIXED
MAP_ANONYMOUS
MAP_ANON
malloc for large blocks. This is not an issue with the GNU C library,
as the included malloc automatically uses mmap where appropriate.
mmap returns the address of the new mapping, or @math{-1} for an
error.
Possible errors include:
EINVAL
EACCES
ENOMEM
ENODEV
ENOEXEC
mmap64 function is equivalent to the mmap function but
the offset parameter is of type off64_t. On 32-bit systems
this allows the file associated with the filedes descriptor to be
larger than 2GB. filedes must be a descriptor returned from a
call to open64 or fopen64 and freopen64 where the
descriptor is retrieved with fileno.
When the sources are translated with _FILE_OFFSET_BITS == 64 this
function is actually available under the name mmap. I.e., the
new, extended API using 64 bit file sizes and offsets transparently
replaces the old API.
munmap removes any memory maps from (addr) to (addr +
length). length should be the length of the mapping.
It is safe to unmap multiple mappings in one command, or include unmapped space in the range. It is also possible to unmap only part of an existing mapping. However, only entire pages can be removed. If length is not an even number of pages, it will be rounded up.
It returns @math{0} for success and @math{-1} for an error.
One error is possible:
EINVAL
When using shared mappings, the kernel can write the file at any time before the mapping is removed. To be certain data has actually been written to the file and will be accessible to non-memory-mapped I/O, it is necessary to use this function.
It operates on the region address to (address + length). It may be used on part of a mapping or multiple mappings, however the region given should not contain any unmapped space.
flags can contain some options:
MS_SYNC
msync only makes sure that accesses to a file with
conventional I/O reflect the recent changes.
MS_ASYNC
msync to begin the synchronization, but not to wait for
it to complete.
msync returns @math{0} for success and @math{-1} for
error. Errors include:
EINVAL
EFAULT
This function can be used to change the size of an existing memory
area. address and length must cover a region entirely mapped
in the same mmap statement. A new mapping with the same
characteristics will be returned with the length new_length.
One option is possible, MREMAP_MAYMOVE. If it is given in
flags, the system may remove the existing mapping and create a new
one of the desired length in another location.
The address of the resulting mapping is returned, or @math{-1}. Possible error codes include:
EFAULT
EINVAL
EAGAIN
ENOMEM
MREMAP_MAYMOVE is not given and the extension would collide with
another mapped region.
This function is only available on a few systems. Except for performing optional optimizations one should not rely on this function.
Not all file descriptors may be mapped. Sockets, pipes, and most devices
only allow sequential access and do not fit into the mapping abstraction.
In addition, some regular files may not be mmapable, and older kernels may
not support mapping at all. Thus, programs using mmap should
have a fallback method to use should it fail. See section `Mmap' in GNU Coding Standards.
Sometimes a program needs to accept input on multiple input channels whenever input arrives. For example, some workstations may have devices such as a digitizing tablet, function button box, or dial box that are connected via normal asynchronous serial interfaces; good user interface style requires responding immediately to input on any device. Another example is a program that acts as a server to several other processes via pipes or sockets.
You cannot normally use read for this purpose, because this
blocks the program until input is available on one particular file
descriptor; input on other channels won't wake it up. You could set
nonblocking mode and poll each file descriptor in turn, but this is very
inefficient.
A better solution is to use the select function. This blocks the
program until input or output is ready on a specified set of file
descriptors, or until a timer expires, whichever comes first. This
facility is declared in the header file `sys/types.h'.
In the case of a server socket (see section Listening for Connections), we say that
"input" is available when there are pending connections that could be
accepted (see section Accepting Connections). accept for server
sockets blocks and interacts with select just as read does
for normal input.
The file descriptor sets for the select function are specified
as fd_set objects. Here is the description of the data type
and some macros for manipulating these objects.
fd_set data type represents file descriptor sets for the
select function. It is actually a bit array.
fd_set object can hold information about. On systems with a
fixed maximum number, FD_SETSIZE is at least that number. On
some systems, including GNU, there is no absolute limit on the number of
descriptors open, but this macro still has a constant value which
controls the number of bits in an fd_set; if you get a file
descriptor with a value as high as FD_SETSIZE, you cannot put
that descriptor into an fd_set.
Next, here is the description of the select function itself.
select function blocks the calling process until there is
activity on any of the specified sets of file descriptors, or until the
timeout period has expired.
The file descriptors specified by the read-fds argument are checked to see if they are ready for reading; the write-fds file descriptors are checked to see if they are ready for writing; and the except-fds file descriptors are checked for exceptional conditions. You can pass a null pointer for any of these arguments if you are not interested in checking for that kind of condition.
A file descriptor is considered ready for reading if it is not at end of
file. A server socket is considered ready for reading if there is a
pending connection which can be accepted with accept;
see section Accepting Connections. A client socket is ready for writing when
its connection is fully established; see section Making a Connection.
"Exceptional conditions" does not mean errors--errors are reported immediately when an erroneous system call is executed, and do not constitute a state of the descriptor. Rather, they include conditions such as the presence of an urgent message on a socket. (See section Sockets, for information on urgent messages.)
The select function checks only the first nfds file
descriptors. The usual thing is to pass FD_SETSIZE as the value
of this argument.
The timeout specifies the maximum time to wait. If you pass a
null pointer for this argument, it means to block indefinitely until one
of the file descriptors is ready. Otherwise, you should provide the
time in struct timeval format; see section High-Resolution Calendar. Specify zero as the time (a struct timeval containing
all zeros) if you want to find out which descriptors are ready without
waiting if none are ready.
The normal return value from select is the total number of ready file
descriptors in all of the sets. Each of the argument sets is overwritten
with information about the descriptors that are ready for the corresponding
operation. Thus, to see if a particular descriptor desc has input,
use FD_ISSET (desc, read-fds) after select returns.
If select returns because the timeout period expires, it returns
a value of zero.
Any signal will cause select to return immediately. So if your
program uses signals, you can't rely on select to keep waiting
for the full time specified. If you want to be sure of waiting for a
particular amount of time, you must check for EINTR and repeat
the select with a newly calculated timeout based on the current
time. See the example below. See also section Primitives Interrupted by Signals.
If an error occurs, select returns -1 and does not modify
the argument file descriptor sets. The following errno error
conditions are defined for this function:
EBADF
EINTR
EINVAL
Portability Note: The select function is a BSD Unix
feature.
Here is an example showing how you can use select to establish a
timeout period for reading from a file descriptor. The input_timeout
function blocks the calling process until input is available on the
file descriptor, or until the timeout period expires.
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>
int
input_timeout (int filedes, unsigned int seconds)
{
fd_set set;
struct timeval timeout;
/* Initialize the file descriptor set. */
FD_ZERO (&set);
FD_SET (filedes, &set);
/* Initialize the timeout data structure. */
timeout.tv_sec = seconds;
timeout.tv_usec = 0;
/* select returns 0 if timeout, 1 if input available, -1 if error. */
return TEMP_FAILURE_RETRY (select (FD_SETSIZE,
&set, NULL, NULL,
&timeout));
}
int
main (void)
{
fprintf (stderr, "select returned %d.\n",
input_timeout (STDIN_FILENO, 5));
return 0;
}
There is another example showing the use of select to multiplex
input from multiple sockets in section Byte Stream Connection Server Example.
In most modern operating systems the normal I/O operations are not
executed synchronously. I.e., even if a write system call
returns this does not mean the data is actually written to the media,
e.g., the disk.
In situations where synchronization points are necessary,you can use special functions which ensure that all operations finish before they return.
A prototype for sync can be found in `unistd.h'.
The return value is zero to indicate no error.
Programs more often want to ensure that data written to a given file is
committed, rather than all data in the system. For this, sync is overkill.
fsync can be used to make sure all data associated with the
open file fildes is written to the device associated with the
descriptor. The function call does not return unless all actions have
finished.
A prototype for fsync can be found in `unistd.h'.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time fsync is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this, calls to fsync should be
protected using cancellation handlers.
The return value of the function is zero if no error occurred. Otherwise it is @math{-1} and the global variable errno is set to the following values:
EBADF
EINVAL
Sometimes it is not even necessary to write all data associated with a file descriptor. E.g., in database files which do not change in size it is enough to write all the file content data to the device. Meta-information like the modification time etc. are not that important and leaving such information uncommitted does not prevent a successful recovering of the file in case of a problem.
fdatasync function returns, it is ensured
that all of the file data is written to the device. For all pending I/O
operations, the parts guaranteeing data integrity finished.
Not all systems implement the fdatasync operation. On systems
missing this functionality fdatasync is emulated by a call to
fsync since the performed actions are a superset of those
required by fdatasyn.
The prototype for fdatasync is in `unistd.h'.
The return value of the function is zero if no error occurred. Otherwise it is @math{-1} and the global variable errno is set to the following values:
EBADF
EINVAL
The POSIX.1b standard defines a new set of I/O operations which can
significantly reduce the time an application spends waiting at I/O. The
new functions allow a program to initiate one or more I/O operations and
then immediately resume normal work while the I/O operations are
executed in parallel. This functionality is available if the
`unistd.h' file defines the symbol _POSIX_ASYNCHRONOUS_IO.
These functions are part of the library with realtime functions named `librt'. They are not actually part of the `libc' binary. The implementation of these functions can be done using support in the kernel (if available) or using an implementation based on threads at userlevel. In the latter case it might be necessary to link applications with the thread library `libpthread' in addition to `librt'.
All AIO operations operate on files which were opened previously. There
might be arbitrarily many operations running for one file. The
asynchronous I/O operations are controlled using a data structure named
struct aiocb (AIO control block). It is defined in
`aio.h' as follows.
struct aiocb structure
contains at least the members described in the following table. There
might be more elements which are used by the implementation, but
depending on these elements is not portable and is highly deprecated.
int aio_fildes
lseek call would lead to an error.
off_t aio_offset
volatile void *aio_buf
size_t aio_nbytes
aio_buf.
int aio_reqprio
_POSIX_PRIORITIZED_IO and
_POSIX_PRIORITY_SCHEDULING the AIO requests are
processed based on the current scheduling priority. The
aio_reqprio element can then be used to lower the priority of the
AIO operation.
struct sigevent aio_sigevent
sigev_notify element is
SIGEV_NONE no notification is send. If it is SIGEV_SIGNAL
the signal determined by sigev_signo is send. Otherwise
sigev_notify must be SIGEV_THREAD. In this case a thread
is created which starts executing the function pointed to by
sigev_notify_function.
int aio_lio_opcode
lio_listio and
lio_listio64 functions. Since these functions allow an
arbitrary number of operations to start at once, and each operation can be
input or output (or nothing), the information must be stored in the
control block. The possible values are:
LIO_READ
aio_offset and store the next aio_nbytes bytes in the
buffer pointed to by aio_buf.
LIO_WRITE
aio_nbytes bytes starting at
aio_buf into the file starting at position aio_offset.
LIO_NOP
struct aiocb values contains holes, i.e., some of the
values must not be handled although the whole array is presented to the
lio_listio function.
When the sources are compiled using _FILE_OFFSET_BITS == 64 on a
32 bit machine this type is in fact struct aiocb64 since the LFS
interface transparently replaces the struct aiocb definition.
For use with the AIO functions defined in the LFS there is a similar type
defined which replaces the types of the appropriate members with larger
types but otherwise is equivalent to struct aiocb. Particularly,
all member names are the same.
int aio_fildes
lseek call would lead to an error.
off64_t aio_offset
volatile void *aio_buf
size_t aio_nbytes
aio_buf.
int aio_reqprio
_POSIX_PRIORITIZED_IO and
_POSIX_PRIORITY_SCHEDULING are defined the AIO requests are
processed based on the current scheduling priority. The
aio_reqprio element can then be used to lower the priority of the
AIO operation.
struct sigevent aio_sigevent
sigev_notify element is
SIGEV_NONE no notification is sent. If it is SIGEV_SIGNAL
the signal determined by sigev_signo is sent. Otherwise
sigev_notify must be SIGEV_THREAD in which case a thread
which starts executing the function pointed to by
sigev_notify_function.
int aio_lio_opcode
lio_listio and
[lio_listio64 functions. Since these functions allow an
arbitrary number of operations to start at once, and since each operation can be
input or output (or nothing), the information must be stored in the
control block. See the description of struct aiocb for a description
of the possible values.
When the sources are compiled using _FILE_OFFSET_BITS == 64 on a
32 bit machine this type is available under the name struct
aiocb64 since the LFS replaces transparently the old interface.
The first aiocbp->aio_nbytes bytes of the file for which
aiocbp->aio_fildes is a descriptor are written to the buffer
starting at aiocbp->aio_buf. Reading starts at the absolute
position aiocbp->aio_offset in the file.
If prioritized I/O is supported by the platform the
aiocbp->aio_reqprio value is used to adjust the priority before
the request is actually enqueued.
The calling process is notified about the termination of the read
request according to the aiocbp->aio_sigevent value.
When aio_read returns, the return value is zero if no error
occurred that can be found before the process is enqueued. If such an
early error is found, the function returns @math{-1} and sets
errno to one of the following values:
EAGAIN
ENOSYS
aio_read function is not implemented.
EBADF
aiocbp->aio_fildes descriptor is not valid. This condition
need not be recognized before enqueueing the request and so this error
might also be signaled asynchronously.
EINVAL
aiocbp->aio_offset or aiocbp->aio_reqpiro value is
invalid. This condition need not be recognized before enqueueing the
request and so this error might also be signaled asynchronously.
If aio_read returns zero, the current status of the request
can be queried using aio_error and aio_return functions.
As long as the value returned by aio_error is EINPROGRESS
the operation has not yet completed. If aio_error returns zero,
the operation successfully terminated, otherwise the value is to be
interpreted as an error code. If the function terminated, the result of
the operation can be obtained using a call to aio_return. The
returned value is the same as an equivalent call to read would
have returned. Possible error codes returned by aio_error are:
EBADF
aiocbp->aio_fildes descriptor is not valid.
ECANCELED
EINVAL
aiocbp->aio_offset value is invalid.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is in fact aio_read64 since the LFS interface transparently
replaces the normal implementation.
aio_read function. The only
difference is that on 32 bit machines the file descriptor should
be opened in the large file mode. Internally aio_read64 uses
functionality equivalent to lseek64 (see section Setting the File Position of a Descriptor) to position the file descriptor correctly for the reading,
as opposed to lseek functionality used in aio_read.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is available under the name aio_read and so transparently
replaces the interface for small files on 32 bit machines.
To write data asynchronously to a file there exists an equivalent pair of functions with a very similar interface.
The first aiocbp->aio_nbytes bytes from the buffer starting at
aiocbp->aio_buf are written to the file for which
aiocbp->aio_fildes is an descriptor, starting at the absolute
position aiocbp->aio_offset in the file.
If prioritized I/O is supported by the platform the
aiocbp->aio_reqprio value is used to adjust the priority before
the request is actually enqueued.
The calling process is notified about the termination of the read
request according to the aiocbp->aio_sigevent value.
When aio_write returns the return value is zero if no error
occurred that can be found before the process is enqueued. If such an
early error is found the function returns @math{-1} and sets
errno to one of the following values.
EAGAIN
ENOSYS
aio_write function is not implemented.
EBADF
aiocbp->aio_fildes descriptor is not valid. This condition
needs not be recognized before enqueueing the request and so this error
might also be signaled asynchronously.
EINVAL
aiocbp->aio_offset or aiocbp->aio_reqpiro value is
invalid. This condition needs not be recognized before enqueueing the
request and so this error might also be signaled asynchronously.
In the case aio_write returns zero the current status of the
request can be queried using aio_error and aio_return
functions. As long as the value returned by aio_error is
EINPROGRESS the operation has not yet completed. If
aio_error returns zero the operation successfully terminated,
otherwise the value is to be interpreted as an error code. If the
function terminated the result of the operation can be get using a call
to aio_return. The returned value is the same as an equivalent
call to read would have returned. Possible error code returned
by aio_error are:
EBADF
aiocbp->aio_fildes descriptor is not valid.
ECANCELED
EINVAL
aiocbp->aio_offset value is invalid.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is in fact aio_write64 since the LFS interface transparently
replaces the normal implementation.
aio_write function. The only
difference is that on 32 bit machines the file descriptor should
be opened in the large file mode. Internally aio_write64 uses
functionality equivalent to lseek64 (see section Setting the File Position of a Descriptor) to position the file descriptor correctly for the writing,
as opposed to lseek functionality used in aio_write.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is available under the name aio_write and so transparently
replaces the interface for small files on 32 bit machines.
Beside these functions with the more or less traditional interface
POSIX.1b also defines a function with can initiate more than one
operation at once and which can handled freely mixed read and write
operation. It is therefore similar to a combination of readv and
writev.
lio_listio function can be used to enqueue an arbitrary
number of read and write requests at one time. The requests can all be
meant for the same file, all for different files or every solution in
between.
lio_listio gets the nent requests from the array pointed to
by list. What operation has to be performed is determined by the
aio_lio_opcode member in each element of list. If this
field is LIO_READ an read operation is queued, similar to a call
of aio_read for this element of the array (except that the way
the termination is signalled is different, as we will see below). If
the aio_lio_opcode member is LIO_WRITE an write operation
is enqueued. Otherwise the aio_lio_opcode must be LIO_NOP
in which case this element of list is simply ignored. This
"operation" is useful in situations where one has a fixed array of
struct aiocb elements from which only a few need to be handled at
a time. Another situation is where the lio_listio call was
cancelled before all requests are processed (see section Cancellation of AIO Operations) and the remaining requests have to be reissued.
The other members of each element of the array pointed to by
list must have values suitable for the operation as described in
the documentation for aio_read and aio_write above.
The mode argument determines how lio_listio behaves after
having enqueued all the requests. If mode is LIO_WAIT it
waits until all requests terminated. Otherwise mode must be
LIO_NOWAIT and in this case the function returns immediately after
having enqueued all the requests. In this case the caller gets a
notification of the termination of all requests according to the
sig parameter. If sig is NULL no notification is
send. Otherwise a signal is sent or a thread is started, just as
described in the description for aio_read or aio_write.
If mode is LIO_WAIT the return value of lio_listio
is @math{0} when all requests completed successfully. Otherwise the
function return @math{-1} and errno is set accordingly. To find
out which request or requests failed one has to use the aio_error
function on all the elements of the array list.
In case mode is LIO_NOWAIT the function return @math{0} if
all requests were enqueued correctly. The current state of the requests
can be found using aio_error and aio_return as described
above. In case lio_listio returns @math{-1} in this mode the
global variable errno is set accordingly. If a request did not
yet terminate a call to aio_error returns EINPROGRESS. If
the value is different the request is finished and the error value (or
@math{0}) is returned and the result of the operation can be retrieved
using aio_return.
Possible values for errno are:
EAGAIN
EINVAL
AIO_LISTIO_MAX.
EIO
ENOSYS
lio_listio function is not supported.
If the mode parameter is LIO_NOWAIT and the caller cancels
an request the error status for this request returned by
aio_error is ECANCELED.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is in fact lio_listio64 since the LFS interface
transparently replaces the normal implementation.
aio_listio function. The only
difference is that only 32 bit machines the file descriptor should
be opened in the large file mode. Internally lio_listio64 uses
functionality equivalent to lseek64 (see section Setting the File Position of a Descriptor) to position the file descriptor correctly for the reading or
writing, as opposed to lseek functionality used in
lio_listio.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is available under the name lio_listio and so
transparently replaces the interface for small files on 32 bit
machines.
As already described in the documentation of the functions in the last
section, it must be possible to get information about the status of an I/O
request. When the operation is performed truly asynchronously (as with
aio_read and aio_write and with aio_listio when the
mode is LIO_NOWAIT) one sometimes needs to know whether a
specific request already terminated and if yes, what the result was.
The following two functions allow you to get this kind of information.
struct aiocb variable pointed to by aiocbp. If the
request has not yet terminated the value returned is always
EINPROGRESS. Once the request has terminated the value
aio_error returns is either @math{0} if the request completed
successfully or it returns the value which would be stored in the
errno variable if the request would have been done using
read, write, or fsync.
The function can return ENOSYS if it is not implemented. It
could also return EINVAL if the aiocbp parameter does not
refer to an asynchronous operation whose return status is not yet known.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is in fact aio_error64 since the LFS interface
transparently replaces the normal implementation.
aio_error with the only difference
that the argument is a reference to a variable of type struct
aiocb64.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is available under the name aio_error and so
transparently replaces the interface for small files on 32 bit
machines.
aio_error is EINPROGRESS the return of this function is
undefined.
Once the request is finished this function can be used exactly once to
retrieve the return value. Following calls might lead to undefined
behaviour. The return value itself is the value which would have been
returned by the read, write, or fsync call.
The function can return ENOSYS if it is not implemented. It
could also return EINVAL if the aiocbp parameter does not
refer to an asynchronous operation whose return status is not yet known.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is in fact aio_return64 since the LFS interface
transparently replaces the normal implementation.
aio_return with the only difference
that the argument is a reference to a variable of type struct
aiocb64.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is available under the name aio_return and so
transparently replaces the interface for small files on 32 bit
machines.
When dealing with asynchronous operations it is sometimes necessary to get into a consistent state. This would mean for AIO that one wants to know whether a certain request or a group of request were processed. This could be done by waiting for the notification sent by the system after the operation terminated, but this sometimes would mean wasting resources (mainly computation time). Instead POSIX.1b defines two functions which will help with most kinds of consistency.
The aio_fsync and aio_fsync64 functions are only available
if in `unistd.h' the symbol _POSIX_SYNCHRONIZED_IO is
defined.
aiocbp->aio_fildes into the synchronized I/O completion state
(see section Synchronizing I/O operations). The aio_fsync function returns
immediately but the notification through the method described in
aiocbp->aio_sigevent will happen only after all requests for this
file descriptor have terminated and the file is synchronized. This also
means that requests for this very same file descriptor which are queued
after the synchronization request are not affected.
If op is O_DSYNC the synchronization happens as with a call
to fdatasync. Otherwise op should be O_SYNC and
the synchronization happens as with fsync.
As long as the synchronization has not happened a call to
aio_error with the reference to the object pointed to by
aiocbp returns EINPROGRESS. Once the synchronization is
done aio_error return @math{0} if the synchronization was not
successful. Otherwise the value returned is the value to which the
fsync or fdatasync function would have set the
errno variable. In this case nothing can be assumed about the
consistency for the data written to this file descriptor.
The return value of this function is @math{0} if the request was
successfully filed. Otherwise the return value is @math{-1} and
errno is set to one of the following values:
EAGAIN
EBADF
aiocbp->aio_fildes is not valid or not open
for writing.
EINVAL
O_DSYNC and O_SYNC.
ENOSYS
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is in fact aio_return64 since the LFS interface
transparently replaces the normal implementation.
aio_fsync with the only difference
that the argument is a reference to a variable of type struct
aiocb64.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is available under the name aio_fsync and so
transparently replaces the interface for small files on 32 bit
machines.
Another method of synchronization is to wait until one or more requests of a
specific set terminated. This could be achieved by the aio_*
functions to notify the initiating process about the termination but in
some situations this is not the ideal solution. In a program which
constantly updates clients somehow connected to the server it is not
always the best solution to go round robin since some connections might
be slow. On the other hand letting the aio_* function notify the
caller might also be not the best solution since whenever the process
works on preparing data for on client it makes no sense to be
interrupted by a notification since the new client will not be handled
before the current client is served. For situations like this
aio_suspend should be used.
aio_suspend is called the function returns
immediately. Whether a request has terminated or not is done by
comparing the error status of the request with EINPROGRESS. If
an element of list is NULL the entry is simply ignored.
If no request has finished the calling process is suspended. If
timeout is NULL the process is not waked until a request
finished. If timeout is not NULL the process remains
suspended at as long as specified in timeout. In this case
aio_suspend returns with an error.
The return value of the function is @math{0} if one or more requests
from the list have terminated. Otherwise the function returns
@math{-1} and errno is set to one of the following values:
EAGAIN
EINTR
aio_suspend function. This signal might
also be sent by the AIO implementation while signalling the termination
of one of the requests.
ENOSYS
aio_suspend function is not implemented.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is in fact aio_suspend64 since the LFS interface
transparently replaces the normal implementation.
aio_suspend with the only difference
that the argument is a reference to a variable of type struct
aiocb64.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is available under the name aio_suspend and so
transparently replaces the interface for small files on 32 bit
machines.
When one or more requests are asynchronously processed it might be useful in some situations to cancel a selected operation, e.g., if it becomes obvious that the written data is not anymore accurate and would have to be overwritten soon. As an example assume an application, which writes data in files in a situation where new incoming data would have to be written in a file which will be updated by an enqueued request. The POSIX AIO implementation provides such a function but this function is not capable to force the cancellation of the request. It is up to the implementation to decide whether it is possible to cancel the operation or not. Therefore using this function is merely a hint.
aio_cancel function can be used to cancel one or more
outstanding requests. If the aiocbp parameter is NULL the
function tries to cancel all outstanding requests which would process
the file descriptor fildes (i.e.,, whose aio_fildes member
is fildes). If aiocbp is not NULL the very specific
request pointed to by aiocbp is tried to be cancelled.
For requests which were successfully cancelled the normal notification
about the termination of the request should take place. I.e., depending
on the struct sigevent object which controls this, nothing
happens, a signal is sent or a thread is started. If the request cannot
be cancelled it terminates the usual way after performing te operation.
After a request is successfully cancelled a call to aio_error with
a reference to this request as the parameter will return
ECANCELED and a call to aio_return will return @math{-1}.
If the request wasn't cancelled and is still running the error status is
still EINPROGRESS.
The return value of the function is AIO_CANCELED if there were
requests which haven't terminated and which successfully were cancelled.
If there is one or more request left which couldn't be cancelled the
return value is AIO_NOTCANCELED. In this case aio_error
must be used to find out which of the perhaps multiple requests (in
aiocbp is NULL) wasn't successfully cancelled. If all
requests already terminated at the time aio_cancel is called the
return value is AIO_ALLDONE.
If an error occurred during the execution of aio_cancel the
function returns @math{-1} and sets errno to one of the following
values.
EBADF
ENOSYS
aio_cancel is not implemented.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is in fact aio_cancel64 since the LFS interface
transparently replaces the normal implementation.
aio_cancel with the only difference
that the argument is a reference to a variable of type struct
aiocb64.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is available under the name aio_cancel and so
transparently replaces the interface for small files on 32 bit
machines.
The POSIX standard does not specify how the AIO functions are implemented. They could be system calls but it is also possible to emulate them at userlevel.
At least the available implementation at the point of this writing is a userlevel implementation which uses threads for handling the enqueued requests. This implementation requires to make some decisions about limitations but hard limitations are something which better should be avoided the GNU C library implementation provides a mean to tune the AIO implementation individually for each use.
aio_init
function.
int aio_threads
int aio_num
int aio_locks
int aio_usedba
int aio_debug
int aio_numusers
int aio_reserved[2]
Before calling the aio_init function the members of a variable of
type struct aioinit must be initialized. Then a reference to
this variable is passed as the parameter to aio_init which itself
may or may not pay attention to the hints.
The function has no return value and no error cases are defined. It is a extension which follows a proposal from the SGI implementation in Irix 6. It is not covered by POSIX.1b or Unix98.
This section describes how you can perform various other operations on
file descriptors, such as inquiring about or setting flags describing
the status of the file descriptor, manipulating record locks, and the
like. All of these operations are performed by the function fcntl.
The second argument to the fcntl function is a command that
specifies which operation to perform. The function and macros that name
various flags that are used with it are declared in the header file
`fcntl.h'. Many of these flags are also used by the open
function; see section Opening and Closing Files.
fcntl function performs the operation specified by
command on the file descriptor filedes. Some commands
require additional arguments to be supplied. These additional arguments
and the return value and error conditions are given in the detailed
descriptions of the individual commands.
Briefly, here is a list of what the various commands are.
F_DUPFD
F_GETFD
F_SETFD
F_GETFL
F_SETFL
F_GETLK
F_SETLK
F_SETLKW
F_SETLK, but wait for completion. See section File Locks.
F_GETOWN
SIGIO signals.
See section Interrupt-Driven Input.
F_SETOWN
SIGIO signals.
See section Interrupt-Driven Input.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time fcntl is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this calls to fcntl should be
protected using cancellation handlers.
You can duplicate a file descriptor, or allocate another file descriptor that refers to the same open file as the original. Duplicate descriptors share one file position and one set of file status flags (see section File Status Flags), but each has its own set of file descriptor flags (see section File Descriptor Flags).
The major use of duplicating a file descriptor is to implement redirection of input or output: that is, to change the file or pipe that a particular file descriptor corresponds to.
You can perform this operation using the fcntl function with the
F_DUPFD command, but there are also convenient functions
dup and dup2 for duplicating descriptors.
The fcntl function and flags are declared in `fcntl.h',
while prototypes for dup and dup2 are in the header file
`unistd.h'.
fcntl (old, F_DUPFD, 0).
If old is an invalid descriptor, then dup2 does nothing; it
does not close new. Otherwise, the new duplicate of old
replaces any previous meaning of descriptor new, as if new
were closed first.
If old and new are different numbers, and old is a
valid descriptor number, then dup2 is equivalent to:
close (new); fcntl (old, F_DUPFD, new)
However, dup2 does this atomically; there is no instant in the
middle of calling dup2 at which new is closed and not yet a
duplicate of old.
fcntl, to
copy the file descriptor given as the first argument.
The form of the call in this case is:
fcntl (old, F_DUPFD, next-filedes)
The next-filedes argument is of type int and specifies that
the file descriptor returned should be the next available one greater
than or equal to this value.
The return value from fcntl with this command is normally the value
of the new file descriptor. A return value of @math{-1} indicates an
error. The following errno error conditions are defined for
this command:
EBADF
EINVAL
EMFILE
RLIMIT_NOFILE limit.
ENFILE is not a possible error code for dup2 because
dup2 does not create a new opening of a file; duplicate
descriptors do not count toward the limit which ENFILE
indicates. EMFILE is possible because it refers to the limit on
distinct descriptor numbers in use in one process.
Here is an example showing how to use dup2 to do redirection.
Typically, redirection of the standard streams (like stdin) is
done by a shell or shell-like program before calling one of the
exec functions (see section Executing a File) to execute a new
program in a child process. When the new program is executed, it
creates and initializes the standard streams to point to the
corresponding file descriptors, before its main function is
invoked.
So, to redirect standard input to a file, the shell could do something like:
pid = fork ();
if (pid == 0)
{
char *filename;
char *program;
int file;
...
file = TEMP_FAILURE_RETRY (open (filename, O_RDONLY));
dup2 (file, STDIN_FILENO);
TEMP_FAILURE_RETRY (close (file));
execv (program, NULL);
}
There is also a more detailed example showing how to implement redirection in the context of a pipeline of processes in section Launching Jobs.
File descriptor flags are miscellaneous attributes of a file descriptor. These flags are associated with particular file descriptors, so that if you have created duplicate file descriptors from a single opening of a file, each descriptor has its own set of flags.
Currently there is just one file descriptor flag: FD_CLOEXEC,
which causes the descriptor to be closed if you use any of the
exec... functions (see section Executing a File).
The symbols in this section are defined in the header file `fcntl.h'.
fcntl, to
specify that it should return the file descriptor flags associated
with the filedes argument.
The normal return value from fcntl with this command is a
nonnegative number which can be interpreted as the bitwise OR of the
individual flags (except that currently there is only one flag to use).
In case of an error, fcntl returns @math{-1}. The following
errno error conditions are defined for this command:
EBADF
fcntl, to
specify that it should set the file descriptor flags associated with the
filedes argument. This requires a third int argument to
specify the new flags, so the form of the call is:
fcntl (filedes, F_SETFD, new-flags)
The normal return value from fcntl with this command is an
unspecified value other than @math{-1}, which indicates an error.
The flags and error conditions are the same as for the F_GETFD
command.
The following macro is defined for use as a file descriptor flag with
the fcntl function. The value is an integer constant usable
as a bit mask value.
exec function is invoked; see section Executing a File. When
a file descriptor is allocated (as with open or dup),
this bit is initially cleared on the new file descriptor, meaning that
descriptor will survive into the new program after exec.
If you want to modify the file descriptor flags, you should get the
current flags with F_GETFD and modify the value. Don't assume
that the flags listed here are the only ones that are implemented; your
program may be run years from now and more flags may exist then. For
example, here is a function to set or clear the flag FD_CLOEXEC
without altering any other flags:
/* Set theFD_CLOEXECflag of desc if value is nonzero, or clear the flag if value is 0. Return 0 on success, or -1 on error witherrnoset. */ int set_cloexec_flag (int desc, int value) { int oldflags = fcntl (desc, F_GETFD, 0); /* If reading the flags failed, return error indication now. if (oldflags < 0) return oldflags; /* Set just the flag we want to set. */ if (value != 0) oldflags |= FD_CLOEXEC; else oldflags &= ~FD_CLOEXEC; /* Store modified flag word in the descriptor. */ return fcntl (desc, F_SETFD, oldflags); }
File status flags are used to specify attributes of the opening of a
file. Unlike the file descriptor flags discussed in section File Descriptor Flags, the file status flags are shared by duplicated file descriptors
resulting from a single opening of the file. The file status flags are
specified with the flags argument to open;
see section Opening and Closing Files.
File status flags fall into three categories, which are described in the following sections.
open and are
returned by fcntl, but cannot be changed.
open will do.
These flags are not preserved after the open call.
read and
write are done. They are set by open, and can be fetched or
changed with fcntl.
The symbols in this section are defined in the header file `fcntl.h'.
The file access modes allow a file descriptor to be used for reading, writing, or both. (In the GNU system, they can also allow none of these, and allow execution of the file as a program.) The access modes are chosen when the file is opened, and never change.
In the GNU system (and not in other systems), O_RDONLY and
O_WRONLY are independent bits that can be bitwise-ORed together,
and it is valid for either bit to be set or clear. This means that
O_RDWR is the same as O_RDONLY|O_WRONLY. A file access
mode of zero is permissible; it allows no operations that do input or
output to the file, but does allow other operations such as
fchmod. On the GNU system, since "read-only" or "write-only"
is a misnomer, `fcntl.h' defines additional names for the file
access modes. These names are preferred when writing GNU-specific code.
But most programs will want to be portable to other POSIX.1 systems and
should use the POSIX.1 names above instead.
To determine the file access mode with fcntl, you must extract
the access mode bits from the retrieved file status flags. In the GNU
system, you can just test the O_READ and O_WRITE bits in
the flags word. But in other POSIX.1 systems, reading and writing
access modes are not stored as distinct bit flags. The portable way to
extract the file access mode bits is with O_ACCMODE.
O_RDONLY, O_WRONLY, or O_RDWR.
(In the GNU system it could also be zero, and it never includes the
O_EXEC bit.)
The open-time flags specify options affecting how open will behave.
These options are not preserved once the file is open. The exception to
this is O_NONBLOCK, which is also an I/O operating mode and so it
is saved. See section Opening and Closing Files, for how to call
open.
There are two sorts of options specified by open-time flags.
open looks up the
file name to locate the file, and whether the file can be created.
open will
perform on the file once it is open.
Here are the file name translation flags.
O_CREAT and O_EXCL are set, then open fails
if the specified file already exists. This is guaranteed to never
clobber an existing file.
open from blocking for a "long time" to open the
file. This is only meaningful for some kinds of files, usually devices
such as serial ports; when it is not meaningful, it is harmless and
ignored. Often opening a port to a modem blocks until the modem reports
carrier detection; if O_NONBLOCK is specified, open will
return immediately without a carrier.
Note that the O_NONBLOCK flag is overloaded as both an I/O operating
mode and a file name translation flag. This means that specifying
O_NONBLOCK in open also sets nonblocking I/O mode;
see section I/O Operating Modes. To open the file without blocking but do normal
I/O that blocks, you must call open with O_NONBLOCK set and
then call fcntl to turn the bit off.
In the GNU system and 4.4 BSD, opening a file never makes it the
controlling terminal and O_NOCTTY is zero. However, other
systems may use a nonzero value for O_NOCTTY and set the
controlling terminal when you open a file that is a terminal device; so
to be portable, use O_NOCTTY when it is important to avoid this.
The following three file name translation flags exist only in the GNU system.
fstat on the new file descriptor will
return the information returned by lstat on the link's name.)
The open-time action flags tell open to do additional operations
which are not really related to opening the file. The reason to do them
as part of open instead of in separate calls is that open
can do them atomically.
O_TRUNC. In
BSD and GNU you must have permission to write the file to truncate it,
but you need not open for write access.
This is the only open-time action flag specified by POSIX.1. There is
no good reason for truncation to be done by open, instead of by
calling ftruncate afterwards. The O_TRUNC flag existed in
Unix before ftruncate was invented, and is retained for backward
compatibility.
The remaining operating modes are BSD extensions. They exist only on some systems. On other systems, these macros are not defined.
flock.
See section File Locks.
If O_CREAT is specified, the locking is done atomically when
creating the file. You are guaranteed that no other process will get
the lock on the new file first.
flock.
See section File Locks. This is atomic like O_SHLOCK.
The operating modes affect how input and output operations using a file
descriptor work. These flags are set by open and can be fetched
and changed with fcntl.
write operations write the data at the end of the file, extending
it, regardless of the current file position. This is the only reliable
way to append to a file. In append mode, you are guaranteed that the
data you write will always go to the current end of the file, regardless
of other processes writing to the file. Conversely, if you simply set
the file position to the end of file and write, then another process can
extend the file after you set the file position but before you write,
resulting in your data appearing someplace before the real end of file.
read requests on the file can return immediately with a failure
status if there is no input immediately available, instead of blocking.
Likewise, write requests can also return immediately with a
failure status if the output can't be written immediately.
Note that the O_NONBLOCK flag is overloaded as both an I/O
operating mode and a file name translation flag; see section Open-time Flags.
O_NONBLOCK, provided for
compatibility with BSD. It is not defined by the POSIX.1 standard.
The remaining operating modes are BSD and GNU extensions. They exist only on some systems. On other systems, these macros are not defined.
SIGIO
signals will be generated when input is available. See section Interrupt-Driven Input.
Asynchronous input mode is a BSD feature.
write call will make sure the data is reliably stored on disk before
returning.
Synchronous writing is a BSD feature.
read will not update the access time of the
file. See section File Times. This is used by programs that do backups, so
that backing a file up does not count as reading it.
Only the owner of the file or the superuser may use this bit.
This is a GNU extension.
The fcntl function can fetch or change file status flags.
fcntl, to
read the file status flags for the open file with descriptor
filedes.
The normal return value from fcntl with this command is a
nonnegative number which can be interpreted as the bitwise OR of the
individual flags. Since the file access modes are not single-bit values,
you can mask off other bits in the returned flags with O_ACCMODE
to compare them.
In case of an error, fcntl returns @math{-1}. The following
errno error conditions are defined for this command:
EBADF
fcntl, to set
the file status flags for the open file corresponding to the
filedes argument. This command requires a third int
argument to specify the new flags, so the call looks like this:
fcntl (filedes, F_SETFL, new-flags)
You can't change the access mode for the file in this way; that is, whether the file descriptor was opened for reading or writing.
The normal return value from fcntl with this command is an
unspecified value other than @math{-1}, which indicates an error. The
error conditions are the same as for the F_GETFL command.
If you want to modify the file status flags, you should get the current
flags with F_GETFL and modify the value. Don't assume that the
flags listed here are the only ones that are implemented; your program
may be run years from now and more flags may exist then. For example,
here is a function to set or clear the flag O_NONBLOCK without
altering any other flags:
/* Set theO_NONBLOCKflag of desc if value is nonzero, or clear the flag if value is 0. Return 0 on success, or -1 on error witherrnoset. */ int set_nonblock_flag (int desc, int value) { int oldflags = fcntl (desc, F_GETFL, 0); /* If reading the flags failed, return error indication now. */ if (oldflags == -1) return -1; /* Set just the flag we want to set. */ if (value != 0) oldflags |= O_NONBLOCK; else oldflags &= ~O_NONBLOCK; /* Store modified flag word in the descriptor. */ return fcntl (desc, F_SETFL, oldflags); }
The remaining fcntl commands are used to support record
locking, which permits multiple cooperating programs to prevent each
other from simultaneously accessing parts of a file in error-prone
ways.
An exclusive or write lock gives a process exclusive access for writing to the specified part of the file. While a write lock is in place, no other process can lock that part of the file.
A shared or read lock prohibits any other process from requesting a write lock on the specified part of the file. However, other processes can request read locks.
The read and write functions do not actually check to see
whether there are any locks in place. If you want to implement a
locking protocol for a file shared by multiple processes, your application
must do explicit fcntl calls to request and clear locks at the
appropriate points.
Locks are associated with processes. A process can only have one kind
of lock set for each byte of a given file. When any file descriptor for
that file is closed by the process, all of the locks that process holds
on that file are released, even if the locks were made using other
descriptors that remain open. Likewise, locks are released when a
process exits, and are not inherited by child processes created using
fork (see section Creating a Process).
When making a lock, use a struct flock to specify what kind of
lock and where. This data type and the associated macros for the
fcntl function are declared in the header file `fcntl.h'.
fcntl function to describe a file
lock. It has these members:
short int l_type
F_RDLCK, F_WRLCK, or
F_UNLCK.
short int l_whence
fseek or
lseek, and specifies what the offset is relative to. Its value
can be one of SEEK_SET, SEEK_CUR, or SEEK_END.
off_t l_start
l_whence member.
off_t l_len
0 is treated specially; it means the region extends to the end of
the file.
pid_t l_pid
fcntl with
the F_GETLK command, but is ignored when making a lock.
fcntl, to
specify that it should get information about a lock. This command
requires a third argument of type struct flock * to be passed
to fcntl, so that the form of the call is:
fcntl (filedes, F_GETLK, lockp)
If there is a lock already in place that would block the lock described
by the lockp argument, information about that lock overwrites
*lockp. Existing locks are not reported if they are
compatible with making a new lock as specified. Thus, you should
specify a lock type of F_WRLCK if you want to find out about both
read and write locks, or F_RDLCK if you want to find out about
write locks only.
There might be more than one lock affecting the region specified by the
lockp argument, but fcntl only returns information about
one of them. The l_whence member of the lockp structure is
set to SEEK_SET and the l_start and l_len fields
set to identify the locked region.
If no lock applies, the only change to the lockp structure is to
update the l_type to a value of F_UNLCK.
The normal return value from fcntl with this command is an
unspecified value other than @math{-1}, which is reserved to indicate an
error. The following errno error conditions are defined for
this command:
EBADF
EINVAL
fcntl, to
specify that it should set or clear a lock. This command requires a
third argument of type struct flock * to be passed to
fcntl, so that the form of the call is:
fcntl (filedes, F_SETLK, lockp)
If the process already has a lock on any part of the region, the old lock
on that part is replaced with the new lock. You can remove a lock
by specifying a lock type of F_UNLCK.
If the lock cannot be set, fcntl returns immediately with a value
of @math{-1}. This function does not block waiting for other processes
to release locks. If fcntl succeeds, it return a value other
than @math{-1}.
The following errno error conditions are defined for this
function:
EAGAIN
EACCES
EAGAIN in this case, and other systems
use EACCES; your program should treat them alike, after
F_SETLK. (The GNU system always uses EAGAIN.)
EBADF
EINVAL
ENOLCK
fcntl, to
specify that it should set or clear a lock. It is just like the
F_SETLK command, but causes the process to block (or wait)
until the request can be specified.
This command requires a third argument of type struct flock *, as
for the F_SETLK command.
The fcntl return values and errors are the same as for the
F_SETLK command, but these additional errno error conditions
are defined for this command:
EINTR
EDEADLK
The following macros are defined for use as values for the l_type
member of the flock structure. The values are integer constants.
F_RDLCK
F_WRLCK
F_UNLCK
As an example of a situation where file locking is useful, consider a program that can be run simultaneously by several different users, that logs status information to a common file. One example of such a program might be a game that uses a file to keep track of high scores. Another example might be a program that records usage or accounting information for billing purposes.
Having multiple copies of the program simultaneously writing to the file could cause the contents of the file to become mixed up. But you can prevent this kind of problem by setting a write lock on the file before actually writing to the file.
If the program also needs to read the file and wants to make sure that the contents of the file are in a consistent state, then it can also use a read lock. While the read lock is set, no other process can lock that part of the file for writing.
Remember that file locks are only a voluntary protocol for controlling access to a file. There is still potential for access to the file by programs that don't use the lock protocol.
If you set the O_ASYNC status flag on a file descriptor
(see section File Status Flags), a SIGIO signal is sent whenever
input or output becomes possible on that file descriptor. The process
or process group to receive the signal can be selected by using the
F_SETOWN command to the fcntl function. If the file
descriptor is a socket, this also selects the recipient of SIGURG
signals that are delivered when out-of-band data arrives on that socket;
see section Out-of-Band Data. (SIGURG is sent in any situation
where select would report the socket as having an "exceptional
condition". See section Waiting for Input or Output.)
If the file descriptor corresponds to a terminal device, then SIGIO
signals are sent to the foreground process group of the terminal.
See section Job Control.
The symbols in this section are defined in the header file `fcntl.h'.
fcntl, to
specify that it should get information about the process or process
group to which SIGIO signals are sent. (For a terminal, this is
actually the foreground process group ID, which you can get using
tcgetpgrp; see section Functions for Controlling Terminal Access.)
The return value is interpreted as a process ID; if negative, its absolute value is the process group ID.
The following errno error condition is defined for this command:
EBADF
fcntl, to
specify that it should set the process or process group to which
SIGIO signals are sent. This command requires a third argument
of type pid_t to be passed to fcntl, so that the form of
the call is:
fcntl (filedes, F_SETOWN, pid)
The pid argument should be a process ID. You can also pass a negative number whose absolute value is a process group ID.
The return value from fcntl with this command is @math{-1}
in case of error and some other value if successful. The following
errno error conditions are defined for this command:
EBADF
ESRCH
The GNU system can handle most input/output operations on many different
devices and objects in terms of a few file primitives - read,
write and lseek. However, most devices also have a few
peculiar operations which do not fit into this model. Such as:
lseek is inapplicable).
Although some such objects such as sockets and terminals (2) have special functions of their own, it would not be practical to create functions for all these cases.
Instead these minor operations, known as IOCTLs, are assigned code
numbers and multiplexed through the ioctl function, defined in
sys/ioctl.h. The code numbers themselves are defined in many
different headers.
The ioctl function performs the generic I/O operation
command on filedes.
A third argument is usually present, either a single number or a pointer to a structure. The meaning of this argument, the returned value, and any error codes depends upon the command used. Often @math{-1} is returned for a failure.
On some systems, IOCTLs used by different devices share the same numbers. Thus, although use of an inappropriate IOCTL usually only produces an error, you should not attempt to use device-specific IOCTLs on an unknown device.
Most IOCTLs are OS-specific and/or only used in special system utilities, and are thus beyond the scope of this document. For an example of the use of an IOCTL, see section Out-of-Band Data.
This chapter describes the GNU C library's functions for manipulating files. Unlike the input and output functions (see section Input/Output on Streams; see section Low-Level Input/Output), these functions are concerned with operating on the files themselves rather than on their contents.
Among the facilities described in this chapter are functions for examining or modifying directories, functions for renaming and deleting files, and functions for examining and setting file attributes such as access permissions and modification times.
Each process has associated with it a directory, called its current working directory or simply working directory, that is used in the resolution of relative file names (see section File Name Resolution).
When you log in and begin a new session, your working directory is
initially set to the home directory associated with your login account
in the system user database. You can find any user's home directory
using the getpwuid or getpwnam functions; see section User Database.
Users can change the working directory using shell commands like
cd. The functions described in this section are the primitives
used by those commands and by other programs for examining and changing
the working directory.
Prototypes for these functions are declared in the header file `unistd.h'.
getcwd function returns an absolute file name representing
the current working directory, storing it in the character array
buffer that you provide. The size argument is how you tell
the system the allocation size of buffer.
The GNU library version of this function also permits you to specify a
null pointer for the buffer argument. Then getcwd
allocates a buffer automatically, as with malloc
(see section Unconstrained Allocation). If the size is greater than
zero, then the buffer is that large; otherwise, the buffer is as large
as necessary to hold the result.
The return value is buffer on success and a null pointer on failure.
The following errno error conditions are defined for this function:
EINVAL
ERANGE
EACCES
You could implement the behavior of GNU's getcwd (NULL, 0)
using only the standard behavior of getcwd:
char *
gnu_getcwd ()
{
size_t size = 100;
while (1)
{
char *buffer = (char *) xmalloc (size);
if (getcwd (buffer, size) == buffer)
return buffer;
free (buffer);
if (errno != ERANGE)
return 0;
size *= 2;
}
}
See section Examples of malloc, for information about xmalloc, which is
not a library function but is a customary name used in most GNU
software.
getcwd, but has no way to specify the size of
the buffer. The GNU library provides getwd only
for backwards compatibility with BSD.
The buffer argument should be a pointer to an array at least
PATH_MAX bytes long (see section Limits on File System Capacity). In the GNU
system there is no limit to the size of a file name, so this is not
necessarily enough space to contain the directory name. That is why
this function is deprecated.
get_current_dir_name function is bascially equivalent to
getcwd (NULL, 0). The only difference is that the value of
the PWD variable is returned if this value is correct. This is a
subtle difference which is visible if the path described by the
PWD value is using one or more symbol links in which case the
value returned by getcwd can resolve the symbol links and
therefore yield a different result.
This function is a GNU extension.
The normal, successful return value from chdir is 0. A
value of -1 is returned to indicate an error. The errno
error conditions defined for this function are the usual file name
syntax errors (see section File Name Errors), plus ENOTDIR if the
file filename is not a directory.
The normal, successful return value from fchdir is 0. A
value of -1 is returned to indicate an error. The following
errno error conditions are defined for this function:
EACCES
dirname.
EBADF
ENOTDIR
EINTR
EIO
The facilities described in this section let you read the contents of a directory file. This is useful if you want your program to list all the files in a directory, perhaps as part of a menu.
The opendir function opens a directory stream whose
elements are directory entries. You use the readdir function on
the directory stream to retrieve these entries, represented as
struct dirent objects. The name of the file for each entry is
stored in the d_name member of this structure. There are obvious
parallels here to the stream facilities for ordinary files, described in
section Input/Output on Streams.
This section describes what you find in a single directory entry, as you might obtain it from a directory stream. All the symbols are declared in the header file `dirent.h'.
char d_name[]
ino_t d_fileno
d_ino. In the GNU system and most POSIX
systems, for most files this the same as the st_ino member that
stat will return for the file. See section File Attributes.
unsigned char d_namlen
unsigned char because that is the integer
type of the appropriate size
unsigned char d_type
DT_UNKNOWN
DT_REG
DT_DIR
DT_FIFO
DT_SOCK
DT_CHR
DT_BLK
_DIRENT_HAVE_D_TYPE
is defined if this member is available. On systems where it is used, it
corresponds to the file type bits in the st_mode member of
struct statbuf. If the value cannot be determine the member
value is DT_UNKNOWN. These two macros convert between d_type
values and st_mode values:
This structure may contain additional members in the future. Their
availability is always announced in the compilation environment by a
macro names _DIRENT_HAVE_D_xxx where xxx is replaced
by the name of the new member. For instance, the member d_reclen
available on some systems is announced through the macro
_DIRENT_HAVE_D_RECLEN.
When a file has multiple names, each name has its own directory entry.
The only way you can tell that the directory entries belong to a
single file is that they have the same value for the d_fileno
field.
File attributes such as size, modification times etc., are part of the file itself, not of any particular directory entry. See section File Attributes.
This section describes how to open a directory stream. All the symbols are declared in the header file `dirent.h'.
You shouldn't ever allocate objects of the struct dirent or
DIR data types, since the directory access functions do that for
you. Instead, you refer to these objects using the pointers returned by
the following functions.
opendir function opens and returns a directory stream for
reading the directory whose file name is dirname. The stream has
type DIR *.
If unsuccessful, opendir returns a null pointer. In addition to
the usual file name errors (see section File Name Errors), the
following errno error conditions are defined for this function:
EACCES
dirname.
EMFILE
ENFILE
The DIR type is typically implemented using a file descriptor,
and the opendir function in terms of the open function.
See section Low-Level Input/Output. Directory streams and the underlying
file descriptors are closed on exec (see section Executing a File).
In some situations it can be desirable to get hold of the file
descriptor which is created by the opendir call. For instance,
to switch the current working directory to the directory just read the
fchdir function could be used. Historically the DIR type
was exposed and programs could access the fields. This does not happen
in the GNU C library. Instead a separate function is provided to allow
access.
dirfd returns the file descriptor associated with
the directory stream dirstream. This descriptor can be used until
the directory is closed with closedir. If the directory stream
implementation is not using file descriptors the return value is
-1.
This section describes how to read directory entries from a directory stream, and how to close the stream when you are done with it. All the symbols are declared in the header file `dirent.h'.
Portability Note: On some systems readdir may not
return entries for `.' and `..', even though these are always
valid file names in any directory. See section File Name Resolution.
If there are no more entries in the directory or an error is detected,
readdir returns a null pointer. The following errno error
conditions are defined for this function:
EBADF
readdir is not thread safe. Multiple threads using
readdir on the same dirstream may overwrite the return
value. Use readdir_r when this is critical.
readdir. Like
readdir it returns the next entry from the directory. But to
prevent conflicts between simultaneously running threads the result is
not stored in statically allocated memory. Instead the argument
entry points to a place to store the result.
The return value is 0 in case the next entry was read
successfully. In this case a pointer to the result is returned in
*result. It is not required that *result is the same as
entry. If something goes wrong while executing readdir_r
the function returns a value indicating the error (as described for
readdir).
If there are no more directory entries, readdir_r's return value is
0, and *result is set to NULL.
Portability Note: On some systems readdir_r may not
return a NUL terminated string for the file name, even when there is no
d_reclen field in struct dirent and the file
name is the maximum allowed size. Modern systems all have the
d_reclen field, and on old systems multi-threading is not
critical. In any case there is no such problem with the readdir
function, so that even on systems without the d_reclen member one
could use multiple threads by using external locking.
It is also important to look at the definition of the struct
dirent type. Simply passing a pointer to an object of this type for
the second parameter of readdir_r might not be enough. Some
systems don't define the d_name element sufficiently long. In
this case the user has to provide additional space. There must be room
for at least NAME_MAX + 1 characters in the d_name array.
Code to call readdir_r could look like this:
union
{
struct dirent d;
char b[offsetof (struct dirent, d_name) + NAME_MAX + 1];
} u;
if (readdir_r (dir, &u.d, &res) == 0)
...
To support large filesystems on 32-bit machines there are LFS variants of the last two functions.
readdir64 function is just like the readdir function
except that it returns a pointer to a record of type struct
dirent64. Some of the members of this data type (notably d_ino)
might have a different size to allow large filesystems.
In all other aspects this function is equivalent to readdir.
readdir64_r function is equivalent to the readdir_r
function except that it takes parameters of base type struct
dirent64 instead of struct dirent in the second and third
position. The same precautions mentioned in the documentation of
readdir_r also apply here.
0 on success and -1 on failure.
The following errno error conditions are defined for this
function:
EBADF
Here's a simple program that prints the names of the files in the current working directory:
#include <stddef.h>
#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>
int
main (void)
{
DIR *dp;
struct dirent *ep;
dp = opendir ("./");
if (dp != NULL)
{
while (ep = readdir (dp))
puts (ep->d_name);
(void) closedir (dp);
}
else
puts ("Couldn't open the directory.");
return 0;
}
The order in which files appear in a directory tends to be fairly random. A more useful program would sort the entries (perhaps by alphabetizing them) before printing them; see section Scanning the Content of a Directory, and section Array Sort Function.
This section describes how to reread parts of a directory that you have already read from an open directory stream. All the symbols are declared in the header file `dirent.h'.
rewinddir function is used to reinitialize the directory
stream dirstream, so that if you call readdir it
returns information about the first entry in the directory again. This
function also notices if files have been added or removed to the
directory since it was opened with opendir. (Entries for these
files might or might not be returned by readdir if they were
added or removed since you last called opendir or
rewinddir.)
telldir function returns the file position of the directory
stream dirstream. You can use this value with seekdir to
restore the directory stream to that position.
seekdir function sets the file position of the directory
stream dirstream to pos. The value pos must be the
result of a previous call to telldir on this particular stream;
closing and reopening the directory can invalidate values returned by
telldir.
A higher-level interface to the directory handling functions is the
scandir function. With its help one can select a subset of the
entries in a directory, possibly sort them and get a list of names as
the result.
The scandir function scans the contents of the directory selected
by dir. The result in *namelist is an array of pointers to
structure of type struct dirent which describe all selected
directory entries and which is allocated using malloc. Instead
of always getting all directory entries returned, the user supplied
function selector can be used to decide which entries are in the
result. Only the entries for which selector returns a non-zero
value are selected.
Finally the entries in *namelist are sorted using the
user-supplied function cmp. The arguments passed to the cmp
function are of type struct dirent **, therefore one cannot
directly use the strcmp or strcoll functions; instead see
the functions alphasort and versionsort below.
The return value of the function is the number of entries placed in
*namelist. If it is -1 an error occurred (either the
directory could not be opened for reading or the malloc call failed) and
the global variable errno contains more information on the error.
As described above the fourth argument to the scandir function
must be a pointer to a sorting function. For the convenience of the
programmer the GNU C library contains implementations of functions which
are very helpful for this purpose.
alphasort function behaves like the strcoll function
(see section String/Array Comparison). The difference is that the arguments
are not string pointers but instead they are of type
struct dirent **.
The return value of alphasort is less than, equal to, or greater
than zero depending on the order of the two entries a and b.
versionsort function is like alphasort except that it
uses the strverscmp function internally.
If the filesystem supports large files we cannot use the scandir
anymore since the dirent structure might not able to contain all
the information. The LFS provides the new type struct
dirent64. To use this we need a new function.
scandir64 function works like the scandir function
except that the directory entries it returns are described by elements
of type struct dirent64. The function pointed to by
selector is again used to select the desired entries, except that
selector now must point to a function which takes a
struct dirent64 * parameter.
Similarly the cmp function should expect its two arguments to be
of type struct dirent64 **.
As cmp is now a function of a different type, the functions
alphasort and versionsort cannot be supplied for that
argument. Instead we provide the two replacement functions below.
alphasort64 function behaves like the strcoll function
(see section String/Array Comparison). The difference is that the arguments
are not string pointers but instead they are of type
struct dirent64 **.
Return value of alphasort64 is less than, equal to, or greater
than zero depending on the order of the two entries a and b.
versionsort64 function is like alphasort64, excepted that it
uses the strverscmp function internally.
It is important not to mix the use of scandir and the 64-bit
comparison functions or vice versa. There are systems on which this
works but on others it will fail miserably.
Here is a revised version of the directory lister found above
(see section Simple Program to List a Directory). Using the scandir function we
can avoid the functions which work directly with the directory contents.
After the call the returned entries are available for direct use.
#include <stdio.h>
#include <dirent.h>
static int
one (struct dirent *unused)
{
return 1;
}
int
main (void)
{
struct dirent **eps;
int n;
n = scandir ("./", &eps, one, alphasort);
if (n >= 0)
{
int cnt;
for (cnt = 0; cnt < n; ++cnt)
puts (eps[cnt]->d_name);
}
else
perror ("Couldn't open the directory");
return 0;
}
Note the simple selector function in this example. Since we want to see
all directory entries we always return 1.
The functions described so far for handling the files in a directory
have allowed you to either retrieve the information bit by bit, or to
process all the files as a group (see scandir). Sometimes it is
useful to process whole hierarchies of directories and their contained
files. The X/Open specification defines two functions to do this. The
simpler form is derived from an early definition in System V systems
and therefore this function is available on SVID-derived systems. The
prototypes and required definitions can be found in the `ftw.h'
header.
There are four functions in this family: ftw, nftw and
their 64-bit counterparts ftw64 and nftw64. These
functions take as one of their arguments a pointer to a callback
function of the appropriate type.
int (*) (const char *, const struct stat *, int)
The type of callback functions given to the ftw function. The
first parameter points to the file name, the second parameter to an
object of type struct stat which is filled in for the file named
in the first parameter.
The last parameter is a flag giving more information about the current file. It can have the following values:
FTW_F
FTW_D
FTW_NS
stat call failed and so the information pointed to by the
second paramater is invalid.
FTW_DNR
FTW_SL
ftw callback function means the referenced
file does not exist. The situation for nftw is different.
This value is only available if the program is compiled with
_BSD_SOURCE or _XOPEN_EXTENDED defined before including
the first header. The original SVID systems do not have symbolic links.
If the sources are compiled with _FILE_OFFSET_BITS == 64 this
type is in fact __ftw64_func_t since this mode changes
struct stat to be struct stat64.
For the LFS interface and for use in the function ftw64, the
header `ftw.h' defines another function type.
int (*) (const char *, const struct stat64 *, int)
This type is used just like __ftw_func_t for the callback
function, but this time is called from ftw64. The second
parameter to the function is a pointer to a variable of type
struct stat64 which is able to represent the larger values.
int (*) (const char *, const struct stat *, int, struct FTW *)
The first three arguments are the same as for the __ftw_func_t
type. However for the third argument some additional values are defined
to allow finer differentiation:
FTW_DP
FTW_D if
the FTW_DEPTH flag is passed to nftw (see below).
FTW_SLN
The last parameter of the callback function is a pointer to a structure with some extra information as described below.
If the sources are compiled with _FILE_OFFSET_BITS == 64 this
type is in fact __nftw64_func_t since this mode changes
struct stat to be struct stat64.
For the LFS interface there is also a variant of this data type
available which has to be used with the nftw64 function.
int (*) (const char *, const struct stat64 *, int, struct FTW *)
This type is used just like __nftw_func_t for the callback
function, but this time is called from nftw64. The second
parameter to the function is this time a pointer to a variable of type
struct stat64 which is able to represent the larger values.
int base
FTW_CHDIR flag was set in calling nftw
since then the current directory is the one the current item is found
in.
int level
ftw function calls the callback function given in the
parameter func for every item which is found in the directory
specified by filename and all directories below. The function
follows symbolic links if necessary but does not process an item twice.
If filename is not a directory then it itself is the only object
returned to the callback function.
The file name passed to the callback function is constructed by taking
the filename parameter and appending the names of all passed
directories and then the local file name. So the callback function can
use this parameter to access the file. ftw also calls
stat for the file and passes that information on to the callback
function. If this stat call was not successful the failure is
indicated by setting the third argument of the callback function to
FTW_NS. Otherwise it is set according to the description given
in the account of __ftw_func_t above.
The callback function is expected to return @math{0} to indicate that no
error occurred and that processing should continue. If an error
occurred in the callback function or it wants ftw to return
immediately, the callback function can return a value other than
@math{0}. This is the only correct way to stop the function. The
program must not use setjmp or similar techniques to continue
from another place. This would leave resources allocated by the
ftw function unfreed.
The descriptors parameter to ftw specifies how many file
descriptors it is allowed to consume. The function runs faster the more
descriptors it can use. For each level in the directory hierarchy at
most one descriptor is used, but for very deep ones any limit on open
file descriptors for the process or the system may be exceeded.
Moreover, file descriptor limits in a multi-threaded program apply to
all the threads as a group, and therefore it is a good idea to supply a
reasonable limit to the number of open descriptors.
The return value of the ftw function is @math{0} if all callback
function calls returned @math{0} and all actions performed by the
ftw succeeded. If a function call failed (other than calling
stat on an item) the function returns @math{-1}. If a callback
function returns a value other than @math{0} this value is returned as
the return value of ftw.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit system this function is in fact ftw64, i.e. the LFS
interface transparently replaces the old interface.
ftw but it can work on filesystems
with large files. File information is reported using a variable of type
struct stat64 which is passed by reference to the callback
function.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit system this function is available under the name ftw and
transparently replaces the old implementation.
nftw function works like the ftw functions. They call
the callback function func for all items found in the directory
filename and below. At most descriptors file descriptors
are consumed during the nftw call.
One difference is that the callback function is of a different type. It
is of type struct FTW * and provides the callback function
with the extra information described above.
A second difference is that nftw takes a fourth argument, which
is @math{0} or a bitwise-OR combination of any of the following values.
FTW_PHYS
FTW_SL value for the type
parameter to the callback function. If the file referenced by a
symbolic link does not exist FTW_SLN is returned instead.
FTW_MOUNT
nftw.
FTW_CHDIR
ntfw finally returns the current directory is restored to
its original value.
FTW_DEPTH
FTW_DP and not FTW_D.
The return value is computed in the same way as for ftw.
nftw returns @math{0} if no failures occurred and all callback
functions returned @math{0}. In case of internal errors, such as memory
problems, the return value is @math{-1} and errno is set
accordingly. If the return value of a callback invocation was non-zero
then that value is returned.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit system this function is in fact nftw64, i.e. the LFS
interface transparently replaces the old interface.
nftw but it can work on filesystems
with large files. File information is reported using a variable of type
struct stat64 which is passed by reference to the callback
function.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit system this function is available under the name nftw and
transparently replaces the old implementation.
In POSIX systems, one file can have many names at the same time. All of the names are equally real, and no one of them is preferred to the others.
To add a name to a file, use the link function. (The new name is
also called a hard link to the file.) Creating a new link to a
file does not copy the contents of the file; it simply makes a new name
by which the file can be known, in addition to the file's existing name
or names.
One file can have names in several directories, so the organization of the file system is not a strict hierarchy or tree.
In most implementations, it is not possible to have hard links to the
same file in multiple file systems. link reports an error if you
try to make a hard link to the file from another file system when this
cannot be done.
The prototype for the link function is declared in the header
file `unistd.h'.
link function makes a new link to the existing file named by
oldname, under the new name newname.
This function returns a value of 0 if it is successful and
-1 on failure. In addition to the usual file name errors
(see section File Name Errors) for both oldname and newname, the
following errno error conditions are defined for this function:
EACCES
EEXIST
EMLINK
LINK_MAX; see
section Limits on File System Capacity.)
ENOENT
ENOSPC
EPERM
EROFS
EXDEV
EIO
The GNU system supports soft links or symbolic links. This is a kind of "file" that is essentially a pointer to another file name. Unlike hard links, symbolic links can be made to directories or across file systems with no restrictions. You can also make a symbolic link to a name which is not the name of any file. (Opening this link will fail until a file by that name is created.) Likewise, if the symbolic link points to an existing file which is later deleted, the symbolic link continues to point to the same file name even though the name no longer names any file.
The reason symbolic links work the way they do is that special things
happen when you try to open the link. The open function realizes
you have specified the name of a link, reads the file name contained in
the link, and opens that file name instead. The stat function
likewise operates on the file that the symbolic link points to, instead
of on the link itself.
By contrast, other operations such as deleting or renaming the file
operate on the link itself. The functions readlink and
lstat also refrain from following symbolic links, because their
purpose is to obtain information about the link. link, the
function that makes a hard link, does too. It makes a hard link to the
symbolic link, which one rarely wants.
Some systems have for some functions operating on files have a limit on how many symbolic links are followed when resolving a path name. The limit if it exists is published in the `sys/param.h' header file.
The macro MAXSYMLINKS specifies how many symlinks some function
will follow before returning ELOOP. Not all functions behave the
same and this value is not the same a that returned for
_SC_SYMLOOP by sysconf. In fact, the sysconf
result can indicate that there is no fixed limit although
MAXSYMLINKS exists and has a finite value.
Prototypes for most of the functions listed in this section are in `unistd.h'.
symlink function makes a symbolic link to oldname named
newname.
The normal return value from symlink is 0. A return value
of -1 indicates an error. In addition to the usual file name
syntax errors (see section File Name Errors), the following errno
error conditions are defined for this function:
EEXIST
EROFS
ENOSPC
EIO
readlink function gets the value of the symbolic link
filename. The file name that the link points to is copied into
buffer. This file name string is not null-terminated;
readlink normally returns the number of characters copied. The
size argument specifies the maximum number of characters to copy,
usually the allocation size of buffer.
If the return value equals size, you cannot tell whether or not
there was room to return the entire name. So make a bigger buffer and
call readlink again. Here is an example:
char *
readlink_malloc (char *filename)
{
int size = 100;
while (1)
{
char *buffer = (char *) xmalloc (size);
int nchars = readlink (filename, buffer, size);
if (nchars < size)
return buffer;
free (buffer);
size *= 2;
}
}
A value of -1 is returned in case of error. In addition to the
usual file name errors (see section File Name Errors), the following
errno error conditions are defined for this function:
EINVAL
EIO
In some situations it is desirable to resolve all the to get the real
name of a file where no prefix names a symbolic link which is followed
and no filename in the path is . or ... This is for
instance desirable if files have to be compare in which case different
names can refer to the same inode.
The canonicalize_file_name function returns the absolute name of
the file named by name which contains no ., ..
components nor any repeated path separators (/) or symlinks. The
result is passed back as the return value of the function in a block of
memory allocated with malloc. If the result is not used anymore
the memory should be freed with a call to free.
In any of the path components except the last one is missing the
function returns a NULL pointer. This is also what is returned if the
length of the path reaches or exceeds PATH_MAX characters. In
any case errno is set accordingly.
ENAMETOOLONG
EACCES
ENOENT
ENOENT
ELOOP
MAXSYMLINKS many symlinks have been followed.
This function is a GNU extension and is declared in `stdlib.h'.
The Unix standard includes a similar function which differs from
canonicalize_file_name in that the user has to provide the buffer
where the result is placed in.
The realpath function behaves just like
canonicalize_file_name but instead of allocating a buffer for the
result it is placed in the buffer pointed to by resolved.
One other difference is that the buffer resolved will contain the
part of the path component which does not exist or is not readable if
the function returns NULL and errno is set to
EACCES or ENOENT.
This function is declared in `stdlib.h'.
The advantage of using this function is that it is more widely available. The drawback is that it reports failures for long path on systems which have no limits on the file name length.
You can delete a file with unlink or remove.
Deletion actually deletes a file name. If this is the file's only name, then the file is deleted as well. If the file has other remaining names (see section Hard Links), it remains accessible under those names.
unlink function deletes the file name filename. If
this is a file's sole name, the file itself is also deleted. (Actually,
if any process has the file open when this happens, deletion is
postponed until all processes have closed the file.)
The function unlink is declared in the header file `unistd.h'.
This function returns 0 on successful completion, and -1
on error. In addition to the usual file name errors
(see section File Name Errors), the following errno error conditions are
defined for this function:
EACCES
EBUSY
ENOENT
EPERM
unlink cannot be used to delete the name of a
directory, or at least can only be used this way by a privileged user.
To avoid such problems, use rmdir to delete directories. (In the
GNU system unlink can never delete the name of a directory.)
EROFS
rmdir function deletes a directory. The directory must be
empty before it can be removed; in other words, it can only contain
entries for `.' and `..'.
In most other respects, rmdir behaves like unlink. There
are two additional errno error conditions defined for
rmdir:
ENOTEMPTY
EEXIST
These two error codes are synonymous; some systems use one, and some use
the other. The GNU system always uses ENOTEMPTY.
The prototype for this function is declared in the header file `unistd.h'.
unlink for files and like rmdir for directories.
remove is declared in `stdio.h'.
The rename function is used to change a file's name.
rename function renames the file oldname to
newname. The file formerly accessible under the name
oldname is afterwards accessible as newname instead. (If
the file had any other names aside from oldname, it continues to
have those names.)
The directory containing the name newname must be on the same file system as the directory containing the name oldname.
One special case for rename is when oldname and
newname are two names for the same file. The consistent way to
handle this case is to delete oldname. However, in this case
POSIX requires that rename do nothing and report success--which
is inconsistent. We don't know what your operating system will do.
If oldname is not a directory, then any existing file named
newname is removed during the renaming operation. However, if
newname is the name of a directory, rename fails in this
case.
If oldname is a directory, then either newname must not
exist or it must name a directory that is empty. In the latter case,
the existing directory named newname is deleted first. The name
newname must not specify a subdirectory of the directory
oldname which is being renamed.
One useful feature of rename is that the meaning of newname
changes "atomically" from any previously existing file by that name to
its new meaning (i.e. the file that was called oldname). There is
no instant at which newname is non-existent "in between" the old
meaning and the new meaning. If there is a system crash during the
operation, it is possible for both names to still exist; but
newname will always be intact if it exists at all.
If rename fails, it returns -1. In addition to the usual
file name errors (see section File Name Errors), the following
errno error conditions are defined for this function:
EACCES
EBUSY
ENOTEMPTY
EEXIST
ENOTEMPTY for this, but some other systems return EEXIST.
EINVAL
EISDIR
EMLINK
ENOENT
ENOSPC
EROFS
EXDEV
Directories are created with the mkdir function. (There is also
a shell command mkdir which does the same thing.)
mkdir function creates a new, empty directory with name
filename.
The argument mode specifies the file permissions for the new directory file. See section The Mode Bits for Access Permission, for more information about this.
A return value of 0 indicates successful completion, and
-1 indicates failure. In addition to the usual file name syntax
errors (see section File Name Errors), the following errno error
conditions are defined for this function:
EACCES
EEXIST
EMLINK
ENOSPC
EROFS
To use this function, your program should include the header file `sys/stat.h'.
When you issue an `ls -l' shell command on a file, it gives you information about the size of the file, who owns it, when it was last modified, etc. These are called the file attributes, and are associated with the file itself and not a particular one of its names.
This section contains information about how you can inquire about and modify the attributes of a file.
When you read the attributes of a file, they come back in a structure
called struct stat. This section describes the names of the
attributes, their data types, and what they mean. For the functions
to read the attributes of a file, see section Reading the Attributes of a File.
The header file `sys/stat.h' declares all the symbols defined in this section.
stat structure type is used to return information about the
attributes of a file. It contains at least the following members:
mode_t st_mode
ino_t st_ino
dev_t st_dev
st_ino and
st_dev, taken together, uniquely identify the file. The
st_dev value is not necessarily consistent across reboots or
system crashes, however.
nlink_t st_nlink
uid_t st_uid
gid_t st_gid
off_t st_size
time_t st_atime
unsigned long int st_atime_usec
time_t st_mtime
unsigned long int st_mtime_usec
time_t st_ctime
unsigned long int st_ctime_usec
blkcnt_t st_blocks
st_size, like this:
(st.st_blocks * 512 < st.st_size)This test is not perfect because a file that is just slightly sparse might not be detected as sparse at all. For practical applications, this is not a problem.
unsigned int st_blksize
st_blocks.)
The extensions for the Large File Support (LFS) require, even on 32-bit
machines, types which can handle file sizes up to @math{2^63}.
Therefore a new definition of struct stat is necessary.
struct stat. The only difference is that the members
st_ino, st_size, and st_blocks have a different
type to support larger values.
mode_t st_mode
ino64_t st_ino
dev_t st_dev
st_ino and
st_dev, taken together, uniquely identify the file. The
st_dev value is not necessarily consistent across reboots or
system crashes, however.
nlink_t st_nlink
uid_t st_uid
gid_t st_gid
off64_t st_size
time_t st_atime
unsigned long int st_atime_usec
time_t st_mtime
unsigned long int st_mtime_usec
time_t st_ctime
unsigned long int st_ctime_usec
blkcnt64_t st_blocks
unsigned int st_blksize
st_blocks.)
Some of the file attributes have special data type names which exist specifically for those attributes. (They are all aliases for well-known integer types that you know and love.) These typedef names are defined in the header file `sys/types.h' as well as in `sys/stat.h'. Here is a list of them.
unsigned int.
unsigned long int.
If the source is compiled with _FILE_OFFSET_BITS == 64 this type
is transparently replaced by ino64_t.
unsigned long longint.
When compiling with _FILE_OFFSET_BITS == 64 this type is
available under the name ino_t.
int.
unsigned short int.
unsigned long int.
If the source is compiled with _FILE_OFFSET_BITS == 64 this type
is transparently replaced by blkcnt64_t.
unsigned
long long int.
When compiling with _FILE_OFFSET_BITS == 64 this type is
available under the name blkcnt_t.
To examine the attributes of files, use the functions stat,
fstat and lstat. They return the attribute information in
a struct stat object. All three functions are declared in the
header file `sys/stat.h'.
stat function returns information about the attributes of the
file named by filename in the structure pointed to by buf.
If filename is the name of a symbolic link, the attributes you get
describe the file that the link points to. If the link points to a
nonexistent file name, then stat fails reporting a nonexistent
file.
The return value is 0 if the operation is successful, or
-1 on failure. In addition to the usual file name errors
(see section File Name Errors, the following errno error conditions
are defined for this function:
ENOENT
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is in fact stat64 since the LFS interface transparently
replaces the normal implementation.
stat but it is also able to work on
files larger then @math{2^31} bytes on 32-bit systems. To be able to do
this the result is stored in a variable of type struct stat64 to
which buf must point.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is available under the name stat and so transparently
replaces the interface for small files on 32-bit machines.
fstat function is like stat, except that it takes an
open file descriptor as an argument instead of a file name.
See section Low-Level Input/Output.
Like stat, fstat returns 0 on success and -1
on failure. The following errno error conditions are defined for
fstat:
EBADF
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is in fact fstat64 since the LFS interface transparently
replaces the normal implementation.
fstat but is able to work on large
files on 32-bit platforms. For large files the file descriptor
filedes should be obtained by open64 or creat64.
The buf pointer points to a variable of type struct stat64
which is able to represent the larger values.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is available under the name fstat and so transparently
replaces the interface for small files on 32-bit machines.
lstat function is like stat, except that it does not
follow symbolic links. If filename is the name of a symbolic
link, lstat returns information about the link itself; otherwise
lstat works like stat. See section Symbolic Links.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is in fact lstat64 since the LFS interface transparently
replaces the normal implementation.
lstat but it is also able to work on
files larger then @math{2^31} bytes on 32-bit systems. To be able to do
this the result is stored in a variable of type struct stat64 to
which buf must point.
When the sources are compiled with _FILE_OFFSET_BITS == 64 this
function is available under the name lstat and so transparently
replaces the interface for small files on 32-bit machines.
The file mode, stored in the st_mode field of the file
attributes, contains two kinds of information: the file type code, and
the access permission bits. This section discusses only the type code,
which you can use to tell whether the file is a directory, socket,
symbolic link, and so on. For details about access permissions see
section The Mode Bits for Access Permission.
There are two ways you can access the file type information in a file mode. Firstly, for each file type there is a predicate macro which examines a given file mode and returns whether it is of that type or not. Secondly, you can mask out the rest of the file mode to leave just the file type code, and compare this against constants for each of the supported file types.
All of the symbols listed in this section are defined in the header file `sys/stat.h'.
The following predicate macros test the type of a file, given the value
m which is the st_mode field returned by stat on
that file:
An alternate non-POSIX method of testing the file type is supported for
compatibility with BSD. The mode can be bitwise AND-ed with
S_IFMT to extract the file type code, and compared to the
appropriate constant. For example,
S_ISCHR (mode)
is equivalent to:
((mode & S_IFMT) == S_IFCHR)
These are the symbolic names for the different file type codes:
S_IFDIR
S_IFCHR
S_IFBLK
S_IFREG
S_IFLNK
S_IFSOCK
S_IFIFO
The POSIX.1b standard introduced a few more objects which possibly can
be implemented as object in the filesystem. These are message queues,
semaphores, and shared memory objects. To allow differentiating these
objects from other files the POSIX standard introduces three new test
macros. But unlike the other macros it does not take the value of the
st_mode field as the parameter. Instead they expect a pointer to
the whole struct stat structure.
Every file has an owner which is one of the registered user names defined on the system. Each file also has a group which is one of the defined groups. The file owner can often be useful for showing you who edited the file (especially when you edit with GNU Emacs), but its main purpose is for access control.
The file owner and group play a role in determining access because the file has one set of access permission bits for the owner, another set that applies to users who belong to the file's group, and a third set of bits that applies to everyone else. See section How Your Access to a File is Decided, for the details of how access is decided based on this data.
When a file is created, its owner is set to the effective user ID of the process that creates it (see section The Persona of a Process). The file's group ID may be set to either the effective group ID of the process, or the group ID of the directory that contains the file, depending on the system where the file is stored. When you access a remote file system, it behaves according to its own rules, not according to the system your program is running on. Thus, your program must be prepared to encounter either kind of behavior no matter what kind of system you run it on.
You can change the owner and/or group owner of an existing file using
the chown function. This is the primitive for the chown
and chgrp shell commands.
The prototype for this function is declared in `unistd.h'.
chown function changes the owner of the file filename to
owner, and its group owner to group.
Changing the owner of the file on certain systems clears the set-user-ID and set-group-ID permission bits. (This is because those bits may not be appropriate for the new owner.) Other file permission bits are not changed.
The return value is 0 on success and -1 on failure.
In addition to the usual file name errors (see section File Name Errors),
the following errno error conditions are defined for this function:
EPERM
_POSIX_CHOWN_RESTRICTED macro.
EROFS
chown, except that it changes the owner of the open
file with descriptor filedes.
The return value from fchown is 0 on success and -1
on failure. The following errno error codes are defined for this
function:
EBADF
EINVAL
EPERM
chmod above.
EROFS
The file mode, stored in the st_mode field of the file
attributes, contains two kinds of information: the file type code, and
the access permission bits. This section discusses only the access
permission bits, which control who can read or write the file.
See section Testing the Type of a File, for information about the file type code.
All of the symbols listed in this section are defined in the header file `sys/stat.h'.
These symbolic constants are defined for the file mode bits that control access permission for the file:
S_IRUSR
S_IREAD
S_IREAD is an obsolete synonym provided for BSD
compatibility.
S_IWUSR
S_IWRITE
S_IWRITE is an obsolete synonym provided for BSD compatibility.
S_IXUSR
S_IEXEC
S_IEXEC is an obsolete
synonym provided for BSD compatibility.
S_IRWXU
S_IRGRP
S_IWGRP
S_IXGRP
S_IRWXG
S_IROTH
S_IWOTH
S_IXOTH
S_IRWXO
S_ISUID
S_ISGID
S_ISVTX
chmod fails with EFTYPE;
see section Assigning File Permissions.
Some systems (particularly SunOS) have yet another use for the sticky
bit. If the sticky bit is set on a file that is not executable,
it means the opposite: never cache the pages of this file at all. The
main use of this is for the files on an NFS server machine which are
used as the swap area of diskless client machines. The idea is that the
pages of the file will be cached in the client's memory, so it is a
waste of the server's memory to cache them a second time. With this
usage the sticky bit also implies that the filesystem may fail to record
the file's modification time onto disk reliably (the idea being that
no-one cares for a swap file).
This bit is only available on BSD systems (and those derived from
them). Therefore one has to use the _BSD_SOURCE feature select
macro to get the definition (see section Feature Test Macros).
The actual bit values of the symbols are listed in the table above so you can decode file mode values when debugging your programs. These bit values are correct for most systems, but they are not guaranteed.
Warning: Writing explicit numbers for file permissions is bad practice. Not only is it not portable, it also requires everyone who reads your program to remember what the bits mean. To make your program clean use the symbolic names.
Recall that the operating system normally decides access permission for a file based on the effective user and group IDs of the process and its supplementary group IDs, together with the file's owner, group and permission bits. These concepts are discussed in detail in section The Persona of a Process.
If the effective user ID of the process matches the owner user ID of the file, then permissions for read, write, and execute/search are controlled by the corresponding "user" (or "owner") bits. Likewise, if any of the effective group ID or supplementary group IDs of the process matches the group owner ID of the file, then permissions are controlled by the "group" bits. Otherwise, permissions are controlled by the "other" bits.
Privileged users, like `root', can access any file regardless of its permission bits. As a special case, for a file to be executable even by a privileged user, at least one of its execute bits must be set.
The primitive functions for creating files (for example, open or
mkdir) take a mode argument, which specifies the file
permissions to give the newly created file. This mode is modified by
the process's file creation mask, or umask, before it is
used.
The bits that are set in the file creation mask identify permissions that are always to be disabled for newly created files. For example, if you set all the "other" access bits in the mask, then newly created files are not accessible at all to processes in the "other" category, even if the mode argument passed to the create function would permit such access. In other words, the file creation mask is the complement of the ordinary access permissions you want to grant.
Programs that create files typically specify a mode argument that includes all the permissions that make sense for the particular file. For an ordinary file, this is typically read and write permission for all classes of users. These permissions are then restricted as specified by the individual user's own file creation mask.
To change the permission of an existing file given its name, call
chmod. This function uses the specified permission bits and
ignores the file creation mask.
In normal use, the file creation mask is initialized by the user's login
shell (using the umask shell command), and inherited by all
subprocesses. Application programs normally don't need to worry about
the file creation mask. It will automatically do what it is supposed to
do.
When your program needs to create a file and bypass the umask for its
access permissions, the easiest way to do this is to use fchmod
after opening the file, rather than changing the umask. In fact,
changing the umask is usually done only by shells. They use the
umask function.
The functions in this section are declared in `sys/stat.h'.
umask function sets the file creation mask of the current
process to mask, and returns the previous value of the file
creation mask.
Here is an example showing how to read the mask with umask
without changing it permanently:
mode_t
read_umask (void)
{
mode_t mask = umask (0);
umask (mask);
return mask;
}
However, it is better to use getumask if you just want to read
the mask value, because it is reentrant (at least if you use the GNU
operating system).
chmod function sets the access permission bits for the file
named by filename to mode.
If filename is a symbolic link, chmod changes the
permissions of the file pointed to by the link, not those of the link
itself.
This function returns 0 if successful and -1 if not. In
addition to the usual file name errors (see section File Name Errors), the following errno error conditions are defined for
this function:
ENOENT
EPERM
EROFS
EFTYPE
S_ISVTX bit (the "sticky bit") set,
and the named file is not a directory. Some systems do not allow setting the
sticky bit on non-directory files, and some do (and only some of those
assign a useful meaning to the bit for non-directory files).
You only get EFTYPE on systems where the sticky bit has no useful
meaning for non-directory files, so it is always safe to just clear the
bit in mode and call chmod again. See section The Mode Bits for Access Permission,
for full details on the sticky bit.
chmod, except that it changes the permissions of the
currently open file given by filedes.
The return value from fchmod is 0 on success and -1
on failure. The following errno error codes are defined for this
function:
EBADF
EINVAL
EPERM
EROFS
In some situations it is desirable to allow programs to access files or
devices even if this is not possible with the permissions granted to the
user. One possible solution is to set the setuid-bit of the program
file. If such a program is started the effective user ID of the
process is changed to that of the owner of the program file. So to
allow write access to files like `/etc/passwd', which normally can
be written only by the super-user, the modifying program will have to be
owned by root and the setuid-bit must be set.
But beside the files the program is intended to change the user should not be allowed to access any file to which s/he would not have access anyway. The program therefore must explicitly check whether the user would have the necessary access to a file, before it reads or writes the file.
To do this, use the function access, which checks for access
permission based on the process's real user ID rather than the
effective user ID. (The setuid feature does not alter the real user ID,
so it reflects the user who actually ran the program.)
There is another way you could check this access, which is easy to
describe, but very hard to use. This is to examine the file mode bits
and mimic the system's own access computation. This method is
undesirable because many systems have additional access control
features; your program cannot portably mimic them, and you would not
want to try to keep track of the diverse features that different systems
have. Using access is simple and automatically does whatever is
appropriate for the system you are using.
access is only only appropriate to use in setuid programs.
A non-setuid program will always use the effective ID rather than the
real ID.
The symbols in this section are declared in `unistd.h'.
access function checks to see whether the file named by
filename can be accessed in the way specified by the how
argument. The how argument either can be the bitwise OR of the
flags R_OK, W_OK, X_OK, or the existence test
F_OK.
This function uses the real user and group IDs of the calling
process, rather than the effective IDs, to check for access
permission. As a result, if you use the function from a setuid
or setgid program (see section How an Application Can Change Persona), it gives
information relative to the user who actually ran the program.
The return value is 0 if the access is permitted, and -1
otherwise. (In other words, treated as a predicate function,
access returns true if the requested access is denied.)
In addition to the usual file name errors (see section File Name Errors), the following errno error conditions are defined for
this function:
EACCES
ENOENT
EROFS
These macros are defined in the header file `unistd.h' for use
as the how argument to the access function. The values
are integer constants.
Each file has three time stamps associated with it: its access time,
its modification time, and its attribute modification time. These
correspond to the st_atime, st_mtime, and st_ctime
members of the stat structure; see section File Attributes.
All of these times are represented in calendar time format, as
time_t objects. This data type is defined in `time.h'.
For more information about representation and manipulation of time
values, see section Calendar Time.
Reading from a file updates its access time attribute, and writing updates its modification time. When a file is created, all three time stamps for that file are set to the current time. In addition, the attribute change time and modification time fields of the directory that contains the new entry are updated.
Adding a new name for a file with the link function updates the
attribute change time field of the file being linked, and both the
attribute change time and modification time fields of the directory
containing the new name. These same fields are affected if a file name
is deleted with unlink, remove or rmdir. Renaming
a file with rename affects only the attribute change time and
modification time fields of the two parent directories involved, and not
the times for the file being renamed.
Changing the attributes of a file (for example, with chmod)
updates its attribute change time field.
You can also change some of the time stamps of a file explicitly using
the utime function--all except the attribute change time. You
need to include the header file `utime.h' to use this facility.
utimbuf structure is used with the utime function to
specify new access and modification times for a file. It contains the
following members:
time_t actime
time_t modtime
If times is a null pointer, then the access and modification times
of the file are set to the current time. Otherwise, they are set to the
values from the actime and modtime members (respectively)
of the utimbuf structure pointed to by times.
The attribute modification time for the file is set to the current time in either case (since changing the time stamps is itself a modification of the file attributes).
The utime function returns 0 if successful and -1
on failure. In addition to the usual file name errors
(see section File Name Errors), the following errno error conditions
are defined for this function:
EACCES
ENOENT
EPERM
EROFS
Each of the three time stamps has a corresponding microsecond part,
which extends its resolution. These fields are called
st_atime_usec, st_mtime_usec, and st_ctime_usec;
each has a value between 0 and 999,999, which indicates the time in
microseconds. They correspond to the tv_usec field of a
timeval structure; see section High-Resolution Calendar.
The utimes function is like utime, but also lets you specify
the fractional part of the file times. The prototype for this function is
in the header file `sys/time.h'.
tvp[0], and the new modification time by
tvp[1]. This function comes from BSD.
The return values and error conditions are the same as for the utime
function.
Normally file sizes are maintained automatically. A file begins with a
size of @math{0} and is automatically extended when data is written past
its end. It is also possible to empty a file completely by an
open or fopen call.
However, sometimes it is necessary to reduce the size of a file.
This can be done with the truncate and ftruncate functions.
They were introduced in BSD Unix. ftruncate was later added to
POSIX.1.
Some systems allow you to extend a file (creating holes) with these
functions. This is useful when using memory-mapped I/O
(see section Memory-mapped I/O), where files are not automatically extended.
However, it is not portable but must be implemented if mmap
allows mapping of files (i.e., _POSIX_MAPPED_FILES is defined).
Using these functions on anything other than a regular file gives undefined results. On many systems, such a call will appear to succeed, without actually accomplishing anything.
The truncate function changes the size of filename to
length. If length is shorter than the previous length, data
at the end will be lost. The file must be writable by the user to
perform this operation.
If length is longer, holes will be added to the end. However, some systems do not support this feature and will leave the file unchanged.
When the source file is compiled with _FILE_OFFSET_BITS == 64 the
truncate function is in fact truncate64 and the type
off_t has 64 bits which makes it possible to handle files up to
@math{2^63} bytes in length.
The return value is @math{0} for success, or @math{-1} for an error. In addition to the usual file name errors, the following errors may occur:
EACCES
EINVAL
EFBIG
EIO
EPERM
EINTR
truncate function. The
difference is that the length argument is 64 bits wide even on 32
bits machines, which allows the handling of files with sizes up to
@math{2^63} bytes.
When the source file is compiled with _FILE_OFFSET_BITS == 64 on a
32 bits machine this function is actually available under the name
truncate and so transparently replaces the 32 bits interface.
This is like truncate, but it works on a file descriptor fd
for an opened file instead of a file name to identify the object. The
file must be opened for writing to successfully carry out the operation.
The POSIX standard leaves it implementation defined what happens if the
specified new length of the file is bigger than the original size.
The ftruncate function might simply leave the file alone and do
nothing or it can increase the size to the desired size. In this later
case the extended area should be zero-filled. So using ftruncate
is no reliable way to increase the file size but if it is possible it is
probably the fastest way. The function also operates on POSIX shared
memory segments if these are implemented by the system.
ftruncate is especially useful in combination with mmap.
Since the mapped region must have a fixed size one cannot enlarge the
file by writing something beyond the last mapped page. Instead one has
to enlarge the file itself and then remap the file with the new size.
The example below shows how this works.
When the source file is compiled with _FILE_OFFSET_BITS == 64 the
ftruncate function is in fact ftruncate64 and the type
off_t has 64 bits which makes it possible to handle files up to
@math{2^63} bytes in length.
The return value is @math{0} for success, or @math{-1} for an error. The following errors may occur:
EBADF
EACCES
EINVAL
EFBIG
EIO
EPERM
EINTR
ftruncate function. The
difference is that the length argument is 64 bits wide even on 32
bits machines which allows the handling of files with sizes up to
@math{2^63} bytes.
When the source file is compiled with _FILE_OFFSET_BITS == 64 on a
32 bits machine this function is actually available under the name
ftruncate and so transparently replaces the 32 bits interface.
As announced here is a little example of how to use ftruncate in
combination with mmap:
int fd;
void *start;
size_t len;
int
add (off_t at, void *block, size_t size)
{
if (at + size > len)
{
/* Resize the file and remap. */
size_t ps = sysconf (_SC_PAGESIZE);
size_t ns = (at + size + ps - 1) & ~(ps - 1);
void *np;
if (ftruncate (fd, ns) < 0)
return -1;
np = mmap (NULL, ns, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
if (np == MAP_FAILED)
return -1;
start = np;
len = ns;
}
memcpy ((char *) start + at, block, size);
return 0;
}
The function add writes a block of memory at an arbitrary
position in the file. If the current size of the file is too small it
is extended. Note the it is extended by a round number of pages. This
is a requirement of mmap. The program has to keep track of the
real size, and when it has finished a final ftruncate call should
set the real size of the file.
The mknod function is the primitive for making special files,
such as files that correspond to devices. The GNU library includes
this function for compatibility with BSD.
The prototype for mknod is declared in `sys/stat.h'.
mknod function makes a special file with name filename.
The mode specifies the mode of the file, and may include the various
special file bits, such as S_IFCHR (for a character special file)
or S_IFBLK (for a block special file). See section Testing the Type of a File.
The dev argument specifies which device the special file refers to. Its exact interpretation depends on the kind of special file being created.
The return value is 0 on success and -1 on error. In addition
to the usual file name errors (see section File Name Errors), the
following errno error conditions are defined for this function:
EPERM
ENOSPC
EROFS
EEXIST
If you need to use a temporary file in your program, you can use the
tmpfile function to open it. Or you can use the tmpnam
(better: tmpnam_r) function to provide a name for a temporary
file and then you can open it in the usual way with fopen.
The tempnam function is like tmpnam but lets you choose
what directory temporary files will go in, and something about what
their file names will look like. Important for multi-threaded programs
is that tempnam is reentrant, while tmpnam is not since it
returns a pointer to a static buffer.
These facilities are declared in the header file `stdio.h'.
fopen with mode "wb+". The file is deleted
automatically when it is closed or when the program terminates. (On
some other ISO C systems the file may fail to be deleted if the program
terminates abnormally).
This function is reentrant.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit system this function is in fact tmpfile64, i.e. the LFS
interface transparently replaces the old interface.
tmpfile, but the stream it returns a
pointer to was opened using tmpfile64. Therefore this stream can
be used for files larger then @math{2^31} bytes on 32-bit machines.
Please note that the return type is still FILE *. There is no
special FILE type for the LFS interface.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name tmpfile
and so transparently replaces the old interface.
L_tmpnam characters, and the
result is written into that array.
It is possible for tmpnam to fail if you call it too many times
without removing previously-created files. This is because the limited
length of the temporary file names gives room for only a finite number
of different names. If tmpnam fails it returns a null pointer.
Warning: Between the time the pathname is constructed and the
file is created another process might have created a file with the same
name using tmpnam, leading to a possible security hole. The
implementation generates names which can hardly be predicted, but when
opening the file you should use the O_EXCL flag. Using
tmpfile or mkstemp is a safe way to avoid this problem.
tmpnam function, except
that if result is a null pointer it returns a null pointer.
This guarantees reentrancy because the non-reentrant situation of
tmpnam cannot happen here.
Warning: This function has the same security problems as
tmpnam.
tmpnam function.
TMP_MAX is a lower bound for how many temporary names
you can create with tmpnam. You can rely on being able to call
tmpnam at least this many times before it might fail saying you
have made too many temporary file names.
With the GNU library, you can create a very large number of temporary
file names. If you actually created the files, you would probably run
out of disk space before you ran out of names. Some other systems have
a fixed, small limit on the number of temporary files. The limit is
never less than 25.
malloc, so you should release its storage with
free when it is no longer needed.
Because the string is dynamically allocated this function is reentrant.
The directory prefix for the temporary file name is determined by testing each of the following in sequence. The directory must exist and be writable.
TMPDIR, if it is defined. For security
reasons this only happens if the program is not SUID or SGID enabled.
P_tmpdir macro.
This function is defined for SVID compatibility.
Warning: Between the time the pathname is constructed and the
file is created another process might have created a file with the same
name using tempnam, leading to a possible security hole. The
implementation generates names which can hardly be predicted, but when
opening the file you should use the O_EXCL flag. Using
tmpfile or mkstemp is a safe way to avoid this problem.
Older Unix systems did not have the functions just described. Instead
they used mktemp and mkstemp. Both of these functions
work by modifying a file name template string you pass. The last six
characters of this string must be `XXXXXX'. These six `X's
are replaced with six characters which make the whole string a unique
file name. Usually the template string is something like
`/tmp/prefixXXXXXX', and each program uses a unique prefix.
Note: Because mktemp and mkstemp modify the
template string, you must not pass string constants to them.
String constants are normally in read-only storage, so your program
would crash when mktemp or mkstemp tried to modify the
string.
mktemp function generates a unique file name by modifying
template as described above. If successful, it returns
template as modified. If mktemp cannot find a unique file
name, it makes template an empty string and returns that. If
template does not end with `XXXXXX', mktemp returns a
null pointer.
Warning: Between the time the pathname is constructed and the
file is created another process might have created a file with the same
name using mktemp, leading to a possible security hole. The
implementation generates names which can hardly be predicted, but when
opening the file you should use the O_EXCL flag. Using
mkstemp is a safe way to avoid this problem.
mkstemp function generates a unique file name just as
mktemp does, but it also opens the file for you with open
(see section Opening and Closing Files). If successful, it modifies
template in place and returns a file descriptor for that file open
for reading and writing. If mkstemp cannot create a
uniquely-named file, it returns -1. If template does not
end with `XXXXXX', mkstemp returns -1 and does not
modify template.
The file is opened using mode 0600. If the file is meant to be
used by other users this mode must be changed explicitly.
Unlike mktemp, mkstemp is actually guaranteed to create a
unique file that cannot possibly clash with any other program trying to
create a temporary file. This is because it works by calling
open with the O_EXCL flag, which says you want to create a
new file and get an error if the file already exists.
mkdtemp function creates a directory with a unique name. If
it succeeds, it overwrites template with the name of the
directory, and returns template. As with mktemp and
mkstemp, template should be a string ending with
`XXXXXX'.
If mkdtemp cannot create an uniquely named directory, it returns
NULL and sets errno appropriately. If template does
not end with `XXXXXX', mkdtemp returns NULL and does
not modify template. errno will be set to EINVAL in
this case.
The directory is created using mode 0700.
The directory created by mkdtemp cannot clash with temporary
files or directories created by other users. This is because directory
creation always works like open with O_EXCL.
See section Creating Directories.
The mkdtemp function comes from OpenBSD.
A pipe is a mechanism for interprocess communication; data written to the pipe by one process can be read by another process. The data is handled in a first-in, first-out (FIFO) order. The pipe has no name; it is created for one use and both ends must be inherited from the single process which created the pipe.
A FIFO special file is similar to a pipe, but instead of being an anonymous, temporary connection, a FIFO has a name or names like any other file. Processes open the FIFO by name in order to communicate through it.
A pipe or FIFO has to be open at both ends simultaneously. If you read
from a pipe or FIFO file that doesn't have any processes writing to it
(perhaps because they have all closed the file, or exited), the read
returns end-of-file. Writing to a pipe or FIFO that doesn't have a
reading process is treated as an error condition; it generates a
SIGPIPE signal, and fails with error code EPIPE if the
signal is handled or blocked.
Neither pipes nor FIFO special files allow file positioning. Both reading and writing operations happen sequentially; reading from the beginning of the file and writing at the end.
The primitive for creating a pipe is the pipe function. This
creates both the reading and writing ends of the pipe. It is not very
useful for a single process to use a pipe to talk to itself. In typical
use, a process creates a pipe just before it forks one or more child
processes (see section Creating a Process). The pipe is then used for
communication either between the parent or child processes, or between
two sibling processes.
The pipe function is declared in the header file
`unistd.h'.
pipe function creates a pipe and puts the file descriptors
for the reading and writing ends of the pipe (respectively) into
filedes[0] and filedes[1].
An easy way to remember that the input end comes first is that file
descriptor 0 is standard input, and file descriptor 1 is
standard output.
If successful, pipe returns a value of 0. On failure,
-1 is returned. The following errno error conditions are
defined for this function:
EMFILE
ENFILE
ENFILE. This error never occurs in
the GNU system.
Here is an example of a simple program that creates a pipe. This program
uses the fork function (see section Creating a Process) to create
a child process. The parent process writes data to the pipe, which is
read by the child process.
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
/* Read characters from the pipe and echo them to stdout. */
void
read_from_pipe (int file)
{
FILE *stream;
int c;
stream = fdopen (file, "r");
while ((c = fgetc (stream)) != EOF)
putchar (c);
fclose (stream);
}
/* Write some random text to the pipe. */
void
write_to_pipe (int file)
{
FILE *stream;
stream = fdopen (file, "w");
fprintf (stream, "hello, world!\n");
fprintf (stream, "goodbye, world!\n");
fclose (stream);
}
int
main (void)
{
pid_t pid;
int mypipe[2];
/* Create the pipe. */
if (pipe (mypipe))
{
fprintf (stderr, "Pipe failed.\n");
return EXIT_FAILURE;
}
/* Create the child process. */
pid = fork ();
if (pid == (pid_t) 0)
{
/* This is the child process.
Close other end first. */
close (mypipe[1]);
read_from_pipe (mypipe[0]);
return EXIT_SUCCESS;
}
else if (pid < (pid_t) 0)
{
/* The fork failed. */
fprintf (stderr, "Fork failed.\n");
return EXIT_FAILURE;
}
else
{
/* This is the parent process.
Close other end first. */
close (mypipe[0]);
write_to_pipe (mypipe[1]);
return EXIT_SUCCESS;
}
}
A common use of pipes is to send data to or receive data from a program
being run as a subprocess. One way of doing this is by using a combination of
pipe (to create the pipe), fork (to create the subprocess),
dup2 (to force the subprocess to use the pipe as its standard input
or output channel), and exec (to execute the new program). Or,
you can use popen and pclose.
The advantage of using popen and pclose is that the
interface is much simpler and easier to use. But it doesn't offer as
much flexibility as using the low-level functions directly.
popen function is closely related to the system
function; see section Running a Command. It executes the shell command
command as a subprocess. However, instead of waiting for the
command to complete, it creates a pipe to the subprocess and returns a
stream that corresponds to that pipe.
If you specify a mode argument of "r", you can read from the
stream to retrieve data from the standard output channel of the subprocess.
The subprocess inherits its standard input channel from the parent process.
Similarly, if you specify a mode argument of "w", you can
write to the stream to send data to the standard input channel of the
subprocess. The subprocess inherits its standard output channel from
the parent process.
In the event of an error popen returns a null pointer. This
might happen if the pipe or stream cannot be created, if the subprocess
cannot be forked, or if the program cannot be executed.
pclose function is used to close a stream created by popen.
It waits for the child process to terminate and returns its status value,
as for the system function.
Here is an example showing how to use popen and pclose to
filter output through another program, in this case the paging program
more.
#include <stdio.h>
#include <stdlib.h>
void
write_data (FILE * stream)
{
int i;
for (i = 0; i < 100; i++)
fprintf (stream, "%d\n", i);
if (ferror (stream))
{
fprintf (stderr, "Output to stream failed.\n");
exit (EXIT_FAILURE);
}
}
int
main (void)
{
FILE *output;
output = popen ("more", "w");
if (!output)
{
fprintf (stderr,
"incorrect parameters or too many files.\n");
return EXIT_FAILURE;
}
write_data (output);
if (pclose (output) != 0)
{
fprintf (stderr,
"Could not run more or other error.\n");
}
return EXIT_SUCCESS;
}
A FIFO special file is similar to a pipe, except that it is created in a
different way. Instead of being an anonymous communications channel, a
FIFO special file is entered into the file system by calling
mkfifo.
Once you have created a FIFO special file in this way, any process can open it for reading or writing, in the same way as an ordinary file. However, it has to be open at both ends simultaneously before you can proceed to do any input or output operations on it. Opening a FIFO for reading normally blocks until some other process opens the same FIFO for writing, and vice versa.
The mkfifo function is declared in the header file
`sys/stat.h'.
mkfifo function makes a FIFO special file with name
filename. The mode argument is used to set the file's
permissions; see section Assigning File Permissions.
The normal, successful return value from mkfifo is 0. In
the case of an error, -1 is returned. In addition to the usual
file name errors (see section File Name Errors), the following
errno error conditions are defined for this function:
EEXIST
ENOSPC
EROFS
Reading or writing pipe data is atomic if the size of data written
is not greater than PIPE_BUF. This means that the data transfer
seems to be an instantaneous unit, in that nothing else in the system
can observe a state in which it is partially complete. Atomic I/O may
not begin right away (it may need to wait for buffer space or for data),
but once it does begin it finishes immediately.
Reading or writing a larger amount of data may not be atomic; for
example, output data from other processes sharing the descriptor may be
interspersed. Also, once PIPE_BUF characters have been written,
further writes will block until some characters are read.
See section Limits on File System Capacity, for information about the PIPE_BUF
parameter.
This chapter describes the GNU facilities for interprocess communication using sockets.
A socket is a generalized interprocess communication channel.
Like a pipe, a socket is represented as a file descriptor. Unlike pipes
sockets support communication between unrelated processes, and even
between processes running on different machines that communicate over a
network. Sockets are the primary means of communicating with other
machines; telnet, rlogin, ftp, talk and the
other familiar network programs use sockets.
Not all operating systems support sockets. In the GNU library, the header file `sys/socket.h' exists regardless of the operating system, and the socket functions always exist, but if the system does not really support sockets these functions always fail.
Incomplete: We do not currently document the facilities for broadcast messages or for configuring Internet interfaces. The reentrant functions and some newer functions that are related to IPv6 aren't documented either so far.
When you create a socket, you must specify the style of communication you want to use and the type of protocol that should implement it. The communication style of a socket defines the user-level semantics of sending and receiving data on the socket. Choosing a communication style specifies the answers to questions such as these:
You must also choose a namespace for naming the socket. A socket name ("address") is meaningful only in the context of a particular namespace. In fact, even the data type to use for a socket name may depend on the namespace. Namespaces are also called "domains", but we avoid that word as it can be confused with other usage of the same term. Each namespace has a symbolic name that starts with `PF_'. A corresponding symbolic name starting with `AF_' designates the address format for that namespace.
Finally you must choose the protocol to carry out the communication. The protocol determines what low-level mechanism is used to transmit and receive data. Each protocol is valid for a particular namespace and communication style; a namespace is sometimes called a protocol family because of this, which is why the namespace names start with `PF_'.
The rules of a protocol apply to the data passing between two programs, perhaps on different computers; most of these rules are handled by the operating system and you need not know about them. What you do need to know about protocols is this:
Throughout the following description at various places
variables/parameters to denote sizes are required. And here the trouble
starts. In the first implementations the type of these variables was
simply int. On most machines at that time an int was 32
bits wide, which created a de facto standard requiring 32-bit
variables. This is important since references to variables of this type
are passed to the kernel.
Then the POSIX people came and unified the interface with the words "all
size values are of type size_t". On 64-bit machines
size_t is 64 bits wide, so pointers to variables were no longer
possible.
The Unix98 specification provides a solution by introducing a type
socklen_t. This type is used in all of the cases that POSIX
changed to use size_t. The only requirement of this type is that
it be an unsigned type of at least 32 bits. Therefore, implementations
which require that references to 32-bit variables be passed can be as
happy as implementations which use 64-bit values.
The GNU library includes support for several different kinds of sockets, each with different characteristics. This section describes the supported socket types. The symbolic constants listed here are defined in `sys/socket.h'.
SOCK_STREAM style is like a pipe (see section Pipes and FIFOs).
It operates over a connection with a particular remote socket and
transmits data reliably as a stream of bytes.
Use of this style is covered in detail in section Using Sockets with Connections.
SOCK_DGRAM style is used for sending
individually-addressed packets unreliably.
It is the diametrical opposite of SOCK_STREAM.
Each time you write data to a socket of this kind, that data becomes
one packet. Since SOCK_DGRAM sockets do not have connections,
you must specify the recipient address with each packet.
The only guarantee that the system makes about your requests to transmit data is that it will try its best to deliver each packet you send. It may succeed with the sixth packet after failing with the fourth and fifth packets; the seventh packet may arrive before the sixth, and may arrive a second time after the sixth.
The typical use for SOCK_DGRAM is in situations where it is
acceptable to simply re-send a packet if no response is seen in a
reasonable amount of time.
See section Datagram Socket Operations, for detailed information about how to use datagram sockets.
The name of a socket is normally called an address. The functions and symbols for dealing with socket addresses were named inconsistently, sometimes using the term "name" and sometimes using "address". You can regard these terms as synonymous where sockets are concerned.
A socket newly created with the socket function has no
address. Other processes can find it for communication only if you
give it an address. We call this binding the address to the
socket, and the way to do it is with the bind function.
You need be concerned with the address of a socket if other processes are to find it and start communicating with it. You can specify an address for other sockets, but this is usually pointless; the first time you send data from a socket, or use it to initiate a connection, the system assigns an address automatically if you have not specified one.
Occasionally a client needs to specify an address because the server
discriminates based on address; for example, the rsh and rlogin
protocols look at the client's socket address and only bypass password
checking if it is less than IPPORT_RESERVED (see section Internet Ports).
The details of socket addresses vary depending on what namespace you are using. See section The Local Namespace, or section The Internet Namespace, for specific information.
Regardless of the namespace, you use the same functions bind and
getsockname to set and examine a socket's address. These
functions use a phony data type, struct sockaddr *, to accept the
address. In practice, the address lives in a structure of some other
data type appropriate to the address format you are using, but you cast
its address to struct sockaddr * when you pass it to
bind.
The functions bind and getsockname use the generic data
type struct sockaddr * to represent a pointer to a socket
address. You can't use this data type effectively to interpret an
address or construct one; for that, you must use the proper data type
for the socket's namespace.
Thus, the usual practice is to construct an address of the proper
namespace-specific type, then cast a pointer to struct sockaddr *
when you call bind or getsockname.
The one piece of information that you can get from the struct
sockaddr data type is the address format designator. This tells
you which data type to use to understand the address fully.
The symbols in this section are defined in the header file `sys/socket.h'.
struct sockaddr type itself has the following members:
short int sa_family
char sa_data[14]
sa_data is essentially arbitrary.
Each address format has a symbolic name which starts with `AF_'. Each of them corresponds to a `PF_' symbol which designates the corresponding namespace. Here is a list of address format names:
AF_LOCAL
PF_LOCAL is the name of that namespace.) See section Details of Local Namespace, for information about this address format.
AF_UNIX
AF_LOCAL. Although AF_LOCAL is
mandated by POSIX.1g, AF_UNIX is portable to more systems.
AF_UNIX was the traditional name stemming from BSD, so even most
POSIX systems support it. It is also the name of choice in the Unix98
specification. (The same is true for PF_UNIX
vs. PF_LOCAL).
AF_FILE
AF_LOCAL, for compatibility.
(PF_FILE is likewise a synonym for PF_LOCAL.)
AF_INET
PF_INET is the name of that namespace.)
See section Internet Socket Address Formats.
AF_INET6
AF_INET, but refers to the IPv6 protocol.
(PF_INET6 is the name of the corresponding namespace.)
AF_UNSPEC
PF_UNSPEC exists
for completeness, but there is no reason to use it in a program.
`sys/socket.h' defines symbols starting with `AF_' for many different kinds of networks, most or all of which are not actually implemented. We will document those that really work as we receive information about how to use them.
Use the bind function to assign an address to a socket. The
prototype for bind is in the header file `sys/socket.h'.
For examples of use, see section Example of Local-Namespace Sockets, or see section Internet Socket Example.
bind function assigns an address to the socket
socket. The addr and length arguments specify the
address; the detailed format of the address depends on the namespace.
The first part of the address is always the format designator, which
specifies a namespace, and says that the address is in the format of
that namespace.
The return value is 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
EBADF
ENOTSOCK
EADDRNOTAVAIL
EADDRINUSE
EINVAL
EACCES
IPPORT_RESERVED minus one; see
section Internet Ports.)
Additional conditions may be possible depending on the particular namespace of the socket.
Use the function getsockname to examine the address of an
Internet socket. The prototype for this function is in the header file
`sys/socket.h'.
getsockname function returns information about the
address of the socket socket in the locations specified by the
addr and length-ptr arguments. Note that the
length-ptr is a pointer; you should initialize it to be the
allocation size of addr, and on return it contains the actual
size of the address data.
The format of the address data depends on the socket namespace. The
length of the information is usually fixed for a given namespace, so
normally you can know exactly how much space is needed and can provide
that much. The usual practice is to allocate a place for the value
using the proper data type for the socket's namespace, then cast its
address to struct sockaddr * to pass it to getsockname.
The return value is 0 on success and -1 on error. The
following errno error conditions are defined for this function:
EBADF
ENOTSOCK
ENOBUFS
You can't read the address of a socket in the file namespace. This is consistent with the rest of the system; in general, there's no way to find a file's name from a descriptor for that file.
Each network interface has a name. This usually consists of a few
letters that relate to the type of interface, which may be followed by a
number if there is more than one interface of that type. Examples
might be lo (the loopback interface) and eth0 (the first
Ethernet interface).
Although such names are convenient for humans, it would be clumsy to have to use them whenever a program needs to refer to an interface. In such situations an interface is referred to by its index, which is an arbitrarily-assigned small positive integer.
The following functions, constants and data types are declared in the header file `net/if.h'.
ifname, which
must be at least IFNAMSIZ bytes in length. If the index was
invalid, the function's return value is a null pointer, otherwise it is
ifname.
unsigned int if_index;
char *if_name
if_nameindex structures, one
for every interface that is present. The end of the list is indicated
by a structure with an interface of 0 and a null name pointer. If an
error occurs, this function returns a null pointer.
The returned structure must be freed with if_freenameindex after
use.
if_nameindex.
This section describes the details of the local namespace, whose
symbolic name (required when you create a socket) is PF_LOCAL.
The local namespace is also known as "Unix domain sockets". Another
name is file namespace since socket addresses are normally implemented
as file names.
In the local namespace socket addresses are file names. You can specify any file name you want as the address of the socket, but you must have write permission on the directory containing it. It's common to put these files in the `/tmp' directory.
One peculiarity of the local namespace is that the name is only used when opening the connection; once open the address is not meaningful and may not exist.
Another peculiarity is that you cannot connect to such a socket from another machine--not even if the other machine shares the file system which contains the name of the socket. You can see the socket in a directory listing, but connecting to it never succeeds. Some programs take advantage of this, such as by asking the client to send its own process ID, and using the process IDs to distinguish between clients. However, we recommend you not use this method in protocols you design, as we might someday permit connections from other machines that mount the same file systems. Instead, send each new client an identifying number if you want it to have one.
After you close a socket in the local namespace, you should delete the
file name from the file system. Use unlink or remove to
do this; see section Deleting Files.
The local namespace supports just one protocol for any communication
style; it is protocol number 0.
To create a socket in the local namespace, use the constant
PF_LOCAL as the namespace argument to socket or
socketpair. This constant is defined in `sys/socket.h'.
PF_Local is the
macro used by Posix.1g.
The structure for specifying socket names in the local namespace is defined in the header file `sys/un.h':
short int sun_family
AF_LOCAL to designate the local
namespace. See section Socket Addresses.
char sun_path[108]
alloca to allocate an appropriate amount of storage based on
the length of the filename.
You should compute the length parameter for a socket address in
the local namespace as the sum of the size of the sun_family
component and the string length (not the allocation size!) of
the file name string. This can be done using the macro SUN_LEN:
Here is an example showing how to create and name a socket in the local namespace.
#include <stddef.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>
int
make_named_socket (const char *filename)
{
struct sockaddr_un name;
int sock;
size_t size;
/* Create the socket. */
sock = socket (PF_LOCAL, SOCK_DGRAM, 0);
if (sock < 0)
{
perror ("socket");
exit (EXIT_FAILURE);
}
/* Bind a name to the socket. */
name.sun_family = AF_LOCAL;
strncpy (name.sun_path, filename, sizeof (name.sun_path));
/* The size of the address is
the offset of the start of the filename,
plus its length,
plus one for the terminating null byte.
Alternatively you can just do:
size = SUN_LEN (&name);
*/
size = (offsetof (struct sockaddr_un, sun_path)
+ strlen (name.sun_path) + 1);
if (bind (sock, (struct sockaddr *) &name, size) < 0)
{
perror ("bind");
exit (EXIT_FAILURE);
}
return sock;
}
This section describes the details of the protocols and socket naming conventions used in the Internet namespace.
Originally the Internet namespace used only IP version 4 (IPv4). With the growing number of hosts on the Internet, a new protocol with a larger address space was necessary: IP version 6 (IPv6). IPv6 introduces 128-bit addresses (IPv4 has 32-bit addresses) and other features, and will eventually replace IPv4.
To create a socket in the IPv4 Internet namespace, use the symbolic name
PF_INET of this namespace as the namespace argument to
socket or socketpair. For IPv6 addresses you need the
macro PF_INET6. These macros are defined in `sys/socket.h'.
A socket address for the Internet namespace includes the following components:
You must ensure that the address and port number are represented in a canonical format called network byte order. See section Byte Order Conversion, for information about this.
In the Internet namespace, for both IPv4 (AF_INET) and IPv6
(AF_INET6), a socket address consists of a host address
and a port on that host. In addition, the protocol you choose serves
effectively as a part of the address because local port numbers are
meaningful only within a particular protocol.
The data types for representing socket addresses in the Internet namespace are defined in the header file `netinet/in.h'.
sa_family_t sin_family
AF_INET in this member.
See section Socket Addresses.
struct in_addr sin_addr
unsigned short int sin_port
When you call bind or getsockname, you should specify
sizeof (struct sockaddr_in) as the length parameter if
you are using an IPv4 Internet namespace socket address.
sa_family_t sin6_family
AF_INET6 in this member.
See section Socket Addresses.
struct in6_addr sin6_addr
uint32_t sin6_flowinfo
uint16_t sin6_port
Each computer on the Internet has one or more Internet addresses, numbers which identify that computer among all those on the Internet. Users typically write IPv4 numeric host addresses as sequences of four numbers, separated by periods, as in `128.52.46.32', and IPv6 numeric host addresses as sequences of up to eight numbers separated by colons, as in `5f03:1200:836f:c100::1'.
Each computer also has one or more host names, which are strings of words separated by periods, as in `mescaline.gnu.org'.
Programs that let the user specify a host typically accept both numeric addresses and host names. To open a connection a program needs a numeric address, and so must convert a host name to the numeric address it stands for.
An IPv4 Internet host address is a number containing four bytes of data. Historically these are divided into two parts, a network number and a local network address number within that network. In the mid-1990s classless addresses were introduced which changed this behaviour. Since some functions implicitly expect the old definitions, we first describe the class-based network and will then describe classless addresses. IPv6 uses only classless addresses and therefore the following paragraphs don't apply.
The class-based IPv4 network number consists of the first one, two or three bytes; the rest of the bytes are the local address.
IPv4 network numbers are registered with the Network Information Center (NIC), and are divided into three classes--A, B and C. The local network address numbers of individual machines are registered with the administrator of the particular network.
Class A networks have single-byte numbers in the range 0 to 127. There are only a small number of Class A networks, but they can each support a very large number of hosts. Medium-sized Class B networks have two-byte network numbers, with the first byte in the range 128 to 191. Class C networks are the smallest; they have three-byte network numbers, with the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes of an Internet address specify a network. The remaining bytes of the Internet address specify the address within that network.
The Class A network 0 is reserved for broadcast to all networks. In addition, the host number 0 within each network is reserved for broadcast to all hosts in that network. These uses are obsolete now but for compatibility reasons you shouldn't use network 0 and host number 0.
The Class A network 127 is reserved for loopback; you can always use the Internet address `127.0.0.1' to refer to the host machine.
Since a single machine can be a member of multiple networks, it can have multiple Internet host addresses. However, there is never supposed to be more than one machine with the same host address.
There are four forms of the standard numbers-and-dots notation for Internet addresses:
a.b.c.d
a.b.c
a.b.
a.b
a
Within each part of the address, the usual C conventions for specifying the radix apply. In other words, a leading `0x' or `0X' implies hexadecimal radix; a leading `0' implies octal; and otherwise decimal radix is assumed.
IPv4 addresses (and IPv6 addresses also) are now considered classless; the distinction between classes A, B and C can be ignored. Instead an IPv4 host address consists of a 32-bit address and a 32-bit mask. The mask contains set bits for the network part and cleared bits for the host part. The network part is contiguous from the left, with the remaining bits representing the host. As a consequence, the netmask can simply be specified as the number of set bits. Classes A, B and C are just special cases of this general rule. For example, class A addresses have a netmask of `255.0.0.0' or a prefix length of 8.
Classless IPv4 network addresses are written in numbers-and-dots notation with the prefix length appended and a slash as separator. For example the class A network 10 is written as `10.0.0.0/8'.
IPv6 addresses contain 128 bits (IPv4 has 32 bits) of data. A host address is usually written as eight 16-bit hexadecimal numbers that are separated by colons. Two colons are used to abbreviate strings of consecutive zeros. For example, the IPv6 loopback address `0:0:0:0:0:0:0:1' can just be written as `::1'.
IPv4 Internet host addresses are represented in some contexts as integers
(type uint32_t). In other contexts, the integer is
packaged inside a structure of type struct in_addr. It would
be better if the usage were made consistent, but it is not hard to extract
the integer from the structure or put the integer into a structure.
You will find older code that uses unsigned long int for
IPv4 Internet host addresses instead of uint32_t or struct
in_addr. Historically unsigned long int was a 32-bit number but
with 64-bit machines this has changed. Using unsigned long int
might break the code if it is used on machines where this type doesn't
have 32 bits. uint32_t is specified by Unix98 and guaranteed to have
32 bits.
IPv6 Internet host addresses have 128 bits and are packaged inside a
structure of type struct in6_addr.
The following basic definitions for Internet addresses are declared in the header file `netinet/in.h':
s_addr, which records
the host address number as an uint32_t.
INADDR_LOOPBACK
specially, avoiding any network traffic for the case of one machine
talking to itself.
sin_addr member of struct
sockaddr_in when you want to accept Internet connections.
IN6ADDR_LOOPBACK_INIT is provided to allow you to initialize your
own variables to this value.
IN6ADDR_ANY_INIT is provided to allow you to initialize your
own variables to this value.
These additional functions for manipulating Internet addresses are declared in the header file `arpa/inet.h'. They represent Internet addresses in network byte order, and network numbers and local-address-within-network numbers in host byte order. See section Byte Order Conversion, for an explanation of network and host byte order.
struct in_addr that addr points to.
inet_aton returns nonzero if the address is valid, zero if not.
inet_addr returns INADDR_NONE. This is an
obsolete interface to inet_aton, described immediately above. It
is obsolete because INADDR_NONE is a valid address
(255.255.255.255), and inet_aton provides a cleaner way to
indicate error return.
inet_network returns
-1.
The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore.
In multi-threaded programs each thread has an own statically-allocated
buffer. But still subsequent calls of inet_ntoa in the same
thread will overwrite the result of the last call.
Instead of inet_ntoa the newer function inet_ntop which is
described below should be used since it handles both IPv4 and IPv6
addresses.
The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore.
The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore.
AF_INET or AF_INET6, as appropriate for the type of
address being converted. cp is a pointer to the input string, and
buf is a pointer to a buffer for the result. It is the caller's
responsibility to make sure the buffer is large enough.
AF_INET or AF_INET6, as appropriate. cp is a
pointer to the address to be converted. buf should be a pointer
to a buffer to hold the result, and len is the length of this
buffer. The return value from the function will be this buffer address.
Besides the standard numbers-and-dots notation for Internet addresses, you can also refer to a host by a symbolic name. The advantage of a symbolic name is that it is usually easier to remember. For example, the machine with Internet address `158.121.106.19' is also known as `alpha.gnu.org'; and other machines in the `gnu.org' domain can refer to it simply as `alpha'.
Internally, the system uses a database to keep track of the mapping between host names and host numbers. This database is usually either the file `/etc/hosts' or an equivalent provided by a name server. The functions and other symbols for accessing this database are declared in `netdb.h'. They are BSD features, defined unconditionally if you include `netdb.h'.
char *h_name
char **h_aliases
int h_addrtype
AF_INET or AF_INET6, with the latter being used for IPv6
hosts. In principle other kinds of addresses could be represented in
the database as well as Internet addresses; if this were done, you
might find a value in this field other than AF_INET or
AF_INET6. See section Socket Addresses.
int h_length
char **h_addr_list
char *h_addr
h_addr_list[0]; in other words, it is the
first host address.
As far as the host database is concerned, each address is just a block
of memory h_length bytes long. But in other contexts there is an
implicit assumption that you can convert IPv4 addresses to a
struct in_addr or an uint32_t. Host addresses in
a struct hostent structure are always given in network byte
order; see section Byte Order Conversion.
You can use gethostbyname, gethostbyname2 or
gethostbyaddr to search the hosts database for information about
a particular host. The information is returned in a
statically-allocated structure; you must copy the information if you
need to save it across calls. You can also use getaddrinfo and
getnameinfo to obtain this information.
gethostbyname function returns information about the host
named name. If the lookup fails, it returns a null pointer.
gethostbyname2 function is like gethostbyname, but
allows the caller to specify the desired address family (e.g.
AF_INET or AF_INET6) of the result.
gethostbyaddr function returns information about the host
with Internet address addr. The parameter addr is not
really a pointer to char - it can be a pointer to an IPv4 or an IPv6
address. The length argument is the size (in bytes) of the address
at addr. format specifies the address format; for an IPv4
Internet address, specify a value of AF_INET; for an IPv6
Internet address, use AF_INET6.
If the lookup fails, gethostbyaddr returns a null pointer.
If the name lookup by gethostbyname or gethostbyaddr
fails, you can find out the reason by looking at the value of the
variable h_errno. (It would be cleaner design for these
functions to set errno, but use of h_errno is compatible
with other systems.)
Here are the error codes that you may find in h_errno:
HOST_NOT_FOUND
TRY_AGAIN
NO_RECOVERY
NO_ADDRESS
The lookup functions above all have one in common: they are not reentrant and therefore unusable in multi-threaded applications. Therefore provides the GNU C library a new set of functions which can be used in this context.
gethostbyname_r function returns information about the host
named name. The caller must pass a pointer to an object of type
struct hostent in the result_buf parameter. In addition
the function may need extra buffer space and the caller must pass an
pointer and the size of the buffer in the buf and buflen
parameters.
A pointer to the buffer, in which the result is stored, is available in
*result after the function call successfully returned. If
an error occurs or if no entry is found, the pointer *result
is a null pointer. Success is signalled by a zero return value. If the
function failed the return value is an error number. In addition to the
errors defined for gethostbyname it can also be ERANGE.
In this case the call should be repeated with a larger buffer.
Additional error information is not stored in the global variable
h_errno but instead in the object pointed to by h_errnop.
Here's a small example:
struct hostent *
gethostname (char *host)
{
struct hostent hostbuf, *hp;
size_t hstbuflen;
char *tmphstbuf;
int res;
int herr;
hstbuflen = 1024;
/* Allocate buffer, remember to free it to avoid a memory leakage. */
tmphstbuf = malloc (hstbuflen);
while ((res = gethostbyname_r (host, &hostbuf, tmphstbuf, hstbuflen,
&hp, &herr)) == ERANGE)
{
/* Enlarge the buffer. */
hstbuflen *= 2;
tmphstbuf = realloc (tmphstbuf, hstbuflen);
}
/* Check for errors. */
if (res || hp == NULL)
return NULL;
return hp;
}
gethostbyname2_r function is like gethostbyname_r, but
allows the caller to specify the desired address family (e.g.
AF_INET or AF_INET6) for the result.
gethostbyaddr_r function returns information about the host
with Internet address addr. The parameter addr is not
really a pointer to char - it can be a pointer to an IPv4 or an IPv6
address. The length argument is the size (in bytes) of the address
at addr. format specifies the address format; for an IPv4
Internet address, specify a value of AF_INET; for an IPv6
Internet address, use AF_INET6.
Similar to the gethostbyname_r function, the caller must provide
buffers for the result and memory used internally. In case of success
the function returns zero. Otherwise the value is an error number where
ERANGE has the special meaning that the caller-provided buffer is
too small.
You can also scan the entire hosts database one entry at a time using
sethostent, gethostent and endhostent. Be careful
when using these functions because they are not reentrant.
gethostent to read the entries.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to gethostbyname or gethostbyaddr will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
A socket address in the Internet namespace consists of a machine's Internet address plus a port number which distinguishes the sockets on a given machine (for a given protocol). Port numbers range from 0 to 65,535.
Port numbers less than IPPORT_RESERVED are reserved for standard
servers, such as finger and telnet. There is a database
that keeps track of these, and you can use the getservbyname
function to map a service name onto a port number; see section The Services Database.
If you write a server that is not one of the standard ones defined in
the database, you must choose a port number for it. Use a number
greater than IPPORT_USERRESERVED; such numbers are reserved for
servers and won't ever be generated automatically by the system.
Avoiding conflicts with servers being run by other users is up to you.
When you use a socket without specifying its address, the system
generates a port number for it. This number is between
IPPORT_RESERVED and IPPORT_USERRESERVED.
On the Internet, it is actually legitimate to have two different
sockets with the same port number, as long as they never both try to
communicate with the same socket address (host address plus port
number). You shouldn't duplicate a port number except in special
circumstances where a higher-level protocol requires it. Normally,
the system won't let you do it; bind normally insists on
distinct port numbers. To reuse a port number, you must set the
socket option SO_REUSEADDR. See section Socket-Level Options.
These macros are defined in the header file `netinet/in.h'.
IPPORT_USERRESERVED are
reserved for explicit use; they will never be allocated automatically.
The database that keeps track of "well-known" services is usually either the file `/etc/services' or an equivalent from a name server. You can use these utilities, declared in `netdb.h', to access the services database.
char *s_name
char **s_aliases
int s_port
char *s_proto
To get information about a particular service, use the
getservbyname or getservbyport functions. The information
is returned in a statically-allocated structure; you must copy the
information if you need to save it across calls.
getservbyname function returns information about the
service named name using protocol proto. If it can't find
such a service, it returns a null pointer.
This function is useful for servers as well as for clients; servers use it to determine which port they should listen on (see section Listening for Connections).
getservbyport function returns information about the
service at port port using protocol proto. If it can't
find such a service, it returns a null pointer.
You can also scan the services database using setservent,
getservent and endservent. Be careful when using these
functions because they are not reentrant.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to getservbyname or getservbyport will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
Different kinds of computers use different conventions for the ordering of bytes within a word. Some computers put the most significant byte within a word first (this is called "big-endian" order), and others put it last ("little-endian" order).
So that machines with different byte order conventions can communicate, the Internet protocols specify a canonical byte order convention for data transmitted over the network. This is known as network byte order.
When establishing an Internet socket connection, you must make sure that
the data in the sin_port and sin_addr members of the
sockaddr_in structure are represented in network byte order.
If you are encoding integer data in the messages sent through the
socket, you should convert this to network byte order too. If you don't
do this, your program may fail when running on or talking to other kinds
of machines.
If you use getservbyname and gethostbyname or
inet_addr to get the port number and host address, the values are
already in network byte order, and you can copy them directly into
the sockaddr_in structure.
Otherwise, you have to convert the values explicitly. Use htons
and ntohs to convert values for the sin_port member. Use
htonl and ntohl to convert IPv4 addresses for the
sin_addr member. (Remember, struct in_addr is equivalent
to uint32_t.) These functions are declared in
`netinet/in.h'.
uint16_t integer hostshort from
host byte order to network byte order.
uint16_t integer netshort from
network byte order to host byte order.
uint32_t integer hostlong from
host byte order to network byte order.
This is used for IPv4 Internet addresses.
uint32_t integer netlong from
network byte order to host byte order.
This is used for IPv4 Internet addresses.
The communications protocol used with a socket controls low-level details of how data are exchanged. For example, the protocol implements things like checksums to detect errors in transmissions, and routing instructions for messages. Normal user programs have little reason to mess with these details directly.
The default communications protocol for the Internet namespace depends on the communication style. For stream communication, the default is TCP ("transmission control protocol"). For datagram communication, the default is UDP ("user datagram protocol"). For reliable datagram communication, the default is RDP ("reliable datagram protocol"). You should nearly always use the default.
Internet protocols are generally specified by a name instead of a
number. The network protocols that a host knows about are stored in a
database. This is usually either derived from the file
`/etc/protocols', or it may be an equivalent provided by a name
server. You look up the protocol number associated with a named
protocol in the database using the getprotobyname function.
Here are detailed descriptions of the utilities for accessing the protocols database. These are declared in `netdb.h'.
char *p_name
char **p_aliases
int p_proto
socket.
You can use getprotobyname and getprotobynumber to search
the protocols database for a specific protocol. The information is
returned in a statically-allocated structure; you must copy the
information if you need to save it across calls.
getprotobyname function returns information about the
network protocol named name. If there is no such protocol, it
returns a null pointer.
getprotobynumber function returns information about the
network protocol with number protocol. If there is no such
protocol, it returns a null pointer.
You can also scan the whole protocols database one protocol at a time by
using setprotoent, getprotoent and endprotoent.
Be careful when using these functions because they are not reentrant.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to getprotobyname or getprotobynumber will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
Here is an example showing how to create and name a socket in the
Internet namespace. The newly created socket exists on the machine that
the program is running on. Rather than finding and using the machine's
Internet address, this example specifies INADDR_ANY as the host
address; the system replaces that with the machine's actual address.
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
make_socket (uint16_t port)
{
int sock;
struct sockaddr_in name;
/* Create the socket. */
sock = socket (PF_INET, SOCK_STREAM, 0);
if (sock < 0)
{
perror ("socket");
exit (EXIT_FAILURE);
}
/* Give the socket a name. */
name.sin_family = AF_INET;
name.sin_port = htons (port);
name.sin_addr.s_addr = htonl (INADDR_ANY);
if (bind (sock, (struct sockaddr *) &name, sizeof (name)) < 0)
{
perror ("bind");
exit (EXIT_FAILURE);
}
return sock;
}
Here is another example, showing how you can fill in a sockaddr_in
structure, given a host name string and a port number:
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
void
init_sockaddr (struct sockaddr_in *name,
const char *hostname,
uint16_t port)
{
struct hostent *hostinfo;
name->sin_family = AF_INET;
name->sin_port = htons (port);
hostinfo = gethostbyname (hostname);
if (hostinfo == NULL)
{
fprintf (stderr, "Unknown host %s.\n", hostname);
exit (EXIT_FAILURE);
}
name->sin_addr = *(struct in_addr *) hostinfo->h_addr;
}
Certain other namespaces and associated protocol families are supported
but not documented yet because they are not often used. PF_NS
refers to the Xerox Network Software protocols. PF_ISO stands
for Open Systems Interconnect. PF_CCITT refers to protocols from
CCITT. `socket.h' defines these symbols and others naming protocols
not actually implemented.
PF_IMPLINK is used for communicating between hosts and Internet
Message Processors. For information on this and PF_ROUTE, an
occasionally-used local area routing protocol, see the GNU Hurd Manual
(to appear in the future).
This section describes the actual library functions for opening and closing sockets. The same functions work for all namespaces and connection styles.
The primitive for creating a socket is the socket function,
declared in `sys/socket.h'.
PF_LOCAL (see section The Local Namespace) or
PF_INET (see section The Internet Namespace). protocol
designates the specific protocol (see section Socket Concepts); zero is
usually right for protocol.
The return value from socket is the file descriptor for the new
socket, or -1 in case of error. The following errno error
conditions are defined for this function:
EPROTONOSUPPORT
EMFILE
ENFILE
EACCESS
ENOBUFS
The file descriptor returned by the socket function supports both
read and write operations. However, like pipes, sockets do not support file
positioning operations.
For examples of how to call the socket function,
see section Example of Local-Namespace Sockets, or section Internet Socket Example.
When you have finished using a socket, you can simply close its
file descriptor with close; see section Opening and Closing Files.
If there is still data waiting to be transmitted over the connection,
normally close tries to complete this transmission. You
can control this behavior using the SO_LINGER socket option to
specify a timeout period; see section Socket Options.
You can also shut down only reception or transmission on a
connection by calling shutdown, which is declared in
`sys/socket.h'.
shutdown function shuts down the connection of socket
socket. The argument how specifies what action to
perform:
0
1
2
The return value is 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
EBADF
ENOTSOCK
ENOTCONN
A socket pair consists of a pair of connected (but unnamed)
sockets. It is very similar to a pipe and is used in much the same
way. Socket pairs are created with the socketpair function,
declared in `sys/socket.h'. A socket pair is much like a pipe; the
main difference is that the socket pair is bidirectional, whereas the
pipe has one input-only end and one output-only end (see section Pipes and FIFOs).
filedes[0] and filedes[1]. The socket pair
is a full-duplex communications channel, so that both reading and writing
may be performed at either end.
The namespace, style and protocol arguments are
interpreted as for the socket function. style should be
one of the communication styles listed in section Communication Styles.
The namespace argument specifies the namespace, which must be
AF_LOCAL (see section The Local Namespace); protocol specifies the
communications protocol, but zero is the only meaningful value.
If style specifies a connectionless communication style, then the two sockets you get are not connected, strictly speaking, but each of them knows the other as the default destination address, so they can send packets to each other.
The socketpair function returns 0 on success and -1
on failure. The following errno error conditions are defined
for this function:
EMFILE
EAFNOSUPPORT
EPROTONOSUPPORT
EOPNOTSUPP
The most common communication styles involve making a connection to a particular other socket, and then exchanging data with that socket over and over. Making a connection is asymmetric; one side (the client) acts to request a connection, while the other side (the server) makes a socket and waits for the connection request.
In making a connection, the client makes a connection while the server
waits for and accepts the connection. Here we discuss what the client
program must do with the connect function, which is declared in
`sys/socket.h'.
connect function initiates a connection from the socket
with file descriptor socket to the socket whose address is
specified by the addr and length arguments. (This socket
is typically on another machine, and it must be already set up as a
server.) See section Socket Addresses, for information about how these
arguments are interpreted.
Normally, connect waits until the server responds to the request
before it returns. You can set nonblocking mode on the socket
socket to make connect return immediately without waiting
for the response. See section File Status Flags, for information about
nonblocking mode.
The normal return value from connect is 0. If an error
occurs, connect returns -1. The following errno
error conditions are defined for this function:
EBADF
ENOTSOCK
EADDRNOTAVAIL
EAFNOSUPPORT
EISCONN
ETIMEDOUT
ECONNREFUSED
ENETUNREACH
EADDRINUSE
EINPROGRESS
select; see section Waiting for Input or Output.
Another connect call on the same socket, before the connection is
completely established, will fail with EALREADY.
EALREADY
EINPROGRESS above).
This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.
Now let us consider what the server process must do to accept
connections on a socket. First it must use the listen function
to enable connection requests on the socket, and then accept each
incoming connection with a call to accept (see section Accepting Connections). Once connection requests are enabled on a server socket,
the select function reports when the socket has a connection
ready to be accepted (see section Waiting for Input or Output).
The listen function is not allowed for sockets using
connectionless communication styles.
You can write a network server that does not even start running until a
connection to it is requested. See section inetd Servers.
In the Internet namespace, there are no special protection mechanisms for controlling access to a port; any process on any machine can make a connection to your server. If you want to restrict access to your server, make it examine the addresses associated with connection requests or implement some other handshaking or identification protocol.
In the local namespace, the ordinary file protection bits control who has access to connect to the socket.
listen function enables the socket socket to accept
connections, thus making it a server socket.
The argument n specifies the length of the queue for pending
connections. When the queue fills, new clients attempting to connect
fail with ECONNREFUSED until the server calls accept to
accept a connection from the queue.
The listen function returns 0 on success and -1
on failure. The following errno error conditions are defined
for this function:
EBADF
ENOTSOCK
EOPNOTSUPP
When a server receives a connection request, it can complete the
connection by accepting the request. Use the function accept
to do this.
A socket that has been established as a server can accept connection
requests from multiple clients. The server's original socket
does not become part of the connection; instead, accept
makes a new socket which participates in the connection.
accept returns the descriptor for this socket. The server's
original socket remains available for listening for further connection
requests.
The number of pending connection requests on a server socket is finite.
If connection requests arrive from clients faster than the server can
act upon them, the queue can fill up and additional requests are refused
with an ECONNREFUSED error. You can specify the maximum length of
this queue as an argument to the listen function, although the
system may also impose its own internal limit on the length of this
queue.
The accept function waits if there are no connections pending,
unless the socket socket has nonblocking mode set. (You can use
select to wait for a pending connection, with a nonblocking
socket.) See section File Status Flags, for information about nonblocking
mode.
The addr and length-ptr arguments are used to return information about the name of the client socket that initiated the connection. See section Socket Addresses, for information about the format of the information.
Accepting a connection does not make socket part of the
connection. Instead, it creates a new socket which becomes
connected. The normal return value of accept is the file
descriptor for the new socket.
After accept, the original socket socket remains open and
unconnected, and continues listening until you close it. You can
accept further connections with socket by calling accept
again.
If an error occurs, accept returns -1. The following
errno error conditions are defined for this function:
EBADF
ENOTSOCK
EOPNOTSUPP
EWOULDBLOCK
This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.
The accept function is not allowed for sockets using
connectionless communication styles.
getpeername function returns the address of the socket that
socket is connected to; it stores the address in the memory space
specified by addr and length-ptr. It stores the length of
the address in *length-ptr.
See section Socket Addresses, for information about the format of the
address. In some operating systems, getpeername works only for
sockets in the Internet domain.
The return value is 0 on success and -1 on error. The
following errno error conditions are defined for this function:
EBADF
ENOTSOCK
ENOTCONN
ENOBUFS
Once a socket has been connected to a peer, you can use the ordinary
read and write operations (see section Input and Output Primitives) to
transfer data. A socket is a two-way communications channel, so read
and write operations can be performed at either end.
There are also some I/O modes that are specific to socket operations.
In order to specify these modes, you must use the recv and
send functions instead of the more generic read and
write functions. The recv and send functions take
an additional argument which you can use to specify various flags to
control special I/O modes. For example, you can specify the
MSG_OOB flag to read or write out-of-band data, the
MSG_PEEK flag to peek at input, or the MSG_DONTROUTE flag
to control inclusion of routing information on output.
The send function is declared in the header file
`sys/socket.h'. If your flags argument is zero, you can just
as well use write instead of send; see section Input and Output Primitives. If the socket was connected but the connection has broken,
you get a SIGPIPE signal for any use of send or
write (see section Miscellaneous Signals).
send function is like write, but with the additional
flags flags. The possible values of flags are described
in section Socket Data Options.
This function returns the number of bytes transmitted, or -1 on
failure. If the socket is nonblocking, then send (like
write) can return after sending just part of the data.
See section File Status Flags, for information about nonblocking mode.
Note, however, that a successful return value merely indicates that the message has been sent without error, not necessarily that it has been received without error.
The following errno error conditions are defined for this function:
EBADF
EINTR
ENOTSOCK
EMSGSIZE
EWOULDBLOCK
send blocks until the operation can be
completed.)
ENOBUFS
ENOTCONN
EPIPE
send generates a SIGPIPE signal first; if that
signal is ignored or blocked, or if its handler returns, then
send fails with EPIPE.
This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.
The recv function is declared in the header file
`sys/socket.h'. If your flags argument is zero, you can
just as well use read instead of recv; see section Input and Output Primitives.
recv function is like read, but with the additional
flags flags. The possible values of flags are described
in section Socket Data Options.
If nonblocking mode is set for socket, and no data are available to
be read, recv fails immediately rather than waiting. See section File Status Flags, for information about nonblocking mode.
This function returns the number of bytes received, or -1 on failure.
The following errno error conditions are defined for this function:
EBADF
ENOTSOCK
EWOULDBLOCK
recv blocks until there is input
available to be read.)
EINTR
ENOTCONN
This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.
The flags argument to send and recv is a bit
mask. You can bitwise-OR the values of the following macros together
to obtain a value for this argument. All are defined in the header
file `sys/socket.h'.
recv, not with
send.
Here is an example client program that makes a connection for a byte stream socket in the Internet namespace. It doesn't do anything particularly interesting once it has connected to the server; it just sends a text string to the server and exits.
This program uses init_sockaddr to set up the socket address; see
section Internet Socket Example.
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define PORT 5555
#define MESSAGE "Yow!!! Are we having fun yet?!?"
#define SERVERHOST "mescaline.gnu.org"
void
write_to_server (int filedes)
{
int nbytes;
nbytes = write (filedes, MESSAGE, strlen (MESSAGE) + 1);
if (nbytes < 0)
{
perror ("write");
exit (EXIT_FAILURE);
}
}
int
main (void)
{
extern void init_sockaddr (struct sockaddr_in *name,
const char *hostname,
uint16_t port);
int sock;
struct sockaddr_in servername;
/* Create the socket. */
sock = socket (PF_INET, SOCK_STREAM, 0);
if (sock < 0)
{
perror ("socket (client)");
exit (EXIT_FAILURE);
}
/* Connect to the server. */
init_sockaddr (&servername, SERVERHOST, PORT);
if (0 > connect (sock,
(struct sockaddr *) &servername,
sizeof (servername)))
{
perror ("connect (client)");
exit (EXIT_FAILURE);
}
/* Send data to the server. */
write_to_server (sock);
close (sock);
exit (EXIT_SUCCESS);
}
The server end is much more complicated. Since we want to allow
multiple clients to be connected to the server at the same time, it
would be incorrect to wait for input from a single client by simply
calling read or recv. Instead, the right thing to do is
to use select (see section Waiting for Input or Output) to wait for input on
all of the open sockets. This also allows the server to deal with
additional connection requests.
This particular server doesn't do anything interesting once it has gotten a message from a client. It does close the socket for that client when it detects an end-of-file condition (resulting from the client shutting down its end of the connection).
This program uses make_socket to set up the socket address; see
section Internet Socket Example.
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define PORT 5555
#define MAXMSG 512
int
read_from_client (int filedes)
{
char buffer[MAXMSG];
int nbytes;
nbytes = read (filedes, buffer, MAXMSG);
if (nbytes < 0)
{
/* Read error. */
perror ("read");
exit (EXIT_FAILURE);
}
else if (nbytes == 0)
/* End-of-file. */
return -1;
else
{
/* Data read. */
fprintf (stderr, "Server: got message: `%s'\n", buffer);
return 0;
}
}
int
main (void)
{
extern int make_socket (uint16_t port);
int sock;
fd_set active_fd_set, read_fd_set;
int i;
struct sockaddr_in clientname;
size_t size;
/* Create the socket and set it up to accept connections. */
sock = make_socket (PORT);
if (listen (sock, 1) < 0)
{
perror ("listen");
exit (EXIT_FAILURE);
}
/* Initialize the set of active sockets. */
FD_ZERO (&active_fd_set);
FD_SET (sock, &active_fd_set);
while (1)
{
/* Block until input arrives on one or more active sockets. */
read_fd_set = active_fd_set;
if (select (FD_SETSIZE, &read_fd_set, NULL, NULL, NULL) < 0)
{
perror ("select");
exit (EXIT_FAILURE);
}
/* Service all the sockets with input pending. */
for (i = 0; i < FD_SETSIZE; ++i)
if (FD_ISSET (i, &read_fd_set))
{
if (i == sock)
{
/* Connection request on original socket. */
int new;
size = sizeof (clientname);
new = accept (sock,
(struct sockaddr *) &clientname,
&size);
if (new < 0)
{
perror ("accept");
exit (EXIT_FAILURE);
}
fprintf (stderr,
"Server: connect from host %s, port %hd.\n",
inet_ntoa (clientname.sin_addr),
ntohs (clientname.sin_port));
FD_SET (new, &active_fd_set);
}
else
{
/* Data arriving on an already-connected socket. */
if (read_from_client (i) < 0)
{
close (i);
FD_CLR (i, &active_fd_set);
}
}
}
}
}
Streams with connections permit out-of-band data that is
delivered with higher priority than ordinary data. Typically the
reason for sending out-of-band data is to send notice of an
exceptional condition. To send out-of-band data use
send, specifying the flag MSG_OOB (see section Sending Data).
Out-of-band data are received with higher priority because the
receiving process need not read it in sequence; to read the next
available out-of-band data, use recv with the MSG_OOB
flag (see section Receiving Data). Ordinary read operations do not read
out-of-band data; they read only ordinary data.
When a socket finds that out-of-band data are on their way, it sends a
SIGURG signal to the owner process or process group of the
socket. You can specify the owner using the F_SETOWN command
to the fcntl function; see section Interrupt-Driven Input. You must
also establish a handler for this signal, as described in section Signal Handling, in order to take appropriate action such as reading the
out-of-band data.
Alternatively, you can test for pending out-of-band data, or wait
until there is out-of-band data, using the select function; it
can wait for an exceptional condition on the socket. See section Waiting for Input or Output, for more information about select.
Notification of out-of-band data (whether with SIGURG or with
select) indicates that out-of-band data are on the way; the data
may not actually arrive until later. If you try to read the
out-of-band data before it arrives, recv fails with an
EWOULDBLOCK error.
Sending out-of-band data automatically places a "mark" in the stream of ordinary data, showing where in the sequence the out-of-band data "would have been". This is useful when the meaning of out-of-band data is "cancel everything sent so far". Here is how you can test, in the receiving process, whether any ordinary data was sent before the mark:
success = ioctl (socket, SIOCATMARK, &atmark);
The integer variable atmark is set to a nonzero value if
the socket's read pointer has reached the "mark".
Here's a function to discard any ordinary data preceding the out-of-band mark:
int
discard_until_mark (int socket)
{
while (1)
{
/* This is not an arbitrary limit; any size will do. */
char buffer[1024];
int atmark, success;
/* If we have reached the mark, return. */
success = ioctl (socket, SIOCATMARK, &atmark);
if (success < 0)
perror ("ioctl");
if (result)
return;
/* Otherwise, read a bunch of ordinary data and discard it.
This is guaranteed not to read past the mark
if it starts before the mark. */
success = read (socket, buffer, sizeof buffer);
if (success < 0)
perror ("read");
}
}
If you don't want to discard the ordinary data preceding the mark, you
may need to read some of it anyway, to make room in internal system
buffers for the out-of-band data. If you try to read out-of-band data
and get an EWOULDBLOCK error, try reading some ordinary data
(saving it so that you can use it when you want it) and see if that
makes room. Here is an example:
struct buffer
{
char *buf;
int size;
struct buffer *next;
};
/* Read the out-of-band data from SOCKET and return it
as a `struct buffer', which records the address of the data
and its size.
It may be necessary to read some ordinary data
in order to make room for the out-of-band data.
If so, the ordinary data are saved as a chain of buffers
found in the `next' field of the value. */
struct buffer *
read_oob (int socket)
{
struct buffer *tail = 0;
struct buffer *list = 0;
while (1)
{
/* This is an arbitrary limit.
Does anyone know how to do this without a limit? */
#define BUF_SZ 1024
char *buf = (char *) xmalloc (BUF_SZ);
int success;
int atmark;
/* Try again to read the out-of-band data. */
success = recv (socket, buf, BUF_SZ, MSG_OOB);
if (success >= 0)
{
/* We got it, so return it. */
struct buffer *link
= (struct buffer *) xmalloc (sizeof (struct buffer));
link->buf = buf;
link->size = success;
link->next = list;
return link;
}
/* If we fail, see if we are at the mark. */
success = ioctl (socket, SIOCATMARK, &atmark);
if (success < 0)
perror ("ioctl");
if (atmark)
{
/* At the mark; skipping past more ordinary data cannot help.
So just wait a while. */
sleep (1);
continue;
}
/* Otherwise, read a bunch of ordinary data and save it.
This is guaranteed not to read past the mark
if it starts before the mark. */
success = read (socket, buf, BUF_SZ);
if (success < 0)
perror ("read");
/* Save this data in the buffer list. */
{
struct buffer *link
= (struct buffer *) xmalloc (sizeof (struct buffer));
link->buf = buf;
link->size = success;
/* Add the new link to the end of the list. */
if (tail)
tail->next = link;
else
list = link;
tail = link;
}
}
}
This section describes how to use communication styles that don't use
connections (styles SOCK_DGRAM and SOCK_RDM). Using
these styles, you group data into packets and each packet is an
independent communication. You specify the destination for each
packet individually.
Datagram packets are like letters: you send each one independently with its own destination address, and they may arrive in the wrong order or not at all.
The listen and accept functions are not allowed for
sockets using connectionless communication styles.
The normal way of sending data on a datagram socket is by using the
sendto function, declared in `sys/socket.h'.
You can call connect on a datagram socket, but this only
specifies a default destination for further data transmission on the
socket. When a socket has a default destination you can use
send (see section Sending Data) or even write (see section Input and Output Primitives) to send a packet there. You can cancel the default
destination by calling connect using an address format of
AF_UNSPEC in the addr argument. See section Making a Connection, for
more information about the connect function.
sendto function transmits the data in the buffer
through the socket socket to the destination address specified
by the addr and length arguments. The size argument
specifies the number of bytes to be transmitted.
The flags are interpreted the same way as for send; see
section Socket Data Options.
The return value and error conditions are also the same as for
send, but you cannot rely on the system to detect errors and
report them; the most common error is that the packet is lost or there
is no-one at the specified address to receive it, and the operating
system on your machine usually does not know this.
It is also possible for one call to sendto to report an error
owing to a problem related to a previous call.
This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.
The recvfrom function reads a packet from a datagram socket and
also tells you where it was sent from. This function is declared in
`sys/socket.h'.
recvfrom function reads one packet from the socket
socket into the buffer buffer. The size argument
specifies the maximum number of bytes to be read.
If the packet is longer than size bytes, then you get the first size bytes of the packet and the rest of the packet is lost. There's no way to read the rest of the packet. Thus, when you use a packet protocol, you must always know how long a packet to expect.
The addr and length-ptr arguments are used to return the address where the packet came from. See section Socket Addresses. For a socket in the local domain the address information won't be meaningful, since you can't read the address of such a socket (see section The Local Namespace). You can specify a null pointer as the addr argument if you are not interested in this information.
The flags are interpreted the same way as for recv
(see section Socket Data Options). The return value and error conditions
are also the same as for recv.
This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.
You can use plain recv (see section Receiving Data) instead of
recvfrom if you don't need to find out who sent the packet
(either because you know where it should come from or because you
treat all possible senders alike). Even read can be used if
you don't want to specify flags (see section Input and Output Primitives).
Here is a set of example programs that send messages over a datagram
stream in the local namespace. Both the client and server programs use
the make_named_socket function that was presented in section Example of Local-Namespace Sockets, to create and name their sockets.
First, here is the server program. It sits in a loop waiting for messages to arrive, bouncing each message back to the sender. Obviously this isn't a particularly useful program, but it does show the general ideas involved.
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>
#define SERVER "/tmp/serversocket"
#define MAXMSG 512
int
main (void)
{
int sock;
char message[MAXMSG];
struct sockaddr_un name;
size_t size;
int nbytes;
/* Remove the filename first, it's ok if the call fails */
unlink (SERVER);
/* Make the socket, then loop endlessly. */
sock = make_named_socket (SERVER);
while (1)
{
/* Wait for a datagram. */
size = sizeof (name);
nbytes = recvfrom (sock, message, MAXMSG, 0,
(struct sockaddr *) & name, &size);
if (nbytes < 0)
{
perror ("recfrom (server)");
exit (EXIT_FAILURE);
}
/* Give a diagnostic message. */
fprintf (stderr, "Server: got message: %s\n", message);
/* Bounce the message back to the sender. */
nbytes = sendto (sock, message, nbytes, 0,
(struct sockaddr *) & name, size);
if (nbytes < 0)
{
perror ("sendto (server)");
exit (EXIT_FAILURE);
}
}
}
Here is the client program corresponding to the server above.
It sends a datagram to the server and then waits for a reply. Notice that the socket for the client (as well as for the server) in this example has to be given a name. This is so that the server can direct a message back to the client. Since the socket has no associated connection state, the only way the server can do this is by referencing the name of the client.
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>
#define SERVER "/tmp/serversocket"
#define CLIENT "/tmp/mysocket"
#define MAXMSG 512
#define MESSAGE "Yow!!! Are we having fun yet?!?"
int
main (void)
{
extern int make_named_socket (const char *name);
int sock;
char message[MAXMSG];
struct sockaddr_un name;
size_t size;
int nbytes;
/* Make the socket. */
sock = make_named_socket (CLIENT);
/* Initialize the server socket address. */
name.sun_family = AF_LOCAL;
strcpy (name.sun_path, SERVER);
size = strlen (name.sun_path) + sizeof (name.sun_family);
/* Send the datagram. */
nbytes = sendto (sock, MESSAGE, strlen (MESSAGE) + 1, 0,
(struct sockaddr *) & name, size);
if (nbytes < 0)
{
perror ("sendto (client)");
exit (EXIT_FAILURE);
}
/* Wait for a reply. */
nbytes = recvfrom (sock, message, MAXMSG, 0, NULL, 0);
if (nbytes < 0)
{
perror ("recfrom (client)");
exit (EXIT_FAILURE);
}
/* Print a diagnostic message. */
fprintf (stderr, "Client: got message: %s\n", message);
/* Clean up. */
remove (CLIENT);
close (sock);
}
Keep in mind that datagram socket communications are unreliable. In
this example, the client program waits indefinitely if the message
never reaches the server or if the server's response never comes
back. It's up to the user running the program to kill and restart
it if desired. A more automatic solution could be to use
select (see section Waiting for Input or Output) to establish a timeout period
for the reply, and in case of timeout either re-send the message or
shut down the socket and exit.
inetd DaemonWe've explained above how to write a server program that does its own listening. Such a server must already be running in order for anyone to connect to it.
Another way to provide a service on an Internet port is to let the daemon
program inetd do the listening. inetd is a program that
runs all the time and waits (using select) for messages on a
specified set of ports. When it receives a message, it accepts the
connection (if the socket style calls for connections) and then forks a
child process to run the corresponding server program. You specify the
ports and their programs in the file `/etc/inetd.conf'.
inetd Servers
Writing a server program to be run by inetd is very simple. Each time
someone requests a connection to the appropriate port, a new server
process starts. The connection already exists at this time; the
socket is available as the standard input descriptor and as the
standard output descriptor (descriptors 0 and 1) in the server
process. Thus the server program can begin reading and writing data
right away. Often the program needs only the ordinary I/O facilities;
in fact, a general-purpose filter program that knows nothing about
sockets can work as a byte stream server run by inetd.
You can also use inetd for servers that use connectionless
communication styles. For these servers, inetd does not try to accept
a connection since no connection is possible. It just starts the
server program, which can read the incoming datagram packet from
descriptor 0. The server program can handle one request and then
exit, or you can choose to write it to keep reading more requests
until no more arrive, and then exit. You must specify which of these
two techniques the server uses when you configure inetd.
inetd
The file `/etc/inetd.conf' tells inetd which ports to listen to
and what server programs to run for them. Normally each entry in the
file is one line, but you can split it onto multiple lines provided
all but the first line of the entry start with whitespace. Lines that
start with `#' are comments.
Here are two standard entries in `/etc/inetd.conf':
ftp stream tcp nowait root /libexec/ftpd ftpd talk dgram udp wait root /libexec/talkd talkd
An entry has this format:
service style protocol wait username program arguments
The service field says which service this program provides. It
should be the name of a service defined in `/etc/services'.
inetd uses service to decide which port to listen on for
this entry.
The fields style and protocol specify the communication style and the protocol to use for the listening socket. The style should be the name of a communication style, converted to lower case and with `SOCK_' deleted--for example, `stream' or `dgram'. protocol should be one of the protocols listed in `/etc/protocols'. The typical protocol names are `tcp' for byte stream connections and `udp' for unreliable datagrams.
The wait field should be either `wait' or `nowait'.
Use `wait' if style is a connectionless style and the
server, once started, handles multiple requests as they come in.
Use `nowait' if inetd should start a new process for each message
or request that comes in. If style uses connections, then
wait must be `nowait'.
user is the user name that the server should run as. inetd runs
as root, so it can set the user ID of its children arbitrarily. It's
best to avoid using `root' for user if you can; but some
servers, such as Telnet and FTP, read a username and password
themselves. These servers need to be root initially so they can log
in as commanded by the data coming over the network.
program together with arguments specifies the command to run to start the server. program should be an absolute file name specifying the executable file to run. arguments consists of any number of whitespace-separated words, which become the command-line arguments of program. The first word in arguments is argument zero, which should by convention be the program name itself (sans directories).
If you edit `/etc/inetd.conf', you can tell inetd to reread the
file and obey its new contents by sending the inetd process the
SIGHUP signal. You'll have to use ps to determine the
process ID of the inetd process as it is not fixed.
This section describes how to read or set various options that modify the behavior of sockets and their underlying communications protocols.
When you are manipulating a socket option, you must specify which level the option pertains to. This describes whether the option applies to the socket interface, or to a lower-level communications protocol interface.
Here are the functions for examining and modifying socket options. They are declared in `sys/socket.h'.
getsockopt function gets information about the value of
option optname at level level for socket socket.
The option value is stored in a buffer that optval points to.
Before the call, you should supply in *optlen-ptr the
size of this buffer; on return, it contains the number of bytes of
information actually stored in the buffer.
Most options interpret the optval buffer as a single int
value.
The actual return value of getsockopt is 0 on success
and -1 on failure. The following errno error conditions
are defined:
EBADF
ENOTSOCK
ENOPROTOOPT
@hfuzz 6pt
The return value and error codes for setsockopt are the same as
for getsockopt.
getsockopt or
setsockopt to manipulate the socket-level options described in
this section.
Here is a table of socket-level option names; all are defined in the header file `sys/socket.h'.
SO_DEBUG
int; a nonzero value means
"yes".
SO_REUSEADDR
bind (see section Setting the Address of a Socket)
should permit reuse of local addresses for this socket. If you enable
this option, you can actually have two sockets with the same Internet
port number; but the system won't allow you to use the two
identically-named sockets in a way that would confuse the Internet. The
reason for this option is that some higher-level Internet protocols,
including FTP, require you to keep reusing the same port number.
The value has type int; a nonzero value means "yes".
SO_KEEPALIVE
int; a nonzero value means
"yes".
SO_DONTROUTE
int; a nonzero
value means "yes".
SO_LINGER
struct linger.
SO_BROADCAST
int; a nonzero value means "yes".
SO_OOBINLINE
read or recv without specifying the MSG_OOB
flag. See section Out-of-Band Data. The value has type int; a
nonzero value means "yes".
SO_SNDBUF
size_t, which is the size in bytes.
SO_RCVBUF
size_t, which is the size in bytes.
SO_STYLE
SO_TYPE
getsockopt only. It is used to
get the socket's communication style. SO_TYPE is the
historical name, and SO_STYLE is the preferred name in GNU.
The value has type int and its value designates a communication
style; see section Communication Styles.
SO_ERROR
getsockopt only. It is used to reset
the error status of the socket. The value is an int, which represents
the previous error status.
Many systems come with a database that records a list of networks known
to the system developer. This is usually kept either in the file
`/etc/networks' or in an equivalent from a name server. This data
base is useful for routing programs such as route, but it is not
useful for programs that simply communicate over the network. We
provide functions to access this database, which are declared in
`netdb.h'.
char *n_name
char **n_aliases
int n_addrtype
AF_INET for Internet networks.
unsigned long int n_net
Use the getnetbyname or getnetbyaddr functions to search
the networks database for information about a specific network. The
information is returned in a statically-allocated structure; you must
copy the information if you need to save it.
getnetbyname function returns information about the network
named name. It returns a null pointer if there is no such
network.
getnetbyaddr function returns information about the network
of type type with number net. You should specify a value of
AF_INET for the type argument for Internet networks.
getnetbyaddr returns a null pointer if there is no such
network.
You can also scan the networks database using setnetent,
getnetent and endnetent. Be careful when using these
functions because they are not reentrant.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to getnetbyname or getnetbyaddr will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
This chapter describes functions that are specific to terminal devices. You can use these functions to do things like turn off input echoing; set serial line characteristics such as line speed and flow control; and change which characters are used for end-of-file, command-line editing, sending signals, and similar control functions.
Most of the functions in this chapter operate on file descriptors. See section Low-Level Input/Output, for more information about what a file descriptor is and how to open a file descriptor for a terminal device.
The functions described in this chapter only work on files that
correspond to terminal devices. You can find out whether a file
descriptor is associated with a terminal by using the isatty
function.
Prototypes for the functions in this section are declared in the header file `unistd.h'.
1 if filedes is a file descriptor
associated with an open terminal device, and @math{0} otherwise.
If a file descriptor is associated with a terminal, you can get its
associated file name using the ttyname function. See also the
ctermid function, described in section Identifying the Controlling Terminal.
ttyname function returns a pointer to a
statically-allocated, null-terminated string containing the file name of
the terminal file. The value is a null pointer if the file descriptor
isn't associated with a terminal, or the file name cannot be determined.
ttyname_r function is similar to the ttyname function
except that it places its result into the user-specified buffer starting
at buf with length len.
The normal return value from ttyname_r is @math{0}. Otherwise an
error number is returned to indicate the error. The following
errno error conditions are defined for this function:
EBADF
ENOTTY
ERANGE
Many of the remaining functions in this section refer to the input and output queues of a terminal device. These queues implement a form of buffering within the kernel independent of the buffering implemented by I/O streams (see section Input/Output on Streams).
The terminal input queue is also sometimes referred to as its typeahead buffer. It holds the characters that have been received from the terminal but not yet read by any process.
The size of the input queue is described by the MAX_INPUT and
_POSIX_MAX_INPUT parameters; see section Limits on File System Capacity. You
are guaranteed a queue size of at least MAX_INPUT, but the queue
might be larger, and might even dynamically change size. If input flow
control is enabled by setting the IXOFF input mode bit
(see section Input Modes), the terminal driver transmits STOP and START
characters to the terminal when necessary to prevent the queue from
overflowing. Otherwise, input may be lost if it comes in too fast from
the terminal. In canonical mode, all input stays in the queue until a
newline character is received, so the terminal input queue can fill up
when you type a very long line. See section Two Styles of Input: Canonical or Not.
The terminal output queue is like the input queue, but for output;
it contains characters that have been written by processes, but not yet
transmitted to the terminal. If output flow control is enabled by
setting the IXON input mode bit (see section Input Modes), the
terminal driver obeys START and STOP characters sent by the terminal to
stop and restart transmission of output.
Clearing the terminal input queue means discarding any characters that have been received but not yet read. Similarly, clearing the terminal output queue means discarding any characters that have been written but not yet transmitted.
POSIX systems support two basic modes of input: canonical and noncanonical.
In canonical input processing mode, terminal input is processed in
lines terminated by newline ('\n'), EOF, or EOL characters. No
input can be read until an entire line has been typed by the user, and
the read function (see section Input and Output Primitives) returns at most a
single line of input, no matter how many bytes are requested.
In canonical input mode, the operating system provides input editing facilities: some characters are interpreted specially to perform editing operations within the current line of text, such as ERASE and KILL. See section Characters for Input Editing.
The constants _POSIX_MAX_CANON and MAX_CANON parameterize
the maximum number of bytes which may appear in a single line of
canonical input. See section Limits on File System Capacity. You are guaranteed a maximum
line length of at least MAX_CANON bytes, but the maximum might be
larger, and might even dynamically change size.
In noncanonical input processing mode, characters are not grouped into lines, and ERASE and KILL processing is not performed. The granularity with which bytes are read in noncanonical input mode is controlled by the MIN and TIME settings. See section Noncanonical Input.
Most programs use canonical input mode, because this gives the user a way to edit input line by line. The usual reason to use noncanonical mode is when the program accepts single-character commands or provides its own editing facilities.
The choice of canonical or noncanonical input is controlled by the
ICANON flag in the c_lflag member of struct termios.
See section Local Modes.
This section describes the various terminal attributes that control how input and output are done. The functions, data structures, and symbolic constants are all declared in the header file `termios.h'.
Don't confuse terminal attributes with file attributes. A device special file which is associated with a terminal has file attributes as described in section File Attributes. These are unrelated to the attributes of the terminal device itself, which are discussed in this section.
The entire collection of attributes of a terminal is stored in a
structure of type struct termios. This structure is used
with the functions tcgetattr and tcsetattr to read
and set the attributes.
tcflag_t c_iflag
tcflag_t c_oflag
tcflag_t c_cflag
tcflag_t c_lflag
cc_t c_cc[NCCS]
The struct termios structure also contains members which
encode input and output transmission speeds, but the representation is
not specified. See section Line Speed, for how to examine and store the
speed values.
The following sections describe the details of the members of the
struct termios structure.
If successful, tcgetattr returns @math{0}. A return value of @math{-1}
indicates an error. The following errno error conditions are
defined for this function:
EBADF
ENOTTY
The when argument specifies how to deal with input and output already queued. It can be one of the following values:
TCSANOW
TCSADRAIN
TCSAFLUSH
TCSADRAIN, but also discards any queued input.
TCSASOFT
TCSASOFT is exactly the same as setting the CIGNORE
bit in the c_cflag member of the structure termios-p points
to. See section Control Modes, for a description of CIGNORE.
If this function is called from a background process on its controlling
terminal, normally all processes in the process group are sent a
SIGTTOU signal, in the same way as if the process were trying to
write to the terminal. The exception is if the calling process itself
is ignoring or blocking SIGTTOU signals, in which case the
operation is performed and no signal is sent. See section Job Control.
If successful, tcsetattr returns @math{0}. A return value of
@math{-1} indicates an error. The following errno error
conditions are defined for this function:
EBADF
ENOTTY
EINVAL
when argument is not valid, or there is
something wrong with the data in the termios-p argument.
Although tcgetattr and tcsetattr specify the terminal
device with a file descriptor, the attributes are those of the terminal
device itself and not of the file descriptor. This means that the
effects of changing terminal attributes are persistent; if another
process opens the terminal file later on, it will see the changed
attributes even though it doesn't have anything to do with the open file
descriptor you originally specified in changing the attributes.
Similarly, if a single process has multiple or duplicated file descriptors for the same terminal device, changing the terminal attributes affects input and output to all of these file descriptors. This means, for example, that you can't open one file descriptor or stream to read from a terminal in the normal line-buffered, echoed mode; and simultaneously have another file descriptor for the same terminal that you use to read from it in single-character, non-echoed mode. Instead, you have to explicitly switch the terminal back and forth between the two modes.
When you set terminal modes, you should call tcgetattr first to
get the current modes of the particular terminal device, modify only
those modes that you are really interested in, and store the result with
tcsetattr.
It's a bad idea to simply initialize a struct termios structure
to a chosen set of attributes and pass it directly to tcsetattr.
Your program may be run years from now, on systems that support members
not documented in this manual. The way to avoid setting these members
to unreasonable values is to avoid changing them.
What's more, different terminal devices may require different mode settings in order to function properly. So you should avoid blindly copying attributes from one terminal device to another.
When a member contains a collection of independent flags, as the
c_iflag, c_oflag and c_cflag members do, even
setting the entire member is a bad idea, because particular operating
systems have their own flags. Instead, you should start with the
current value of the member and alter only the flags whose values matter
in your program, leaving any other flags unchanged.
Here is an example of how to set one flag (ISTRIP) in the
struct termios structure while properly preserving all the other
data in the structure:
int
set_istrip (int desc, int value)
{
struct termios settings;
int result;
result = tcgetattr (desc, &settings);
if (result < 0)
{
perror ("error in tcgetattr");
return 0;
}
settings.c_iflag &= ~ISTRIP;
if (value)
settings.c_iflag |= ISTRIP;
result = tcsetattr (desc, TCSANOW, &settings);
if (result < 0)
{
perror ("error in tcgetattr");
return;
}
return 1;
}
This section describes the terminal attribute flags that control fairly low-level aspects of input processing: handling of parity errors, break signals, flow control, and RET and LFD characters.
All of these flags are bits in the c_iflag member of the
struct termios structure. The member is an integer, and you
change flags using the operators &, | and ^. Don't
try to specify the entire value for c_iflag---instead, change
only specific flags and leave the rest untouched (see section Setting Terminal Modes Properly).
Parity checking on input processing is independent of whether parity
detection and generation on the underlying terminal hardware is enabled;
see section Control Modes. For example, you could clear the INPCK
input mode flag and set the PARENB control mode flag to ignore
parity errors on input, but still generate parity on output.
If this bit is set, what happens when a parity error is detected depends
on whether the IGNPAR or PARMRK bits are set. If neither
of these bits are set, a byte with a parity error is passed to the
application as a '\0' character.
INPCK is also set.
INPCK is set and IGNPAR is not set.
The way erroneous bytes are marked is with two preceding bytes,
377 and 0. Thus, the program actually reads three bytes
for one erroneous byte received from the terminal.
If a valid byte has the value 0377, and ISTRIP (see below)
is not set, the program might confuse it with the prefix that marks a
parity error. So a valid byte 0377 is passed to the program as
two bytes, 0377 0377, in this case.
A break condition is defined in the context of asynchronous serial data transmission as a series of zero-value bits longer than a single byte.
IGNBRK is not set, a break condition
clears the terminal input and output queues and raises a SIGINT
signal for the foreground process group associated with the terminal.
If neither BRKINT nor IGNBRK are set, a break condition is
passed to the application as a single '\0' character if
PARMRK is not set, or otherwise as a three-character sequence
'\377', '\0', '\0'.
'\r') are
discarded on input. Discarding carriage return may be useful on
terminals that send both carriage return and linefeed when you type the
RET key.
IGNCR is not set, carriage return characters
('\r') received as input are passed to the application as newline
characters ('\n').
'\n') received as input
are passed to the application as carriage return characters ('\r').
This is a BSD extension; it exists only on BSD systems and the GNU system.
007) to the terminal to ring the bell.
This is a BSD extension.
This section describes the terminal flags and fields that control how
output characters are translated and padded for display. All of these
are contained in the c_oflag member of the struct termios
structure.
The c_oflag member itself is an integer, and you change the flags
and fields using the operators &, |, and ^. Don't
try to specify the entire value for c_oflag---instead, change
only specific flags and leave the rest untouched (see section Setting Terminal Modes Properly).
'\n') onto
carriage return and linefeed pairs.
If this bit isn't set, the characters are transmitted as-is.
The following three bits are BSD features, and they exist only BSD
systems and the GNU system. They are effective only if OPOST is
set.
004) on
output. These characters cause many dial-up terminals to disconnect.
This section describes the terminal flags and fields that control
parameters usually associated with asynchronous serial data
transmission. These flags may not make sense for other kinds of
terminal ports (such as a network connection pseudo-terminal). All of
these are contained in the c_cflag member of the struct
termios structure.
The c_cflag member itself is an integer, and you change the flags
and fields using the operators &, |, and ^. Don't
try to specify the entire value for c_cflag---instead, change
only specific flags and leave the rest untouched (see section Setting Terminal Modes Properly).
On many systems if this bit is not set and you call open without
the O_NONBLOCK flag set, open blocks until a modem
connection is established.
If this bit is not set and a modem disconnect is detected, a
SIGHUP signal is sent to the controlling process group for the
terminal (if it has one). Normally, this causes the process to exit;
see section Signal Handling. Reading from the terminal after a disconnect
causes an end-of-file condition, and writing causes an EIO error
to be returned. The terminal device must be closed and reopened to
clear the condition.
If this bit is not set, no parity bit is added to output characters, and input characters are not checked for correct parity.
PARENB is set. If PARODD is set,
odd parity is used, otherwise even parity is used.
The control mode flags also includes a field for the number of bits per
character. You can use the CSIZE macro as a mask to extract the
value, like this: settings.c_cflag & CSIZE.
The following four bits are BSD extensions; this exist only on BSD systems and the GNU system.
tcsetattr.
The c_cflag member and the line speed values returned by
cfgetispeed and cfgetospeed will be unaffected by the
call. CIGNORE is useful if you want to set all the software
modes in the other members, but leave the hardware details in
c_cflag unchanged. (This is how the TCSASOFT flag to
tcsettattr works.)
This bit is never set in the structure filled in by tcgetattr.
This section describes the flags for the c_lflag member of the
struct termios structure. These flags generally control
higher-level aspects of input processing than the input modes flags
described in section Input Modes, such as echoing, signals, and the choice
of canonical or noncanonical input.
The c_lflag member itself is an integer, and you change the flags
and fields using the operators &, |, and ^. Don't
try to specify the entire value for c_lflag---instead, change
only specific flags and leave the rest untouched (see section Setting Terminal Modes Properly).
This bit only controls the display behavior; the ICANON bit by
itself controls actual recognition of the ERASE character and erasure of
input, without which ECHOE is simply irrelevant.
ECHOE, enables display of the ERASE character in
a way that is geared to a hardcopy terminal. When you type the ERASE
character, a `\' character is printed followed by the first
character erased. Typing the ERASE character again just prints the next
character erased. Then, the next time you type a normal character, a
`/' character is printed before the character echoes.
This is a BSD extension, and exists only in BSD systems and the GNU system.
ECHOKE (below) is nicer to look at.
If this bit is not set, the KILL character echoes just as it would if it were not the KILL character. Then it is up to the user to remember that the KILL character has erased the preceding input; there is no indication of this on the screen.
This bit only controls the display behavior; the ICANON bit by
itself controls actual recognition of the KILL character and erasure of
input, without which ECHOK is simply irrelevant.
ECHOK. It enables special display of the
KILL character by erasing on the screen the entire line that has been
killed. This is a BSD extension, and exists only in BSD systems and the
GNU system.
ICANON bit is also set, then the
newline ('\n') character is echoed even if the ECHO bit
is not set.
ECHO bit is also set, echo control
characters with `^' followed by the corresponding text character.
Thus, control-A echoes as `^A'. This is usually the preferred mode
for interactive input, because echoing a control character back to the
terminal could have some undesired effect on the terminal.
This is a BSD extension, and exists only in BSD systems and the GNU system.
You should use caution when disabling recognition of these characters. Programs that cannot be interrupted interactively are very user-unfriendly. If you clear this bit, your program should provide some alternate interface that allows the user to interactively send the signals associated with these characters, or to escape from the program.
See section Characters that Cause Signals.
IEXTEN implementation-defined meaning,
so you cannot rely on this interpretation on all systems.
On BSD systems and the GNU system, it enables the LNEXT and DISCARD characters. See section Other Special Characters.
SIGTTOU signals are generated by background processes that
attempt to write to the terminal. See section Access to the Controlling Terminal.
The following bits are BSD extensions; they exist only in BSD systems and the GNU system.
If this bit is clear, then the beginning of a word is a nonwhitespace character following a whitespace character. If the bit is set, then the beginning of a word is an alphanumeric character or underscore following a character which is none of those.
See section Characters for Input Editing, for more information about the WERASE character.
The terminal line speed tells the computer how fast to read and write data on the terminal.
If the terminal is connected to a real serial line, the terminal speed you specify actually controls the line--if it doesn't match the terminal's own idea of the speed, communication does not work. Real serial ports accept only certain standard speeds. Also, particular hardware may not support even all the standard speeds. Specifying a speed of zero hangs up a dialup connection and turns off modem control signals.
If the terminal is not a real serial line (for example, if it is a network connection), then the line speed won't really affect data transmission speed, but some programs will use it to determine the amount of padding needed. It's best to specify a line speed value that matches the actual speed of the actual terminal, but you can safely experiment with different values to vary the amount of padding.
There are actually two line speeds for each terminal, one for input and one for output. You can set them independently, but most often terminals use the same speed for both directions.
The speed values are stored in the struct termios structure, but
don't try to access them in the struct termios structure
directly. Instead, you should use the following functions to read and
store them:
*termios-p.
*termios-p.
*termios-p as the output
speed. The normal return value is @math{0}; a value of @math{-1}
indicates an error. If speed is not a speed, cfsetospeed
returns @math{-1}.
*termios-p as the input
speed. The normal return value is @math{0}; a value of @math{-1}
indicates an error. If speed is not a speed, cfsetospeed
returns @math{-1}.
*termios-p as both the
input and output speeds. The normal return value is @math{0}; a value
of @math{-1} indicates an error. If speed is not a speed,
cfsetspeed returns @math{-1}. This function is an extension in
4.4 BSD.
The functions cfsetospeed and cfsetispeed report errors
only for speed values that the system simply cannot handle. If you
specify a speed value that is basically acceptable, then those functions
will succeed. But they do not check that a particular hardware device
can actually support the specified speeds--in fact, they don't know
which device you plan to set the speed for. If you use tcsetattr
to set the speed of a particular device to a value that it cannot
handle, tcsetattr returns @math{-1}.
Portability note: In the GNU library, the functions above
accept speeds measured in bits per second as input, and return speed
values measured in bits per second. Other libraries require speeds to
be indicated by special codes. For POSIX.1 portability, you must use
one of the following symbols to represent the speed; their precise
numeric values are system-dependent, but each name has a fixed meaning:
B110 stands for 110 bps, B300 for 300 bps, and so on.
There is no portable way to represent any speed but these, but these are
the only speeds that typical serial lines can support.
B0 B50 B75 B110 B134 B150 B200 B300 B600 B1200 B1800 B2400 B4800 B9600 B19200 B38400 B57600 B115200 B230400 B460800
BSD defines two additional speed symbols as aliases: EXTA is an
alias for B19200 and EXTB is an alias for B38400.
These aliases are obsolete.
In canonical input, the terminal driver recognizes a number of special
characters which perform various control functions. These include the
ERASE character (usually DEL) for editing input, and other editing
characters. The INTR character (normally C-c) for sending a
SIGINT signal, and other signal-raising characters, may be
available in either canonical or noncanonical input mode. All these
characters are described in this section.
The particular characters used are specified in the c_cc member
of the struct termios structure. This member is an array; each
element specifies the character for a particular role. Each element has
a symbolic constant that stands for the index of that element--for
example, VINTR is the index of the element that specifies the INTR
character, so storing '=' in termios.c_cc[VINTR]
specifies `=' as the INTR character.
On some systems, you can disable a particular special character function
by specifying the value _POSIX_VDISABLE for that role. This
value is unequal to any possible character code. See section Optional Features in File Support, for more information about how to tell whether the operating
system you are using supports _POSIX_VDISABLE.
These special characters are active only in canonical input mode. See section Two Styles of Input: Canonical or Not.
termios.c_cc[VEOF] holds the character
itself.
The EOF character is recognized only in canonical input mode. It acts
as a line terminator in the same way as a newline character, but if the
EOF character is typed at the beginning of a line it causes read
to return a byte count of zero, indicating end-of-file. The EOF
character itself is discarded.
Usually, the EOF character is C-d.
termios.c_cc[VEOL] holds the character
itself.
The EOL character is recognized only in canonical input mode. It acts as a line terminator, just like a newline character. The EOL character is not discarded; it is read as the last character in the input line.
You don't need to use the EOL character to make RET end a line. Just set the ICRNL flag. In fact, this is the default state of affairs.
termios.c_cc[VEOL2] holds the character
itself.
The EOL2 character works just like the EOL character (see above), but it can be a different character. Thus, you can specify two characters to terminate an input line, by setting EOL to one of them and EOL2 to the other.
The EOL2 character is a BSD extension; it exists only on BSD systems and the GNU system.
termios.c_cc[VERASE] holds the
character itself.
The ERASE character is recognized only in canonical input mode. When the user types the erase character, the previous character typed is discarded. (If the terminal generates multibyte character sequences, this may cause more than one byte of input to be discarded.) This cannot be used to erase past the beginning of the current line of text. The ERASE character itself is discarded.
Usually, the ERASE character is DEL.
termios.c_cc[VWERASE] holds the character
itself.
The WERASE character is recognized only in canonical mode. It erases an entire word of prior input, and any whitespace after it; whitespace characters before the word are not erased.
The definition of a "word" depends on the setting of the
ALTWERASE mode; see section Local Modes.
If the ALTWERASE mode is not set, a word is defined as a sequence
of any characters except space or tab.
If the ALTWERASE mode is set, a word is defined as a sequence of
characters containing only letters, numbers, and underscores, optionally
followed by one character that is not a letter, number, or underscore.
The WERASE character is usually C-w.
This is a BSD extension.
termios.c_cc[VKILL] holds the character
itself.
The KILL character is recognized only in canonical input mode. When the user types the kill character, the entire contents of the current line of input are discarded. The kill character itself is discarded too.
The KILL character is usually C-u.
termios.c_cc[VREPRINT] holds the character
itself.
The REPRINT character is recognized only in canonical mode. It reprints the current input line. If some asynchronous output has come while you are typing, this lets you see the line you are typing clearly again.
The REPRINT character is usually C-r.
This is a BSD extension.
These special characters may be active in either canonical or noncanonical
input mode, but only when the ISIG flag is set (see section Local Modes).
termios.c_cc[VINTR] holds the character
itself.
The INTR (interrupt) character raises a SIGINT signal for all
processes in the foreground job associated with the terminal. The INTR
character itself is then discarded. See section Signal Handling, for more
information about signals.
Typically, the INTR character is C-c.
termios.c_cc[VQUIT] holds the character
itself.
The QUIT character raises a SIGQUIT signal for all processes in
the foreground job associated with the terminal. The QUIT character
itself is then discarded. See section Signal Handling, for more information
about signals.
Typically, the QUIT character is C-\.
termios.c_cc[VSUSP] holds the character
itself.
The SUSP (suspend) character is recognized only if the implementation
supports job control (see section Job Control). It causes a SIGTSTP
signal to be sent to all processes in the foreground job associated with
the terminal. The SUSP character itself is then discarded.
See section Signal Handling, for more information about signals.
Typically, the SUSP character is C-z.
Few applications disable the normal interpretation of the SUSP
character. If your program does this, it should provide some other
mechanism for the user to stop the job. When the user invokes this
mechanism, the program should send a SIGTSTP signal to the
process group of the process, not just to the process itself.
See section Signaling Another Process.
termios.c_cc[VDSUSP] holds the character
itself.
The DSUSP (suspend) character is recognized only if the implementation
supports job control (see section Job Control). It sends a SIGTSTP
signal, like the SUSP character, but not right away--only when the
program tries to read it as input. Not all systems with job control
support DSUSP; only BSD-compatible systems (including the GNU system).
See section Signal Handling, for more information about signals.
Typically, the DSUSP character is C-y.
These special characters may be active in either canonical or noncanonical
input mode, but their use is controlled by the flags IXON and
IXOFF (see section Input Modes).
termios.c_cc[VSTART] holds the
character itself.
The START character is used to support the IXON and IXOFF
input modes. If IXON is set, receiving a START character resumes
suspended output; the START character itself is discarded. If
IXANY is set, receiving any character at all resumes suspended
output; the resuming character is not discarded unless it is the START
character. IXOFF is set, the system may also transmit START
characters to the terminal.
The usual value for the START character is C-q. You may not be able to change this value--the hardware may insist on using C-q regardless of what you specify.
termios.c_cc[VSTOP] holds the character
itself.
The STOP character is used to support the IXON and IXOFF
input modes. If IXON is set, receiving a STOP character causes
output to be suspended; the STOP character itself is discarded. If
IXOFF is set, the system may also transmit STOP characters to the
terminal, to prevent the input queue from overflowing.
The usual value for the STOP character is C-s. You may not be able to change this value--the hardware may insist on using C-s regardless of what you specify.
These special characters exist only in BSD systems and the GNU system.
termios.c_cc[VLNEXT] holds the character
itself.
The LNEXT character is recognized only when IEXTEN is set, but in
both canonical and noncanonical mode. It disables any special
significance of the next character the user types. Even if the
character would normally perform some editing function or generate a
signal, it is read as a plain character. This is the analogue of the
C-q command in Emacs. "LNEXT" stands for "literal next."
The LNEXT character is usually C-v.
termios.c_cc[VDISCARD] holds the character
itself.
The DISCARD character is recognized only when IEXTEN is set, but
in both canonical and noncanonical mode. Its effect is to toggle the
discard-output flag. When this flag is set, all program output is
discarded. Setting the flag also discards all output currently in the
output buffer. Typing any other character resets the flag.
termios.c_cc[VSTATUS] holds the character
itself.
The STATUS character's effect is to print out a status message about how the current process is running.
The STATUS character is recognized only in canonical mode, and only if
NOKERNINFO is not set.
In noncanonical input mode, the special editing characters such as ERASE and KILL are ignored. The system facilities for the user to edit input are disabled in noncanonical mode, so that all input characters (unless they are special for signal or flow-control purposes) are passed to the application program exactly as typed. It is up to the application program to give the user ways to edit the input, if appropriate.
Noncanonical mode offers special parameters called MIN and TIME for controlling whether and how long to wait for input to be available. You can even use them to avoid ever waiting--to return immediately with whatever input is available, or with no input.
The MIN and TIME are stored in elements of the c_cc array, which
is a member of the struct termios structure. Each element of
this array has a particular role, and each element has a symbolic
constant that stands for the index of that element. VMIN and
VMAX are the names for the indices in the array of the MIN and
TIME slots.
c_cc array. Thus,
termios.c_cc[VMIN] is the value itself.
The MIN slot is only meaningful in noncanonical input mode; it
specifies the minimum number of bytes that must be available in the
input queue in order for read to return.
c_cc array. Thus,
termios.c_cc[VTIME] is the value itself.
The TIME slot is only meaningful in noncanonical input mode; it specifies how long to wait for input before returning, in units of 0.1 seconds.
The MIN and TIME values interact to determine the criterion for when
read should return; their precise meanings depend on which of
them are nonzero. There are four possible cases:
read keeps waiting until either MIN bytes have arrived in all, or
TIME elapses with no further input.
read always blocks until the first character arrives, even if
TIME elapses first. read can return more than MIN characters if
more than MIN happen to be in the queue.
read always returns immediately with as many
characters as are available in the queue, up to the number requested.
If no input is immediately available, read returns a value of
zero.
read waits for time TIME for input to become
available; the availability of a single byte is enough to satisfy the
read request and cause read to return. When it returns, it
returns as many characters as are available, up to the number requested.
If no input is available before the timer expires, read returns a
value of zero.
read waits until at least MIN bytes are available
in the queue. At that time, read returns as many characters as
are available, up to the number requested. read can return more
than MIN characters if more than MIN happen to be in the queue.
What happens if MIN is 50 and you ask to read just 10 bytes?
Normally, read waits until there are 50 bytes in the buffer (or,
more generally, the wait condition described above is satisfied), and
then reads 10 of them, leaving the other 40 buffered in the operating
system for a subsequent call to read.
Portability note: On some systems, the MIN and TIME slots are actually the same as the EOF and EOL slots. This causes no serious problem because the MIN and TIME slots are used only in noncanonical input and the EOF and EOL slots are used only in canonical input, but it isn't very clean. The GNU library allocates separate slots for these uses.
*termios-p for
what has traditionally been called "raw mode" in BSD. This uses
noncanonical input, and turns off most processing to give an unmodified
channel to the terminal.
It does exactly this:
termios-p->c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP
|INLCR|IGNCR|ICRNL|IXON);
termios-p->c_oflag &= ~OPOST;
termios-p->c_lflag &= ~(ECHO|ECHONL|ICANON|ISIG|IEXTEN);
termios-p->c_cflag &= ~(CSIZE|PARENB);
termios-p->c_cflag |= CS8;
The usual way to get and set terminal modes is with the functions described
in section Terminal Modes. However, on some systems you can use the
BSD-derived functions in this section to do some of the same thing. On
many systems, these functions do not exist. Even with the GNU C library,
the functions simply fail with errno = ENOSYS with many
kernels, including Linux.
The symbols used in this section are declared in `sgtty.h'.
gtty and
stty.
char sg_ispeed
char sg_ospeed
char sg_erase
char sg_kill
int sg_flags
gtty sets *attributes to describe the terminal attributes
of the terminal which is open with file descriptor filedes.
This function sets the attributes of a terminal.
stty sets the terminal attributes of the terminal which is open with
file descriptor filedes to those described by *filedes.
These functions perform miscellaneous control actions on terminal
devices. As regards terminal access, they are treated like doing
output: if any of these functions is used by a background process on its
controlling terminal, normally all processes in the process group are
sent a SIGTTOU signal. The exception is if the calling process
itself is ignoring or blocking SIGTTOU signals, in which case the
operation is performed and no signal is sent. See section Job Control.
This function does nothing if the terminal is not an asynchronous serial data port.
The return value is normally zero. In the event of an error, a value
of @math{-1} is returned. The following errno error conditions
are defined for this function:
EBADF
ENOTTY
tcdrain function waits until all queued
output to the terminal filedes has been transmitted.
This function is a cancelation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time tcdrain is
called. If the thread gets canceled these resources stay allocated
until the program ends. To avoid this calls to tcdrain should be
protected using cancelation handlers.
The return value is normally zero. In the event of an error, a value
of @math{-1} is returned. The following errno error conditions
are defined for this function:
EBADF
ENOTTY
EINTR
tcflush function is used to clear the input and/or output
queues associated with the terminal file filedes. The queue
argument specifies which queue(s) to clear, and can be one of the
following values:
TCIFLUSH
TCOFLUSH
TCIOFLUSH
The return value is normally zero. In the event of an error, a value
of @math{-1} is returned. The following errno error conditions
are defined for this function:
EBADF
ENOTTY
EINVAL
It is unfortunate that this function is named tcflush, because
the term "flush" is normally used for quite another operation--waiting
until all output is transmitted--and using it for discarding input or
output would be confusing. Unfortunately, the name tcflush comes
from POSIX and we cannot change it.
tcflow function is used to perform operations relating to
XON/XOFF flow control on the terminal file specified by filedes.
The action argument specifies what operation to perform, and can be one of the following values:
TCOOFF
TCOON
TCIOFF
TCION
For more information about the STOP and START characters, see section Special Characters.
The return value is normally zero. In the event of an error, a value
of @math{-1} is returned. The following errno error conditions
are defined for this function:
Here is an example program that shows how you can set up a terminal device to read single characters in noncanonical input mode, without echo.
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
/* Use this variable to remember original terminal attributes. */
struct termios saved_attributes;
void
reset_input_mode (void)
{
tcsetattr (STDIN_FILENO, TCSANOW, &saved_attributes);
}
void
set_input_mode (void)
{
struct termios tattr;
char *name;
/* Make sure stdin is a terminal. */
if (!isatty (STDIN_FILENO))
{
fprintf (stderr, "Not a terminal.\n");
exit (EXIT_FAILURE);
}
/* Save the terminal attributes so we can restore them later. */
tcgetattr (STDIN_FILENO, &saved_attributes);
atexit (reset_input_mode);
/* Set the funny terminal modes. */
tcgetattr (STDIN_FILENO, &tattr);
tattr.c_lflag &= ~(ICANON|ECHO); /* Clear ICANON and ECHO. */
tattr.c_cc[VMIN] = 1;
tattr.c_cc[VTIME] = 0;
tcsetattr (STDIN_FILENO, TCSAFLUSH, &tattr);
}
int
main (void)
{
char c;
set_input_mode ();
while (1)
{
read (STDIN_FILENO, &c, 1);
if (c == '\004') /* C-d */
break;
else
putchar (c);
}
return EXIT_SUCCESS;
}
This program is careful to restore the original terminal modes before
exiting or terminating with a signal. It uses the atexit
function (see section Cleanups on Exit) to make sure this is done
by exit.
The shell is supposed to take care of resetting the terminal modes when a process is stopped or continued; see section Job Control. But some existing shells do not actually do this, so you may wish to establish handlers for job control signals that reset terminal modes. The above example does so.
A pseudo-terminal is a special interprocess communication channel that acts like a terminal. One end of the channel is called the master side or master pseudo-terminal device, the other side is called the slave side. Data written to the master side is received by the slave side as if it was the result of a user typing at an ordinary terminal, and data written to the slave side is sent to the master side as if it was written on an ordinary terminal.
Pseudo terminals are the way programs like xterm and emacs
implement their terminal emulation functionality.
This subsection describes functions for allocating a pseudo-terminal, and for making this pseudo-terminal available for actual use. These functions are declared in the header file `stdlib.h'.
getpt function returns a new file descriptor for the next
available master pseudo-terminal. The normal return value from
getpt is a non-negative integer file descriptor. In the case of
an error, a value of @math{-1} is returned instead. The following
errno conditions are defined for this function:
ENOENT
This function is a GNU extension.
grantpt function changes the ownership and access permission
of the slave pseudo-terminal device corresponding to the master
pseudo-terminal device associated with the file descriptor
filedes. The owner is set from the real user ID of the calling
process (see section The Persona of a Process), and the group is set to a special
group (typically tty) or from the real group ID of the calling
process. The access permission is set such that the file is both
readable and writable by the owner and only writable by the group.
On some systems this function is implemented by invoking a special
setuid root program (see section How an Application Can Change Persona). As a
consequence, installing a signal handler for the SIGCHLD signal
(see section Job Control Signals) may interfere with a call to
grantpt.
The normal return value from grantpt is @math{0}; a value of
@math{-1} is returned in case of failure. The following errno
error conditions are defined for this function:
EBADF
ENINVAL
EACCESS
unlockpt function unlocks the slave pseudo-terminal device
corresponding to the master pseudo-terminal device associated with the
file descriptor filedes. On many systems, the slave can only be
opened after unlocking, so portable applications should always call
unlockpt before trying to open the slave.
The normal return value from unlockpt is @math{0}; a value of
@math{-1} is returned in case of failure. The following errno
error conditions are defined for this function:
EBADF
EINVAL
ptsname function returns a
pointer to a statically-allocated, null-terminated string containing the
file name of the associated slave pseudo-terminal file. This string
might be overwritten by subsequent calls to ptsname.
ptsname_r function is similar to the ptsname function
except that it places its result into the user-specified buffer starting
at buf with length len.
This function is a GNU extension.
Portability Note: On System V derived systems, the file
returned by the ptsname and ptsname_r functions may be
STREAMS-based, and therefore require additional processing after opening
before it actually behaves as a pseudo terminal.
Typical usage of these functions is illustrated by the following example:
int
open_pty_pair (int *amaster, int *aslave)
{
int master, slave;
char *name;
master = getpt ();
if (master < 0)
return 0;
if (grantpt (master) < 0 || unlockpt (master) < 0)
goto close_master;
name = ptsname (master);
if (name == NULL)
goto close_master;
slave = open (name, O_RDWR);
if (slave == -1)
goto close_master;
if (isastream (slave))
{
if (ioctl (slave, I_PUSH, "ptem") < 0
|| ioctl (slave, I_PUSH, "ldterm") < 0)
goto close_slave;
}
*amaster = master;
*aslave = slave;
return 1;
close_slave:
close (slave);
close_master:
close (master);
return 0;
}
These functions, derived from BSD, are available in the separate `libutil' library, and declared in `pty.h'.
*name. If termp is not a null pointer,
the terminal attributes of the slave are set to the ones specified in
the structure that termp points to (see section Terminal Modes).
Likewise, if the winp is not a null pointer, the screen size of
the slave is set to the values specified in the structure that
winp points to.
The normal return value from openpty is @math{0}; a value of
@math{-1} is returned in case of failure. The following errno
conditions are defined for this function:
ENOENT
Warning: Using the openpty function with name not
set to NULL is very dangerous because it provides no
protection against overflowing the string name. You should use
the ttyname function on the file descriptor returned in
*slave to find out the file name of the slave pseudo-terminal
device instead.
openpty function, but in
addition, forks a new process (see section Creating a Process) and makes the
newly opened slave pseudo-terminal device the controlling terminal
(see section Controlling Terminal of a Process) for the child process.
If the operation is successful, there are then both parent and child
processes and both see forkpty return, but with different values:
it returns a value of @math{0} in the child process and returns the child's
process ID in the parent process.
If the allocation of a pseudo-terminal pair or the process creation
failed, forkpty returns a value of @math{-1} in the parent
process.
Warning: The forkpty function has the same problems with
respect to the name argument as openpty.
This chapter describes facilities for issuing and logging messages of system administration interest. This chapter has nothing to do with programs issuing messages to their own users or keeping private logs (One would typically do that with the facilities described in section Input/Output on Streams).
Most systems have a facility called "Syslog" that allows programs to submit messages of interest to system administrators and can be configured to pass these messages on in various ways, such as printing on the console, mailing to a particular person, or recording in a log file for future reference.
A program uses the facilities in this chapter to submit such messages.
System administrators have to deal with lots of different kinds of messages from a plethora of subsystems within each system, and usually lots of systems as well. For example, an FTP server might report every connection it gets. The kernel might report hardware failures on a disk drive. A DNS server might report usage statistics at regular intervals.
Some of these messages need to be brought to a system administrator's attention immediately. And it may not be just any system administrator -- there may be a particular system administrator who deals with a particular kind of message. Other messages just need to be recorded for future reference if there is a problem. Still others may need to have information extracted from them by an automated process that generates monthly reports.
To deal with these messages, most Unix systems have a facility called "Syslog." It is generally based on a daemon called "Syslogd" Syslogd listens for messages on a Unix domain socket named `/dev/log'. Based on classification information in the messages and its configuration file (usually `/etc/syslog.conf'), Syslogd routes them in various ways. Some of the popular routings are:
Syslogd can also handle messages from other systems. It listens on the
syslog UDP port as well as the local socket for messages.
Syslog can handle messages from the kernel itself. But the kernel doesn't write to `/dev/log'; rather, another daemon (sometimes called "Klogd") extracts messages from the kernel and passes them on to Syslog as any other process would (and it properly identifies them as messages from the kernel).
Syslog can even handle messages that the kernel issued before Syslogd or Klogd was running. A Linux kernel, for example, stores startup messages in a kernel message ring and they are normally still there when Klogd later starts up. Assuming Syslogd is running by the time Klogd starts, Klogd then passes everything in the message ring to it.
In order to classify messages for disposition, Syslog requires any process that submits a message to it to provide two pieces of classification information with it:
A "facility/priority" is a number that indicates both the facility and the priority.
Warning: This terminology is not universal. Some people use "level" to refer to the priority and "priority" to refer to the combination of facility and priority. A Linux kernel has a concept of a message "level," which corresponds both to a Syslog priority and to a Syslog facility/priority (It can be both because the facility code for the kernel is zero, and that makes priority and facility/priority the same value).
The GNU C library provides functions to submit messages to Syslog. They do it by writing to the `/dev/log' socket. See section Submitting Syslog Messages.
The GNU C library functions only work to submit messages to the Syslog
facility on the same system. To submit a message to the Syslog facility
on another system, use the socket I/O functions to write a UDP datagram
to the syslog UDP port on that system. See section Sockets.
The GNU C library provides functions to submit messages to the Syslog facility:
These functions only work to submit messages to the Syslog facility on
the same system. To submit a message to the Syslog facility on another
system, use the socket I/O functions to write a UDP datagram to the
syslog UDP port on that system. See section Sockets.
The symbols referred to in this section are declared in the file `syslog.h'.
openlog opens or reopens a connection to Syslog in preparation
for submitting messages.
ident is an arbitrary identification string which future
syslog invocations will prefix to each message. This is intended
to identify the source of the message, and people conventionally set it
to the name of the program that will submit the messages.
openlog may or may not open the `/dev/log' socket, depending
on option. If it does, it tries to open it and connect it as a
stream socket. If that doesn't work, it tries to open it and connect it
as a datagram socket. The socket has the "Close on Exec" attribute,
so the kernel will close it if the process performs an exec.
You don't have to use openlog. If you call syslog without
having called openlog, syslog just opens the connection
implicitly and uses defaults for the information in ident and
options.
options is a bit string, with the bits as defined by the following single bit masks:
LOG_PERROR
openlog sets up the connection so that any syslog
on this connection writes its message to the calling process' Standard
Error stream in addition to submitting it to Syslog. If off, syslog
does not write the message to Standard Error.
LOG_CONS
openlog sets up the connection so that a syslog on
this connection that fails to submit a message to Syslog writes the
message instead to system console. If off, syslog does not write
to the system console (but of course Syslog may write messages it
receives to the console).
LOG_PID
openlog sets up the connection so that a syslog
on this connection inserts the calling process' Process ID (PID) into
the message. When off, openlog does not insert the PID.
LOG_NDELAY
openlog opens and connects the `/dev/log' socket.
When off, a future syslog call must open and connect the socket.
Portability note: In early systems, the sense of this bit was
exactly the opposite.
LOG_ODELAY
If any other bit in options is on, the result is undefined.
facility is the default facility code for this connection. A
syslog on this connection that specifies default facility causes
this facility to be associated with the message. See syslog for
possible values. A value of zero means the default default, which is
LOG_USER.
If a Syslog connection is already open when you call openlog,
openlog "reopens" the connection. Reopening is like opening
except that if you specify zero for the default facility code, the
default facility code simply remains unchanged and if you specify
LOG_NDELAY and the socket is already open and connected, openlog
just leaves it that way.
The symbols referred to in this section are declared in the file `syslog.h'.
syslog submits a message to the Syslog facility. It does this by
writing to the Unix domain socket /dev/log.
syslog submits the message with the facility and priority indicated
by facility_priority. The macro LOG_MAKEPRI generates a
facility/priority from a facility and a priority, as in the following
example:
LOG_MAKEPRI(LOG_USER, LOG_WARNING)
The possible values for the facility code are (macros):
LOG_USER
LOG_MAIL
LOG_DAEMON
LOG_AUTH
LOG_SYSLOG
LOG_LPR
LOG_NEWS
LOG_UUCP
LOG_CRON
LOG_AUTHPRIV
LOG_FTP
LOG_LOCAL0
LOG_LOCAL1
LOG_LOCAL2
LOG_LOCAL3
LOG_LOCAL4
LOG_LOCAL5
LOG_LOCAL6
LOG_LOCAL7
Results are undefined if the facility code is anything else.
note: syslog recognizes one other facility code: that of
the kernel. But you can't specify that facility code with these
functions. If you try, it looks the same to syslog as if you are
requesting the default facility. But you wouldn't want to anyway,
because any program that uses the GNU C library is not the kernel.
You can use just a priority code as facility_priority. In that
case, syslog assumes the default facility established when the
Syslog connection was opened. See section Syslog Example.
The possible values for the priority code are (macros):
LOG_EMERG
LOG_ALERT
LOG_CRIT
LOG_ERR
LOG_WARNING
LOG_NOTICE
LOG_INFO
LOG_DEBUG
Results are undefined if the priority code is anything else.
If the process does not presently have a Syslog connection open (i.e.
it did not call openlog), syslog implicitly opens the
connection the same as openlog would, with the following defaults
for information that would otherwise be included in an openlog
call: The default identification string is the program name. The
default default facility is LOG_USER. The default for all the
connection options in options is as if those bits were off.
syslog leaves the Syslog connection open.
If the `dev/log' socket is not open and connected, syslog
opens and connects it, the same as openlog with the
LOG_NDELAY option would.
syslog leaves `/dev/log' open and connected unless its attempt
to send the message failed, in which case syslog closes it (with the
hope that a future implicit open will restore the Syslog connection to a
usable state).
Example:
#include <syslog.h>
syslog (LOG_MAKEPRI(LOG_LOCAL1, LOG_ERROR),
"Unable to make network connection to %s. Error=%m", host);
This is functionally identical to syslog, with the BSD style variable
length argument.
The symbols referred to in this section are declared in the file `syslog.h'.
closelog closes the current Syslog connection, if there is one.
This include closing the `dev/log' socket, if it is open.
There is very little reason to use this function. It does not flush any
buffers; you can reopen a Syslog connection without closing it first;
The connection gets closed automatically on exec or exit.
closelog has primarily aesthetic value.
The symbols referred to in this section are declared in the file `syslog.h'.
setlogmask sets a mask (the "logmask") that determines which
future syslog calls shall be ignored. If a program has not
called setlogmask, syslog doesn't ignore any calls. You
can use setlogmask to specify that messages of particular
priorities shall be ignored in the future.
A setlogmask call overrides any previous setlogmask call.
Note that the logmask exists entirely independently of opening and closing of Syslog connections.
Setting the logmask has a similar effect to, but is not the same as, configuring Syslog. The Syslog configuration may cause Syslog to discard certain messages it receives, but the logmask causes certain messages never to get submitted to Syslog in the first place.
mask is a bit string with one bit corresponding to each of the
possible message priorities. If the bit is on, syslog handles
messages of that priority normally. If it is off, syslog
discards messages of that priority. Use the message priority macros
described in section syslog, vsyslog and the LOG_MASK to construct
an appropriate mask value, as in this example:
LOG_MASK(LOG_EMERG) | LOG_MASK(LOG_ERROR)
or
~(LOG_MASK(LOG_INFO))
There is also a LOG_UPTO macro, which generates a mask with the bits
on for a certain priority and all priorities above it:
LOG_UPTO(LOG_ERROR)
The unfortunate naming of the macro is due to the fact that internally, higher numbers are used for lower message priorities.
Here is an example of openlog, syslog, and closelog:
This example sets the logmask so that debug and informational messages
get discarded without ever reaching Syslog. So the second syslog
in the example does nothing.
#include <syslog.h>
setlogmask (LOG_UPTO (LOG_NOTICE));
openlog ("exampleprog", LOG_CONS | LOG_PID | LOG_NDELAY, LOG_LOCAL1);
syslog (LOG_NOTICE, "Program started by User %d", getuid ());
syslog (LOG_INFO, "A tree falls in a forest");
closelog ();
@set mult ·
@set infty ∞
@set pie π
@macro mul @cdot @macro infinity @infty @ifnottex @macro pi
This chapter contains information about functions for performing mathematical computations, such as trigonometric functions. Most of these functions have prototypes declared in the header file `math.h'. The complex-valued functions are defined in `complex.h'.
All mathematical functions which take a floating-point argument
have three variants, one each for double, float, and
long double arguments. The double versions are mostly
defined in ISO C89. The float and long double
versions are from the numeric extensions to C included in ISO C99.
Which of the three versions of a function should be used depends on the
situation. For most calculations, the float functions are the
fastest. On the other hand, the long double functions have the
highest precision. double is somewhere in between. It is
usually wise to pick the narrowest type that can accommodate your data.
Not all machines have a distinct long double type; it may be the
same as double.
The header `math.h' defines several useful mathematical constants.
All values are defined as preprocessor macros starting with M_.
The values provided are:
M_E
M_LOG2E
2 of M_E.
M_LOG10E
10 of M_E.
M_LN2
2.
M_LN10
10.
M_PI
M_PI_2
M_PI_4
M_1_PI
M_2_PI
M_2_SQRTPI
M_SQRT2
M_SQRT1_2
These constants come from the Unix98 standard and were also available in
4.4BSD; therefore they are only defined if _BSD_SOURCE or
_XOPEN_SOURCE=500, or a more general feature select macro, is
defined. The default set of features includes these constants.
See section Feature Test Macros.
All values are of type double. As an extension, the GNU C
library also defines these constants with type long double. The
long double macros have a lowercase `l' appended to their
names: M_El, M_PIl, and so forth. These are only
available if _GNU_SOURCE is defined.
Note: Some programs use a constant named PI which has the
same value as M_PI. This constant is not standard; it may have
appeared in some old AT&T headers, and is mentioned in Stroustrup's book
on C++. It infringes on the user's name space, so the GNU C library
does not define it. Fixing programs written to expect it is simple:
replace PI with M_PI throughout, or put `-DPI=M_PI'
on the compiler command line.
These are the familiar sin, cos, and tan functions.
The arguments to all of these functions are in units of radians; recall
that pi radians equals 180 degrees.
The math library normally defines M_PI to a double
approximation of pi. If strict ISO and/or POSIX compliance
are requested this constant is not defined, but you can easily define it
yourself:
#define M_PI 3.14159265358979323846264338327
You can also compute the value of pi with the expression acos
(-1.0).
-1 to 1.
-1 to 1.
Mathematically, the tangent function has singularities at odd multiples
of pi/2. If the argument x is too close to one of these
singularities, tan will signal overflow.
In many applications where sin and cos are used, the sine
and cosine of the same angle are needed at the same time. It is more
efficient to compute them simultaneously, so the library provides a
function to do that.
*sinx and the
cosine of x in *cos, where x is given in
radians. Both values, *sinx and *cosx, are in
the range of -1 to 1.
This function is a GNU extension. Portable programs should be prepared to cope with its absence.
ISO C99 defines variants of the trig functions which work on complex numbers. The GNU C library provides these functions, but they are only useful if your compiler supports the new complex types defined by the standard. (As of this writing GCC supports complex numbers, but there are bugs in the implementation.)
@ifnottex @math{sin (z) = 1/(2*i) * (exp (z*i) - exp (-z*i))}.
@ifnottex @math{cos (z) = 1/2 * (exp (z*i) + exp (-z*i))}
@ifnottex @math{tan (z) = -i * (exp (z*i) - exp (-z*i)) / (exp (z*i) + exp (-z*i))}
The complex tangent has poles at @math{pi/2 + 2n}, where @math{n} is an
integer. ctan may signal overflow if z is too close to a
pole.
These are the usual arc sine, arc cosine and arc tangent functions, which are the inverses of the sine, cosine and tangent functions respectively.
-pi/2 and pi/2 (inclusive).
The arc sine function is defined mathematically only
over the domain -1 to 1. If x is outside the
domain, asin signals a domain error.
0 and pi (inclusive).
The arc cosine function is defined mathematically only
over the domain -1 to 1. If x is outside the
domain, acos signals a domain error.
-pi/2 and pi/2 (inclusive).
-pi to pi, inclusive.
If x and y are coordinates of a point in the plane,
atan2 returns the signed angle between the line from the origin
to that point and the x-axis. Thus, atan2 is useful for
converting Cartesian coordinates to polar coordinates. (To compute the
radial coordinate, use hypot; see section Exponentiation and Logarithms.)
If both x and y are zero, atan2 returns zero.
ISO C99 defines complex versions of the inverse trig functions.
Unlike the real-valued functions, casin is defined for all
values of z.
Unlike the real-valued functions, cacos is defined for all
values of z.
e (the base of natural logarithms) raised
to the power x.
If the magnitude of the result is too large to be representable,
exp signals overflow.
2 raised to the power x.
Mathematically, exp2 (x) is the same as exp (x * log (2)).
10 raised to the power x.
Mathematically, exp10 (x) is the same as exp (x * log (10)).
These functions are GNU extensions. The name exp10 is
preferred, since it is analogous to exp and exp2.
exp (log
(x)) equals x, exactly in mathematics and approximately in
C.
If x is negative, log signals a domain error. If x
is zero, it returns negative infinity; if x is too close to zero,
it may signal overflow.
log10 (x) equals log (x) / log (10).
log2 (x) equals log (x) / log (2).
FLT_RADIX is two, logb is equal
to floor (log2 (x)), except it's probably faster.
If x is de-normalized, logb returns the exponent x
would have if it were normalized. If x is infinity (positive or
negative), logb returns @math{@infinity{}}. If x is zero,
logb returns @math{@infinity{}}. It does not signal.
logb
functions except that they return signed integer values.
Since integers cannot represent infinity and NaN, ilogb instead
returns an integer that can't be the exponent of a normal floating-point
number. `math.h' defines constants so you can check for this.
ilogb returns this value if its argument is 0. The
numeric value is either INT_MIN or -INT_MAX.
This macro is defined in ISO C99.
ilogb returns this value if its argument is NaN. The
numeric value is either INT_MIN or INT_MAX.
This macro is defined in ISO C99.
These values are system specific. They might even be the same. The
proper way to test the result of ilogb is as follows:
i = ilogb (f);
if (i == FP_ILOGB0 || i == FP_ILOGBNAN)
{
if (isnan (f))
{
/* Handle NaN. */
}
else if (f == 0.0)
{
/* Handle 0.0. */
}
else
{
/* Some other value with large exponent,
perhaps +Inf. */
}
}
Mathematically, pow would return a complex number when base
is negative and power is not an integral value. pow can't
do that, so instead it signals a domain error. pow may also
underflow or overflow the destination type.
If x is negative, sqrt signals a domain error.
Mathematically, it should return a complex number.
sqrt (x*x +
y*y). This is the length of the hypotenuse of a right
triangle with sides of length x and y, or the distance
of the point (x, y) from the origin. Using this function
instead of the direct formula is wise, since the error is
much smaller. See also the function cabs in section Absolute Value.
exp (x) - 1.
They are computed in a way that is accurate even if x is
near zero--a case where exp (x) - 1 would be inaccurate owing
to subtraction of two numbers that are nearly equal.
log (1 + x).
They are computed in a way that is accurate even if x is
near zero.
ISO C99 defines complex variants of some of the exponentiation and logarithm functions.
e (the base of natural
logarithms) raised to the power of z.
Mathematically, this corresponds to the value
@ifnottex @math{exp (z) = exp (creal (z)) * (cos (cimag (z)) + I * sin (cimag (z)))}
@ifnottex @math{log (z) = log (cabs (z)) + I * carg (z)}
clog has a pole at 0, and will signal overflow if z equals
or is very close to 0. It is well-defined for all other values of
z.
@ifnottex @math{log (z) = log10 (cabs (z)) + I * carg (z)}
These functions are GNU extensions.
cexp (y * clog (x))
The functions in this section are related to the exponential functions; see section Exponentiation and Logarithms.
(exp (x) - exp (-x)) / 2. They
may signal overflow if x is too large.
(exp (x) + exp (-x)) / 2.
They may signal overflow if x is too large.
sinh (x) / cosh (x).
They may signal overflow if x is too large.
There are counterparts for the hyperbolic functions which take complex arguments.
(exp (z) - exp (-z)) / 2.
(exp (z) + exp (-z)) / 2.
csinh (z) / ccosh (z).
1, acosh signals a domain error.
1, atanh signals a domain error;
if it is equal to 1, atanh returns infinity.
These are some more exotic mathematical functions which are sometimes useful. Currently they only have real-valued versions.
erf returns the error function of x. The error
function is defined as
@ifnottex
erf (x) = 2/sqrt(pi) * integral from 0 to x of exp(-t^2) dt
erfc returns 1.0 - erf(x), but computed in a
fashion that avoids round-off error when x is large.
lgamma returns the natural logarithm of the absolute value of
the gamma function of x. The gamma function is defined as
@ifnottex
gamma (x) = integral from 0 to @infinity{} of t^(x-1) e^-t dt
The sign of the gamma function is stored in the global variable
signgam, which is declared in `math.h'. It is 1 if
the intermediate result was positive or zero, or -1 if it was
negative.
To compute the real gamma function you can use the tgamma
function or you can compute the values as follows:
lgam = lgamma(x); gam = signgam*exp(lgam);
The gamma function has singularities at the non-positive integers.
lgamma will raise the zero divide exception if evaluated at a
singularity.
lgamma_r is just like lgamma, but it stores the sign of
the intermediate result in the variable pointed to by signp
instead of in the signgam global. This means it is reentrant.
lgamma etc. It is better to use lgamma since for one the
name reflects better the actual computation, moreover lgamma is
standardized in ISO C99 while gamma is not.
tgamma applies the gamma function to x. The gamma
function is defined as
@ifnottex
gamma (x) = integral from 0 to @infinity{} of t^(x-1) e^-t dt
This function was introduced in ISO C99.
j0 returns the Bessel function of the first kind of order 0 of
x. It may signal underflow if x is too large.
j1 returns the Bessel function of the first kind of order 1 of
x. It may signal underflow if x is too large.
jn returns the Bessel function of the first kind of order
n of x. It may signal underflow if x is too large.
y0 returns the Bessel function of the second kind of order 0 of
x. It may signal underflow if x is too large. If x
is negative, y0 signals a domain error; if it is zero,
y0 signals overflow and returns @math{-@infinity}.
y1 returns the Bessel function of the second kind of order 1 of
x. It may signal underflow if x is too large. If x
is negative, y1 signals a domain error; if it is zero,
y1 signals overflow and returns @math{-@infinity}.
yn returns the Bessel function of the second kind of order n of
x. It may signal underflow if x is too large. If x
is negative, yn signals a domain error; if it is zero,
yn signals overflow and returns @math{-@infinity}.
This section lists the known errors of the functions in the math library. Errors are measured in "units of the last place". This is a measure for the relative error. For a number @math{z} with the representation @math{d.d...d@mul{}2^e} (we assume IEEE floating-point numbers with base 2) the ULP is represented by
@ifnottex
|d.d...d - (z / 2^e)| / 2^(p - 1)
where @math{p} is the number of bits in the mantissa of the floating-point number representation. Ideally the error for all functions is always less than 0.5ulps. Using rounding bits this is also possible and normally implemented for the basic operations. To achieve the same for the complex math functions requires a lot more work and this was not spend so far.
Therefore many of the functions in the math library have errors. The table lists the maximum error for each function which is exposed by one of the existing tests in the test suite. It is tried to cover as much as possible and really list the maximum error (or at least a ballpark figure) but this is often not achieved due to the large search space.
The table lists the ULP values for different architectures. Different architectures have different results since their hardware support for floating-point operations varies and also the existing hardware support is different.
@multitable {nexttowardf} {1000 + i 1000} {1000 + i 1000} {1000 + i 1000} {1000 + i 1000} {1000 + i 1000} {1000 + i 1000} {1000 + i 1000} {1000 + i 1000} {1000 + i 1000} {1000 + i 1000} {1000 + i 1000} {1000 + i 1000}
This section describes the GNU facilities for generating a series of pseudo-random numbers. The numbers generated are not truly random; typically, they form a sequence that repeats periodically, with a period so large that you can ignore it for ordinary purposes. The random number generator works by remembering a seed value which it uses to compute the next random number and also to compute a new seed.
Although the generated numbers look unpredictable within one run of a program, the sequence of numbers is exactly the same from one run to the next. This is because the initial seed is always the same. This is convenient when you are debugging a program, but it is unhelpful if you want the program to behave unpredictably. If you want a different pseudo-random series each time your program runs, you must specify a different seed each time. For ordinary purposes, basing the seed on the current time works well.
You can obtain repeatable sequences of numbers on a particular machine type by specifying the same initial seed value for the random number generator. There is no standard meaning for a particular seed value; the same seed, used in different C libraries or on different CPU types, will give you different random numbers.
The GNU library supports the standard ISO C random number functions
plus two other sets derived from BSD and SVID. The BSD and ISO C
functions provide identical, somewhat limited functionality. If only a
small number of random bits are required, we recommend you use the
ISO C interface, rand and srand. The SVID functions
provide a more flexible interface, which allows better random number
generator algorithms, provides more random bits (up to 48) per call, and
can provide random floating-point numbers. These functions are required
by the XPG standard and therefore will be present in all modern Unix
systems.
This section describes the random number functions that are part of the ISO C standard.
To use these facilities, you should include the header file `stdlib.h' in your program.
rand function can return. In the GNU library, it is
2147483647, which is the largest signed integer representable in
32 bits. In other libraries, it may be as low as 32767.
rand function returns the next pseudo-random number in the
series. The value ranges from 0 to RAND_MAX.
rand before a seed has been
established with srand, it uses the value 1 as a default
seed.
To produce a different pseudo-random series each time your program is
run, do srand (time (0)).
POSIX.1 extended the C standard functions to support reproducible random numbers in multi-threaded programs. However, the extension is badly designed and unsuitable for serious work.
RAND_MAX
just as rand does. However, all its state is stored in the
seed argument. This means the RNG's state can only have as many
bits as the type unsigned int has. This is far too few to
provide a good RNG.
If your program requires a reentrant RNG, we recommend you use the reentrant GNU extensions to the SVID random number generator. The POSIX.1 interface should only be used when the GNU extensions are not available.
This section describes a set of random number generation functions that are derived from BSD. There is no advantage to using these functions with the GNU C library; we support them for BSD compatibility only.
The prototypes for these functions are in `stdlib.h'.
0 to RAND_MAX.
Note: Temporarily this function was defined to return a
int32_t value to indicate that the return value always contains
32 bits even if long int is wider. The standard demands it
differently. Users must always be aware of the 32-bit limitation,
though.
srandom function sets the state of the random number
generator based on the integer seed. If you supply a seed value
of 1, this will cause random to reproduce the default set
of random numbers.
To produce a different set of pseudo-random numbers each time your
program runs, do srandom (time (0)).
initstate function is used to initialize the random number
generator state. The argument state is an array of size
bytes, used to hold the state information. It is initialized based on
seed. The size must be between 8 and 256 bytes, and should be a
power of two. The bigger the state array, the better.
The return value is the previous value of the state information array.
You can use this value later as an argument to setstate to
restore that state.
setstate function restores the random number state
information state. The argument must have been the result of
a previous call to initstate or setstate.
The return value is the previous value of the state information array.
You can use this value later as an argument to setstate to
restore that state.
If the function fails the return value is NULL.
The four functions described so far in this section all work on a state which is shared by all threads. The state is not directly accessible to the user and can only be modified by these functions. This makes it hard to deal with situations where each thread should have its own pseudo-random number generator.
The GNU C library contains four additional functions which contain the state as an explicit parameter and therefore make it possible to handle thread-local PRNGs. Beside this there are no difference. In fact, the four functions already discussed are implemented internally using the following interfaces.
The `stdlib.h' header contains a definition of the following type:
Objects of type struct random_data contain the information
necessary to represent the state of the PRNG. Although a complete
definition of the type is present the type should be treated as opaque.
The functions modifying the state follow exactly the already described functions.
random_r function behaves exactly like the random
function except that it uses and modifies the state in the object
pointed to by the first parameter instead of the global state.
srandom_r function behaves exactly like the srandom
function except that it uses and modifies the state in the object
pointed to by the second parameter instead of the global state.
initstate_r function behaves exactly like the initstate
function except that it uses and modifies the state in the object
pointed to by the fourth parameter instead of the global state.
setstate_r function behaves exactly like the setstate
function except that it uses and modifies the state in the object
pointed to by the first parameter instead of the global state.
The C library on SVID systems contains yet another kind of random number generator functions. They use a state of 48 bits of data. The user can choose among a collection of functions which return the random bits in different forms.
Generally there are two kinds of function. The first uses a state of the random number generator which is shared among several functions and by all threads of the process. The second requires the user to handle the state.
All functions have in common that they use the same congruential formula with the same constants. The formula is
Y = (a * X + c) mod m
where X is the state of the generator at the beginning and
Y the state at the end. a and c are constants
determining the way the generator works. By default they are
a = 0x5DEECE66D = 25214903917 c = 0xb = 11
but they can also be changed by the user. m is of course 2^48
since the state consists of a 48-bit array.
The prototypes for these functions are in `stdlib.h'.
double value in the range of 0.0
to 1.0 (exclusive). The random bits are determined by the global
state of the random number generator in the C library.
Since the double type according to IEEE 754 has a 52-bit
mantissa this means 4 bits are not initialized by the random number
generator. These are (of course) chosen to be the least significant
bits and they are initialized to 0.
double value in the range of 0.0
to 1.0 (exclusive), similarly to drand48. The argument is
an array describing the state of the random number generator.
This function can be called subsequently since it updates the array to guarantee random numbers. The array should have been initialized before initial use to obtain reproducible results.
lrand48 function returns an integer value in the range of
0 to 2^31 (exclusive). Even if the size of the long
int type can take more than 32 bits, no higher numbers are returned.
The random bits are determined by the global state of the random number
generator in the C library.
lrand48 function in that it
returns a number in the range of 0 to 2^31 (exclusive) but
the state of the random number generator used to produce the random bits
is determined by the array provided as the parameter to the function.
The numbers in the array are updated afterwards so that subsequent calls to this function yield different results (as is expected of a random number generator). The array should have been initialized before the first call to obtain reproducible results.
mrand48 function is similar to lrand48. The only
difference is that the numbers returned are in the range -2^31 to
2^31 (exclusive).
jrand48 function is similar to nrand48. The only
difference is that the numbers returned are in the range -2^31 to
2^31 (exclusive). For the xsubi parameter the same
requirements are necessary.
The internal state of the random number generator can be initialized in several ways. The methods differ in the completeness of the information provided.
srand48 function sets the most significant 32 bits of the
internal state of the random number generator to the least
significant 32 bits of the seedval parameter. The lower 16 bits
are initialized to the value 0x330E. Even if the long
int type contains more than 32 bits only the lower 32 bits are used.
Owing to this limitation, initialization of the state of this
function is not very useful. But it makes it easy to use a construct
like srand48 (time (0)).
A side-effect of this function is that the values a and c
from the internal state, which are used in the congruential formula,
are reset to the default values given above. This is of importance once
the user has called the lcong48 function (see below).
seed48 function initializes all 48 bits of the state of the
internal random number generator from the contents of the parameter
seed16v. Here the lower 16 bits of the first element of
see16v initialize the least significant 16 bits of the internal
state, the lower 16 bits of seed16v[1] initialize the mid-order
16 bits of the state and the 16 lower bits of seed16v[2]
initialize the most significant 16 bits of the state.
Unlike srand48 this function lets the user initialize all 48 bits
of the state.
The value returned by seed48 is a pointer to an array containing
the values of the internal state before the change. This might be
useful to restart the random number generator at a certain state.
Otherwise the value can simply be ignored.
As for srand48, the values a and c from the
congruential formula are reset to the default values.
There is one more function to initialize the random number generator which enables you to specify even more information by allowing you to change the parameters in the congruential formula.
lcong48 function allows the user to change the complete state
of the random number generator. Unlike srand48 and
seed48, this function also changes the constants in the
congruential formula.
From the seven elements in the array param the least significant
16 bits of the entries param[0] to param[2]
determine the initial state, the least significant 16 bits of
param[3] to param[5] determine the 48 bit
constant a and param[6] determines the 16-bit value
c.
All the above functions have in common that they use the global parameters for the congruential formula. In multi-threaded programs it might sometimes be useful to have different parameters in different threads. For this reason all the above functions have a counterpart which works on a description of the random number generator in the user-supplied buffer instead of the global state.
Please note that it is no problem if several threads use the global state if all threads use the functions which take a pointer to an array containing the state. The random numbers are computed following the same loop but if the state in the array is different all threads will obtain an individual random number generator.
The user-supplied buffer must be of type struct drand48_data.
This type should be regarded as opaque and not manipulated directly.
drand48 function with the
difference that it does not modify the global random number generator
parameters but instead the parameters in the buffer supplied through the
pointer buffer. The random number is returned in the variable
pointed to by result.
The return value of the function indicates whether the call succeeded.
If the value is less than 0 an error occurred and errno is
set to indicate the problem.
This function is a GNU extension and should not be used in portable programs.
erand48_r function works like erand48, but in addition
it takes an argument buffer which describes the random number
generator. The state of the random number generator is taken from the
xsubi array, the parameters for the congruential formula from the
global random number generator data. The random number is returned in
the variable pointed to by result.
The return value is non-negative if the call succeeded.
This function is a GNU extension and should not be used in portable programs.
lrand48, but in addition it takes a
pointer to a buffer describing the state of the random number generator
just like drand48.
If the return value of the function is non-negative the variable pointed to by result contains the result. Otherwise an error occurred.
This function is a GNU extension and should not be used in portable programs.
nrand48_r function works like nrand48 in that it
produces a random number in the range 0 to 2^31. But instead
of using the global parameters for the congruential formula it uses the
information from the buffer pointed to by buffer. The state is
described by the values in xsubi.
If the return value is non-negative the variable pointed to by result contains the result.
This function is a GNU extension and should not be used in portable programs.
mrand48 but like the other reentrant
functions it uses the random number generator described by the value in
the buffer pointed to by buffer.
If the return value is non-negative the variable pointed to by result contains the result.
This function is a GNU extension and should not be used in portable programs.
jrand48_r function is similar to jrand48. Like the
other reentrant functions of this function family it uses the
congruential formula parameters from the buffer pointed to by
buffer.
If the return value is non-negative the variable pointed to by result contains the result.
This function is a GNU extension and should not be used in portable programs.
Before any of the above functions are used the buffer of type
struct drand48_data should be initialized. The easiest way to do
this is to fill the whole buffer with null bytes, e.g. by
memset (buffer, '\0', sizeof (struct drand48_data));
Using any of the reentrant functions of this family now will automatically initialize the random number generator to the default values for the state and the parameters of the congruential formula.
The other possibility is to use any of the functions which explicitly initialize the buffer. Though it might be obvious how to initialize the buffer from looking at the parameter to the function, it is highly recommended to use these functions since the result might not always be what you expect.
srand48 does. The state is initialized from the parameter
seedval and the parameters for the congruential formula are
initialized to their default values.
If the return value is non-negative the function call succeeded.
This function is a GNU extension and should not be used in portable programs.
srand48_r but like seed48 it
initializes all 48 bits of the state from the parameter seed16v.
If the return value is non-negative the function call succeeded. It
does not return a pointer to the previous state of the random number
generator like the seed48 function does. If the user wants to
preserve the state for a later re-run s/he can copy the whole buffer
pointed to by buffer.
This function is a GNU extension and should not be used in portable programs.
If the return value is non-negative the function call succeeded.
This function is a GNU extension and should not be used in portable programs.
If an application uses many floating point functions it is often the case that the cost of the function calls themselves is not negligible. Modern processors can often execute the operations themselves very fast, but the function call disrupts the instruction pipeline.
For this reason the GNU C Library provides optimizations for many of the frequently-used math functions. When GNU CC is used and the user activates the optimizer, several new inline functions and macros are defined. These new functions and macros have the same names as the library functions and so are used instead of the latter. In the case of inline functions the compiler will decide whether it is reasonable to use them, and this decision is usually correct.
This means that no calls to the library functions may be necessary, and can increase the speed of generated code significantly. The drawback is that code size will increase, and the increase is not always negligible.
There are two kind of inline functions: Those that give the same result
as the library functions and others that might not set errno and
might have a reduced precision and/or argument range in comparison with
the library functions. The latter inline functions are only available
if the flag -ffast-math is given to GNU CC.
In cases where the inline functions and macros are not wanted the symbol
__NO_MATH_INLINES should be defined before any system header is
included. This will ensure that only library functions are used. Of
course, it can be determined for each file in the project whether
giving this option is preferable or not.
Not all hardware implements the entire IEEE 754 standard, and even if it does there may be a substantial performance penalty for using some of its features. For example, enabling traps on some processors forces the FPU to run un-pipelined, which can more than double calculation time.
This chapter contains information about functions for doing basic arithmetic operations, such as splitting a float into its integer and fractional parts or retrieving the imaginary part of a complex value. These functions are declared in the header files `math.h' and `complex.h'.
The C language defines several integer data types: integer, short integer, long integer, and character, all in both signed and unsigned varieties. The GNU C compiler extends the language to contain long long integers as well.
The C integer types were intended to allow code to be portable among machines with different inherent data sizes (word sizes), so each type may have different ranges on different machines. The problem with this is that a program often needs to be written for a particular range of integers, and sometimes must be written for a particular size of storage, regardless of what machine the program runs on.
To address this problem, the GNU C library contains C type definitions you can use to declare integers that meet your exact needs. Because the GNU C library header files are customized to a specific machine, your program source code doesn't have to be.
These typedefs are in `stdint.h'.
If you require that an integer be represented in exactly N bits, use one of the following types, with the obvious mapping to bit size and signedness:
If your C compiler and target machine do not allow integers of a certain size, the corresponding above type does not exist.
If you don't need a specific storage size, but want the smallest data structure with at least N bits, use one of these:
If you don't need a specific storage size, but want the data structure that allows the fastest access while having at least N bits (and among data structures with the same access speed, the smallest one), use one of these:
If you want an integer with the widest range possible on the platform on which it is being used, use one of the following. If you use these, you should write code that takes into account the variable size and range of the integer.
The GNU C library also provides macros that tell you the maximum and
minimum possible values for each integer data type. The macro names
follow these examples: INT32_MAX, UINT8_MAX,
INT_FAST32_MIN, INT_LEAST64_MIN, UINTMAX_MAX,
INTMAX_MAX, INTMAX_MIN. Note that there are no macros for
unsigned integer minima. These are always zero.
There are similar macros for use with C's built in integer types which should come with your C compiler. These are described in section Data Type Measurements.
Don't forget you can use the C sizeof function with any of these
data types to get the number of bytes of storage each uses.
This section describes functions for performing integer division. These
functions are redundant when GNU CC is used, because in GNU C the
`/' operator always rounds towards zero. But in other C
implementations, `/' may round differently with negative arguments.
div and ldiv are useful because they specify how to round
the quotient: towards zero. The remainder has the same sign as the
numerator.
These functions are specified to return a result r such that the value
r.quot*denominator + r.rem equals
numerator.
To use these facilities, you should include the header file `stdlib.h' in your program.
div
function. It has the following members:
int quot
int rem
div computes the quotient and remainder from
the division of numerator by denominator, returning the
result in a structure of type div_t.
If the result cannot be represented (as in a division by zero), the behavior is undefined.
Here is an example, albeit not a very useful one.
div_t result; result = div (20, -6);
Now result.quot is -3 and result.rem is 2.
ldiv
function. It has the following members:
long int quot
long int rem
(This is identical to div_t except that the components are of
type long int rather than int.)
ldiv function is similar to div, except that the
arguments are of type long int and the result is returned as a
structure of type ldiv_t.
lldiv
function. It has the following members:
long long int quot
long long int rem
(This is identical to div_t except that the components are of
type long long int rather than int.)
lldiv function is like the div function, but the
arguments are of type long long int and the result is returned as
a structure of type lldiv_t.
The lldiv function was added in ISO C99.
imaxdiv
function. It has the following members:
intmax_t quot
intmax_t rem
(This is identical to div_t except that the components are of
type intmax_t rather than int.)
See section Integers for a description of the intmax_t type.
imaxdiv function is like the div function, but the
arguments are of type intmax_t and the result is returned as
a structure of type imaxdiv_t.
See section Integers for a description of the intmax_t type.
The imaxdiv function was added in ISO C99.
Most computer hardware has support for two different kinds of numbers: integers (@math{...-3, -2, -1, 0, 1, 2, 3...}) and floating-point numbers. Floating-point numbers have three parts: the mantissa, the exponent, and the sign bit. The real number represented by a floating-point value is given by @ifnottex @math{(s ? -1 : 1) @mul{} 2^e @mul{} M} where @math{s} is the sign bit, @math{e} the exponent, and @math{M} the mantissa. See section Floating Point Representation Concepts, for details. (It is possible to have a different base for the exponent, but all modern hardware uses @math{2}.)
Floating-point numbers can represent a finite subset of the real numbers. While this subset is large enough for most purposes, it is important to remember that the only reals that can be represented exactly are rational numbers that have a terminating binary expansion shorter than the width of the mantissa. Even simple fractions such as @math{1/5} can only be approximated by floating point.
Mathematical operations and functions frequently need to produce values that are not representable. Often these values can be approximated closely enough for practical purposes, but sometimes they can't. Historically there was no way to tell when the results of a calculation were inaccurate. Modern computers implement the IEEE 754 standard for numerical computations, which defines a framework for indicating to the program when the results of calculation are not trustworthy. This framework consists of a set of exceptions that indicate why a result could not be represented, and the special values infinity and not a number (NaN).
ISO C99 defines macros that let you determine what sort of floating-point number a variable holds.
int. The possible values are:
FP_NAN
FP_INFINITE
FP_ZERO
FP_SUBNORMAL
fpclassify returns this value
for values of x in this alternate format.
FP_NORMAL
fpclassify is most useful if more than one property of a number
must be tested. There are more specific macros which only test one
property at a time. Generally these macros execute faster than
fpclassify, since there is special hardware support for them.
You should therefore use the specific macros whenever possible.
(fpclassify (x) != FP_NAN && fpclassify (x) != FP_INFINITE)
isfinite is implemented as a macro which accepts any
floating-point type.
(fpclassify (x) == FP_NORMAL)
(fpclassify (x) == FP_NAN)
Another set of floating-point classification functions was provided by BSD. The GNU C library also supports these functions; however, we recommend that you use the ISO C99 macros in new code. Those are standard and will be available more widely. Also, since they are macros, you do not have to worry about the type of their argument.
-1 if x represents negative infinity,
1 if x represents positive infinity, and 0 otherwise.
Note: The isnan macro defined by ISO C99 overrides
the BSD function. This is normally not a problem, because the two
routines behave identically. However, if you really need to get the BSD
function for some reason, you can write
(isnan) (x)
EDOM or ERANGE; infnan returns the
value that a math function would return if it set errno to that
value. See section Error Reporting by Mathematical Functions. -ERANGE is also acceptable
as an argument, and corresponds to -HUGE_VAL as a value.
In the BSD library, on certain machines, infnan raises a fatal
signal in all cases. The GNU library does not do likewise, because that
does not fit the ISO C specification.
Portability Note: The functions listed in this section are BSD extensions.
The IEEE 754 standard defines five exceptions that can occur during a calculation. Each corresponds to a particular sort of error, such as overflow.
When exceptions occur (when exceptions are raised, in the language of the standard), one of two things can happen. By default the exception is simply noted in the floating-point status word, and the program continues as if nothing had happened. The operation produces a default value, which depends on the exception (see the table below). Your program can check the status word to find out which exceptions happened.
Alternatively, you can enable traps for exceptions. In that case,
when an exception is raised, your program will receive the SIGFPE
signal. The default action for this signal is to terminate the
program. See section Signal Handling, for how you can change the effect of
the signal.
In the System V math library, the user-defined function matherr
is called when certain exceptions occur inside math library functions.
However, the Unix98 standard deprecates this interface. We support it
for historical compatibility, but recommend that you do not use it in
new programs.
The exceptions defined in IEEE 754 are:
IEEE 754 floating point numbers can represent positive or negative infinity, and NaN (not a number). These three values arise from calculations whose result is undefined or cannot be represented accurately. You can also deliberately set a floating-point variable to any of them, which is sometimes useful. Some examples of calculations that produce infinity or NaN:
@ifnottex
@math{1/0 = @infinity{}}
@math{log (0) = -@infinity{}}
@math{sqrt (-1) = NaN}
When a calculation produces any of these values, an exception also occurs; see section FP Exceptions.
The basic operations and math functions all accept infinity and NaN and produce sensible output. Infinities propagate through calculations as one would expect: for example, @math{2 + @infinity{} = @infinity{}}, @math{4/@infinity{} = 0}, atan @math{(@infinity{}) = @pi{}/2}. NaN, on the other hand, infects any calculation that involves it. Unless the calculation would produce the same result no matter what real value replaced NaN, the result is NaN.
In comparison operations, positive infinity is larger than all values
except itself and NaN, and negative infinity is smaller than all values
except itself and NaN. NaN is unordered: it is not equal to,
greater than, or less than anything, including itself. x ==
x is false if the value of x is NaN. You can use this to test
whether a value is NaN or not, but the recommended way to test for NaN
is with the isnan function (see section Floating-Point Number Classification Functions). In
addition, <, >, <=, and >= will raise an
exception when applied to NaNs.
`math.h' defines macros that allow you to explicitly set a variable to infinity or NaN.
1.0 / 0.0.
-INFINITY represents negative infinity.
You can test whether a floating-point value is infinite by comparing it
to this macro. However, this is not recommended; you should use the
isfinite macro instead. See section Floating-Point Number Classification Functions.
This macro was introduced in the ISO C99 standard.
You can use `#ifdef NAN' to test whether the machine supports
NaN. (Of course, you must arrange for GNU extensions to be visible,
such as by defining _GNU_SOURCE, and then you must include
`math.h'.)
IEEE 754 also allows for another unusual value: negative zero. This
value is produced when you divide a positive number by negative
infinity, or when a negative result is smaller than the limits of
representation. Negative zero behaves identically to zero in all
calculations, unless you explicitly test the sign bit with
signbit or copysign.
ISO C99 defines functions to query and manipulate the floating-point status word. You can use these functions to check for untrapped exceptions when it's convenient, rather than worrying about them in the middle of a calculation.
These constants represent the various IEEE 754 exceptions. Not all FPUs report all the different exceptions. Each constant is defined if and only if the FPU you are compiling for supports that exception, so you can test for FPU support with `#ifdef'. They are defined in `fenv.h'.
FE_INEXACT
FE_DIVBYZERO
FE_UNDERFLOW
FE_OVERFLOW
FE_INVALID
The macro FE_ALL_EXCEPT is the bitwise OR of all exception macros
which are supported by the FP implementation.
These functions allow you to clear exception flags, test for exceptions, and save and restore the set of exceptions flagged.
The function returns zero in case the operation was successful, a non-zero value otherwise.
FE_OVERFLOW) or underflow (FE_UNDERFLOW) are
raised before inexact (FE_INEXACT). Whether for overflow or
underflow the inexact exception is also raised is also implementation
dependent.
The function returns zero in case the operation was successful, a non-zero value otherwise.
To understand these functions, imagine that the status word is an
integer variable named status. feclearexcept is then
equivalent to `status &= ~excepts' and fetestexcept is
equivalent to `(status & excepts)'. The actual implementation may
be very different, of course.
Exception flags are only cleared when the program explicitly requests it,
by calling feclearexcept. If you want to check for exceptions
from a set of calculations, you should clear all the flags first. Here
is a simple example of the way to use fetestexcept:
{
double f;
int raised;
feclearexcept (FE_ALL_EXCEPT);
f = compute ();
raised = fetestexcept (FE_OVERFLOW | FE_INVALID);
if (raised & FE_OVERFLOW) { /* ... */ }
if (raised & FE_INVALID) { /* ... */ }
/* ... */
}
You cannot explicitly set bits in the status word. You can, however, save the entire status word and restore it later. This is done with the following functions:
The function returns zero in case the operation was successful, a non-zero value otherwise.
The function returns zero in case the operation was successful, a non-zero value otherwise.
Note that the value stored in fexcept_t bears no resemblance to
the bit mask returned by fetestexcept. The type may not even be
an integer. Do not attempt to modify an fexcept_t variable.
Many of the math functions are defined only over a subset of the real or complex numbers. Even if they are mathematically defined, their result may be larger or smaller than the range representable by their return type. These are known as domain errors, overflows, and underflows, respectively. Math functions do several things when one of these errors occurs. In this manual we will refer to the complete response as signalling a domain error, overflow, or underflow.
When a math function suffers a domain error, it raises the invalid
exception and returns NaN. It also sets errno to EDOM;
this is for compatibility with old systems that do not support IEEE
754 exception handling. Likewise, when overflow occurs, math
functions raise the overflow exception and return @math{@infinity{}} or
@math{-@infinity{}} as appropriate. They also set errno to
ERANGE. When underflow occurs, the underflow exception is
raised, and zero (appropriately signed) is returned. errno may be
set to ERANGE, but this is not guaranteed.
Some of the math functions are defined mathematically to result in a
complex value over parts of their domains. The most familiar example of
this is taking the square root of a negative number. The complex math
functions, such as csqrt, will return the appropriate complex value
in this case. The real-valued functions, such as sqrt, will
signal a domain error.
Some older hardware does not support infinities. On that hardware, overflows instead return a particular very large number (usually the largest representable number). `math.h' defines macros you can use to test for overflow on both old and new hardware.
HUGE_VAL is infinity.
On other machines, it's typically the largest positive number that can
be represented.
Mathematical functions return the appropriately typed version of
HUGE_VAL or -HUGE_VAL when the result is too large
to be represented.
Floating-point calculations are carried out internally with extra precision, and then rounded to fit into the destination type. This ensures that results are as precise as the input data. IEEE 754 defines four possible rounding modes:
FLT_EPSILON.
`fenv.h' defines constants which you can use to refer to the various rounding modes. Each one will be defined if and only if the FPU supports the corresponding rounding mode.
FE_TONEAREST
FE_UPWARD
FE_DOWNWARD
FE_TOWARDZERO
Underflow is an unusual case. Normally, IEEE 754 floating point
numbers are always normalized (see section Floating Point Representation Concepts).
Numbers smaller than @math{2^r} (where @math{r} is the minimum exponent,
FLT_MIN_RADIX-1 for float) cannot be represented as
normalized numbers. Rounding all such numbers to zero or @math{2^r}
would cause some algorithms to fail at 0. Therefore, they are left in
denormalized form. That produces loss of precision, since some bits of
the mantissa are stolen to indicate the decimal point.
If a result is too small to be represented as a denormalized number, it
is rounded to zero. However, the sign of the result is preserved; if
the calculation was negative, the result is negative zero.
Negative zero can also result from some operations on infinity, such as
@math{4/-@infinity{}}. Negative zero behaves identically to zero except
when the copysign or signbit functions are used to check
the sign bit directly.
At any time one of the above four rounding modes is selected. You can find out which one with this function:
To change the rounding mode, use this function:
fesetround returns zero if it changed the
rounding mode, a nonzero value if the mode is not supported.
You should avoid changing the rounding mode if possible. It can be an expensive operation; also, some hardware requires you to compile your program differently for it to work. The resulting code may run slower. See your compiler documentation for details.
IEEE 754 floating-point implementations allow the programmer to
decide whether traps will occur for each of the exceptions, by setting
bits in the control word. In C, traps result in the program
receiving the SIGFPE signal; see section Signal Handling.
Note: IEEE 754 says that trap handlers are given details of the exceptional situation, and can set the result value. C signals do not provide any mechanism to pass this information back and forth. Trapping exceptions in C is therefore not very useful.
It is sometimes necessary to save the state of the floating-point unit while you perform some calculation. The library provides functions which save and restore the exception flags, the set of exceptions that generate traps, and the rounding mode. This information is known as the floating-point environment.
The functions to save and restore the floating-point environment all use
a variable of type fenv_t to store information. This type is
defined in `fenv.h'. Its size and contents are
implementation-defined. You should not attempt to manipulate a variable
of this type directly.
To save the state of the FPU, use one of these functions:
The function returns zero in case the operation was successful, a non-zero value otherwise.
feholdexcept cannot set this mode, it returns nonzero value. If it
succeeds, it returns zero.
The functions which restore the floating-point environment can take these kinds of arguments:
fenv_t objects, which were initialized previously by a
call to fegetenv or feholdexcept.
FE_DFL_ENV which represents the floating-point
environment as it was available at program start.
FE_ and
having type fenv_t *.
If possible, the GNU C Library defines a macro FE_NOMASK_ENV
which represents an environment where every exception raised causes a
trap to occur. You can test for this macro using #ifdef. It is
only defined if _GNU_SOURCE is defined.
Some platforms might define other predefined environments.
To set the floating-point environment, you can use either of these functions:
The function returns zero in case the operation was successful, a non-zero value otherwise.
fesetenv, this function sets the floating-point environment
to that described by envp. However, if any exceptions were
flagged in the status word before feupdateenv was called, they
remain flagged after the call. In other words, after feupdateenv
is called, the status word is the bitwise OR of the previous status word
and the one saved in envp.
The function returns zero in case the operation was successful, a non-zero value otherwise.
To control for individual exceptions if raising them causes a trap to occur, you can use the following two functions.
Portability Note: These functions are all GNU extensions.
The function returns the previous enabled exceptions in case the
operation was successful, -1 otherwise.
The function returns the previous enabled exceptions in case the
operation was successful, -1 otherwise.
-1 in case of failure.
The C library provides functions to do basic operations on floating-point numbers. These include absolute value, maximum and minimum, normalization, bit twiddling, rounding, and a few others.
These functions are provided for obtaining the absolute value (or
magnitude) of a number. The absolute value of a real number
x is x if x is positive, -x if x is
negative. For a complex number z, whose real part is x and
whose imaginary part is y, the absolute value is sqrt
(x*x + y*y).
Prototypes for abs, labs and llabs are in `stdlib.h';
imaxabs is declared in `inttypes.h';
fabs, fabsf and fabsl are declared in `math.h'.
cabs, cabsf and cabsl are declared in `complex.h'.
Most computers use a two's complement integer representation, in which
the absolute value of INT_MIN (the smallest possible int)
cannot be represented; thus, abs (INT_MIN) is not defined.
llabs and imaxdiv are new to ISO C99.
See section Integers for a description of the intmax_t type.
sqrt (creal (z) * creal (z) + cimag (z) * cimag (z))
This function should always be used instead of the direct formula
because it takes special care to avoid losing precision. It may also
take advantage of hardware support for this operation. See hypot
in section Exponentiation and Logarithms.
The functions described in this section are primarily provided as a way to efficiently perform certain low-level manipulations on floating point numbers that are represented internally using a binary radix; see section Floating Point Representation Concepts. These functions are required to have equivalent behavior even if the representation does not use a radix of 2, but of course they are unlikely to be particularly efficient in those cases.
All these functions are declared in `math.h'.
If the argument value is not zero, the return value is value
times a power of two, and is always in the range 1/2 (inclusive) to 1
(exclusive). The corresponding exponent is stored in
*exponent; the return value multiplied by 2 raised to this
exponent equals the original number value.
For example, frexp (12.8, &exponent) returns 0.8 and
stores 4 in exponent.
If value is zero, then the return value is zero and
zero is stored in *exponent.
frexp.)
For example, ldexp (0.8, 4) returns 12.8.
The following functions, which come from BSD, provide facilities
equivalent to those of ldexp and frexp.
double. This is
the highest integer power of 2 contained in x. The sign of
x is ignored. For example, logb (3.5) is 1.0 and
logb (4.0) is 2.0.
When 2 raised to this power is divided into x, it gives a
quotient between 1 (inclusive) and 2 (exclusive).
If x is zero, the return value is minus infinity if the machine supports infinities, and a very small number if it does not. If x is infinity, the return value is infinity.
For finite x, the value returned by logb is one less than
the value that frexp would store into *exponent.
scalb function is the BSD name for ldexp.
scalbn is identical to scalb, except that the exponent
n is an int instead of a floating-point number.
scalbln is identical to scalb, except that the exponent
n is a long int instead of a floating-point number.
significand returns the mantissa of x scaled to the range
@math{[1, 2)}.
It is equivalent to scalb (x, (double) -ilogb (x)).
This function exists mainly for use in certain standardized tests of IEEE 754 conformance.
The functions listed here perform operations such as rounding and truncation of floating-point values. Some of these functions convert floating point numbers to integer values. They are all declared in `math.h'.
You can also convert floating-point numbers to integers simply by
casting them to int. This discards the fractional part,
effectively rounding towards zero. However, this only works if the
result can actually be represented as an int---for very large
numbers, this is impossible. The functions listed here return the
result as a double instead to get around this problem.
double. Thus, ceil (1.5)
is 2.0.
double. Thus, floor
(1.5) is 1.0 and floor (-1.5) is -2.0.
trunc functions round x towards zero to the nearest
integer (returned in floating-point format). Thus, trunc (1.5)
is 1.0 and trunc (-1.5) is -1.0.
If x was not initially an integer, these functions raise the inexact exception.
rint functions, but
do not raise the inexact exception if x is not an integer.
rint, but they round halfway
cases away from zero instead of to the nearest even integer.
rint, but they return a
long int instead of a floating-point number.
rint, but they return a
long long int instead of a floating-point number.
round, but they return a
long int instead of a floating-point number.
round, but they return a
long long int instead of a floating-point number.
-1 and 1, exclusive). Their sum
equals value. Each of the parts has the same sign as value,
and the integer part is always rounded toward zero.
modf stores the integer part in *integer-part, and
returns the fractional part. For example, modf (2.5, &intpart)
returns 0.5 and stores 2.0 into intpart.
The functions in this section compute the remainder on division of two floating-point numbers. Each is a little different; pick the one that suits your problem.
numerator - n * denominator, where n
is the quotient of numerator divided by denominator, rounded
towards zero to an integer. Thus, fmod (6.5, 2.3) returns
1.9, which is 6.5 minus 4.6.
The result has the same sign as the numerator and has magnitude less than the magnitude of the denominator.
If denominator is zero, fmod signals a domain error.
fmod except that they rounds the
internal quotient n to the nearest integer instead of towards zero
to an integer. For example, drem (6.5, 2.3) returns -0.4,
which is 6.5 minus 6.9.
The absolute value of the result is less than or equal to half the
absolute value of the denominator. The difference between
fmod (numerator, denominator) and drem
(numerator, denominator) is always either
denominator, minus denominator, or zero.
If denominator is zero, drem signals a domain error.
drem.
There are some operations that are too complicated or expensive to perform by hand on floating-point numbers. ISO C99 defines functions to do these operations, which mostly involve changing single bits.
copysign never raises an exception.
This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).
signbit is a generic macro which can work on all floating-point
types. It returns a nonzero value if the value of x has its sign
bit set.
This is not the same as x < 0.0, because IEEE 754 floating
point allows zero to be signed. The comparison -0.0 < 0.0 is
false, but signbit (-0.0) will return a nonzero value.
nextafter function returns the next representable neighbor of
x in the direction towards y. The size of the step between
x and the result depends on the type of the result. If
@math{x = y} the function simply returns y. If either
value is NaN, NaN is returned. Otherwise
a value corresponding to the value of the least significant bit in the
mantissa is added or subtracted, depending on the direction.
nextafter will signal overflow or underflow if the result goes
outside of the range of normalized numbers.
This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).
nextafter except that their second argument is a long
double.
nan function returns a representation of NaN, provided that
NaN is supported by the target platform.
nan ("n-char-sequence") is equivalent to
strtod ("NAN(n-char-sequence)").
The argument tagp is used in an unspecified manner. On IEEE 754 systems, there are many representations of NaN, and tagp selects one. On other systems it may do nothing.
The standard C comparison operators provoke exceptions when one or other of the operands is NaN. For example,
int v = a < 1.0;
will raise an exception if a is NaN. (This does not
happen with == and !=; those merely return false and true,
respectively, when NaN is examined.) Frequently this exception is
undesirable. ISO C99 therefore defines comparison functions that
do not raise exceptions when NaN is examined. All of the functions are
implemented as macros which allow their arguments to be of any
floating-point type. The macros are guaranteed to evaluate their
arguments only once.
(x) > (y), but no
exception is raised if x or y are NaN.
(x) >= (y), but no
exception is raised if x or y are NaN.
(x) < (y), but no exception is
raised if x or y are NaN.
(x) <= (y), but no
exception is raised if x or y are NaN.
(x) < (y) ||
(x) > (y) (although it only evaluates x and y
once), but no exception is raised if x or y are NaN.
This macro is not equivalent to x != y, because that
expression is true if x or y are NaN.
Not all machines provide hardware support for these operations. On machines that don't, the macros can be very slow. Therefore, you should not use these functions when NaN is not a concern.
Note: There are no macros isequal or isunequal.
They are unnecessary, because the == and != operators do
not throw an exception if one or both of the operands are NaN.
The functions in this section perform miscellaneous but common operations that are awkward to express with C operators. On some processors these functions can use special machine instructions to perform these operations faster than the equivalent C code.
fmin function returns the lesser of the two values x
and y. It is similar to the expression
((x) < (y) ? (x) : (y))
except that x and y are only evaluated once.
If an argument is NaN, the other argument is returned. If both arguments are NaN, NaN is returned.
fmax function returns the greater of the two values x
and y.
If an argument is NaN, the other argument is returned. If both arguments are NaN, NaN is returned.
fdim function returns the positive difference between
x and y. The positive difference is @math{x -
y} if x is greater than y, and @math{0} otherwise.
If x, y, or both are NaN, NaN is returned.
fma function performs floating-point multiply-add. This is
the operation @math{(x @mul{} y) + z}, but the
intermediate result is not rounded to the destination type. This can
sometimes improve the precision of a calculation.
This function was introduced because some processors have a special
instruction to perform multiply-add. The C compiler cannot use it
directly, because the expression `x*y + z' is defined to round the
intermediate result. fma lets you choose when you want to round
only once.
On processors which do not implement multiply-add in hardware,
fma can be very slow since it must avoid intermediate rounding.
`math.h' defines the symbols FP_FAST_FMA,
FP_FAST_FMAF, and FP_FAST_FMAL when the corresponding
version of fma is no slower than the expression `x*y + z'.
In the GNU C library, this always means the operation is implemented in
hardware.
ISO C99 introduces support for complex numbers in C. This is done
with a new type qualifier, complex. It is a keyword if and only
if `complex.h' has been included. There are three complex types,
corresponding to the three real types: float complex,
double complex, and long double complex.
To construct complex numbers you need a way to indicate the imaginary part of a number. There is no standard notation for an imaginary floating point constant. Instead, `complex.h' defines two macros that can be used to create complex numbers.
_Complex_I gives a
complex number whose value is purely imaginary. You can use this to
construct complex constants:
@math{3.0 + 4.0i} = 3.0 + 4.0 * _Complex_I
Note that _Complex_I * _Complex_I has the value -1, but
the type of that value is complex.
_Complex_I is a bit of a mouthful. `complex.h' also defines
a shorter name for the same constant.
_Complex_I. Most of the
time it is preferable. However, it causes problems if you want to use
the identifier I for something else. You can safely write
#include <complex.h> #undef I
if you need I for your own purposes. (In that case we recommend
you also define some other short name for _Complex_I, such as
J.)
ISO C99 also defines functions that perform basic operations on complex numbers, such as decomposition and conjugation. The prototypes for all these functions are in `complex.h'. All functions are available in three variants, one for each of the three complex types.
carg has a branch cut along the positive real axis.
INFINITY + I * copysign (0.0, cimag (z))
This section describes functions for "reading" integer and
floating-point numbers from a string. It may be more convenient in some
cases to use sscanf or one of the related functions; see
section Formatted Input. But often you can make a program more robust by
finding the tokens in the string by hand, then converting the numbers
one by one.
The `str' functions are declared in `stdlib.h' and those
beginning with `wcs' are declared in `wchar.h'. One might
wonder about the use of restrict in the prototypes of the
functions in this section. It is seemingly useless but the ISO C
standard uses it (for the functions defined there) so we have to do it
as well.
strtol ("string-to-long") function converts the initial
part of string to a signed integer, which is returned as a value
of type long int.
This function attempts to decompose string as follows:
isspace function
(see section Classification of Characters). These are discarded.
2 and 36.
If base is 16, the digits may optionally be preceded by
`0x' or `0X'. If base has no legal value the value returned
is 0l and the global variable errno is set to EINVAL.
strtol stores a pointer to this tail in
*tailptr.
If the string is empty, contains only whitespace, or does not contain an
initial substring that has the expected syntax for an integer in the
specified base, no conversion is performed. In this case,
strtol returns a value of zero and the value stored in
*tailptr is the value of string.
In a locale other than the standard "C" locale, this function
may recognize additional implementation-dependent syntax.
If the string has valid syntax for an integer but the value is not
representable because of overflow, strtol returns either
LONG_MAX or LONG_MIN (see section Range of an Integer Type), as
appropriate for the sign of the value. It also sets errno
to ERANGE to indicate there was overflow.
You should not check for errors by examining the return value of
strtol, because the string might be a valid representation of
0l, LONG_MAX, or LONG_MIN. Instead, check whether
tailptr points to what you expect after the number
(e.g. '\0' if the string should end after the number). You also
need to clear errno before the call and check it afterward, in
case there was overflow.
There is an example at the end of this section.
wcstol function is equivalent to the strtol function
in nearly all aspects but handles wide character strings.
The wcstol function was introduced in Amendment 1 of ISO C90.
strtoul ("string-to-unsigned-long") function is like
strtol except it converts to an unsigned long int value.
The syntax is the same as described above for strtol. The value
returned on overflow is ULONG_MAX (see section Range of an Integer Type).
If string depicts a negative number, strtoul acts the same
as strtol but casts the result to an unsigned integer. That means
for example that strtoul on "-1" returns ULONG_MAX
and an input more negative than LONG_MIN returns
(ULONG_MAX + 1) / 2.
strtoul sets errno to EINVAL if base is out of
range, or ERANGE on overflow.
wcstoul function is equivalent to the strtoul function
in nearly all aspects but handles wide character strings.
The wcstoul function was introduced in Amendment 1 of ISO C90.
strtoll function is like strtol except that it returns
a long long int value, and accepts numbers with a correspondingly
larger range.
If the string has valid syntax for an integer but the value is not
representable because of overflow, strtoll returns either
LONG_LONG_MAX or LONG_LONG_MIN (see section Range of an Integer Type), as
appropriate for the sign of the value. It also sets errno to
ERANGE to indicate there was overflow.
The strtoll function was introduced in ISO C99.
wcstoll function is equivalent to the strtoll function
in nearly all aspects but handles wide character strings.
The wcstoll function was introduced in Amendment 1 of ISO C90.
strtoq ("string-to-quad-word") is the BSD name for strtoll.
wcstoq function is equivalent to the strtoq function
in nearly all aspects but handles wide character strings.
The wcstoq function is a GNU extension.
strtoull function is related to strtoll the same way
strtoul is related to strtol.
The strtoull function was introduced in ISO C99.
wcstoull function is equivalent to the strtoull function
in nearly all aspects but handles wide character strings.
The wcstoull function was introduced in Amendment 1 of ISO C90.
strtouq is the BSD name for strtoull.
wcstouq function is equivalent to the strtouq function
in nearly all aspects but handles wide character strings.
The wcstoq function is a GNU extension.
strtoimax function is like strtol except that it returns
a intmax_t value, and accepts numbers of a corresponding range.
If the string has valid syntax for an integer but the value is not
representable because of overflow, strtoimax returns either
INTMAX_MAX or INTMAX_MIN (see section Integers), as
appropriate for the sign of the value. It also sets errno to
ERANGE to indicate there was overflow.
See section Integers for a description of the intmax_t type. The
strtoimax function was introduced in ISO C99.
wcstoimax function is equivalent to the strtoimax function
in nearly all aspects but handles wide character strings.
The wcstoimax function was introduced in ISO C99.
strtoumax function is related to strtoimax
the same way that strtoul is related to strtol.
See section Integers for a description of the intmax_t type. The
strtoumax function was introduced in ISO C99.
wcstoumax function is equivalent to the strtoumax function
in nearly all aspects but handles wide character strings.
The wcstoumax function was introduced in ISO C99.
strtol function with a base
argument of 10, except that it need not detect overflow errors.
The atol function is provided mostly for compatibility with
existing code; using strtol is more robust.
atol, except that it returns an int.
The atoi function is also considered obsolete; use strtol
instead.
atol, except it returns a long
long int.
The atoll function was introduced in ISO C99. It too is
obsolete (despite having just been added); use strtoll instead.
All the functions mentioned in this section so far do not handle
alternative representations of characters as described in the locale
data. Some locales specify thousands separator and the way they have to
be used which can help to make large numbers more readable. To read
such numbers one has to use the scanf functions with the `''
flag.
Here is a function which parses a string as a sequence of integers and returns the sum of them:
int
sum_ints_from_string (char *string)
{
int sum = 0;
while (1) {
char *tail;
int next;
/* Skip whitespace by hand, to detect the end. */
while (isspace (*string)) string++;
if (*string == 0)
break;
/* There is more nonwhitespace, */
/* so it ought to be another number. */
errno = 0;
/* Parse it. */
next = strtol (string, &tail, 0);
/* Add it in, if not overflow. */
if (errno)
printf ("Overflow\n");
else
sum += next;
/* Advance past it. */
string = tail;
}
return sum;
}
The `str' functions are declared in `stdlib.h' and those
beginning with `wcs' are declared in `wchar.h'. One might
wonder about the use of restrict in the prototypes of the
functions in this section. It is seemingly useless but the ISO C
standard uses it (for the functions defined there) so we have to do it
as well.
strtod ("string-to-double") function converts the initial
part of string to a floating-point number, which is returned as a
value of type double.
This function attempts to decompose string as follows:
isspace function
(see section Classification of Characters). These are discarded.
*tailptr.
If the string is empty, contains only whitespace, or does not contain an
initial substring that has the expected syntax for a floating-point
number, no conversion is performed. In this case, strtod returns
a value of zero and the value returned in *tailptr is the
value of string.
In a locale other than the standard "C" or "POSIX" locales,
this function may recognize additional locale-dependent syntax.
If the string has valid syntax for a floating-point number but the value
is outside the range of a double, strtod will signal
overflow or underflow as described in section Error Reporting by Mathematical Functions.
strtod recognizes four special input strings. The strings
"inf" and "infinity" are converted to @math{@infinity{}},
or to the largest representable value if the floating-point format
doesn't support infinities. You can prepend a "+" or "-"
to specify the sign. Case is ignored when scanning these strings.
The strings "nan" and "nan(chars...)" are converted
to NaN. Again, case is ignored. If chars... are provided, they
are used in some unspecified fashion to select a particular
representation of NaN (there can be several).
Since zero is a valid result as well as the value returned on error, you
should check for errors in the same way as for strtol, by
examining errno and tailptr.
strtod, but return float
and long double values respectively. They report errors in the
same way as strtod. strtof can be substantially faster
than strtod, but has less precision; conversely, strtold
can be much slower but has more precision (on systems where long
double is a separate type).
These functions have been GNU extensions and are new to ISO C99.
wcstod, wcstof, and wcstol functions are
equivalent in nearly all aspect to the strtod, strtof, and
strtold functions but it handles wide character string.
The wcstod function was introduced in Amendment 1 of ISO
C90. The wcstof and wcstold functions were introduced in
ISO C99.
strtod function, except that it
need not detect overflow and underflow errors. The atof function
is provided mostly for compatibility with existing code; using
strtod is more robust.
The GNU C library also provides `_l' versions of these functions, which take an additional argument, the locale to use in conversion. See section Parsing of Integers.
The old System V C library provided three functions to convert numbers to strings, with unusual and hard-to-use semantics. The GNU C library also provides these functions and some natural extensions.
These functions are only available in glibc and on systems descended
from AT&T Unix. Therefore, unless these functions do precisely what you
need, it is better to use sprintf, which is standard.
All these functions are defined in `stdlib.h'.
ecvt converts the floating-point number value
to a string with at most ndigit decimal digits. The
returned string contains no decimal point or sign. The first digit of
the string is non-zero (unless value is actually zero) and the
last digit is rounded to nearest. *decpt is set to the
index in the string of the first digit after the decimal point.
*neg is set to a nonzero value if value is negative,
zero otherwise.
If ndigit decimal digits would exceed the precision of a
double it is reduced to a system-specific value.
The returned string is statically allocated and overwritten by each call
to ecvt.
If value is zero, it is implementation defined whether
*decpt is 0 or 1.
For example: ecvt (12.3, 5, &d, &n) returns "12300"
and sets d to 2 and n to 0.
fcvt is like ecvt, but ndigit specifies
the number of digits after the decimal point. If ndigit is less
than zero, value is rounded to the @math{ndigit+1}'th place to the
left of the decimal point. For example, if ndigit is -1,
value will be rounded to the nearest 10. If ndigit is
negative and larger than the number of digits to the left of the decimal
point in value, value will be rounded to one significant digit.
If ndigit decimal digits would exceed the precision of a
double it is reduced to a system-specific value.
The returned string is statically allocated and overwritten by each call
to fcvt.
gcvt is functionally equivalent to `sprintf(buf, "%*g",
ndigit, value'. It is provided only for compatibility's sake. It
returns buf.
If ndigit decimal digits would exceed the precision of a
double it is reduced to a system-specific value.
As extensions, the GNU C library provides versions of these three
functions that take long double arguments.
ecvt except that it takes a
long double for the first parameter and that ndigit is
restricted by the precision of a long double.
fcvt except that it
takes a long double for the first parameter and that ndigit is
restricted by the precision of a long double.
gcvt except that it takes a
long double for the first parameter and that ndigit is
restricted by the precision of a long double.
The ecvt and fcvt functions, and their long double
equivalents, all return a string located in a static buffer which is
overwritten by the next call to the function. The GNU C library
provides another set of extended functions which write the converted
string into a user-supplied buffer. These have the conventional
_r suffix.
gcvt_r is not necessary, because gcvt already uses a
user-supplied buffer.
ecvt_r function is the same as ecvt, except
that it places its result into the user-specified buffer pointed to by
buf, with length len.
This function is a GNU extension.
fcvt_r function is the same as fcvt, except
that it places its result into the user-specified buffer pointed to by
buf, with length len.
This function is a GNU extension.
qecvt_r function is the same as qecvt, except
that it places its result into the user-specified buffer pointed to by
buf, with length len.
This function is a GNU extension.
qfcvt_r function is the same as qfcvt, except
that it places its result into the user-specified buffer pointed to by
buf, with length len.
This function is a GNU extension.
This chapter describes functions for manipulating dates and times, including functions for determining what time it is and conversion between different time representations.
Discussing time in a technical manual can be difficult because the word "time" in English refers to lots of different things. In this manual, we use a rigorous terminology to avoid confusion, and the only thing we use the simple word "time" for is to talk about the abstract concept.
A calendar time is a point in the time continuum, for example November 4, 1990 at 18:02.5 UTC. Sometimes this is called "absolute time".
We don't speak of a "date", because that is inherent in a calendar time.
An interval is a contiguous part of the time continuum between two calendar times, for example the hour between 9:00 and 10:00 on July 4, 1980.
An elapsed time is the length of an interval, for example, 35 minutes. People sometimes sloppily use the word "interval" to refer to the elapsed time of some interval.
An amount of time is a sum of elapsed times, which need not be of any specific intervals. For example, the amount of time it takes to read a book might be 9 hours, independently of when and in how many sittings it is read.
A period is the elapsed time of an interval between two events, especially when they are part of a sequence of regularly repeating events.
CPU time is like calendar time, except that it is based on the subset of the time continuum when a particular process is actively using a CPU. CPU time is, therefore, relative to a process.
Processor time is an amount of time that a CPU is in use. In fact, it's a basic system resource, since there's a limit to how much can exist in any given interval (that limit is the elapsed time of the interval times the number of CPUs in the processor). People often call this CPU time, but we reserve the latter term in this manual for the definition above.
One way to represent an elapsed time is with a simple arithmetic data type, as with the following function to compute the elapsed time between two calendar times. This function is declared in `time.h'.
difftime function returns the number of seconds of elapsed
time between calendar time time1 and calendar time time0, as
a value of type double. The difference ignores leap seconds
unless leap second support is enabled.
In the GNU system, you can simply subtract time_t values. But on
other systems, the time_t data type might use some other encoding
where subtraction doesn't work directly.
The GNU C library provides two data types specifically for representing an elapsed time. They are used by various GNU C library functions, and you can use them for your own purposes too. They're exactly the same except that one has a resolution in microseconds, and the other, newer one, is in nanoseconds.
struct timeval structure represents an elapsed time. It is
declared in `sys/time.h' and has the following members:
long int tv_sec
long int tv_usec
struct timespec structure represents an elapsed time. It is
declared in `time.h' and has the following members:
long int tv_sec
long int tv_nsec
It is often necessary to subtract two values of type struct
timeval or struct timespec. Here is the best way to do
this. It works even on some peculiar operating systems where the
tv_sec member has an unsigned type.
/* Subtract the `struct timeval' values X and Y,
storing the result in RESULT.
Return 1 if the difference is negative, otherwise 0. */
int
timeval_subtract (result, x, y)
struct timeval *result, *x, *y;
{
/* Perform the carry for the later subtraction by updating y. */
if (x->tv_usec < y->tv_usec) {
int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
y->tv_usec -= 1000000 * nsec;
y->tv_sec += nsec;
}
if (x->tv_usec - y->tv_usec > 1000000) {
int nsec = (x->tv_usec - y->tv_usec) / 1000000;
y->tv_usec += 1000000 * nsec;
y->tv_sec -= nsec;
}
/* Compute the time remaining to wait.
tv_usec is certainly positive. */
result->tv_sec = x->tv_sec - y->tv_sec;
result->tv_usec = x->tv_usec - y->tv_usec;
/* Return 1 if result is negative. */
return x->tv_sec < y->tv_sec;
}
Common functions that use struct timeval are gettimeofday
and settimeofday.
There are no GNU C library functions specifically oriented toward dealing with elapsed times, but the calendar time, processor time, and alarm and sleeping functions have a lot to do with them.
If you're trying to optimize your program or measure its efficiency, it's very useful to know how much processor time it uses. For that, calendar time and elapsed times are useless because a process may spend time waiting for I/O or for other processes to use the CPU. However, you can get the information with the functions in this section.
CPU time (see section Time Basics) is represented by the data type
clock_t, which is a number of clock ticks. It gives the
total amount of time a process has actively used a CPU since some
arbitrary event. On the GNU system, that event is the creation of the
process. While arbitrary in general, the event is always the same event
for any particular process, so you can always measure how much time on
the CPU a particular computation takes by examinining the process' CPU
time before and after the computation.
In the GNU system, clock_t is equivalent to long int and
CLOCKS_PER_SEC is an integer value. But in other systems, both
clock_t and the macro CLOCKS_PER_SEC can be either integer
or floating-point types. Casting CPU time values to double, as
in the example above, makes sure that operations such as arithmetic and
printing work properly and consistently no matter what the underlying
representation is.
Note that the clock can wrap around. On a 32bit system with
CLOCKS_PER_SEC set to one million this function will return the
same value approximately every 72 minutes.
For additional functions to examine a process' use of processor time, and to control it, See section Resource Usage And Limitation.
To get a process' CPU time, you can use the clock function. This
facility is declared in the header file `time.h'.
In typical usage, you call the clock function at the beginning
and end of the interval you want to time, subtract the values, and then
divide by CLOCKS_PER_SEC (the number of clock ticks per second)
to get processor time, like this:
#include <time.h> clock_t start, end; double cpu_time_used; start = clock(); ... /* Do the work. */ end = clock(); cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
Do not use a single CPU time as an amount of time; it doesn't work that way. Either do a subtraction as shown above or query processor time directly. See section Processor Time Inquiry.
Different computers and operating systems vary wildly in how they keep track of CPU time. It's common for the internal processor clock to have a resolution somewhere between a hundredth and millionth of a second.
clock function. POSIX requires that this value be one
million independent of the actual resolution.
clock function.
Values of type clock_t are numbers of clock ticks.
clock returns the
value (clock_t)(-1).
The times function returns information about a process'
consumption of processor time in a struct tms object, in
addition to the process' CPU time. See section Time Basics. You should
include the header file `sys/times.h' to use this facility.
tms structure is used to return information about process
times. It contains at least the following members:
clock_t tms_utime
clock_t tms_stime
clock_t tms_cutime
tms_utime values and the tms_cutime
values of all terminated child processes of the calling process, whose
status has been reported to the parent process by wait or
waitpid; see section Process Completion. In other words, it
represents the total processor time used in executing the instructions
of all the terminated child processes of the calling process, excluding
child processes which have not yet been reported by wait or
waitpid.
clock_t tms_cstime
tms_cutime, but represents the total processor
time system has used on behalf of all the terminated child processes
of the calling process.
All of the times are given in numbers of clock ticks. Unlike CPU time, these are the actual amounts of time; not relative to any event. See section Creating a Process.
times function stores the processor time information for
the calling process in buffer.
The return value is the calling process' CPU time (the same value you
get from clock(). times returns (clock_t)(-1) to
indicate failure.
Portability Note: The clock function described in
section CPU Time Inquiry is specified by the ISO C standard. The
times function is a feature of POSIX.1. In the GNU system, the
CPU time is defined to be equivalent to the sum of the tms_utime
and tms_stime fields returned by times.
This section describes facilities for keeping track of calendar time. See section Time Basics.
The GNU C library represents calendar time three ways:
time_t data type) is a compact
representation, typically giving the number of seconds of elapsed time
since some implementation-specific base time.
struct
timeval data type, which includes fractions of a second. Use this time
representation instead of simple time when you need greater precision.
struct tm data
type) represents a calendar time as a set of components specifying the
year, month, and so on in the Gregorian calendar, for a specific time
zone. This calendar time representation is usually used only to
communicate with people.
This section describes the time_t data type for representing calendar
time as simple time, and the functions which operate on simple time objects.
These facilities are declared in the header file `time.h'.
TZ to certain values (see section Specifying the Time Zone with TZ).
Note that a simple time has no concept of local time zone. Calendar Time T is the same instant in time regardless of where on the globe the computer is.
In the GNU C library, time_t is equivalent to long int.
In other systems, time_t might be either an integer or
floating-point type.
The function difftime tells you the elapsed time between two
simple calendar times, which is not always as easy to compute as just
subtracting. See section Elapsed Time.
time function returns the current calendar time as a value of
type time_t. If the argument result is not a null pointer,
the calendar time value is also stored in *result. If the
current calendar time is not available, the value
(time_t)(-1) is returned.
stime sets the system clock, i.e. it tells the system that the
current calendar time is newtime, where newtime is
interpreted as described in the above definition of time_t.
settimeofday is a newer function which sets the system clock to
better than one second precision. settimeofday is generally a
better choice than stime. See section High-Resolution Calendar.
Only the superuser can set the system clock.
If the function succeeds, the return value is zero. Otherwise, it is
-1 and errno is set accordingly:
EPERM
The time_t data type used to represent simple times has a
resolution of only one second. Some applications need more precision.
So, the GNU C library also contains functions which are capable of representing calendar times to a higher resolution than one second. The functions and the associated data types described in this section are declared in `sys/time.h'.
struct timezone structure is used to hold minimal information
about the local time zone. It has the following members:
int tz_minuteswest
int tz_dsttime
The struct timezone type is obsolete and should never be used.
Instead, use the facilities described in section Functions and Variables for Time Zones.
gettimeofday function returns the current calendar time as
the elapsed time since the epoch in the struct timeval structure
indicated by tp. (see section Elapsed Time for a description of
struct timespec). Information about the time zone is returned in
the structure pointed at tzp. If the tzp argument is a null
pointer, time zone information is ignored.
The return value is 0 on success and -1 on failure. The
following errno error condition is defined for this function:
ENOSYS
struct timezone to represent time zone
information; that is an obsolete feature of 4.3 BSD.
Instead, use the facilities described in section Functions and Variables for Time Zones.
settimeofday function sets the current calendar time in the
system clock according to the arguments. As for gettimeofday,
the calendar time is represented as the elapsed time since the epoch.
As for gettimeofday, time zone information is ignored if
tzp is a null pointer.
You must be a privileged user in order to use settimeofday.
Some kernels automatically set the system clock from some source such as
a hardware clock when they start up. Others, including Linux, place the
system clock in an "invalid" state (in which attempts to read the clock
fail). A call of stime removes the system clock from an invalid
state, and system startup scripts typically run a program that calls
stime.
settimeofday causes a sudden jump forwards or backwards, which
can cause a variety of problems in a system. Use adjtime (below)
to make a smooth transition from one time to another by temporarily
speeding up or slowing down the clock.
With a Linux kernel, adjtimex does the same thing and can also
make permanent changes to the speed of the system clock so it doesn't
need to be corrected as often.
The return value is 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
EPERM
ENOSYS
The delta argument specifies a relative adjustment to be made to the clock time. If negative, the system clock is slowed down for a while until it has lost this much elapsed time. If positive, the system clock is speeded up for a while.
If the olddelta argument is not a null pointer, the adjtime
function returns information about any previous time adjustment that
has not yet completed.
This function is typically used to synchronize the clocks of computers in a local network. You must be a privileged user to use it.
With a Linux kernel, you can use the adjtimex function to
permanently change the clock speed.
The return value is 0 on success and -1 on failure. The
following errno error condition is defined for this function:
EPERM
Portability Note: The gettimeofday, settimeofday,
and adjtime functions are derived from BSD.
Symbols for the following function are declared in `sys/timex.h'.
adjtimex is functionally identical to ntp_adjtime.
See section High Accuracy Clock.
This function is present only with a Linux kernel.
Calendar time is represented by the usual GNU C library functions as an elapsed time since a fixed base calendar time. This is convenient for computation, but has no relation to the way people normally think of calendar time. By contrast, broken-down time is a binary representation of calendar time separated into year, month, day, and so on. Broken-down time values are not useful for calculations, but they are useful for printing human readable time information.
A broken-down time value is always relative to a choice of time zone, and it also indicates which time zone that is.
The symbols in this section are declared in the header file `time.h'.
int tm_sec
0 through 59, but the actual upper limit is
60, to allow for leap seconds if leap second support is
available).
int tm_min
0 through 59).
int tm_hour
0 through
23).
int tm_mday
1 through 31).
Watch out for this one! As the only ordinal number in the structure, it is
inconsistent with the rest of the structure.
int tm_mon
0 through 11). Watch out for this one!
People usually use ordinal numbers for month-of-year (where January = 1).
int tm_year
int tm_wday
0 through
6).
int tm_yday
0 through 365).
int tm_isdst
long int tm_gmtoff
-5*60*60.
The tm_gmtoff field is derived from BSD and is a GNU library
extension; it is not visible in a strict ISO C environment.
const char *tm_zone
tm_gmtoff, this field is a BSD and
GNU extension, and is not visible in a strict ISO C environment.
localtime function converts the simple time pointed to by
time to broken-down time representation, expressed relative to the
user's specified time zone.
The return value is a pointer to a static broken-down time structure, which
might be overwritten by subsequent calls to ctime, gmtime,
or localtime. (But no other library function overwrites the contents
of this object.)
The return value is the null pointer if time cannot be represented
as a broken-down time; typically this is because the year cannot fit into
an int.
Calling localtime has one other effect: it sets the variable
tzname with information about the current time zone. See section Functions and Variables for Time Zones.
Using the localtime function is a big problem in multi-threaded
programs. The result is returned in a static buffer and this is used in
all threads. POSIX.1c introduced a variant of this function.
localtime_r function works just like the localtime
function. It takes a pointer to a variable containing a simple time
and converts it to the broken-down time format.
But the result is not placed in a static buffer. Instead it is placed
in the object of type struct tm to which the parameter
resultp points.
If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.
localtime, except that the broken-down
time is expressed as Coordinated Universal Time (UTC) (formerly called
Greenwich Mean Time (GMT)) rather than relative to a local time zone.
As for the localtime function we have the problem that the result
is placed in a static variable. POSIX.1c also provides a replacement for
gmtime.
localtime_r, except that it converts
just like gmtime the given time as Coordinated Universal Time.
If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.
mktime function is used to convert a broken-down time structure
to a simple time representation. It also "normalizes" the contents of
the broken-down time structure, by filling in the day of week and day of
year based on the other date and time components.
The mktime function ignores the specified contents of the
tm_wday and tm_yday members of the broken-down time
structure. It uses the values of the other components to determine the
calendar time; it's permissible for these components to have
unnormalized values outside their normal ranges. The last thing that
mktime does is adjust the components of the brokentime
structure (including the tm_wday and tm_yday).
If the specified broken-down time cannot be represented as a simple time,
mktime returns a value of (time_t)(-1) and does not modify
the contents of brokentime.
Calling mktime also sets the variable tzname with
information about the current time zone. See section Functions and Variables for Time Zones.
timelocal is functionally identical to mktime, but more
mnemonically named. Note that it is the inverse of the localtime
function.
Portability note: mktime is essentially universally
available. timelocal is rather rare.
timegm is functionally identical to mktime except it
always takes the input values to be Coordinated Universal Time (UTC)
regardless of any local time zone setting.
Note that timegm is the inverse of gmtime.
Portability note: mktime is essentially universally
available. timegm is rather rare. For the most portable
conversion from a UTC broken-down time to a simple time, set
the TZ environment variable to UTC, call mktime, then set
TZ back.
The ntp_gettime and ntp_adjtime functions provide an
interface to monitor and manipulate the system clock to maintain high
accuracy time. For example, you can fine tune the speed of the clock
or synchronize it with another time source.
A typical use of these functions is by a server implementing the Network Time Protocol to synchronize the clocks of multiple systems and high precision clocks.
These functions are declared in `sys/timex.h'.
struct timeval time
struct timeval data type is described in
section Elapsed Time.
long int maxerror
ntp_adjtime periodically, this value will reach some
platform-specific maximum value.
long int esterror
ntp_adjtime to indicate the estimated offset of the
system clock from the true calendar time.
ntp_gettime function sets the structure pointed to by
tptr to current values. The elements of the structure afterwards
contain the values the timer implementation in the kernel assumes. They
might or might not be correct. If they are not a ntp_adjtime
call is necessary.
The return value is 0 on success and other values on failure. The
following errno error conditions are defined for this function:
TIME_ERROR
unsigned int modes
MOD_.
long int offset
MOD_OFFSET is set in modes, the offset (and possibly other
dependent values) can be set. The offset's absolute value must not
exceed MAXPHASE.
long int frequency
1 <<
SHIFT_USEC. The value can be set with bit MOD_FREQUENCY, but
the absolute value must not exceed MAXFREQ.
long int maxerror
MOD_MAXERROR. Unless updated via
ntp_adjtime periodically, this value will increase steadily
and reach some platform-specific maximum value.
long int esterror
MOD_ESTERROR.
int status
STA_. Some of these flags can be updated using the
MOD_STATUS bit.
long int constant
MOD_TIMECONST.
long int precision
long int tolerance
maxerror every
second.
struct timeval time
long int tick
long int ppsfreq
long int jitter
int shift
PPS_SHIFT to PPS_SHIFTMAX.
long int stabil
long int jitcnt
MAXTIME.
long int calcnt
long int errcnt
long int stbcnt
ntp_adjtime function sets the structure specified by
tptr to current values.
In addition, ntp_adjtime updates some settings to match what you
pass to it in *tptr. Use the modes element of *tptr
to select what settings to update. You can set offset,
freq, maxerror, esterror, status,
constant, and tick.
modes = zero means set nothing.
Only the superuser can update settings.
The return value is 0 on success and other values on failure. The
following errno error conditions are defined for this function:
TIME_ERROR
EPERM
For more details see RFC1305 (Network Time Protocol, Version 3) and related documents.
Portability note: Early versions of the GNU C library did not
have this function but did have the synonymous adjtimex.
The functions described in this section format calendar time values as strings. These functions are declared in the header file `time.h'.
asctime function converts the broken-down time value that
brokentime points to into a string in a standard format:
"Tue May 21 13:46:22 1991\n"
The abbreviations for the days of week are: `Sun', `Mon', `Tue', `Wed', `Thu', `Fri', and `Sat'.
The abbreviations for the months are: `Jan', `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and `Dec'.
The return value points to a statically allocated string, which might be
overwritten by subsequent calls to asctime or ctime.
(But no other library function overwrites the contents of this
string.)
asctime but instead of placing the
result in a static buffer it writes the string in the buffer pointed to
by the parameter buffer. This buffer should have room
for at least 26 bytes, including the terminating null.
If no error occurred the function returns a pointer to the string the
result was written into, i.e., it returns buffer. Otherwise
return NULL.
ctime function is similar to asctime, except that you
specify the calendar time argument as a time_t simple time value
rather than in broken-down local time format. It is equivalent to
asctime (localtime (time))
ctime sets the variable tzname, because localtime
does so. See section Functions and Variables for Time Zones.
ctime, but places the result in the
string pointed to by buffer. It is equivalent to (written using
gcc extensions, see section `Statement Exprs' in Porting and Using gcc):
({ struct tm tm; asctime_r (localtime_r (time, &tm), buf); })
If no error occurred the function returns a pointer to the string the
result was written into, i.e., it returns buffer. Otherwise
return NULL.
sprintf function (see section Formatted Input), but the conversion specifications that can appear in the format
template template are specialized for printing components of the date
and time brokentime according to the locale currently specified for
time conversion (see section Locales and Internationalization).
Ordinary characters appearing in the template are copied to the output string s; this can include multibyte character sequences. Conversion specifiers are introduced by a `%' character, followed by an optional flag which can be one of the following. These flags are all GNU extensions. The first three affect only the output of numbers:
_
-
0
^
The default action is to pad the number with zeros to keep it a constant width. Numbers that do not have a range indicated below are never padded, since there is no natural width for them.
Following the flag an optional specification of the width is possible. This is specified in decimal notation. If the natural size of the output is of the field has less than the specified number of characters, the result is written right adjusted and space padded to the given size.
An optional modifier can follow the optional flag and width specification. The modifiers, which are POSIX.2 extensions, are:
E
%c, %C, %x, %X,
%y and %Y format specifiers. In a Japanese locale, for
example, %Ex might yield a date format based on the Japanese
Emperors' reigns.
O
If the format supports the modifier but no alternate representation is available, it is ignored.
The conversion specifier ends with a format specifier taken from the following list. The whole `%' sequence is replaced in the output string as follows:
%a
%A
%b
%B
%c
%C
%d
01 through 31).
%D
%m/%d/%y.
This format is a POSIX.2 extension and also appears in ISO C99.
%e
%d, but padded with blank (range
1 through 31).
This format is a POSIX.2 extension and also appears in ISO C99.
%F
%Y-%m-%d. This is the form specified
in the ISO 8601 standard and is the preferred form for all uses.
This format is a ISO C99 extension.
%g
00 through 99). This has the same format and value
as %y, except that if the ISO week number (see %V) belongs
to the previous or next year, that year is used instead.
This format was introduced in ISO C99.
%G
%Y, except that if the ISO week number (see
%V) belongs to the previous or next year, that year is used
instead.
This format was introduced in ISO C99 but was previously available
as a GNU extension.
%h
%b.
This format is a POSIX.2 extension and also appears in ISO C99.
%H
00 through
23).
%I
01 through
12).
%j
001 through 366).
%k
%H, but
padded with blank (range 0 through 23).
This format is a GNU extension.
%l
%I, but
padded with blank (range 1 through 12).
This format is a GNU extension.
%m
01 through 12).
%M
00 through 59).
%n
%p
%P
%r
%R
%H:%M.
This format was introduced in ISO C99 but was previously available
as a GNU extension.
%s
%S
00 through 60).
%t
%T
%H:%M:%S.
This format is a POSIX.2 extension.
%u
1 through
7), Monday being 1.
This format is a POSIX.2 extension and also appears in ISO C99.
%U
00
through 53), starting with the first Sunday as the first day of
the first week. Days preceding the first Sunday in the year are
considered to be in week 00.
%V
01
through 53). ISO weeks start with Monday and end with Sunday.
Week 01 of a year is the first week which has the majority of its
days in that year; this is equivalent to the week containing the year's
first Thursday, and it is also equivalent to the week containing January
4. Week 01 of a year can contain days from the previous year.
The week before week 01 of a year is the last week (52 or
53) of the previous year even if it contains days from the new
year.
This format is a POSIX.2 extension and also appears in ISO C99.
%w
0 through
6), Sunday being 0.
%W
00
through 53), starting with the first Monday as the first day of
the first week. All days preceding the first Monday in the year are
considered to be in week 00.
%x
%X
%y
00 through
99). This is equivalent to the year modulo 100.
%Y
1 are numbered 0, -1, and so on.
%z
-0600 or +0100), or nothing if no time zone is
determinable.
This format was introduced in ISO C99 but was previously available
as a GNU extension.
A full RFC 822 timestamp is generated by the format
`"%a, %d %b %Y %H:%M:%S %z"' (or the equivalent
`"%a, %d %b %Y %T %z"').
%Z
%%
The size parameter can be used to specify the maximum number of
characters to be stored in the array s, including the terminating
null character. If the formatted time requires more than size
characters, strftime returns zero and the contents of the array
s are undefined. Otherwise the return value indicates the
number of characters placed in the array s, not including the
terminating null character.
Warning: This convention for the return value which is prescribed
in ISO C can lead to problems in some situations. For certain
format strings and certain locales the output really can be the empty
string and this cannot be discovered by testing the return value only.
E.g., in most locales the AM/PM time format is not supported (most of
the world uses the 24 hour time representation). In such locales
"%p" will return the empty string, i.e., the return value is
zero. To detect situations like this something similar to the following
code should be used:
buf[0] = '\1';
len = strftime (buf, bufsize, format, tp);
if (len == 0 && buf[0] != '\0')
{
/* Something went wrong in the strftime call. */
...
}
If s is a null pointer, strftime does not actually write
anything, but instead returns the number of characters it would have written.
According to POSIX.1 every call to strftime implies a call to
tzset. So the contents of the environment variable TZ
is examined before any output is produced.
For an example of strftime, see section Time Functions Example.
wcsftime function is equivalent to the strftime
function with the difference that it operates on wide character
strings. The buffer where the result is stored, pointed to by s,
must be an array of wide characters. The parameter size which
specifies the size of the output buffer gives the number of wide
character, not the number of bytes.
Also the format string template is a wide character string. Since
all characters needed to specify the format string are in the basic
character set it is portably possible to write format strings in the C
source code using the L"..." notation. The parameter
brokentime has the same meaning as in the strftime call.
The wcsftime function supports the same flags, modifiers, and
format specifiers as the strftime function.
The return value of wcsftime is the number of wide characters
stored in s. When more characters would have to be written than
can be placed in the buffer s the return value is zero, with the
same problems indicated in the strftime documentation.
The ISO C standard does not specify any functions which can convert
the output of the strftime function back into a binary format.
This led to a variety of more-or-less successful implementations with
different interfaces over the years. Then the Unix standard was
extended by the addition of two functions: strptime and
getdate. Both have strange interfaces but at least they are
widely available.
he first function is rather low-level. It is nevertheless frequently
used in software since it is better known. Its interface and
implementation are heavily influenced by the getdate function,
which is defined and implemented in terms of calls to strptime.
strptime function parses the input string s according
to the format string fmt and stores its results in the
structure tp.
The input string could be generated by a strftime call or
obtained any other way. It does not need to be in a human-recognizable
format; e.g. a date passed as "02:1999:9" is acceptable, even
though it is ambiguous without context. As long as the format string
fmt matches the input string the function will succeed.
The format string consists of the same components as the format string
of the strftime function. The only difference is that the flags
_, -, 0, and ^ are not allowed.
Several of the distinct formats of strftime do the same work in
strptime since differences like case of the input do not matter.
For reasons of symmetry all formats are supported, though.
The modifiers E and O are also allowed everywhere the
strftime function allows them.
The formats are:
%a
%A
%b
%B
%h
%c
%Ec
%c but the locale's alternative date and time format is used.
%C
%y format.
%EC
%C it sometimes makes sense to use this format since some
cultures represent years relative to the beginning of eras instead of
using the Gregorian years.
%d
%e
1 through 31).
Leading zeroes are permitted but not required.
%Od
%Oe
%d but using the locale's alternative numeric symbols.
Leading zeroes are permitted but not required.
%D
%m/%d/%y.
%F
%Y-%m-%d, which is the ISO 8601 date
format.
This is a GNU extension following an ISO C99 extension to
strftime.
%g
00 through 99).
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
This format is a GNU extension following a GNU extension of strftime.
%G
strftime.
%H
%k
00 through
23).
%k is a GNU extension following a GNU extension of strftime.
%OH
%H but using the locale's alternative numeric symbols.
%I
%l
01 through
12).
%l is a GNU extension following a GNU extension of strftime.
%OI
%I but using the locale's alternative numeric symbols.
%j
1 through 366).
Leading zeroes are permitted but not required.
%m
1 through 12).
Leading zeroes are permitted but not required.
%Om
%m but using the locale's alternative numeric symbols.
%M
0 through 59).
Leading zeroes are permitted but not required.
%OM
%M but using the locale's alternative numeric symbols.
%n
%t
%p
%P
%I or %l is also used.
Another complication is that the locale might not define these values at
all and therefore the conversion fails.
%P is a GNU extension following a GNU extension to strftime.
%r
%R
%H:%M.
%R is a GNU extension following a GNU extension to strftime.
%s
%s is a GNU extension following a GNU extension to strftime.
%S
0 through 60).
Leading zeroes are permitted but not required.
Note: The Unix specification says the upper bound on this value
is 61, a result of a decision to allow double leap seconds. You
will not see the value 61 because no minute has more than one
leap second, but the myth persists.
%OS
%S but using the locale's alternative numeric symbols.
%T
%H:%M:%S in this place.
%u
1 through
7), Monday being 1.
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
%U
0
through 53).
Leading zeroes are permitted but not required.
%OU
%U but using the locale's alternative numeric symbols.
%V
1
through 53).
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
%w
0 through
6), Sunday being 0.
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
%Ow
%w but using the locale's alternative numeric symbols.
%W
0
through 53).
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
%OW
%W but using the locale's alternative numeric symbols.
%x
%Ex
%x but the locale's alternative data representation is used.
%X
%EX
%X but the locale's alternative time representation is used.
%y
0 through
99).
Leading zeroes are permitted but not required.
Note that it is questionable to use this format without
the %C format. The strptime function does regard input
values in the range @math{68} to @math{99} as the years @math{1969} to
@math{1999} and the values @math{0} to @math{68} as the years
@math{2000} to @math{2068}. But maybe this heuristic fails for some
input data.
Therefore it is best to avoid %y completely and use %Y
instead.
%Ey
%EC in the locale's alternative representation.
%Oy
%C) using the locale's alternative
numeric symbols.
%Y
%EY
%z
%a, %d %b %Y %H:%M:%S %z in this place.
This is the full ISO 8601 date and time format.
%Z
%%
All other characters in the format string must have a matching character in the input string. Exceptions are white spaces in the input string which can match zero or more white space characters in the format string.
The strptime function processes the input string from right to
left. Each of the three possible input elements (white space, literal,
or format) are handled one after the other. If the input cannot be
matched to the format string the function stops. The remainder of the
format and input strings are not processed.
The function returns a pointer to the first character it was unable to
process. If the input string contains more characters than required by
the format string the return value points right after the last consumed
input character. If the whole input string is consumed the return value
points to the NULL byte at the end of the string. If an error
occurs, i.e. strptime fails to match all of the format string,
the function returns NULL.
The specification of the function in the XPG standard is rather vague, leaving out a few important pieces of information. Most importantly, it does not specify what happens to those elements of tm which are not directly initialized by the different formats. The implementations on different Unix systems vary here.
The GNU libc implementation does not touch those fields which are not
directly initialized. Exceptions are the tm_wday and
tm_yday elements, which are recomputed if any of the year, month,
or date elements changed. This has two implications:
strptime function for a new input string, you
should prepare the tm structure you pass. Normally this will mean
initializing all values are to zero. Alternatively, you can set all
fields to values like INT_MAX, allowing you to determine which
elements were set by the function call. Zero does not work here since
it is a valid value for many of the fields.
Careful initialization is necessary if you want to find out whether a
certain field in tm was initialized by the function call.
struct tm value with several consecutive
strptime calls. A useful application of this is e.g. the parsing
of two separate strings, one containing date information and the other
time information. By parsing one after the other without clearing the
structure in-between, you can construct a complete broken-down time.
The following example shows a function which parses a string which is contains the date information in either US style or ISO 8601 form:
const char *
parse_date (const char *input, struct tm *tm)
{
const char *cp;
/* First clear the result structure. */
memset (tm, '\0', sizeof (*tm));
/* Try the ISO format first. */
cp = strptime (input, "%F", tm);
if (cp == NULL)
{
/* Does not match. Try the US form. */
cp = strptime (input, "%D", tm);
}
return cp;
}
The Unix standard defines another function for parsing date strings. The interface is weird, but if the function happens to suit your application it is just fine. It is problematic to use this function in multi-threaded programs or libraries, since it returns a pointer to a static variable, and uses a global variable and global state (an environment variable).
int contains the error code of the last
unsuccessful call to getdate. Defined values are:
DATEMSK is not defined or null.
DATEMSK environment variable
cannot be opened.
time_t variable.
getdate is the simplest possible for a function
to parse a string and return the value. string is the input
string and the result is returned in a statically-allocated variable.
The details about how the string is processed are hidden from the user.
In fact, they can be outside the control of the program. Which formats
are recognized is controlled by the file named by the environment
variable DATEMSK. This file should contain
lines of valid format strings which could be passed to strptime.
The getdate function reads these format strings one after the
other and tries to match the input string. The first line which
completely matches the input string is used.
Elements not initialized through the format string retain the values
present at the time of the getdate function call.
The formats recognized by getdate are the same as for
strptime. See above for an explanation. There are only a few
extensions to the strptime behavior:
%Z format is given the broken-down time is based on the
current time of the timezone matched, not of the current timezone of the
runtime environment.
Note: This is not implemented (currently). The problem is that
timezone names are not unique. If a fixed timezone is assumed for a
given string (say EST meaning US East Coast time), then uses for
countries other than the USA will fail. So far we have found no good
solution to this.
tm_wday
value the current week's day is chosen, otherwise the day next week is chosen.
It should be noted that the format in the template file need not only contain format elements. The following is a list of possible format strings (taken from the Unix standard):
%m %A %B %d, %Y %H:%M:%S %A %B %m/%d/%y %I %p %d,%m,%Y %H:%M at %A the %dst of %B in %Y run job at %I %p,%B %dnd %A den %d. %B %Y %H.%M Uhr
As you can see, the template list can contain very specific strings like
run job at %I %p,%B %dnd. Using the above list of templates and
assuming the current time is Mon Sep 22 12:19:47 EDT 1986 we can obtain the
following results for the given input.
@multitable {xxxxxxxxxxxx} {xxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx}
struct tm, or a null pointer if an error occurred. The
result is only valid until the next getdate call, making this
function unusable in multi-threaded applications.
The errno variable is not changed. Error conditions are
stored in the global variable getdate_err. See the
description above for a list of the possible error values.
Warning: The getdate function should never be
used in SUID-programs. The reason is obvious: using the
DATEMSK environment variable you can get the function to open
any arbitrary file and chances are high that with some bogus input
(such as a binary file) the program will crash.
getdate_r function is the reentrant counterpart of
getdate. It does not use the global variable getdate_err
to signal an error, but instead returns an error code. The same error
codes as described in the getdate_err documentation above are
used, with 0 meaning success.
Moreover, getdate_r stores the broken-down time in the variable
of type struct tm pointed to by the second argument, rather than
in a static variable.
This function is not defined in the Unix standard. Nevertheless it is
available on some other Unix systems as well.
The warning against using getdate in SUID-programs applies to
getdate_r as well.
TZ
In POSIX systems, a user can specify the time zone by means of the
TZ environment variable. For information about how to set
environment variables, see section Environment Variables. The functions
for accessing the time zone are declared in `time.h'.
You should not normally need to set TZ. If the system is
configured properly, the default time zone will be correct. You might
set TZ if you are using a computer over a network from a
different time zone, and would like times reported to you in the time
zone local to you, rather than what is local to the computer.
In POSIX.1 systems the value of the TZ variable can be in one of
three formats. With the GNU C library, the most common format is the
last one, which can specify a selection from a large database of time
zone information for many regions of the world. The first two formats
are used to describe the time zone information directly, which is both
more cumbersome and less precise. But the POSIX.1 standard only
specifies the details of the first two formats, so it is good to be
familiar with them in case you come across a POSIX.1 system that doesn't
support a time zone information database.
The first format is used when there is no Daylight Saving Time (or summer time) in the local time zone:
std offset
The std string specifies the name of the time zone. It must be three or more characters long and must not contain a leading colon, embedded digits, commas, nor plus and minus signs. There is no space character separating the time zone name from the offset, so these restrictions are necessary to parse the specification correctly.
The offset specifies the time value you must add to the local time
to get a Coordinated Universal Time value. It has syntax like
[+|-]hh[:mm[:ss]]. This
is positive if the local time zone is west of the Prime Meridian and
negative if it is east. The hour must be between 0 and
23, and the minute and seconds between 0 and 59.
For example, here is how we would specify Eastern Standard Time, but without any Daylight Saving Time alternative:
EST+5
The second format is used when there is Daylight Saving Time:
std offset dst [offset],start[/time],end[/time]
The initial std and offset specify the standard time zone, as described above. The dst string and offset specify the name and offset for the corresponding Daylight Saving Time zone; if the offset is omitted, it defaults to one hour ahead of standard time.
The remainder of the specification describes when Daylight Saving Time is in effect. The start field is when Daylight Saving Time goes into effect and the end field is when the change is made back to standard time. The following formats are recognized for these fields:
Jn
1 and 365.
February 29 is never counted, even in leap years.
n
0 and 365.
February 29 is counted in leap years.
Mm.w.d
0 (Sunday) and 6. The week
w must be between 1 and 5; week 1 is the
first week in which day d occurs, and week 5 specifies the
last d day in the month. The month m should be
between 1 and 12.
The time fields specify when, in the local time currently in
effect, the change to the other time occurs. If omitted, the default is
02:00:00.
For example, here is how you would specify the Eastern time zone in the United States, including the appropriate Daylight Saving Time and its dates of applicability. The normal offset from UTC is 5 hours; since this is west of the prime meridian, the sign is positive. Summer time begins on the first Sunday in April at 2:00am, and ends on the last Sunday in October at 2:00am.
EST+5EDT,M4.1.0/2,M10.5.0/2
The schedule of Daylight Saving Time in any particular jurisdiction has changed over the years. To be strictly correct, the conversion of dates and times in the past should be based on the schedule that was in effect then. However, this format has no facilities to let you specify how the schedule has changed from year to year. The most you can do is specify one particular schedule--usually the present day schedule--and this is used to convert any date, no matter when. For precise time zone specifications, it is best to use the time zone information database (see below).
The third format looks like this:
:characters
Each operating system interprets this format differently; in the GNU C library, characters is the name of a file which describes the time zone.
If the TZ environment variable does not have a value, the
operation chooses a time zone by default. In the GNU C library, the
default time zone is like the specification `TZ=:/etc/localtime'
(or `TZ=:/usr/local/etc/localtime', depending on how GNU C library
was configured; see section Installing the GNU C Library). Other C libraries use their own
rule for choosing the default time zone, so there is little we can say
about them.
If characters begins with a slash, it is an absolute file name; otherwise the library looks for the file `/share/lib/zoneinfo/characters'. The `zoneinfo' directory contains data files describing local time zones in many different parts of the world. The names represent major cities, with subdirectories for geographical areas; for example, `America/New_York', `Europe/London', `Asia/Hong_Kong'. These data files are installed by the system administrator, who also sets `/etc/localtime' to point to the data file for the local time zone. The GNU C library comes with a large database of time zone information for most regions of the world, which is maintained by a community of volunteers and put in the public domain.
tzname contains two strings, which are the standard
names of the pair of time zones (standard and Daylight
Saving) that the user has selected. tzname[0] is the name of
the standard time zone (for example, "EST"), and tzname[1]
is the name for the time zone when Daylight Saving Time is in use (for
example, "EDT"). These correspond to the std and dst
strings (respectively) from the TZ environment variable. If
Daylight Saving Time is never used, tzname[1] is the empty string.
The tzname array is initialized from the TZ environment
variable whenever tzset, ctime, strftime,
mktime, or localtime is called. If multiple abbreviations
have been used (e.g. "EWT" and "EDT" for U.S. Eastern War
Time and Eastern Daylight Time), the array contains the most recent
abbreviation.
The tzname array is required for POSIX.1 compatibility, but in
GNU programs it is better to use the tm_zone member of the
broken-down time structure, since tm_zone reports the correct
abbreviation even when it is not the latest one.
Though the strings are declared as char * the user must refrain
from modifying these strings. Modifying the strings will almost certainly
lead to trouble.
tzset function initializes the tzname variable from
the value of the TZ environment variable. It is not usually
necessary for your program to call this function, because it is called
automatically when you use the other time conversion functions that
depend on the time zone.
The following variables are defined for compatibility with System V
Unix. Like tzname, these variables are set by calling
tzset or the other time conversion functions.
5*60*60. Unlike the tm_gmtoff member
of the broken-down time structure, this value is not adjusted for
daylight saving, and its sign is reversed. In GNU programs it is better
to use tm_gmtoff, since it contains the correct offset even when
it is not the latest one.
Here is an example program showing the use of some of the calendar time functions.
#include <time.h>
#include <stdio.h>
#define SIZE 256
int
main (void)
{
char buffer[SIZE];
time_t curtime;
struct tm *loctime;
/* Get the current time. */
curtime = time (NULL);
/* Convert it to local time representation. */
loctime = localtime (&curtime);
/* Print out the date and time in the standard format. */
fputs (asctime (loctime), stdout);
/* Print it out in a nice format. */
strftime (buffer, SIZE, "Today is %A, %B %d.\n", loctime);
fputs (buffer, stdout);
strftime (buffer, SIZE, "The time is %I:%M %p.\n", loctime);
fputs (buffer, stdout);
return 0;
}
It produces output like this:
Wed Jul 31 13:02:36 1991 Today is Wednesday, July 31. The time is 01:02 PM.
The alarm and setitimer functions provide a mechanism for a
process to interrupt itself in the future. They do this by setting a
timer; when the timer expires, the process receives a signal.
Each process has three independent interval timers available:
SIGALRM signal to the process when it expires.
SIGVTALRM signal to the process when it expires.
SIGPROF signal to the process when it expires.
This timer is useful for profiling in interpreters. The interval timer
mechanism does not have the fine granularity necessary for profiling
native code.
You can only have one timer of each kind set at any given time. If you set a timer that has not yet expired, that timer is simply reset to the new value.
You should establish a handler for the appropriate alarm signal using
signal or sigaction before issuing a call to
setitimer or alarm. Otherwise, an unusual chain of events
could cause the timer to expire before your program establishes the
handler. In this case it would be terminated, since termination is the
default action for the alarm signals. See section Signal Handling.
The setitimer function is the primary means for setting an alarm.
This facility is declared in the header file `sys/time.h'. The
alarm function, declared in `unistd.h', provides a somewhat
simpler interface for setting the real-time timer.
struct timeval it_interval
struct timeval it_value
The struct timeval data type is described in section Elapsed Time.
setitimer function sets the timer specified by which
according to new. The which argument can have a value of
ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF.
If old is not a null pointer, setitimer returns information
about any previous unexpired timer of the same kind in the structure it
points to.
The return value is 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
EINVAL
getitimer function stores information about the timer specified
by which in the structure pointed at by old.
The return value and error conditions are the same as for setitimer.
ITIMER_REAL
setitimer and getitimer functions to specify the real-time
timer.
ITIMER_VIRTUAL
setitimer and getitimer functions to specify the virtual
timer.
ITIMER_PROF
setitimer and getitimer functions to specify the profiling
timer.
alarm function sets the real-time timer to expire in
seconds seconds. If you want to cancel any existing alarm, you
can do this by calling alarm with a seconds argument of
zero.
The return value indicates how many seconds remain before the previous
alarm would have been sent. If there is no previous alarm, alarm
returns zero.
The alarm function could be defined in terms of setitimer
like this:
unsigned int
alarm (unsigned int seconds)
{
struct itimerval old, new;
new.it_interval.tv_usec = 0;
new.it_interval.tv_sec = 0;
new.it_value.tv_usec = 0;
new.it_value.tv_sec = (long int) seconds;
if (setitimer (ITIMER_REAL, &new, &old) < 0)
return 0;
else
return old.it_value.tv_sec;
}
There is an example showing the use of the alarm function in
section Signal Handlers that Return.
If you simply want your process to wait for a given number of seconds,
you should use the sleep function. See section Sleeping.
You shouldn't count on the signal arriving precisely when the timer expires. In a multiprocessing environment there is typically some amount of delay involved.
Portability Note: The setitimer and getitimer
functions are derived from BSD Unix, while the alarm function is
specified by the POSIX.1 standard. setitimer is more powerful than
alarm, but alarm is more widely used.
The function sleep gives a simple way to make the program wait
for a short interval. If your program doesn't use signals (except to
terminate), then you can expect sleep to wait reliably throughout
the specified interval. Otherwise, sleep can return sooner if a
signal arrives; if you want to wait for a given interval regardless of
signals, use select (see section Waiting for Input or Output) and don't specify
any descriptors to wait for.
sleep function waits for seconds or until a signal
is delivered, whichever happens first.
If sleep function returns because the requested interval is over,
it returns a value of zero. If it returns because of delivery of a
signal, its return value is the remaining time in the sleep interval.
The sleep function is declared in `unistd.h'.
Resist the temptation to implement a sleep for a fixed amount of time by
using the return value of sleep, when nonzero, to call
sleep again. This will work with a certain amount of accuracy as
long as signals arrive infrequently. But each signal can cause the
eventual wakeup time to be off by an additional second or so. Suppose a
few signals happen to arrive in rapid succession by bad luck--there is
no limit on how much this could shorten or lengthen the wait.
Instead, compute the calendar time at which the program should stop
waiting, and keep trying to wait until that calendar time. This won't
be off by more than a second. With just a little more work, you can use
select and make the waiting period quite accurate. (Of course,
heavy system load can cause additional unavoidable delays--unless the
machine is dedicated to one application, there is no way you can avoid
this.)
On some systems, sleep can do strange things if your program uses
SIGALRM explicitly. Even if SIGALRM signals are being
ignored or blocked when sleep is called, sleep might
return prematurely on delivery of a SIGALRM signal. If you have
established a handler for SIGALRM signals and a SIGALRM
signal is delivered while the process is sleeping, the action taken
might be just to cause sleep to return instead of invoking your
handler. And, if sleep is interrupted by delivery of a signal
whose handler requests an alarm or alters the handling of SIGALRM,
this handler and sleep will interfere.
On the GNU system, it is safe to use sleep and SIGALRM in
the same program, because sleep does not work by means of
SIGALRM.
nanosleep function can
be used. As the name suggests the sleep interval can be specified in
nanoseconds. The actual elapsed time of the sleep interval might be
longer since the system rounds the elapsed time you request up to the
next integer multiple of the actual resolution the system can deliver.
*requested_time is the elapsed time of the interval you want to
sleep.
The function returns as *remaining the elapsed time left in the
interval for which you requested to sleep. If the interval completed
without getting interrupted by a signal, this is zero.
struct timespec is described in See section Elapsed Time.
If the function returns because the interval is over the return value is zero. If the function returns @math{-1} the global variable errno is set to the following values:
EINTR
EINVAL
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time nanosleep is
called. If the thread gets canceled these resources stay allocated
until the program ends. To avoid this calls to nanosleep should
be protected using cancellation handlers.
The nanosleep function is declared in `time.h'.
This chapter describes functions for examining how much of various kinds of resources (CPU time, memory, etc.) a process has used and getting and setting limits on future usage.
The function getrusage and the data type struct rusage
are used to examine the resource usage of a process. They are declared
in `sys/resource.h'.
*rusage.
In most systems, processes has only two valid values:
RUSAGE_SELF
RUSAGE_CHILDREN
In the GNU system, you can also inquire about a particular child process by specifying its process ID.
The return value of getrusage is zero for success, and -1
for failure.
EINVAL
One way of getting resource usage for a particular child process is with
the function wait4, which returns totals for a child when it
terminates. See section BSD Process Wait Functions.
struct timeval ru_utime
struct timeval ru_stime
long int ru_maxrss
long int ru_ixrss
long int ru_idrss
long int ru_isrss
long int ru_minflt
long int ru_majflt
long int ru_nswap
long int ru_inblock
long int ru_oublock
long int ru_msgsnd
long int ru_msgrcv
long int ru_nsignals
long int ru_nvcsw
long int ru_nivcsw
vtimes is a historical function that does some of what
getrusage does. getrusage is a better choice.
vtimes and its vtimes data structure are declared in
`sys/vtimes.h'.
vtimes reports resource usage totals for a process.
If current is non-null, vtimes stores resource usage totals for
the invoking process alone in the structure to which it points. If
child is non-null, vtimes stores resource usage totals for all
past children (which have terminated) of the invoking process in the structure
to which it points.
struct rusage data type
described above.
vm_utime
ru_utime in struct rusage
vm_stime
ru_stime in struct rusage
vm_idsrss
ru_idrss and ru_isrss in struct rusage
vm_ixrss
ru_ixrss in struct rusage
vm_maxrss
ru_maxrss in
struct rusage
vm_majflt
ru_majflt in struct rusage
vm_minflt
ru_minflt in struct rusage
vm_nswap
ru_nswap in struct rusage
vm_inblk
ru_inblk in struct rusage
vm_oublk
ru_oublk in struct rusage
The return value is zero if the function succeeds; -1 otherwise.
An additional historical function for examining resource usage,
vtimes, is supported but not documented here. It is declared in
`sys/vtimes.h'.
You can specify limits for the resource usage of a process. When the process tries to exceed a limit, it may get a signal, or the system call by which it tried to do so may fail, depending on the resource. Each process initially inherits its limit values from its parent, but it can subsequently change them.
There are two per-process limits associated with a resource:
The symbols for use with getrlimit, setrlimit,
getrlimit64, and seterlimit64 are defined in
`sys/resource.h'.
*rlp.
The return value is 0 on success and -1 on failure. The
only possible errno error condition is EFAULT.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit system this function is in fact getrlimit64. Thus, the
LFS interface transparently replaces the old interface.
getrlimit but its second parameter is
a pointer to a variable of type struct rlimit64, which allows it
to read values which wouldn't fit in the member of a struct
rlimit.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit machine, this function is available under the name
getrlimit and so transparently replaces the old interface.
*rlp.
The return value is 0 on success and -1 on failure. The
following errno error condition is possible:
EPERM
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit system this function is in fact setrlimit64. Thus, the
LFS interface transparently replaces the old interface.
setrlimit but its second parameter is
a pointer to a variable of type struct rlimit64 which allows it
to set values which wouldn't fit in the member of a struct
rlimit.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit machine this function is available under the name
setrlimit and so transparently replaces the old interface.
getrlimit to receive limit values,
and with setrlimit to specify limit values for a particular process
and resource. It has two fields:
rlim_t rlim_cur
rlim_t rlim_max
For getrlimit, the structure is an output; it receives the current
values. For setrlimit, it specifies the new values.
For the LFS functions a similar type is defined in `sys/resource.h'.
rlimit structure above, but
its components have wider ranges. It has two fields:
rlim64_t rlim_cur
rlimit.rlim_cur, but with a different type.
rlim64_t rlim_max
rlimit.rlim_max, but with a different type.
Here is a list of resources for which you can specify a limit. Memory and file sizes are measured in bytes.
RLIMIT_CPU
SIGXCPU. The value is
measured in seconds. See section Operation Error Signals.
RLIMIT_FSIZE
SIGXFSZ. See section Operation Error Signals.
RLIMIT_DATA
RLIMIT_STACK
SIGSEGV signal.
See section Program Error Signals.
RLIMIT_CORE
RLIMIT_RSS
RLIMIT_MEMLOCK
RLIMIT_NPROC
fork will fail
with EAGAIN. See section Creating a Process.
RLIMIT_NOFILE
RLIMIT_OFILE
errno
EMFILE. See section Error Codes. Not all systems support this limit;
GNU does, and 4.4 BSD does.
RLIMIT_AS
brk, malloc, mmap or sbrk, the
allocation function fails.
RLIM_NLIMITS
RLIM_NLIMITS.
setrlimit.
The following are historical functions to do some of what the functions above do. The functions above are better choices.
ulimit and the command symbols are declared in `ulimit.h'.
ulimit gets the current limit or sets the current and maximum
limit for a particular resource for the calling process according to the
command cmd.a
If you are getting a limit, the command argument is the only argument.
If you are setting a limit, there is a second argument:
long int limit which is the value to which you are setting
the limit.
The cmd values and the operations they specify are:
GETFSIZE
SETFSIZE
There are also some other cmd values that may do things on some systems, but they are not supported.
Only the superuser may increase a maximum limit.
When you successfully get a limit, the return value of ulimit is
that limit, which is never negative. When you successfully set a limit,
the return value is zero. When the function fails, the return value is
-1 and errno is set according to the reason:
EPERM
vlimit and its resource symbols are declared in `sys/vlimit.h'.
vlimit sets the current limit for a resource for a process.
resource identifies the resource:
LIM_CPU
RLIMIT_CPU for setrlimit.
LIM_FSIZE
RLIMIT_FSIZE for setrlimit.
LIM_DATA
RLIMIT_DATA for setrlimit.
LIM_STACK
RLIMIT_STACK for setrlimit.
LIM_CORE
RLIMIT_COR for setrlimit.
LIM_MAXRSS
RLIMIT_RSS for setrlimit.
The return value is zero for success, and -1 with errno set
accordingly for failure:
EPERM
When multiple processes simultaneously require CPU time, the system's scheduling policy and process CPU priorities determine which processes get it. This section describes how that determination is made and GNU C library functions to control it.
It is common to refer to CPU scheduling simply as scheduling and a process' CPU priority simply as the process' priority, with the CPU resource being implied. Bear in mind, though, that CPU time is not the only resource a process uses or that processes contend for. In some cases, it is not even particularly important. Giving a process a high "priority" may have very little effect on how fast a process runs with respect to other processes. The priorities discussed in this section apply only to CPU time.
CPU scheduling is a complex issue and different systems do it in wildly different ways. New ideas continually develop and find their way into the intricacies of the various systems' scheduling algorithms. This section discusses the general concepts, some specifics of systems that commonly use the GNU C library, and some standards.
For simplicity, we talk about CPU contention as if there is only one CPU in the system. But all the same principles apply when a processor has multiple CPUs, and knowing that the number of processes that can run at any one time is equal to the number of CPUs, you can easily extrapolate the information.
The functions described in this section are all defined by the POSIX.1
and POSIX.1b standards (the sched... functions are POSIX.1b).
However, POSIX does not define any semantics for the values that these
functions get and set. In this chapter, the semantics are based on the
Linux kernel's implementation of the POSIX standard. As you will see,
the Linux implementation is quite the inverse of what the authors of the
POSIX syntax had in mind.
Every process has an absolute priority, and it is represented by a number. The higher the number, the higher the absolute priority.
On systems of the past, and most systems today, all processes have absolute priority 0 and this section is irrelevant. In that case, See section Traditional Scheduling. Absolute priorities were invented to accomodate realtime systems, in which it is vital that certain processes be able to respond to external events happening in real time, which means they cannot wait around while some other process that wants to, but doesn't need to run occupies the CPU.
When two processes are in contention to use the CPU at any instant, the one with the higher absolute priority always gets it. This is true even if the process with the lower priority is already using the CPU (i.e. the scheduling is preemptive). Of course, we're only talking about processes that are running or "ready to run," which means they are ready to execute instructions right now. When a process blocks to wait for something like I/O, its absolute priority is irrelevant.
Note: The term "runnable" is a synonym for "ready to run."
When two processes are running or ready to run and both have the same absolute priority, it's more interesting. In that case, who gets the CPU is determined by the scheduling policy. If the processeses have absolute priority 0, the traditional scheduling policy described in section Traditional Scheduling applies. Otherwise, the policies described in section Realtime Scheduling apply.
You normally give an absolute priority above 0 only to a process that can be trusted not to hog the CPU. Such processes are designed to block (or terminate) after relatively short CPU runs.
A process begins life with the same absolute priority as its parent process. Functions described in section Basic Scheduling Functions can change it.
Only a privileged process can change a process' absolute priority to
something other than 0. Only a privileged process or the
target process' owner can change its absolute priority at all.
POSIX requires absolute priority values used with the realtime
scheduling policies to be consecutive with a range of at least 32. On
Linux, they are 1 through 99. The functions
sched_get_priority_max and sched_set_priority_min portably
tell you what the range is on a particular system.
One thing you must keep in mind when designing real time applications is that having higher absolute priority than any other process doesn't guarantee the process can run continuously. Two things that can wreck a good CPU run are interrupts and page faults.
Interrupt handlers live in that limbo between processes. The CPU is executing instructions, but they aren't part of any process. An interrupt will stop even the highest priority process. So you must allow for slight delays and make sure that no device in the system has an interrupt handler that could cause too long a delay between instructions for your process.
Similarly, a page fault causes what looks like a straightforward
sequence of instructions to take a long time. The fact that other
processes get to run while the page faults in is of no consequence,
because as soon as the I/O is complete, the high priority process will
kick them out and run again, but the wait for the I/O itself could be a
problem. To neutralize this threat, use mlock or
mlockall.
There are a few ramifications of the absoluteness of this priority on a single-CPU system that you need to keep in mind when you choose to set a priority and also when you're working on a program that runs with high absolute priority. Consider a process that has higher absolute priority than any other process in the system and due to a bug in its program, it gets into an infinite loop. It will never cede the CPU. You can't run a command to kill it because your command would need to get the CPU in order to run. The errant program is in complete control. It controls the vertical, it controls the horizontal.
There are two ways to avoid this: 1) keep a shell running somewhere with a higher absolute priority. 2) keep a controlling terminal attached to the high priority process group. All the priority in the world won't stop an interrupt handler from running and delivering a signal to the process if you hit Control-C.
Some systems use absolute priority as a means of allocating a fixed per centage of CPU time to a process. To do this, a super high priority privileged process constantly monitors the process' CPU usage and raises its absolute priority when the process isn't getting its entitled share and lowers it when the process is exceeding it.
Note: The absolute priority is sometimes called the "static priority." We don't use that term in this manual because it misses the most important feature of the absolute priority: its absoluteness.
Whenever two processes with the same absolute priority are ready to run, the kernel has a decision to make, because only one can run at a time. If the processes have absolute priority 0, the kernel makes this decision as described in section Traditional Scheduling. Otherwise, the decision is as described in this section.
If two processes are ready to run but have different absolute priorities, the decision is much simpler, and is described in section Absolute Priority.
Each process has a scheduling policy. For processes with absolute priority other than zero, there are two available:
The most sensible case is where all the processes with a certain absolute priority have the same scheduling policy. We'll discuss that first.
In Round Robin, processes share the CPU, each one running for a small quantum of time ("time slice") and then yielding to another in a circular fashion. Of course, only processes that are ready to run and have the same absolute priority are in this circle.
In First Come First Served, the process that has been waiting the longest to run gets the CPU, and it keeps it until it voluntarily relinquishes the CPU, runs out of things to do (blocks), or gets preempted by a higher priority process.
First Come First Served, along with maximal absolute priority and careful control of interrupts and page faults, is the one to use when a process absolutely, positively has to run at full CPU speed or not at all.
Judicious use of sched_yield function invocations by processes
with First Come First Served scheduling policy forms a good compromise
between Round Robin and First Come First Served.
To understand how scheduling works when processes of different scheduling policies occupy the same absolute priority, you have to know the nitty gritty details of how processes enter and exit the ready to run list:
In both cases, the ready to run list is organized as a true queue, where a process gets pushed onto the tail when it becomes ready to run and is popped off the head when the scheduler decides to run it. Note that ready to run and running are two mutually exclusive states. When the scheduler runs a process, that process is no longer ready to run and no longer in the ready to run list. When the process stops running, it may go back to being ready to run again.
The only difference between a process that is assigned the Round Robin scheduling policy and a process that is assigned First Come First Serve is that in the former case, the process is automatically booted off the CPU after a certain amount of time. When that happens, the process goes back to being ready to run, which means it enters the queue at the tail. The time quantum we're talking about is small. Really small. This is not your father's timesharing. For example, with the Linux kernel, the round robin time slice is a thousand times shorter than its typical time slice for traditional scheduling.
A process begins life with the same scheduling policy as its parent process. Functions described in section Basic Scheduling Functions can change it.
Only a privileged process can set the scheduling policy of a process that has absolute priority higher than 0.
This section describes functions in the GNU C library for setting the absolute priority and scheduling policy of a process.
Portability Note: On systems that have the functions in this section, the macro _POSIX_PRIORITY_SCHEDULING is defined in `<unistd.h>'.
For the case that the scheduling policy is traditional scheduling, more functions to fine tune the scheduling are in section Traditional Scheduling.
Don't try to make too much out of the naming and structure of these functions. They don't match the concepts described in this manual because the functions are as defined by POSIX.1b, but the implementation on systems that use the GNU C library is the inverse of what the POSIX structure contemplates. The POSIX scheme assumes that the primary scheduling parameter is the scheduling policy and that the priority value, if any, is a parameter of the scheduling policy. In the implementation, though, the priority value is king and the scheduling policy, if anything, only fine tunes the effect of that priority.
The symbols in this section are declared by including file `sched.h'.
int sched_priority
This function sets both the absolute priority and the scheduling policy for a process.
It assigns the absolute priority value given by param and the
scheduling policy policy to the process with Process ID pid,
or the calling process if pid is zero. If policy is
negative, sched_setschedule keeps the existing scheduling policy.
The following macros represent the valid values for policy:
SCHED_OTHER
SCHED_FIFO
SCHED_RR
On success, the return value is 0. Otherwise, it is -1
and ERRNO is set accordingly. The errno values specific
to this function are:
EPERM
CAP_SYS_NICE permission and
policy is not SCHED_OTHER (or it's negative and the
existing policy is not SCHED_OTHER.
CAP_SYS_NICE permission and its
owner is not the target process' owner. I.e. the effective uid of the
calling process is neither the effective nor the real uid of process
pid.
ESRCH
EINVAL
sched_get_priority_max and sched_get_priority_min
tell you what the valid range is.
This function returns the scheduling policy assigned to the process with Process ID (pid) pid, or the calling process if pid is zero.
The return value is the scheduling policy. See
sched_setscheduler for the possible values.
If the function fails, the return value is instead -1 and
errno is set accordingly.
The errno values specific to this function are:
ESRCH
EINVAL
Note that this function is not an exact mate to sched_setscheduler
because while that function sets the scheduling policy and the absolute
priority, this function gets only the scheduling policy. To get the
absolute priority, use sched_getparam.
This function sets a process' absolute priority.
It is functionally identical to sched_setscheduler with
policy = -1.
This function returns a process' absolute priority.
pid is the Process ID (pid) of the process whose absolute priority you want to know.
param is a pointer to a structure in which the function stores the absolute priority of the process.
On success, the return value is 0. Otherwise, it is -1
and ERRNO is set accordingly. The errno values specific
to this function are:
ESRCH
EINVAL
This function returns the lowest absolute priority value that is allowable for a process with scheduling policy policy.
On Linux, it is 0 for SCHED_OTHER and 1 for everything else.
On success, the return value is 0. Otherwise, it is -1
and ERRNO is set accordingly. The errno values specific
to this function are:
EINVAL
This function returns the highest absolute priority value that is allowable for a process that with scheduling policy policy.
On Linux, it is 0 for SCHED_OTHER and 99 for everything else.
On success, the return value is 0. Otherwise, it is -1
and ERRNO is set accordingly. The errno values specific
to this function are:
EINVAL
This function returns the length of the quantum (time slice) used with the Round Robin scheduling policy, if it is used, for the process with Process ID pid.
It returns the length of time as interval.
With a Linux kernel, the round robin time slice is always 150 microseconds, and pid need not even be a real pid.
The return value is 0 on success and in the pathological case
that it fails, the return value is -1 and errno is set
accordingly. There is nothing specific that can go wrong with this
function, so there are no specific errno values.
This function voluntarily gives up the process' claim on the CPU.
Technically, sched_yield causes the calling process to be made
immediately ready to run (as opposed to running, which is what it was
before). This means that if it has absolute priority higher than 0, it
gets pushed onto the tail of the queue of processes that share its
absolute priority and are ready to run, and it will run again when its
turn next arrives. If its absolute priority is 0, it is more
complicated, but still has the effect of yielding the CPU to other
processes.
If there are no other processes that share the calling process' absolute priority, this function doesn't have any effect.
To the extent that the containing program is oblivious to what other processes in the system are doing and how fast it executes, this function appears as a no-op.
The return value is 0 on success and in the pathological case
that it fails, the return value is -1 and errno is set
accordingly. There is nothing specific that can go wrong with this
function, so there are no specific errno values.
This section is about the scheduling among processes whose absolute priority is 0. When the system hands out the scraps of CPU time that are left over after the processes with higher absolulte priority have taken all they want, the scheduling described herein determines who among the great unwashed processes gets them.
Long before there was absolute priority (See section Absolute Priority), Unix systems were scheduling the CPU using this system. When Posix came in like the Romans and imposed absolute priorities to accomodate the needs of realtime processing, it left the indigenous Absolute Priority Zero processes to govern themselves by their own familiar scheduling policy.
Indeed, absolute priorities higher than zero are not available on many systems today and are not typically used when they are, being intended mainly for computers that do realtime processing. So this section describes the only scheduling many programmers need to be concerned about.
But just to be clear about the scope of this scheduling: Any time a process with a absolute priority of 0 and a process with an absolute priority higher than 0 are ready to run at the same time, the one with absolute priority 0 does not run. If it's already running when the higher priority ready-to-run process comes into existence, it stops immediately.
In addition to its absolute priority of zero, every process has another priority, which we will refer to as "dynamic priority" because it changes over time. The dynamic priority is meaningless for processes with an absolute priority higher than zero.
The dynamic priority sometimes determines who gets the next turn on the CPU. Sometimes it determines how long turns last. Sometimes it determines whether a process can kick another off the CPU.
In Linux, the value is a combination of these things, but mostly it is just determines the length of the time slice. The higher a process' dynamic priority, the longer a shot it gets on the CPU when it gets one. If it doesn't use up its time slice before giving up the CPU to do something like wait for I/O, it is favored for getting the CPU back when it's ready for it, to finish out its time slice. Other than that, selection of processes for new time slices is basically round robin. But the scheduler does throw a bone to the low priority processes: A process' dynamic priority rises every time it is snubbed in the scheduling process. In Linux, even the fat kid gets to play.
The fluctuation of a process' dynamic priority is regulated by another value: The "nice" value. The nice value is an integer, usually in the range -20 to 20, and represents an upper limit on a process' dynamic priority. The higher the nice number, the lower that limit.
On a typical Linux system, for example, a process with a nice value of 20 can get only 10 milliseconds on the CPU at a time, whereas a process with a nice value of -20 can achieve a high enough priority to get 400 milliseconds.
The idea of the nice value is deferential courtesy. In the beginning, in the Unix garden of Eden, all processes shared equally in the bounty of the computer system. But not all processes really need the same share of CPU time, so the nice value gave a courteous process the ability to refuse its equal share of CPU time that others might prosper. Hence, the higher a process' nice value, the nicer the process is. (Then a snake came along and offered some process a negative nice value and the system became the crass resource allocation system we know today).
Dynamic priorities tend upward and downward with an objective of smoothing out allocation of CPU time and giving quick response time to infrequent requests. But they never exceed their nice limits, so on a heavily loaded CPU, the nice value effectively determines how fast a process runs.
In keeping with the socialistic heritage of Unix process priority, a process begins life with the same nice value as its parent process and can raise it at will. A process can also raise the nice value of any other process owned by the same user (or effective user). But only a privileged process can lower its nice value. A privileged process can also raise or lower another process' nice value.
GNU C Library functions for getting and setting nice values are described in See section Functions For Traditional Scheduling.
This section describes how you can read and set the nice value of a process. All these symbols are declared in `sys/resource.h'.
The function and macro names are defined by POSIX, and refer to "priority," but the functions actually have to do with nice values, as the terms are used both in the manual and POSIX.
The range of valid nice values depends on the kernel, but typically it
runs from -20 to 20. A lower nice value corresponds to
higher priority for the process. These constants describe the range of
priority values:
On success, the return value is 0. Otherwise, it is -1
and ERRNO is set accordingly. The errno values specific
to this function are:
ESRCH
EINVAL
If the return value is -1, it could indicate failure, or it could
be the nice value. The only way to make certain is to set errno =
0 before calling getpriority, then use errno != 0
afterward as the criterion for failure.
The return value is the nice value on success, and -1 on
failure. The following errno error condition are possible for
this function:
ESRCH
EINVAL
EPERM
CAP_SYS_NICE permission.
EACCES
CAP_SYS_NICE permission.
The arguments class and id together specify a set of processes in which you are interested. These are the possible values of class:
PRIO_PROCESS
PRIO_PGRP
PRIO_USER
If the argument id is 0, it stands for the calling process, its process group, or its owner (real uid), according to class.
setpriority.
Here is an equivalent definition of nice:
int
nice (int increment)
{
int old = getpriority (PRIO_PROCESS, 0);
return setpriority (PRIO_PROCESS, 0, old + increment);
}
The amount of memory available in the system and the way it is organized
determines oftentimes the way programs can and have to work. For
functions like mmap it is necessary to know about the size of
individual memory pages and knowing how much memory is available enables
a program to select appropriate sizes for, say, caches. Before we get
into these details a few words about memory subsystems in traditional
Unix systems will be given.
Unix systems normally provide processes virtual address spaces. This means that the addresses of the memory regions do not have to correspond directly to the addresses of the actual physical memory which stores the data. An extra level of indirection is introduced which translates virtual addresses into physical addresses. This is normally done by the hardware of the processor.
Using a virtual address space has several advantage. The most important is process isolation. The different processes running on the system cannot interfere directly with each other. No process can write into the address space of another process (except when shared memory is used but then it is wanted and controlled).
Another advantage of virtual memory is that the address space the processes see can actually be larger than the physical memory available. The physical memory can be extended by storage on an external media where the content of currently unused memory regions is stored. The address translation can then intercept accesses to these memory regions and make memory content available again by loading the data back into memory. This concept makes it necessary that programs which have to use lots of memory know the difference between available virtual address space and available physical memory. If the working set of virtual memory of all the processes is larger than the available physical memory the system will slow down dramatically due to constant swapping of memory content from the memory to the storage media and back. This is called "thrashing".
A final aspect of virtual memory which is important and follows from what is said in the last paragraph is the granularity of the virtual address space handling. When we said that the virtual address handling stores memory content externally it cannot do this on a byte-by-byte basis. The administrative overhead does not allow this (leaving alone the processor hardware). Instead several thousand bytes are handled together and form a page. The size of each page is always a power of two byte. The smallest page size in use today is 4096, with 8192, 16384, and 65536 being other popular sizes.
The page size of the virtual memory the process sees is essential to
know in several situations. Some programming interface (e.g.,
mmap, see section Memory-mapped I/O) require the user to provide
information adjusted to the page size. In the case of mmap is it
necessary to provide a length argument which is a multiple of the page
size. Another place where the knowledge about the page size is useful
is in memory allocation. If one allocates pieces of memory in larger
chunks which are then subdivided by the application code it is useful to
adjust the size of the larger blocks to the page size. If the total
memory requirement for the block is close (but not larger) to a multiple
of the page size the kernel's memory handling can work more effectively
since it only has to allocate memory pages which are fully used. (To do
this optimization it is necessary to know a bit about the memory
allocator which will require a bit of memory itself for each block and
this overhead must not push the total size over the page size multiple.
The page size traditionally was a compile time constant. But recent development of processors changed this. Processors now support different page sizes and they can possibly even vary among different processes on the same system. Therefore the system should be queried at runtime about the current page size and no assumptions (except about it being a power of two) should be made.
The correct interface to query about the page size is sysconf
(see section Definition of sysconf) with the parameter _SC_PAGESIZE.
There is a much older interface available, too.
getpagesize function returns the page size of the process.
This value is fixed for the runtime of the process but can vary in
different runs of the application.
The function is declared in `unistd.h'.
Widely available on System V derived systems is a method to get information about the physical memory the system has. The call
sysconf (_SC_PHYS_PAGES)
returns the total number of pages of physical the system has. This does not mean all this memory is available. This information can be found using
sysconf (_SC_AVPHYS_PAGES)
These two values help to optimize applications. The value returned for
_SC_AVPHYS_PAGES is the amount of memory the application can use
without hindering any other process (given that no other process
increases its memory usage). The value returned for
_SC_PHYS_PAGES is more or less a hard limit for the working set.
If all applications together constantly use more than that amount of
memory the system is in trouble.
The GNU C library provides in addition to these already described way to
get this information two functions. They are declared in the file
`sys/sysinfo.h'. Programmers should prefer to use the
sysconf method described above.
get_phys_pages function returns the total number of pages of
physical the system has. To get the amount of memory this number has to
be multiplied by the page size.
This function is a GNU extension.
get_phys_pages function returns the number of available pages of
physical the system has. To get the amount of memory this number has to
be multiplied by the page size.
This function is a GNU extension.
The use of threads or processes with shared memory allows an application to take advantage of all the processing power a system can provide. If the task can be parallelized the optimal way to write an application is to have at any time as many processes running as there are processors. To determine the number of processors available to the system one can run
sysconf (_SC_NPROCESSORS_CONF)
which returns the number of processors the operating system configured. But it might be possible for the operating system to disable individual processors and so the call
sysconf (_SC_NPROCESSORS_ONLN)
returns the number of processors which are currently inline (i.e., available).
For these two pieces of information the GNU C library also provides functions to get the information directly. The functions are declared in `sys/sysinfo.h'.
get_nprocs_conf function returns the number of processors the
operating system configured.
This function is a GNU extension.
get_nprocs function returns the number of available processors.
This function is a GNU extension.
Before starting more threads it should be checked whether the processors are not already overused. Unix systems calculate something called the load average. This is a number indicating how many processes were running. This number is average over different periods of times (normally 1, 5, and 15 minutes).
getloadavg will
place at most nelem elements into the array but never more than
three elements. The return value is the number of elements written to
loadavg, or -1 on error.
This function is declared in `stdlib.h'.
Sometimes when your program detects an unusual situation inside a deeply
nested set of function calls, you would like to be able to immediately
return to an outer level of control. This section describes how you can
do such non-local exits using the setjmp and longjmp
functions.
As an example of a situation where a non-local exit can be useful, suppose you have an interactive program that has a "main loop" that prompts for and executes commands. Suppose the "read" command reads input from a file, doing some lexical analysis and parsing of the input while processing it. If a low-level input error is detected, it would be useful to be able to return immediately to the "main loop" instead of having to make each of the lexical analysis, parsing, and processing phases all have to explicitly deal with error situations initially detected by nested calls.
(On the other hand, if each of these phases has to do a substantial amount of cleanup when it exits--such as closing files, deallocating buffers or other data structures, and the like--then it can be more appropriate to do a normal return and have each phase do its own cleanup, because a non-local exit would bypass the intervening phases and their associated cleanup code entirely. Alternatively, you could use a non-local exit but do the cleanup explicitly either before or after returning to the "main loop".)
In some ways, a non-local exit is similar to using the `return' statement to return from a function. But while `return' abandons only a single function call, transferring control back to the point at which it was called, a non-local exit can potentially abandon many levels of nested function calls.
You identify return points for non-local exits by calling the function
setjmp. This function saves information about the execution
environment in which the call to setjmp appears in an object of
type jmp_buf. Execution of the program continues normally after
the call to setjmp, but if an exit is later made to this return
point by calling longjmp with the corresponding jmp_buf
object, control is transferred back to the point where setjmp was
called. The return value from setjmp is used to distinguish
between an ordinary return and a return made by a call to
longjmp, so calls to setjmp usually appear in an `if'
statement.
Here is how the example program described above might be set up:
#include <setjmp.h>
#include <stdlib.h>
#include <stdio.h>
jmp_buf main_loop;
void
abort_to_main_loop (int status)
{
longjmp (main_loop, status);
}
int
main (void)
{
while (1)
if (setjmp (main_loop))
puts ("Back at main loop....");
else
do_command ();
}
void
do_command (void)
{
char buffer[128];
if (fgets (buffer, 128, stdin) == NULL)
abort_to_main_loop (-1);
else
exit (EXIT_SUCCESS);
}
The function abort_to_main_loop causes an immediate transfer of
control back to the main loop of the program, no matter where it is
called from.
The flow of control inside the main function may appear a little
mysterious at first, but it is actually a common idiom with
setjmp. A normal call to setjmp returns zero, so the
"else" clause of the conditional is executed. If
abort_to_main_loop is called somewhere within the execution of
do_command, then it actually appears as if the same call
to setjmp in main were returning a second time with a value
of -1.
So, the general pattern for using setjmp looks something like:
if (setjmp (buffer)) /* Code to clean up after premature return. */ ... else /* Code to be executed normally after setting up the return point. */ ...
Here are the details on the functions and data structures used for performing non-local exits. These facilities are declared in `setjmp.h'.
jmp_buf hold the state information to
be restored by a non-local exit. The contents of a jmp_buf
identify a specific place to return to.
setjmp stores information about the
execution state of the program in state and returns zero. If
longjmp is later used to perform a non-local exit to this
state, setjmp returns a nonzero value.
setjmp that
established that return point. Returning from setjmp by means of
longjmp returns the value argument that was passed to
longjmp, rather than 0. (But if value is given as
0, setjmp returns 1).
There are a lot of obscure but important restrictions on the use of
setjmp and longjmp. Most of these restrictions are
present because non-local exits require a fair amount of magic on the
part of the C compiler and can interact with other parts of the language
in strange ways.
The setjmp function is actually a macro without an actual
function definition, so you shouldn't try to `#undef' it or take
its address. In addition, calls to setjmp are safe in only the
following contexts:
Return points are valid only during the dynamic extent of the function
that called setjmp to establish them. If you longjmp to
a return point that was established in a function that has already
returned, unpredictable and disastrous things are likely to happen.
You should use a nonzero value argument to longjmp. While
longjmp refuses to pass back a zero argument as the return value
from setjmp, this is intended as a safety net against accidental
misuse and is not really good programming style.
When you perform a non-local exit, accessible objects generally retain
whatever values they had at the time longjmp was called. The
exception is that the values of automatic variables local to the
function containing the setjmp call that have been changed since
the call to setjmp are indeterminate, unless you have declared
them volatile.
In BSD Unix systems, setjmp and longjmp also save and
restore the set of blocked signals; see section Blocking Signals. However,
the POSIX.1 standard requires setjmp and longjmp not to
change the set of blocked signals, and provides an additional pair of
functions (sigsetjmp and siglongjmp) to get the BSD
behavior.
The behavior of setjmp and longjmp in the GNU library is
controlled by feature test macros; see section Feature Test Macros. The
default in the GNU system is the POSIX.1 behavior rather than the BSD
behavior.
The facilities in this section are declared in the header file `setjmp.h'.
jmp_buf, except that it can also store state
information about the set of blocked signals.
setjmp. If savesigs is nonzero, the set
of blocked signals is saved in state and will be restored if a
siglongjmp is later performed with this state.
longjmp except for the type of its state
argument. If the sigsetjmp call that set this state used a
nonzero savesigs flag, siglongjmp also restores the set of
blocked signals.
The Unix standard one more set of function to control the execution path and these functions are more powerful than those discussed in this chapter so far. These function were part of the original System V API and by this route were added to the Unix API. Beside on branded Unix implementations these interfaces are not widely available. Not all platforms and/or architectures the GNU C Library is available on provide this interface. Use `configure' to detect the availability.
Similar to the jmp_buf and sigjmp_buf types used for the
variables to contain the state of the longjmp functions the
interfaces of interest here have an appropriate type as well. Objects
of this type are normally much larger since more information is
contained. The type is also used in a few more places as we will see.
The types and functions described in this section are all defined and
declared respectively in the `ucontext.h' header file.
The ucontext_t type is defined as a structure with as least the
following elements:
ucontext_t *uc_link
sigset_t uc_sigmask
stack_t uc_stack
mcontext_t uc_mcontext
mcontext_t type is also defined in this header but the definition
should be treated as opaque. Any use of knowledge of the type makes
applications less portable.
Objects of this type have to be created by the user. The initialization and modification happens through one of the following functions:
getcontext function initializes the variable pointed to by
ucp with the context of the calling thread. The context contains
the content of the registers, the signal mask, and the current stack.
Executing the contents would start at the point where the
getcontext call just returned.
The function returns 0 if succesful. Otherwise it returns
-1 and sets errno accordingly.
The getcontext function is similar to setjmp but it does
not provide an indication of whether the function returns for the first
time or whether the initialized context was used and the execution is
resumed at just that point. If this is necessary the user has to take
determine this herself. This must be done carefully since the context
contains registers which might contain register variables. This is a
good situation to define variables with volatile.
Once the context variable is initialized it can be used as is or it can
be modified. The latter is normally done to implement co-routines or
similar constructs. The makecontext function is what has to be
used to do that.
The ucp parameter passed to the makecontext shall be
initialized by a call to getcontext. The context will be
modified to in a way so that if the context is resumed it will start by
calling the function func which gets argc integer arguments
passed. The integer arguments which are to be passed should follow the
argc parameter in the call to makecontext.
Before the call to this function the uc_stack and uc_link
element of the ucp structure should be initialized. The
uc_stack element describes the stack which is used for this
context. No two contexts which are used at the same time should use the
same memory region for a stack.
The uc_link element of the object pointed to by ucp should
be a pointer to the context to be executed when the function func
returns or it should be a null pointer. See setcontext for more
information about the exact use.
While allocating the memory for the stack one has to be careful. Most modern processors keep track of whether a certain memory region is allowed to contain code which is executed or not. Data segments and heap memory is normally not tagged to allow this. The result is that programs would fail. Examples for such code include the calling sequences the GNU C compiler generates for calls to nested functions. Safe ways to allocate stacks correctly include using memory on the original threads stack or explicitly allocate memory tagged for execution using (see section Memory-mapped I/O).
Compatibility note: The current Unix standard is very imprecise
about the way the stack is allocated. All implementations seem to agree
that the uc_stack element must be used but the values stored in
the elements of the stack_t value are unclear. The GNU C library
and most other Unix implementations require the ss_sp value of
the uc_stack element to point to the base of the memory region
allocated for the stack and the size of the memory region is stored in
ss_size. There are implements out there which require
ss_sp to be set to the value the stack pointer will have (which
can depending on the direction the stack grows be different). This
difference makes the makecontext function hard to use and it
requires detection of the platform at compile time.
The setcontext function restores the context described by
ucp. The context is not modified and can be reused as often as
wanted.
If the context was created by getcontext execution resumes with
the registers filled with the same values and the same stack as if the
getcontext call just returned.
If the context was modified with a call to makecontext execution
continues with the function passed to makecontext which gets the
specified parameters passed. If this function returns execution is
resumed in the context which was referenced by the uc_link
element of the context structure passed to makecontext at the
time of the call. If uc_link was a null pointer the application
terminates in this case.
Since the context contains information about the stack no two threads should use the same context at the same time. The result in most cases would be disastrous.
The setcontext function does not return unless an error occurred
in which case it returns -1.
The setcontext function simply replaces the current context with
the one described by the ucp parameter. This is often useful but
there are situations where the current context has to be preserved.
The swapcontext function is similar to setcontext but
instead of just replacing the current context the latter is first saved
in the object pointed to by oucp as if this was a call to
getcontext. The saved context would resume after the call to
swapcontext.
Once the current context is saved the context described in ucp is installed and execution continues as described in this context.
If swapcontext succeeds the function does not return unless the
context oucp is used without prior modification by
makecontext. The return value in this case is 0. If the
function fails it returns -1 and set errno accordingly.
The easiest way to use the context handling functions is as a
replacement for setjmp and longjmp. The context contains
on most platforms more information which might lead to less surprises
but this also means using these functions is more expensive (beside
being less portable).
int
random_search (int n, int (*fp) (int, ucontext_t *))
{
volatile int cnt = 0;
ucontext_t uc;
/* Safe current context. */
if (getcontext (&uc) < 0)
return -1;
/* If we have not tried n times try again. */
if (cnt++ < n)
/* Call the function with a new random number
and the context. */
if (fp (rand (), &uc) != 0)
/* We found what we were looking for. */
return 1;
/* Not found. */
return 0;
}
Using contexts in such a way enables emulating exception handling. The search functions passed in the fp parameter could be very large, nested, and complex which would make it complicated (or at least would require a lot of code) to leave the function with an error value which has to be passed down to the caller. By using the context it is possible to leave the search function in one step and allow restarting the search which also has the nice side effect that it can be significantly faster.
Something which is harder to implement with setjmp and
longjmp is to switch temporarily to a different execution path
and then resume where execution was stopped.
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>
#include <sys/time.h>
/* Set by the signal handler. */
static volatile int expired;
/* The contexts. */
static ucontext_t uc[3];
/* We do only a certain number of switches. */
static int switches;
/* This is the function doing the work. It is just a
skeleton, real code has to be filled in. */
static void
f (int n)
{
int m = 0;
while (1)
{
/* This is where the work would be done. */
if (++m % 100 == 0)
{
putchar ('.');
fflush (stdout);
}
/* Regularly the expire variable must be checked. */
if (expired)
{
/* We do not want the program to run forever. */
if (++switches == 20)
return;
printf ("\nswitching from %d to %d\n", n, 3 - n);
expired = 0;
/* Switch to the other context, saving the current one. */
swapcontext (&uc[n], &uc[3 - n]);
}
}
}
/* This is the signal handler which simply set the variable. */
void
handler (int signal)
{
expired = 1;
}
int
main (void)
{
struct sigaction sa;
struct itimerval it;
char st1[8192];
char st2[8192];
/* Initialize the data structures for the interval timer. */
sa.sa_flags = SA_RESTART;
sigfillset (&sa.sa_mask);
sa.sa_handler = handler;
it.it_interval.tv_sec = 0;
it.it_interval.tv_usec = 1;
it.it_value = it.it_interval;
/* Install the timer and get the context we can manipulate. */
if (sigaction (SIGPROF, &sa, NULL) < 0
|| setitimer (ITIMER_PROF, &it, NULL) < 0
|| getcontext (&uc[1]) == -1
|| getcontext (&uc[2]) == -1)
abort ();
/* Create a context with a separate stack which causes the
function f to be call with the parameter 1.
Note that the uc_link points to the main context
which will cause the program to terminate once the function
return. */
uc[1].uc_link = &uc[0];
uc[1].uc_stack.ss_sp = st1;
uc[1].uc_stack.ss_size = sizeof st1;
makecontext (&uc[1], (void (*) (void)) f, 1, 1);
/* Similarly, but 2 is passed as the parameter to f. */
uc[2].uc_link = &uc[0];
uc[2].uc_stack.ss_sp = st2;
uc[2].uc_stack.ss_size = sizeof st2;
makecontext (&uc[2], (void (*) (void)) f, 1, 2);
/* Start running. */
swapcontext (&uc[0], &uc[1]);
putchar ('\n');
return 0;
}
This an example how the context functions can be used to implement
co-routines or cooperative multi-threading. All that has to be done is
to call every once in a while swapcontext to continue running a
different context. It is not allowed to do the context switching from
the signal handler directly since neither setcontext nor
swapcontext are functions which can be called from a signal
handler. But setting a variable in the signal handler and checking it
in the body of the functions which are executed. Since
swapcontext is saving the current context it is possible to have
multiple different scheduling points in the code. Execution will always
resume where it was left.
A signal is a software interrupt delivered to a process. The operating system uses signals to report exceptional situations to an executing program. Some signals report errors such as references to invalid memory addresses; others report asynchronous events, such as disconnection of a phone line.
The GNU C library defines a variety of signal types, each for a particular kind of event. Some kinds of events make it inadvisable or impossible for the program to proceed as usual, and the corresponding signals normally abort the program. Other kinds of signals that report harmless events are ignored by default.
If you anticipate an event that causes signals, you can define a handler function and tell the operating system to run it when that particular type of signal arrives.
Finally, one process can send a signal to another process; this allows a parent process to abort a child, or two related processes to communicate and synchronize.
This section explains basic concepts of how signals are generated, what happens after a signal is delivered, and how programs can handle signals.
A signal reports the occurrence of an exceptional event. These are some of the events that can cause (or generate, or raise) a signal:
kill or raise by the same process.
kill from another process. Signals are a limited but
useful form of interprocess communication.
Each of these kinds of events (excepting explicit calls to kill
and raise) generates its own particular kind of signal. The
various kinds of signals are listed and described in detail in
section Standard Signals.
In general, the events that generate signals fall into three major categories: errors, external events, and explicit requests.
An error means that a program has done something invalid and cannot
continue execution. But not all kinds of errors generate signals--in
fact, most do not. For example, opening a nonexistent file is an error,
but it does not raise a signal; instead, open returns -1.
In general, errors that are necessarily associated with certain library
functions are reported by returning a value that indicates an error.
The errors which raise signals are those which can happen anywhere in
the program, not just in library calls. These include division by zero
and invalid memory addresses.
An external event generally has to do with I/O or other processes. These include the arrival of input, the expiration of a timer, and the termination of a child process.
An explicit request means the use of a library function such as
kill whose purpose is specifically to generate a signal.
Signals may be generated synchronously or asynchronously. A synchronous signal pertains to a specific action in the program, and is delivered (unless blocked) during that action. Most errors generate signals synchronously, and so do explicit requests by a process to generate a signal for that same process. On some machines, certain kinds of hardware errors (usually floating-point exceptions) are not reported completely synchronously, but may arrive a few instructions later.
Asynchronous signals are generated by events outside the control of the process that receives them. These signals arrive at unpredictable times during execution. External events generate signals asynchronously, and so do explicit requests that apply to some other process.
A given type of signal is either typically synchronous or typically asynchronous. For example, signals for errors are typically synchronous because errors generate signals synchronously. But any type of signal can be generated synchronously or asynchronously with an explicit request.
When a signal is generated, it becomes pending. Normally it remains pending for just a short period of time and then is delivered to the process that was signaled. However, if that kind of signal is currently blocked, it may remain pending indefinitely--until signals of that kind are unblocked. Once unblocked, it will be delivered immediately. See section Blocking Signals.
When the signal is delivered, whether right away or after a long delay,
the specified action for that signal is taken. For certain
signals, such as SIGKILL and SIGSTOP, the action is fixed,
but for most signals, the program has a choice: ignore the signal,
specify a handler function, or accept the default action for
that kind of signal. The program specifies its choice using functions
such as signal or sigaction (see section Specifying Signal Actions). We
sometimes say that a handler catches the signal. While the
handler is running, that particular signal is normally blocked.
If the specified action for a kind of signal is to ignore it, then any such signal which is generated is discarded immediately. This happens even if the signal is also blocked at the time. A signal discarded in this way will never be delivered, not even if the program subsequently specifies a different action for that kind of signal and then unblocks it.
If a signal arrives which the program has neither handled nor ignored, its default action takes place. Each kind of signal has its own default action, documented below (see section Standard Signals). For most kinds of signals, the default action is to terminate the process. For certain kinds of signals that represent "harmless" events, the default action is to do nothing.
When a signal terminates a process, its parent process can determine the
cause of termination by examining the termination status code reported
by the wait or waitpid functions. (This is discussed in
more detail in section Process Completion.) The information it can get
includes the fact that termination was due to a signal and the kind of
signal involved. If a program you run from a shell is terminated by a
signal, the shell typically prints some kind of error message.
The signals that normally represent program errors have a special property: when one of these signals terminates the process, it also writes a core dump file which records the state of the process at the time of termination. You can examine the core dump with a debugger to investigate what caused the error.
If you raise a "program error" signal by explicit request, and this terminates the process, it makes a core dump file just as if the signal had been due directly to an error.
This section lists the names for various standard kinds of signals and describes what kind of event they mean. Each signal name is a macro which stands for a positive integer--the signal number for that kind of signal. Your programs should never make assumptions about the numeric code for a particular kind of signal, but rather refer to them always by the names defined here. This is because the number for a given kind of signal can vary from system to system, but the meanings of the names are standardized and fairly uniform.
The signal names are defined in the header file `signal.h'.
NSIG is also one greater than the largest defined signal number.
The following signals are generated when a serious program error is detected by the operating system or the computer itself. In general, all of these signals are indications that your program is seriously broken in some way, and there's usually no way to continue the computation which encountered the error.
Some programs handle program error signals in order to tidy up before terminating; for example, programs that turn off echoing of terminal input should handle program error signals in order to turn echoing back on. The handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (See section Handlers That Terminate the Process.)
Termination is the sensible ultimate outcome from a program error in
most programs. However, programming systems such as Lisp that can load
compiled user programs might need to keep executing even if a user
program incurs an error. These programs have handlers which use
longjmp to return control to the command level.
The default action for all of these signals is to cause the process to
terminate. If you block or ignore these signals or establish handlers
for them that return normally, your program will probably break horribly
when such signals happen, unless they are generated by raise or
kill instead of a real error.
When one of these program error signals terminates a process, it also
writes a core dump file which records the state of the process at
the time of termination. The core dump file is named `core' and is
written in whichever directory is current in the process at the time.
(On the GNU system, you can specify the file name for core dumps with
the environment variable COREFILE.) The purpose of core dump
files is so that you can examine them with a debugger to investigate
what caused the error.
SIGFPE signal reports a fatal arithmetic error. Although the
name is derived from "floating-point exception", this signal actually
covers all arithmetic errors, including division by zero and overflow.
If a program stores integer data in a location which is then used in a
floating-point operation, this often causes an "invalid operation"
exception, because the processor cannot recognize the data as a
floating-point number.
Actual floating-point exceptions are a complicated subject because there
are many types of exceptions with subtly different meanings, and the
SIGFPE signal doesn't distinguish between them. The IEEE
Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985
and ANSI/IEEE Std 854-1987)
defines various floating-point exceptions and requires conforming
computer systems to report their occurrences. However, this standard
does not specify how the exceptions are reported, or what kinds of
handling and control the operating system can offer to the programmer.
BSD systems provide the SIGFPE handler with an extra argument
that distinguishes various causes of the exception. In order to access
this argument, you must define the handler to accept two arguments,
which means you must cast it to a one-argument function type in order to
establish the handler. The GNU library does provide this extra
argument, but the value is meaningful only on operating systems that
provide the information (BSD systems and GNU systems).
FPE_INTOVF_TRAP
FPE_INTDIV_TRAP
FPE_SUBRNG_TRAP
FPE_FLTOVF_TRAP
FPE_FLTDIV_TRAP
FPE_FLTUND_TRAP
FPE_DECOVF_TRAP
SIGILL typically indicates that the executable file is corrupted,
or that you are trying to execute data. Some common ways of getting
into the latter situation are by passing an invalid object where a
pointer to a function was expected, or by writing past the end of an
automatic array (or similar problems with pointers to automatic
variables) and corrupting other data on the stack such as the return
address of a stack frame.
SIGILL can also be generated when the stack overflows, or when
the system has trouble running the handler for a signal.
Common ways of getting a SIGSEGV condition include dereferencing
a null or uninitialized pointer, or when you use a pointer to step
through an array, but fail to check for the end of the array. It varies
among systems whether dereferencing a null pointer generates
SIGSEGV or SIGBUS.
SIGSEGV, this signal is typically the result of dereferencing an
uninitialized pointer. The difference between the two is that
SIGSEGV indicates an invalid access to valid memory, while
SIGBUS indicates an access to an invalid address. In particular,
SIGBUS signals often result from dereferencing a misaligned
pointer, such as referring to a four-word integer at an address not
divisible by four. (Each kind of computer has its own requirements for
address alignment.)
The name of this signal is an abbreviation for "bus error".
abort. See section Aborting a Program.
SIGABRT.
SIGTRAP if it is somehow executing bad
instructions.
These signals are all used to tell a process to terminate, in one way or another. They have different names because they're used for slightly different purposes, and programs might want to handle them differently.
The reason for handling these signals is usually so your program can tidy up as appropriate before actually terminating. For example, you might want to save state information, delete temporary files, or restore the previous terminal modes. Such a handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (See section Handlers That Terminate the Process.)
The (obvious) default action for all of these signals is to cause the process to terminate.
SIGTERM signal is a generic signal used to cause program
termination. Unlike SIGKILL, this signal can be blocked,
handled, and ignored. It is the normal way to politely ask a program to
terminate.
SIGINT ("program interrupt") signal is sent when the user
types the INTR character (normally C-c). See section Special Characters, for information about terminal driver support for
C-c.
SIGQUIT signal is similar to SIGINT, except that it's
controlled by a different key--the QUIT character, usually
C-\---and produces a core dump when it terminates the process,
just like a program error signal. You can think of this as a
program error condition "detected" by the user.
See section Program Error Signals, for information about core dumps. See section Special Characters, for information about terminal driver support.
Certain kinds of cleanups are best omitted in handling SIGQUIT.
For example, if the program creates temporary files, it should handle
the other termination requests by deleting the temporary files. But it
is better for SIGQUIT not to delete them, so that the user can
examine them in conjunction with the core dump.
SIGKILL signal is used to cause immediate program termination.
It cannot be handled or ignored, and is therefore always fatal. It is
also not possible to block this signal.
This signal is usually generated only by explicit request. Since it
cannot be handled, you should generate it only as a last resort, after
first trying a less drastic method such as C-c or SIGTERM.
If a process does not respond to any other termination signals, sending
it a SIGKILL signal will almost always cause it to go away.
In fact, if SIGKILL fails to terminate a process, that by itself
constitutes an operating system bug which you should report.
The system will generate SIGKILL for a process itself under some
unusual conditions where the program cannot possibly continue to run
(even to run a signal handler).
SIGHUP ("hang-up") signal is used to report that the user's
terminal is disconnected, perhaps because a network or telephone
connection was broken. For more information about this, see section Control Modes.
This signal is also used to report the termination of the controlling process on a terminal to jobs associated with that session; this termination effectively disconnects all processes in the session from the controlling terminal. For more information, see section Termination Internals.
These signals are used to indicate the expiration of timers. See section Setting an Alarm, for information about functions that cause these signals to be sent.
The default behavior for these signals is to cause program termination. This default is rarely useful, but no other default would be useful; most of the ways of using these signals would require handler functions in any case.
alarm function, for example.
The signals listed in this section are used in conjunction with
asynchronous I/O facilities. You have to take explicit action by
calling fcntl to enable a particular file descriptor to generate
these signals (see section Interrupt-Driven Input). The default action for these
signals is to ignore them.
On most operating systems, terminals and sockets are the only kinds of
files that can generate SIGIO; other kinds, including ordinary
files, never generate SIGIO even if you ask them to.
In the GNU system SIGIO will always be generated properly
if you successfully set asynchronous mode with fcntl.
SIGIO.
It is defined only for compatibility.
These signals are used to support job control. If your system doesn't support job control, then these macros are defined but the signals themselves can't be raised or handled.
You should generally leave these signals alone unless you really understand how job control works. See section Job Control.
The default action for this signal is to ignore it. If you establish a
handler for this signal while there are child processes that have
terminated but not reported their status via wait or
waitpid (see section Process Completion), whether your new handler
applies to those processes or not depends on the particular operating
system.
SIGCONT signal to a process to make it continue.
This signal is special--it always makes the process continue if it is
stopped, before the signal is delivered. The default behavior is to do
nothing else. You cannot block this signal. You can set a handler, but
SIGCONT always makes the process continue regardless.
Most programs have no reason to handle SIGCONT; they simply
resume execution without realizing they were ever stopped. You can use
a handler for SIGCONT to make a program do something special when
it is stopped and continued--for example, to reprint a prompt when it
is suspended while waiting for input.
SIGTSTP signal is an interactive stop signal. Unlike
SIGSTOP, this signal can be handled and ignored.
Your program should handle this signal if you have a special need to
leave files or system tables in a secure state when a process is
stopped. For example, programs that turn off echoing should handle
SIGTSTP so they can turn echoing back on before stopping.
This signal is generated when the user types the SUSP character (normally C-z). For more information about terminal driver support, see section Special Characters.
SIGTTIN signal. The default action for this signal is to
stop the process. For more information about how this interacts with
the terminal driver, see section Access to the Controlling Terminal.
SIGTTIN, but is generated when a process in a
background job attempts to write to the terminal or set its modes.
Again, the default action is to stop the process. SIGTTOU is
only generated for an attempt to write to the terminal if the
TOSTOP output mode is set; see section Output Modes.
While a process is stopped, no more signals can be delivered to it until
it is continued, except SIGKILL signals and (obviously)
SIGCONT signals. The signals are marked as pending, but not
delivered until the process is continued. The SIGKILL signal
always causes termination of the process and can't be blocked, handled
or ignored. You can ignore SIGCONT, but it always causes the
process to be continued anyway if it is stopped. Sending a
SIGCONT signal to a process causes any pending stop signals for
that process to be discarded. Likewise, any pending SIGCONT
signals for a process are discarded when it receives a stop signal.
When a process in an orphaned process group (see section Orphaned Process Groups) receives a SIGTSTP, SIGTTIN, or SIGTTOU
signal and does not handle it, the process does not stop. Stopping the
process would probably not be very useful, since there is no shell
program that will notice it stop and allow the user to continue it.
What happens instead depends on the operating system you are using.
Some systems may do nothing; others may deliver another signal instead,
such as SIGKILL or SIGHUP. In the GNU system, the process
dies with SIGKILL; this avoids the problem of many stopped,
orphaned processes lying around the system.
These signals are used to report various errors generated by an operation done by the program. They do not necessarily indicate a programming error in the program, but an error that prevents an operating system call from completing. The default action for all of them is to cause the process to terminate.
SIGPIPE signal. If SIGPIPE is blocked, handled or
ignored, the offending call fails with EPIPE instead.
Pipes and FIFO special files are discussed in more detail in section Pipes and FIFOs.
Another cause of SIGPIPE is when you try to output to a socket
that isn't connected. See section Sending Data.
In the GNU system, SIGLOST is generated when any server program
dies unexpectedly. It is usually fine to ignore the signal; whatever
call was made to the server that died just returns an error.
These signals are used for various other purposes. In general, they will not affect your program unless it explicitly uses them for something.
SIGUSR1 and SIGUSR2 signals are set aside for you to
use any way you want. They're useful for simple interprocess
communication, if you write a signal handler for them in the program
that receives the signal.
There is an example showing the use of SIGUSR1 and SIGUSR2
in section Signaling Another Process.
The default action is to terminate the process.
If a program does full-screen display, it should handle SIGWINCH.
When the signal arrives, it should fetch the new screen size and
reformat its display accordingly.
If the process is the leader of the process group, the default action is to print some status information about the system and what the process is doing. Otherwise the default is to do nothing.
We mentioned above that the shell prints a message describing the signal
that terminated a child process. The clean way to print a message
describing a signal is to use the functions strsignal and
psignal. These functions use a signal number to specify which
kind of signal to describe. The signal number may come from the
termination status of a child process (see section Process Completion) or it
may come from a signal handler in the same process.
This function is a GNU extension, declared in the header file `string.h'.
stderr; see section Standard Streams.
If you call psignal with a message that is either a null
pointer or an empty string, psignal just prints the message
corresponding to signum, adding a trailing newline.
If you supply a non-null message argument, then psignal
prefixes its output with this string. It adds a colon and a space
character to separate the message from the string corresponding
to signum.
This function is a BSD feature, declared in the header file `signal.h'.
There is also an array sys_siglist which contains the messages
for the various signal codes. This array exists on BSD systems, unlike
strsignal.
The simplest way to change the action for a signal is to use the
signal function. You can specify a built-in action (such as to
ignore the signal), or you can establish a handler.
The GNU library also implements the more versatile sigaction
facility. This section describes both facilities and gives suggestions
on which to use when.
The signal function provides a simple interface for establishing
an action for a particular signal. The function and associated macros
are declared in the header file `signal.h'.
void. So, you should define handler functions like this:
void handler (int signum) { ... }
The name sighandler_t for this data type is a GNU extension.
signal function establishes action as the action for
the signal signum.
The first argument, signum, identifies the signal whose behavior you want to control, and should be a signal number. The proper way to specify a signal number is with one of the symbolic signal names (see section Standard Signals)---don't use an explicit number, because the numerical code for a given kind of signal may vary from operating system to operating system.
The second argument, action, specifies the action to use for the signal signum. This can be one of the following:
SIG_DFL
SIG_DFL specifies the default action for the particular signal.
The default actions for various kinds of signals are stated in
section Standard Signals.
SIG_IGN
SIG_IGN specifies that the signal should be ignored.
Your program generally should not ignore signals that represent serious
events or that are normally used to request termination. You cannot
ignore the SIGKILL or SIGSTOP signals at all. You can
ignore program error signals like SIGSEGV, but ignoring the error
won't enable the program to continue executing meaningfully. Ignoring
user requests such as SIGINT, SIGQUIT, and SIGTSTP
is unfriendly.
When you do not wish signals to be delivered during a certain part of
the program, the thing to do is to block them, not ignore them.
See section Blocking Signals.
handler
If you set the action for a signal to SIG_IGN, or if you set it
to SIG_DFL and the default action is to ignore that signal, then
any pending signals of that type are discarded (even if they are
blocked). Discarding the pending signals means that they will never be
delivered, not even if you subsequently specify another action and
unblock this kind of signal.
The signal function returns the action that was previously in
effect for the specified signum. You can save this value and
restore it later by calling signal again.
If signal can't honor the request, it returns SIG_ERR
instead. The following errno error conditions are defined for
this function:
EINVAL
SIGKILL or SIGSTOP.
Compatibility Note: A problem encountered when working with the
signal function is that it has different semantics on BSD and
SVID systems. The difference is that on SVID systems the signal handler
is deinstalled after signal delivery. On BSD systems the
handler must be explicitly deinstalled. In the GNU C Library we use the
BSD version by default. To use the SVID version you can either use the
function sysv_signal (see below) or use the _XOPEN_SOURCE
feature select macro (see section Feature Test Macros). In general, use of these
functions should be avoided because of compatibility problems. It
is better to use sigaction if it is available since the results
are much more reliable.
Here is a simple example of setting up a handler to delete temporary files when certain fatal signals happen:
#include <signal.h>
void
termination_handler (int signum)
{
struct temp_file *p;
for (p = temp_file_list; p; p = p->next)
unlink (p->name);
}
int
main (void)
{
...
if (signal (SIGINT, termination_handler) == SIG_IGN)
signal (SIGINT, SIG_IGN);
if (signal (SIGHUP, termination_handler) == SIG_IGN)
signal (SIGHUP, SIG_IGN);
if (signal (SIGTERM, termination_handler) == SIG_IGN)
signal (SIGTERM, SIG_IGN);
...
}
Note that if a given signal was previously set to be ignored, this code avoids altering that setting. This is because non-job-control shells often ignore certain signals when starting children, and it is important for the children to respect this.
We do not handle SIGQUIT or the program error signals in this
example because these are designed to provide information for debugging
(a core dump), and the temporary files may give useful information.
sysv_signal implements the behaviour of the standard
signal function as found on SVID systems. The difference to BSD
systems is that the handler is deinstalled after a delivery of a signal.
Compatibility Note: As said above for signal, this
function should be avoided when possible. sigaction is the
preferred method.
ssignal function does the same thing as signal; it is
provided only for compatibility with SVID.
signal
to indicate an error.
The sigaction function has the same basic effect as
signal: to specify how a signal should be handled by the process.
However, sigaction offers more control, at the expense of more
complexity. In particular, sigaction allows you to specify
additional flags to control when the signal is generated and how the
handler is invoked.
The sigaction function is declared in `signal.h'.
struct sigaction are used in the
sigaction function to specify all the information about how to
handle a particular signal. This structure contains at least the
following members:
sighandler_t sa_handler
signal function. The value can be SIG_DFL,
SIG_IGN, or a function pointer. See section Basic Signal Handling.
sigset_t sa_mask
sa_mask. If you want that signal not to be blocked within its
handler, you must write code in the handler to unblock it.
int sa_flags
sigaction.
signal function's return value--you can check to see what the
old action in effect for the signal was, and restore it later if you
want.)
Either action or old-action can be a null pointer. If old-action is a null pointer, this simply suppresses the return of information about the old action. If action is a null pointer, the action associated with the signal signum is unchanged; this allows you to inquire about how a signal is being handled without changing that handling.
The return value from sigaction is zero if it succeeds, and
-1 on failure. The following errno error conditions are
defined for this function:
EINVAL
SIGKILL or SIGSTOP.
signal and sigaction
It's possible to use both the signal and sigaction
functions within a single program, but you have to be careful because
they can interact in slightly strange ways.
The sigaction function specifies more information than the
signal function, so the return value from signal cannot
express the full range of sigaction possibilities. Therefore, if
you use signal to save and later reestablish an action, it may
not be able to reestablish properly a handler that was established with
sigaction.
To avoid having problems as a result, always use sigaction to
save and restore a handler if your program uses sigaction at all.
Since sigaction is more general, it can properly save and
reestablish any action, regardless of whether it was established
originally with signal or sigaction.
On some systems if you establish an action with signal and then
examine it with sigaction, the handler address that you get may
not be the same as what you specified with signal. It may not
even be suitable for use as an action argument with signal. But
you can rely on using it as an argument to sigaction. This
problem never happens on the GNU system.
So, you're better off using one or the other of the mechanisms consistently within a single program.
Portability Note: The basic signal function is a feature
of ISO C, while sigaction is part of the POSIX.1 standard. If
you are concerned about portability to non-POSIX systems, then you
should use the signal function instead.
sigaction Function Example
In section Basic Signal Handling, we gave an example of establishing a
simple handler for termination signals using signal. Here is an
equivalent example using sigaction:
#include <signal.h>
void
termination_handler (int signum)
{
struct temp_file *p;
for (p = temp_file_list; p; p = p->next)
unlink (p->name);
}
int
main (void)
{
...
struct sigaction new_action, old_action;
/* Set up the structure to specify the new action. */
new_action.sa_handler = termination_handler;
sigemptyset (&new_action.sa_mask);
new_action.sa_flags = 0;
sigaction (SIGINT, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGINT, &new_action, NULL);
sigaction (SIGHUP, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGHUP, &new_action, NULL);
sigaction (SIGTERM, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGTERM, &new_action, NULL);
...
}
The program just loads the new_action structure with the desired
parameters and passes it in the sigaction call. The usage of
sigemptyset is described later; see section Blocking Signals.
As in the example using signal, we avoid handling signals
previously set to be ignored. Here we can avoid altering the signal
handler even momentarily, by using the feature of sigaction that
lets us examine the current action without specifying a new one.
Here is another example. It retrieves information about the current
action for SIGINT without changing that action.
struct sigaction query_action; if (sigaction (SIGINT, NULL, &query_action) < 0) /*sigactionreturns -1 in case of error. */ else if (query_action.sa_handler == SIG_DFL) /*SIGINTis handled in the default, fatal manner. */ else if (query_action.sa_handler == SIG_IGN) /*SIGINTis ignored. */ else /* A programmer-defined signal handler is in effect. */
sigaction
The sa_flags member of the sigaction structure is a
catch-all for special features. Most of the time, SA_RESTART is
a good value to use for this field.
The value of sa_flags is interpreted as a bit mask. Thus, you
should choose the flags you want to set, OR those flags together,
and store the result in the sa_flags member of your
sigaction structure.
Each signal number has its own set of flags. Each call to
sigaction affects one particular signal number, and the flags
that you specify apply only to that particular signal.
In the GNU C library, establishing a handler with signal sets all
the flags to zero except for SA_RESTART, whose value depends on
the settings you have made with siginterrupt. See section Primitives Interrupted by Signals, to see what this is about.
These macros are defined in the header file `signal.h'.
SIGCHLD signal. When the
flag is set, the system delivers the signal for a terminated child
process but not for one that is stopped. By default, SIGCHLD is
delivered for both terminated children and stopped children.
Setting this flag for a signal other than SIGCHLD has no effect.
SIGILL.
open, read or write),
and the signal handler returns normally. There are two alternatives:
the library function can resume, or it can return failure with error
code EINTR.
The choice is controlled by the SA_RESTART flag for the
particular kind of signal that was delivered. If the flag is set,
returning from a handler resumes the library function. If the flag is
clear, returning from a handler makes the function fail.
See section Primitives Interrupted by Signals.
When a new process is created (see section Creating a Process), it inherits
handling of signals from its parent process. However, when you load a
new process image using the exec function (see section Executing a File), any signals that you've defined your own handlers for revert to
their SIG_DFL handling. (If you think about it a little, this
makes sense; the handler functions from the old program are specific to
that program, and aren't even present in the address space of the new
program image.) Of course, the new program can establish its own
handlers.
When a program is run by a shell, the shell normally sets the initial
actions for the child process to SIG_DFL or SIG_IGN, as
appropriate. It's a good idea to check to make sure that the shell has
not set up an initial action of SIG_IGN before you establish your
own signal handlers.
Here is an example of how to establish a handler for SIGHUP, but
not if SIGHUP is currently ignored:
...
struct sigaction temp;
sigaction (SIGHUP, NULL, &temp);
if (temp.sa_handler != SIG_IGN)
{
temp.sa_handler = handle_sighup;
sigemptyset (&temp.sa_mask);
sigaction (SIGHUP, &temp, NULL);
}
This section describes how to write a signal handler function that can
be established with the signal or sigaction functions.
A signal handler is just a function that you compile together with the
rest of the program. Instead of directly invoking the function, you use
signal or sigaction to tell the operating system to call
it when a signal arrives. This is known as establishing the
handler. See section Specifying Signal Actions.
There are two basic strategies you can use in signal handler functions:
You need to take special care in writing handler functions because they can be called asynchronously. That is, a handler might be called at any point in the program, unpredictably. If two signals arrive during a very short interval, one handler can run within another. This section describes what your handler should do, and what you should avoid.
Handlers which return normally are usually used for signals such as
SIGALRM and the I/O and interprocess communication signals. But
a handler for SIGINT might also return normally after setting a
flag that tells the program to exit at a convenient time.
It is not safe to return normally from the handler for a program error signal, because the behavior of the program when the handler function returns is not defined after a program error. See section Program Error Signals.
Handlers that return normally must modify some global variable in order
to have any effect. Typically, the variable is one that is examined
periodically by the program during normal operation. Its data type
should be sig_atomic_t for reasons described in section Atomic Data Access and Signal Handling.
Here is a simple example of such a program. It executes the body of
the loop until it has noticed that a SIGALRM signal has arrived.
This technique is useful because it allows the iteration in progress
when the signal arrives to complete before the loop exits.
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
/* This flag controls termination of the main loop. */
volatile sig_atomic_t keep_going = 1;
/* The signal handler just clears the flag and re-enables itself. */
void
catch_alarm (int sig)
{
keep_going = 0;
signal (sig, catch_alarm);
}
void
do_stuff (void)
{
puts ("Doing stuff while waiting for alarm....");
}
int
main (void)
{
/* Establish a handler for SIGALRM signals. */
signal (SIGALRM, catch_alarm);
/* Set an alarm to go off in a little while. */
alarm (2);
/* Check the flag once in a while to see when to quit. */
while (keep_going)
do_stuff ();
return EXIT_SUCCESS;
}
Handler functions that terminate the program are typically used to cause orderly cleanup or recovery from program error signals and interactive interrupts.
The cleanest way for a handler to terminate the process is to raise the same signal that ran the handler in the first place. Here is how to do this:
volatile sig_atomic_t fatal_error_in_progress = 0;
void
fatal_error_signal (int sig)
{
/* Since this handler is established for more than one kind of signal,
it might still get invoked recursively by delivery of some other kind
of signal. Use a static variable to keep track of that. */
if (fatal_error_in_progress)
raise (sig);
fatal_error_in_progress = 1;
/* Now do the clean up actions:
- reset terminal modes
- kill child processes
- remove lock files */
...
/* Now reraise the signal. We reactivate the signal's
default handling, which is to terminate the process.
We could just call exit or abort,
but reraising the signal sets the return status
from the process correctly. */
signal (sig, SIG_DFL);
raise (sig);
}
You can do a nonlocal transfer of control out of a signal handler using
the setjmp and longjmp facilities (see section Non-Local Exits).
When the handler does a nonlocal control transfer, the part of the program that was running will not continue. If this part of the program was in the middle of updating an important data structure, the data structure will remain inconsistent. Since the program does not terminate, the inconsistency is likely to be noticed later on.
There are two ways to avoid this problem. One is to block the signal for the parts of the program that update important data structures. Blocking the signal delays its delivery until it is unblocked, once the critical updating is finished. See section Blocking Signals.
The other way to re-initialize the crucial data structures in the signal handler, or make their values consistent.
Here is a rather schematic example showing the reinitialization of one global variable.
#include <signal.h>
#include <setjmp.h>
jmp_buf return_to_top_level;
volatile sig_atomic_t waiting_for_input;
void
handle_sigint (int signum)
{
/* We may have been waiting for input when the signal arrived,
but we are no longer waiting once we transfer control. */
waiting_for_input = 0;
longjmp (return_to_top_level, 1);
}
int
main (void)
{
...
signal (SIGINT, sigint_handler);
...
while (1) {
prepare_for_command ();
if (setjmp (return_to_top_level) == 0)
read_and_execute_command ();
}
}
/* Imagine this is a subroutine used by various commands. */
char *
read_data ()
{
if (input_from_terminal) {
waiting_for_input = 1;
...
waiting_for_input = 0;
} else {
...
}
}
What happens if another signal arrives while your signal handler function is running?
When the handler for a particular signal is invoked, that signal is
automatically blocked until the handler returns. That means that if two
signals of the same kind arrive close together, the second one will be
held until the first has been handled. (The handler can explicitly
unblock the signal using sigprocmask, if you want to allow more
signals of this type to arrive; see section Process Signal Mask.)
However, your handler can still be interrupted by delivery of another
kind of signal. To avoid this, you can use the sa_mask member of
the action structure passed to sigaction to explicitly specify
which signals should be blocked while the signal handler runs. These
signals are in addition to the signal for which the handler was invoked,
and any other signals that are normally blocked by the process.
See section Blocking Signals for a Handler.
When the handler returns, the set of blocked signals is restored to the
value it had before the handler ran. So using sigprocmask inside
the handler only affects what signals can arrive during the execution of
the handler itself, not what signals can arrive once the handler returns.
Portability Note: Always use sigaction to establish a
handler for a signal that you expect to receive asynchronously, if you
want your program to work properly on System V Unix. On this system,
the handling of a signal whose handler was established with
signal automatically sets the signal's action back to
SIG_DFL, and the handler must re-establish itself each time it
runs. This practice, while inconvenient, does work when signals cannot
arrive in succession. However, if another signal can arrive right away,
it may arrive before the handler can re-establish itself. Then the
second signal would receive the default handling, which could terminate
the process.
If multiple signals of the same type are delivered to your process before your signal handler has a chance to be invoked at all, the handler may only be invoked once, as if only a single signal had arrived. In effect, the signals merge into one. This situation can arise when the signal is blocked, or in a multiprocessing environment where the system is busy running some other processes while the signals are delivered. This means, for example, that you cannot reliably use a signal handler to count signals. The only distinction you can reliably make is whether at least one signal has arrived since a given time in the past.
Here is an example of a handler for SIGCHLD that compensates for
the fact that the number of signals received may not equal the number of
child processes that generate them. It assumes that the program keeps track
of all the child processes with a chain of structures as follows:
struct process
{
struct process *next;
/* The process ID of this child. */
int pid;
/* The descriptor of the pipe or pseudo terminal
on which output comes from this child. */
int input_descriptor;
/* Nonzero if this process has stopped or terminated. */
sig_atomic_t have_status;
/* The status of this child; 0 if running,
otherwise a status value from waitpid. */
int status;
};
struct process *process_list;
This example also uses a flag to indicate whether signals have arrived since some time in the past--whenever the program last cleared it to zero.
/* Nonzero means some child's status has changed
so look at process_list for the details. */
int process_status_change;
Here is the handler itself:
void
sigchld_handler (int signo)
{
int old_errno = errno;
while (1) {
register int pid;
int w;
struct process *p;
/* Keep asking for a status until we get a definitive result. */
do
{
errno = 0;
pid = waitpid (WAIT_ANY, &w, WNOHANG | WUNTRACED);
}
while (pid <= 0 && errno == EINTR);
if (pid <= 0) {
/* A real failure means there are no more
stopped or terminated child processes, so return. */
errno = old_errno;
return;
}
/* Find the process that signaled us, and record its status. */
for (p = process_list; p; p = p->next)
if (p->pid == pid) {
p->status = w;
/* Indicate that the status field
has data to look at. We do this only after storing it. */
p->have_status = 1;
/* If process has terminated, stop waiting for its output. */
if (WIFSIGNALED (w) || WIFEXITED (w))
if (p->input_descriptor)
FD_CLR (p->input_descriptor, &input_wait_mask);
/* The program should check this flag from time to time
to see if there is any news in process_list. */
++process_status_change;
}
/* Loop around to handle all the processes
that have something to tell us. */
}
}
Here is the proper way to check the flag process_status_change:
if (process_status_change) {
struct process *p;
process_status_change = 0;
for (p = process_list; p; p = p->next)
if (p->have_status) {
... Examine p->status ...
}
}
It is vital to clear the flag before examining the list; otherwise, if a signal were delivered just before the clearing of the flag, and after the appropriate element of the process list had been checked, the status change would go unnoticed until the next signal arrived to set the flag again. You could, of course, avoid this problem by blocking the signal while scanning the list, but it is much more elegant to guarantee correctness by doing things in the right order.
The loop which checks process status avoids examining p->status
until it sees that status has been validly stored. This is to make sure
that the status cannot change in the middle of accessing it. Once
p->have_status is set, it means that the child process is stopped
or terminated, and in either case, it cannot stop or terminate again
until the program has taken notice. See section Atomic Usage Patterns, for more
information about coping with interruptions during accesses of a
variable.
Here is another way you can test whether the handler has run since the last time you checked. This technique uses a counter which is never changed outside the handler. Instead of clearing the count, the program remembers the previous value and sees whether it has changed since the previous check. The advantage of this method is that different parts of the program can check independently, each part checking whether there has been a signal since that part last checked.
sig_atomic_t process_status_change;
sig_atomic_t last_process_status_change;
...
{
sig_atomic_t prev = last_process_status_change;
last_process_status_change = process_status_change;
if (last_process_status_change != prev) {
struct process *p;
for (p = process_list; p; p = p->next)
if (p->have_status) {
... Examine p->status ...
}
}
}
Handler functions usually don't do very much. The best practice is to
write a handler that does nothing but set an external variable that the
program checks regularly, and leave all serious work to the program.
This is best because the handler can be called asynchronously, at
unpredictable times--perhaps in the middle of a primitive function, or
even between the beginning and the end of a C operator that requires
multiple instructions. The data structures being manipulated might
therefore be in an inconsistent state when the handler function is
invoked. Even copying one int variable into another can take two
instructions on most machines.
This means you have to be very careful about what you do in a signal handler.
volatile. This tells the compiler that
the value of the variable might change asynchronously, and inhibits
certain optimizations that would be invalidated by such modifications.
A function can be non-reentrant if it uses memory that is not on the stack.
gethostbyname.
This function returns its value in a static object, reusing the same
object each time. If the signal happens to arrive during a call to
gethostbyname, or even after one (while the program is still
using the value), it will clobber the value that the program asked for.
However, if the program does not use gethostbyname or any other
function that returns information in the same object, or if it always
blocks signals around each use, then you are safe.
There are a large number of library functions that return values in a
fixed object, always reusing the same object in this fashion, and all of
them cause the same problem. Function descriptions in this manual
always mention this behavior.
fprintf. Suppose that the
program was in the middle of an fprintf call using the same
stream when the signal was delivered. Both the signal handler's message
and the program's data could be corrupted, because both calls operate on
the same data structure--the stream itself.
However, if you know that the stream that the handler uses cannot
possibly be used by the program at a time when signals can arrive, then
you are safe. It is no problem if the program uses some other stream.
malloc and free are not reentrant,
because they use a static data structure which records what memory
blocks are free. As a result, no library functions that allocate or
free memory are reentrant. This includes functions that allocate space
to store a result.
The best way to avoid the need to allocate memory in a handler is to
allocate in advance space for signal handlers to use.
The best way to avoid freeing memory in a handler is to flag or record
the objects to be freed, and have the program check from time to time
whether anything is waiting to be freed. But this must be done with
care, because placing an object on a chain is not atomic, and if it is
interrupted by another signal handler that does the same thing, you
could "lose" one of the objects.
errno is non-reentrant, but you can
correct for this: in the handler, save the original value of
errno and restore it before returning normally. This prevents
errors that occur within the signal handler from being confused with
errors from system calls at the point the program is interrupted to run
the handler.
This technique is generally applicable; if you want to call in a handler
a function that modifies a particular object in memory, you can make
this safe by saving and restoring that object.
Whether the data in your application concerns atoms, or mere text, you have to be careful about the fact that access to a single datum is not necessarily atomic. This means that it can take more than one instruction to read or write a single object. In such cases, a signal handler might be invoked in the middle of reading or writing the object.
There are three ways you can cope with this problem. You can use data types that are always accessed atomically; you can carefully arrange that nothing untoward happens if an access is interrupted, or you can block all signals around any access that had better not be interrupted (see section Blocking Signals).
Here is an example which shows what can happen if a signal handler runs in the middle of modifying a variable. (Interrupting the reading of a variable can also lead to paradoxical results, but here we only show writing.)
#include <signal.h>
#include <stdio.h>
struct two_words { int a, b; } memory;
void
handler(int signum)
{
printf ("%d,%d\n", memory.a, memory.b);
alarm (1);
}
int
main (void)
{
static struct two_words zeros = { 0, 0 }, ones = { 1, 1 };
signal (SIGALRM, handler);
memory = zeros;
alarm (1);
while (1)
{
memory = zeros;
memory = ones;
}
}
This program fills memory with zeros, ones, zeros, ones,
alternating forever; meanwhile, once per second, the alarm signal handler
prints the current contents. (Calling printf in the handler is
safe in this program because it is certainly not being called outside
the handler when the signal happens.)
Clearly, this program can print a pair of zeros or a pair of ones. But
that's not all it can do! On most machines, it takes several
instructions to store a new value in memory, and the value is
stored one word at a time. If the signal is delivered in between these
instructions, the handler might find that memory.a is zero and
memory.b is one (or vice versa).
On some machines it may be possible to store a new value in
memory with just one instruction that cannot be interrupted. On
these machines, the handler will always print two zeros or two ones.
To avoid uncertainty about interrupting access to a variable, you can
use a particular data type for which access is always atomic:
sig_atomic_t. Reading and writing this data type is guaranteed
to happen in a single instruction, so there's no way for a handler to
run "in the middle" of an access.
The type sig_atomic_t is always an integer data type, but which
one it is, and how many bits it contains, may vary from machine to
machine.
In practice, you can assume that int and other integer types no
longer than int are atomic. You can also assume that pointer
types are atomic; that is very convenient. Both of these assumptions
are true on all of the machines that the GNU C library supports and on
all POSIX systems we know of.
Certain patterns of access avoid any problem even if an access is interrupted. For example, a flag which is set by the handler, and tested and cleared by the main program from time to time, is always safe even if access actually requires two instructions. To show that this is so, we must consider each access that could be interrupted, and show that there is no problem if it is interrupted.
An interrupt in the middle of testing the flag is safe because either it's recognized to be nonzero, in which case the precise value doesn't matter, or it will be seen to be nonzero the next time it's tested.
An interrupt in the middle of clearing the flag is no problem because either the value ends up zero, which is what happens if a signal comes in just before the flag is cleared, or the value ends up nonzero, and subsequent events occur as if the signal had come in just after the flag was cleared. As long as the code handles both of these cases properly, it can also handle a signal in the middle of clearing the flag. (This is an example of the sort of reasoning you need to do to figure out whether non-atomic usage is safe.)
Sometimes you can insure uninterrupted access to one object by protecting its use with another object, perhaps one whose type guarantees atomicity. See section Signals Close Together Merge into One, for an example.
A signal can arrive and be handled while an I/O primitive such as
open or read is waiting for an I/O device. If the signal
handler returns, the system faces the question: what should happen next?
POSIX specifies one approach: make the primitive fail right away. The
error code for this kind of failure is EINTR. This is flexible,
but usually inconvenient. Typically, POSIX applications that use signal
handlers must check for EINTR after each library function that
can return it, in order to try the call again. Often programmers forget
to check, which is a common source of error.
The GNU library provides a convenient way to retry a call after a
temporary failure, with the macro TEMP_FAILURE_RETRY:
EINTR, TEMP_FAILURE_RETRY evaluates it again,
and over and over until the result is not a temporary failure.
The value returned by TEMP_FAILURE_RETRY is whatever value
expression produced.
BSD avoids EINTR entirely and provides a more convenient
approach: to restart the interrupted primitive, instead of making it
fail. If you choose this approach, you need not be concerned with
EINTR.
You can choose either approach with the GNU library. If you use
sigaction to establish a signal handler, you can specify how that
handler should behave. If you specify the SA_RESTART flag,
return from that handler will resume a primitive; otherwise, return from
that handler will cause EINTR. See section Flags for sigaction.
Another way to specify the choice is with the siginterrupt
function. See section BSD Function to Establish a Handler.
When you don't specify with sigaction or siginterrupt what
a particular handler should do, it uses a default choice. The default
choice in the GNU library depends on the feature test macros you have
defined. If you define _BSD_SOURCE or _GNU_SOURCE before
calling signal, the default is to resume primitives; otherwise,
the default is to make them fail with EINTR. (The library
contains alternate versions of the signal function, and the
feature test macros determine which one you really call.) See section Feature Test Macros.
The description of each primitive affected by this issue
lists EINTR among the error codes it can return.
There is one situation where resumption never happens no matter which
choice you make: when a data-transfer function such as read or
write is interrupted by a signal after transferring part of the
data. In this case, the function returns the number of bytes already
transferred, indicating partial success.
This might at first appear to cause unreliable behavior on
record-oriented devices (including datagram sockets; see section Datagram Socket Operations),
where splitting one read or write into two would read or
write two records. Actually, there is no problem, because interruption
after a partial transfer cannot happen on such devices; they always
transfer an entire record in one burst, with no waiting once data
transfer has started.
Besides signals that are generated as a result of a hardware trap or interrupt, your program can explicitly send signals to itself or to another process.
A process can send itself a signal with the raise function. This
function is declared in `signal.h'.
raise function sends the signal signum to the calling
process. It returns zero if successful and a nonzero value if it fails.
About the only reason for failure would be if the value of signum
is invalid.
gsignal function does the same thing as raise; it is
provided only for compatibility with SVID.
One convenient use for raise is to reproduce the default behavior
of a signal that you have trapped. For instance, suppose a user of your
program types the SUSP character (usually C-z; see section Special Characters) to send it an interactive stop signal
(SIGTSTP), and you want to clean up some internal data buffers
before stopping. You might set this up like this:
#include <signal.h>
/* When a stop signal arrives, set the action back to the default
and then resend the signal after doing cleanup actions. */
void
tstp_handler (int sig)
{
signal (SIGTSTP, SIG_DFL);
/* Do cleanup actions here. */
...
raise (SIGTSTP);
}
/* When the process is continued again, restore the signal handler. */
void
cont_handler (int sig)
{
signal (SIGCONT, cont_handler);
signal (SIGTSTP, tstp_handler);
}
/* Enable both handlers during program initialization. */
int
main (void)
{
signal (SIGCONT, cont_handler);
signal (SIGTSTP, tstp_handler);
...
}
Portability note: raise was invented by the ISO C
committee. Older systems may not support it, so using kill may
be more portable. See section Signaling Another Process.
The kill function can be used to send a signal to another process.
In spite of its name, it can be used for a lot of things other than
causing a process to terminate. Some examples of situations where you
might want to send signals between processes are:
This section assumes that you know a little bit about how processes work. For more information on this subject, see section Processes.
The kill function is declared in `signal.h'.
kill function sends the signal signum to the process
or process group specified by pid. Besides the signals listed in
section Standard Signals, signum can also have a value of zero to
check the validity of the pid.
The pid specifies the process or process group to receive the signal:
pid > 0
pid == 0
pid < -1
pid == -1
A process can send a signal to itself with a call like kill
(getpid(), signum). If kill is used by a process to send
a signal to itself, and the signal is not blocked, then kill
delivers at least one signal (which might be some other pending
unblocked signal instead of the signal signum) to that process
before it returns.
The return value from kill is zero if the signal can be sent
successfully. Otherwise, no signal is sent, and a value of -1 is
returned. If pid specifies sending a signal to several processes,
kill succeeds if it can send the signal to at least one of them.
There's no way you can tell which of the processes got the signal
or whether all of them did.
The following errno error conditions are defined for this function:
EINVAL
EPERM
ESCRH
kill, but sends signal signum to the
process group pgid. This function is provided for compatibility
with BSD; using kill to do this is more portable.
As a simple example of kill, the call kill (getpid (),
sig) has the same effect as raise (sig).
kill
There are restrictions that prevent you from using kill to send
signals to any random process. These are intended to prevent antisocial
behavior such as arbitrarily killing off processes belonging to another
user. In typical use, kill is used to pass signals between
parent, child, and sibling processes, and in these situations you
normally do have permission to send signals. The only common exception
is when you run a setuid program in a child process; if the program
changes its real UID as well as its effective UID, you may not have
permission to send a signal. The su program does this.
Whether a process has permission to send a signal to another process is determined by the user IDs of the two processes. This concept is discussed in detail in section The Persona of a Process.
Generally, for a process to be able to send a signal to another process, either the sending process must belong to a privileged user (like `root'), or the real or effective user ID of the sending process must match the real or effective user ID of the receiving process. If the receiving process has changed its effective user ID from the set-user-ID mode bit on its process image file, then the owner of the process image file is used in place of its current effective user ID. In some implementations, a parent process might be able to send signals to a child process even if the user ID's don't match, and other implementations might enforce other restrictions.
The SIGCONT signal is a special case. It can be sent if the
sender is part of the same session as the receiver, regardless of
user IDs.
kill for Communication
Here is a longer example showing how signals can be used for
interprocess communication. This is what the SIGUSR1 and
SIGUSR2 signals are provided for. Since these signals are fatal
by default, the process that is supposed to receive them must trap them
through signal or sigaction.
In this example, a parent process forks a child process and then waits
for the child to complete its initialization. The child process tells
the parent when it is ready by sending it a SIGUSR1 signal, using
the kill function.
#include <signal.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
/* When a SIGUSR1 signal arrives, set this variable. */
volatile sig_atomic_t usr_interrupt = 0;
void
synch_signal (int sig)
{
usr_interrupt = 1;
}
/* The child process executes this function. */
void
child_function (void)
{
/* Perform initialization. */
printf ("I'm here!!! My pid is %d.\n", (int) getpid ());
/* Let parent know you're done. */
kill (getppid (), SIGUSR1);
/* Continue with execution. */
puts ("Bye, now....");
exit (0);
}
int
main (void)
{
struct sigaction usr_action;
sigset_t block_mask;
pid_t child_id;
/* Establish the signal handler. */
sigfillset (&block_mask);
usr_action.sa_handler = synch_signal;
usr_action.sa_mask = block_mask;
usr_action.sa_flags = 0;
sigaction (SIGUSR1, &usr_action, NULL);
/* Create the child process. */
child_id = fork ();
if (child_id == 0)
child_function (); /* Does not return. */
/* Busy wait for the child to send a signal. */
while (!usr_interrupt)
;
/* Now continue execution. */
puts ("That's all, folks!");
return 0;
}
This example uses a busy wait, which is bad, because it wastes CPU cycles that other programs could otherwise use. It is better to ask the system to wait until the signal arrives. See the example in section Waiting for a Signal.
Blocking a signal means telling the operating system to hold it and
deliver it later. Generally, a program does not block signals
indefinitely--it might as well ignore them by setting their actions to
SIG_IGN. But it is useful to block signals briefly, to prevent
them from interrupting sensitive operations. For instance:
sigprocmask function to block signals while you
modify global variables that are also modified by the handlers for these
signals.
sa_mask in your sigaction call to block
certain signals while a particular signal handler runs. This way, the
signal handler can run without being interrupted itself by signals.
Temporary blocking of signals with sigprocmask gives you a way to
prevent interrupts during critical parts of your code. If signals
arrive in that part of the program, they are delivered later, after you
unblock them.
One example where this is useful is for sharing data between a signal
handler and the rest of the program. If the type of the data is not
sig_atomic_t (see section Atomic Data Access and Signal Handling), then the signal
handler could run when the rest of the program has only half finished
reading or writing the data. This would lead to confusing consequences.
To make the program reliable, you can prevent the signal handler from running while the rest of the program is examining or modifying that data--by blocking the appropriate signal around the parts of the program that touch the data.
Blocking signals is also necessary when you want to perform a certain
action only if a signal has not arrived. Suppose that the handler for
the signal sets a flag of type sig_atomic_t; you would like to
test the flag and perform the action if the flag is not set. This is
unreliable. Suppose the signal is delivered immediately after you test
the flag, but before the consequent action: then the program will
perform the action even though the signal has arrived.
The only way to test reliably for whether a signal has yet arrived is to test while the signal is blocked.
All of the signal blocking functions use a data structure called a signal set to specify what signals are affected. Thus, every activity involves two stages: creating the signal set, and then passing it as an argument to a library function.
These facilities are declared in the header file `signal.h'.
sigset_t data type is used to represent a signal set.
Internally, it may be implemented as either an integer or structure
type.
For portability, use only the functions described in this section to
initialize, change, and retrieve information from sigset_t
objects--don't try to manipulate them directly.
There are two ways to initialize a signal set. You can initially
specify it to be empty with sigemptyset and then add specified
signals individually. Or you can specify it to be full with
sigfillset and then delete specified signals individually.
You must always initialize the signal set with one of these two
functions before using it in any other way. Don't try to set all the
signals explicitly because the sigset_t object might include some
other information (like a version field) that needs to be initialized as
well. (In addition, it's not wise to put into your program an
assumption that the system has no signals aside from the ones you know
about.)
0.
0.
sigaddset does is modify set; it does not block or
unblock any signals.
The return value is 0 on success and -1 on failure.
The following errno error condition is defined for this function:
EINVAL
sigdelset does is modify set; it does not
block or unblock any signals. The return value and error conditions are
the same as for sigaddset.
Finally, there is a function to test what signals are in a signal set:
sigismember function tests whether the signal signum is
a member of the signal set set. It returns 1 if the signal
is in the set, 0 if not, and -1 if there is an error.
The following errno error condition is defined for this function:
EINVAL
The collection of signals that are currently blocked is called the signal mask. Each process has its own signal mask. When you create a new process (see section Creating a Process), it inherits its parent's mask. You can block or unblock signals with total flexibility by modifying the signal mask.
The prototype for the sigprocmask function is in `signal.h'.
sigprocmask function is used to examine or change the calling
process's signal mask. The how argument determines how the signal
mask is changed, and must be one of the following values:
SIG_BLOCK
set---add them to the existing mask. In
other words, the new mask is the union of the existing mask and
set.
SIG_UNBLOCK
SIG_SETMASK
The last argument, oldset, is used to return information about the
old process signal mask. If you just want to change the mask without
looking at it, pass a null pointer as the oldset argument.
Similarly, if you want to know what's in the mask without changing it,
pass a null pointer for set (in this case the how argument
is not significant). The oldset argument is often used to
remember the previous signal mask in order to restore it later. (Since
the signal mask is inherited over fork and exec calls, you
can't predict what its contents are when your program starts running.)
If invoking sigprocmask causes any pending signals to be
unblocked, at least one of those signals is delivered to the process
before sigprocmask returns. The order in which pending signals
are delivered is not specified, but you can control the order explicitly
by making multiple sigprocmask calls to unblock various signals
one at a time.
The sigprocmask function returns 0 if successful, and -1
to indicate an error. The following errno error conditions are
defined for this function:
EINVAL
You can't block the SIGKILL and SIGSTOP signals, but
if the signal set includes these, sigprocmask just ignores
them instead of returning an error status.
Remember, too, that blocking program error signals such as SIGFPE
leads to undesirable results for signals generated by an actual program
error (as opposed to signals sent with raise or kill).
This is because your program may be too broken to be able to continue
executing to a point where the signal is unblocked again.
See section Program Error Signals.
Now for a simple example. Suppose you establish a handler for
SIGALRM signals that sets a flag whenever a signal arrives, and
your main program checks this flag from time to time and then resets it.
You can prevent additional SIGALRM signals from arriving in the
meantime by wrapping the critical part of the code with calls to
sigprocmask, like this:
/* This variable is set by the SIGALRM signal handler. */
volatile sig_atomic_t flag = 0;
int
main (void)
{
sigset_t block_alarm;
...
/* Initialize the signal mask. */
sigemptyset (&block_alarm);
sigaddset (&block_alarm, SIGALRM);
while (1)
{
/* Check if a signal has arrived; if so, reset the flag. */
sigprocmask (SIG_BLOCK, &block_alarm, NULL);
if (flag)
{
actions-if-not-arrived
flag = 0;
}
sigprocmask (SIG_UNBLOCK, &block_alarm, NULL);
...
}
}
When a signal handler is invoked, you usually want it to be able to finish without being interrupted by another signal. From the moment the handler starts until the moment it finishes, you must block signals that might confuse it or corrupt its data.
When a handler function is invoked on a signal, that signal is
automatically blocked (in addition to any other signals that are already
in the process's signal mask) during the time the handler is running.
If you set up a handler for SIGTSTP, for instance, then the
arrival of that signal forces further SIGTSTP signals to wait
during the execution of the handler.
However, by default, other kinds of signals are not blocked; they can arrive during handler execution.
The reliable way to block other kinds of signals during the execution of
the handler is to use the sa_mask member of the sigaction
structure.
Here is an example:
#include <signal.h>
#include <stddef.h>
void catch_stop ();
void
install_handler (void)
{
struct sigaction setup_action;
sigset_t block_mask;
sigemptyset (&block_mask);
/* Block other terminal-generated signals while handler runs. */
sigaddset (&block_mask, SIGINT);
sigaddset (&block_mask, SIGQUIT);
setup_action.sa_handler = catch_stop;
setup_action.sa_mask = block_mask;
setup_action.sa_flags = 0;
sigaction (SIGTSTP, &setup_action, NULL);
}
This is more reliable than blocking the other signals explicitly in the code for the handler. If you block signals explicitly in the handler, you can't avoid at least a short interval at the beginning of the handler where they are not yet blocked.
You cannot remove signals from the process's current mask using this
mechanism. However, you can make calls to sigprocmask within
your handler to block or unblock signals as you wish.
In any case, when the handler returns, the system restores the mask that was in place before the handler was entered. If any signals that become unblocked by this restoration are pending, the process will receive those signals immediately, before returning to the code that was interrupted.
You can find out which signals are pending at any time by calling
sigpending. This function is declared in `signal.h'.
sigpending function stores information about pending signals
in set. If there is a pending signal that is blocked from
delivery, then that signal is a member of the returned set. (You can
test whether a particular signal is a member of this set using
sigismember; see section Signal Sets.)
The return value is 0 if successful, and -1 on failure.
Testing whether a signal is pending is not often useful. Testing when that signal is not blocked is almost certainly bad design.
Here is an example.
#include <signal.h>
#include <stddef.h>
sigset_t base_mask, waiting_mask;
sigemptyset (&base_mask);
sigaddset (&base_mask, SIGINT);
sigaddset (&base_mask, SIGTSTP);
/* Block user interrupts while doing other processing. */
sigprocmask (SIG_SETMASK, &base_mask, NULL);
...
/* After a while, check to see whether any signals are pending. */
sigpending (&waiting_mask);
if (sigismember (&waiting_mask, SIGINT)) {
/* User has tried to kill the process. */
}
else if (sigismember (&waiting_mask, SIGTSTP)) {
/* User has tried to stop the process. */
}
Remember that if there is a particular signal pending for your process,
additional signals of that same type that arrive in the meantime might
be discarded. For example, if a SIGINT signal is pending when
another SIGINT signal arrives, your program will probably only
see one of them when you unblock this signal.
Portability Note: The sigpending function is new in
POSIX.1. Older systems have no equivalent facility.
Instead of blocking a signal using the library facilities, you can get almost the same results by making the handler set a flag to be tested later, when you "unblock". Here is an example:
/* If this flag is nonzero, don't handle the signal right away. */
volatile sig_atomic_t signal_pending;
/* This is nonzero if a signal arrived and was not handled. */
volatile sig_atomic_t defer_signal;
void
handler (int signum)
{
if (defer_signal)
signal_pending = signum;
else
... /* "Really" handle the signal. */
}
...
void
update_mumble (int frob)
{
/* Prevent signals from having immediate effect. */
defer_signal++;
/* Now update mumble, without worrying about interruption. */
mumble.a = 1;
mumble.b = hack ();
mumble.c = frob;
/* We have updated mumble. Handle any signal that came in. */
defer_signal--;
if (defer_signal == 0 && signal_pending != 0)
raise (signal_pending);
}
Note how the particular signal that arrives is stored in
signal_pending. That way, we can handle several types of
inconvenient signals with the same mechanism.
We increment and decrement defer_signal so that nested critical
sections will work properly; thus, if update_mumble were called
with signal_pending already nonzero, signals would be deferred
not only within update_mumble, but also within the caller. This
is also why we do not check signal_pending if defer_signal
is still nonzero.
The incrementing and decrementing of defer_signal each require more
than one instruction; it is possible for a signal to happen in the
middle. But that does not cause any problem. If the signal happens
early enough to see the value from before the increment or decrement,
that is equivalent to a signal which came before the beginning of the
increment or decrement, which is a case that works properly.
It is absolutely vital to decrement defer_signal before testing
signal_pending, because this avoids a subtle bug. If we did
these things in the other order, like this,
if (defer_signal == 1 && signal_pending != 0)
raise (signal_pending);
defer_signal--;
then a signal arriving in between the if statement and the decrement
would be effectively "lost" for an indefinite amount of time. The
handler would merely set defer_signal, but the program having
already tested this variable, it would not test the variable again.
Bugs like these are called timing errors. They are especially bad because they happen only rarely and are nearly impossible to reproduce. You can't expect to find them with a debugger as you would find a reproducible bug. So it is worth being especially careful to avoid them.
(You would not be tempted to write the code in this order, given the use
of defer_signal as a counter which must be tested along with
signal_pending. After all, testing for zero is cleaner than
testing for one. But if you did not use defer_signal as a
counter, and gave it values of zero and one only, then either order
might seem equally simple. This is a further advantage of using a
counter for defer_signal: it will reduce the chance you will
write the code in the wrong order and create a subtle bug.)
If your program is driven by external events, or uses signals for synchronization, then when it has nothing to do it should probably wait until a signal arrives.
pause
The simple way to wait until a signal arrives is to call pause.
Please read about its disadvantages, in the following section, before
you use it.
pause function suspends program execution until a signal
arrives whose action is either to execute a handler function, or to
terminate the process.
If the signal causes a handler function to be executed, then
pause returns. This is considered an unsuccessful return (since
"successful" behavior would be to suspend the program forever), so the
return value is -1. Even if you specify that other primitives
should resume when a system handler returns (see section Primitives Interrupted by Signals), this has no effect on pause; it always fails when a
signal is handled.
The following errno error conditions are defined for this function:
EINTR
If the signal causes program termination, pause doesn't return
(obviously).
This function is a cancellation point in multithreaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time pause is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this calls to pause should be
protected using cancellation handlers.
The pause function is declared in `unistd.h'.
pause
The simplicity of pause can conceal serious timing errors that
can make a program hang mysteriously.
It is safe to use pause if the real work of your program is done
by the signal handlers themselves, and the "main program" does nothing
but call pause. Each time a signal is delivered, the handler
will do the next batch of work that is to be done, and then return, so
that the main loop of the program can call pause again.
You can't safely use pause to wait until one more signal arrives,
and then resume real work. Even if you arrange for the signal handler
to cooperate by setting a flag, you still can't use pause
reliably. Here is an example of this problem:
/* usr_interrupt is set by the signal handler. */
if (!usr_interrupt)
pause ();
/* Do work once the signal arrives. */
...
This has a bug: the signal could arrive after the variable
usr_interrupt is checked, but before the call to pause.
If no further signals arrive, the process would never wake up again.
You can put an upper limit on the excess waiting by using sleep
in a loop, instead of using pause. (See section Sleeping, for more
about sleep.) Here is what this looks like:
/* usr_interrupt is set by the signal handler.
while (!usr_interrupt)
sleep (1);
/* Do work once the signal arrives. */
...
For some purposes, that is good enough. But with a little more
complexity, you can wait reliably until a particular signal handler is
run, using sigsuspend.
sigsuspend
The clean and reliable way to wait for a signal to arrive is to block it
and then use sigsuspend. By using sigsuspend in a loop,
you can wait for certain kinds of signals, while letting other kinds of
signals be handled by their handlers.
If the process is woken up by delivery of a signal that invokes a handler
function, and the handler function returns, then sigsuspend also
returns.
The mask remains set only as long as sigsuspend is waiting.
The function sigsuspend always restores the previous signal mask
when it returns.
The return value and error conditions are the same as for pause.
With sigsuspend, you can replace the pause or sleep
loop in the previous section with something completely reliable:
sigset_t mask, oldmask; ... /* Set up the mask of signals to temporarily block. */ sigemptyset (&mask); sigaddset (&mask, SIGUSR1); ... /* Wait for a signal to arrive. */ sigprocmask (SIG_BLOCK, &mask, &oldmask); while (!usr_interrupt) sigsuspend (&oldmask); sigprocmask (SIG_UNBLOCK, &mask, NULL);
This last piece of code is a little tricky. The key point to remember
here is that when sigsuspend returns, it resets the process's
signal mask to the original value, the value from before the call to
sigsuspend---in this case, the SIGUSR1 signal is once
again blocked. The second call to sigprocmask is
necessary to explicitly unblock this signal.
One other point: you may be wondering why the while loop is
necessary at all, since the program is apparently only waiting for one
SIGUSR1 signal. The answer is that the mask passed to
sigsuspend permits the process to be woken up by the delivery of
other kinds of signals, as well--for example, job control signals. If
the process is woken up by a signal that doesn't set
usr_interrupt, it just suspends itself again until the "right"
kind of signal eventually arrives.
This technique takes a few more lines of preparation, but that is needed just once for each kind of wait criterion you want to use. The code that actually waits is just four lines.
A signal stack is a special area of memory to be used as the execution
stack during signal handlers. It should be fairly large, to avoid any
danger that it will overflow in turn; the macro SIGSTKSZ is
defined to a canonical size for signal stacks. You can use
malloc to allocate the space for the stack. Then call
sigaltstack or sigstack to tell the system to use that
space for the signal stack.
You don't need to write signal handlers differently in order to use a signal stack. Switching from one stack to the other happens automatically. (Some non-GNU debuggers on some machines may get confused if you examine a stack trace while a handler that uses the signal stack is running.)
There are two interfaces for telling the system to use a separate signal
stack. sigstack is the older interface, which comes from 4.2
BSD. sigaltstack is the newer interface, and comes from 4.4
BSD. The sigaltstack interface has the advantage that it does
not require your program to know which direction the stack grows, which
depends on the specific machine and operating system.
void *ss_sp
size_t ss_size
SIGSTKSZ
MINSIGSTKSZ
SIGSTKSZ for ss_size is
sufficient. But if you know how much stack space your program's signal
handlers will need, you may want to use a different size. In this case,
you should allocate MINSIGSTKSZ additional bytes for the signal
stack and increase ss_size accordingly.
int ss_flags
sigaltstack function specifies an alternate stack for use
during signal handling. When a signal is received by the process and
its action indicates that the signal stack is used, the system arranges
a switch to the currently installed signal stack while the handler for
that signal is executed.
If oldstack is not a null pointer, information about the currently installed signal stack is returned in the location it points to. If stack is not a null pointer, then this is installed as the new stack for use by signal handlers.
The return value is 0 on success and -1 on failure. If
sigaltstack fails, it sets errno to one of these values:
EINVAL
ENOMEM
MINSIGSTKSZ.
Here is the older sigstack interface. You should use
sigaltstack instead on systems that have it.
void *ss_sp
int ss_onstack
sigstack function specifies an alternate stack for use during
signal handling. When a signal is received by the process and its
action indicates that the signal stack is used, the system arranges a
switch to the currently installed signal stack while the handler for
that signal is executed.
If oldstack is not a null pointer, information about the currently installed signal stack is returned in the location it points to. If stack is not a null pointer, then this is installed as the new stack for use by signal handlers.
The return value is 0 on success and -1 on failure.
This section describes alternative signal handling functions derived from BSD Unix. These facilities were an advance, in their time; today, they are mostly obsolete, and supported mainly for compatibility with BSD Unix.
There are many similarities between the BSD and POSIX signal handling facilities, because the POSIX facilities were inspired by the BSD facilities. Besides having different names for all the functions to avoid conflicts, the main differences between the two are:
int bit mask, rather than
as a sigset_t object.
The BSD facilities are declared in `signal.h'.
struct sigaction
(see section Advanced Signal Handling); it is used to specify signal actions
to the sigvec function. It contains the following members:
sighandler_t sv_handler
int sv_mask
int sv_flags
sv_onstack.
These symbolic constants can be used to provide values for the
sv_flags field of a sigvec structure. This field is a bit
mask value, so you bitwise-OR the flags of interest to you together.
sv_flags field of a sigvec
structure, it means to use the signal stack when delivering the signal.
sv_flags field of a sigvec
structure, it means that system calls interrupted by this kind of signal
should not be restarted if the handler returns; instead, the system
calls should return with a EINTR error status. See section Primitives Interrupted by Signals.
sv_flags field of a sigvec
structure, it means to reset the action for the signal back to
SIG_DFL when the signal is received.
sigaction (see section Advanced Signal Handling); it installs the action action for the signal signum,
returning information about the previous action in effect for that signal
in old-action.
EINTR. See section Primitives Interrupted by Signals.
sigmask
together to specify more than one signal. For example,
(sigmask (SIGTSTP) | sigmask (SIGSTOP) | sigmask (SIGTTIN) | sigmask (SIGTTOU))
specifies a mask that includes all the job-control stop signals.
sigprocmask (see section Process Signal Mask) with a how argument of SIG_BLOCK: it adds the
signals specified by mask to the calling process's set of blocked
signals. The return value is the previous set of blocked signals.
sigprocmask (see section Process Signal Mask) with a how argument of SIG_SETMASK: it sets
the calling process's signal mask to mask. The return value is
the previous set of blocked signals.
sigsuspend (see section Waiting for a Signal): it sets the calling process's signal mask to mask,
and waits for a signal to arrive. On return the previous set of blocked
signals is restored.
Processes are the primitive units for allocation of system resources. Each process has its own address space and (usually) one thread of control. A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies. Though it may have multiple threads of control within the same program and a program may be composed of multiple logically separate modules, a process always executes exactly one program.
Note that we are using a specific definition of "program" for the purposes of this manual, which corresponds to a common definition in the context of Unix system. In popular usage, "program" enjoys a much broader definition; it can refer for example to a system's kernel, an editor macro, a complex package of software, or a discrete section of code executing within a process.
Writing the program is what this manual is all about. This chapter explains the most basic interface between your program and the system that runs, or calls, it. This includes passing of parameters (arguments and environment) from the system, requesting basic services from the system, and telling the system the program is done.
A program starts another program with the exec family of system calls.
This chapter looks at program startup from the execee's point of view. To
see the event from the execor's point of view, See section Executing a File.
The system starts a C program by calling the function main. It
is up to you to write a function named main---otherwise, you
won't even be able to link your program without errors.
In ISO C you can define main either to take no arguments, or to
take two arguments that represent the command line arguments to the
program, like this:
int main (int argc, char *argv[])
The command line arguments are the whitespace-separated tokens given in
the shell command used to invoke the program; thus, in `cat foo
bar', the arguments are `foo' and `bar'. The only way a
program can look at its command line arguments is via the arguments of
main. If main doesn't take arguments, then you cannot get
at the command line.
The value of the argc argument is the number of command line
arguments. The argv argument is a vector of C strings; its
elements are the individual command line argument strings. The file
name of the program being run is also included in the vector as the
first element; the value of argc counts this element. A null
pointer always follows the last element: argv[argc]
is this null pointer.
For the command `cat foo bar', argc is 3 and argv has
three elements, "cat", "foo" and "bar".
In Unix systems you can define main a third way, using three arguments:
int main (int argc, char *argv[], char *envp[])
The first two arguments are just the same. The third argument
envp gives the program's environment; it is the same as the value
of environ. See section Environment Variables. POSIX.1 does not
allow this three-argument form, so to be portable it is best to write
main to take two arguments, and use the value of environ.
POSIX recommends these conventions for command line arguments.
getopt (see section Parsing program options using getopt) and argp_parse (see section Parsing Program Options with Argp) make
it easy to implement them.
isalnum;
see section Classification of Characters).
ld command requires an argument--an output file name.
getopt and argp_parse in the GNU C
library normally make it appear as if all the option arguments were
specified before all the non-option arguments for the purposes of
parsing, even if the user of your program intermixed option and
non-option arguments. They do this by reordering the elements of the
argv array. This behavior is nonstandard; if you want to suppress
it, define the _POSIX_OPTION_ORDER environment variable.
See section Standard Environment Variables.
GNU adds long options to these conventions. Long options consist of `--' followed by a name made of alphanumeric characters and dashes. Option names are typically one to three words long, with hyphens to separate words. Users can abbreviate the option names as long as the abbreviations are unique.
To specify an argument for a long option, write `--name=value'. This syntax enables a long option to accept an argument that is itself optional.
Eventually, the GNU system will provide completion for long option names in the shell.
If the syntax for the command line arguments to your program is simple
enough, you can simply pick the arguments off from argv by hand.
But unless your program takes a fixed number of arguments, or all of the
arguments are interpreted in the same way (as file names, for example),
you are usually better off using getopt (see section Parsing program options using getopt) or
argp_parse (see section Parsing Program Options with Argp) to do the parsing.
getopt is more standard (the short-option only version of it is a
part of the POSIX standard), but using argp_parse is often
easier, both for very simple and very complex option structures, because
it does more of the dirty work for you.
getopt
The getopt and getopt_long functions automate some of the
chore involved in parsing typical unix command line options.
getopt function
Here are the details about how to call the getopt function. To
use this facility, your program must include the header file
`unistd.h'.
getopt prints an
error message to the standard error stream if it encounters an unknown
option character or an option with a missing required argument. This is
the default behavior. If you set this variable to zero, getopt
does not print any messages, but it still returns the character ?
to indicate an error.
getopt encounters an unknown option character or an option
with a missing required argument, it stores that option character in
this variable. You can use this for providing your own diagnostic
messages.
getopt to the index of the next element
of the argv array to be processed. Once getopt has found
all of the option arguments, you can use this variable to determine
where the remaining non-option arguments begin. The initial value of
this variable is 1.
getopt to point at the value of the
option argument, for those options that accept arguments.
getopt function gets the next option argument from the
argument list specified by the argv and argc arguments.
Normally these values come directly from the arguments received by
main.
The options argument is a string that specifies the option characters that are valid for this program. An option character in this string can be followed by a colon (`:') to indicate that it takes a required argument. If an option character is followed by two colons (`::'), its argument is optional; this is a GNU extension.
getopt has three ways to deal with options that follow
non-options argv elements. The special argument `--' forces
in all cases the end of option scanning.
POSIXLY_CORRECT or beginning the options argument
string with a plus sign (`+').
The getopt function returns the option character for the next
command line option. When no more option arguments are available, it
returns -1. There may still be more non-option arguments; you
must compare the external variable optind against the argc
parameter to check this.
If the option has an argument, getopt returns the argument by
storing it in the variable optarg. You don't ordinarily need to
copy the optarg string, since it is a pointer into the original
argv array, not into a static area that might be overwritten.
If getopt finds an option character in argv that was not
included in options, or a missing option argument, it returns
`?' and sets the external variable optopt to the actual
option character. If the first character of options is a colon
(`:'), then getopt returns `:' instead of `?' to
indicate a missing option argument. In addition, if the external
variable opterr is nonzero (which is the default), getopt
prints an error message.
getopt
Here is an example showing how getopt is typically used. The
key points to notice are:
getopt is called in a loop. When getopt returns
-1, indicating no more options are present, the loop terminates.
switch statement is used to dispatch on the return value from
getopt. In typical use, each case just sets a variable that
is used later in the program.
#include <unistd.h>
#include <stdio.h>
int
main (int argc, char **argv)
{
int aflag = 0;
int bflag = 0;
char *cvalue = NULL;
int index;
int c;
opterr = 0;
while ((c = getopt (argc, argv, "abc:")) != -1)
switch (c)
{
case 'a':
aflag = 1;
break;
case 'b':
bflag = 1;
break;
case 'c':
cvalue = optarg;
break;
case '?':
if (isprint (optopt))
fprintf (stderr, "Unknown option `-%c'.\n", optopt);
else
fprintf (stderr,
"Unknown option character `\\x%x'.\n",
optopt);
return 1;
default:
abort ();
}
printf ("aflag = %d, bflag = %d, cvalue = %s\n",
aflag, bflag, cvalue);
for (index = optind; index < argc; index++)
printf ("Non-option argument %s\n", argv[index]);
return 0;
}
Here are some examples showing what this program prints with different combinations of arguments:
% testopt aflag = 0, bflag = 0, cvalue = (null) % testopt -a -b aflag = 1, bflag = 1, cvalue = (null) % testopt -ab aflag = 1, bflag = 1, cvalue = (null) % testopt -c foo aflag = 0, bflag = 0, cvalue = foo % testopt -cfoo aflag = 0, bflag = 0, cvalue = foo % testopt arg1 aflag = 0, bflag = 0, cvalue = (null) Non-option argument arg1 % testopt -a arg1 aflag = 1, bflag = 0, cvalue = (null) Non-option argument arg1 % testopt -c foo arg1 aflag = 0, bflag = 0, cvalue = foo Non-option argument arg1 % testopt -a -- -b aflag = 1, bflag = 0, cvalue = (null) Non-option argument -b % testopt -a - aflag = 1, bflag = 0, cvalue = (null) Non-option argument -
getopt_long
To accept GNU-style long options as well as single-character options,
use getopt_long instead of getopt. This function is
declared in `getopt.h', not `unistd.h'. You should make every
program accept long options if it uses any options, for this takes
little extra work and helps beginners remember how to use the program.
getopt_long. The argument longopts must be an array of
these structures, one for each long option. Terminate the array with an
element containing all zeros.
The struct option structure has these fields:
const char *name
int has_arg
no_argument,
required_argument and optional_argument.
int *flag
int val
flag is a null pointer, then the val is a value which
identifies this option. Often these values are chosen to uniquely
identify particular long options.
If flag is not a null pointer, it should be the address of an
int variable which is the flag for this option. The value in
val is the value to store in the flag to indicate that the option
was seen.
getopt. The argument longopts describes the long
options to accept (see above).
When getopt_long encounters a short option, it does the same
thing that getopt would do: it returns the character code for the
option, and stores the options argument (if it has one) in optarg.
When getopt_long encounters a long option, it takes actions based
on the flag and val fields of the definition of that
option.
If flag is a null pointer, then getopt_long returns the
contents of val to indicate which option it found. You should
arrange distinct values in the val field for options with
different meanings, so you can decode these values after
getopt_long returns. If the long option is equivalent to a short
option, you can use the short option's character code in val.
If flag is not a null pointer, that means this option should just
set a flag in the program. The flag is a variable of type int
that you define. Put the address of the flag in the flag field.
Put in the val field the value you would like this option to
store in the flag. In this case, getopt_long returns 0.
For any long option, getopt_long tells you the index in the array
longopts of the options definition, by storing it into
*indexptr. You can get the name of the option with
longopts[*indexptr].name. So you can distinguish among
long options either by the values in their val fields or by their
indices. You can also distinguish in this way among long options that
set flags.
When a long option has an argument, getopt_long puts the argument
value in the variable optarg before returning. When the option
has no argument, the value in optarg is a null pointer. This is
how you can tell whether an optional argument was supplied.
When getopt_long has no more options to handle, it returns
-1, and leaves in the variable optind the index in
argv of the next remaining argument.
Since long option names were used before before the getopt_long
options was invented there are program interfaces which require programs
to recognize options like `-option value' instead of
`--option value'. To enable these programs to use the GNU
getopt functionality there is one more function available.
The getopt_long_only function is equivalent to the
getopt_long function but it allows to specify the user of the
application to pass long options with only `-' instead of
`--'. The `--' prefix is still recognized but instead of
looking through the short options if a `-' is seen it is first
tried whether this parameter names a long option. If not, it is parsed
as a short option.
Assuming getopt_long_only is used starting an application with
app -foo
the getopt_long_only will first look for a long option named
`foo'. If this is not found, the short options `f', `o',
and again `o' are recognized.
getopt_long
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
/* Flag set by `--verbose'. */
static int verbose_flag;
int
main (argc, argv)
int argc;
char **argv;
{
int c;
while (1)
{
static struct option long_options[] =
{
/* These options set a flag. */
{"verbose", no_argument, &verbose_flag, 1},
{"brief", no_argument, &verbose_flag, 0},
/* These options don't set a flag.
We distinguish them by their indices. */
{"add", required_argument, 0, 'a'},
{"append", no_argument, 0, 'b'},
{"delete", required_argument, 0, 'd'},
{"create", no_argument, 0, 'c'},
{"file", required_argument, 0, 'f'},
{0, 0, 0, 0}
};
/* getopt_long stores the option index here. */
int option_index = 0;
c = getopt_long (argc, argv, "abc:d:f:",
long_options, &option_index);
/* Detect the end of the options. */
if (c == -1)
break;
switch (c)
{
case 0:
/* If this option set a flag, do nothing else now. */
if (long_options[option_index].flag != 0)
break;
printf ("option %s", long_options[option_index].name);
if (optarg)
printf (" with arg %s", optarg);
printf ("\n");
break;
case 'a':
puts ("option -a\n");
break;
case 'b':
puts ("option -b\n");
break;
case 'c':
printf ("option -c with value `%s'\n", optarg);
break;
case 'd':
printf ("option -d with value `%s'\n", optarg);
break;
case 'f':
printf ("option -f with value `%s'\n", optarg);
break;
case '?':
/* getopt_long already printed an error message. */
break;
default:
abort ();
}
}
/* Instead of reporting `--verbose'
and `--brief' as they are encountered,
we report the final status resulting from them. */
if (verbose_flag)
puts ("verbose flag is set");
/* Print any remaining command line arguments (not options). */
if (optind < argc)
{
printf ("non-option ARGV-elements: ");
while (optind < argc)
printf ("%s ", argv[optind++]);
putchar ('\n');
}
exit (0);
}
Argp is an interface for parsing unix-style argument vectors (see section Program Arguments).
Unlike the more common getopt interface, it provides many related
convenience features in addition to parsing options, such as
automatically producing output in response to `--help' and
`--version' options (as defined by the GNU coding standards).
Doing these things in argp results in a more consistent look for
programs that use it, and makes less likely that implementors will
neglect to implement them or keep them up-to-date.
Argp also provides the ability to merge several independently defined option parsers into one, mediating conflicts between them, and making the result appear seamless. A library can export an argp option parser, which programs can easily use in conjunction with their own option parser. This results in less work for user programs (indeed, some may use only argument parsers exported by libraries, and have no options of their own), and more consistent option-parsing for the abstractions implemented by the library.
The header file `<argp.h>' should be included to use argp.
argp_parse Function
The main interface to argp is the argp_parse function; often, a
call to argp_parse is the only argument-parsing code needed in
main (see section Program Arguments).
argp_parse function parses the arguments in argv, of
length argc, using the argp parser argp (see section Specifying Argp Parsers); a value of zero is the same as a struct argp
containing all zeros. flags is a set of flag bits that modify the
parsing behavior (see section Flags for argp_parse). input is passed through to
the argp parser argp, and has meaning defined by it; a typical
usage is to pass a pointer to a structure which can be used for
specifying parameters to the parser and passing back results from it.
Unless the ARGP_NO_EXIT or ARGP_NO_HELP flags are included
in flags, calling argp_parse may result in the program
exiting--for instance when an unknown option is encountered.
See section Program Termination.
If arg_index is non-null, the index of the first unparsed option in argv is returned in it.
The return value is zero for successful parsing, or an error code
(see section Error Codes) if an error was detected. Different argp parsers
may return arbitrary error codes, but standard ones are ENOMEM if
a memory allocation error occurred, or EINVAL if an unknown option
or option argument was encountered.
These variables make it very easy for every user program to implement the `--version' option and provide a bug-reporting address in the `--help' output (which is implemented by argp regardless).
argp_parse
(unless the ARGP_NO_HELP flag is used), which will print this
string followed by a newline and exit (unless the ARGP_NO_EXIT
flag is used).
argp_program_bug_address should point to a string that is the
bug-reporting address for the program. It will be printed at the end of
the standard output for the `--help' option, embedded in a sentence
that says something like `Report bugs to address.'.
argp_parse
(unless the ARGP_NO_HELP flag is used), which calls this function
to print the version, and then exits with a status of 0 (unless the
ARGP_NO_EXIT flag is used). It should point to a function with
the following type signature:
void print-version (FILE *stream, struct argp_state *state)
See section Argp Parsing State, for an explanation of state.
This variable takes precedent over argp_program_version, and is
useful if a program has version information that cannot be easily
specified as a simple string.
EX_USAGE from `<sysexits.h>'.
The first argument to the argp_parse function is a pointer to a
struct argp, which known as an argp parser:
const struct argp_option *options
argp_option structures specifying which
options this argp parser understands; it may be zero if there are no
options at all. See section Specifying Options in an Argp Parser.
argp_parser_t parser
ARGP_ERR_UNKNOWN.
See section Argp Parser Functions.
const char *args_doc
const char *doc
'\v', '\013') character. By
convention, the documentation before the options is just a short string
saying what the program does, and that afterwards is longer, describing
the behavior in more detail.
const struct argp_child *children
argp_children structures specifying
additional argp parsers that should be combined with this one.
See section Combining Multiple Argp Parsers.
char *(*help_filter)(int key, const char *text, void *input)
const char *argp_domain
The options, parser, args_doc, and doc
fields are usually all that are needed. If an argp parser is defined as
an initialized C variable, only the used fields need be specified in
the initializer--the rest will default to zero due to the way C
structure initialization works (this fact is exploited for most argp
structures, grouping the most-used fields near the beginning, so that
unused fields can simply be left unspecified).
The options field in a struct argp points to a vector of
struct argp_option structures, each of which specifies an option
that argp parser supports (actually, sometimes multiple entries may used
for a single option if it has many names). It should be terminated by
an entry with zero in all fields (note that when using an initialized C
array for options, writing { 0 } is enough to achieve this).
const char *name
OPTION_ALIAS flag set (see section Flags for Argp Options).
int key
isascii (key) is
true), it also specifies a short option `-char', where
char is the ASCII character with the code key.
const char *arg
OPTION_ARG_OPTIONAL flag (see section Flags for Argp Options) is set, in which case it may be provided.
int flags
const char *doc
name and key fields are zero, this string
will be printed out-dented from the normal option column, making it
useful as a group header (it will be the first thing printed in its
group); in this usage, it's conventional to end the string with a
`:' character.
int group
name and
key fields both zero), in which case, the previous entry + 1 is
the default. Automagic options such as `--help' are put into group
-1.
Note that because of C structure initialization rules, this field
often need not be specified, because 0 is the right value.
The following flags may be or'd together in the flags field of a
struct argp_option, and control various aspects of how that
option is parsed or displayed in help messages:
OPTION_ARG_OPTIONAL
OPTION_HIDDEN
OPTION_ALIAS
name and key from the aliased option.
OPTION_DOC
name field is displayed
unmodified (e.g., no `--' prefix is added) at the left-margin
(where a short option would normally be displayed), and the
documentation string in the normal place. For purposes of sorting, any
leading whitespace and punctuation is ignored, except that if the first
non-whitespace character is not `-', this entry is displayed after
all options (and OPTION_DOC entries with a leading `-') in
the same group.
OPTION_NO_USAGE
args_doc field
(see section Specifying Argp Parsers), in which case including the option
in the generic usage list would be redundant.
For instance, if args_doc is "FOO BAR\n-x BLAH", and the
`-x' option's purpose is to distinguish these two cases, `-x'
should probably be marked OPTION_NO_USAGE.
The function pointed to by the parser field in a struct
argp (see section Specifying Argp Parsers) defines what actions take place in response
to each option or argument that is parsed, and is also used as a hook,
to allow a parser to do something at certain other points during
parsing.
Argp parser functions have the following type signature:
error_t parser (int key, char *arg, struct argp_state *state)
where the arguments are as follows:
key field in the option vector
(see section Specifying Options in an Argp Parser). parser is also called at other
times with special reserved keys, such as ARGP_KEY_ARG for
non-option arguments. See section Special Keys for Argp Parser Functions.
arg
field can ever have a value, and those must always have a value,
unless the OPTION_ARG_OPTIONAL flag was specified (if the input
being parsed specifies a value for an option that doesn't allow one, an
error results before parser ever gets called).
If key is ARGP_KEY_ARG, arg is a non-option argument;
other special keys always have a zero arg.
struct argp_state, containing useful
information about the current parsing state for use by parser.
See section Argp Parsing State.
When parser is called, it should perform whatever action is
appropriate for key, and return either 0 for success,
ARGP_ERR_UNKNOWN, if the value of key is not handled by
this parser function, or a unix error code if a real error occurred
(see section Error Codes).
ARGP_ERR_UNKNOWN for any
key value they do not recognize, or for non-option arguments
(key == ARGP_KEY_ARG) that they do not wish to handle.
A typical parser function uses a switch statement on key:
error_t
parse_opt (int key, char *arg, struct argp_state *state)
{
switch (key)
{
case option_key:
action
break;
...
default:
return ARGP_ERR_UNKNOWN;
}
return 0;
}
In addition to key values corresponding to user options, the key argument to argp parser functions may have a number of other special values (arg and state refer to parser function arguments; see section Argp Parser Functions):
ARGP_KEY_ARG
ARGP_ERR_UNKNOWN; if an argument is handled by no one,
argp_parse immediately returns success, without parsing any more
arguments.
Once a parser function returns success for this key, that fact is
recorded, and the ARGP_KEY_NO_ARGS case won't be used.
However, if while processing the argument, a parser function
decrements the next field of its state argument, the option
won't be considered processed; this is to allow you to actually modify
the argument (perhaps into an option), and have it processed again.
ARGP_KEY_ARGS
ARGP_ERR_UNKNOWN for
ARGP_KEY_ARG, it is immediately called again with the key
ARGP_KEY_ARGS, which has a similar meaning, but is slightly more
convenient for consuming all remaining arguments. arg is 0, and
the tail of the argument vector may be found at state->argv
+ state->next. If success is returned for this key, and
state->next is unchanged, then all remaining arguments are
considered to have been consumed, otherwise, the amount by which
state->next has been adjust indicates how many were used.
For instance, here's an example that uses both, for different args:
...
case ARGP_KEY_ARG:
if (state->arg_num == 0)
/* First argument */
first_arg = arg;
else
/* Let the next case parse it. */
return ARGP_KEY_UNKNOWN;
break;
case ARGP_KEY_ARGS:
remaining_args = state->argv + state->next;
num_remaining_args = state->argc - state->next;
break;
ARGP_KEY_END
ARGP_KEY_NO_ARGS
ARGP_KEY_END (where more general validity checks on
previously parsed arguments can take place).
ARGP_KEY_INIT
child_input field of state, if any, are
copied to each child's state to be the initial value of the input
when their parsers are called.
ARGP_KEY_SUCCESS
ARGP_KEY_ERROR
ARGP_KEY_SUCCESS is never made).
ARGP_KEY_FINI
ARGP_KEY_SUCCESS and ARGP_KEY_ERROR). Any resources
allocated by ARGP_KEY_INIT may be freed here (although sometimes
certain resources allocated there are to be returned to the caller after
a successful parse; in that case, those particular resources can be
freed in the ARGP_KEY_ERROR case).
In all cases, ARGP_KEY_INIT is the first key seen by parser
functions, and ARGP_KEY_FINI the last (unless an error was
returned by the parser for ARGP_KEY_INIT). Other keys can occur
in one the following orders (opt refers to an arbitrary option
key):
ARGP_KEY_NO_ARGS ARGP_KEY_END ARGP_KEY_SUCCESS
ARGP_KEY_ARG )... ARGP_KEY_END ARGP_KEY_SUCCESS
ARGP_KEY_ARG )... ARGP_KEY_SUCCESS
ARGP_KEY_UNKNOWN
for an argument, in which case parsing stops at that argument. If
arg_index is a null pointer otherwise an error occurs.
In all cases, if a non-null value for arg_index was passed to
argp_parse, the index of the first unparsed command-line argument
is passed back in it.
If an error occurs (either detected by argp, or because a parser
function returned an error value), then each parser is called with
ARGP_KEY_ERROR, and no further calls are made except the final
call with ARGP_KEY_FINI.
Argp provides a number of functions for the user of argp parser functions (see section Argp Parser Functions), mostly for producing error messages. These take as their first argument the state argument to the parser function (see section Argp Parsing State).
state->err_stream and terminate the program
with exit (argp_err_exit_status) (see section Argp Global Variables).
argp_err_exit_status (see section Argp Global Variables).
error,
print the printf format string fmt and following args, preceded by
the program name and `:', and followed by the standard unix error
text for errnum if it is non-zero; then if status is
non-zero, terminate the program with that as its exit status.
The difference between this function and argp_error is that
argp_error is for parsing errors, whereas
argp_failure is for other problems that occur during parsing but
don't reflect a syntactic problem with the input--such as illegal
values for options, bad phase of the moon, etc.
argp_help Function.
Error output is sent to state->err_stream, and the program
name printed is state->name.
The output or program termination behavior of these functions may be
suppressed if the ARGP_NO_EXIT or ARGP_NO_ERRS flags,
respectively, were passed to argp_parse. See section Flags for argp_parse.
This behavior is useful if an argp parser is exported for use by other programs (e.g., by a library), and may be used in a context where it is not desirable to terminate the program in response to parsing errors. In argp parsers intended for such general use, calls to any of these functions should be followed by code return of an appropriate error code for the case where the program doesn't terminate; for example:
if (bad argument syntax)
{
argp_usage (state);
return EINVAL;
}
If it's known that a parser function will only be used when
ARGP_NO_EXIT is not set, the return may be omitted.
The third argument to argp parser functions (see section Argp Parser Functions) is a pointer to a struct argp_state, which contains
information about the state of the option parsing.
const struct argp *const root_argp
struct argp passed into argp_parse by
the invoking program (see section Parsing Program Options with Argp), but instead an internal argp parser
that contains options implemented by argp_parse itself (such as
`--help').
int argc
char **argv
int next
argv of the next argument to be parsed. May be modified.
One way to consume all remaining arguments in the input is to set
state->next = state->argc (perhaps after recording
the value of the next field to find the consumed arguments).
Also, you can cause the current option to be re-parsed by decrementing
this field, and then modifying
state->argv[state->next] to be the option that should
be reexamined.
unsigned flags
argp_parse. May be modified, although some
flags may only take effect when argp_parse is first invoked.
See section Flags for argp_parse.
unsigned arg_num
ARGP_KEY_ARG, this is the number of the current arg, starting at
0, and incremented after each such call returns. At all other times,
this is the number of such arguments that have been processed.
int quoted
argv of the first argument following a
special `--' argument (which prevents anything following being
interpreted as an option). Only set once argument parsing has proceeded
past this point.
void *input
argp_parse, in
the input argument.
void **child_inputs
state->child_inputs[i] as
its state->input field, where i is the index
of the child in the this parser's children field. See section Combining Multiple Argp Parsers.
void *hook
char *name
argv[0], or program_invocation_name if that is
unavailable.
FILE *err_stream
FILE *out_stream
err_stream, and all other output (such as
`--help' output) to out_stream. These are initialized to
stderr and stdout respectively (see section Standard Streams).
void *pstate
The children field in a struct argp allows other argp
parsers to be combined with the referencing one to parse a single set of
arguments. It should point to a vector of struct argp_child,
terminated by an entry having a value of zero in the argp field.
Where conflicts between combined parsers arise (for instance, if two specify an option with the same name), they are resolved in favor of the parent argp parsers, or earlier argp parsers in the list of children.
children field in a struct argp. The fields are as follows:
const struct argp *argp
int flags
const char *header
"". As with header
strings specified in an option entry, the value conventionally has
`:' as the last character. See section Specifying Options in an Argp Parser.
int group
group field in struct argp_option (see section Specifying Options in an Argp Parser), but all child-groupings follow parent options at a particular
group level. If both this field and header are zero, then the
child's options aren't grouped together at all, but rather merged with
the parent options (merging the child's grouping levels with the
parents).
argp_parse
The default behavior of argp_parse is designed to be convenient
for the most common case of parsing program command line argument. To
modify these defaults, the following flags may be or'd together in the
flags argument to argp_parse:
ARGP_PARSE_ARGV0
argp_parse. Normally (and always unless ARGP_NO_ERRS is
set) the first element of the argument vector is skipped for option
parsing purposes, as it corresponds to the program name in a command
line.
ARGP_NO_ERRS
stderr; unless
this flag is set, ARGP_PARSE_ARGV0 is ignored, as argv[0]
is used as the program name in the error messages. This flag implies
ARGP_NO_EXIT (on the assumption that silent exiting upon errors
is bad behaviour).
ARGP_NO_ARGS
ARGP_KEY_ARG, and the
actual arg as the value. This flag needn't normally be set, as the
normal behavior is to stop parsing as soon as some argument isn't
accepted by a parsing function. See section Argp Parser Functions.
ARGP_IN_ORDER
ARGP_NO_HELP
stdout,
and exit (0) called.
ARGP_NO_EXIT
ARGP_LONG_ONLY
ARGP_SILENT
ARGP_NO_EXIT, ARGP_NO_ERRS, and ARGP_NO_HELP.
The help_filter field in a struct argp is a pointer to a
function to filter the text of help messages before displaying them.
They have a function signature like:
char *help-filter (int key, const char *text, void *input)
where key is either a key from an option, in which case text is that option's help text (see section Specifying Options in an Argp Parser), or one of the special keys with names beginning with `ARGP_KEY_HELP_', describing which other help text text is (see section Special Keys for Argp Help Filter Functions).
The function should return either text, if it should be used
as-is, a replacement string, which should be allocated using
malloc, and will be freed by argp, or zero, meaning `print
nothing'. The value of text supplied is after any
translation has been done, so if any of the replacement text also needs
translation, that should be done by the filter function. input is
either the input supplied to argp_parse, or zero, if
argp_help was called directly by the user.
The following special values may be passed to an argp help filter function as the first argument, in addition to key values for user options, and specify which help text the text argument contains:
ARGP_KEY_HELP_PRE_DOC
ARGP_KEY_HELP_POST_DOC
ARGP_KEY_HELP_HEADER
ARGP_KEY_HELP_EXTRA
ARGP_KEY_HELP_DUP_ARGS_NOTE
ARGP_KEY_HELP_ARGS_DOC
args_doc field from the argp parser;
see section Specifying Argp Parsers).
argp_help Function
Normally programs using argp need not worry too much about printing
argument-usage-type help messages, because the standard `--help'
option is handled automatically by argp, and the typical error cases can
be handled using argp_usage and argp_error (see section Functions For Use in Argp Parsers).
However, if it's desirable to print a standard help message in some
context other than parsing the program options, argp offers the
argp_help interface.
Any options such as `--help' that are implemented automatically by
argp itself will not be present in the help output; for this
reason, it is better to use argp_state_help if calling from
within an argp parser function. See section Functions For Use in Argp Parsers.
argp_help Function
When calling argp_help (see section The argp_help Function), or
argp_state_help (see section Functions For Use in Argp Parsers), exactly what is
output is determined by the flags argument, which should consist
of any of the following flags, or'd together:
ARGP_HELP_USAGE
ARGP_HELP_SHORT_USAGE
ARGP_HELP_SEE
ARGP_HELP_LONG
ARGP_HELP_PRE_DOC
ARGP_HELP_POST_DOC
ARGP_HELP_DOC
(ARGP_HELP_PRE_DOC | ARGP_HELP_POST_DOC)
ARGP_HELP_BUG_ADDR
argp_program_bug_address variable contains one.
ARGP_HELP_LONG_ONLY
ARGP_LONG_ONLY mode.
The following flags are only understood when used with
argp_state_help, and control whether the function returns after
printing its output, or terminates the program:
ARGP_HELP_EXIT_ERR
exit (argp_err_exit_status).
ARGP_HELP_EXIT_OK
exit (0).
The following flags are combinations of the basic ones for printing standard messages:
ARGP_HELP_STD_ERR
ARGP_HELP_STD_USAGE
ARGP_HELP_STD_HELP
These example programs demonstrate the basic usage of argp.
This is (probably) the smallest possible program that uses argp. It won't do much except give an error messages and exit when there are any arguments, and print a (rather pointless) message for `--help'.
/* Argp example #1 -- a minimal program using argp */
/* This is (probably) the smallest possible program that
uses argp. It won't do much except give an error
messages and exit when there are any arguments, and print
a (rather pointless) messages for --help. */
#include <argp.h>
int main (int argc, char **argv)
{
argp_parse (0, argc, argv, 0, 0, 0);
exit (0);
}
This program doesn't use any options or arguments, but uses argp to be compliant with the GNU standard command line format.
In addition to making sure no arguments are given, and implementing a `--help' option, this example will have a `--version' option, and will put the given documentation string and bug address in the `--help' output, as per GNU standards.
The variable argp contains the argument parser specification;
adding fields to this structure is the way most parameters are passed to
argp_parse (the first three fields are usually used, but not in
this small program). There are also two global variables that argp
knows about defined here, argp_program_version and
argp_program_bug_address (they are global variables because they
will almost always be constant for a given program, even if it uses
different argument parsers for various tasks).
/* Argp example #2 -- a pretty minimal program using argp */ /* This program doesn't use any options or arguments, but uses argp to be compliant with the GNU standard command line format. In addition to making sure no arguments are given, and implementing a --help option, this example will have a --version option, and will put the given documentation string and bug address in the --help output, as per GNU standards. The variable ARGP contains the argument parser specification; adding fields to this structure is the way most parameters are passed to argp_parse (the first three fields are usually used, but not in this small program). There are also two global variables that argp knows about defined here, ARGP_PROGRAM_VERSION and ARGP_PROGRAM_BUG_ADDRESS (they are global variables becuase they will almost always be constant for a given program, even if it uses different argument parsers for various tasks). */ #include <argp.h> const char *argp_program_version = "argp-ex2 1.0"; const char *argp_program_bug_address = "<bug-gnu-utils@gnu.org>"; /* Program documentation. */ static char doc[] = "Argp example #2 -- a pretty minimal program using argp"; /* Our argument parser. Theoptions,parser, andargs_docfields are zero because we have neither options or arguments;docandargp_program_bug_addresswill be used in the output for `--help', and the `--version' option will print outargp_program_version. */ static struct argp argp = { 0, 0, 0, doc }; int main (int argc, char **argv) { argp_parse (&argp, argc, argv, 0, 0, 0); exit (0); }
This program uses the same features as example 2, and adds user options and arguments.
We now use the first four fields in argp (see section Specifying Argp Parsers),
and specifies parse_opt as the parser function (see section Argp Parser Functions).
Note that in this example, main uses a structure to communicate
with the parse_opt function, a pointer to which it passes in the
input argument to argp_parse (see section Parsing Program Options with Argp), and is
retrieved by parse_opt through the input field in its
state argument (see section Argp Parsing State). Of course, it's
also possible to use global variables instead, but using a structure
like this is somewhat more flexible and clean.
/* Argp example #3 -- a program with options and arguments using argp */
/* This program uses the same features as example 2, and uses options and
arguments.
We now use the first four fields in ARGP, so here's a description of them:
OPTIONS -- A pointer to a vector of struct argp_option (see below)
PARSER -- A function to parse a single option, called by argp
ARGS_DOC -- A string describing how the non-option arguments should look
DOC -- A descriptive string about this program; if it contains a
vertical tab character (\v), the part after it will be
printed *following* the options
The function PARSER takes the following arguments:
KEY -- An integer specifying which option this is (taken
from the KEY field in each struct argp_option), or
a special key specifying something else; the only
special keys we use here are ARGP_KEY_ARG, meaning
a non-option argument, and ARGP_KEY_END, meaning
that all arguments have been parsed
ARG -- For an option KEY, the string value of its
argument, or NULL if it has none
STATE-- A pointer to a struct argp_state, containing
various useful information about the parsing state; used here
are the INPUT field, which reflects the INPUT argument to
argp_parse, and the ARG_NUM field, which is the number of the
current non-option argument being parsed
It should return either 0, meaning success, ARGP_ERR_UNKNOWN, meaning the
given KEY wasn't recognized, or an errno value indicating some other
error.
Note that in this example, main uses a structure to communicate with the
parse_opt function, a pointer to which it passes in the INPUT argument to
argp_parse. Of course, it's also possible to use global variables
instead, but this is somewhat more flexible.
The OPTIONS field contains a pointer to a vector of struct argp_option's;
that structure has the following fields (if you assign your option
structures using array initialization like this example, unspecified
fields will be defaulted to 0, and need not be specified):
NAME -- The name of this option's long option (may be zero)
KEY -- The KEY to pass to the PARSER function when parsing this option,
*and* the name of this option's short option, if it is a
printable ascii character
ARG -- The name of this option's argument, if any
FLAGS -- Flags describing this option; some of them are:
OPTION_ARG_OPTIONAL -- The argument to this option is optional
OPTION_ALIAS -- This option is an alias for the
previous option
OPTION_HIDDEN -- Don't show this option in --help output
DOC -- A documentation string for this option, shown in --help output
An options vector should be terminated by an option with all fields zero. */
#include <argp.h>
const char *argp_program_version =
"argp-ex3 1.0";
const char *argp_program_bug_address =
"<bug-gnu-utils@gnu.org>";
/* Program documentation. */
static char doc[] =
"Argp example #3 -- a program with options and arguments using argp";
/* A description of the arguments we accept. */
static char args_doc[] = "ARG1 ARG2";
/* The options we understand. */
static struct argp_option options[] = {
{"verbose", 'v', 0, 0, "Produce verbose output" },
{"quiet", 'q', 0, 0, "Don't produce any output" },
{"silent", 's', 0, OPTION_ALIAS },
{"output", 'o', "FILE", 0,
"Output to FILE instead of standard output" },
{ 0 }
};
/* Used by main to communicate with parse_opt. */
struct arguments
{
char *args[2]; /* arg1 & arg2 */
int silent, verbose;
char *output_file;
};
/* Parse a single option. */
static error_t
parse_opt (int key, char *arg, struct argp_state *state)
{
/* Get the input argument from argp_parse, which we
know is a pointer to our arguments structure. */
struct arguments *arguments = state->input;
switch (key)
{
case 'q': case 's':
arguments->silent = 1;
break;
case 'v':
arguments->verbose = 1;
break;
case 'o':
arguments->output_file = arg;
break;
case ARGP_KEY_ARG:
if (state->arg_num >= 2)
/* Too many arguments. */
argp_usage (state);
arguments->args[state->arg_num] = arg;
break;
case ARGP_KEY_END:
if (state->arg_num < 2)
/* Not enough arguments. */
argp_usage (state);
break;
default:
return ARGP_ERR_UNKNOWN;
}
return 0;
}
/* Our argp parser. */
static struct argp argp = { options, parse_opt, args_doc, doc };
int main (int argc, char **argv)
{
struct arguments arguments;
/* Default values. */
arguments.silent = 0;
arguments.verbose = 0;
arguments.output_file = "-";
/* Parse our arguments; every option seen by parse_opt will
be reflected in arguments. */
argp_parse (&argp, argc, argv, 0, 0, &arguments);
printf ("ARG1 = %s\nARG2 = %s\nOUTPUT_FILE = %s\n"
"VERBOSE = %s\nSILENT = %s\n",
arguments.args[0], arguments.args[1],
arguments.output_file,
arguments.verbose ? "yes" : "no",
arguments.silent ? "yes" : "no");
exit (0);
}
This program uses the same features as example 3, but has more options,
and somewhat more structure in the `--help' output. It also shows
how you can `steal' the remainder of the input arguments past a certain
point, for programs that accept a list of items, and the special
key value ARGP_KEY_NO_ARGS, which is only given if no
non-option arguments were supplied to the program (see section Special Keys for Argp Parser Functions).
For structuring the help output, two features are used: headers,
which are entries in the options vector (see section Specifying Options in an Argp Parser)
with the first four fields being zero, and a two part documentation
string (in the variable doc), which allows documentation both
before and after the options (see section Specifying Argp Parsers); the
two parts of doc are separated by a vertical-tab character
('\v', or '\013'). By convention, the documentation
before the options is just a short string saying what the program does,
and that afterwards is longer, describing the behavior in more detail.
All documentation strings are automatically filled for output, although
newlines may be included to force a line break at a particular point.
All documentation strings are also passed to the gettext
function, for possible translation into the current locale.
/* Argp example #4 -- a program with somewhat more complicated options */
/* This program uses the same features as example 3, but has more
options, and somewhat more structure in the -help output. It
also shows how you can `steal' the remainder of the input
arguments past a certain point, for programs that accept a
list of items. It also shows the special argp KEY value
ARGP_KEY_NO_ARGS, which is only given if no non-option
arguments were supplied to the program.
For structuring the help output, two features are used,
*headers* which are entries in the options vector with the
first four fields being zero, and a two part documentation
string (in the variable DOC), which allows documentation both
before and after the options; the two parts of DOC are
separated by a vertical-tab character ('\v', or '\013'). By
convention, the documentation before the options is just a
short string saying what the program does, and that afterwards
is longer, describing the behavior in more detail. All
documentation strings are automatically filled for output,
although newlines may be included to force a line break at a
particular point. All documentation strings are also passed to
the `gettext' function, for possible translation into the
current locale. */
#include <stdlib.h>
#include <error.h>
#include <argp.h>
const char *argp_program_version =
"argp-ex4 1.0";
const char *argp_program_bug_address =
"<bug-gnu-utils@prep.ai.mit.edu>";
/* Program documentation. */
static char doc[] =
"Argp example #4 -- a program with somewhat more complicated\
options\
\vThis part of the documentation comes *after* the options;\
note that the text is automatically filled, but it's possible\
to force a line-break, e.g.\n<-- here.";
/* A description of the arguments we accept. */
static char args_doc[] = "ARG1 [STRING...]";
/* Keys for options without short-options. */
#define OPT_ABORT 1 /* --abort */
/* The options we understand. */
static struct argp_option options[] = {
{"verbose", 'v', 0, 0, "Produce verbose output" },
{"quiet", 'q', 0, 0, "Don't produce any output" },
{"silent", 's', 0, OPTION_ALIAS },
{"output", 'o', "FILE", 0,
"Output to FILE instead of standard output" },
{0,0,0,0, "The following options should be grouped together:" },
{"repeat", 'r', "COUNT", OPTION_ARG_OPTIONAL,
"Repeat the output COUNT (default 10) times"},
{"abort", OPT_ABORT, 0, 0, "Abort before showing any output"},
{ 0 }
};
/* Used by main to communicate with parse_opt. */
struct arguments
{
char *arg1; /* arg1 */
char **strings; /* [string...] */
int silent, verbose, abort; /* `-s', `-v', `--abort' */
char *output_file; /* file arg to `--output' */
int repeat_count; /* count arg to `--repeat' */
};
/* Parse a single option. */
static error_t
parse_opt (int key, char *arg, struct argp_state *state)
{
/* Get the input argument from argp_parse, which we
know is a pointer to our arguments structure. */
struct arguments *arguments = state->input;
switch (key)
{
case 'q': case 's':
arguments->silent = 1;
break;
case 'v':
arguments->verbose = 1;
break;
case 'o':
arguments->output_file = arg;
break;
case 'r':
arguments->repeat_count = arg ? atoi (arg) : 10;
break;
case OPT_ABORT:
arguments->abort = 1;
break;
case ARGP_KEY_NO_ARGS:
argp_usage (state);
case ARGP_KEY_ARG:
/* Here we know that state->arg_num == 0, since we
force argument parsing to end before any more arguments can
get here. */
arguments->arg1 = arg;
/* Now we consume all the rest of the arguments.
state->next is the index in state->argv of the
next argument to be parsed, which is the first string
we're interested in, so we can just use
&state->argv[state->next] as the value for
arguments->strings.
In addition, by setting state->next to the end
of the arguments, we can force argp to stop parsing here and
return. */
arguments->strings = &state->argv[state->next];
state->next = state->argc;
break;
default:
return ARGP_ERR_UNKNOWN;
}
return 0;
}
/* Our argp parser. */
static struct argp argp = { options, parse_opt, args_doc, doc };
int main (int argc, char **argv)
{
int i, j;
struct arguments arguments;
/* Default values. */
arguments.silent = 0;
arguments.verbose = 0;
arguments.output_file = "-";
arguments.repeat_count = 1;
arguments.abort = 0;
/* Parse our arguments; every option seen by parse_opt will be
reflected in arguments. */
argp_parse (&argp, argc, argv, 0, 0, &arguments);
if (arguments.abort)
error (10, 0, "ABORTED");
for (i = 0; i < arguments.repeat_count; i++)
{
printf ("ARG1 = %s\n", arguments.arg1);
printf ("STRINGS = ");
for (j = 0; arguments.strings[j]; j++)
printf (j == 0 ? "%s" : ", %s", arguments.strings[j]);
printf ("\n");
printf ("OUTPUT_FILE = %s\nVERBOSE = %s\nSILENT = %s\n",
arguments.output_file,
arguments.verbose ? "yes" : "no",
arguments.silent ? "yes" : "no");
}
exit (0);
}
The way formatting of argp `--help' output may be controlled to
some extent by a program's users, by setting the ARGP_HELP_FMT
environment variable to a comma-separated list (whitespace is ignored)
of the following tokens:
Having a single level of options is sometimes not enough. There might be too many options which have to be available or a set of options is closely related.
For this case some programs use suboptions. One of the most prominent
programs is certainly mount(8). The -o option take one
argument which itself is a comma separated list of options. To ease the
programming of code like this the function getsubopt is
available.
The optionp parameter must be a pointer to a variable containing the address of the string to process. When the function returns the reference is updated to point to the next suboption or to the terminating `\0' character if there is no more suboption available.
The tokens parameter references an array of strings containing the
known suboptions. All strings must be `\0' terminated and to mark
the end a null pointer must be stored. When getsubopt finds a
possible legal suboption it compares it with all strings available in
the tokens array and returns the index in the string as the
indicator.
In case the suboption has an associated value introduced by a `=' character, a pointer to the value is returned in valuep. The string is `\0' terminated. If no argument is available valuep is set to the null pointer. By doing this the caller can check whether a necessary value is given or whether no unexpected value is present.
In case the next suboption in the string is not mentioned in the tokens array the starting address of the suboption including a possible value is returned in valuep and the return value of the function is `-1'.
The code which might appear in the mount(8) program is a perfect
example of the use of getsubopt:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int do_all;
const char *type;
int read_size;
int write_size;
int read_only;
enum
{
RO_OPTION = 0,
RW_OPTION,
READ_SIZE_OPTION,
WRITE_SIZE_OPTION,
THE_END
};
const char *mount_opts[] =
{
[RO_OPTION] = "ro",
[RW_OPTION] = "rw",
[READ_SIZE_OPTION] = "rsize",
[WRITE_SIZE_OPTION] = "wsize",
[THE_END] = NULL
};
int
main (int argc, char *argv[])
{
char *subopts, *value;
int opt;
while ((opt = getopt (argc, argv, "at:o:")) != -1)
switch (opt)
{
case 'a':
do_all = 1;
break;
case 't':
type = optarg;
break;
case 'o':
subopts = optarg;
while (*subopts != '\0')
switch (getsubopt (&subopts, mount_opts, &value))
{
case RO_OPTION:
read_only = 1;
break;
case RW_OPTION:
read_only = 0;
break;
case READ_SIZE_OPTION:
if (value == NULL)
abort ();
read_size = atoi (value);
break;
case WRITE_SIZE_OPTION:
if (value == NULL)
abort ();
write_size = atoi (value);
break;
default:
/* Unknown suboption. */
printf ("Unknown suboption `%s'\n", value);
break;
}
break;
default:
abort ();
}
/* Do the real work. */
return 0;
}
When a program is executed, it receives information about the context in
which it was invoked in two ways. The first mechanism uses the
argv and argc arguments to its main function, and is
discussed in section Program Arguments. The second mechanism uses
environment variables and is discussed in this section.
The argv mechanism is typically used to pass command-line arguments specific to the particular program being invoked. The environment, on the other hand, keeps track of information that is shared by many programs, changes infrequently, and that is less frequently used.
The environment variables discussed in this section are the same
environment variables that you set using assignments and the
export command in the shell. Programs executed from the shell
inherit all of the environment variables from the shell.
Standard environment variables are used for information about the user's home directory, terminal type, current locale, and so on; you can define additional variables for other purposes. The set of all environment variables that have values is collectively known as the environment.
Names of environment variables are case-sensitive and must not contain the character `='. System-defined environment variables are invariably uppercase.
The values of environment variables can be anything that can be represented as a string. A value must not contain an embedded null character, since this is assumed to terminate the string.
The value of an environment variable can be accessed with the
getenv function. This is declared in the header file
`stdlib.h'. All of the following functions can be safely used in
multi-threaded programs. It is made sure that concurrent modifications
to the environment do not lead to errors.
getenv (but not by any other library function). If the
environment variable name is not defined, the value is a null
pointer.
putenv function adds or removes definitions from the environment.
If the string is of the form `name=value', the
definition is added to the environment. Otherwise, the string is
interpreted as the name of an environment variable, and any definition
for this variable in the environment is removed.
The difference to the setenv function is that the exact string
given as the parameter string is put into the environment. If the
user should change the string after the putenv call this will
reflect in automatically in the environment. This also requires that
string is no automatic variable which scope is left before the
variable is removed from the environment. The same applies of course to
dynamically allocated variables which are freed later.
This function is part of the extended Unix interface. Since it was also available in old SVID libraries you should define either _XOPEN_SOURCE or _SVID_SOURCE before including any header.
setenv function can be used to add a new definition to the
environment. The entry with the name name is replaced by the
value `name=value'. Please note that this is also true
if value is the empty string. To do this a new string is created
and the strings name and value are copied. A null pointer
for the value parameter is illegal. If the environment already
contains an entry with key name the replace parameter
controls the action. If replace is zero, nothing happens. Otherwise
the old entry is replaced by the new one.
Please note that you cannot remove an entry completely using this function.
This function was originally part of the BSD library but is now part of the Unix standard.
putenv when the value part of the
string is empty.
The function return -1 if name is a null pointer, points to
an empty string, or points to a string containing a = character.
It returns 0 if the call succeeded.
This function was originall part of the BSD library but is now part of the Unix standard. The BSD version had no return value, though.
There is one more function to modify the whole environment. This function is said to be used in the POSIX.9 (POSIX bindings for Fortran 77) and so one should expect it did made it into POSIX.1. But this never happened. But we still provide this function as a GNU extension to enable writing standard compliant Fortran environments.
clearenv function removes all entries from the environment.
Using putenv and setenv new entries can be added again
later.
If the function is successful it returns 0. Otherwise the return
value is nonzero.
You can deal directly with the underlying representation of environment objects to add more variables to the environment (for example, to communicate with another program you are about to execute; see section Executing a File).
This variable is declared in the header file `unistd.h'.
If you just want to get the value of an environment variable, use
getenv.
Unix systems, and the GNU system, pass the initial value of
environ as the third argument to main.
See section Program Arguments.
These environment variables have standard meanings. This doesn't mean that they are always present in the environment; but if these variables are present, they have these meanings. You shouldn't try to use these environment variable names for some other purpose.
HOME
HOME to any value.
If you need to make sure to obtain the proper home directory
for a particular user, you should not use HOME; instead,
look up the user's name in the user database (see section User Database).
For most purposes, it is better to use HOME, precisely because
this lets the user specify the value.
LOGNAME
getlogin (see section Identifying Who Logged In) is better for that purpose.
For most purposes, it is better to use LOGNAME, precisely because
this lets the user specify the value.
PATH
PATH holds a path used
for searching for programs to be run.
The execlp and execvp functions (see section Executing a File)
use this environment variable, as do many shells and other utilities
which are implemented in terms of those functions.
The syntax of a path is a sequence of directory names separated by
colons. An empty string instead of a directory name stands for the
current directory (see section Working Directory).
A typical value for this environment variable might be a string like:
:/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local/binThis means that if the user tries to execute a program named
foo,
the system will look for files named `foo', `/bin/foo',
`/etc/foo', and so on. The first of these files that exists is
the one that is executed.
TERM
TERM environment variable, for example.
TZ
TZ, for information about
the format of this string and how it is used.
LANG
LC_ALL nor the specific environment variable for that
category is set. See section Locales and Internationalization, for more information about
locales.
LC_ALL
LC_* environment variables. The
value of the other LC_* environment variables is simply ignored
in this case.
LC_COLLATE
LC_CTYPE
LC_MESSAGES
LC_MONETARY
LC_NUMERIC
LC_TIME
NLSPATH
catopen function
looks for message translation catalogs.
_POSIX_OPTION_ORDER
getopt and
argp_parse. See section Program Argument Syntax Conventions.
A system call is a request for service that a program makes of the
kernel. The service is generally something that only the kernel has
the privilege to do, such as doing I/O. Programmers don't normally
need to be concerned with system calls because there are functions in
the GNU C library to do virtually everything that system calls do.
These functions work by making system calls themselves. For example,
there is a system call that changes the permissions of a file, but
you don't need to know about it because you can just use the GNU C
library's chmod function.
System calls are sometimes called kernel calls.
However, there are times when you want to make a system call explicitly,
and for that, the GNU C library provides the syscall function.
syscall is harder to use and less portable than functions like
chmod, but easier and more portable than coding the system call
in assembler instructions.
syscall is most useful when you are working with a system call
which is special to your system or is newer than the GNU C library you
are using. syscall is implemented in an entirely generic way;
the function does not know anything about what a particular system
call does or even if it is valid.
The description of syscall in this section assumes a certain
protocol for system calls on the various platforms on which the GNU C
library runs. That protocol is not defined by any strong authority, but
we won't describe it here either because anyone who is coding
syscall probably won't accept anything less than kernel and C
library source code as a specification of the interface between them
anyway.
syscall is declared in `unistd.h'.
syscall performs a generic system call.
sysno is the system call number. Each kind of system call is identified by a number. Macros for all the possible system call numbers are defined in `sys/syscall.h'
The remaining arguments are the arguments for the system call, in order, and their meanings depend on the kind of system call. Each kind of system call has a definite number of arguments, from zero to five. If you code more arguments than the system call takes, the extra ones to the right are ignored.
The return value is the return value from the system call, unless the
system call failed. In that case, syscall returns -1 and
sets errno to an error code that the system call returned. Note
that system calls do not return -1 when they succeed.
If you specify an invalid sysno, syscall returns -1
with errno = ENOSYS.
Example:
#include <unistd.h> #include <sys/syscall.h> #include <errno.h> ... int rc; rc = syscall(SYS_chmod, "/etc/passwd", 0444); if (rc == -1) fprintf(stderr, "chmod failed, errno = %d\n", errno);
This, if all the compatibility stars are aligned, is equivalent to the following preferable code:
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
...
int rc;
rc = chmod("/etc/passwd", 0444);
if (rc == -1)
fprintf(stderr, "chmod failed, errno = %d\n", errno);
The usual way for a program to terminate is simply for its main
function to return. The exit status value returned from the
main function is used to report information back to the process's
parent process or shell.
A program can also terminate normally by calling the exit
function.
In addition, programs can be terminated by signals; this is discussed in
more detail in section Signal Handling. The abort function causes
a signal that kills the program.
A process terminates normally when its program signals it is done by
calling exit. Returning from main is equivalent to
calling exit, and the value that main returns is used as
the argument to exit.
exit function tells the system that the program is done, which
causes it to terminate the process.
status is the program's exit status, which becomes part of the process' termination status. This function does not return.
Normal termination causes the following actions:
atexit or on_exit
functions are called in the reverse order of their registration. This
mechanism allows your application to specify its own "cleanup" actions
to be performed at program termination. Typically, this is used to do
things like saving program state information in a file, or unlocking
locks in shared data bases.
tmpfile function are removed; see section Temporary Files.
_exit is called, terminating the program. See section Termination Internals.
When a program exits, it can return to the parent process a small
amount of information about the cause of termination, using the
exit status. This is a value between 0 and 255 that the exiting
process passes as an argument to exit.
Normally you should use the exit status to report very broad information about success or failure. You can't provide a lot of detail about the reasons for the failure, and most parent processes would not want much detail anyway.
There are conventions for what sorts of status values certain programs should return. The most common convention is simply 0 for success and 1 for failure. Programs that perform comparison use a different convention: they use status 1 to indicate a mismatch, and status 2 to indicate an inability to compare. Your program should follow an existing convention if an existing convention makes sense for it.
A general convention reserves status values 128 and up for special purposes. In particular, the value 128 is used to indicate failure to execute another program in a subprocess. This convention is not universally obeyed, but it is a good idea to follow it in your programs.
Warning: Don't try to use the number of errors as the exit status. This is actually not very useful; a parent process would generally not care how many errors occurred. Worse than that, it does not work, because the status value is truncated to eight bits. Thus, if the program tried to report 256 errors, the parent would receive a report of 0 errors--that is, success.
For the same reason, it does not work to use the value of errno
as the exit status--these can exceed 255.
Portability note: Some non-POSIX systems use different
conventions for exit status values. For greater portability, you can
use the macros EXIT_SUCCESS and EXIT_FAILURE for the
conventional status value for success and failure, respectively. They
are declared in the file `stdlib.h'.
exit function to indicate
successful program completion.
On POSIX systems, the value of this macro is 0. On other
systems, the value might be some other (possibly non-constant) integer
expression.
exit function to indicate
unsuccessful program completion in a general sense.
On POSIX systems, the value of this macro is 1. On other
systems, the value might be some other (possibly non-constant) integer
expression. Other nonzero status values also indicate failures. Certain
programs use different nonzero status values to indicate particular
kinds of "non-success". For example, diff uses status value
1 to mean that the files are different, and 2 or more to
mean that there was difficulty in opening the files.
Don't confuse a program's exit status with a process' termination status.
There are lots of ways a process can terminate besides having it's program
finish. In the event that the process termination is caused by program
termination (i.e. exit), though, the program's exit status becomes
part of the process' termination status.
Your program can arrange to run its own cleanup functions if normal
termination happens. If you are writing a library for use in various
application programs, then it is unreliable to insist that all
applications call the library's cleanup functions explicitly before
exiting. It is much more robust to make the cleanup invisible to the
application, by setting up a cleanup function in the library itself
using atexit or on_exit.
atexit function registers the function function to be
called at normal program termination. The function is called with
no arguments.
The return value from atexit is zero on success and nonzero if
the function cannot be registered.
atexit. It
accepts two arguments, a function function and an arbitrary
pointer arg. At normal program termination, the function is
called with two arguments: the status value passed to exit,
and the arg.
This function is included in the GNU C library only for compatibility for SunOS, and may not be supported by other implementations.
Here's a trivial program that illustrates the use of exit and
atexit:
#include <stdio.h>
#include <stdlib.h>
void
bye (void)
{
puts ("Goodbye, cruel world....");
}
int
main (void)
{
atexit (bye);
exit (EXIT_SUCCESS);
}
When this program is executed, it just prints the message and exits.
You can abort your program using the abort function. The prototype
for this function is in `stdlib.h'.
abort function causes abnormal program termination. This
does not execute cleanup functions registered with atexit or
on_exit.
This function actually terminates the process by raising a
SIGABRT signal, and your program can include a handler to
intercept this signal; see section Signal Handling.
Future Change Warning: Proposed Federal censorship regulations may prohibit us from giving you information about the possibility of calling this function. We would be required to say that this is not an acceptable way of terminating a program.
The _exit function is the primitive used for process termination
by exit. It is declared in the header file `unistd.h'.
_exit function is the primitive for causing a process to
terminate with status status. Calling this function does not
execute cleanup functions registered with atexit or
on_exit.
_Exit function is the ISO C equivalent to _exit.
The ISO C committee members were not sure whether the definitions of
_exit and _Exit were compatible so they have not used the
POSIX name.
This function was introduced in ISO C99 and is declared in `stdlib.h'.
When a process terminates for any reason--either because the program terminates, or as a result of a signal--the following things happen:
wait or waitpid; see section Process Completion. If the
program exited, this status includes as its low-order 8 bits the program
exit status.
init
process, with process ID 1.)
SIGCHLD signal is sent to the parent process.
SIGHUP signal is sent to each process in the foreground job,
and the controlling terminal is disassociated from that session.
See section Job Control.
SIGHUP
signal and a SIGCONT signal are sent to each process in the
group. See section Job Control.
Processes are the primitive units for allocation of system resources. Each process has its own address space and (usually) one thread of control. A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies.
Processes are organized hierarchically. Each process has a parent process which explicitly arranged to create it. The processes created by a given parent are called its child processes. A child inherits many of its attributes from the parent process.
This chapter describes how a program can create, terminate, and control child processes. Actually, there are three distinct operations involved: creating a new child process, causing the new process to execute a program, and coordinating the completion of the child process with the original program.
The system function provides a simple, portable mechanism for
running another program; it does all three steps automatically. If you
need more control over the details of how this is done, you can use the
primitive functions to do each step individually instead.
The easy way to run another program is to use the system
function. This function does all the work of running a subprogram, but
it doesn't give you much control over the details: you have to wait
until the subprogram terminates before you can do anything else.
sh to run the command.
In particular, it searches the directories in PATH to find
programs to execute. The return value is -1 if it wasn't
possible to create the shell process, and otherwise is the status of the
shell process. See section Process Completion, for details on how this
status code can be interpreted.
If the command argument is a null pointer, a return value of zero indicates that no command processor is available.
This function is a cancelation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time system is
called. If the thread gets canceled these resources stay allocated
until the program ends. To avoid this calls to system should be
protected using cancelation handlers.
The system function is declared in the header file
`stdlib.h'.
Portability Note: Some C implementations may not have any
notion of a command processor that can execute other programs. You can
determine whether a command processor exists by executing
system (NULL); if the return value is nonzero, a command
processor is available.
The popen and pclose functions (see section Pipe to a Subprocess) are closely related to the system function. They
allow the parent process to communicate with the standard input and
output channels of the command being executed.
This section gives an overview of processes and of the steps involved in creating a process and making it run another program.
Each process is named by a process ID number. A unique process ID is allocated to each process when it is created. The lifetime of a process ends when its termination is reported to its parent process; at that time, all of the process resources, including its process ID, are freed.
Processes are created with the fork system call (so the operation
of creating a new process is sometimes called forking a process).
The child process created by fork is a copy of the original
parent process, except that it has its own process ID.
After forking a child process, both the parent and child processes
continue to execute normally. If you want your program to wait for a
child process to finish executing before continuing, you must do this
explicitly after the fork operation, by calling wait or
waitpid (see section Process Completion). These functions give you
limited information about why the child terminated--for example, its
exit status code.
A newly forked child process continues to execute the same program as
its parent process, at the point where the fork call returns.
You can use the return value from fork to tell whether the program
is running in the parent process or the child.
Having several processes run the same program is only occasionally
useful. But the child can execute another program using one of the
exec functions; see section Executing a File. The program that the
process is executing is called its process image. Starting
execution of a new program causes the process to forget all about its
previous process image; when the new program exits, the process exits
too, instead of returning to the previous process image.
The pid_t data type represents process IDs. You can get the
process ID of a process by calling getpid. The function
getppid returns the process ID of the parent of the current
process (this is also known as the parent process ID). Your
program should include the header files `unistd.h' and
`sys/types.h' to use these functions.
pid_t data type is a signed integer type which is capable
of representing a process ID. In the GNU library, this is an int.
getppid function returns the process ID of the parent of the
current process.
The fork function is the primitive for creating a process.
It is declared in the header file `unistd.h'.
fork function creates a new process.
If the operation is successful, there are then both parent and child
processes and both see fork return, but with different values: it
returns a value of 0 in the child process and returns the child's
process ID in the parent process.
If process creation failed, fork returns a value of -1 in
the parent process. The following errno error conditions are
defined for fork:
EAGAIN
RLIMIT_NPROC resource limit, which can usually be increased;
see section Limiting Resource Usage.
ENOMEM
The specific attributes of the child process that differ from the parent process are:
vfork function is similar to fork but on some systems
it is more efficient; however, there are restrictions you must follow to
use it safely.
While fork makes a complete copy of the calling process's address
space and allows both the parent and child to execute independently,
vfork does not make this copy. Instead, the child process
created with vfork shares its parent's address space until it
calls _exit or one of the exec functions. In the
meantime, the parent process suspends execution.
You must be very careful not to allow the child process created with
vfork to modify any global data or even local variables shared
with the parent. Furthermore, the child process cannot return from (or
do a long jump out of) the function that called vfork! This
would leave the parent process's control information very confused. If
in doubt, use fork instead.
Some operating systems don't really implement vfork. The GNU C
library permits you to use vfork on all systems, but actually
executes fork if vfork isn't available. If you follow
the proper precautions for using vfork, your program will still
work even if the system uses fork instead.
This section describes the exec family of functions, for executing
a file as a process image. You can use these functions to make a child
process execute a new program after it has been forked.
To see the effects of exec from the point of view of the called
program, See section The Basic Program/System Interface.
The functions in this family differ in how you specify the arguments, but otherwise they all do the same thing. They are declared in the header file `unistd.h'.
execv function executes the file named by filename as a
new process image.
The argv argument is an array of null-terminated strings that is
used to provide a value for the argv argument to the main
function of the program to be executed. The last element of this array
must be a null pointer. By convention, the first element of this array
is the file name of the program sans directory names. See section Program Arguments, for full details on how programs can access these arguments.
The environment for the new process image is taken from the
environ variable of the current process image; see
section Environment Variables, for information about environments.
execv, but the argv strings are
specified individually instead of as an array. A null pointer must be
passed as the last such argument.
execv, but permits you to specify the environment
for the new program explicitly as the env argument. This should
be an array of strings in the same format as for the environ
variable; see section Environment Access.
execl, but permits you to specify the
environment for the new program explicitly. The environment argument is
passed following the null pointer that marks the last argv
argument, and should be an array of strings in the same format as for
the environ variable.
execvp function is similar to execv, except that it
searches the directories listed in the PATH environment variable
(see section Standard Environment Variables) to find the full file name of a
file from filename if filename does not contain a slash.
This function is useful for executing system utility programs, because it looks for them in the places that the user has chosen. Shells use it to run the commands that users type.
execl, except that it performs the same
file name searching as the execvp function.
The size of the argument list and environment list taken together must
not be greater than ARG_MAX bytes. See section General Capacity Limits. In
the GNU system, the size (which compares against ARG_MAX)
includes, for each string, the number of characters in the string, plus
the size of a char *, plus one, rounded up to a multiple of the
size of a char *. Other systems may have somewhat different
rules for counting.
These functions normally don't return, since execution of a new program
causes the currently executing program to go away completely. A value
of -1 is returned in the event of a failure. In addition to the
usual file name errors (see section File Name Errors), the following
errno error conditions are defined for these functions:
E2BIG
ARG_MAX bytes. The GNU system has no
specific limit on the argument list size, so this error code cannot
result, but you may get ENOMEM instead if the arguments are too
big for available memory.
ENOEXEC
ENOMEM
If execution of the new file succeeds, it updates the access time field of the file as if the file had been read. See section File Times, for more details about access times of files.
The point at which the file is closed again is not specified, but is at some point before the process exits or before another process image is executed.
Executing a new process image completely changes the contents of memory, copying only the argument and environment strings to new locations. But many other attributes of the process are unchanged:
If the set-user-ID and set-group-ID mode bits of the process image file are set, this affects the effective user ID and effective group ID (respectively) of the process. These concepts are discussed in detail in section The Persona of a Process.
Signals that are set to be ignored in the existing process image are also set to be ignored in the new process image. All other signals are set to the default action in the new process image. For more information about signals, see section Signal Handling.
File descriptors open in the existing process image remain open in the
new process image, unless they have the FD_CLOEXEC
(close-on-exec) flag set. The files that remain open inherit all
attributes of the open file description from the existing process image,
including file locks. File descriptors are discussed in section Low-Level Input/Output.
Streams, by contrast, cannot survive through exec functions,
because they are located in the memory of the process itself. The new
process image has no streams except those it creates afresh. Each of
the streams in the pre-exec process image has a descriptor inside
it, and these descriptors do survive through exec (provided that
they do not have FD_CLOEXEC set). The new process image can
reconnect these to new streams using fdopen (see section Descriptors and Streams).
The functions described in this section are used to wait for a child process to terminate or stop, and determine its status. These functions are declared in the header file `sys/wait.h'.
waitpid function is used to request status information from a
child process whose process ID is pid. Normally, the calling
process is suspended until the child process makes status information
available by terminating.
Other values for the pid argument have special interpretations. A
value of -1 or WAIT_ANY requests status information for
any child process; a value of 0 or WAIT_MYPGRP requests
information for any child process in the same process group as the
calling process; and any other negative value - pgid
requests information for any child process whose process group ID is
pgid.
If status information for a child process is available immediately, this
function returns immediately without waiting. If more than one eligible
child process has status information available, one of them is chosen
randomly, and its status is returned immediately. To get the status
from the other eligible child processes, you need to call waitpid
again.
The options argument is a bit mask. Its value should be the
bitwise OR (that is, the `|' operator) of zero or more of the
WNOHANG and WUNTRACED flags. You can use the
WNOHANG flag to indicate that the parent process shouldn't wait;
and the WUNTRACED flag to request status information from stopped
processes as well as processes that have terminated.
The status information from the child process is stored in the object that status-ptr points to, unless status-ptr is a null pointer.
This function is a cancelation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time waitpid is
called. If the thread gets canceled these resources stay allocated
until the program ends. To avoid this calls to waitpid should be
protected using cancelation handlers.
The return value is normally the process ID of the child process whose
status is reported. If there are child processes but none of them is
waiting to be noticed, waitpid will block until one is. However,
if the WNOHANG option was specified, waitpid will return
zero instead of blocking.
If a specific PID to wait for was given to waitpid, it will
ignore all other children (if any). Therefore if there are children
waiting to be noticed but the child whose PID was specified is not one
of them, waitpid will block or return zero as described above.
A value of -1 is returned in case of error. The following
errno error conditions are defined for this function:
EINTR
ECHILD
EINVAL
These symbolic constants are defined as values for the pid argument
to the waitpid function.
WAIT_ANY
-1) specifies that
waitpid should return status information about any child process.
WAIT_MYPGRP
0) specifies that waitpid should
return status information about any child process in the same process
group as the calling process.
These symbolic constants are defined as flags for the options
argument to the waitpid function. You can bitwise-OR the flags
together to obtain a value to use as the argument.
WNOHANG
waitpid should return immediately
instead of waiting, if there is no child process ready to be noticed.
WUNTRACED
waitpid should report the status of any
child processes that have been stopped as well as those that have
terminated.
waitpid, and is used to wait
until any one child process terminates. The call:
wait (&status)
is exactly equivalent to:
waitpid (-1, &status, 0)
This function is a cancelation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time wait is
called. If the thread gets canceled these resources stay allocated
until the program ends. To avoid this calls to wait should be
protected using cancelation handlers.
wait4 is equivalent to
waitpid (pid, status-ptr, options).
If usage is not null, wait4 stores usage figures for the
child process in *rusage (but only if the child has
terminated, not if it has stopped). See section Resource Usage.
This function is a BSD extension.
Here's an example of how to use waitpid to get the status from
all child processes that have terminated, without ever waiting. This
function is designed to be a handler for SIGCHLD, the signal that
indicates that at least one child process has terminated.
void
sigchld_handler (int signum)
{
int pid, status, serrno;
serrno = errno;
while (1)
{
pid = waitpid (WAIT_ANY, &status, WNOHANG);
if (pid < 0)
{
perror ("waitpid");
break;
}
if (pid == 0)
break;
notice_termination (pid, status);
}
errno = serrno;
}
If the exit status value (see section Program Termination) of the child
process is zero, then the status value reported by waitpid or
wait is also zero. You can test for other kinds of information
encoded in the returned status value using the following macros.
These macros are defined in the header file `sys/wait.h'.
exit or _exit.
WIFEXITED is true of status, this macro returns the
low-order 8 bits of the exit status value from the child process.
See section Exit Status.
WIFSIGNALED is true of status, this macro returns the
signal number of the signal that terminated the child process.
WIFSTOPPED is true of status, this macro returns the
signal number of the signal that caused the child process to stop.
The GNU library also provides these related facilities for compatibility
with BSD Unix. BSD uses the union wait data type to represent
status values rather than an int. The two representations are
actually interchangeable; they describe the same bit patterns. The GNU
C Library defines macros such as WEXITSTATUS so that they will
work on either kind of object, and the wait function is defined
to accept either type of pointer as its status-ptr argument.
These functions are declared in `sys/wait.h'.
int w_termsig
WTERMSIG macro.
int w_coredump
WCOREDUMP macro.
int w_retcode
WEXITSTATUS macro.
int w_stopsig
WSTOPSIG macro.
Instead of accessing these members directly, you should use the equivalent macros.
The wait3 function is the predecessor to wait4, which is
more flexible. wait3 is now obsolete.
wait3 is equivalent to
waitpid (-1, status-ptr, options).
If usage is not null, wait3 stores usage figures for the
child process in *rusage (but only if the child has
terminated, not if it has stopped). See section Resource Usage.
Here is an example program showing how you might write a function
similar to the built-in system. It executes its command
argument using the equivalent of `sh -c command'.
#include <stddef.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
/* Execute the command using this shell program. */
#define SHELL "/bin/sh"
int
my_system (const char *command)
{
int status;
pid_t pid;
pid = fork ();
if (pid == 0)
{
/* This is the child process. Execute the shell command. */
execl (SHELL, SHELL, "-c", command, NULL);
_exit (EXIT_FAILURE);
}
else if (pid < 0)
/* The fork failed. Report failure. */
status = -1;
else
/* This is the parent process. Wait for the child to complete. */
if (waitpid (pid, &status, 0) != pid)
status = -1;
return status;
}
There are a couple of things you should pay attention to in this example.
Remember that the first argv argument supplied to the program
represents the name of the program being executed. That is why, in the
call to execl, SHELL is supplied once to name the program
to execute and a second time to supply a value for argv[0].
The execl call in the child process doesn't return if it is
successful. If it fails, you must do something to make the child
process terminate. Just returning a bad status code with return
would leave two processes running the original program. Instead, the
right behavior is for the child process to report failure to its parent
process.
Call _exit to accomplish this. The reason for using _exit
instead of exit is to avoid flushing fully buffered streams such
as stdout. The buffers of these streams probably contain data
that was copied from the parent process by the fork, data that
will be output eventually by the parent process. Calling exit in
the child would output the data twice. See section Termination Internals.
Job control refers to the protocol for allowing a user to move between multiple process groups (or jobs) within a single login session. The job control facilities are set up so that appropriate behavior for most programs happens automatically and they need not do anything special about job control. So you can probably ignore the material in this chapter unless you are writing a shell or login program.
You need to be familiar with concepts relating to process creation (see section Process Creation Concepts) and signal handling (see section Signal Handling) in order to understand this material presented in this chapter.
The fundamental purpose of an interactive shell is to read
commands from the user's terminal and create processes to execute the
programs specified by those commands. It can do this using the
fork (see section Creating a Process) and exec
(see section Executing a File) functions.
A single command may run just one process--but often one command uses
several processes. If you use the `|' operator in a shell command,
you explicitly request several programs in their own processes. But
even if you run just one program, it can use multiple processes
internally. For example, a single compilation command such as `cc
-c foo.c' typically uses four processes (though normally only two at any
given time). If you run make, its job is to run other programs
in separate processes.
The processes belonging to a single command are called a process
group or job. This is so that you can operate on all of them at
once. For example, typing C-c sends the signal SIGINT to
terminate all the processes in the foreground process group.
A session is a larger group of processes. Normally all the processes that stem from a single login belong to the same session.
Every process belongs to a process group. When a process is created, it
becomes a member of the same process group and session as its parent
process. You can put it in another process group using the
setpgid function, provided the process group belongs to the same
session.
The only way to put a process in a different session is to make it the
initial process of a new session, or a session leader, using the
setsid function. This also puts the session leader into a new
process group, and you can't move it out of that process group again.
Usually, new sessions are created by the system login program, and the session leader is the process running the user's login shell.
A shell that supports job control must arrange to control which job can use the terminal at any time. Otherwise there might be multiple jobs trying to read from the terminal at once, and confusion about which process should receive the input typed by the user. To prevent this, the shell must cooperate with the terminal driver using the protocol described in this chapter.
The shell can give unlimited access to the controlling terminal to only one process group at a time. This is called the foreground job on that controlling terminal. Other process groups managed by the shell that are executing without such access to the terminal are called background jobs.
If a background job needs to read from its controlling
terminal, it is stopped by the terminal driver; if the
TOSTOP mode is set, likewise for writing. The user can stop
a foreground job by typing the SUSP character (see section Special Characters) and a program can stop any job by sending it a
SIGSTOP signal. It's the responsibility of the shell to notice
when jobs stop, to notify the user about them, and to provide mechanisms
for allowing the user to interactively continue stopped jobs and switch
jobs between foreground and background.
See section Access to the Controlling Terminal, for more information about I/O to the controlling terminal,
Not all operating systems support job control. The GNU system does support job control, but if you are using the GNU library on some other system, that system may not support job control itself.
You can use the _POSIX_JOB_CONTROL macro to test at compile-time
whether the system supports job control. See section Overall System Options.
If job control is not supported, then there can be only one process
group per session, which behaves as if it were always in the foreground.
The functions for creating additional process groups simply fail with
the error code ENOSYS.
The macros naming the various job control signals (see section Job Control Signals) are defined even if job control is not supported. However, the system never generates these signals, and attempts to send a job control signal or examine or specify their actions report errors or do nothing.
One of the attributes of a process is its controlling terminal. Child
processes created with fork inherit the controlling terminal from
their parent process. In this way, all the processes in a session
inherit the controlling terminal from the session leader. A session
leader that has control of a terminal is called the controlling
process of that terminal.
You generally do not need to worry about the exact mechanism used to allocate a controlling terminal to a session, since it is done for you by the system when you log in.
An individual process disconnects from its controlling terminal when it
calls setsid to become the leader of a new session.
See section Process Group Functions.
Processes in the foreground job of a controlling terminal have unrestricted access to that terminal; background processes do not. This section describes in more detail what happens when a process in a background job tries to access its controlling terminal.
When a process in a background job tries to read from its controlling
terminal, the process group is usually sent a SIGTTIN signal.
This normally causes all of the processes in that group to stop (unless
they handle the signal and don't stop themselves). However, if the
reading process is ignoring or blocking this signal, then read
fails with an EIO error instead.
Similarly, when a process in a background job tries to write to its
controlling terminal, the default behavior is to send a SIGTTOU
signal to the process group. However, the behavior is modified by the
TOSTOP bit of the local modes flags (see section Local Modes). If
this bit is not set (which is the default), then writing to the
controlling terminal is always permitted without sending a signal.
Writing is also permitted if the SIGTTOU signal is being ignored
or blocked by the writing process.
Most other terminal operations that a program can do are treated as reading or as writing. (The description of each operation should say which.)
For more information about the primitive read and write
functions, see section Input and Output Primitives.
When a controlling process terminates, its terminal becomes free and a new session can be established on it. (In fact, another user could log in on the terminal.) This could cause a problem if any processes from the old session are still trying to use that terminal.
To prevent problems, process groups that continue running even after the session leader has terminated are marked as orphaned process groups.
When a process group becomes an orphan, its processes are sent a
SIGHUP signal. Ordinarily, this causes the processes to
terminate. However, if a program ignores this signal or establishes a
handler for it (see section Signal Handling), it can continue running as in
the orphan process group even after its controlling process terminates;
but it still cannot access the terminal any more.
This section describes what a shell must do to implement job control, by presenting an extensive sample program to illustrate the concepts involved.
All of the program examples included in this chapter are part of a simple shell program. This section presents data structures and utility functions which are used throughout the example.
The sample shell deals mainly with two data structures. The
job type contains information about a job, which is a
set of subprocesses linked together with pipes. The process type
holds information about a single subprocess. Here are the relevant
data structure declarations:
/* A process is a single process. */
typedef struct process
{
struct process *next; /* next process in pipeline */
char **argv; /* for exec */
pid_t pid; /* process ID */
char completed; /* true if process has completed */
char stopped; /* true if process has stopped */
int status; /* reported status value */
} process;
/* A job is a pipeline of processes. */
typedef struct job
{
struct job *next; /* next active job */
char *command; /* command line, used for messages */
process *first_process; /* list of processes in this job */
pid_t pgid; /* process group ID */
char notified; /* true if user told about stopped job */
struct termios tmodes; /* saved terminal modes */
int stdin, stdout, stderr; /* standard i/o channels */
} job;
/* The active jobs are linked into a list. This is its head. */
job *first_job = NULL;
Here are some utility functions that are used for operating on job
objects.
/* Find the active job with the indicated pgid. */
job *
find_job (pid_t pgid)
{
job *j;
for (j = first_job; j; j = j->next)
if (j->pgid == pgid)
return j;
return NULL;
}
/* Return true if all processes in the job have stopped or completed. */
int
job_is_stopped (job *j)
{
process *p;
for (p = j->first_process; p; p = p->next)
if (!p->completed && !p->stopped)
return 0;
return 1;
}
/* Return true if all processes in the job have completed. */
int
job_is_completed (job *j)
{
process *p;
for (p = j->first_process; p; p = p->next)
if (!p->completed)
return 0;
return 1;
}
When a shell program that normally performs job control is started, it has to be careful in case it has been invoked from another shell that is already doing its own job control.
A subshell that runs interactively has to ensure that it has been placed
in the foreground by its parent shell before it can enable job control
itself. It does this by getting its initial process group ID with the
getpgrp function, and comparing it to the process group ID of the
current foreground job associated with its controlling terminal (which
can be retrieved using the tcgetpgrp function).
If the subshell is not running as a foreground job, it must stop itself
by sending a SIGTTIN signal to its own process group. It may not
arbitrarily put itself into the foreground; it must wait for the user to
tell the parent shell to do this. If the subshell is continued again,
it should repeat the check and stop itself again if it is still not in
the foreground.
Once the subshell has been placed into the foreground by its parent
shell, it can enable its own job control. It does this by calling
setpgid to put itself into its own process group, and then
calling tcsetpgrp to place this process group into the
foreground.
When a shell enables job control, it should set itself to ignore all the
job control stop signals so that it doesn't accidentally stop itself.
You can do this by setting the action for all the stop signals to
SIG_IGN.
A subshell that runs non-interactively cannot and should not support job control. It must leave all processes it creates in the same process group as the shell itself; this allows the non-interactive shell and its child processes to be treated as a single job by the parent shell. This is easy to do--just don't use any of the job control primitives--but you must remember to make the shell do it.
Here is the initialization code for the sample shell that shows how to do all of this.
/* Keep track of attributes of the shell. */
#include <sys/types.h>
#include <termios.h>
#include <unistd.h>
pid_t shell_pgid;
struct termios shell_tmodes;
int shell_terminal;
int shell_is_interactive;
/* Make sure the shell is running interactively as the foreground job
before proceeding. */
void
init_shell ()
{
/* See if we are running interactively. */
shell_terminal = STDIN_FILENO;
shell_is_interactive = isatty (shell_terminal);
if (shell_is_interactive)
{
/* Loop until we are in the foreground. */
while (tcgetpgrp (shell_terminal) != (shell_pgid = getpgrp ()))
kill (- shell_pgid, SIGTTIN);
/* Ignore interactive and job-control signals. */
signal (SIGINT, SIG_IGN);
signal (SIGQUIT, SIG_IGN);
signal (SIGTSTP, SIG_IGN);
signal (SIGTTIN, SIG_IGN);
signal (SIGTTOU, SIG_IGN);
signal (SIGCHLD, SIG_IGN);
/* Put ourselves in our own process group. */
shell_pgid = getpid ();
if (setpgid (shell_pgid, shell_pgid) < 0)
{
perror ("Couldn't put the shell in its own process group");
exit (1);
}
/* Grab control of the terminal. */
tcsetpgrp (shell_terminal, shell_pgid);
/* Save default terminal attributes for shell. */
tcgetattr (shell_terminal, &shell_tmodes);
}
}
Once the shell has taken responsibility for performing job control on its controlling terminal, it can launch jobs in response to commands typed by the user.
To create the processes in a process group, you use the same fork
and exec functions described in section Process Creation Concepts.
Since there are multiple child processes involved, though, things are a
little more complicated and you must be careful to do things in the
right order. Otherwise, nasty race conditions can result.
You have two choices for how to structure the tree of parent-child relationships among the processes. You can either make all the processes in the process group be children of the shell process, or you can make one process in group be the ancestor of all the other processes in that group. The sample shell program presented in this chapter uses the first approach because it makes bookkeeping somewhat simpler.
As each process is forked, it should put itself in the new process group
by calling setpgid; see section Process Group Functions. The first
process in the new group becomes its process group leader, and its
process ID becomes the process group ID for the group.
The shell should also call setpgid to put each of its child
processes into the new process group. This is because there is a
potential timing problem: each child process must be put in the process
group before it begins executing a new program, and the shell depends on
having all the child processes in the group before it continues
executing. If both the child processes and the shell call
setpgid, this ensures that the right things happen no matter which
process gets to it first.
If the job is being launched as a foreground job, the new process group
also needs to be put into the foreground on the controlling terminal
using tcsetpgrp. Again, this should be done by the shell as well
as by each of its child processes, to avoid race conditions.
The next thing each child process should do is to reset its signal actions.
During initialization, the shell process set itself to ignore job
control signals; see section Initializing the Shell. As a result, any child
processes it creates also ignore these signals by inheritance. This is
definitely undesirable, so each child process should explicitly set the
actions for these signals back to SIG_DFL just after it is forked.
Since shells follow this convention, applications can assume that they
inherit the correct handling of these signals from the parent process.
But every application has a responsibility not to mess up the handling
of stop signals. Applications that disable the normal interpretation of
the SUSP character should provide some other mechanism for the user to
stop the job. When the user invokes this mechanism, the program should
send a SIGTSTP signal to the process group of the process, not
just to the process itself. See section Signaling Another Process.
Finally, each child process should call exec in the normal way.
This is also the point at which redirection of the standard input and
output channels should be handled. See section Duplicating Descriptors,
for an explanation of how to do this.
Here is the function from the sample shell program that is responsible for launching a program. The function is executed by each child process immediately after it has been forked by the shell, and never returns.
void
launch_process (process *p, pid_t pgid,
int infile, int outfile, int errfile,
int foreground)
{
pid_t pid;
if (shell_is_interactive)
{
/* Put the process into the process group and give the process group
the terminal, if appropriate.
This has to be done both by the shell and in the individual
child processes because of potential race conditions. */
pid = getpid ();
if (pgid == 0) pgid = pid;
setpgid (pid, pgid);
if (foreground)
tcsetpgrp (shell_terminal, pgid);
/* Set the handling for job control signals back to the default. */
signal (SIGINT, SIG_DFL);
signal (SIGQUIT, SIG_DFL);
signal (SIGTSTP, SIG_DFL);
signal (SIGTTIN, SIG_DFL);
signal (SIGTTOU, SIG_DFL);
signal (SIGCHLD, SIG_DFL);
}
/* Set the standard input/output channels of the new process. */
if (infile != STDIN_FILENO)
{
dup2 (infile, STDIN_FILENO);
close (infile);
}
if (outfile != STDOUT_FILENO)
{
dup2 (outfile, STDOUT_FILENO);
close (outfile);
}
if (errfile != STDERR_FILENO)
{
dup2 (errfile, STDERR_FILENO);
close (errfile);
}
/* Exec the new process. Make sure we exit. */
execvp (p->argv[0], p->argv);
perror ("execvp");
exit (1);
}
If the shell is not running interactively, this function does not do anything with process groups or signals. Remember that a shell not performing job control must keep all of its subprocesses in the same process group as the shell itself.
Next, here is the function that actually launches a complete job. After creating the child processes, this function calls some other functions to put the newly created job into the foreground or background; these are discussed in section Foreground and Background.
void
launch_job (job *j, int foreground)
{
process *p;
pid_t pid;
int mypipe[2], infile, outfile;
infile = j->stdin;
for (p = j->first_process; p; p = p->next)
{
/* Set up pipes, if necessary. */
if (p->next)
{
if (pipe (mypipe) < 0)
{
perror ("pipe");
exit (1);
}
outfile = mypipe[1];
}
else
outfile = j->stdout;
/* Fork the child processes. */
pid = fork ();
if (pid == 0)
/* This is the child process. */
launch_process (p, j->pgid, infile,
outfile, j->stderr, foreground);
else if (pid < 0)
{
/* The fork failed. */
perror ("fork");
exit (1);
}
else
{
/* This is the parent process. */
p->pid = pid;
if (shell_is_interactive)
{
if (!j->pgid)
j->pgid = pid;
setpgid (pid, j->pgid);
}
}
/* Clean up after pipes. */
if (infile != j->stdin)
close (infile);
if (outfile != j->stdout)
close (outfile);
infile = mypipe[0];
}
format_job_info (j, "launched");
if (!shell_is_interactive)
wait_for_job (j);
else if (foreground)
put_job_in_foreground (j, 0);
else
put_job_in_background (j, 0);
}
Now let's consider what actions must be taken by the shell when it launches a job into the foreground, and how this differs from what must be done when a background job is launched.
When a foreground job is launched, the shell must first give it access
to the controlling terminal by calling tcsetpgrp. Then, the
shell should wait for processes in that process group to terminate or
stop. This is discussed in more detail in section Stopped and Terminated Jobs.
When all of the processes in the group have either completed or stopped,
the shell should regain control of the terminal for its own process
group by calling tcsetpgrp again. Since stop signals caused by
I/O from a background process or a SUSP character typed by the user
are sent to the process group, normally all the processes in the job
stop together.
The foreground job may have left the terminal in a strange state, so the
shell should restore its own saved terminal modes before continuing. In
case the job is merely stopped, the shell should first save the current
terminal modes so that it can restore them later if the job is
continued. The functions for dealing with terminal modes are
tcgetattr and tcsetattr; these are described in
section Terminal Modes.
Here is the sample shell's function for doing all of this.
/* Put job j in the foreground. If cont is nonzero,
restore the saved terminal modes and send the process group a
SIGCONT signal to wake it up before we block. */
void
put_job_in_foreground (job *j, int cont)
{
/* Put the job into the foreground. */
tcsetpgrp (shell_terminal, j->pgid);
/* Send the job a continue signal, if necessary. */
if (cont)
{
tcsetattr (shell_terminal, TCSADRAIN, &j->tmodes);
if (kill (- j->pgid, SIGCONT) < 0)
perror ("kill (SIGCONT)");
}
/* Wait for it to report. */
wait_for_job (j);
/* Put the shell back in the foreground. */
tcsetpgrp (shell_terminal, shell_pgid);
/* Restore the shell's terminal modes. */
tcgetattr (shell_terminal, &j->tmodes);
tcsetattr (shell_terminal, TCSADRAIN, &shell_tmodes);
}
If the process group is launched as a background job, the shell should remain in the foreground itself and continue to read commands from the terminal.
In the sample shell, there is not much that needs to be done to put a job into the background. Here is the function it uses:
/* Put a job in the background. If the cont argument is true, send
the process group a SIGCONT signal to wake it up. */
void
put_job_in_background (job *j, int cont)
{
/* Send the job a continue signal, if necessary. */
if (cont)
if (kill (-j->pgid, SIGCONT) < 0)
perror ("kill (SIGCONT)");
}
When a foreground process is launched, the shell must block until all of
the processes in that job have either terminated or stopped. It can do
this by calling the waitpid function; see section Process Completion. Use the WUNTRACED option so that status is reported
for processes that stop as well as processes that terminate.
The shell must also check on the status of background jobs so that it
can report terminated and stopped jobs to the user; this can be done by
calling waitpid with the WNOHANG option. A good place to
put a such a check for terminated and stopped jobs is just before
prompting for a new command.
The shell can also receive asynchronous notification that there is
status information available for a child process by establishing a
handler for SIGCHLD signals. See section Signal Handling.
In the sample shell program, the SIGCHLD signal is normally
ignored. This is to avoid reentrancy problems involving the global data
structures the shell manipulates. But at specific times when the shell
is not using these data structures--such as when it is waiting for
input on the terminal--it makes sense to enable a handler for
SIGCHLD. The same function that is used to do the synchronous
status checks (do_job_notification, in this case) can also be
called from within this handler.
Here are the parts of the sample shell program that deal with checking the status of jobs and reporting the information to the user.
/* Store the status of the process pid that was returned by waitpid.
Return 0 if all went well, nonzero otherwise. */
int
mark_process_status (pid_t pid, int status)
{
job *j;
process *p;
if (pid > 0)
{
/* Update the record for the process. */
for (j = first_job; j; j = j->next)
for (p = j->first_process; p; p = p->next)
if (p->pid == pid)
{
p->status = status;
if (WIFSTOPPED (status))
p->stopped = 1;
else
{
p->completed = 1;
if (WIFSIGNALED (status))
fprintf (stderr, "%d: Terminated by signal %d.\n",
(int) pid, WTERMSIG (p->status));
}
return 0;
}
fprintf (stderr, "No child process %d.\n", pid);
return -1;
}
else if (pid == 0 || errno == ECHILD)
/* No processes ready to report. */
return -1;
else {
/* Other weird errors. */
perror ("waitpid");
return -1;
}
}
/* Check for processes that have status information available,
without blocking. */
void
update_status (void)
{
int status;
pid_t pid;
do
pid = waitpid (WAIT_ANY, &status, WUNTRACED|WNOHANG);
while (!mark_process_status (pid, status));
}
/* Check for processes that have status information available,
blocking until all processes in the given job have reported. */
void
wait_for_job (job *j)
{
int status;
pid_t pid;
do
pid = waitpid (WAIT_ANY, &status, WUNTRACED);
while (!mark_process_status (pid, status)
&& !job_is_stopped (j)
&& !job_is_completed (j));
}
/* Format information about job status for the user to look at. */
void
format_job_info (job *j, const char *status)
{
fprintf (stderr, "%ld (%s): %s\n", (long)j->pgid, status, j->command);
}
/* Notify the user about stopped or terminated jobs.
Delete terminated jobs from the active job list. */
void
do_job_notification (void)
{
job *j, *jlast, *jnext;
process *p;
/* Update status information for child processes. */
update_status ();
jlast = NULL;
for (j = first_job; j; j = jnext)
{
jnext = j->next;
/* If all processes have completed, tell the user the job has
completed and delete it from the list of active jobs. */
if (job_is_completed (j)) {
format_job_info (j, "completed");
if (jlast)
jlast->next = jnext;
else
first_job = jnext;
free_job (j);
}
/* Notify the user about stopped jobs,
marking them so that we won't do this more than once. */
else if (job_is_stopped (j) && !j->notified) {
format_job_info (j, "stopped");
j->notified = 1;
jlast = j;
}
/* Don't say anything about jobs that are still running. */
else
jlast = j;
}
}
The shell can continue a stopped job by sending a SIGCONT signal
to its process group. If the job is being continued in the foreground,
the shell should first invoke tcsetpgrp to give the job access to
the terminal, and restore the saved terminal settings. After continuing
a job in the foreground, the shell should wait for the job to stop or
complete, as if the job had just been launched in the foreground.
The sample shell program handles both newly created and continued jobs
with the same pair of functions, put_job_in_foreground and
put_job_in_background. The definitions of these functions
were given in section Foreground and Background. When continuing a
stopped job, a nonzero value is passed as the cont argument to
ensure that the SIGCONT signal is sent and the terminal modes
reset, as appropriate.
This leaves only a function for updating the shell's internal bookkeeping about the job being continued:
/* Mark a stopped job J as being running again. */
void
mark_job_as_running (job *j)
{
Process *p;
for (p = j->first_process; p; p = p->next)
p->stopped = 0;
j->notified = 0;
}
/* Continue the job J. */
void
continue_job (job *j, int foreground)
{
mark_job_as_running (j);
if (foreground)
put_job_in_foreground (j, 1);
else
put_job_in_background (j, 1);
}
The code extracts for the sample shell included in this chapter are only
a part of the entire shell program. In particular, nothing at all has
been said about how job and program data structures are
allocated and initialized.
Most real shells provide a complex user interface that has support for a command language; variables; abbreviations, substitutions, and pattern matching on file names; and the like. All of this is far too complicated to explain here! Instead, we have concentrated on showing how to implement the core process creation and job control functions that can be called from such a shell.
Here is a table summarizing the major entry points we have presented:
void init_shell (void)
void launch_job (job *j, int foreground)
void do_job_notification (void)
SIGCHLD signals.
See section Stopped and Terminated Jobs.
void continue_job (job *j, int foreground)
Of course, a real shell would also want to provide other functions for
managing jobs. For example, it would be useful to have commands to list
all active jobs or to send a signal (such as SIGKILL) to a job.
This section contains detailed descriptions of the functions relating to job control.
You can use the ctermid function to get a file name that you can
use to open the controlling terminal. In the GNU library, it returns
the same string all the time: "/dev/tty". That is a special
"magic" file name that refers to the controlling terminal of the
current process (if it has one). To find the name of the specific
terminal device, use ttyname; see section Identifying Terminals.
The function ctermid is declared in the header file
`stdio.h'.
ctermid function returns a string containing the file name of
the controlling terminal for the current process. If string is
not a null pointer, it should be an array that can hold at least
L_ctermid characters; the string is returned in this array.
Otherwise, a pointer to a string in a static area is returned, which
might get overwritten on subsequent calls to this function.
An empty string is returned if the file name cannot be determined for any reason. Even if a file name is returned, access to the file it represents is not guaranteed.
ctermid.
See also the isatty and ttyname functions, in
section Identifying Terminals.
Here are descriptions of the functions for manipulating process groups. Your program should include the header files `sys/types.h' and `unistd.h' to use these functions.
setsid function creates a new session. The calling process
becomes the session leader, and is put in a new process group whose
process group ID is the same as the process ID of that process. There
are initially no other processes in the new process group, and no other
process groups in the new session.
This function also makes the calling process have no controlling terminal.
The setsid function returns the new process group ID of the
calling process if successful. A return value of -1 indicates an
error. The following errno error conditions are defined for this
function:
EPERM
The getsid function returns the process group ID of the session
leader of the specified process. If a pid is 0, the
process group ID of the session leader of the current process is
returned.
In case of error -1 is returned and errno is set. The
following errno error conditions are defined for this function:
ESRCH
EPERM
The getpgrp function has two definitions: one derived from BSD
Unix, and one from the POSIX.1 standard. The feature test macros you
have selected (see section Feature Test Macros) determine which definition
you get. Specifically, you get the BSD version if you define
_BSD_SOURCE; otherwise, you get the POSIX version if you define
_POSIX_SOURCE or _GNU_SOURCE. Programs written for old
BSD systems will not include `unistd.h', which defines
getpgrp specially under _BSD_SOURCE. You must link such
programs with the -lbsd-compat option to get the BSD definition.
getpgrp returns the process group ID of
the calling process.
getpgrp returns the process group ID of the
process pid. You can supply a value of 0 for the pid
argument to get information about the calling process.
getpgid is the same as the BSD function getpgrp. It
returns the process group ID of the process pid. You can supply a
value of 0 for the pid argument to get information about
the calling process.
In case of error -1 is returned and errno is set. The
following errno error conditions are defined for this function:
ESRCH
setpgid function puts the process pid into the process
group pgid. As a special case, either pid or pgid can
be zero to indicate the process ID of the calling process.
This function fails on a system that does not support job control. See section Job Control is Optional, for more information.
If the operation is successful, setpgid returns zero. Otherwise
it returns -1. The following errno error conditions are
defined for this function:
EACCES
exec
function since it was forked.
EINVAL
ENOSYS
EPERM
ESRCH
setpgid. Both functions do exactly
the same thing.
These are the functions for reading or setting the foreground process group of a terminal. You should include the header files `sys/types.h' and `unistd.h' in your application to use these functions.
Although these functions take a file descriptor argument to specify the terminal device, the foreground job is associated with the terminal file itself and not a particular open file descriptor.
If there is no foreground process group, the return value is a number
greater than 1 that does not match the process group ID of any
existing process group. This can happen if all of the processes in the
job that was formerly the foreground job have terminated, and no other
job has yet been moved into the foreground.
In case of an error, a value of -1 is returned. The
following errno error conditions are defined for this function:
EBADF
ENOSYS
ENOTTY
For terminal access purposes, this function is treated as output. If it
is called from a background process on its controlling terminal,
normally all processes in the process group are sent a SIGTTOU
signal. The exception is if the calling process itself is ignoring or
blocking SIGTTOU signals, in which case the operation is
performed and no signal is sent.
If successful, tcsetpgrp returns 0. A return value of
-1 indicates an error. The following errno error
conditions are defined for this function:
EBADF
EINVAL
ENOSYS
ENOTTY
EPERM
(pid_t) -1 and the global variable errno
is set to the following value:
EBADF
ENOTTY
Various functions in the C Library need to be configured to work correctly in the local environment. Traditionally, this was done by using files (e.g., `/etc/passwd'), but other nameservices (like the Network Information Service (NIS) and the Domain Name Service (DNS)) became popular, and were hacked into the C library, usually with a fixed search order (see section `frobnicate' in The Jargon File).
The GNU C Library contains a cleaner solution of this problem. It is designed after a method used by Sun Microsystems in the C library of Solaris 2. GNU C Library follows their name and calls this scheme Name Service Switch (NSS).
Though the interface might be similar to Sun's version there is no common code. We never saw any source code of Sun's implementation and so the internal interface is incompatible. This also manifests in the file names we use as we will see later.
The basic idea is to put the implementation of the different services offered to access the databases in separate modules. This has some advantages:
To fulfill the first goal above the ABI of the modules will be described below. For getting the implementation of a new service right it is important to understand how the functions in the modules get called. They are in no way designed to be used by the programmer directly. Instead the programmer should only use the documented and standardized functions to access the databases.
The databases available in the NSS are
aliases
ethers
group
hosts
netgroup
networks
protocols
passwd
rpc
services
shadow
There will be some more added later (automount, bootparams,
netmasks, and publickey).
Somehow the NSS code must be told about the wishes of the user. For this reason there is the file `/etc/nsswitch.conf'. For each database this file contain a specification how the lookup process should work. The file could look like this:
# /etc/nsswitch.conf # # Name Service Switch configuration file. # passwd: db files nis shadow: files group: db files nis hosts: files nisplus nis dns networks: nisplus [NOTFOUND=return] files ethers: nisplus [NOTFOUND=return] db files protocols: nisplus [NOTFOUND=return] db files rpc: nisplus [NOTFOUND=return] db files services: nisplus [NOTFOUND=return] db files
The first column is the database as you can guess from the table above. The rest of the line specifies how the lookup process works. Please note that you specify the way it works for each database individually. This cannot be done with the old way of a monolithic implementation.
The configuration specification for each database can contain two different items:
files, db, or nis.
[NOTFOUND=return].
The above example file mentions four different services: files,
db, nis, and nisplus. This does not mean these
services are available on all sites and it does also not mean these are
all the services which will ever be available.
In fact, these names are simply strings which the NSS code uses to find the implicitly addressed functions. The internal interface will be described later. Visible to the user are the modules which implement an individual service.
Assume the service name shall be used for a lookup. The code for this service is implemented in a module called `libnss_name'. On a system supporting shared libraries this is in fact a shared library with the name (for example) `libnss_name.so.2'. The number at the end is the currently used version of the interface which will not change frequently. Normally the user should not have to be cognizant of these files since they should be placed in a directory where they are found automatically. Only the names of all available services are important.
The second item in the specification gives the user much finer control on the lookup process. Action items are placed between two service names and are written within brackets. The general form is
[(!? status=action )+]
where
status => success | notfound | unavail | tryagain action => return | continue
The case of the keywords is insignificant. The status values are the results of a call to a lookup function of a specific service. They mean
return.
continue.
continue.
continue.
If we have a line like
ethers: nisplus [NOTFOUND=return] db files
this is equivalent to
ethers: nisplus [SUCCESS=return NOTFOUND=return UNAVAIL=continue
TRYAGAIN=continue]
db [SUCCESS=return NOTFOUND=continue UNAVAIL=continue
TRYAGAIN=continue]
files
(except that it would have to be written on one line). The default value for the actions are normally what you want, and only need to be changed in exceptional cases.
If the optional ! is placed before the status this means
the following action is used for all statuses but status itself.
I.e., ! is negation as in the C language (and others).
Before we explain the exception which makes this action item necessary
one more remark: obviously it makes no sense to add another action
item after the files service. Since there is no other service
following the action always is return.
Now, why is this [NOTFOUND=return] action useful? To understand
this we should know that the nisplus service is often
complete; i.e., if an entry is not available in the NIS+ tables it is
not available anywhere else. This is what is expressed by this action
item: it is useless to examine further services since they will not give
us a result.
The situation would be different if the NIS+ service is not available
because the machine is booting. In this case the return value of the
lookup function is not notfound but instead unavail. And
as you can see in the complete form above: in this situation the
db and files services are used. Neat, isn't it? The
system administrator need not pay special care for the time the system
is not completely ready to work (while booting or shutdown or
network problems).
Finally a few more hints. The NSS implementation is not completely helpless if `/etc/nsswitch.conf' does not exist. For all supported databases there is a default value so it should normally be possible to get the system running even if the file is corrupted or missing.
For the hosts and networks databases the default value is
dns [!UNAVAIL=return] files. I.e., the system is prepared for
the DNS service not to be available but if it is available the answer it
returns is definitive.
The passwd, group, and shadow databases are
traditionally handled in a special way. The appropriate files in the
`/etc' directory are read but if an entry with a name starting
with a + character is found NIS is used. This kind of lookup
remains possible by using the special lookup service compat
and the default value for the three databases above is
compat [NOTFOUND=return] files.
For all other databases the default value is
nis [NOTFOUND=return] files. This solution give the best
chance to be correct since NIS and file based lookup is used.
A second point is that the user should try to optimize the lookup
process. The different service have different response times.
A simple file look up on a local file could be fast, but if the file
is long and the needed entry is near the end of the file this may take
quite some time. In this case it might be better to use the db
service which allows fast local access to large data sets.
Often the situation is that some global information like NIS must be
used. So it is unavoidable to use service entries like nis etc.
But one should avoid slow services like this if possible.
Now it is time to describe what the modules look like. The functions contained in a module are identified by their names. I.e., there is no jump table or the like. How this is done is of no interest here; those interested in this topic should read about Dynamic Linking.
The name of each function consist of various parts:
_nss_service_function
service of course corresponds to the name of the module this
function is found in.(3) The function part is derived
from the interface function in the C library itself. If the user calls
the function gethostbyname and the service used is files
the function
_nss_files_gethostbyname_r
in the module
libnss_files.so.2
is used. You see, what is explained above in not the whole truth. In
fact the NSS modules only contain reentrant versions of the lookup
functions. I.e., if the user would call the gethostbyname_r
function this also would end in the above function. For all user
interface functions the C library maps this call to a call to the
reentrant function. For reentrant functions this is trivial since the
interface is (nearly) the same. For the non-reentrant version The
library keeps internal buffers which are used to replace the user
supplied buffer.
I.e., the reentrant functions can have counterparts. No service
module is forced to have functions for all databases and all kinds to
access them. If a function is not available it is simply treated as if
the function would return unavail
(see section Actions in the NSS configuration).
The file name `libnss_files.so.2' would be on a Solaris 2 system `nss_files.so.2'. This is the difference mentioned above. Sun's NSS modules are usable as modules which get indirectly loaded only.
The NSS modules in the GNU C Library are prepared to be used as normal libraries themselves. This is not true at the moment, though. However, the organization of the name space in the modules does not make it impossible like it is for Solaris. Now you can see why the modules are still libraries.(4)
Now we know about the functions contained in the modules. It is now time to describe the types. When we mentioned the reentrant versions of the functions above, this means there are some additional arguments (compared with the standard, non-reentrant version). The prototypes for the non-reentrant and reentrant versions of our function above are:
struct hostent *gethostbyname (const char *name)
int gethostbyname_r (const char *name, struct hostent *result_buf,
char *buf, size_t buflen, struct hostent **result,
int *h_errnop)
The actual prototype of the function in the NSS modules in this case is
enum nss_status _nss_files_gethostbyname_r (const char *name,
struct hostent *result_buf,
char *buf, size_t buflen,
int *errnop, int *h_errnop)
I.e., the interface function is in fact the reentrant function with the
change of the return value and the omission of the result
parameter. While the user-level function returns a pointer to the
result the reentrant function return an enum nss_status value:
NSS_STATUS_TRYAGAIN
-2
NSS_STATUS_UNAVAIL
-1
NSS_STATUS_NOTFOUND
0
NSS_STATUS_SUCCESS
1
Now you see where the action items of the `/etc/nsswitch.conf' file are used.
If you study the source code you will find there is a fifth value:
NSS_STATUS_RETURN. This is an internal use only value, used by a
few functions in places where none of the above value can be used. If
necessary the source code should be examined to learn about the details.
In case the interface function has to return an error it is important
that the correct error code is stored in *errnop. Some
return status value have only one associated error code, others have
more.
@multitable @columnfractions .3 .2 .50
NSS_STATUS_TRYAGAIN @tab
EAGAIN @tab One of the functions used ran temporarily out of
resources or a service is currently not available.
ERANGE @tab The provided buffer is not large enough.
The function should be called again with a larger buffer.
NSS_STATUS_UNAVAIL @tab
ENOENT @tab A necessary input file cannot be found.
NSS_STATUS_NOTFOUND @tab
ENOENT @tab The requested entry is not available.
These are proposed values. There can be other error codes and the
described error codes can have different meaning. With one
exception: when returning NSS_STATUS_TRYAGAIN the error code
ERANGE must mean that the user provided buffer is too
small. Everything is non-critical.
The above function has something special which is missing for almost all
the other module functions. There is an argument h_errnop. This
points to a variable which will be filled with the error code in case
the execution of the function fails for some reason. The reentrant
function cannot use the global variable h_errno;
gethostbyname calls gethostbyname_r with the last argument
set to &h_errno.
The getXXXbyYYY functions are the most important
functions in the NSS modules. But there are others which implement
the other ways to access system databases (say for the
password database, there are setpwent, getpwent, and
endpwent). These will be described in more detail later.
Here we give a general way to determine the
signature of the module function:
int;
STRUCT_TYPE *result_buf
STRUCT_TYPE is
normally a struct which corresponds to the database.
char *buffer
size_t buflen
set...ent
and end...ent functions.
One of the advantages of NSS mentioned above is that it can be extended quite easily. There are two ways in which the extension can happen: adding another database or adding another service. The former is normally done only by the C library developers. It is here only important to remember that adding another database is independent from adding another service because a service need not support all databases or lookup functions.
A designer/implementor of a new service is therefore free to choose the databases s/he is interested in and leave the rest for later (or completely aside).
The sources for a new service need not (and should not) be part of the GNU C Library itself. The developer retains complete control over the sources and its development. The links between the C library and the new service module consists solely of the interface functions.
Each module is designed following a specific interface specification.
For now the version is 2 (the interface in version 1 was not adequate)
and this manifests in the version number of the shared library object of
the NSS modules: they have the extension .2. If the interface
changes again in an incompatible way, this number will be increased.
Modules using the old interface will still be usable.
Developers of a new service will have to make sure that their module is created using the correct interface number. This means the file itself must have the correct name and on ElF systems the soname (Shared Object Name) must also have this number. Building a module from a bunch of object files on an ELF system using GNU CC could be done like this:
gcc -shared -o libnss_NAME.so.2 -Wl,-soname,libnss_NAME.so.2 OBJECTS
section `Link Options' in GNU CC, to learn more about this command line.
To use the new module the library must be able to find it. This can be
achieved by using options for the dynamic linker so that it will search
the directory where the binary is placed. For an ELF system this could be
done by adding the wanted directory to the value of
LD_LIBRARY_PATH.
But this is not always possible since some programs (those which run
under IDs which do not belong to the user) ignore this variable.
Therefore the stable version of the module should be placed into a
directory which is searched by the dynamic linker. Normally this should
be the directory `$prefix/lib', where `$prefix' corresponds to
the value given to configure using the --prefix option. But be
careful: this should only be done if it is clear the module does not
cause any harm. System administrators should be careful.
Until now we only provided the syntactic interface for the functions in the NSS module. In fact there is not much more we can say since the implementation obviously is different for each function. But a few general rules must be followed by all functions.
In fact there are four kinds of different functions which may appear in
the interface. All derive from the traditional ones for system databases.
db in the following table is normally an abbreviation for the
database (e.g., it is pw for the password database).
enum nss_status _nss_database_setdbent (void)
int setdbent (int)). section Host Names, which describes the
sethostent function.
The return value should be NSS_STATUS_SUCCESS or according to the
table above in case of an error (see section The Interface of the Function in NSS Modules).
enum nss_status _nss_database_enddbent (void)
enum nss_status _nss_database_getdbent_r (STRUCTURE *result, char *buffer, size_t buflen, int *errnop)
host and networks.
The function shall return NSS_STATUS_SUCCESS as long as there are
more entries. When the last entry was read it should return
NSS_STATUS_NOTFOUND. When the buffer given as an argument is too
small for the data to be returned NSS_STATUS_TRYAGAIN should be
returned. When the service was not formerly initialized by a call to
_nss_DATABASE_setdbent all return value allowed for
this function can also be returned here.
enum nss_status _nss_DATABASE_getdbbyXX_r (PARAMS, STRUCTURE *result, char *buffer, size_t buflen, int *errnop)
setDBent function whenever this makes sense.
Before the function returns the implementation should store the value of
the local errno variable in the variable pointed to be
errnop. This is important to guarantee the module working in
statically linked programs.
Again, this function takes an additional last argument for the
host and networks database.
The return value should as always follow the rules given above
(see section The Interface of the Function in NSS Modules).
Every user who can log in on the system is identified by a unique number called the user ID. Each process has an effective user ID which says which user's access permissions it has.
Users are classified into groups for access control purposes. Each process has one or more group ID values which say which groups the process can use for access to files.
The effective user and group IDs of a process collectively form its persona. This determines which files the process can access. Normally, a process inherits its persona from the parent process, but under special circumstances a process can change its persona and thus change its access permissions.
Each file in the system also has a user ID and a group ID. Access control works by comparing the user and group IDs of the file with those of the running process.
The system keeps a database of all the registered users, and another database of all the defined groups. There are library functions you can use to examine these databases.
Each user account on a computer system is identified by a user name (or login name) and user ID. Normally, each user name has a unique user ID, but it is possible for several login names to have the same user ID. The user names and corresponding user IDs are stored in a data base which you can access as described in section User Database.
Users are classified in groups. Each user name belongs to one default group and may also belong to any number of supplementary groups. Users who are members of the same group can share resources (such as files) that are not accessible to users who are not a member of that group. Each group has a group name and group ID. See section Group Database, for how to find information about a group ID or group name.
At any time, each process has an effective user ID, a effective group ID, and a set of supplementary group IDs. These IDs determine the privileges of the process. They are collectively called the persona of the process, because they determine "who it is" for purposes of access control.
Your login shell starts out with a persona which consists of your user ID, your default group ID, and your supplementary group IDs (if you are in more than one group). In normal circumstances, all your other processes inherit these values.
A process also has a real user ID which identifies the user who created the process, and a real group ID which identifies that user's default group. These values do not play a role in access control, so we do not consider them part of the persona. But they are also important.
Both the real and effective user ID can be changed during the lifetime of a process. See section Why Change the Persona of a Process?.
For details on how a process's effective user ID and group IDs affect its permission to access files, see section How Your Access to a File is Decided.
The effective user ID of a process also controls permissions for sending
signals using the kill function. See section Signaling Another Process.
Finally, there are many operations which can only be performed by a
process whose effective user ID is zero. A process with this user ID is
a privileged process. Commonly the user name root is
associated with user ID 0, but there may be other user names with this
ID.
The most obvious situation where it is necessary for a process to change
its user and/or group IDs is the login program. When
login starts running, its user ID is root. Its job is to
start a shell whose user and group IDs are those of the user who is
logging in. (To accomplish this fully, login must set the real
user and group IDs as well as its persona. But this is a special case.)
The more common case of changing persona is when an ordinary user program needs access to a resource that wouldn't ordinarily be accessible to the user actually running it.
For example, you may have a file that is controlled by your program but that shouldn't be read or modified directly by other users, either because it implements some kind of locking protocol, or because you want to preserve the integrity or privacy of the information it contains. This kind of restricted access can be implemented by having the program change its effective user or group ID to match that of the resource.
Thus, imagine a game program that saves scores in a file. The game
program itself needs to be able to update this file no matter who is
running it, but if users can write the file without going through the
game, they can give themselves any scores they like. Some people
consider this undesirable, or even reprehensible. It can be prevented
by creating a new user ID and login name (say, games) to own the
scores file, and make the file writable only by this user. Then, when
the game program wants to update this file, it can change its effective
user ID to be that for games. In effect, the program must
adopt the persona of games so it can write the scores file.
The ability to change the persona of a process can be a source of unintentional privacy violations, or even intentional abuse. Because of the potential for problems, changing persona is restricted to special circumstances.
You can't arbitrarily set your user ID or group ID to anything you want; only privileged processes can do that. Instead, the normal way for a program to change its persona is that it has been set up in advance to change to a particular user or group. This is the function of the setuid and setgid bits of a file's access mode. See section The Mode Bits for Access Permission.
When the setuid bit of an executable file is on, executing that file gives the process a third user ID: the file user ID. This ID is set to the owner ID of the file. The system then changes the effective user ID to the file user ID. The real user ID remains as it was. Likewise, if the setgid bit is on, the process is given a file group ID equal to the group ID of the file, and its effective group ID is changed to the file group ID.
If a process has a file ID (user or group), then it can at any time change its effective ID to its real ID and back to its file ID. Programs use this feature to relinquish their special privileges except when they actually need them. This makes it less likely that they can be tricked into doing something inappropriate with their privileges.
Portability Note: Older systems do not have file IDs.
To determine if a system has this feature, you can test the compiler
define _POSIX_SAVED_IDS. (In the POSIX standard, file IDs are
known as saved IDs.)
See section File Attributes, for a more general discussion of file modes and accessibility.
Here are detailed descriptions of the functions for reading the user and group IDs of a process, both real and effective. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'.
unsigned int.
unsigned int.
getgroups function is used to inquire about the supplementary
group IDs of the process. Up to count of these group IDs are
stored in the array groups; the return value from the function is
the number of group IDs actually stored. If count is smaller than
the total number of supplementary group IDs, then getgroups
returns a value of -1 and errno is set to EINVAL.
If count is zero, then getgroups just returns the total
number of supplementary group IDs. On systems that do not support
supplementary groups, this will always be zero.
Here's how to use getgroups to read all the supplementary group
IDs:
gid_t *
read_all_groups (void)
{
int ngroups = getgroups (0, NULL);
gid_t *groups
= (gid_t *) xmalloc (ngroups * sizeof (gid_t));
int val = getgroups (ngroups, groups);
if (val < 0)
{
free (groups);
return NULL;
}
return groups;
}
This section describes the functions for altering the user ID (real and/or effective) of a process. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'.
The seteuid function returns a value of 0 to indicate
successful completion, and a value of -1 to indicate an error.
The following errno error conditions are defined for this
function:
EINVAL
EPERM
Older systems (those without the _POSIX_SAVED_IDS feature) do not
have this function.
If the process is not privileged, and the system supports the
_POSIX_SAVED_IDS feature, then this function behaves like
seteuid.
The return values and error conditions are the same as for seteuid.
-1, it means
not to change the real user ID; likewise if euid is -1, it
means not to change the effective user ID.
The setreuid function exists for compatibility with 4.3 BSD Unix,
which does not support file IDs. You can use this function to swap the
effective and real user IDs of the process. (Privileged processes are
not limited to this particular usage.) If file IDs are supported, you
should use that feature instead of this function. See section Enabling and Disabling Setuid Access.
The return value is 0 on success and -1 on failure.
The following errno error conditions are defined for this
function:
EPERM
This section describes the functions for altering the group IDs (real and effective) of a process. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'.
seteuid, if the process is privileged it may
change its effective group ID to any value; if it isn't, but it has a
file group ID, then it may change to its real group ID or file group ID;
otherwise it may not change its effective group ID.
Note that a process is only privileged if its effective user ID is zero. The effective group ID only affects access permissions.
The return values and error conditions for setegid are the same
as those for seteuid.
This function is only present if _POSIX_SAVED_IDS is defined.
If the process is not privileged, then setgid behaves like
setegid.
The return values and error conditions for setgid are the same
as those for seteuid.
-1, it
means not to change the real group ID; likewise if egid is
-1, it means not to change the effective group ID.
The setregid function is provided for compatibility with 4.3 BSD
Unix, which does not support file IDs. You can use this function to
swap the effective and real group IDs of the process. (Privileged
processes are not limited to this usage.) If file IDs are supported,
you should use that feature instead of using this function.
See section Enabling and Disabling Setuid Access.
The return values and error conditions for setregid are the same
as those for setreuid.
setuid and setgid behave differently depending on whether
the effective user ID at the time is zero. If it is not zero, they
behave like seteuid and setegid. If it is, they change
both effective and real IDs and delete the file ID. To avoid confusion,
we recommend you always use seteuid and setegid except
when you know the effective user ID is zero and your intent is to change
the persona permanently. This case is rare--most of the programs that
need it, such as login and su, have already been written.
Note that if your program is setuid to some user other than root,
there is no way to drop privileges permanently.
The system also lets privileged processes change their supplementary
group IDs. To use setgroups or initgroups, your programs
should include the header file `grp.h'.
This function returns 0 if successful and -1 on error.
The following errno error conditions are defined for this
function:
EPERM
initgroups function sets the process's supplementary group
IDs to be the normal default for the user name user. If gid
is not -1, it includes that group also.
This function works by scanning the group database for all the groups
user belongs to. It then calls setgroups with the list it
has constructed.
The return values and error conditions are the same as for
setgroups.
A typical setuid program does not need its special access all of the time. It's a good idea to turn off this access when it isn't needed, so it can't possibly give unintended access.
If the system supports the _POSIX_SAVED_IDS feature, you can
accomplish this with seteuid. When the game program starts, its
real user ID is jdoe, its effective user ID is games, and
its saved user ID is also games. The program should record both
user ID values once at the beginning, like this:
user_user_id = getuid (); game_user_id = geteuid ();
Then it can turn off game file access with
seteuid (user_user_id);
and turn it on with
seteuid (game_user_id);
Throughout this process, the real user ID remains jdoe and the
file user ID remains games, so the program can always set its
effective user ID to either one.
On other systems that don't support file user IDs, you can
turn setuid access on and off by using setreuid to swap the real
and effective user IDs of the process, as follows:
setreuid (geteuid (), getuid ());
This special case is always allowed--it cannot fail.
Why does this have the effect of toggling the setuid access? Suppose a
game program has just started, and its real user ID is jdoe while
its effective user ID is games. In this state, the game can
write the scores file. If it swaps the two uids, the real becomes
games and the effective becomes jdoe; now the program has
only jdoe access. Another swap brings games back to
the effective user ID and restores access to the scores file.
In order to handle both kinds of systems, test for the saved user ID feature with a preprocessor conditional, like this:
#ifdef _POSIX_SAVED_IDS setuid (user_user_id); #else setreuid (geteuid (), getuid ()); #endif
Here's an example showing how to set up a program that changes its effective user ID.
This is part of a game program called caber-toss that manipulates
a file `scores' that should be writable only by the game program
itself. The program assumes that its executable file will be installed
with the setuid bit set and owned by the same user as the `scores'
file. Typically, a system administrator will set up an account like
games for this purpose.
The executable file is given mode 4755, so that doing an
`ls -l' on it produces output like:
-rwsr-xr-x 1 games 184422 Jul 30 15:17 caber-toss
The setuid bit shows up in the file modes as the `s'.
The scores file is given mode 644, and doing an `ls -l' on
it shows:
-rw-r--r-- 1 games 0 Jul 31 15:33 scores
Here are the parts of the program that show how to set up the changed
user ID. This program is conditionalized so that it makes use of the
file IDs feature if it is supported, and otherwise uses setreuid
to swap the effective and real user IDs.
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
/* Remember the effective and real UIDs. */
static uid_t euid, ruid;
/* Restore the effective UID to its original value. */
void
do_setuid (void)
{
int status;
#ifdef _POSIX_SAVED_IDS
status = seteuid (euid);
#else
status = setreuid (ruid, euid);
#endif
if (status < 0) {
fprintf (stderr, "Couldn't set uid.\n");
exit (status);
}
}
/* Set the effective UID to the real UID. */
void
undo_setuid (void)
{
int status;
#ifdef _POSIX_SAVED_IDS
status = seteuid (ruid);
#else
status = setreuid (euid, ruid);
#endif
if (status < 0) {
fprintf (stderr, "Couldn't set uid.\n");
exit (status);
}
}
/* Main program. */
int
main (void)
{
/* Remember the real and effective user IDs. */
ruid = getuid ();
euid = geteuid ();
undo_setuid ();
/* Do the game and record the score. */
...
}
Notice how the first thing the main function does is to set the
effective user ID back to the real user ID. This is so that any other
file accesses that are performed while the user is playing the game use
the real user ID for determining permissions. Only when the program
needs to open the scores file does it switch back to the file user ID,
like this:
/* Record the score. */
int
record_score (int score)
{
FILE *stream;
char *myname;
/* Open the scores file. */
do_setuid ();
stream = fopen (SCORES_FILE, "a");
undo_setuid ();
/* Write the score to the file. */
if (stream)
{
myname = cuserid (NULL);
if (score < 0)
fprintf (stream, "%10s: Couldn't lift the caber.\n", myname);
else
fprintf (stream, "%10s: %d feet.\n", myname, score);
fclose (stream);
return 0;
}
else
return -1;
}
It is easy for setuid programs to give the user access that isn't intended--in fact, if you want to avoid this, you need to be careful. Here are some guidelines for preventing unintended access and minimizing its consequences when it does occur:
setuid programs with privileged user IDs such as
root unless it is absolutely necessary. If the resource is
specific to your particular program, it's better to define a new,
nonprivileged user ID or group ID just to manage that resource.
It's better if you can write your program to use a special group than a
special user.
exec functions in combination with
changing the effective user ID. Don't let users of your program execute
arbitrary programs under a changed user ID. Executing a shell is
especially bad news. Less obviously, the execlp and execvp
functions are a potential risk (since the program they execute depends
on the user's PATH environment variable).
If you must exec another program under a changed ID, specify an
absolute file name (see section File Name Resolution) for the executable,
and make sure that the protections on that executable and all
containing directories are such that ordinary users cannot replace it
with some other program.
You should also check the arguments passed to the program to make sure
they do not have unexpected effects. Likewise, you should examine the
environment variables. Decide which arguments and variables are safe,
and reject all others.
You should never use system in a privileged program, because it
invokes a shell.
setuid part of your program needs to access other files
besides the controlled resource, it should verify that the real user
would ordinarily have permission to access those files. You can use the
access function (see section How Your Access to a File is Decided) to check this; it
uses the real user and group IDs, rather than the effective IDs.
You can use the functions listed in this section to determine the login
name of the user who is running a process, and the name of the user who
logged in the current session. See also the function getuid and
friends (see section Reading the Persona of a Process). How this information is collected by
the system and how to control/add/remove information from the background
storage is described in section The User Accounting Database.
The getlogin function is declared in `unistd.h', while
cuserid and L_cuserid are declared in `stdio.h'.
getlogin function returns a pointer to a string containing the
name of the user logged in on the controlling terminal of the process,
or a null pointer if this information cannot be determined. The string
is statically allocated and might be overwritten on subsequent calls to
this function or to cuserid.
cuserid function returns a pointer to a string containing a
user name associated with the effective ID of the process. If
string is not a null pointer, it should be an array that can hold
at least L_cuserid characters; the string is returned in this
array. Otherwise, a pointer to a string in a static area is returned.
This string is statically allocated and might be overwritten on
subsequent calls to this function or to getlogin.
The use of this function is deprecated since it is marked to be withdrawn in XPG4.2 and has already been removed from newer revisions of POSIX.1.
These functions let your program identify positively the user who is running or the user who logged in this session. (These can differ when setuid programs are involved; see section The Persona of a Process.) The user cannot do anything to fool these functions.
For most purposes, it is more useful to use the environment variable
LOGNAME to find out who the user is. This is more flexible
precisely because the user can set LOGNAME arbitrarily.
See section Standard Environment Variables.
Most Unix-like operating systems keep track of logged in users by maintaining a user accounting database. This user accounting database stores for each terminal, who has logged on, at what time, the process ID of the user's login shell, etc., etc., but also stores information about the run level of the system, the time of the last system reboot, and possibly more.
The user accounting database typically lives in `/etc/utmp', `/var/adm/utmp' or `/var/run/utmp'. However, these files should never be accessed directly. For reading information from and writing information to the user accounting database, the functions described in this section should be used.
These functions and the corresponding data structures are declared in the header file `utmp.h'.
exit_status data structure is used to hold information about
the exit status of processes marked as DEAD_PROCESS in the user
accounting database.
short int e_termination
short int e_exit
utmp data structure is used to hold information about entries
in the user accounting database. On the GNU system it has the following
members:
short int ut_type
EMPTY, RUN_LVL,
BOOT_TIME, OLD_TIME, NEW_TIME, INIT_PROCESS,
LOGIN_PROCESS, USER_PROCESS, DEAD_PROCESS or
ACCOUNTING.
pid_t ut_pid
char ut_line[]
char ut_id[]
char ut_user[]
char ut_host[]
struct exit_status ut_exit
DEAD_PROCESS.
long ut_session
struct timeval ut_tv
OLD_TIME this is
the time when the system clock changed, and for entries of type
NEW_TIME this is the time the system clock was set to.
int32_t ut_addr_v6[4]
The ut_type, ut_pid, ut_id, ut_tv, and
ut_host fields are not available on all systems. Portable
applications therefore should be prepared for these situations. To help
doing this the `utmp.h' header provides macros
_HAVE_UT_TYPE, _HAVE_UT_PID, _HAVE_UT_ID,
_HAVE_UT_TV, and _HAVE_UT_HOST if the respective field is
available. The programmer can handle the situations by using
#ifdef in the program code.
The following macros are defined for use as values for the
ut_type member of the utmp structure. The values are
integer constants.
EMPTY
RUN_LVL
BOOT_TIME
OLD_TIME
NEW_TIME
INIT_PROCESS
LOGIN_PROCESS
USER_PROCESS
DEAD_PROCESS
ACCOUNTING
The size of the ut_line, ut_id, ut_user and
ut_host arrays can be found using the sizeof operator.
Many older systems have, instead of an ut_tv member, an
ut_time member, usually of type time_t, for representing
the time associated with the entry. Therefore, for backwards
compatibility only, `utmp.h' defines ut_time as an alias for
ut_tv.tv_sec.
getutent, getutid or getutline to
read entries and pututline to write entries.
If the database is already open, it resets the input to the beginning of the database.
getutent function reads the next entry from the user
accounting database. It returns a pointer to the entry, which is
statically allocated and may be overwritten by subsequent calls to
getutent. You must copy the contents of the structure if you
wish to save the information or you can use the getutent_r
function which stores the data in a user-provided buffer.
A null pointer is returned in case no further entry is available.
ut_type member of the
id structure is one of RUN_LVL, BOOT_TIME,
OLD_TIME or NEW_TIME the entries match if the
ut_type members are identical. If the ut_type member of
the id structure is INIT_PROCESS, LOGIN_PROCESS,
USER_PROCESS or DEAD_PROCESS, the entries match if the
ut_type member of the entry read from the database is one of
these four, and the ut_id members match. However if the
ut_id member of either the id structure or the entry read
from the database is empty it checks if the ut_line members match
instead. If a matching entry is found, getutid returns a pointer
to the entry, which is statically allocated, and may be overwritten by a
subsequent call to getutent, getutid or getutline.
You must copy the contents of the structure if you wish to save the
information.
A null pointer is returned in case the end of the database is reached without a match.
The getutid function may cache the last read entry. Therefore,
if you are using getutid to search for multiple occurrences, it
is necessary to zero out the static data after each call. Otherwise
getutid could just return a pointer to the same entry over and
over again.
ut_type value is
LOGIN_PROCESS or USER_PROCESS, and whose ut_line
member matches the ut_line member of the line structure.
If it finds such an entry, it returns a pointer to the entry which is
statically allocated, and may be overwritten by a subsequent call to
getutent, getutid or getutline. You must copy the
contents of the structure if you wish to save the information.
A null pointer is returned in case the end of the database is reached without a match.
The getutline function may cache the last read entry. Therefore
if you are using getutline to search for multiple occurrences, it
is necessary to zero out the static data after each call. Otherwise
getutline could just return a pointer to the same entry over and
over again.
pututline function inserts the entry *utmp at
the appropriate place in the user accounting database. If it finds that
it is not already at the correct place in the database, it uses
getutid to search for the position to insert the entry, however
this will not modify the static structure returned by getutent,
getutid and getutline. If this search fails, the entry
is appended to the database.
The pututline function returns a pointer to a copy of the entry
inserted in the user accounting database, or a null pointer if the entry
could not be added. The following errno error conditions are
defined for this function:
EPERM
All the get* functions mentioned before store the information
they return in a static buffer. This can be a problem in multi-threaded
programs since the data returned for the request is overwritten by the
return value data in another thread. Therefore the GNU C Library
provides as extensions three more functions which return the data in a
user-provided buffer.
getutent_r is equivalent to the getutent function. It
returns the next entry from the database. But instead of storing the
information in a static buffer it stores it in the buffer pointed to by
the parameter buffer.
If the call was successful, the function returns 0 and the
pointer variable pointed to by the parameter result contains a
pointer to the buffer which contains the result (this is most probably
the same value as buffer). If something went wrong during the
execution of getutent_r the function returns -1.
This function is a GNU extension.
getutid the next entry matching
the information stored in id. But the result is stored in the
buffer pointed to by the parameter buffer.
If successful the function returns 0 and the pointer variable
pointed to by the parameter result contains a pointer to the
buffer with the result (probably the same as result. If not
successful the function return -1.
This function is a GNU extension.
getutline the next entry
matching the information stored in line. But the result is stored
in the buffer pointed to by the parameter buffer.
If successful the function returns 0 and the pointer variable
pointed to by the parameter result contains a pointer to the
buffer with the result (probably the same as result. If not
successful the function return -1.
This function is a GNU extension.
In addition to the user accounting database, most systems keep a number of similar databases. For example most systems keep a log file with all previous logins (usually in `/etc/wtmp' or `/var/log/wtmp').
For specifying which database to examine, the following function should be used.
utmpname function changes the name of the database to be
examined to file, and closes any previously opened database. By
default getutent, getutid, getutline and
pututline read from and write to the user accounting database.
The following macros are defined for use as the file argument:
The utmpname function returns a value of 0 if the new name
was successfully stored, and a value of -1 to indicate an error.
Note that utmpname does not try to open the database, and that
therefore the return value does not say anything about whether the
database can be successfully opened.
Specially for maintaining log-like databases the GNU C Library provides the following function:
updwtmp function appends the entry *utmp to the
database specified by wtmp_file. For possible values for the
wtmp_file argument see the utmpname function.
Portability Note: Although many operating systems provide a
subset of these functions, they are not standardized. There are often
subtle differences in the return types, and there are considerable
differences between the various definitions of struct utmp. When
programming for the GNU system, it is probably best to stick
with the functions described in this section. If however, you want your
program to be portable, consider using the XPG functions described in
section XPG User Accounting Database Functions, or take a look at the BSD compatible functions in
section Logging In and Out.
These functions, described in the X/Open Portability Guide, are declared in the header file `utmpx.h'.
utmpx data structure contains at least the following members:
short int ut_type
EMPTY, RUN_LVL,
BOOT_TIME, OLD_TIME, NEW_TIME, INIT_PROCESS,
LOGIN_PROCESS, USER_PROCESS or DEAD_PROCESS.
pid_t ut_pid
char ut_line[]
char ut_id[]
char ut_user[]
struct timeval ut_tv
OLD_TIME this is
the time when the system clock changed, and for entries of type
NEW_TIME this is the time the system clock was set to.
On the GNU system, struct utmpx is identical to struct
utmp except for the fact that including `utmpx.h' does not make
visible the declaration of struct exit_status.
The following macros are defined for use as values for the
ut_type member of the utmpx structure. The values are
integer constants and are, on the GNU system, identical to the
definitions in `utmp.h'.
EMPTY
RUN_LVL
BOOT_TIME
OLD_TIME
NEW_TIME
INIT_PROCESS
LOGIN_PROCESS
USER_PROCESS
DEAD_PROCESS
The size of the ut_line, ut_id and ut_user arrays
can be found using the sizeof operator.
setutent. On the GNU system it is
simply an alias for setutent.
getutxent function is similar to getutent, but returns
a pointer to a struct utmpx instead of struct utmp. On
the GNU system it simply is an alias for getutent.
endutent. On the GNU system it is
simply an alias for endutent.
getutid, but uses struct utmpx
instead of struct utmp. On the GNU system it is simply an alias
for getutid.
getutid, but uses struct utmpx
instead of struct utmp. On the GNU system it is simply an alias
for getutline.
pututxline function is functionally identical to
pututline, but uses struct utmpx instead of struct
utmp. On the GNU system, pututxline is simply an alias for
pututline.
utmpxname function is functionally identical to
utmpname. On the GNU system, utmpxname is simply an
alias for utmpname.
You can translate between a traditional struct utmp and an XPG
struct utmpx with the following functions. On the GNU system,
these functions are merely copies, since the two structures are
identical.
getutmp copies the information, insofar as the structures are
compatible, from utmpx to utmp.
getutmpx copies the information, insofar as the structures are
compatible, from utmp to utmpx.
These functions, derived from BSD, are available in the separate `libutil' library, and declared in `utmp.h'.
Note that the ut_user member of struct utmp is called
ut_name in BSD. Therefore, ut_name is defined as an alias
for ut_user in `utmp.h'.
This function returns 0 on successful completion, and -1
on error.
login functions inserts an entry into the user accounting
database. The ut_line member is set to the name of the terminal
on standard input. If standard input is not a terminal login
uses standard output or standard error output to determine the name of
the terminal. If struct utmp has a ut_type member,
login sets it to USER_PROCESS, and if there is an
ut_pid member, it will be set to the process ID of the current
process. The remaining entries are copied from entry.
A copy of the entry is written to the user accounting log file.
The logout function returns 1 if the entry was successfully
written to the database, or 0 on error.
logwtmp function appends an entry to the user accounting log
file, for the current time and the information provided in the
ut_line, ut_name and ut_host arguments.
Portability Note: The BSD struct utmp only has the
ut_line, ut_name, ut_host and ut_time
members. Older systems do not even have the ut_host member.
This section describes how to search and scan the database of registered users. The database itself is kept in the file `/etc/passwd' on most systems, but on some systems a special network server gives access to it.
The functions and data structures for accessing the system user database are declared in the header file `pwd.h'.
passwd data structure is used to hold information about
entries in the system user data base. It has at least the following members:
char *pw_name
char *pw_passwd.
uid_t pw_uid
gid_t pw_gid
char *pw_gecos
char *pw_dir
char *pw_shell
You can search the system user database for information about a
specific user using getpwuid or getpwnam. These
functions are declared in `pwd.h'.
getpwuid.
A null pointer value indicates there is no user in the data base with user ID uid.
getpwuid in that it returns
information about the user whose user ID is uid. However, it
fills the user supplied structure pointed to by result_buf with
the information instead of using a static buffer. The first
buflen bytes of the additional buffer pointed to by buffer
are used to contain additional information, normally strings which are
pointed to by the elements of the result structure.
If a user with ID uid is found, the pointer returned in
result points to the record which contains the wanted data (i.e.,
result contains the value result_buf). If no user is found
or if an error occurred, the pointer returned in result is a null
pointer. The function returns zero or an error code. If the buffer
buffer is too small to contain all the needed information, the
error code ERANGE is returned and errno is set to
ERANGE.
getpwnam.
A null pointer return indicates there is no user named name.
getpwnam in that is returns
information about the user whose user name is name. However, like
getpwuid_r, it fills the user supplied buffers in
result_buf and buffer with the information instead of using
a static buffer.
The return values are the same as for getpwuid_r.
This section explains how a program can read the list of all users in the system, one user at a time. The functions described here are declared in `pwd.h'.
You can use the fgetpwent function to read user entries from a
particular file.
fgetpwent. You must copy the
contents of the structure if you wish to save the information.
The stream must correspond to a file in the same format as the standard password database file.
fgetpwent in that it reads the next
user entry from stream. But the result is returned in the
structure pointed to by result_buf. The
first buflen bytes of the additional buffer pointed to by
buffer are used to contain additional information, normally
strings which are pointed to by the elements of the result structure.
The stream must correspond to a file in the same format as the standard password database file.
If the function returns zero result points to the structure with the wanted data (normally this is in result_buf). If errors occurred the return value is nonzero and result contains a null pointer.
The way to scan all the entries in the user database is with
setpwent, getpwent, and endpwent.
getpwent and
getpwent_r use to read the user database.
getpwent function reads the next entry from the stream
initialized by setpwent. It returns a pointer to the entry. The
structure is statically allocated and is rewritten on subsequent calls
to getpwent. You must copy the contents of the structure if you
wish to save the information.
A null pointer is returned when no more entries are available.
getpwent in that it returns the next
entry from the stream initialized by setpwent. Like
fgetpwent_r, it uses the user-supplied buffers in
result_buf and buffer to return the information requested.
The return values are the same as for fgetpwent_r.
getpwent or
getpwent_r.
*p to the stream
stream, in the format used for the standard user database
file. The return value is zero on success and nonzero on failure.
This function exists for compatibility with SVID. We recommend that you
avoid using it, because it makes sense only on the assumption that the
struct passwd structure has no members except the standard ones;
on a system which merges the traditional Unix data base with other
extended information about users, adding an entry using this function
would inevitably leave out much of the important information.
The function putpwent is declared in `pwd.h'.
This section describes how to search and scan the database of registered groups. The database itself is kept in the file `/etc/group' on most systems, but on some systems a special network service provides access to it.
The functions and data structures for accessing the system group database are declared in the header file `grp.h'.
group structure is used to hold information about an entry in
the system group database. It has at least the following members:
char *gr_name
gid_t gr_gid
char **gr_mem
You can search the group database for information about a specific
group using getgrgid or getgrnam. These functions are
declared in `grp.h'.
getgrgid.
A null pointer indicates there is no group with ID gid.
getgrgid in that it returns
information about the group whose group ID is gid. However, it
fills the user supplied structure pointed to by result_buf with
the information instead of using a static buffer. The first
buflen bytes of the additional buffer pointed to by buffer
are used to contain additional information, normally strings which are
pointed to by the elements of the result structure.
If a group with ID gid is found, the pointer returned in
result points to the record which contains the wanted data (i.e.,
result contains the value result_buf). If no group is found
or if an error occurred, the pointer returned in result is a null
pointer. The function returns zero or an error code. If the buffer
buffer is too small to contain all the needed information, the
error code ERANGE is returned and errno is set to
ERANGE.
getgrnam.
A null pointer indicates there is no group named name.
getgrnam in that is returns
information about the group whose group name is name. Like
getgrgid_r, it uses the user supplied buffers in
result_buf and buffer, not a static buffer.
The return values are the same as for getgrgid_r
ERANGE.
This section explains how a program can read the list of all groups in the system, one group at a time. The functions described here are declared in `grp.h'.
You can use the fgetgrent function to read group entries from a
particular file.
fgetgrent function reads the next entry from stream.
It returns a pointer to the entry. The structure is statically
allocated and is overwritten on subsequent calls to fgetgrent. You
must copy the contents of the structure if you wish to save the
information.
The stream must correspond to a file in the same format as the standard group database file.
fgetgrent in that it reads the next
user entry from stream. But the result is returned in the
structure pointed to by result_buf. The first buflen bytes
of the additional buffer pointed to by buffer are used to contain
additional information, normally strings which are pointed to by the
elements of the result structure.
This stream must correspond to a file in the same format as the standard group database file.
If the function returns zero result points to the structure with the wanted data (normally this is in result_buf). If errors occurred the return value is non-zero and result contains a null pointer.
The way to scan all the entries in the group database is with
setgrent, getgrent, and endgrent.
getgrent or getgrent_r.
getgrent function reads the next entry from the stream
initialized by setgrent. It returns a pointer to the entry. The
structure is statically allocated and is overwritten on subsequent calls
to getgrent. You must copy the contents of the structure if you
wish to save the information.
getgrent in that it returns the next
entry from the stream initialized by setgrent. Like
fgetgrent_r, it places the result in user-supplied buffers
pointed to result_buf and buffer.
If the function returns zero result contains a pointer to the data (normally equal to result_buf). If errors occurred the return value is non-zero and result contains a null pointer.
getgrent or
getgrent_r.
Here is an example program showing the use of the system database inquiry functions. The program prints some information about the user running the program.
#include <grp.h>
#include <pwd.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
int
main (void)
{
uid_t me;
struct passwd *my_passwd;
struct group *my_group;
char **members;
/* Get information about the user ID. */
me = getuid ();
my_passwd = getpwuid (me);
if (!my_passwd)
{
printf ("Couldn't find out about user %d.\n", (int) me);
exit (EXIT_FAILURE);
}
/* Print the information. */
printf ("I am %s.\n", my_passwd->pw_gecos);
printf ("My login name is %s.\n", my_passwd->pw_name);
printf ("My uid is %d.\n", (int) (my_passwd->pw_uid));
printf ("My home directory is %s.\n", my_passwd->pw_dir);
printf ("My default shell is %s.\n", my_passwd->pw_shell);
/* Get information about the default group ID. */
my_group = getgrgid (my_passwd->pw_gid);
if (!my_group)
{
printf ("Couldn't find out about group %d.\n",
(int) my_passwd->pw_gid);
exit (EXIT_FAILURE);
}
/* Print the information. */
printf ("My default group is %s (%d).\n",
my_group->gr_name, (int) (my_passwd->pw_gid));
printf ("The members of this group are:\n");
members = my_group->gr_mem;
while (*members)
{
printf (" %s\n", *(members));
members++;
}
return EXIT_SUCCESS;
}
Here is some output from this program:
I am Throckmorton Snurd. My login name is snurd. My uid is 31093. My home directory is /home/fsg/snurd. My default shell is /bin/sh. My default group is guest (12). The members of this group are: friedman tami
Sometimes it is useful to group users according to other criteria (see section Group Database). E.g., it is useful to associate a certain group of users with a certain machine. On the other hand grouping of host names is not supported so far.
In Sun Microsystems SunOS appeared a new kind of database, the netgroup database. It allows grouping hosts, users, and domains freely, giving them individual names. To be more concrete, a netgroup is a list of triples consisting of a host name, a user name, and a domain name where any of the entries can be a wildcard entry matching all inputs. A last possibility is that names of other netgroups can also be given in the list specifying a netgroup. So one can construct arbitrary hierarchies without loops.
Sun's implementation allows netgroups only for the nis or
nisplus service, see section Services in the NSS configuration File. The
implementation in the GNU C library has no such restriction. An entry
in either of the input services must have the following form:
groupname ( groupname |(hostname,username,domainname))+
Any of the fields in the triple can be empty which means anything
matches. While describing the functions we will see that the opposite
case is useful as well. I.e., there may be entries which will not
match any input. For entries like this, a name consisting of the single
character - shall be used.
The lookup functions for netgroups are a bit different to all other system database handling functions. Since a single netgroup can contain many entries a two-step process is needed. First a single netgroup is selected and then one can iterate over all entries in this netgroup. These functions are declared in `netdb.h'.
getnetgrent to iterate over all entries
in the netgroup with name netgroup.
When the call is successful (i.e., when a netgroup with this name exists)
the return value is 1. When the return value is 0 no
netgroup of this name is known or some other error occurred.
It is important to remember that there is only one single state for
iterating the netgroups. Even if the programmer uses the
getnetgrent_r function the result is not really reentrant since
always only one single netgroup at a time can be processed. If the
program needs to process more than one netgroup simultaneously she
must protect this by using external locking. This problem was
introduced in the original netgroups implementation in SunOS and since
we must stay compatible it is not possible to change this.
Some other functions also use the netgroups state. Currently these are
the innetgr function and parts of the implementation of the
compat service part of the NSS implementation.
NULL.
The returned string pointers are only valid if none of the netgroup
related functions are called.
The return value is 1 if the next entry was successfully read. A
value of 0 means no further entries exist or internal errors occurred.
getnetgrent with only one exception:
the strings the three string pointers hostp, userp, and
domainp point to, are placed in the buffer of buflen bytes
starting at buffer. This means the returned values are valid
even after other netgroup related functions are called.
The return value is 1 if the next entry was successfully read and
the buffer contains enough room to place the strings in it. 0 is
returned in case no more entries are found, the buffer is too small, or
internal errors occurred.
This function is a GNU extension. The original implementation in the SunOS libc does not provide this function.
getnetgrent are invalid afterwards.
It is often not necessary to scan the whole netgroup since often the only interesting question is whether a given entry is part of the selected netgroup.
set/get/endnetgrent functions.
Any of the pointers hostp, userp, and domainp can be
NULL which means any value is accepted in this position. This is
also true for the name - which should not match any other string
otherwise.
The return value is 1 if an entry matching the given triple is
found in the netgroup. The return value is 0 if the netgroup
itself is not found, the netgroup does not contain the triple or
internal errors occurred.
This chapter describes facilities for controlling the system that underlies a process (including the operating system and hardware) and for getting information about it. Anyone can generally use the informational facilities, but usually only a properly privileged process can make changes.
To get information on parameters of the system that are built into the system, such as the maximum length of a filename, section System Configuration Parameters.
This section explains how to identify the particular system on which your program is running. First, let's review the various ways computer systems are named, which is a little complicated because of the history of the development of the Internet.
Every Unix system (also known as a host) has a host name, whether it's connected to a network or not. In its simplest form, as used before computer networks were an issue, it's just a word like `chicken'.
But any system attached to the Internet or any network like it conforms to a more rigorous naming convention as part of the Domain Name System (DNS). In DNS, every host name is composed of two parts:
You will note that "hostname" looks a lot like "host name", but is not the same thing, and that people often incorrectly refer to entire host names as "domain names."
In DNS, the full host name is properly called the FQDN (Fully Qualified Domain Name) and consists of the hostname, then a period, then the domain name. The domain name itself usually has multiple components separated by periods. So for example, a system's hostname may be `chicken' and its domain name might be `ai.mit.edu', so its FQDN (which is its host name) is `chicken.ai.mit.edu'.
Adding to the confusion, though, is that DNS is not the only name space in which a computer needs to be known. Another name space is the NIS (aka YP) name space. For NIS purposes, there is another domain name, which is called the NIS domain name or the YP domain name. It need not have anything to do with the DNS domain name.
Confusing things even more is the fact that in DNS, it is possible for multiple FQDNs to refer to the same system. However, there is always exactly one of them that is the true host name, and it is called the canonical FQDN.
In some contexts, the host name is called a "node name."
For more information on DNS host naming, See section Host Names.
Prototypes for these functions appear in `unistd.h'.
The programs hostname, hostid, and domainname work
by calling these functions.
The return value is 0 on success and -1 on failure. In
the GNU C library, gethostname fails if size is not large
enough; then you can try again with a larger array. The following
errno error condition is defined for this function:
ENAMETOOLONG
On some systems, there is a symbol for the maximum possible host name
length: MAXHOSTNAMELEN. It is defined in `sys/param.h'.
But you can't count on this to exist, so it is cleaner to handle
failure and try again.
gethostname stores the beginning of the host name in name
even if the host name won't entirely fit. For some purposes, a
truncated host name is good enough. If it is, you can ignore the
error code.
sethostname function sets the host name of the system that
calls it to name, a string with length length. Only
privileged processes are permitted to do this.
Usually sethostname gets called just once, at system boot time.
Often, the program that calls it sets it to the value it finds in the
file /etc/hostname.
Be sure to set the host name to the full host name, not just the DNS hostname (see above).
The return value is 0 on success and -1 on failure.
The following errno error condition is defined for this function:
EPERM
getdomainname returns the NIS (aka YP) domain name of the system
on which it is called. Note that this is not the more popular DNS
domain name. Get that with gethostname.
The specifics of this function are analogous to gethostname, above.
getdomainname sets the NIS (aka YP) domain name of the system
on which it is called. Note that this is not the more popular DNS
domain name. Set that with sethostname.
The specifics of this function are analogous to sethostname, above.
long int. However, on some
systems it is a meaningless but unique number which is hard-coded for
each machine.
This is not widely used. It arose in BSD 4.2, but was dropped in BSD 4.4. It is not required by POSIX.
The proper way to query the IP address is to use gethostbyname
on the results of gethostname. For more information on IP addresses,
See section Host Addresses.
sethostid function sets the "host ID" of the host machine
to id. Only privileged processes are permitted to do this. Usually
it happens just once, at system boot time.
The proper way to establish the primary IP address of a system
is to configure the IP address resolver to associate that IP address with
the system's host name as returned by gethostname. For example,
put a record for the system in `/etc/hosts'.
See gethostid above for more information on host ids.
The return value is 0 on success and -1 on failure.
The following errno error conditions are defined for this function:
EPERM
ENOSYS
You can use the uname function to find out some information about
the type of computer your program is running on. This function and the
associated data type are declared in the header file
`sys/utsname.h'.
As a bonus, uname also gives some information identifying the
particular system your program is running on. This is the same information
which you can get with functions targetted to this purpose described in
section Host Identification.
utsname structure is used to hold information returned
by the uname function. It has the following members:
char sysname[]
char release[]
char version[]
char machine[]
machine is supposed to describe just the
hardware, it consists of the first two parts of the configuration name:
`cpu-manufacturer'. For example, it might be one of these:
"sparc-sun","i386-anything","m68k-hp","m68k-sony","m68k-sun","mips-dec"
char nodename[]
gethostname;
see section Host Identification.
gethostname() is implemented with a call to uname().
char domainname[]
getdomainname; see section Host Identification. This element
is a relatively recent invention and use of it is not as portable as
use of the rest of the structure.
uname function fills in the structure pointed to by
info with information about the operating system and host machine.
A non-negative value indicates that the data was successfully stored.
-1 as the value indicates an error. The only error possible is
EFAULT, which we normally don't mention as it is always a
possibility.
All files are in filesystems, and before you can access any file, its filesystem must be mounted. Because of Unix's concept of Everything is a file, mounting of filesystems is central to doing almost anything. This section explains how to find out what filesystems are currently mounted and what filesystems are available for mounting, and how to change what is mounted.
The classic filesystem is the contents of a disk drive. The concept is considerably more abstract, though, and lots of things other than disk drives can be mounted.
Some block devices don't correspond to traditional devices like disk drives. For example, a loop device is a block device whose driver uses a regular file in another filesystem as its medium. So if that regular file contains appropriate data for a filesystem, you can by mounting the loop device essentially mount a regular file.
Some filesystems aren't based on a device of any kind. The "proc" filesystem, for example, contains files whose data is made up by the filesystem driver on the fly whenever you ask for it. And when you write to it, the data you write causes changes in the system. No data gets stored.
For some programs it is desirable and necessary to access information about whether a certain filesystem is mounted and, if it is, where, or simply to get lists of all the available filesystems. The GNU libc provides some functions to retrieve this information portably.
Traditionally Unix systems have a file named `/etc/fstab' which
describes all possibly mounted filesystems. The mount program
uses this file to mount at startup time of the system all the necessary
filesystems. The information about all the filesystems actually mounted
is normally kept in a file named `/etc/mtab'. Both files share
the same syntax and it is crucial that this syntax is followed all the
time. Therefore it is best to never directly write the files. The
functions described in this section can do this and they also provide
the functionality to convert the external textual representation to the
internal representation.
Note that the `fstab' and `mtab' files are maintained on a system by convention. It is possible for the files not to exist or not to be consistent with what is really mounted or available to mount, if the system's administration policy allows it. But programs that mount and unmount filesystems typically maintain and use these files as described herein.
The filenames given above should never be used directly. The portable
way to handle these file is to use the macros _PATH_FSTAB,
defined in `fstab.h' and _PATH_MNTTAB, defined in
`mntent.h', respectively. There are also two alternate macro names
FSTAB and _PATH_MOUNTED defined but both names are
deprecated and kept only for backward compatibility. The two former
names should always be used.
The internal representation for entries of the file is struct
fstab, defined in `fstab.h'.
getfsent, getfsspec, and
getfsfile functions.
char *fs_spec
const it shouldn't be
modified. The missing const has historic reasons, since this
function predates ISO C. The same is true for the other string
elements of this structure.
char *fs_file
char *fs_vfstype
char *fs_mntops
mount call. Again, this can be almost anything. There can be
more than one option, separated from the others by a comma. Each option
consists of a name and an optional value part, introduced by an =
character.
If the value of this element must be processed it should ideally be done
using the getsubopt function; see section Parsing of Suboptions.
const char *fs_type
fs_mntops string) which describes the modes with which the
filesystem is mounted. `fstab' defines five macros to describe the
possible values:
FSTAB_RW
FSTAB_RQ
FSTAB_RO
FSTAB_SW
FSTAB_XX
strcmp
since these are all strings. Comparing the pointer will probably always
fail.
int fs_freq
int fs_passno
dump utility used on Unix systems.
To read the entire content of the of the `fstab' file the GNU libc contains a set of three functions which are designed in the usual way.
Since the file handle is internal to the libc this function is not thread-safe.
This function returns a non-zero value if the operation was successful
and the getfs* functions can be used to read the entries of the
file.
setfsent (explicitly or implicitly by calling getfsent) are
freed.
endfsent, the file will be
opened.
The function returns a pointer to a variable of type struct
fstab. This variable is shared by all threads and therefore this
function is not thread-safe. If an error occurred getfsent
returns a NULL pointer.
fs_spec element.
Since there is normally exactly one entry for each special device it
makes no sense to call this function more than once for the same
argument. If this is the first call to any of the functions handling
`fstab' since program start or the last call of endfsent,
the file will be opened.
The function returns a pointer to a variable of type struct
fstab. This variable is shared by all threads and therefore this
function is not thread-safe. If an error occurred getfsent
returns a NULL pointer.
fs_file element.
Since there is normally exactly one entry for each mount point it
makes no sense to call this function more than once for the same
argument. If this is the first call to any of the functions handling
`fstab' since program start or the last call of endfsent,
the file will be opened.
The function returns a pointer to a variable of type struct
fstab. This variable is shared by all threads and therefore this
function is not thread-safe. If an error occurred getfsent
returns a NULL pointer.
The following functions and data structure access the `mtab' file.
getmntent, getmntent_t,
addmntent, and hasmntopt functions.
char *mnt_fsname
fs_spec element in struct fstab.
char *mnt_dir
fs_file element in
struct fstab.
char *mnt_type
mnt_type describes the filesystem type and is therefore
equivalent to fs_vfstype in struct fstab. `mntent.h'
defines a few symbolic names for some of the values this string can have.
But since the kernel can support arbitrary filesystems it does not
make much sense to give them symbolic names. If one knows the symbol
name one also knows the filesystem name. Nevertheless here follows the
list of the symbols provided in `mntent.h'.
MNTTYPE_IGNORE
"ignore". The value is sometime used in
`fstab' files to make sure entries are not used without removing them.
MNTTYPE_NFS
"nfs". Using this macro sometimes could make sense
since it names the default NFS implementation, in case both version 2
and 3 are supported.
MNTTYPE_SWAP
"swap". It names the special `fstab'
entry which names one of the possibly multiple swap partitions.
char *mnt_opts
fs_mntops of
struct fstab it is best to use the function getsubopt
(see section Parsing of Suboptions) to access the parts of this string.
The `mntent.h' file defines a number of macros with string values
which correspond to some of the options understood by the kernel. There
might be many more options which are possible so it doesn't make much sense
to rely on these macros but to be consistent here is the list:
MNTOPT_DEFAULTS
"defaults". This option should be used alone since it
indicates all values for the customizable values are chosen to be the
default.
MNTOPT_RO
"ro". See the FSTAB_RO value, it means the
filesystem is mounted read-only.
MNTOPT_RW
"rw". See the FSTAB_RW value, it means the
filesystem is mounted with read and write permissions.
MNTOPT_SUID
"suid". This means that the SUID bit (see section How an Application Can Change Persona) is respected when a program from the filesystem is
started.
MNTOPT_NOSUID
"nosuid". This is the opposite of MNTOPT_SUID,
the SUID bit for all files from the filesystem is ignored.
MNTOPT_NOAUTO
"noauto". At startup time the mount program
will ignore this entry if it is started with the -a option to
mount all filesystems mentioned in the `fstab' file.
FSTAB_* entries introduced above it is important to
use strcmp to check for equality.
mnt_freq
fs_freq and also specifies the
frequency in days in which dumps are made.
mnt_passno
fs_passno with the same meaning
which is uninteresting for all programs beside dump.
For accessing the `mtab' file there is again a set of three functions to access all entries in a row. Unlike the functions to handle `fstab' these functions do not access a fixed file and there is even a thread safe variant of the get function. Beside this the GNU libc contains functions to alter the file and test for specific options.
setmntent function prepares the file named FILE which
must be in the format of a `fstab' and `mtab' file for the
upcoming processing through the other functions of the family. The
mode parameter can be chosen in the way the opentype
parameter for fopen (see section Opening Streams) can be chosen. If
the file is opened for writing the file is also allowed to be empty.
If the file was successfully opened setmntent returns a file
descriptor for future use. Otherwise the return value is NULL
and errno is set accordingly.
setmntent call.
endmntent closes the stream and frees all resources.
The return value is @math{1} unless an error occurred in which case it is @math{0}.
getmntent function takes as the parameter a file handle
previously returned by successful call to setmntent. It returns
a pointer to a static variable of type struct mntent which is
filled with the information from the next entry from the file currently
read.
The file format used prescribes the use of spaces or tab characters to
separate the fields. This makes it harder to use name containing one of
these characters (e.g., mount points using spaces). Therefore these
characters are encoded in the files and the getmntent function
takes care of the decoding while reading the entries back in.
'\040' is used to encode a space character, '\012' to
encode a tab character and '\\' to encode a backslash.
If there was an error or the end of the file is reached the return value
is NULL.
This function is not thread-safe since all calls to this function return
a pointer to the same static variable. getmntent_r should be
used in situations where multiple threads access the file.
getmntent_r function is the reentrant variant of
getmntent. It also returns the next entry from the file and
returns a pointer. The actual variable the values are stored in is not
static, though. Instead the function stores the values in the variable
pointed to by the result parameter. Additional information (e.g.,
the strings pointed to by the elements of the result) are kept in the
buffer of size bufsize pointed to by buffer.
Escaped characters (space, tab, backslash) are converted back in the
same way as it happens for getmentent.
The function returns a NULL pointer in error cases. Errors could be:
addmntent function allows adding a new entry to the file
previously opened with setmntent. The new entries are always
appended. I.e., even if the position of the file descriptor is not at
the end of the file this function does not overwrite an existing entry
following the current position.
The implication of this is that to remove an entry from a file one has to create a new file while leaving out the entry to be removed and after closing the file remove the old one and rename the new file to the chosen name.
This function takes care of spaces and tab characters in the names to be
written to the file. It converts them and the backslash character into
the format describe in the getmntent description above.
This function returns @math{0} in case the operation was successful.
Otherwise the return value is @math{1} and errno is set
appropriately.
mnt_opts element of the variable pointed to by mnt contains
the option opt. If this is true a pointer to the beginning of the
option in the mnt_opts element is returned. If no such option
exists the function returns NULL.
This function is useful to test whether a specific option is present but
when all options have to be processed one is better off with using the
getsubopt function to iterate over all options in the string.
On a system with a Linux kernel and the proc filesystem, you can
get information on currently mounted filesystems from the file
`mounts' in the proc filesystem. Its format is similar to
that of the `mtab' file, but represents what is truly mounted
without relying on facilities outside the kernel to keep `mtab' up
to date.
This section describes the functions for mounting, unmounting, and remounting filesystems.
Only the superuser can mount, unmount, or remount a filesystem.
These functions do not access the `fstab' and `mtab' files. You should maintain and use these separately. See section Mount Information.
The symbols in this section are declared in `sys/mount.h'.
mount mounts or remounts a filesystem. The two operations are
quite different and are merged rather unnnaturally into this one function.
The MS_REMOUNT option, explained below, determines whether
mount mounts or remounts.
For a mount, the filesystem on the block device represented by the device special file named special_file gets mounted over the mount point dir. This means that the directory dir (along with any files in it) is no longer visible; in its place (and still with the name dir) is the root directory of the filesystem on the device.
As an exception, if the filesystem type (see below) is one which is not
based on a device (e.g. "proc"), mount instantiates a
filesystem and mounts it over dir and ignores special_file.
For a remount, dir specifies the mount point where the filesystem to be remounted is (and remains) mounted and special_file is ignored. Remounting a filesystem means changing the options that control operations on the filesystem while it is mounted. It does not mean unmounting and mounting again.
For a mount, you must identify the type of the filesystem as
fstype. This type tells the kernel how to access the filesystem
and can be thought of as the name of a filesystem driver. The
acceptable values are system dependent. On a system with a Linux kernel
and the proc filesystem, the list of possible values is in the
file `filesystems' in the proc filesystem (e.g. type
cat /proc/filesystems to see the list). With a Linux kernel, the
types of filesystems that mount can mount, and their type names,
depends on what filesystem drivers are configured into the kernel or
loaded as loadable kernel modules. An example of a common value for
fstype is ext2.
For a remount, mount ignores fstype.
options specifies a variety of options that apply until the
filesystem is unmounted or remounted. The precise meaning of an option
depends on the filesystem and with some filesystems, an option may have
no effect at all. Furthermore, for some filesystems, some of these
options (but never MS_RDONLY) can be overridden for individual
file accesses via ioctl.
options is a bit string with bit fields defined using the following mask and masked value macros:
MS_MGC_MASK
MS_MGC_VAL, mount assumes all the following bits are zero and
the data argument is a null string, regardless of their actual values.
MS_REMOUNT
MS_RDONLY
ioctl. This
option is available on nearly all filesystems.
S_IMMUTABLE
ioctl.
This option is a relatively new invention and is not available on many
filesystems.
S_APPEND
ioctl. This is a relatively new invention and is not
available on many filesystems.
MS_NOSUID
MS_NOEXEC
MS_NODEV
MS_SYNCHRONOUS
MS_MANDLOCK
MS_NOATIME
MS_NODIRATIME
Any bits not covered by the above masks should be set off; otherwise, results are undefined.
The meaning of data depends on the filesystem type and is controlled entirely by the filesystem driver in the kernel.
Example:
#include <sys/mount.h>
mount("/dev/hdb", "/cdrom", MS_MGC_VAL | MS_RDONLY | MS_NOSUID, "");
mount("/dev/hda2", "/mnt", MS_MGC_VAL | MS_REMOUNT, "");
Appropriate arguments for mount are conventionally recorded in
the `fstab' table. See section Mount Information.
The return value is zero if the mount or remount is successful. Otherwise,
it is -1 and errno is set appropriately. The values of
errno are filesystem dependent, but here is a general list:
EPERM
ENODEV
ENOTBLK
EBUSY
EINVAL
EACCESS
MS_RDONLY bit off).
MS_NODEV option.
EM_FILE
mount needs to create a
dummy device (aka "unnamed" device) if the filesystem being mounted is
not one that uses a device.
umount2 unmounts a filesystem.
You can identify the filesystem to unmount either by the device special file that contains the filesystem or by the mount point. The effect is the same. Specify either as the string file.
flags contains the one-bit field identified by the following mask macro:
MNT_FORCE
umount2 fails with errno = EBUSY. Depending
on the filesystem, this may override all, some, or no busy conditions.
All other bits in flags should be set to zero; otherwise, the result is undefined.
Example:
#include <sys/mount.h>
umount2("/mnt", MNT_FORCE);
umount2("/dev/hdd1", 0);
After the filesystem is unmounted, the directory that was the mount point is visible, as are any files in it.
As part of unmounting, umount2 syncs the filesystem.
If the unmounting is successful, the return value is zero. Otherwise, it
is -1 and errno is set accordingly:
EPERM
EBUSY
MNT_FORCE option.
EINVAL
This function is not available on all systems.
umount does the same thing as umount2 with flags set
to zeroes. It is more widely available than umount2 but since it
lacks the possibility to forcefully unmount a filesystem is deprecated
when umount2 is also available.
This section describes the sysctl function, which gets and sets
a variety of system parameters.
The symbols used in this section are declared in the file `sysctl.h'.
sysctl gets or sets a specified system parameter. There are so
many of these parameters that it is not practical to list them all here,
but here are some examples:
The set of available parameters depends on the kernel configuration and can change while the system is running, particularly when you load and unload loadable kernel modules.
The system parameters with which syslog is concerned are arranged
in a hierarchical structure like a hierarchical filesystem. To identify
a particular parameter, you specify a path through the structure in a
way analogous to specifying the pathname of a file. Each component of
the path is specified by an integer and each of these integers has a
macro defined for it by `sysctl.h'. names is the path, in
the form of an array of integers. Each component of the path is one
element of the array, in order. nlen is the number of components
in the path.
For example, the first component of the path for all the paging
parameters is the value CTL_VM. For the free page thresholds, the
second component of the path is VM_FREEPG. So to get the free
page threshold values, make names an array containing the two
elements CTL_VM and VM_FREEPG and make nlen = 2.
The format of the value of a parameter depends on the parameter. Sometimes it is an integer; sometimes it is an ASCII string; sometimes it is an elaborate structure. In the case of the free page thresholds used in the example above, the parameter value is a structure containing several integers.
In any case, you identify a place to return the parameter's value with oldval and specify the amount of storage available at that location as *oldlenp. *oldlenp does double duty because it is also the output location that contains the actual length of the returned value.
If you don't want the parameter value returned, specify a null pointer for oldval.
To set the parameter, specify the address and length of the new value as newval and newlen. If you don't want to set the parameter, specify a null pointer as newval.
If you get and set a parameter in the same sysctl call, the value
returned is the value of the parameter before it was set.
Each system parameter has a set of permissions similar to the permissions for a file (including the permissions on directories in its path) that determine whether you may get or set it. For the purposes of these permissions, every parameter is considered to be owned by the superuser and Group 0 so processes with that effective uid or gid may have more access to system parameters. Unlike with files, the superuser does not invariably have full permission to all system parameters, because some of them are designed not to be changed ever.
sysctl returns a zero return value if it succeeds. Otherwise, it
returns -1 and sets errno appropriately. Besides the
failures that apply to all system calls, the following are the
errno codes for all possible failures:
EPERM
ENOTDIR
EFAULT
EINVAL
ENOMEM
EINVAL in some
cases where the space provided for the return of the system parameter is too
small.
If you have a Linux kernel with the proc filesystem, you can get
and set most of the same parameters by reading and writing to files in
the sys directory of the proc filesystem. In the sys
directory, the directory structure represents the hierarchical structure
of the parameters. E.g. you can display the free page thresholds with
cat /proc/sys/vm/freepages
Some more traditional and more widely available, though less general, GNU C library functions for getting and setting some of the same system parameters are:
getdomainname, setdomainname
gethostname, sethostname (See section Host Identification.)
uname (See section Platform Type Identification.)
bdflush
The functions and macros listed in this chapter give information about configuration parameters of the operating system--for example, capacity limits, presence of optional POSIX features, and the default path for executable files (see section String-Valued Parameters).
The POSIX.1 and POSIX.2 standards specify a number of parameters that describe capacity limitations of the system. These limits can be fixed constants for a given operating system, or they can vary from machine to machine. For example, some limit values may be configurable by the system administrator, either at run time or by rebuilding the kernel, and this should not require recompiling application programs.
Each of the following limit parameters has a macro that is defined in
`limits.h' only if the system has a fixed, uniform limit for the
parameter in question. If the system allows different file systems or
files to have different limits, then the macro is undefined; use
sysconf to find out the limit that applies at a particular time
on a particular machine. See section Using sysconf.
Each of these parameters also has another macro, with a name starting with `_POSIX', which gives the lowest value that the limit is allowed to have on any POSIX system. See section Minimum Values for General Capacity Limits.
exec functions.
RLIMIT_NPROC resource limit; see section Limiting Resource Usage.
RLIMIT_NOFILE resource limit; see section Limiting Resource Usage.
These limit macros are always defined in `limits.h'.
The value of this macro is actually a lower bound for the maximum. That
is, you can count on being able to have that many supplementary group
IDs, but a particular machine might let you have even more. You can use
sysconf to see whether a particular machine will let you have
more (see section Using sysconf).
ssize_t.
Effectively, this is the limit on the number of bytes that can be read
or written in a single operation.
This macro is defined in all POSIX systems because this limit is never configurable.
The value of this macro is actually a lower bound for the maximum. That
is, you can count on being able to have that many repetitions, but a
particular machine might let you have even more. You can use
sysconf to see whether a particular machine will let you have
more (see section Using sysconf). And even the value that sysconf tells
you is just a lower bound--larger values might work.
This macro is defined in all POSIX.2 systems, because POSIX.2 says it should always be defined even if there is no specific imposed limit.
POSIX defines certain system-specific options that not all POSIX systems support. Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee any of these features is supported; it depends on the system you are using.
You can test for the availability of a given option using the macros in
this section, together with the function sysconf. The macros are
defined only if you include `unistd.h'.
For the following macros, if the macro is defined in `unistd.h',
then the option is supported. Otherwise, the option may or may not be
supported; use sysconf to find out. See section Using sysconf.
For the following macros, if the macro is defined in `unistd.h',
then its value indicates whether the option is supported. A value of
-1 means no, and any other value means yes. If the macro is not
defined, then the option may or may not be supported; use sysconf
to find out. See section Using sysconf.
c89. The GNU C library always defines this
as 1, on the assumption that you would not have installed it if
you didn't have a C compiler.
fort77. The GNU C library never
defines this, because we don't know what the system has.
asa command to interpret Fortran carriage control. The GNU C
library never defines this, because we don't know what the system has.
localedef command. The GNU C library never defines this, because
we don't know what the system has.
ar, make, and strip. The GNU C library
always defines this as 1, on the assumption that you had to have
ar and make to install the library, and it's unlikely that
strip would be absent when those are present.
199506L.
_POSIX_VERSION is always defined (in `unistd.h') in any
POSIX system.
Usage Note: Don't try to test whether the system supports POSIX
by including `unistd.h' and then checking whether
_POSIX_VERSION is defined. On a non-POSIX system, this will
probably fail because there is no `unistd.h'. We do not know of
any way you can reliably test at compilation time whether your
target system supports POSIX or whether `unistd.h' exists.
The GNU C compiler predefines the symbol __POSIX__ if the target
system is a POSIX system. Provided you do not use any other compilers
on POSIX systems, testing defined (__POSIX__) will reliably
detect such systems.
The value of this symbol says nothing about the utilities installed on the system.
Usage Note: You can use this macro to tell whether a POSIX.1
system library supports POSIX.2 as well. Any POSIX.1 system contains
`unistd.h', so include that file and then test defined
(_POSIX2_C_VERSION).
sysconf
When your system has configurable system limits, you can use the
sysconf function to find out the value that applies to any
particular machine. The function and the associated parameter
constants are declared in the header file `unistd.h'.
sysconf
The normal return value from sysconf is the value you requested.
A value of -1 is returned both if the implementation does not
impose a limit, and in case of an error.
The following errno error conditions are defined for this function:
EINVAL
sysconf Parameters
Here are the symbolic constants for use as the parameter argument
to sysconf. The values are all integer constants (more
specifically, enumeration type values).
_SC_ARG_MAX
ARG_MAX.
_SC_CHILD_MAX
CHILD_MAX.
_SC_OPEN_MAX
OPEN_MAX.
_SC_STREAM_MAX
STREAM_MAX.
_SC_TZNAME_MAX
TZNAME_MAX.
_SC_NGROUPS_MAX
NGROUPS_MAX.
_SC_JOB_CONTROL
_POSIX_JOB_CONTROL.
_SC_SAVED_IDS
_POSIX_SAVED_IDS.
_SC_VERSION
_POSIX_VERSION.
_SC_CLK_TCK
CLOCKS_PER_SEC;
see section CPU Time Inquiry.
_SC_CHARCLASS_NAME_MAX
_SC_REALTIME_SIGNALS
_POSIX_REALTIME_SIGNALS.
_SC_PRIORITY_SCHEDULING
_POSIX_PRIORITY_SCHEDULING.
_SC_TIMERS
_POSIX_TIMERS.
_SC_ASYNCHRONOUS_IO
_POSIX_ASYNCHRONOUS_IO.
_SC_PRIORITIZED_IO
_POSIX_PRIORITIZED_IO.
_SC_SYNCHRONIZED_IO
_POSIX_SYNCHRONIZED_IO.
_SC_FSYNC
_POSIX_FSYNC.
_SC_MAPPED_FILES
_POSIX_MAPPED_FILES.
_SC_MEMLOCK
_POSIX_MEMLOCK.
_SC_MEMLOCK_RANGE
_POSIX_MEMLOCK_RANGE.
_SC_MEMORY_PROTECTION
_POSIX_MEMORY_PROTECTION.
_SC_MESSAGE_PASSING
_POSIX_MESSAGE_PASSING.
_SC_SEMAPHORES
_POSIX_SEMAPHORES.
_SC_SHARED_MEMORY_OBJECTS
_POSIX_SHARED_MEMORY_OBJECTS.
_SC_AIO_LISTIO_MAX
_POSIX_AIO_LISTIO_MAX.
_SC_AIO_MAX
_POSIX_AIO_MAX.
_SC_AIO_PRIO_DELTA_MAX
AIO_PRIO_DELTA_MAX.
_SC_DELAYTIMER_MAX
_POSIX_DELAYTIMER_MAX.
_SC_MQ_OPEN_MAX
_POSIX_MQ_OPEN_MAX.
_SC_MQ_PRIO_MAX
_POSIX_MQ_PRIO_MAX.
_SC_RTSIG_MAX
_POSIX_RTSIG_MAX.
_SC_SEM_NSEMS_MAX
_POSIX_SEM_NSEMS_MAX.
_SC_SEM_VALUE_MAX
_POSIX_SEM_VALUE_MAX.
_SC_SIGQUEUE_MAX
_POSIX_SIGQUEUE_MAX.
_SC_TIMER_MAX
_POSIX_TIMER_MAX.
_SC_PII
_POSIX_PII.
_SC_PII_XTI
_POSIX_PII_XTI.
_SC_PII_SOCKET
_POSIX_PII_SOCKET.
_SC_PII_INTERNET
_POSIX_PII_INTERNET.
_SC_PII_OSI
_POSIX_PII_OSI.
_SC_SELECT
_POSIX_SELECT.
_SC_UIO_MAXIOV
_POSIX_UIO_MAXIOV.
_SC_PII_INTERNET_STREAM
_POSIX_PII_INTERNET_STREAM.
_SC_PII_INTERNET_DGRAM
_POSIX_PII_INTERNET_DGRAM.
_SC_PII_OSI_COTS
_POSIX_PII_OSI_COTS.
_SC_PII_OSI_CLTS
_POSIX_PII_OSI_CLTS.
_SC_PII_OSI_M
_POSIX_PII_OSI_M.
_SC_T_IOV_MAX
T_IOV_MAX
variable.
_SC_THREADS
_POSIX_THREADS.
_SC_THREAD_SAFE_FUNCTIONS
_POSIX_THREAD_SAFE_FUNCTIONS.
_SC_GETGR_R_SIZE_MAX
_POSIX_GETGR_R_SIZE_MAX.
_SC_GETPW_R_SIZE_MAX
_POSIX_GETPW_R_SIZE_MAX.
_SC_LOGIN_NAME_MAX
_POSIX_LOGIN_NAME_MAX.
_SC_TTY_NAME_MAX
_POSIX_TTY_NAME_MAX.
_SC_THREAD_DESTRUCTOR_ITERATIONS
_POSIX_THREAD_DESTRUCTOR_ITERATIONS.
_SC_THREAD_KEYS_MAX
_POSIX_THREAD_KEYS_MAX.
_SC_THREAD_STACK_MIN
_POSIX_THREAD_STACK_MIN.
_SC_THREAD_THREADS_MAX
_POSIX_THREAD_THREADS_MAX.
_SC_THREAD_ATTR_STACKADDR
_POSIX_THREAD_ATTR_STACKADDR.
_SC_THREAD_ATTR_STACKSIZE
_POSIX_THREAD_ATTR_STACKSIZE.
_SC_THREAD_PRIORITY_SCHEDULING
_POSIX_THREAD_PRIORITY_SCHEDULING.
_SC_THREAD_PRIO_INHERIT
_POSIX_THREAD_PRIO_INHERIT.
_SC_THREAD_PRIO_PROTECT
_POSIX_THREAD_PRIO_PROTECT.
_SC_THREAD_PROCESS_SHARED
_POSIX_THREAD_PROCESS_SHARED.
_SC_2_C_DEV
c89.
_SC_2_FORT_DEV
fort77.
_SC_2_FORT_RUN
asa command to
interpret Fortran carriage control.
_SC_2_LOCALEDEF
localedef
command.
_SC_2_SW_DEV
ar,
make, and strip.
_SC_BC_BASE_MAX
obase in the bc
utility.
_SC_BC_DIM_MAX
bc
utility.
_SC_BC_SCALE_MAX
scale in the bc
utility.
_SC_BC_STRING_MAX
bc utility.
_SC_COLL_WEIGHTS_MAX
_SC_EXPR_NEST_MAX
expr utility.
_SC_LINE_MAX
_SC_EQUIV_CLASS_MAX
LC_COLLATE category `order' keyword in a locale
definition. The GNU C library does not presently support locale
definitions.
_SC_VERSION
_SC_2_VERSION
_SC_PAGESIZE
getpagesize returns the same value (see section How to get information about the memory subsystem?).
_SC_NPROCESSORS_CONF
_SC_NPROCESSORS_ONLN
_SC_PHYS_PAGES
_SC_AVPHYS_PAGES
_SC_ATEXIT_MAX
atexit; see section Cleanups on Exit.
_SC_XOPEN_VERSION
_XOPEN_VERSION.
_SC_XOPEN_XCU_VERSION
_XOPEN_XCU_VERSION.
_SC_XOPEN_UNIX
_XOPEN_UNIX.
_SC_XOPEN_REALTIME
_XOPEN_REALTIME.
_SC_XOPEN_REALTIME_THREADS
_XOPEN_REALTIME_THREADS.
_SC_XOPEN_LEGACY
_XOPEN_LEGACY.
_SC_XOPEN_CRYPT
_XOPEN_CRYPT.
_SC_XOPEN_ENH_I18N
_XOPEN_ENH_I18N.
_SC_XOPEN_SHM
_XOPEN_SHM.
_SC_XOPEN_XPG2
_XOPEN_XPG2.
_SC_XOPEN_XPG3
_XOPEN_XPG3.
_SC_XOPEN_XPG4
_XOPEN_XPG4.
_SC_CHAR_BIT
char.
_SC_CHAR_MAX
char.
_SC_CHAR_MIN
char.
_SC_INT_MAX
int.
_SC_INT_MIN
int.
_SC_LONG_BIT
long int.
_SC_WORD_BIT
_SC_MB_LEN_MAX
_SC_NZERO
SC_SSIZE_MAX
ssize_t.
_SC_SCHAR_MAX
signed char.
_SC_SCHAR_MIN
signed char.
_SC_SHRT_MAX
short int.
_SC_SHRT_MIN
short int.
_SC_UCHAR_MAX
unsigned char.
_SC_UINT_MAX
unsigned int.
_SC_ULONG_MAX
unsigned long int.
_SC_USHRT_MAX
unsigned short int.
_SC_NL_ARGMAX
NL_ARGMAX.
_SC_NL_LANGMAX
NL_LANGMAX.
_SC_NL_MSGMAX
NL_MSGMAX.
_SC_NL_NMAX
NL_NMAX.
_SC_NL_SETMAX
NL_SETMAX.
_SC_NL_TEXTMAX
NL_TEXTMAX.
sysconf
We recommend that you first test for a macro definition for the
parameter you are interested in, and call sysconf only if the
macro is not defined. For example, here is how to test whether job
control is supported:
int
have_job_control (void)
{
#ifdef _POSIX_JOB_CONTROL
return 1;
#else
int value = sysconf (_SC_JOB_CONTROL);
if (value < 0)
/* If the system is that badly wedged,
there's no use trying to go on. */
fatal (strerror (errno));
return value;
#endif
}
Here is how to get the value of a numeric limit:
int
get_child_max ()
{
#ifdef CHILD_MAX
return CHILD_MAX;
#else
int value = sysconf (_SC_CHILD_MAX);
if (value < 0)
fatal (strerror (errno));
return value;
#endif
}
Here are the names for the POSIX minimum upper bounds for the system limit parameters. The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far.
_POSIX_AIO_LISTIO_MAX
2; thus you can add up to two new entries
of the list of outstanding operations.
_POSIX_AIO_MAX
1. So you cannot expect that you can issue more than one
operation and immediately continue with the normal work, receiving the
notifications asynchronously.
_POSIX_ARG_MAX
exec functions.
Its value is 4096.
_POSIX_CHILD_MAX
6.
_POSIX_NGROUPS_MAX
0.
_POSIX_OPEN_MAX
16.
_POSIX_SSIZE_MAX
ssize_t. Its value is 32767.
_POSIX_STREAM_MAX
8.
_POSIX_TZNAME_MAX
3.
_POSIX2_RE_DUP_MAX
255.
The POSIX.1 standard specifies a number of parameters that describe the limitations of the file system. It's possible for the system to have a fixed, uniform limit for a parameter, but this isn't the usual case. On most systems, it's possible for different file systems (and, for some parameters, even different files) to have different maximum limits. For example, this is very likely if you use NFS to mount some of the file systems from other machines.
Each of the following macros is defined in `limits.h' only if the
system has a fixed, uniform limit for the parameter in question. If the
system allows different file systems or files to have different limits,
then the macro is undefined; use pathconf or fpathconf to
find out the limit that applies to a particular file. See section Using pathconf.
Each parameter also has another macro, with a name starting with `_POSIX', which gives the lowest value that the limit is allowed to have on any POSIX system. See section Minimum Values for File System Limits.
open).
These are alternative macro names for some of the same information.
Unlike PATH_MAX, this macro is defined even if there is no actual
limit imposed. In such a case, its value is typically a very large
number. This is always the case on the GNU system.
Usage Note: Don't use FILENAME_MAX as the size of an
array in which to store a file name! You can't possibly make an array
that big! Use dynamic allocation (see section Allocating Storage For Program Data) instead.
POSIX defines certain system-specific options in the system calls for operating on files. Some systems support these options and others do not. Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee that any of these features is supported; it depends on the system you are using. They can also vary between file systems on a single machine.
This section describes the macros you can test to determine whether a
particular option is supported on your machine. If a given macro is
defined in `unistd.h', then its value says whether the
corresponding feature is supported. (A value of -1 indicates no;
any other value indicates yes.) If the macro is undefined, it means
particular files may or may not support the feature.
Since all the machines that support the GNU C library also support NFS,
one can never make a general statement about whether all file systems
support the _POSIX_CHOWN_RESTRICTED and _POSIX_NO_TRUNC
features. So these names are never defined as macros in the GNU C
library.
chown function is restricted so
that the only changes permitted to nonprivileged processes is to change
the group owner of a file to either be the effective group ID of the
process, or one of its supplementary group IDs. See section File Owner.
NAME_MAX generate an ENAMETOOLONG error. Otherwise, file
name components that are too long are silently truncated.
If one of these macros is undefined, that means that the option might be
in effect for some files and not for others. To inquire about a
particular file, call pathconf or fpathconf.
See section Using pathconf.
Here are the names for the POSIX minimum upper bounds for some of the above parameters. The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far. In most cases GNU systems do not have these strict limitations. The actual limit should be requested if necessary.
_POSIX_LINK_MAX
8; thus, you
can always make up to eight names for a file without running into a
system limit.
_POSIX_MAX_CANON
255.
_POSIX_MAX_INPUT
255.
_POSIX_NAME_MAX
14.
_POSIX_PATH_MAX
256.
_POSIX_PIPE_BUF
512.
SYMLINK_MAX
POSIX_REC_INCR_XFER_SIZE
POSIX_REC_MIN_XFER_SIZE and POSIX_REC_MAX_XFER_SIZE
values.
POSIX_REC_MAX_XFER_SIZE
POSIX_REC_MIN_XFER_SIZE
POSIX_REC_XFER_ALIGN
pathconfWhen your machine allows different files to have different values for a file system parameter, you can use the functions in this section to find out the value that applies to any particular file.
These functions and the associated constants for the parameter argument are declared in the header file `unistd.h'.
The parameter argument should be one of the `_PC_' constants listed below.
The normal return value from pathconf is the value you requested.
A value of -1 is returned both if the implementation does not
impose a limit, and in case of an error. In the former case,
errno is not set, while in the latter case, errno is set
to indicate the cause of the problem. So the only way to use this
function robustly is to store 0 into errno just before
calling it.
Besides the usual file name errors (see section File Name Errors), the following error condition is defined for this function:
EINVAL
pathconf except that an open file descriptor
is used to specify the file for which information is requested, instead
of a file name.
The following errno error conditions are defined for this function:
EBADF
EINVAL
Here are the symbolic constants that you can use as the parameter
argument to pathconf and fpathconf. The values are all
integer constants.
_PC_LINK_MAX
LINK_MAX.
_PC_MAX_CANON
MAX_CANON.
_PC_MAX_INPUT
MAX_INPUT.
_PC_NAME_MAX
NAME_MAX.
_PC_PATH_MAX
PATH_MAX.
_PC_PIPE_BUF
PIPE_BUF.
_PC_CHOWN_RESTRICTED
_POSIX_CHOWN_RESTRICTED.
_PC_NO_TRUNC
_POSIX_NO_TRUNC.
_PC_VDISABLE
_POSIX_VDISABLE.
_PC_SYNC_IO
_POSIX_SYNC_IO.
_PC_ASYNC_IO
_POSIX_ASYNC_IO.
_PC_PRIO_IO
_POSIX_PRIO_IO.
_PC_SOCK_MAXBUF
_POSIX_PIPE_BUF.
_PC_FILESIZEBITS
_PC_REC_INCR_XFER_SIZE
POSIX_REC_INCR_XFER_SIZE.
_PC_REC_MAX_XFER_SIZE
POSIX_REC_MAX_XFER_SIZE.
_PC_REC_MIN_XFER_SIZE
POSIX_REC_MIN_XFER_SIZE.
_PC_REC_XFER_ALIGN
POSIX_REC_XFER_ALIGN.
The POSIX.2 standard specifies certain system limits that you can access
through sysconf that apply to utility behavior rather than the
behavior of the library or the operating system.
The GNU C library defines macros for these limits, and sysconf
returns values for them if you ask; but these values convey no
meaningful information. They are simply the smallest values that
POSIX.2 permits.
bc utility
is guaranteed to support.
bc utility is guaranteed to support.
expr utility.
LC_COLLATE category `order' keyword in a locale definition.
The GNU C library does not presently support locale definitions.
_POSIX2_BC_BASE_MAX
obase in the bc utility. Its value is 99.
_POSIX2_BC_DIM_MAX
bc utility. Its value is 2048.
_POSIX2_BC_SCALE_MAX
scale in the bc utility. Its value is 99.
_POSIX2_BC_STRING_MAX
bc utility. Its value is 1000.
_POSIX2_COLL_WEIGHTS_MAX
2.
_POSIX2_EXPR_NEST_MAX
expr utility.
Its value is 32.
_POSIX2_LINE_MAX
2048.
_POSIX2_EQUIV_CLASS_MAX
LC_COLLATE
category `order' keyword in a locale definition. Its value is
2. The GNU C library does not presently support locale
definitions.
POSIX.2 defines a way to get string-valued parameters from the operating
system with the function confstr:
The normal return value from confstr is the length of the string
value that you asked for. If you supply a null pointer for buf,
then confstr does not try to store the string; it just returns
its length. A value of 0 indicates an error.
If the string you asked for is too long for the buffer (that is, longer
than len - 1), then confstr stores just that much
(leaving room for the terminating null character). You can tell that
this has happened because confstr returns a value greater than or
equal to len.
The following errno error conditions are defined for this function:
EINVAL
Currently there is just one parameter you can read with confstr:
_CS_PATH
_CS_LFS_CFLAGS
_LARGEFILE_SOURCE feature select macro; see section Feature Test Macros.
_CS_LFS_LDFLAGS
_LARGEFILE_SOURCE feature select macro; see section Feature Test Macros.
_CS_LFS_LIBS
_LARGEFILE_SOURCE feature select macro; see section Feature Test Macros.
_CS_LFS_LINTFLAGS
_LARGEFILE_SOURCE feature select macro; see section Feature Test Macros.
_CS_LFS64_CFLAGS
_LARGEFILE64_SOURCE feature select macro; see section Feature Test Macros.
_CS_LFS64_LDFLAGS
_LARGEFILE64_SOURCE feature select macro; see section Feature Test Macros.
_CS_LFS64_LIBS
_LARGEFILE64_SOURCE feature select macro; see section Feature Test Macros.
_CS_LFS64_LINTFLAGS
_LARGEFILE64_SOURCE feature select macro; see section Feature Test Macros.
The way to use confstr without any arbitrary limit on string size
is to call it twice: first call it to get the length, allocate the
buffer accordingly, and then call confstr again to fill the
buffer, like this:
char *
get_default_path (void)
{
size_t len = confstr (_CS_PATH, NULL, 0);
char *buffer = (char *) xmalloc (len);
if (confstr (_CS_PATH, buf, len + 1) == 0)
{
free (buffer);
return NULL;
}
return buffer;
}
On many systems, it is unnecessary to have any kind of user authentication; for instance, a workstation which is not connected to a network probably does not need any user authentication, because to use the machine an intruder must have physical access.
Sometimes, however, it is necessary to be sure that a user is authorised to use some service a machine provides--for instance, to log in as a particular user id (see section Users and Groups). One traditional way of doing this is for each user to choose a secret password; then, the system can ask someone claiming to be a user what the user's password is, and if the person gives the correct password then the system can grant the appropriate privileges.
If all the passwords are just stored in a file somewhere, then this file has to be very carefully protected. To avoid this, passwords are run through a one-way function, a function which makes it difficult to work out what its input was by looking at its output, before storing in the file.
The GNU C library already provides a one-way function based on MD5 and for compatibility with Unix systems the standard one-way function based on the Data Encryption Standard.
It also provides support for Secure RPC, and some library functions that can be used to perform normal DES encryption.
Because of the continuously changing state of the law, it's not possible to provide a definitive survey of the laws affecting cryptography. Instead, this section warns you of some of the known trouble spots; this may help you when you try to find out what the laws of your country are.
Some countries require that you have a licence to use, posess, or import cryptography. These countries are believed to include Byelorussia, Burma, India, Indonesia, Israel, Kazakhstan, Pakistan, Russia, and Saudi Arabia.
Some countries restrict the transmission of encrypted messages by radio; some telecommunications carriers restrict the transmission of encrypted messages over their network.
Many countries have some form of export control for encryption software. The Wassenaar Arrangement is a multilateral agreement between 33 countries (Argentina, Australia, Austria, Belgium, Bulgaria, Canada, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Japan, Luxembourg, the Netherlands, New Zealand, Norway, Poland, Portugal, the Republic of Korea, Romania, the Russian Federation, the Slovak Republic, Spain, Sweden, Switzerland, Turkey, Ukraine, the United Kingdom and the United States) which restricts some kinds of encryption exports. Different countries apply the arrangement in different ways; some do not allow the exception for certain kinds of "public domain" software (which would include this library), some only restrict the export of software in tangible form, and others impose significant additional restrictions.
The United States has additional rules. This software would generally be exportable under 15 CFR 740.13(e), which permits exports of "encryption source code" which is "publicly available" and which is "not subject to an express agreement for the payment of a licensing fee or royalty for commercial production or sale of any product developed with the source code" to most countries.
The rules in this area are continuously changing. If you know of any
information in this manual that is out-of-date, please report it using
the glibcbug script. See section Reporting Bugs.
When reading in a password, it is desirable to avoid displaying it on the screen, to help keep it secret. The following function handles this in a convenient way.
getpass outputs prompt, then reads a string in from the
terminal without echoing it. It tries to connect to the real terminal,
`/dev/tty', if possible, to encourage users not to put plaintext
passwords in files; otherwise, it uses stdin and stderr.
getpass also disables the INTR, QUIT, and SUSP characters on the
terminal using the ISIG terminal attribute (see section Local Modes).
The terminal is flushed before and after getpass, so that
characters of a mistyped password are not accidentally visible.
In other C libraries, getpass may only return the first
PASS_MAX bytes of a password. The GNU C library has no limit, so
PASS_MAX is undefined.
The prototype for this function is in `unistd.h'. PASS_MAX
would be defined in `limits.h'.
This precise set of operations may not suit all possible situations. In
this case, it is recommended that users write their own getpass
substitute. For instance, a very simple substitute is as follows:
#include <termios.h>
#include <stdio.h>
ssize_t
my_getpass (char **lineptr, size_t *n, FILE *stream)
{
struct termios old, new;
int nread;
/* Turn echoing off and fail if we can't. */
if (tcgetattr (fileno (stream), &old) != 0)
return -1;
new = old;
new.c_lflag &= ~ECHO;
if (tcsetattr (fileno (stream), TCSAFLUSH, &new) != 0)
return -1;
/* Read the password. */
nread = getline (lineptr, n, stream);
/* Restore terminal. */
(void) tcsetattr (fileno (stream), TCSAFLUSH, &old);
return nread;
}
The substitute takes the same parameters as getline
(see section Line-Oriented Input); the user must print any prompt desired.
The crypt function takes a password, key, as a string, and
a salt character array which is described below, and returns a
printable ASCII string which starts with another salt. It is believed
that, given the output of the function, the best way to find a key
that will produce that output is to guess values of key until the
original value of key is found.
The salt parameter does two things. Firstly, it selects which
algorithm is used, the MD5-based one or the DES-based one. Secondly, it
makes life harder for someone trying to guess passwords against a file
containing many passwords; without a salt, an intruder can make a
guess, run crypt on it once, and compare the result with all the
passwords. With a salt, the intruder must run crypt once
for each different salt.
For the MD5-based algorithm, the salt should consist of the string
$1$, followed by up to 8 characters, terminated by either
another $ or the end of the string. The result of crypt
will be the salt, followed by a $ if the salt didn't end
with one, followed by 22 characters from the alphabet
./0-9A-Za-z, up to 34 characters total. Every character in the
key is significant.
For the DES-based algorithm, the salt should consist of two
characters from the alphabet ./0-9A-Za-z, and the result of
crypt will be those two characters followed by 11 more from the
same alphabet, 13 in total. Only the first 8 characters in the
key are significant.
The MD5-based algorithm has no limit on the useful length of the password used, and is slightly more secure. It is therefore preferred over the DES-based algorithm.
When the user enters their password for the first time, the salt
should be set to a new string which is reasonably random. To verify a
password against the result of a previous call to crypt, pass
the result of the previous call as the salt.
The following short program is an example of how to use crypt the
first time a password is entered. Note that the salt generation
is just barely acceptable; in particular, it is not unique between
machines, and in many applications it would not be acceptable to let an
attacker know what time the user's password was last set.
#include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <crypt.h>
int
main(void)
{
unsigned long seed[2];
char salt[] = "$1$........";
const char *const seedchars =
"./0123456789ABCDEFGHIJKLMNOPQRST"
"UVWXYZabcdefghijklmnopqrstuvwxyz";
char *password;
int i;
/* Generate a (not very) random seed.
You should do it better than this... */
seed[0] = time(NULL);
seed[1] = getpid() ^ (seed[0] >> 14 & 0x30000);
/* Turn it into printable characters from `seedchars'. */
for (i = 0; i < 8; i++)
salt[3+i] = seedchars[(seed[i/5] >> (i%5)*6) & 0x3f];
/* Read in the user's password and encrypt it. */
password = crypt(getpass("Password:"), salt);
/* Print the results. */
puts(password);
return 0;
}
The next program shows how to verify a password. It prompts the user
for a password and prints "Access granted." if the user types
GNU libc manual.
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <crypt.h>
int
main(void)
{
/* Hashed form of "GNU libc manual". */
const char *const pass = "$1$/iSaq7rB$EoUw5jJPPvAPECNaaWzMK/";
char *result;
int ok;
/* Read in the user's password and encrypt it,
passing the expected password in as the salt. */
result = crypt(getpass("Password:"), pass);
/* Test the result. */
ok = strcmp (result, pass) == 0;
puts(ok ? "Access granted." : "Access denied.");
return ok ? 0 : 1;
}
The crypt_r function does the same thing as crypt, but
takes an extra parameter which includes space for its result (among
other things), so it can be reentrant. data->initialized must be
cleared to zero before the first time crypt_r is called.
The crypt_r function is a GNU extension.
The crypt and crypt_r functions are prototyped in the
header `crypt.h'.
The Data Encryption Standard is described in the US Government Federal Information Processing Standards (FIPS) 46-3 published by the National Institute of Standards and Technology. The DES has been very thoroughly analysed since it was developed in the late 1970s, and no new significant flaws have been found.
However, the DES uses only a 56-bit key (plus 8 parity bits), and a machine has been built in 1998 which can search through all possible keys in about 6 days, which cost about US$200000; faster searches would be possible with more money. This makes simple DES unsecure for most purposes, and NIST no longer permits new US government systems to use simple DES.
For serious encryption functionality, it is recommended that one of the many free encryption libraries be used instead of these routines.
The DES is a reversible operation which takes a 64-bit block and a 64-bit key, and produces another 64-bit block. Usually the bits are numbered so that the most-significant bit, the first bit, of each block is numbered 1.
Under that numbering, every 8th bit of the key (the 8th, 16th, and so on) is not used by the encryption algorithm itself. But the key must have odd parity; that is, out of bits 1 through 8, and 9 through 16, and so on, there must be an odd number of `1' bits, and this completely specifies the unused bits.
The setkey function sets an internal data structure to be an
expanded form of key. key is specified as an array of 64
bits each stored in a char, the first bit is key[0] and
the 64th bit is key[63]. The key should have the correct
parity.
The encrypt function encrypts block if
edflag is 0, otherwise it decrypts block, using a key
previously set by setkey. The result is
placed in block.
Like setkey, block is specified as an array of 64 bits each
stored in a char, but there are no parity bits in block.
These are reentrant versions of setkey and encrypt. The
only difference is the extra parameter, which stores the expanded
version of key. Before calling setkey_r the first time,
data->initialised must be cleared to zero.
The setkey_r and encrypt_r functions are GNU extensions.
setkey, encrypt, setkey_r, and encrypt_r are
defined in `crypt.h'.
The function ecb_crypt encrypts or decrypts one or more blocks
using DES. Each block is encrypted independently.
The blocks and the key are stored packed in 8-bit bytes, so
that the first bit of the key is the most-significant bit of
key[0] and the 63rd bit of the key is stored as the
least-significant bit of key[7]. The key should have the
correct parity.
len is the number of bytes in blocks. It should be a
multiple of 8 (so that there is a whole number of blocks to encrypt).
len is limited to a maximum of DES_MAXDATA bytes.
The result of the encryption replaces the input in blocks.
The mode parameter is the bitwise OR of two of the following:
DES_ENCRYPT
DES_DECRYPT
DES_HW
DES_SW
The result of the function will be one of these values:
DESERR_NONE
DESERR_NOHWDEVICE
DESERR_HWERROR
DESERR_BADPARAM
DES_MAXDATA.
ecb_crypt or cbc_crypt, and 0 otherwise.
The function cbc_crypt encrypts or decrypts one or more blocks
using DES in Cipher Block Chaining mode.
For encryption in CBC mode, each block is exclusive-ored with ivec before being encrypted, then ivec is replaced with the result of the encryption, then the next block is processed. Decryption is the reverse of this process.
This has the advantage that blocks which are the same before being encrypted are very unlikely to be the same after being encrypted, making it much harder to detect patterns in the data.
Usually, ivec is set to 8 random bytes before encryption starts. Then the 8 random bytes are transmitted along with the encrypted data (without themselves being encrypted), and passed back in as ivec for decryption. Another possibility is to set ivec to 8 zeroes initially, and have the first the block encrypted consist of 8 random bytes.
Otherwise, all the parameters are similar to those for ecb_crypt.
The function des_setparity changes the 64-bit key, stored
packed in 8-bit bytes, to have odd parity by altering the low bits of
each byte.
The ecb_crypt, cbc_crypt, and des_setparity
functions and their accompanying macros are all defined in the header
`rpc/des_crypt.h'.
Applications are usually debugged using dedicated debugger programs. But sometimes this is not possible and, in any case, it is useful to provide the developer with as much information as possible at the time the problems are experienced. For this reason a few functions are provided which a program can use to help the developer more easily locate the problem.
A backtrace is a list of the function calls that are currently active in a thread. The usual way to inspect a backtrace of a program is to use an external debugger such as gdb. However, sometimes it is useful to obtain a backtrace programatically from within a program, e.g., for the purposes of logging or diagnostics.
The header file `execinfo.h' declares three functions that obtain and manipulate backtraces of the current thread.
backtrace function obtains a backtrace for the current
thread, as a list of pointers, and places the information into
buffer. The argument size should be the number of
void * elements that will fit into buffer. The return
value is the actual number of entries of buffer that are obtained,
and is at most size.
The pointers placed in buffer are actually return addresses obtained by inspecting the stack, one return address per stack frame.
Note that certain compiler optimisations may interfere with obtaining a
valid backtrace. Function inlining causes the inlined function to not
have a stack frame; tail call optimisation replaces one stack frame with
another; frame pointer elimination will stop backtrace from
interpreting the stack contents correctly.
backtrace_symbols function translates the information
obtained from the backtrace function into an array of strings.
The argument buffer should be a pointer to an array of addresses
obtained via the backtrace function, and size is the number
of entries in that array (the return value of backtrace).
The return value is a pointer to an array of strings, which has size entries just like the array buffer. Each string contains a printable representation of the corresponding element of buffer. It includes the function name (if this can be determined), an offset into the function, and the actual return address (in hexadecimal).
Currently, the function name and offset only be obtained on systems that
use the ELF binary format for programs and libraries. On other systems,
only the hexadecimal return address will be present. Also, you may need
to pass additional flags to the linker to make the function names
available to the program. (For example, on systems using GNU ld, you
must pass (-rdynamic.)
The return value of backtrace_symbols is a pointer obtained via
the malloc function, and it is the responsibility of the caller
to free that pointer. Note that only the return value need be
freed, not the individual strings.
The return value is NULL if sufficient memory for the strings
cannot be obtained.
backtrace_symbols_fd function performs the same translation
as the function backtrace_symbols function. Instead of returning
the strings to the caller, it writes the strings to the file descriptor
fd, one per line. It does not use the malloc function, and
can therefore be used in situations where that function might fail.
The following program illustrates the use of these functions. Note that
the array to contain the return addresses returned by backtrace
is allocated on the stack. Therefore code like this can be used in
situations where the memory handling via malloc does not work
anymore (in which case the backtrace_symbols has to be replaced
by a backtrace_symbols_fd call as well). The number of return
addresses is normally not very large. Even complicated programs rather
seldom have a nesting level of more than, say, 50 and with 200 possible
entries probably all programs should be covered.
#include <execinfo.h>
#include <stdio.h>
#include <stdlib.h>
/* Obtain a backtrace and print it to stdout. */
void
print_trace (void)
{
void *array[10];
size_t size;
char **strings;
size_t i;
size = backtrace (array, 10);
strings = backtrace_symbols (array, size);
printf ("Obtained %zd stack frames.\n", size);
for (i = 0; i < size; i++)
printf ("%s\n", strings[i]);
free (strings);
}
/* A dummy function to make the backtrace more interesting. */
void
dummy_function (void)
{
print_trace ();
}
int
main (void)
{
dummy_function ();
return 0;
}
Some of the facilities implemented by the C library really should be thought of as parts of the C language itself. These facilities ought to be documented in the C Language Manual, not in the library manual; but since we don't have the language manual yet, and documentation for these features has been written, we are publishing it here.
When you're writing a program, it's often a good idea to put in checks at strategic places for "impossible" errors or violations of basic assumptions. These kinds of checks are helpful in debugging problems with the interfaces between different parts of the program, for example.
The assert macro, defined in the header file `assert.h',
provides a convenient way to abort the program while printing a message
about where in the program the error was detected.
Once you think your program is debugged, you can disable the error
checks performed by the assert macro by recompiling with the
macro NDEBUG defined. This means you don't actually have to
change the program source code to disable these checks.
But disabling these consistency checks is undesirable unless they make the program significantly slower. All else being equal, more error checking is good no matter who is running the program. A wise user would rather have a program crash, visibly, than have it return nonsense without indicating anything might be wrong.
If NDEBUG is not defined, assert tests the value of
expression. If it is false (zero), assert aborts the
program (see section Aborting a Program) after printing a message of the
form:
`file':linenum: function: Assertion `expression' failed.
on the standard error stream stderr (see section Standard Streams).
The filename and line number are taken from the C preprocessor macros
__FILE__ and __LINE__ and specify where the call to
assert was made. When using the GNU C compiler, the name of
the function which calls assert is taken from the built-in
variable __PRETTY_FUNCTION__; with older compilers, the function
name and following colon are omitted.
If the preprocessor macro NDEBUG is defined before
`assert.h' is included, the assert macro is defined to do
absolutely nothing.
Warning: Even the argument expression expression is not
evaluated if NDEBUG is in effect. So never use assert
with arguments that involve side effects. For example, assert
(++i > 0); is a bad idea, because i will not be incremented if
NDEBUG is defined.
Sometimes the "impossible" condition you want to check for is an error
return from an operating system function. Then it is useful to display
not only where the program crashes, but also what error was returned.
The assert_perror macro makes this easy.
assert, but verifies that errnum is zero.
If NDEBUG is defined, assert_perror tests the value of
errnum. If it is nonzero, assert_perror aborts the program
after printing a message of the form:
`file':linenum: function: error text
on the standard error stream. The file name, line number, and function
name are as for assert. The error text is the result of
strerror (errnum). See section Error Messages.
Like assert, if NDEBUG is defined before `assert.h'
is included, the assert_perror macro does absolutely nothing. It
does not evaluate the argument, so errnum should not have any side
effects. It is best for errnum to be just a simple variable
reference; often it will be errno.
This macro is a GNU extension.
Usage note: The assert facility is designed for
detecting internal inconsistency; it is not suitable for
reporting invalid input or improper usage by the user of the
program.
The information in the diagnostic messages printed by the assert
and assert_perror macro is intended to help you, the programmer,
track down the cause of a bug, but is not really useful for telling a user
of your program why his or her input was invalid or why a command could not
be carried out. What's more, your program should not abort when given
invalid input, as assert would do--it should exit with nonzero
status (see section Exit Status) after printing its error messages, or perhaps
read another command or move on to the next input file.
See section Error Messages, for information on printing error messages for problems that do not represent bugs in the program.
ISO C defines a syntax for declaring a function to take a variable number or type of arguments. (Such functions are referred to as varargs functions or variadic functions.) However, the language itself provides no mechanism for such functions to access their non-required arguments; instead, you use the variable arguments macros defined in `stdarg.h'.
This section describes how to declare variadic functions, how to write them, and how to call them properly.
Compatibility Note: Many older C dialects provide a similar, but incompatible, mechanism for defining functions with variable numbers of arguments, using `varargs.h'.
Ordinary C functions take a fixed number of arguments. When you define
a function, you specify the data type for each argument. Every call to
the function should supply the expected number of arguments, with types
that can be converted to the specified ones. Thus, if the function
`foo' is declared with int foo (int, char *); then you must
call it with two arguments, a number (any kind will do) and a string
pointer.
But some functions perform operations that can meaningfully accept an unlimited number of arguments.
In some cases a function can handle any number of values by operating on
all of them as a block. For example, consider a function that allocates
a one-dimensional array with malloc to hold a specified set of
values. This operation makes sense for any number of values, as long as
the length of the array corresponds to that number. Without facilities
for variable arguments, you would have to define a separate function for
each possible array size.
The library function printf (see section Formatted Output) is an
example of another class of function where variable arguments are
useful. This function prints its arguments (which can vary in type as
well as number) under the control of a format template string.
These are good reasons to define a variadic function which can handle as many arguments as the caller chooses to pass.
Some functions such as open take a fixed set of arguments, but
occasionally ignore the last few. Strict adherence to ISO C requires
these functions to be defined as variadic; in practice, however, the GNU
C compiler and most other C compilers let you define such a function to
take a fixed set of arguments--the most it can ever use--and then only
declare the function as variadic (or not declare its arguments
at all!).
Defining and using a variadic function involves three steps:
A function that accepts a variable number of arguments must be declared with a prototype that says so. You write the fixed arguments as usual, and then tack on `...' to indicate the possibility of additional arguments. The syntax of ISO C requires at least one fixed argument before the `...'. For example,
int
func (const char *a, int b, ...)
{
...
}
defines a function func which returns an int and takes two
required arguments, a const char * and an int. These are
followed by any number of anonymous arguments.
Portability note: For some C compilers, the last required
argument must not be declared register in the function
definition. Furthermore, this argument's type must be
self-promoting: that is, the default promotions must not change
its type. This rules out array and function types, as well as
float, char (whether signed or not) and short int
(whether signed or not). This is actually an ISO C requirement.
Ordinary fixed arguments have individual names, and you can use these names to access their values. But optional arguments have no names--nothing but `...'. How can you access them?
The only way to access them is sequentially, in the order they were written, and you must use special macros from `stdarg.h' in the following three step process:
va_list using
va_start. The argument pointer when initialized points to the
first optional argument.
va_arg.
The first call to va_arg gives you the first optional argument,
the next call gives you the second, and so on.
You can stop at any time if you wish to ignore any remaining optional
arguments. It is perfectly all right for a function to access fewer
arguments than were supplied in the call, but you will get garbage
values if you try to access too many arguments.
va_end.
(In practice, with most C compilers, calling va_end does nothing.
This is always true in the GNU C compiler. But you might as well call
va_end just in case your program is someday compiled with a peculiar
compiler.)
See section Argument Access Macros, for the full definitions of va_start,
va_arg and va_end.
Steps 1 and 3 must be performed in the function that accepts the
optional arguments. However, you can pass the va_list variable
as an argument to another function and perform all or part of step 2
there.
You can perform the entire sequence of three steps multiple times within a single function invocation. If you want to ignore the optional arguments, you can do these steps zero times.
You can have more than one argument pointer variable if you like. You
can initialize each variable with va_start when you wish, and
then you can fetch arguments with each argument pointer as you wish.
Each argument pointer variable will sequence through the same set of
argument values, but at its own pace.
Portability note: With some compilers, once you pass an
argument pointer value to a subroutine, you must not keep using the same
argument pointer value after that subroutine returns. For full
portability, you should just pass it to va_end. This is actually
an ISO C requirement, but most ANSI C compilers work happily
regardless.
There is no general way for a function to determine the number and type of the optional arguments it was called with. So whoever designs the function typically designs a convention for the caller to specify the number and type of arguments. It is up to you to define an appropriate calling convention for each variadic function, and write all calls accordingly.
One kind of calling convention is to pass the number of optional arguments as one of the fixed arguments. This convention works provided all of the optional arguments are of the same type.
A similar alternative is to have one of the required arguments be a bit mask, with a bit for each possible purpose for which an optional argument might be supplied. You would test the bits in a predefined sequence; if the bit is set, fetch the value of the next argument, otherwise use a default value.
A required argument can be used as a pattern to specify both the number
and types of the optional arguments. The format string argument to
printf is one example of this (see section Formatted Output Functions).
Another possibility is to pass an "end marker" value as the last
optional argument. For example, for a function that manipulates an
arbitrary number of pointer arguments, a null pointer might indicate the
end of the argument list. (This assumes that a null pointer isn't
otherwise meaningful to the function.) The execl function works
in just this way; see section Executing a File.
You don't have to do anything special to call a variadic function. Just put the arguments (required arguments, followed by optional ones) inside parentheses, separated by commas, as usual. But you must declare the function with a prototype and know how the argument values are converted.
In principle, functions that are defined to be variadic must also be declared to be variadic using a function prototype whenever you call them. (See section Syntax for Variable Arguments, for how.) This is because some C compilers use a different calling convention to pass the same set of argument values to a function depending on whether that function takes variable arguments or fixed arguments.
In practice, the GNU C compiler always passes a given set of argument
types in the same way regardless of whether they are optional or
required. So, as long as the argument types are self-promoting, you can
safely omit declaring them. Usually it is a good idea to declare the
argument types for variadic functions, and indeed for all functions.
But there are a few functions which it is extremely convenient not to
have to declare as variadic--for example, open and
printf.
Since the prototype doesn't specify types for optional arguments, in a
call to a variadic function the default argument promotions are
performed on the optional argument values. This means the objects of
type char or short int (whether signed or not) are
promoted to either int or unsigned int, as
appropriate; and that objects of type float are promoted to type
double. So, if the caller passes a char as an optional
argument, it is promoted to an int, and the function can access
it with va_arg (ap, int).
Conversion of the required arguments is controlled by the function prototype in the usual way: the argument expression is converted to the declared argument type as if it were being assigned to a variable of that type.
Here are descriptions of the macros used to retrieve variable arguments. These macros are defined in the header file `stdarg.h'.
See section Old-Style Variadic Functions, for an alternate definition of va_start
found in the header file `varargs.h'.
va_arg macro returns the value of the next optional argument,
and modifies the value of ap to point to the subsequent argument.
Thus, successive uses of va_arg return successive optional
arguments.
The type of the value returned by va_arg is type as
specified in the call. type must be a self-promoting type (not
char or short int or float) that matches the type
of the actual argument.
va_end call, further
va_arg calls with the same ap may not work. You should invoke
va_end before returning from the function in which va_start
was invoked with the same ap argument.
In the GNU C library, va_end does nothing, and you need not ever
use it except for reasons of portability.
Sometimes it is necessary to parse the list of parameters more than once
or one wants to remember a certain position in the parameter list. To
do this, one will have to make a copy of the current value of the
argument. But va_list is an opaque type and one cannot necessarily
assign the value of one variable of type va_list to another variable
of the same type.
__va_copy macro allows copying of objects of type
va_list even if this is not an integral type. The argument pointer
in dest is initialized to point to the same argument as the
pointer in src.
This macro is a GNU extension but it will hopefully also be available in the next update of the ISO C standard.
If you want to use __va_copy you should always be prepared for the
possibility that this macro will not be available. On architectures where a
simple assignment is invalid, hopefully __va_copy will be available,
so one should always write something like this:
{
va_list ap, save;
...
#ifdef __va_copy
__va_copy (save, ap);
#else
save = ap;
#endif
...
}
Here is a complete sample function that accepts a variable number of arguments. The first argument to the function is the count of remaining arguments, which are added up and the result returned. While trivial, this function is sufficient to illustrate how to use the variable arguments facility.
#include <stdarg.h>
#include <stdio.h>
int
add_em_up (int count,...)
{
va_list ap;
int i, sum;
va_start (ap, count); /* Initialize the argument list. */
sum = 0;
for (i = 0; i < count; i++)
sum += va_arg (ap, int); /* Get the next argument value. */
va_end (ap); /* Clean up. */
return sum;
}
int
main (void)
{
/* This call prints 16. */
printf ("%d\n", add_em_up (3, 5, 5, 6));
/* This call prints 55. */
printf ("%d\n", add_em_up (10, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10));
return 0;
}
Before ISO C, programmers used a slightly different facility for writing variadic functions. The GNU C compiler still supports it; currently, it is more portable than the ISO C facility, since support for ISO C is still not universal. The header file which defines the old-fashioned variadic facility is called `varargs.h'.
Using `varargs.h' is almost the same as using `stdarg.h'. There is no difference in how you call a variadic function; see section Calling Variadic Functions. The only difference is in how you define them. First of all, you must use old-style non-prototype syntax, like this:
tree
build (va_alist)
va_dcl
{
Secondly, you must give va_start only one argument, like this:
va_list p; va_start (p);
These are the special macros used for defining old-style variadic functions:
The other argument macros, va_arg and va_end, are the same
in `varargs.h' as in `stdarg.h'; see section Argument Access Macros, for
details.
It does not work to include both `varargs.h' and `stdarg.h' in
the same compilation; they define va_start in conflicting ways.
The null pointer constant is guaranteed not to point to any real object.
You can assign it to any pointer variable since it has type void
*. The preferred way to write a null pointer constant is with
NULL.
You can also use 0 or (void *)0 as a null pointer
constant, but using NULL is cleaner because it makes the purpose
of the constant more evident.
If you use the null pointer constant as a function argument, then for complete portability you should make sure that the function has a prototype declaration. Otherwise, if the target machine has two different pointer representations, the compiler won't know which representation to use for that argument. You can avoid the problem by explicitly casting the constant to the proper pointer type, but we recommend instead adding a prototype for the function you are calling.
The result of subtracting two pointers in C is always an integer, but the
precise data type varies from C compiler to C compiler. Likewise, the
data type of the result of sizeof also varies between compilers.
ISO defines standard aliases for these two types, so you can refer to
them in a portable fashion. They are defined in the header file
`stddef.h'.
char *p1, *p2;, the
expression p2 - p1 is of type ptrdiff_t. This will
probably be one of the standard signed integer types (short
int, int or long int), but might be a nonstandard
type that exists only for this purpose.
sizeof operator is of this type, and functions
such as malloc (see section Unconstrained Allocation) and
memcpy (see section Copying and Concatenation) accept arguments of
this type to specify object sizes.
Usage Note: size_t is the preferred way to declare any
arguments or variables that hold the size of an object.
In the GNU system size_t is equivalent to either
unsigned int or unsigned long int. These types
have identical properties on the GNU system and, for most purposes, you
can use them interchangeably. However, they are distinct as data types,
which makes a difference in certain contexts.
For example, when you specify the type of a function argument in a
function prototype, it makes a difference which one you use. If the
system header files declare malloc with an argument of type
size_t and you declare malloc with an argument of type
unsigned int, you will get a compilation error if size_t
happens to be unsigned long int on your system. To avoid any
possibility of error, when a function argument or value is supposed to
have type size_t, never declare its type in any other way.
Compatibility Note: Implementations of C before the advent of
ISO C generally used unsigned int for representing object sizes
and int for pointer subtraction results. They did not
necessarily define either size_t or ptrdiff_t. Unix
systems did define size_t, in `sys/types.h', but the
definition was usually a signed type.
Most of the time, if you choose the proper C data type for each object in your program, you need not be concerned with just how it is represented or how many bits it uses. When you do need such information, the C language itself does not provide a way to get it. The header files `limits.h' and `float.h' contain macros which give you this information in full detail.
The most common reason that a program needs to know how many bits are in
an integer type is for using an array of long int as a bit vector.
You can access the bit at index n with
vector[n / LONGBITS] & (1 << (n % LONGBITS))
provided you define LONGBITS as the number of bits in a
long int.
There is no operator in the C language that can give you the number of
bits in an integer data type. But you can compute it from the macro
CHAR_BIT, defined in the header file `limits.h'.
CHAR_BIT
char---eight, on most systems.
The value has type int.
You can compute the number of bits in any data type type like
this:
sizeof (type) * CHAR_BIT
Suppose you need to store an integer value which can range from zero to one million. Which is the smallest type you can use? There is no general rule; it depends on the C compiler and target machine. You can use the `MIN' and `MAX' macros in `limits.h' to determine which type will work.
Each signed integer type has a pair of macros which give the smallest and largest values that it can hold. Each unsigned integer type has one such macro, for the maximum value; the minimum value is, of course, zero.
The values of these macros are all integer constant expressions. The
`MAX' and `MIN' macros for char and short
int types have values of type int. The `MAX' and
`MIN' macros for the other types have values of the same type
described by the macro--thus, ULONG_MAX has type
unsigned long int.
SCHAR_MIN
signed char.
SCHAR_MAX
UCHAR_MAX
signed char and unsigned char, respectively.
CHAR_MIN
char.
It's equal to SCHAR_MIN if char is signed, or zero
otherwise.
CHAR_MAX
char.
It's equal to SCHAR_MAX if char is signed, or
UCHAR_MAX otherwise.
SHRT_MIN
signed
short int. On most machines that the GNU C library runs on,
short integers are 16-bit quantities.
SHRT_MAX
USHRT_MAX
signed short int and unsigned short int,
respectively.
INT_MIN
signed
int. On most machines that the GNU C system runs on, an int is
a 32-bit quantity.
INT_MAX
UINT_MAX
signed int and the type unsigned int.
LONG_MIN
signed
long int. On most machines that the GNU C system runs on, long
integers are 32-bit quantities, the same size as int.
LONG_MAX
ULONG_MAX
signed long int and unsigned long int, respectively.
LONG_LONG_MIN
signed
long long int. On most machines that the GNU C system runs on,
long long integers are 64-bit quantities.
LONG_LONG_MAX
ULONG_LONG_MAX
signed
long long int and unsigned long long int, respectively.
WCHAR_MAX
wchar_t.
See section Introduction to Extended Characters.
The header file `limits.h' also defines some additional constants that parameterize various operating system and file system limits. These constants are described in section System Configuration Parameters.
The specific representation of floating point numbers varies from machine to machine. Because floating point numbers are represented internally as approximate quantities, algorithms for manipulating floating point data often need to take account of the precise details of the machine's floating point representation.
Some of the functions in the C library itself need this information; for example, the algorithms for printing and reading floating point numbers (see section Input/Output on Streams) and for calculating trigonometric and irrational functions (see section Mathematics) use it to avoid round-off error and loss of accuracy. User programs that implement numerical analysis techniques also often need this information in order to minimize or compute error bounds.
The header file `float.h' describes the format used by your machine.
This section introduces the terminology for describing floating point representations.
You are probably already familiar with most of these concepts in terms
of scientific or exponential notation for floating point numbers. For
example, the number 123456.0 could be expressed in exponential
notation as 1.23456e+05, a shorthand notation indicating that the
mantissa 1.23456 is multiplied by the base 10 raised to
power 5.
More formally, the internal representation of a floating point number can be characterized in terms of the following parameters:
-1 or 1.
1. This is a constant for a particular representation.
The mantissa of a floating point number represents an implicit fraction
whose denominator is the base raised to the power of the precision. Since
the largest representable mantissa is one less than this denominator, the
value of the fraction is always strictly less than 1. The
mathematical value of a floating point number is then the product of this
fraction, the sign, and the base raised to the exponent.
We say that the floating point number is normalized if the
fraction is at least 1/b, where b is the base. In
other words, the mantissa would be too large to fit if it were
multiplied by the base. Non-normalized numbers are sometimes called
denormal; they contain less precision than the representation
normally can hold.
If the number is not normalized, then you can subtract 1 from the
exponent while multiplying the mantissa by the base, and get another
floating point number with the same value. Normalization consists
of doing this repeatedly until the number is normalized. Two distinct
normalized floating point numbers cannot be equal in value.
(There is an exception to this rule: if the mantissa is zero, it is
considered normalized. Another exception happens on certain machines
where the exponent is as small as the representation can hold. Then
it is impossible to subtract 1 from the exponent, so a number
may be normalized even if its fraction is less than 1/b.)
These macro definitions can be accessed by including the header file `float.h' in your program.
Macro names starting with `FLT_' refer to the float type,
while names beginning with `DBL_' refer to the double type
and names beginning with `LDBL_' refer to the long double
type. (If GCC does not support long double as a distinct data
type on a target machine then the values for the `LDBL_' constants
are equal to the corresponding constants for the double type.)
Of these macros, only FLT_RADIX is guaranteed to be a constant
expression. The other macros listed here cannot be reliably used in
places that require constant expressions, such as `#if'
preprocessing directives or in the dimensions of static arrays.
Although the ISO C standard specifies minimum and maximum values for most of these parameters, the GNU C implementation uses whatever values describe the floating point representation of the target machine. So in principle GNU C actually satisfies the ISO C requirements only if the target machine is suitable. In practice, all the machines currently supported are suitable.
FLT_ROUNDS
-1
0
1
2
3
1, in accordance with the IEEE
standard for floating point.
Here is a table showing how certain values round for each possible value
of FLT_ROUNDS, if the other aspects of the representation match
the IEEE single-precision standard.
0 1 2 3
1.00000003 1.0 1.0 1.00000012 1.0
1.00000007 1.0 1.00000012 1.00000012 1.0
-1.00000003 -1.0 -1.0 -1.0 -1.00000012
-1.00000007 -1.0 -1.00000012 -1.0 -1.00000012
FLT_RADIX
FLT_MANT_DIG
FLT_RADIX digits in the floating point
mantissa for the float data type. The following expression
yields 1.0 (even though mathematically it should not) due to the
limited number of mantissa digits:
float radix = FLT_RADIX; 1.0f + 1.0f / radix / radix / ... / radixwhere
radix appears FLT_MANT_DIG times.
DBL_MANT_DIG
LDBL_MANT_DIG
FLT_RADIX digits in the floating point
mantissa for the data types double and long double,
respectively.
FLT_DIG
float
data type. Technically, if p and b are the precision and
base (respectively) for the representation, then the decimal precision
q is the maximum number of decimal digits such that any floating
point number with q base 10 digits can be rounded to a floating
point number with p base b digits and back again, without
change to the q decimal digits.
The value of this macro is supposed to be at least 6, to satisfy
ISO C.
DBL_DIG
LDBL_DIG
FLT_DIG, but for the data types
double and long double, respectively. The values of these
macros are supposed to be at least 10.
FLT_MIN_EXP
float.
More precisely, is the minimum negative integer such that the value
FLT_RADIX raised to this power minus 1 can be represented as a
normalized floating point number of type float.
DBL_MIN_EXP
LDBL_MIN_EXP
FLT_MIN_EXP, but for the data types
double and long double, respectively.
FLT_MIN_10_EXP
10 raised to this
power minus 1 can be represented as a normalized floating point number
of type float. This is supposed to be -37 or even less.
DBL_MIN_10_EXP
LDBL_MIN_10_EXP
FLT_MIN_10_EXP, but for the data types
double and long double, respectively.
FLT_MAX_EXP
float. More
precisely, this is the maximum positive integer such that value
FLT_RADIX raised to this power minus 1 can be represented as a
floating point number of type float.
DBL_MAX_EXP
LDBL_MAX_EXP
FLT_MAX_EXP, but for the data types
double and long double, respectively.
FLT_MAX_10_EXP
10 raised to this
power minus 1 can be represented as a normalized floating point number
of type float. This is supposed to be at least 37.
DBL_MAX_10_EXP
LDBL_MAX_10_EXP
FLT_MAX_10_EXP, but for the data types
double and long double, respectively.
FLT_MAX
float. It is supposed to be at least 1E+37. The value
has type float.
The smallest representable number is - FLT_MAX.
DBL_MAX
LDBL_MAX
FLT_MAX, but for the data types
double and long double, respectively. The type of the
macro's value is the same as the type it describes.
FLT_MIN
float. It is supposed
to be no more than 1E-37.
DBL_MIN
LDBL_MIN
FLT_MIN, but for the data types
double and long double, respectively. The type of the
macro's value is the same as the type it describes.
FLT_EPSILON
float
such that 1.0 + FLT_EPSILON != 1.0 is true. It's supposed to
be no greater than 1E-5.
DBL_EPSILON
LDBL_EPSILON
FLT_EPSILON, but for the data types
double and long double, respectively. The type of the
macro's value is the same as the type it describes. The values are not
supposed to be greater than 1E-9.
Here is an example showing how the floating type measurements come out for the most common floating point representation, specified by the IEEE Standard for Binary Floating Point Arithmetic (ANSI/IEEE Std 754-1985). Nearly all computers designed since the 1980s use this format.
The IEEE single-precision float representation uses a base of 2. There is a sign bit, a mantissa with 23 bits plus one hidden bit (so the total precision is 24 base-2 digits), and an 8-bit exponent that can represent values in the range -125 to 128, inclusive.
So, for an implementation that uses this representation for the
float data type, appropriate values for the corresponding
parameters are:
FLT_RADIX 2 FLT_MANT_DIG 24 FLT_DIG 6 FLT_MIN_EXP -125 FLT_MIN_10_EXP -37 FLT_MAX_EXP 128 FLT_MAX_10_EXP +38 FLT_MIN 1.17549435E-38F FLT_MAX 3.40282347E+38F FLT_EPSILON 1.19209290E-07F
Here are the values for the double data type:
DBL_MANT_DIG 53 DBL_DIG 15 DBL_MIN_EXP -1021 DBL_MIN_10_EXP -307 DBL_MAX_EXP 1024 DBL_MAX_10_EXP 308 DBL_MAX 1.7976931348623157E+308 DBL_MIN 2.2250738585072014E-308 DBL_EPSILON 2.2204460492503131E-016
You can use offsetof to measure the location within a structure
type of a particular structure member.
offsetof (struct s, elem) is the offset, in bytes,
of the member elem in a struct s.
This macro won't work if member is a bit field; you get an error from the C compiler in that case.
This appendix is a complete list of the facilities declared within the header files supplied with the GNU C library. Each entry also lists the standard or other source from which each facility is derived, and tells you where in the manual you can find more information about how to use it.
@smallfonts @rm
long int a64l (const char *string)
void abort (void)
int abs (int number)
int accept (int socket, struct sockaddr *addr, socklen_t *length_ptr)
int access (const char *filename, int how)
ACCOUNTING
double acos (double x)
float acosf (float x)
double acosh (double x)
float acoshf (float x)
long double acoshl (long double x)
long double acosl (long double x)
int addmntent (FILE *stream, const struct mntent *mnt)
int adjtime (const struct timeval *delta, struct timeval *olddelta)
int adjtimex (struct timex *timex)
AF_FILE
AF_INET
AF_INET6
AF_LOCAL
AF_UNIX
AF_UNSPEC
int aio_cancel (int fildes, struct aiocb *aiocbp)
int aio_cancel64 (int fildes, struct aiocb *aiocbp)
int aio_error (const struct aiocb *aiocbp)
int aio_error64 (const struct aiocb64 *aiocbp)
int aio_fsync (int op, struct aiocb *aiocbp)
int aio_fsync64 (int op, struct aiocb64 *aiocbp)
void aio_init (const struct aioinit *init)
int aio_read (struct aiocb *aiocbp)
int aio_read64 (struct aiocb *aiocbp)
ssize_t aio_return (const struct aiocb *aiocbp)
int aio_return64 (const struct aiocb64 *aiocbp)
int aio_suspend (const struct aiocb *const list[], int nent, const struct timespec *timeout)
int aio_suspend64 (const struct aiocb64 *const list[], int nent, const struct timespec *timeout)
int aio_write (struct aiocb *aiocbp)
int aio_write64 (struct aiocb *aiocbp)
unsigned int alarm (unsigned int seconds)
void * alloca (size_t size);
int alphasort (const void *a, const void *b)
int alphasort64 (const void *a, const void *b)
tcflag_t ALTWERASE
int ARG_MAX
error_t argp_err_exit_status
void argp_error (const struct argp_state *state, const char *fmt, ...)
int ARGP_ERR_UNKNOWN
void argp_failure (const struct argp_state *state, int status, int errnum, const char *fmt, ...)
void argp_help (const struct argp *argp, FILE *stream, unsigned flags, char *name)
argp_help Function.
ARGP_IN_ORDER
argp_parse.
ARGP_KEY_ARG
ARGP_KEY_ARGS
ARGP_KEY_END
ARGP_KEY_ERROR
ARGP_KEY_FINI
ARGP_KEY_HELP_ARGS_DOC
ARGP_KEY_HELP_DUP_ARGS_NOTE
ARGP_KEY_HELP_EXTRA
ARGP_KEY_HELP_HEADER
ARGP_KEY_HELP_POST_DOC
ARGP_KEY_HELP_PRE_DOC
ARGP_KEY_INIT
ARGP_KEY_NO_ARGS
ARGP_KEY_SUCCESS
ARGP_LONG_ONLY
argp_parse.
ARGP_NO_ARGS
argp_parse.
ARGP_NO_ERRS
argp_parse.
ARGP_NO_EXIT
argp_parse.
ARGP_NO_HELP
argp_parse.
error_t argp_parse (const struct argp *argp, int argc, char **argv, unsigned flags, int *arg_index, void *input)
ARGP_PARSE_ARGV0
argp_parse.
const char * argp_program_bug_address
const char * argp_program_version
argp_program_version_hook
ARGP_SILENT
argp_parse.
void argp_state_help (const struct argp_state *state, FILE *stream, unsigned flags)
void argp_usage (const struct argp_state *state)
error_t argz_add (char **argz, size_t *argz_len, const char *str)
error_t argz_add_sep (char **argz, size_t *argz_len, const char *str, int delim)
error_t argz_append (char **argz, size_t *argz_len, const char *buf, size_t buf_len)
size_t argz_count (const char *argz, size_t arg_len)
error_t argz_create (char *const argv[], char **argz, size_t *argz_len)
error_t argz_create_sep (const char *string, int sep, char **argz, size_t *argz_len)
error_t argz_delete (char **argz, size_t *argz_len, char *entry)
void argz_extract (char *argz, size_t argz_len, char **argv)
error_t argz_insert (char **argz, size_t *argz_len, char *before, const char *entry)
char * argz_next (char *argz, size_t argz_len, const char *entry)
error_t argz_replace (char **argz, size_t *argz_len, const char *str, const char *with, unsigned *replace_count)
void argz_stringify (char *argz, size_t len, int sep)
char * asctime (const struct tm *brokentime)
char * asctime_r (const struct tm *brokentime, char *buffer)
double asin (double x)
float asinf (float x)
double asinh (double x)
float asinhf (float x)
long double asinhl (long double x)
long double asinl (long double x)
int asprintf (char **ptr, const char *template, ...)
void assert (int expression)
void assert_perror (int errnum)
double atan (double x)
double atan2 (double y, double x)
float atan2f (float y, float x)
long double atan2l (long double y, long double x)
float atanf (float x)
double atanh (double x)
float atanhf (float x)
long double atanhl (long double x)
long double atanl (long double x)
int atexit (void (*function) (void))
double atof (const char *string)
int atoi (const char *string)
long int atol (const char *string)
long long int atoll (const char *string)
B0
B110
B115200
B1200
B134
B150
B1800
B19200
B200
B230400
B2400
B300
B38400
B460800
B4800
B50
B57600
B600
B75
B9600
int backtrace (void **buffer, int size)
char ** backtrace_symbols (void *const *buffer, int size)
void backtrace_symbols_fd (void *const *buffer, int size, int fd)
char * basename (char *path)
char * basename (const char *filename)
int BC_BASE_MAX
int BC_DIM_MAX
int bcmp (const void *a1, const void *a2, size_t size)
void bcopy (const void *from, void *to, size_t size)
int BC_SCALE_MAX
int BC_STRING_MAX
int bind (int socket, struct sockaddr *addr, socklen_t length)
char * bindtextdomain (const char *domainname, const char *dirname)
char * bind_textdomain_codeset (const char *domainname, const char *codeset)
gettext uses.
blkcnt64_t
blkcnt_t
BOOT_TIME
BOOT_TIME
int brk (void *addr)
tcflag_t BRKINT
_BSD_SOURCE
void * bsearch (const void *key, const void *array, size_t count, size_t size, comparison_fn_t compare)
wint_t btowc (int c)
int BUFSIZ
void bzero (void *block, size_t size)
double cabs (complex double z)
float cabsf (complex float z)
long double cabsl (complex long double z)
complex double cacos (complex double z)
complex float cacosf (complex float z)
complex double cacosh (complex double z)
complex float cacoshf (complex float z)
complex long double cacoshl (complex long double z)
complex long double cacosl (complex long double z)
void * calloc (size_t count, size_t eltsize)
char * canonicalize_file_name (const char *name)
double carg (complex double z)
float cargf (complex float z)
long double cargl (complex long double z)
complex double casin (complex double z)
complex float casinf (complex float z)
complex double casinh (complex double z)
complex float casinhf (complex float z)
complex long double casinhl (complex long double z)
complex long double casinl (complex long double z)
complex double catan (complex double z)
complex float catanf (complex float z)
complex double catanh (complex double z)
complex float catanhf (complex float z)
complex long double catanhl (complex long double z)
complex long double catanl (complex long double z)
nl_catd catopen (const char *cat_name, int flag)
catgets function family.
int cbc_crypt (char *key, char *blocks, unsigned len, unsigned mode, char *ivec)
double cbrt (double x)
float cbrtf (float x)
long double cbrtl (long double x)
complex double ccos (complex double z)
complex float ccosf (complex float z)
complex double ccosh (complex double z)
complex float ccoshf (complex float z)
complex long double ccoshl (complex long double z)
complex long double ccosl (complex long double z)
cc_t
tcflag_t CCTS_OFLOW
double ceil (double x)
float ceilf (float x)
long double ceill (long double x)
complex double cexp (complex double z)
complex float cexpf (complex float z)
complex long double cexpl (complex long double z)
speed_t cfgetispeed (const struct termios *termios-p)
speed_t cfgetospeed (const struct termios *termios-p)
int cfmakeraw (struct termios *termios-p)
void cfree (void *ptr)
malloc.
int cfsetispeed (struct termios *termios-p, speed_t speed)
int cfsetospeed (struct termios *termios-p, speed_t speed)
int cfsetspeed (struct termios *termios-p, speed_t speed)
CHAR_BIT
CHAR_MAX
CHAR_MIN
int chdir (const char *filename)
int CHILD_MAX
int chmod (const char *filename, mode_t mode)
int chown (const char *filename, uid_t owner, gid_t group)
tcflag_t CIGNORE
double cimag (complex double z)
float cimagf (complex float z)
long double cimagl (complex long double z)
int clearenv (void)
void clearerr (FILE *stream)
void clearerr_unlocked (FILE *stream)
int CLK_TCK
tcflag_t CLOCAL
clock_t clock (void)
int CLOCKS_PER_SEC
clock_t
complex double clog (complex double z)
complex double clog10 (complex double z)
complex float clog10f (complex float z)
complex long double clog10l (complex long double z)
complex float clogf (complex float z)
complex long double clogl (complex long double z)
int close (int filedes)
int closedir (DIR *dirstream)
void closelog (void)
int COLL_WEIGHTS_MAX
size_t confstr (int parameter, char *buf, size_t len)
complex double conj (complex double z)
complex float conjf (complex float z)
complex long double conjl (complex long double z)
int connect (int socket, struct sockaddr *addr, socklen_t length)
cookie_close_function
cookie_io_functions_t
cookie_read_function
cookie_seek_function
cookie_write_function
double copysign (double x, double y)
float copysignf (float x, float y)
long double copysignl (long double x, long double y)
double cos (double x)
float cosf (float x)
double cosh (double x)
float coshf (float x)
long double coshl (long double x)
long double cosl (long double x)
complex double cpow (complex double base, complex double power)
complex float cpowf (complex float base, complex float power)
complex long double cpowl (complex long double base, complex long double power)
complex double cproj (complex double z)
complex float cprojf (complex float z)
complex long double cprojl (complex long double z)
tcflag_t CREAD
double creal (complex double z)
float crealf (complex float z)
long double creall (complex long double z)
int creat (const char *filename, mode_t mode)
int creat64 (const char *filename, mode_t mode)
tcflag_t CRTS_IFLOW
char * crypt (const char *key, const char *salt)
char * crypt_r (const char *key, const char *salt, struct crypt_data * data)
tcflag_t CS5
tcflag_t CS6
tcflag_t CS7
tcflag_t CS8
complex double csin (complex double z)
complex float csinf (complex float z)
complex double csinh (complex double z)
complex float csinhf (complex float z)
complex long double csinhl (complex long double z)
complex long double csinl (complex long double z)
tcflag_t CSIZE
_CS_LFS64_CFLAGS
_CS_LFS64_LDFLAGS
_CS_LFS64_LIBS
_CS_LFS64_LINTFLAGS
_CS_LFS_CFLAGS
_CS_LFS_LDFLAGS
_CS_LFS_LIBS
_CS_LFS_LINTFLAGS
_CS_PATH
complex double csqrt (complex double z)
complex float csqrtf (complex float z)
complex long double csqrtl (complex long double z)
tcflag_t CSTOPB
complex double ctan (complex double z)
complex float ctanf (complex float z)
complex double ctanh (complex double z)
complex float ctanhf (complex float z)
complex long double ctanhl (complex long double z)
complex long double ctanl (complex long double z)
char * ctermid (char *string)
char * ctime (const time_t *time)
char * ctime_r (const time_t *time, char *buffer)
char * cuserid (char *string)
int daylight
DBL_DIG
DBL_EPSILON
DBL_MANT_DIG
DBL_MAX
DBL_MAX_10_EXP
DBL_MAX_EXP
DBL_MIN
DBL_MIN_10_EXP
DBL_MIN_EXP
char * dcgettext (const char *domainname, const char *msgid, int category)
char * dcngettext (const char *domain, const char *msgid1, const char *msgid2, unsigned long int n, int category)
DEAD_PROCESS
DEAD_PROCESS
DES_DECRYPT
DES_ENCRYPT
DESERR_BADPARAM
DESERR_HWERROR
DESERR_NOHWDEVICE
DESERR_NONE
int DES_FAILED (int err)
DES_HW
void des_setparity (char *key)
DES_SW
dev_t
char * dgettext (const char *domainname, const char *msgid)
double difftime (time_t time1, time_t time0)
DIR
int dirfd (DIR *dirstream)
char * dirname (char *path)
div_t div (int numerator, int denominator)
div_t
char * dngettext (const char *domain, const char *msgid1, const char *msgid2, unsigned long int n)
double drand48 (void)
int drand48_r (struct drand48_data *buffer, double *result)
double drem (double numerator, double denominator)
float dremf (float numerator, float denominator)
long double dreml (long double numerator, long double denominator)
mode_t DTTOIF (int dtype)
int dup (int old)
int dup2 (int old, int new)
int E2BIG
int EACCES
int EADDRINUSE
int EADDRNOTAVAIL
int EADV
int EAFNOSUPPORT
int EAGAIN
int EALREADY
int EAUTH
int EBACKGROUND
int EBADE
int EBADF
int EBADFD
int EBADMSG
int EBADR
int EBADRPC
int EBADRQC
int EBADSLT
int EBFONT
int EBUSY
int ecb_crypt (char *key, char *blocks, unsigned len, unsigned mode)
int ECHILD
tcflag_t ECHO
tcflag_t ECHOCTL
tcflag_t ECHOE
tcflag_t ECHOK
tcflag_t ECHOKE
tcflag_t ECHONL
tcflag_t ECHOPRT
int ECHRNG
int ECOMM
int ECONNABORTED
int ECONNREFUSED
int ECONNRESET
char * ecvt (double value, int ndigit, int *decpt, int *neg)
char * ecvt_r (double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
int ED
int EDEADLK
int EDEADLOCK
int EDESTADDRREQ
int EDIED
int EDOM
int EDOTDOT
int EDQUOT
int EEXIST
int EFAULT
int EFBIG
int EFTYPE
int EGRATUITOUS
int EGREGIOUS
int EHOSTDOWN
int EHOSTUNREACH
int EIDRM
int EIEIO
int EILSEQ
int EINPROGRESS
int EINTR
int EINVAL
int EIO
int EISCONN
int EISDIR
int EISNAM
int EL2HLT
int EL2NSYNC
int EL3HLT
int EL3RST
int ELIBACC
int ELIBBAD
int ELIBEXEC
int ELIBMAX
int ELIBSCN
int ELNRNG
int ELOOP
int EMEDIUMTYPE
int EMFILE
int EMLINK
EMPTY
EMPTY
int EMSGSIZE
int EMULTIHOP
int ENAMETOOLONG
int ENAVAIL
void encrypt (char *block, int edflag)
void encrypt_r (char *block, int edflag, struct crypt_data * data)
void endfsent (void)
void endgrent (void)
void endhostent (void)
int endmntent (FILE *stream)
void endnetent (void)
void endnetgrent (void)
void endprotoent (void)
void endpwent (void)
void endservent (void)
void endutent (void)
void endutxent (void)
int ENEEDAUTH
int ENETDOWN
int ENETRESET
int ENETUNREACH
int ENFILE
int ENOANO
int ENOBUFS
int ENOCSI
int ENODATA
int ENODEV
int ENOENT
int ENOEXEC
int ENOLCK
int ENOLINK
int ENOMEDIUM
int ENOMEM
int ENOMSG
int ENONET
int ENOPKG
int ENOPROTOOPT
int ENOSPC
int ENOSR
int ENOSTR
int ENOSYS
int ENOTBLK
int ENOTCONN
int ENOTDIR
int ENOTEMPTY
int ENOTNAM
int ENOTSOCK
int ENOTSUP
int ENOTTY
int ENOTUNIQ
char ** environ
error_t envz_add (char **envz, size_t *envz_len, const char *name, const char *value)
char * envz_entry (const char *envz, size_t envz_len, const char *name)
char * envz_get (const char *envz, size_t envz_len, const char *name)
error_t envz_merge (char **envz, size_t *envz_len, const char *envz2, size_t envz2_len, int override)
void envz_strip (char **envz, size_t *envz_len)
int ENXIO
int EOF
int EOPNOTSUPP
int EOVERFLOW
int EPERM
int EPFNOSUPPORT
int EPIPE
int EPROCLIM
int EPROCUNAVAIL
int EPROGMISMATCH
int EPROGUNAVAIL
int EPROTO
int EPROTONOSUPPORT
int EPROTOTYPE
int EQUIV_CLASS_MAX
double erand48 (unsigned short int xsubi[3])
int erand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, double *result)
int ERANGE
int EREMCHG
int EREMOTE
int EREMOTEIO
int ERESTART
double erf (double x)
double erfc (double x)
float erfcf (float x)
long double erfcl (long double x)
float erff (float x)
long double erfl (long double x)
int EROFS
int ERPCMISMATCH
volatile int errno
int ESHUTDOWN
int ESOCKTNOSUPPORT
int ESPIPE
int ESRCH
int ESRMNT
int ESTALE
int ESTRPIPE
int ETIME
int ETIMEDOUT
int ETOOMANYREFS
int ETXTBSY
int EUCLEAN
int EUNATCH
int EUSERS
int EWOULDBLOCK
int EXDEV
int execl (const char *filename, const char *arg0, ...)
int execle (const char *filename, const char *arg0, char *const env[], ...)
int execlp (const char *filename, const char *arg0, ...)
int execv (const char *filename, char *const argv[])
int execve (const char *filename, char *const argv[], char *const env[])
int execvp (const char *filename, char *const argv[])
int EXFULL
void _Exit (int status)
void _exit (int status)
void exit (int status)
int EXIT_FAILURE
int EXIT_SUCCESS
double exp (double x)
double exp10 (double x)
float exp10f (float x)
long double exp10l (long double x)
double exp2 (double x)
float exp2f (float x)
long double exp2l (long double x)
float expf (float x)
long double expl (long double x)
double expm1 (double x)
float expm1f (float x)
long double expm1l (long double x)
int EXPR_NEST_MAX
double fabs (double number)
float fabsf (float number)
long double fabsl (long double number)
size_t __fbufsize (FILE *stream)
int fchdir (int filedes)
int fchmod (int filedes, int mode)
int fchown (int filedes, int owner, int group)
int fclean (FILE *stream)
int fclose (FILE *stream)
int fcloseall (void)
int fcntl (int filedes, int command, ...)
char * fcvt (double value, int ndigit, int *decpt, int *neg)
char * fcvt_r (double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
int fdatasync (int fildes)
int FD_CLOEXEC
void FD_CLR (int filedes, fd_set *set)
double fdim (double x, double y)
float fdimf (float x, float y)
long double fdiml (long double x, long double y)
int FD_ISSET (int filedes, fd_set *set)
FILE * fdopen (int filedes, const char *opentype)
void FD_SET (int filedes, fd_set *set)
fd_set
int FD_SETSIZE
int F_DUPFD
void FD_ZERO (fd_set *set)
int feclearexcept (int excepts)
int fedisableexcept (int excepts)
FE_DIVBYZERO
FE_DOWNWARD
int feenableexcept (int excepts)
int fegetenv (fenv_t *envp)
int fegetexcept (int excepts)
int fegetexceptflag (fexcept_t *flagp, int excepts)
int fegetround (void)
int feholdexcept (fenv_t *envp)
FE_INEXACT
FE_INVALID
int feof (FILE *stream)
int feof_unlocked (FILE *stream)
FE_OVERFLOW
int feraiseexcept (int excepts)
int ferror (FILE *stream)
int ferror_unlocked (FILE *stream)
int fesetenv (const fenv_t *envp)
int fesetexceptflag (const fexcept_t *flagp, int
int fesetround (int round)
int fetestexcept (int excepts)
FE_TONEAREST
FE_TOWARDZERO
FE_UNDERFLOW
int feupdateenv (const fenv_t *envp)
FE_UPWARD
int fflush (FILE *stream)
int fflush_unlocked (FILE *stream)
int fgetc (FILE *stream)
int fgetc_unlocked (FILE *stream)
int F_GETFD
int F_GETFL
struct group * fgetgrent (FILE *stream)
int fgetgrent_r (FILE *stream, struct group *result_buf, char *buffer, size_t buflen, struct group **result)
int F_GETLK
int F_GETOWN
int fgetpos (FILE *stream, fpos_t *position)
int fgetpos64 (FILE *stream, fpos64_t *position)
struct passwd * fgetpwent (FILE *stream)
int fgetpwent_r (FILE *stream, struct passwd *result_buf, char *buffer, size_t buflen, struct passwd **result)
char * fgets (char *s, int count, FILE *stream)
char * fgets_unlocked (char *s, int count, FILE *stream)
wint_t fgetwc (FILE *stream)
wint_t fgetwc_unlocked (FILE *stream)
wchar_t * fgetws (wchar_t *ws, int count, FILE *stream)
wchar_t * fgetws_unlocked (wchar_t *ws, int count, FILE *stream)
FILE
int FILENAME_MAX
int fileno (FILE *stream)
int fileno_unlocked (FILE *stream)
int finite (double x)
int finitef (float x)
int finitel (long double x)
int __flbf (FILE *stream)
void flockfile (FILE *stream)
double floor (double x)
float floorf (float x)
long double floorl (long double x)
FLT_DIG
FLT_EPSILON
FLT_MANT_DIG
FLT_MAX
FLT_MAX_10_EXP
FLT_MAX_EXP
FLT_MIN
FLT_MIN_10_EXP
FLT_MIN_EXP
FLT_RADIX
FLT_ROUNDS
void _flushlbf (void)
tcflag_t FLUSHO
double fma (double x, double y, double z)
float fmaf (float x, float y, float z)
long double fmal (long double x, long double y, long double z)
double fmax (double x, double y)
float fmaxf (float x, float y)
long double fmaxl (long double x, long double y)
FILE * fmemopen (void *buf, size_t size, const char *opentype)
double fmin (double x, double y)
float fminf (float x, float y)
long double fminl (long double x, long double y)
double fmod (double numerator, double denominator)
float fmodf (float numerator, float denominator)
long double fmodl (long double numerator, long double denominator)
int fmtmsg (long int classification, const char *label, int severity, const char *text, const char *action, const char *tag)
int fnmatch (const char *pattern, const char *string, int flags)
FNM_CASEFOLD
FNM_EXTMATCH
FNM_FILE_NAME
FNM_LEADING_DIR
FNM_NOESCAPE
FNM_PATHNAME
FNM_PERIOD
int F_OK
FILE * fopen (const char *filename, const char *opentype)
FILE * fopen64 (const char *filename, const char *opentype)
FILE * fopencookie (void *cookie, const char *opentype, cookie_io_functions_t io-functions)
int FOPEN_MAX
pid_t fork (void)
int forkpty (int *amaster, char *name, struct termios *termp, struct winsize *winp)
long int fpathconf (int filedes, int parameter)
pathconf.
int fpclassify (float-type x)
FPE_DECOVF_TRAP
FPE_FLTDIV_FAULT
FPE_FLTDIV_TRAP
FPE_FLTOVF_FAULT
FPE_FLTOVF_TRAP
FPE_FLTUND_FAULT
FPE_FLTUND_TRAP
FPE_INTDIV_TRAP
FPE_INTOVF_TRAP
size_t __fpending (FILE *stream) The __fpending
FPE_SUBRNG_TRAP
int FP_ILOGB0
int FP_ILOGBNAN
fpos64_t
fpos_t
int fprintf (FILE *stream, const char *template, ...)
void __fpurge (FILE *stream)
int fputc (int c, FILE *stream)
int fputc_unlocked (int c, FILE *stream)
int fputs (const char *s, FILE *stream)
int fputs_unlocked (const char *s, FILE *stream)
wint_t fputwc (wchar_t wc, FILE *stream)
wint_t fputwc_unlocked (wint_t wc, FILE *stream)
int fputws (const wchar_t *ws, FILE *stream)
int fputws_unlocked (const wchar_t *ws, FILE *stream)
F_RDLCK
size_t fread (void *data, size_t size, size_t count, FILE *stream)
int __freadable (FILE *stream)
int __freading (FILE *stream)
size_t fread_unlocked (void *data, size_t size, size_t count, FILE *stream)
void free (void *ptr)
malloc.
__free_hook
FILE * freopen (const char *filename, const char *opentype, FILE *stream)
FILE * freopen64 (const char *filename, const char *opentype, FILE *stream)
double frexp (double value, int *exponent)
float frexpf (float value, int *exponent)
long double frexpl (long double value, int *exponent)
int fscanf (FILE *stream, const char *template, ...)
int fseek (FILE *stream, long int offset, int whence)
int fseeko (FILE *stream, off_t offset, int whence)
int fseeko64 (FILE *stream, off64_t offset, int whence)
int F_SETFD
int F_SETFL
int F_SETLK
int F_SETLKW
int __fsetlocking (FILE *stream, int type)
int F_SETOWN
int fsetpos (FILE *stream, const fpos_t *position)
int fsetpos64 (FILE *stream, const fpos64_t *position)
int fstat (int filedes, struct stat *buf)
int fstat64 (int filedes, struct stat64 *buf)
int fsync (int fildes)
long int ftell (FILE *stream)
off_t ftello (FILE *stream)
off64_t ftello64 (FILE *stream)
int ftruncate (int fd, off_t length)
int ftruncate64 (int id, off64_t length)
int ftrylockfile (FILE *stream)
int ftw (const char *filename, __ftw_func_t func, int descriptors)
int ftw64 (const char *filename, __ftw64_func_t func, int descriptors)
__ftw64_func_t
__ftw_func_t
F_UNLCK
void funlockfile (FILE *stream)
int fwide (FILE *stream, int mode)
int fwprintf (FILE *stream, const wchar_t *template, ...)
int __fwritable (FILE *stream)
size_t fwrite (const void *data, size_t size, size_t count, FILE *stream)
size_t fwrite_unlocked (const void *data, size_t size, size_t count, FILE *stream)
int __fwriting (FILE *stream)
F_WRLCK
int fwscanf (FILE *stream, const wchar_t *template, ...)
double gamma (double x)
float gammaf (float x)
long double gammal (long double x)
void (*__gconv_end_fct) (struct gconv_step *)
iconv Implementation in the GNU C library.
int (*__gconv_fct) (struct __gconv_step *, struct __gconv_step_data *, const char **, const char *, size_t *, int)
iconv Implementation in the GNU C library.
int (*__gconv_init_fct) (struct __gconv_step *)
iconv Implementation in the GNU C library.
char * gcvt (double value, int ndigit, char *buf)
long int get_avphys_pages (void)
int getc (FILE *stream)
int getchar (void)
int getchar_unlocked (void)
int getcontext (ucontext_t *ucp)
int getc_unlocked (FILE *stream)
char * get_current_dir_name (void)
char * getcwd (char *buffer, size_t size)
struct tm * getdate (const char *string)
getdate_err
int getdate_r (const char *string, struct tm *tp)
ssize_t getdelim (char **lineptr, size_t *n, int delimiter, FILE *stream)
int getdomainnname (char *name, size_t length)
gid_t getegid (void)
char * getenv (const char *name)
uid_t geteuid (void)
struct fstab * getfsent (void)
struct fstab * getfsfile (const char *name)
struct fstab * getfsspec (const char *name)
gid_t getgid (void)
struct group * getgrent (void)
int getgrent_r (struct group *result_buf, char *buffer, size_t buflen, struct group **result)
struct group * getgrgid (gid_t gid)
int getgrgid_r (gid_t gid, struct group *result_buf, char *buffer, size_t buflen, struct group **result)
struct group * getgrnam (const char *name)
int getgrnam_r (const char *name, struct group *result_buf, char *buffer, size_t buflen, struct group **result)
int getgroups (int count, gid_t *groups)
struct hostent * gethostbyaddr (const char *addr, size_t length, int format)
int gethostbyaddr_r (const char *addr, size_t length, int format, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)
struct hostent * gethostbyname (const char *name)
struct hostent * gethostbyname2 (const char *name, int af)
int gethostbyname2_r (const char *name, int af, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)
int gethostbyname_r (const char *restrict name, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)
struct hostent * gethostent (void)
long int gethostid (void)
int gethostname (char *name, size_t size)
int getitimer (int which, struct itimerval *old)
ssize_t getline (char **lineptr, size_t *n, FILE *stream)
int getloadavg (double loadavg[], int nelem)
char * getlogin (void)
struct mntent * getmntent (FILE *stream)
struct mntent * getmntent_r (FILE *stream, struct mentent *result, char *buffer, int bufsize)
struct netent * getnetbyaddr (unsigned long int net, int type)
struct netent * getnetbyname (const char *name)
struct netent * getnetent (void)
int getnetgrent (char **hostp, char **userp, char **domainp)
int getnetgrent_r (char **hostp, char **userp, char **domainp, char *buffer, int buflen)
int get_nprocs (void)
int get_nprocs_conf (void)
int getopt (int argc, char **argv, const char *options)
getopt function.
int getopt_long (int argc, char *const *argv, const char *shortopts, struct option *longopts, int *indexptr)
getopt_long.
int getopt_long_only (int argc, char *const *argv, const char *shortopts, struct option *longopts, int *indexptr)
getopt_long.
int getpagesize (void)
char * getpass (const char *prompt)
int getpeername (int socket, struct sockaddr *addr, socklen_t *length-ptr)
int getpgid (pid_t pid)
pid_t getpgrp (pid_t pid)
pid_t getpgrp (void)
long int get_phys_pages (void)
pid_t getpid (void)
pid_t getppid (void)
int getpriority (int class, int id)
struct protoent * getprotobyname (const char *name)
struct protoent * getprotobynumber (int protocol)
struct protoent * getprotoent (void)
int getpt (void)
struct passwd * getpwent (void)
int getpwent_r (struct passwd *result_buf, char *buffer, int buflen, struct passwd **result)
struct passwd * getpwnam (const char *name)
int getpwnam_r (const char *name, struct passwd *result_buf, char *buffer, size_t buflen, struct passwd **result)
struct passwd * getpwuid (uid_t uid)
int getpwuid_r (uid_t uid, struct passwd *result_buf, char *buffer, size_t buflen, struct passwd **result)
int getrlimit (int resource, struct rlimit *rlp)
int getrlimit64 (int resource, struct rlimit64 *rlp)
int getrusage (int processes, struct rusage *rusage)
char * gets (char *s)
struct servent * getservbyname (const char *name, const char *proto)
struct servent * getservbyport (int port, const char *proto)
struct servent * getservent (void)
pid_t getsid (pid_t pid)
int getsockname (int socket, struct sockaddr *addr, socklen_t *length-ptr)
int getsockopt (int socket, int level, int optname, void *optval, socklen_t *optlen-ptr)
int getsubopt (char **optionp, const char* const *tokens, char **valuep)
char * gettext (const char *msgid)
int gettimeofday (struct timeval *tp, struct timezone *tzp)
uid_t getuid (void)
mode_t getumask (void)
struct utmp * getutent (void)
int getutent_r (struct utmp *buffer, struct utmp **result)
struct utmp * getutid (const struct utmp *id)
int getutid_r (const struct utmp *id, struct utmp *buffer, struct utmp **result)
struct utmp * getutline (const struct utmp *line)
int getutline_r (const struct utmp *line, struct utmp *buffer, struct utmp **result)
int getutmp (const struct utmpx *utmpx, struct utmp *utmp)
int getutmpx (const struct utmp *utmp, struct utmpx *utmpx)
struct utmpx * getutxent (void)
struct utmpx * getutxid (const struct utmpx *id)
struct utmpx * getutxline (const struct utmpx *line)
int getw (FILE *stream)
wint_t getwc (FILE *stream)
wint_t getwchar (void)
wint_t getwchar_unlocked (void)
wint_t getwc_unlocked (FILE *stream)
char * getwd (char *buffer)
gid_t
int glob (const char *pattern, int flags, int (*errfunc) (const char *filename, int error-code), glob_t *vector-ptr)
glob.
int glob64 (const char *pattern, int flags, int (*errfunc) (const char *filename, int error-code), glob64_t *vector-ptr)
glob.
glob64_t
glob.
GLOB_ABORTED
glob.
GLOB_ALTDIRFUNC
GLOB_APPEND
GLOB_BRACE
GLOB_DOOFFS
GLOB_ERR
void globfree (glob_t *pglob)
void globfree64 (glob64_t *pglob)
GLOB_MAGCHAR
GLOB_MARK
GLOB_NOCHECK
GLOB_NOESCAPE
GLOB_NOMAGIC
GLOB_NOMATCH
glob.
GLOB_NOSORT
GLOB_NOSPACE
glob.
GLOB_ONLYDIR
GLOB_PERIOD
glob_t
glob.
GLOB_TILDE
GLOB_TILDE_CHECK
struct tm * gmtime (const time_t *time)
struct tm * gmtime_r (const time_t *time, struct tm *resultp)
_GNU_SOURCE
int grantpt (int filedes)
int gsignal (int signum)
int gtty (int filedes, struct sgttyb *attributes)
char * hasmntopt (const struct mntent *mnt, const char *opt)
int hcreate (size_t nel)
hsearch function..
int hcreate_r (size_t nel, struct hsearch_data *htab)
hsearch function..
void hdestroy (void)
hsearch function..
void hdestroy_r (struct hsearch_data *htab)
hsearch function..
HOST_NOT_FOUND
ENTRY * hsearch (ENTRY item, ACTION action)
hsearch function..
int hsearch_r (ENTRY item, ACTION action, ENTRY **retval, struct hsearch_data *htab)
hsearch function..
uint32_t htonl (uint32_t hostlong)
uint16_t htons (uint16_t hostshort)
double HUGE_VAL
float HUGE_VALF
long double HUGE_VALL
tcflag_t HUPCL
double hypot (double x, double y)
float hypotf (float x, float y)
long double hypotl (long double x, long double y)
tcflag_t ICANON
size_t iconv (iconv_t cd, char **inbuf, size_t *inbytesleft, char **outbuf, size_t *outbytesleft)
int iconv_close (iconv_t cd)
iconv_t iconv_open (const char *tocode, const char *fromcode)
iconv_t
tcflag_t ICRNL
tcflag_t IEXTEN
void if_freenameindex (struct if_nameindex *ptr)
char * if_indextoname (unsigned int ifindex, char *ifname)
struct if_nameindex * if_nameindex (void)
unsigned int if_nametoindex (const char *ifname)
size_t IFNAMSIZ
int IFTODT (mode_t mode)
tcflag_t IGNBRK
tcflag_t IGNCR
tcflag_t IGNPAR
int ilogb (double x)
int ilogbf (float x)
int ilogbl (long double x)
intmax_t imaxabs (intmax_t number)
tcflag_t IMAXBEL
imaxdiv_t imaxdiv (intmax_t numerator, intmax_t denominator)
imaxdiv_t
struct in6_addr in6addr_any
struct in6_addr in6addr_loopback
uint32_t INADDR_ANY
uint32_t INADDR_BROADCAST
uint32_t INADDR_LOOPBACK
uint32_t INADDR_NONE
char * index (const char *string, int c)
uint32_t inet_addr (const char *name)
int inet_aton (const char *name, struct in_addr *addr)
uint32_t inet_lnaof (struct in_addr addr)
struct in_addr inet_makeaddr (uint32_t net, uint32_t local)
uint32_t inet_netof (struct in_addr addr)
uint32_t inet_network (const char *name)
char * inet_ntoa (struct in_addr addr)
const char * inet_ntop (int af, const void *cp, char *buf, size_t len)
int inet_pton (int af, const char *cp, void *buf)
float INFINITY
double infnan (int error)
int initgroups (const char *user, gid_t gid)
INIT_PROCESS
INIT_PROCESS
void * initstate (unsigned int seed, void *state, size_t size)
int initstate_r (unsigned int seed, char *restrict statebuf, size_t statelen, struct random_data *restrict buf)
tcflag_t INLCR
int innetgr (const char *netgroup, const char *host, const char *user, const char *domain)
ino64_t
ino_t
tcflag_t INPCK
INT_MAX
INT_MIN
int _IOFBF
int _IOLBF
int _IONBF
int IPPORT_RESERVED
int IPPORT_USERRESERVED
int isalnum (int c)
int isalpha (int c)
int isascii (int c)
int isatty (int filedes)
int isblank (int c)
int iscntrl (int c)
int isdigit (int c)
int isfinite (float-type x)
int isgraph (int c)
int isgreater (real-floating x, real-floating y)
int isgreaterequal (real-floating x, real-floating y)
tcflag_t ISIG
int isinf (double x)
int isinff (float x)
int isinfl (long double x)
int isless (real-floating x, real-floating y)
int islessequal (real-floating x, real-floating y)
int islessgreater (real-floating x, real-floating y)
int islower (int c)
int isnan (float-type x)
int isnan (double x)
int isnanf (float x)
int isnanl (long double x)
int isnormal (float-type x)
_ISOC99_SOURCE
int isprint (int c)
int ispunct (int c)
int isspace (int c)
tcflag_t ISTRIP
int isunordered (real-floating x, real-floating y)
int isupper (int c)
int iswalnum (wint_t wc)
int iswalpha (wint_t wc)
int iswblank (wint_t wc)
int iswcntrl (wint_t wc)
int iswctype (wint_t wc, wctype_t desc)
int iswdigit (wint_t wc)
int iswgraph (wint_t wc)
int iswlower (wint_t wc)
int iswprint (wint_t wc)
int iswpunct (wint_t wc)
int iswspace (wint_t wc)
int iswupper (wint_t wc)
int iswxdigit (wint_t wc)
int isxdigit (int c)
ITIMER_PROF
ITIMER_REAL
ITIMER_VIRTUAL
tcflag_t IXANY
tcflag_t IXOFF
tcflag_t IXON
double j0 (double x)
float j0f (float x)
long double j0l (long double x)
double j1 (double x)
float j1f (float x)
long double j1l (long double x)
jmp_buf
double jn (int n, double x)
float jnf (int n, float x)
long double jnl (int n, long double x)
long int jrand48 (unsigned short int xsubi[3])
int jrand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, long int *result)
int kill (pid_t pid, int signum)
int killpg (int pgid, int signum)
char * l64a (long int n)
long int labs (long int number)
LANG
LC_ALL
LC_COLLATE
LC_CTYPE
LC_MESSAGES
LC_MONETARY
LC_NUMERIC
void lcong48 (unsigned short int param[7])
int lcong48_r (unsigned short int param[7], struct drand48_data *buffer)
int L_ctermid
LC_TIME
int L_cuserid
double ldexp (double value, int exponent)
float ldexpf (float value, int exponent)
long double ldexpl (long double value, int exponent)
ldiv_t ldiv (long int numerator, long int denominator)
ldiv_t
void * lfind (const void *key, void *base, size_t *nmemb, size_t size, comparison_fn_t compar)
double lgamma (double x)
float lgammaf (float x)
float lgammaf_r (float x, int *signp)
long double lgammal (long double x)
long double lgammal_r (long double x, int *signp)
double lgamma_r (double x, int *signp)
L_INCR
int LINE_MAX
int link (const char *oldname, const char *newname)
int LINK_MAX
int lio_listio (int mode, struct aiocb *const list[], int nent, struct sigevent *sig)
int lio_listio64 (int mode, struct aiocb *const list, int nent, struct sigevent *sig)
int listen (int socket, unsigned int n)
long long int llabs (long long int number)
lldiv_t lldiv (long long int numerator, long long int denominator)
lldiv_t
long long int llrint (double x)
long long int llrintf (float x)
long long int llrintl (long double x)
long long int llround (double x)
long long int llroundf (float x)
long long int llroundl (long double x)
struct lconv * localeconv (void)
localeconv: It is portable but ....
struct tm * localtime (const time_t *time)
struct tm * localtime_r (const time_t *time, struct tm *resultp)
double log (double x)
double log10 (double x)
float log10f (float x)
long double log10l (long double x)
double log1p (double x)
float log1pf (float x)
long double log1pl (long double x)
double log2 (double x)
float log2f (float x)
long double log2l (long double x)
double logb (double x)
double logb (double x)
float logbf (float x)
float logbf (float x)
long double logbl (long double x)
long double logbl (long double x)
float logf (float x)
void login (const struct utmp *entry)
LOGIN_PROCESS
LOGIN_PROCESS
int login_tty (int filedes)
long double logl (long double x)
int logout (const char *ut_line)
void logwtmp (const char *ut_line, const char *ut_name, const char *ut_host)
void longjmp (jmp_buf state, int value)
LONG_LONG_MAX
LONG_LONG_MIN
LONG_MAX
LONG_MIN
long int lrand48 (void)
int lrand48_r (struct drand48_data *buffer, double *result)
long int lrint (double x)
long int lrintf (float x)
long int lrintl (long double x)
long int lround (double x)
long int lroundf (float x)
long int lroundl (long double x)
void * lsearch (const void *key, void *base, size_t *nmemb, size_t size, comparison_fn_t compar)
off_t lseek (int filedes, off_t offset, int whence)
off64_t lseek64 (int filedes, off64_t offset, int whence)
L_SET
int lstat (const char *filename, struct stat *buf)
int lstat64 (const char *filename, struct stat64 *buf)
int L_tmpnam
L_XTND
void makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
struct mallinfo mallinfo (void)
malloc.
void * malloc (size_t size)
__malloc_hook
__malloc_initialize_hook
int MAX_CANON
int MAX_INPUT
int MAXNAMLEN
int MAXSYMLINKS
int MB_CUR_MAX
int mblen (const char *string, size_t size)
int MB_LEN_MAX
size_t mbrlen (const char *restrict s, size_t n, mbstate_t *ps)
size_t mbrtowc (wchar_t *restrict pwc, const char *restrict s, size_t n, mbstate_t *restrict ps)
int mbsinit (const mbstate_t *ps)
size_t mbsnrtowcs (wchar_t *restrict dst, const char **restrict src, size_t nmc, size_t len, mbstate_t *restrict ps)
size_t mbsrtowcs (wchar_t *restrict dst, const char **restrict src, size_t len, mbstate_t *restrict ps)
mbstate_t
size_t mbstowcs (wchar_t *wstring, const char *string, size_t size)
int mbtowc (wchar_t *restrict result, const char *restrict string, size_t size)
int mcheck (void (*abortfn) (enum mcheck_status status))
tcflag_t MDMBUF
void * memalign (size_t boundary, size_t size)
__memalign_hook
void * memccpy (void *restrict to, const void *restrict from, int c, size_t size)
void * memchr (const void *block, int c, size_t size)
int memcmp (const void *a1, const void *a2, size_t size)
void * memcpy (void *restrict to, const void *restrict from, size_t size)
void * memfrob (void *mem, size_t length)
void * memmem (const void *haystack, size_t haystack-len,
const void *needle, size_t needle-len)
void * memmove (void *to, const void *from, size_t size)
void * mempcpy (void *restrict to, const void *restrict from, size_t size)
void * memrchr (const void *block, int c, size_t size)
void * memset (void *block, int c, size_t size)
int mkdir (const char *filename, mode_t mode)
char * mkdtemp (char *template)
int mkfifo (const char *filename, mode_t mode)
int mknod (const char *filename, int mode, int dev)
int mkstemp (char *template)
char * mktemp (char *template)
time_t mktime (struct tm *brokentime)
int mlock (const void *addr, size_t len)
int mlockall (int flags)
void * mmap (void *address, size_t length,int protect, int flags, int filedes, off_t offset)
void * mmap64 (void *address, size_t length,int protect, int flags, int filedes, off64_t offset)
mode_t
double modf (double value, double *integer-part)
float modff (float value, float *integer-part)
long double modfl (long double value, long double *integer-part)
int mount (const char *special_file, const char *dir, const char *fstype, unsigned long int options, const void *data)
long int mrand48 (void)
int mrand48_r (struct drand48_data *buffer, double *result)
void * mremap (void *address, size_t length, size_t new_length, int flag)
int MSG_DONTROUTE
int MSG_OOB
int MSG_PEEK
int msync (void *address, size_t length, int flags)
void mtrace (void)
int munlock (const void *addr, size_t len)
int munlockall (void)
int munmap (void *addr, size_t length)
void muntrace (void)
int NAME_MAX
float NAN
double nan (const char *tagp)
float nanf (const char *tagp)
long double nanl (const char *tagp)
int nanosleep (const struct timespec *requested_time, struct timespec *remaining)
int NCCS
double nearbyint (double x)
float nearbyintf (float x)
long double nearbyintl (long double x)
NEW_TIME
NEW_TIME
double nextafter (double x, double y)
float nextafterf (float x, float y)
long double nextafterl (long double x, long double y)
double nexttoward (double x, long double y)
float nexttowardf (float x, long double y)
long double nexttowardl (long double x, long double y)
int nftw (const char *filename, __nftw_func_t func, int descriptors, int flag)
int nftw64 (const char *filename, __nftw64_func_t func, int descriptors, int flag)
__nftw64_func_t
__nftw_func_t
char * ngettext (const char *msgid1, const char *msgid2, unsigned long int n)
int NGROUPS_MAX
int nice (int increment)
nlink_t
char * nl_langinfo (nl_item item)
NO_ADDRESS
tcflag_t NOFLSH
tcflag_t NOKERNINFO
NO_RECOVERY
long int nrand48 (unsigned short int xsubi[3])
int nrand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, long int *result)
int NSIG
uint32_t ntohl (uint32_t netlong)
uint16_t ntohs (uint16_t netshort)
int ntp_adjtime (struct timex *tptr)
int ntp_gettime (struct ntptimeval *tptr)
void * NULL
int O_ACCMODE
int O_APPEND
int O_ASYNC
void obstack_1grow (struct obstack *obstack-ptr, char c)
void obstack_1grow_fast (struct obstack *obstack-ptr, char c)
int obstack_alignment_mask (struct obstack *obstack-ptr)
void * obstack_alloc (struct obstack *obstack-ptr, int size)
obstack_alloc_failed_handler
void * obstack_base (struct obstack *obstack-ptr)
void obstack_blank (struct obstack *obstack-ptr, int size)
void obstack_blank_fast (struct obstack *obstack-ptr, int size)
int obstack_chunk_size (struct obstack *obstack-ptr)
void * obstack_copy (struct obstack *obstack-ptr, void *address, int size)
void * obstack_copy0 (struct obstack *obstack-ptr, void *address, int size)
void * obstack_finish (struct obstack *obstack-ptr)
void obstack_free (struct obstack *obstack-ptr, void *object)
void obstack_grow (struct obstack *obstack-ptr, void *data, int size)
void obstack_grow0 (struct obstack *obstack-ptr, void *data, int size)
int obstack_init (struct obstack *obstack-ptr)
void obstack_int_grow (struct obstack *obstack-ptr, int data)
void obstack_int_grow_fast (struct obstack *obstack-ptr, int data)
void * obstack_next_free (struct obstack *obstack-ptr)
int obstack_object_size (struct obstack *obstack-ptr)
int obstack_object_size (struct obstack *obstack-ptr)
int obstack_printf (struct obstack *obstack, const char *template, ...)
void obstack_ptr_grow (struct obstack *obstack-ptr, void *data)
void obstack_ptr_grow_fast (struct obstack *obstack-ptr, void *data)
int obstack_room (struct obstack *obstack-ptr)
int obstack_vprintf (struct obstack *obstack, const char *template, va_list ap)
int O_CREAT
int O_EXCL
int O_EXEC
int O_EXLOCK
off64_t
size_t offsetof (type, member)
off_t
int O_FSYNC
int O_IGNORE_CTTY
OLD_TIME
OLD_TIME
int O_NDELAY
int on_exit (void (*function)(int status, void *arg), void *arg)
tcflag_t ONLCR
int O_NOATIME
int O_NOCTTY
tcflag_t ONOEOT
int O_NOLINK
int O_NONBLOCK
int O_NONBLOCK
int O_NOTRANS
int open (const char *filename, int flags[, mode_t mode])
int open64 (const char *filename, int flags[, mode_t mode])
DIR * opendir (const char *dirname)
void openlog (char *ident, int option,
int OPEN_MAX
FILE * open_memstream (char **ptr, size_t *sizeloc)
FILE * open_obstack_stream (struct obstack *obstack)
int openpty (int *amaster, int *aslave, char *name, struct termios *termp, struct winsize *winp)
tcflag_t OPOST
char * optarg
getopt function.
int opterr
getopt function.
int optind
getopt function.
OPTION_ALIAS
OPTION_ARG_OPTIONAL
OPTION_DOC
OPTION_HIDDEN
OPTION_NO_USAGE
int optopt
getopt function.
int O_RDONLY
int O_RDWR
int O_READ
int O_SHLOCK
int O_SYNC
int O_TRUNC
int O_WRITE
int O_WRONLY
tcflag_t OXTABS
PA_CHAR
PA_DOUBLE
PA_FLAG_LONG
PA_FLAG_LONG_DOUBLE
PA_FLAG_LONG_LONG
int PA_FLAG_MASK
PA_FLAG_PTR
PA_FLAG_SHORT
PA_FLOAT
PA_INT
PA_LAST
PA_POINTER
tcflag_t PARENB
tcflag_t PARMRK
tcflag_t PARODD
size_t parse_printf_format (const char *template, size_t n, int *argtypes)
PA_STRING
long int pathconf (const char *filename, int parameter)
pathconf.
int PATH_MAX
int pause ()
pause.
_PC_ASYNC_IO
pathconf.
_PC_CHOWN_RESTRICTED
pathconf.
_PC_FILESIZEBITS
pathconf.
_PC_LINK_MAX
pathconf.
int pclose (FILE *stream)
_PC_MAX_CANON
pathconf.
_PC_MAX_INPUT
pathconf.
_PC_NAME_MAX
pathconf.
_PC_NO_TRUNC
pathconf.
_PC_PATH_MAX
pathconf.
_PC_PIPE_BUF
pathconf.
_PC_PRIO_IO
pathconf.
_PC_REC_INCR_XFER_SIZE
pathconf.
_PC_REC_MAX_XFER_SIZE
pathconf.
_PC_REC_MIN_XFER_SIZE
pathconf.
_PC_REC_XFER_ALIGN
pathconf.
_PC_SOCK_MAXBUF
pathconf.
_PC_SYNC_IO
pathconf.
_PC_VDISABLE
pathconf.
tcflag_t PENDIN
void perror (const char *message)
int PF_FILE
int PF_INET
int PF_INET6
int PF_LOCAL
int PF_UNIX
pid_t
int pipe (int filedes[2])
int PIPE_BUF
FILE * popen (const char *command, const char *mode)
_POSIX2_BC_BASE_MAX
_POSIX2_BC_DIM_MAX
_POSIX2_BC_SCALE_MAX
_POSIX2_BC_STRING_MAX
int _POSIX2_C_DEV
_POSIX2_COLL_WEIGHTS_MAX
long int _POSIX2_C_VERSION
_POSIX2_EQUIV_CLASS_MAX
_POSIX2_EXPR_NEST_MAX
int _POSIX2_FORT_DEV
int _POSIX2_FORT_RUN
_POSIX2_LINE_MAX
int _POSIX2_LOCALEDEF
_POSIX2_RE_DUP_MAX
int _POSIX2_SW_DEV
_POSIX_AIO_LISTIO_MAX
_POSIX_AIO_MAX
_POSIX_ARG_MAX
_POSIX_CHILD_MAX
int _POSIX_CHOWN_RESTRICTED
_POSIX_C_SOURCE
int _POSIX_JOB_CONTROL
_POSIX_LINK_MAX
_POSIX_MAX_CANON
_POSIX_MAX_INPUT
int posix_memalign (void **memptr, size_t alignment, size_t size)
_POSIX_NAME_MAX
_POSIX_NGROUPS_MAX
int _POSIX_NO_TRUNC
_POSIX_OPEN_MAX
_POSIX_PATH_MAX
_POSIX_PIPE_BUF
POSIX_REC_INCR_XFER_SIZE
POSIX_REC_MAX_XFER_SIZE
POSIX_REC_MIN_XFER_SIZE
POSIX_REC_XFER_ALIGN
int _POSIX_SAVED_IDS
_POSIX_SOURCE
_POSIX_SSIZE_MAX
_POSIX_STREAM_MAX
_POSIX_TZNAME_MAX
unsigned char _POSIX_VDISABLE
long int _POSIX_VERSION
double pow (double base, double power)
double pow10 (double x)
float pow10f (float x)
long double pow10l (long double x)
float powf (float base, float power)
long double powl (long double base, long double power)
ssize_t pread (int filedes, void *buffer, size_t size, off_t offset)
ssize_t pread64 (int filedes, void *buffer, size_t size, off64_t offset)
int printf (const char *template, ...)
printf_arginfo_function
printf_function
int printf_size (FILE *fp, const struct printf_info *info, const void *const *args)
printf Handlers.
int printf_size_info (const struct printf_info *info, size_t n, int *argtypes)
printf Handlers.
PRIO_MAX
PRIO_MIN
PRIO_PGRP
PRIO_PROCESS
PRIO_USER
char * program_invocation_name
char * program_invocation_short_name
void psignal (int signum, const char *message)
char * P_tmpdir
ptrdiff_t
char * ptsname (int filedes)
int ptsname_r (int filedes, char *buf, size_t len)
int putc (int c, FILE *stream)
int putchar (int c)
int putchar_unlocked (int c)
int putc_unlocked (int c, FILE *stream)
int putenv (char *string)
int putpwent (const struct passwd *p, FILE *stream)
int puts (const char *s)
struct utmp * pututline (const struct utmp *utmp)
struct utmpx * pututxline (const struct utmpx *utmp)
int putw (int w, FILE *stream)
wint_t putwc (wchar_t wc, FILE *stream)
wint_t putwchar (wchar_t wc)
wint_t putwchar_unlocked (wchar_t wc)
wint_t putwc_unlocked (wchar_t wc, FILE *stream)
ssize_t pwrite (int filedes, const void *buffer, size_t size, off_t offset)
ssize_t pwrite64 (int filedes, const void *buffer, size_t size, off64_t offset)
char * qecvt (long double value, int ndigit, int *decpt, int *neg)
char * qecvt_r (long double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
char * qfcvt (long double value, int ndigit, int *decpt, int *neg)
char * qfcvt_r (long double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
char * qgcvt (long double value, int ndigit, char *buf)
void qsort (void *array, size_t count, size_t size, comparison_fn_t compare)
int raise (int signum)
int rand (void)
int RAND_MAX
long int random (void)
int random_r (struct random_data *restrict buf, int32_t *restrict result)
int rand_r (unsigned int *seed)
void * rawmemchr (const void *block, int c)
ssize_t read (int filedes, void *buffer, size_t size)
struct dirent * readdir (DIR *dirstream)
struct dirent64 * readdir64 (DIR *dirstream)
int readdir64_r (DIR *dirstream, struct dirent64 *entry, struct dirent64 **result)
int readdir_r (DIR *dirstream, struct dirent *entry, struct dirent **result)
int readlink (const char *filename, char *buffer, size_t size)
ssize_t readv (int filedes, const struct iovec *vector, int count)
void * realloc (void *ptr, size_t newsize)
__realloc_hook
char * realpath (const char *restrict name, char *restrict resolved)
int recv (int socket, void *buffer, size_t size, int flags)
int recvfrom (int socket, void *buffer, size_t size, int flags, struct sockaddr *addr, socklen_t *length-ptr)
int recvmsg (int socket, struct msghdr *message, int flags)
int RE_DUP_MAX
_REENTRANT
REG_BADBR
REG_BADPAT
REG_BADRPT
int regcomp (regex_t *compiled, const char *pattern, int cflags)
REG_EBRACE
REG_EBRACK
REG_ECOLLATE
REG_ECTYPE
REG_EESCAPE
REG_EPAREN
REG_ERANGE
size_t regerror (int errcode, regex_t *compiled, char *buffer, size_t length)
REG_ESPACE
REG_ESPACE
REG_ESUBREG
int regexec (regex_t *compiled, char *string, size_t nmatch, regmatch_t matchptr [], int eflags)
regex_t
REG_EXTENDED
void regfree (regex_t *compiled)
REG_ICASE
int register_printf_function (int spec, printf_function handler-function, printf_arginfo_function arginfo-function)
regmatch_t
REG_NEWLINE
REG_NOMATCH
REG_NOSUB
REG_NOTBOL
REG_NOTEOL
regoff_t
double remainder (double numerator, double denominator)
float remainderf (float numerator, float denominator)
long double remainderl (long double numerator, long double denominator)
int remove (const char *filename)
int rename (const char *oldname, const char *newname)
void rewind (FILE *stream)
void rewinddir (DIR *dirstream)
char * rindex (const char *string, int c)
double rint (double x)
float rintf (float x)
long double rintl (long double x)
int RLIM_INFINITY
RLIMIT_AS
RLIMIT_CORE
RLIMIT_CPU
RLIMIT_DATA
RLIMIT_FSIZE
RLIMIT_MEMLOCK
RLIMIT_NOFILE
RLIMIT_NPROC
RLIMIT_RSS
RLIMIT_STACK
RLIM_NLIMITS
int rmdir (const char *filename)
int R_OK
double round (double x)
float roundf (float x)
long double roundl (long double x)
RUN_LVL
RUN_LVL
RUSAGE_CHILDREN
RUSAGE_SELF
int SA_NOCLDSTOP
sigaction.
int SA_ONSTACK
sigaction.
int SA_RESTART
sigaction.
int sbrk (ptrdiff_t delta)
_SC_2_C_DEV
sysconf Parameters.
_SC_2_FORT_DEV
sysconf Parameters.
_SC_2_FORT_RUN
sysconf Parameters.
_SC_2_LOCALEDEF
sysconf Parameters.
_SC_2_SW_DEV
sysconf Parameters.
_SC_2_VERSION
sysconf Parameters.
_SC_AIO_LISTIO_MAX
sysconf Parameters.
_SC_AIO_MAX
sysconf Parameters.
_SC_AIO_PRIO_DELTA_MAX
sysconf Parameters.
double scalb (double value, int exponent)
float scalbf (float value, int exponent)
long double scalbl (long double value, int exponent)
long long int scalbln (double x, long int n)
long long int scalblnf (float x, long int n)
long long int scalblnl (long double x, long int n)
long long int scalbn (double x, int n)
long long int scalbnf (float x, int n)
long long int scalbnl (long double x, int n)
int scandir (const char *dir, struct dirent ***namelist, int (*selector) (const struct dirent *), int (*cmp) (const void *, const void *))
int scandir64 (const char *dir, struct dirent64 ***namelist, int (*selector) (const struct dirent64 *), int (*cmp) (const void *, const void *))
int scanf (const char *template, ...)
_SC_ARG_MAX
sysconf Parameters.
_SC_ASYNCHRONOUS_IO
sysconf Parameters.
_SC_ATEXIT_MAX
sysconf Parameters.
_SC_AVPHYS_PAGES
sysconf Parameters.
_SC_BC_BASE_MAX
sysconf Parameters.
_SC_BC_DIM_MAX
sysconf Parameters.
_SC_BC_SCALE_MAX
sysconf Parameters.
_SC_BC_STRING_MAX
sysconf Parameters.
_SC_CHAR_BIT
sysconf Parameters.
_SC_CHARCLASS_NAME_MAX
sysconf Parameters.
_SC_CHAR_MAX
sysconf Parameters.
_SC_CHAR_MIN
sysconf Parameters.
_SC_CHILD_MAX
sysconf Parameters.
_SC_CLK_TCK
sysconf Parameters.
_SC_COLL_WEIGHTS_MAX
sysconf Parameters.
_SC_DELAYTIMER_MAX
sysconf Parameters.
_SC_EQUIV_CLASS_MAX
sysconf Parameters.
_SC_EXPR_NEST_MAX
sysconf Parameters.
_SC_FSYNC
sysconf Parameters.
_SC_GETGR_R_SIZE_MAX
sysconf Parameters.
_SC_GETPW_R_SIZE_MAX
sysconf Parameters.
SCHAR_MAX
SCHAR_MIN
int sched_getparam (pid_t pid, const struct sched_param *param)
int sched_get_priority_max (int *policy);
int sched_get_priority_min (int *policy);
int sched_getscheduler (pid_t pid)
int sched_rr_get_interval (pid_t pid, struct timespec *interval)
int sched_setparam (pid_t pid, const struct sched_param *param)
int sched_setscheduler (pid_t pid, int policy, const struct sched_param *param)
int sched_yield (void)
_SC_INT_MAX
sysconf Parameters.
_SC_INT_MIN
sysconf Parameters.
_SC_JOB_CONTROL
sysconf Parameters.
_SC_LINE_MAX
sysconf Parameters.
_SC_LOGIN_NAME_MAX
sysconf Parameters.
_SC_LONG_BIT
sysconf Parameters.
_SC_MAPPED_FILES
sysconf Parameters.
_SC_MB_LEN_MAX
sysconf Parameters.
_SC_MEMLOCK
sysconf Parameters.
_SC_MEMLOCK_RANGE
sysconf Parameters.
_SC_MEMORY_PROTECTION
sysconf Parameters.
_SC_MESSAGE_PASSING
sysconf Parameters.
_SC_MQ_OPEN_MAX
sysconf Parameters.
_SC_MQ_PRIO_MAX
sysconf Parameters.
_SC_NGROUPS_MAX
sysconf Parameters.
_SC_NL_ARGMAX
sysconf Parameters.
_SC_NL_LANGMAX
sysconf Parameters.
_SC_NL_MSGMAX
sysconf Parameters.
_SC_NL_NMAX
sysconf Parameters.
_SC_NL_SETMAX
sysconf Parameters.
_SC_NL_TEXTMAX
sysconf Parameters.
_SC_NPROCESSORS_CONF
sysconf Parameters.
_SC_NPROCESSORS_ONLN
sysconf Parameters.
_SC_NZERO
sysconf Parameters.
_SC_OPEN_MAX
sysconf Parameters.
_SC_PAGESIZE
sysconf Parameters.
_SC_PHYS_PAGES
sysconf Parameters.
_SC_PII
sysconf Parameters.
_SC_PII_INTERNET
sysconf Parameters.
_SC_PII_INTERNET_DGRAM
sysconf Parameters.
_SC_PII_INTERNET_STREAM
sysconf Parameters.
_SC_PII_OSI
sysconf Parameters.
_SC_PII_OSI_CLTS
sysconf Parameters.
_SC_PII_OSI_COTS
sysconf Parameters.
_SC_PII_OSI_M
sysconf Parameters.
_SC_PII_SOCKET
sysconf Parameters.
_SC_PII_XTI
sysconf Parameters.
_SC_PRIORITIZED_IO
sysconf Parameters.
_SC_PRIORITY_SCHEDULING
sysconf Parameters.
_SC_REALTIME_SIGNALS
sysconf Parameters.
_SC_RTSIG_MAX
sysconf Parameters.
_SC_SAVED_IDS
sysconf Parameters.
_SC_SCHAR_MAX
sysconf Parameters.
_SC_SCHAR_MIN
sysconf Parameters.
_SC_SELECT
sysconf Parameters.
_SC_SEMAPHORES
sysconf Parameters.
_SC_SEM_NSEMS_MAX
sysconf Parameters.
_SC_SEM_VALUE_MAX
sysconf Parameters.
_SC_SHARED_MEMORY_OBJECTS
sysconf Parameters.
_SC_SHRT_MAX
sysconf Parameters.
_SC_SHRT_MIN
sysconf Parameters.
_SC_SIGQUEUE_MAX
sysconf Parameters.
SC_SSIZE_MAX
sysconf Parameters.
_SC_STREAM_MAX
sysconf Parameters.
_SC_SYNCHRONIZED_IO
sysconf Parameters.
_SC_THREAD_ATTR_STACKADDR
sysconf Parameters.
_SC_THREAD_ATTR_STACKSIZE
sysconf Parameters.
_SC_THREAD_DESTRUCTOR_ITERATIONS
sysconf Parameters.
_SC_THREAD_KEYS_MAX
sysconf Parameters.
_SC_THREAD_PRIO_INHERIT
sysconf Parameters.
_SC_THREAD_PRIO_PROTECT
sysconf Parameters.
_SC_THREAD_PRIORITY_SCHEDULING
sysconf Parameters.
_SC_THREAD_PROCESS_SHARED
sysconf Parameters.
_SC_THREADS
sysconf Parameters.
_SC_THREAD_SAFE_FUNCTIONS
sysconf Parameters.
_SC_THREAD_STACK_MIN
sysconf Parameters.
_SC_THREAD_THREADS_MAX
sysconf Parameters.
_SC_TIMER_MAX
sysconf Parameters.
_SC_TIMERS
sysconf Parameters.
_SC_T_IOV_MAX
sysconf Parameters.
_SC_TTY_NAME_MAX
sysconf Parameters.
_SC_TZNAME_MAX
sysconf Parameters.
_SC_UCHAR_MAX
sysconf Parameters.
_SC_UINT_MAX
sysconf Parameters.
_SC_UIO_MAXIOV
sysconf Parameters.
_SC_ULONG_MAX
sysconf Parameters.
_SC_USHRT_MAX
sysconf Parameters.
_SC_VERSION
sysconf Parameters.
_SC_VERSION
sysconf Parameters.
_SC_WORD_BIT
sysconf Parameters.
_SC_XOPEN_CRYPT
sysconf Parameters.
_SC_XOPEN_ENH_I18N
sysconf Parameters.
_SC_XOPEN_LEGACY
sysconf Parameters.
_SC_XOPEN_REALTIME
sysconf Parameters.
_SC_XOPEN_REALTIME_THREADS
sysconf Parameters.
_SC_XOPEN_SHM
sysconf Parameters.
_SC_XOPEN_UNIX
sysconf Parameters.
_SC_XOPEN_VERSION
sysconf Parameters.
_SC_XOPEN_XCU_VERSION
sysconf Parameters.
_SC_XOPEN_XPG2
sysconf Parameters.
_SC_XOPEN_XPG3
sysconf Parameters.
_SC_XOPEN_XPG4
sysconf Parameters.
unsigned short int * seed48 (unsigned short int seed16v[3])
int seed48_r (unsigned short int seed16v[3], struct drand48_data *buffer)
int SEEK_CUR
void seekdir (DIR *dirstream, off_t pos)
int SEEK_END
int SEEK_SET
int select (int nfds, fd_set *read-fds, fd_set *write-fds, fd_set *except-fds, struct timeval *timeout)
int send (int socket, void *buffer, size_t size, int flags)
int sendmsg (int socket, const struct msghdr *message, int flags)
int sendto (int socket, void *buffer. size_t size, int flags, struct sockaddr *addr, socklen_t length)
void setbuf (FILE *stream, char *buf)
void setbuffer (FILE *stream, char *buf, size_t size)
int setcontext (const ucontext_t *ucp)
int setdomainname (const char *name, size_t length)
int setegid (gid_t newgid)
int setenv (const char *name, const char *value, int replace)
int seteuid (uid_t neweuid)
int setfsent (void)
int setgid (gid_t newgid)
void setgrent (void)
int setgroups (size_t count, gid_t *groups)
void sethostent (int stayopen)
int sethostid (long int id)
int sethostname (const char *name, size_t length)
int setitimer (int which, struct itimerval *new, struct itimerval *old)
int setjmp (jmp_buf state)
void setkey (const char *key)
void setkey_r (const char *key, struct crypt_data * data)
void setlinebuf (FILE *stream)
char * setlocale (int category, const char *locale)
int setlogmask (int mask)
FILE * setmntent (const char *file, const char *mode)
void setnetent (int stayopen)
int setnetgrent (const char *netgroup)
int setpgid (pid_t pid, pid_t pgid)
int setpgrp (pid_t pid, pid_t pgid)
int setpriority (int class, int id, int niceval)
void setprotoent (int stayopen)
void setpwent (void)
int setregid (gid_t rgid, gid_t egid)
int setreuid (uid_t ruid, uid_t euid)
int setrlimit (int resource, const struct rlimit *rlp)
int setrlimit64 (int resource, const struct rlimit64 *rlp)
void setservent (int stayopen)
pid_t setsid (void)
int setsockopt (int socket, int level, int optname, void *optval, socklen_t optlen)
void * setstate (void *state)
int setstate_r (char *restrict statebuf, struct random_data *restrict buf)
int settimeofday (const struct timeval *tp, const struct timezone *tzp)
int setuid (uid_t newuid)
void setutent (void)
void setutxent (void)
int setvbuf (FILE *stream, char *buf, int mode, size_t size)
SHRT_MAX
SHRT_MIN
int shutdown (int socket, int how)
S_IEXEC
S_IFBLK
S_IFCHR
S_IFDIR
S_IFIFO
S_IFLNK
int S_IFMT
S_IFREG
S_IFSOCK
int SIGABRT
int sigaction (int signum, const struct sigaction *restrict action, struct sigaction *restrict old-action)
int sigaddset (sigset_t *set, int signum)
int SIGALRM
int sigaltstack (const stack_t *restrict stack, stack_t *restrict oldstack)
sig_atomic_t
SIG_BLOCK
int sigblock (int mask)
int SIGBUS
int SIGCHLD
int SIGCLD
int SIGCONT
int sigdelset (sigset_t *set, int signum)
int sigemptyset (sigset_t *set)
int SIGEMT
sighandler_t SIG_ERR
int sigfillset (sigset_t *set)
int SIGFPE
sighandler_t
int SIGHUP
int SIGILL
int SIGINFO
int SIGINT
int siginterrupt (int signum, int failflag)
int SIGIO
int SIGIOT
int sigismember (const sigset_t *set, int signum)
sigjmp_buf
int SIGKILL
void siglongjmp (sigjmp_buf state, int value)
int SIGLOST
int sigmask (int signum)
sighandler_t signal (int signum, sighandler_t action)
int signbit (float-type x)
long long int significand (double x)
long long int significandf (float x)
long long int significandl (long double x)
int sigpause (int mask)
int sigpending (sigset_t *set)
int SIGPIPE
int SIGPOLL
int sigprocmask (int how, const sigset_t *restrict set, sigset_t *restrict oldset)
int SIGPROF
int SIGQUIT
int SIGSEGV
int sigsetjmp (sigjmp_buf state, int savesigs)
SIG_SETMASK
int sigsetmask (int mask)
sigset_t
int sigstack (const struct sigstack *stack, struct sigstack *oldstack)
int SIGSTOP
int sigsuspend (const sigset_t *set)
sigsuspend.
int SIGSYS
int SIGTERM
int SIGTRAP
int SIGTSTP
int SIGTTIN
int SIGTTOU
SIG_UNBLOCK
int SIGURG
int SIGUSR1
int SIGUSR2
int sigvec (int signum, const struct sigvec *action,struct sigvec *old-action)
int SIGVTALRM
int SIGWINCH
int SIGXCPU
int SIGXFSZ
double sin (double x)
void sincos (double x, double *sinx, double *cosx)
void sincosf (float x, float *sinx, float *cosx)
void sincosl (long double x, long double *sinx, long double *cosx)
float sinf (float x)
double sinh (double x)
float sinhf (float x)
long double sinhl (long double x)
long double sinl (long double x)
S_IREAD
S_IRGRP
S_IROTH
S_IRUSR
S_IRWXG
S_IRWXO
S_IRWXU
int S_ISBLK (mode_t m)
int S_ISCHR (mode_t m)
int S_ISDIR (mode_t m)
int S_ISFIFO (mode_t m)
S_ISGID
int S_ISLNK (mode_t m)
int S_ISREG (mode_t m)
int S_ISSOCK (mode_t m)
S_ISUID
S_ISVTX
S_IWGRP
S_IWOTH
S_IWRITE
S_IWUSR
S_IXGRP
S_IXOTH
S_IXUSR
size_t
unsigned int sleep (unsigned int seconds)
int snprintf (char *s, size_t size, const char *template, ...)
SO_BROADCAST
int SOCK_DGRAM
int socket (int namespace, int style, int protocol)
int socketpair (int namespace, int style, int protocol, int filedes[2])
int SOCK_RAW
int SOCK_RDM
int SOCK_SEQPACKET
int SOCK_STREAM
SO_DEBUG
SO_DONTROUTE
SO_ERROR
SO_KEEPALIVE
SO_LINGER
int SOL_SOCKET
SO_OOBINLINE
SO_RCVBUF
SO_REUSEADDR
SO_SNDBUF
SO_STYLE
SO_TYPE
speed_t
int sprintf (char *s, const char *template, ...)
double sqrt (double x)
float sqrtf (float x)
long double sqrtl (long double x)
void srand (unsigned int seed)
void srand48 (long int seedval)
int srand48_r (long int seedval, struct drand48_data *buffer)
void srandom (unsigned int seed)
int srandom_r (unsigned int seed, struct random_data *buf)
int sscanf (const char *s, const char *template, ...)
sighandler_t ssignal (int signum, sighandler_t action)
int SSIZE_MAX
ssize_t
stack_t
int stat (const char *filename, struct stat *buf)
int stat64 (const char *filename, struct stat64 *buf)
FILE * stderr
STDERR_FILENO
FILE * stdin
STDIN_FILENO
FILE * stdout
STDOUT_FILENO
int stime (time_t *newtime)
char * stpcpy (char *restrict to, const char *restrict from)
char * stpncpy (char *restrict to, const char *restrict from, size_t size)
int strcasecmp (const char *s1, const char *s2)
char * strcasestr (const char *haystack, const char *needle)
char * strcat (char *restrict to, const char *restrict from)
char * strchr (const char *string, int c)
char * strchrnul (const char *string, int c)
int strcmp (const char *s1, const char *s2)
int strcoll (const char *s1, const char *s2)
char * strcpy (char *restrict to, const char *restrict from)
size_t strcspn (const char *string, const char *stopset)
char * strdup (const char *s)
char * strdupa (const char *s)
int STREAM_MAX
char * strerror (int errnum)
char * strerror_r (int errnum, char *buf, size_t n)
char * strfry (char *string)
size_t strftime (char *s, size_t size, const char *template, const struct tm *brokentime)
size_t strlen (const char *s)
int strncasecmp (const char *s1, const char *s2, size_t n)
char * strncat (char *restrict to, const char *restrict from, size_t size)
int strncmp (const char *s1, const char *s2, size_t size)
char * strncpy (char *restrict to, const char *restrict from, size_t size)
char * strndup (const char *s, size_t size)
char * strndupa (const char *s, size_t size)
size_t strnlen (const char *s, size_t maxlen)
char * strpbrk (const char *string, const char *stopset)
char * strptime (const char *s, const char *fmt, struct tm *tp)
char * strrchr (const char *string, int c)
char * strsep (char **string_ptr, const char *delimiter)
char * strsignal (int signum)
size_t strspn (const char *string, const char *skipset)
char * strstr (const char *haystack, const char *needle)
double strtod (const char *restrict string, char **restrict tailptr)
float strtof (const char *string, char **tailptr)
intmax_t strtoimax (const char *restrict string, char **restrict tailptr, int base)
char * strtok (char *restrict newstring, const char *restrict delimiters)
char * strtok_r (char *newstring, const char *delimiters, char **save_ptr)
long int strtol (const char *restrict string, char **restrict tailptr, int base)
long double strtold (const char *string, char **tailptr)
long long int strtoll (const char *restrict string, char **restrict tailptr, int base)
long long int strtoq (const char *restrict string, char **restrict tailptr, int base)
unsigned long int strtoul (const char *retrict string, char **restrict tailptr, int base)
unsigned long long int strtoull (const char *restrict string, char **restrict tailptr, int base)
uintmax_t strtoumax (const char *restrict string, char **restrict tailptr, int base)
unsigned long long int strtouq (const char *restrict string, char **restrict tailptr, int base)
struct aiocb
struct aiocb64
struct aioinit
struct argp
struct argp_child
struct argp_option
struct argp_state
struct dirent
struct exit_status
struct flock
struct fstab
struct FTW
struct __gconv_step
iconv Implementation in the GNU C library.
struct __gconv_step_data
iconv Implementation in the GNU C library.
struct group
struct hostent
struct if_nameindex
struct in6_addr
struct in_addr
struct iovec
struct itimerval
struct lconv
localeconv: It is portable but ....
struct linger
struct mallinfo
malloc.
struct mntent
struct msghdr
struct netent
struct obstack
struct option
getopt_long.
struct passwd
struct printf_info
struct protoent
struct random_data
struct rlimit
struct rlimit64
struct rusage
struct sched_param
struct servent
struct sgttyb
struct sigaction
struct sigstack
struct sigvec
struct sockaddr
struct sockaddr_in
struct sockaddr_un
struct stat
struct stat64
struct termios
struct timespec
struct timeval
struct timezone
struct tm
struct tms
struct utimbuf
struct utsname
int strverscmp (const char *s1, const char *s2)
size_t strxfrm (char *restrict to, const char *restrict from, size_t size)
int stty (int filedes, struct sgttyb * attributes)
int S_TYPEISMQ (struct stat *s)
int S_TYPEISSEM (struct stat *s)
int S_TYPEISSHM (struct stat *s)
int SUN_LEN (struct sockaddr_un * ptr)
_SVID_SOURCE
int SV_INTERRUPT
int SV_ONSTACK
int SV_RESETHAND
int swapcontext (ucontext_t *restrict oucp, const ucontext_t *restrict ucp)
int swprintf (wchar_t *s, size_t size, const wchar_t *template, ...)
int swscanf (const wchar_t *ws, const char *template, ...)
int symlink (const char *oldname, const char *newname)
SYMLINK_MAX
int sync (void)
long int syscall (long int sysno, ...)
long int sysconf (int parameter)
sysconf.
int sysctl (int *names, int nlen, void *oldval,
void syslog (int facility_priority, char *format, ...)
int system (const char *command)
sighandler_t sysv_signal (int signum, sighandler_t action)
double tan (double x)
float tanf (float x)
double tanh (double x)
float tanhf (float x)
long double tanhl (long double x)
long double tanl (long double x)
int tcdrain (int filedes)
tcflag_t
int tcflow (int filedes, int action)
int tcflush (int filedes, int queue)
int tcgetattr (int filedes, struct termios *termios-p)
pid_t tcgetpgrp (int filedes)
pid_t tcgetsid (int fildes)
TCSADRAIN
TCSAFLUSH
TCSANOW
TCSASOFT
int tcsendbreak (int filedes, int duration)
int tcsetattr (int filedes, int when, const struct termios *termios-p)
int tcsetpgrp (int filedes, pid_t pgid)
void * tdelete (const void *key, void **rootp, comparison_fn_t compar)
tsearch function..
void tdestroy (void *vroot, __free_fn_t freefct)
tsearch function..
off_t telldir (DIR *dirstream)
TEMP_FAILURE_RETRY (expression)
char * tempnam (const char *dir, const char *prefix)
char * textdomain (const char *domainname)
void * tfind (const void *key, void *const *rootp, comparison_fn_t compar)
tsearch function..
double tgamma (double x)
float tgammaf (float x)
long double tgammal (long double x)
time_t time (time_t *result)
time_t timegm (struct tm *brokentime)
time_t timelocal (struct tm *brokentime)
clock_t times (struct tms *buffer)
time_t
long int timezone
FILE * tmpfile (void)
FILE * tmpfile64 (void)
int TMP_MAX
char * tmpnam (char *result)
char * tmpnam_r (char *result)
int toascii (int c)
int _tolower (int c)
int tolower (int c)
tcflag_t TOSTOP
int _toupper (int c)
int toupper (int c)
wint_t towctrans (wint_t wc, wctrans_t desc)
wint_t towlower (wint_t wc)
wint_t towupper (wint_t wc)
double trunc (double x)
int truncate (const char *filename, off_t length)
int truncate64 (const char *name, off64_t length)
float truncf (float x)
long double truncl (long double x)
TRY_AGAIN
void * tsearch (const void *key, void **rootp, comparison_fn_t compar)
tsearch function..
char * ttyname (int filedes)
int ttyname_r (int filedes, char *buf, size_t len)
void twalk (const void *root, __action_fn_t action)
tsearch function..
char * tzname [2]
int TZNAME_MAX
void tzset (void)
UCHAR_MAX
ucontext_t
uid_t
UINT_MAX
int ulimit (int cmd, ...)
ULONG_LONG_MAX
ULONG_MAX
mode_t umask (mode_t mask)
int umount (const char *file)
int umount2 (const char *file, int flags)
int uname (struct utsname *info)
int ungetc (int c, FILE *stream)
ungetc To Do Unreading.
wint_t ungetwc (wint_t wc, FILE *stream)
ungetc To Do Unreading.
union wait
int unlink (const char *filename)
int unlockpt (int filedes)
int unsetenv (const char *name)
void updwtmp (const char *wtmp_file, const struct utmp *utmp)
USER_PROCESS
USER_PROCESS
USHRT_MAX
int utime (const char *filename, const struct utimbuf *times)
int utimes (const char *filename, struct timeval tvp[2])
int utmpname (const char *file)
int utmpxname (const char *file)
va_alist
type va_arg (va_list ap, type)
void __va_copy (va_list dest, va_list src)
va_dcl
void va_end (va_list ap)
va_list
void * valloc (size_t size)
int vasprintf (char **ptr, const char *template, va_list ap)
void va_start (va_list ap)
void va_start (va_list ap, last-required)
int VDISCARD
int VDSUSP
int VEOF
int VEOL
int VEOL2
int VERASE
int versionsort (const void *a, const void *b)
int versionsort64 (const void *a, const void *b)
pid_t vfork (void)
int vfprintf (FILE *stream, const char *template, va_list ap)
int vfscanf (FILE *stream, const char *template, va_list ap)
int vfwprintf (FILE *stream, const wchar_t *template, va_list ap)
int vfwscanf (FILE *stream, const wchar_t *template, va_list ap)
int VINTR
int VKILL
int vlimit (int resource, int limit)
int VLNEXT
int VMIN
int vprintf (const char *template, va_list ap)
int VQUIT
int VREPRINT
int vscanf (const char *template, va_list ap)
int vsnprintf (char *s, size_t size, const char *template, va_list ap)
int vsprintf (char *s, const char *template, va_list ap)
int vsscanf (const char *s, const char *template, va_list ap)
int VSTART
int VSTATUS
int VSTOP
int VSUSP
int vswprintf (wchar_t *s, size_t size, const wchar_t *template, va_list ap)
int vswscanf (const wchar_t *s, const wchar_t *template, va_list ap)
void vsyslog (int facility_priority, char *format, va_list arglist)
int VTIME
int vtimes (struct vtimes current, struct vtimes child)
int VWERASE
int vwprintf (const wchar_t *template, va_list ap)
int vwscanf (const wchar_t *template, va_list ap)
pid_t wait (int *status-ptr)
pid_t wait3 (union wait *status-ptr, int options, struct rusage *usage)
pid_t wait4 (pid_t pid, int *status-ptr, int options, struct rusage *usage)
pid_t waitpid (pid_t pid, int *status-ptr, int options)
WCHAR_MAX
wint_t WCHAR_MAX
wint_t WCHAR_MIN
wchar_t
int WCOREDUMP (int status)
wchar_t * wcpcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom)
wchar_t * wcpncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)
size_t wcrtomb (char *restrict s, wchar_t wc, mbstate_t *restrict ps)
int wcscasecmp (const wchar_t *ws1, const wchar_T *ws2)
wchar_t * wcscat (wchar_t *restrict wto, const wchar_t *restrict wfrom)
wchar_t * wcschr (const wchar_t *wstring, int wc)
wchar_t * wcschrnul (const wchar_t *wstring, wchar_t wc)
int wcscmp (const wchar_t *ws1, const wchar_t *ws2)
int wcscoll (const wchar_t *ws1, const wchar_t *ws2)
wchar_t * wcscpy (wchar_t *restrict wto, const wchar_t *restrict wfrom)
size_t wcscspn (const wchar_t *wstring, const wchar_t *stopset)
wchar_t * wcsdup (const wchar_t *ws)
size_t wcsftime (wchar_t *s, size_t size, const wchar_t *template, const struct tm *brokentime)
size_t wcslen (const wchar_t *ws)
int wcsncasecmp (const wchar_t *ws1, const wchar_t *s2, size_t n)
wchar_t * wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)
int wcsncmp (const wchar_t *ws1, const wchar_t *ws2, size_t size)
wchar_t * wcsncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)
size_t wcsnlen (const wchar_t *ws, size_t maxlen)
size_t wcsnrtombs (char *restrict dst, const wchar_t **restrict src, size_t nwc, size_t len, mbstate_t *restrict ps)
wchar_t * wcspbrk (const wchar_t *wstring, const wchar_t *stopset)
wchar_t * wcsrchr (const wchar_t *wstring, wchar_t c)
size_t wcsrtombs (char *restrict dst, const wchar_t **restrict src, size_t len, mbstate_t *restrict ps)
size_t wcsspn (const wchar_t *wstring, const wchar_t *skipset)
wchar_t * wcsstr (const wchar_t *haystack, const wchar_t *needle)
double wcstod (const wchar_t *restrict string, wchar_t **restrict tailptr)
float wcstof (const wchar_t *string, wchar_t **tailptr)
intmax_t wcstoimax (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
wchar_t * wcstok (wchar_t *newstring, const char *delimiters)
long int wcstol (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
long double wcstold (const wchar_t *string, wchar_t **tailptr)
long long int wcstoll (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
size_t wcstombs (char *string, const wchar_t *wstring, size_t size)
long long int wcstoq (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
unsigned long int wcstoul (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
unsigned long long int wcstoull (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
uintmax_t wcstoumax (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
unsigned long long int wcstouq (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
wchar_t * wcswcs (const wchar_t *haystack, const wchar_t *needle)
size_t wcsxfrm (wchar_t *restrict wto, const wchar_t *wfrom, size_t size)
int wctob (wint_t c)
int wctomb (char *string, wchar_t wchar)
wctrans_t wctrans (const char *property)
wctrans_t
wctype_t wctype (const char *property)
wctype_t
int WEOF
wint_t WEOF
int WEXITSTATUS (int status)
int WIFEXITED (int status)
int WIFSIGNALED (int status)
int WIFSTOPPED (int status)
wint_t
wchar_t * wmemchr (const wchar_t *block, wchar_t wc, size_t size)
int wmemcmp (const wchar_t *a1, const wchar_t *a2, size_t size)
wchar_t * wmemcpy (wchar_t *restrict wto, const wchar_t *restruct wfrom, size_t size)
wchar_t * wmemmove (wchar *wto, const wchar_t *wfrom, size_t size)
wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)
wchar_t * wmemset (wchar_t *block, wchar_t wc, size_t size)
int W_OK
int wordexp (const char *words, wordexp_t *word-vector-ptr, int flags)
wordexp.
wordexp_t
wordexp.
void wordfree (wordexp_t *word-vector-ptr)
wordexp.
int wprintf (const wchar_t *template, ...)
WRDE_APPEND
WRDE_BADCHAR
wordexp.
WRDE_BADVAL
wordexp.
WRDE_CMDSUB
wordexp.
WRDE_DOOFFS
WRDE_NOCMD
WRDE_NOSPACE
wordexp.
WRDE_REUSE
WRDE_SHOWERR
WRDE_SYNTAX
wordexp.
WRDE_UNDEF
ssize_t write (int filedes, const void *buffer, size_t size)
ssize_t writev (int filedes, const struct iovec *vector, int count)
int wscanf (const wchar_t *template, ...)
int WSTOPSIG (int status)
int WTERMSIG (int status)
int X_OK
_XOPEN_SOURCE
_XOPEN_SOURCE_EXTENDED
double y0 (double x)
float y0f (float x)
long double y0l (long double x)
double y1 (double x)
float y1f (float x)
long double y1l (long double x)
double yn (int n, double x)
float ynf (int n, float x)
long double ynl (int n, long double x)
@textfonts @rm
Before you do anything else, you should read the file `FAQ' found at the top level of the source tree. This file answers common questions and describes problems you may experience with compilation and installation. It is updated more frequently than this manual.
Features can be added to GNU Libc via add-on bundles. These are
separate tarfiles which you unpack into the top level of the source
tree. Then you give configure the `--enable-add-ons' option
to activate them, and they will be compiled into the library. As of the
2.2 release, one important component of glibc is distributed as
"official" add-ons: the linuxthreads add-on. Unless you are doing an
unusual installation, you should get this.
Support for POSIX threads is maintained by someone else, so it's in a separate package. It is only available for Linux systems, but this will change in the future. Get it from the same place you got the main bundle; the file is `glibc-linuxthreads-VERSION.tar.gz'.
You will need recent versions of several GNU tools: definitely GCC and GNU Make, and possibly others. See section Recommended Tools for Compilation, below.
GNU libc can be compiled in the source directory, but we strongly advise to build it in a separate build directory. For example, if you have unpacked the glibc sources in `/src/gnu/glibc-2.2.0', create a directory `/src/gnu/glibc-build' to put the object files in. This allows removing the whole build directory in case an error occurs, which is the safest way to get a fresh start and should always be done.
From your object directory, run the shell script `configure' found at the top level of the source tree. In the scenario above, you'd type
$ ../glibc-2.2.0/configure args...
Please note that even if you're building in a separate build directory, the compilation needs to modify a few files in the source directory, especially some files in the manual subdirectory.
configure takes many options, but you can get away with knowing
only two: `--prefix' and `--enable-add-ons'. The
--prefix option tells configure where you want glibc installed.
This defaults to `/usr/local'. The `--enable-add-ons' option
tells configure to use all the add-on bundles it finds in the source
directory. Since important functionality is provided in add-ons, you
should always specify this option.
It may also be useful to set the CC and CFLAGS variables in
the environment when running configure. CC selects the C
compiler that will be used, and CFLAGS sets optimization options
for the compiler.
The following list describes all of the available options for configure:
configure will detect the problem and
suppress these constructs, so that the library will still be usable, but
functionality may be lost--for example, you can't build a shared libc
with old binutils.
configure
will prepare to cross-compile glibc from build-system to be used
on host-system. You'll probably need the `--with-headers'
option too, and you may have to override configure's selection of
the compiler and/or binutils.
If you only specify `--host', configure will prepare for a native
compile but use what you specify instead of guessing what your system is.
This is most useful to change the CPU submodel. For example, if
configure guesses your machine as i586-pc-linux-gnu but you want
to compile a library for 386es, give `--host=i386-pc-linux-gnu' or
just `--host=i386-linux' and add the appropriate compiler flags
(`-mcpu=i386' will do the trick) to CFLAGS.
If you specify just `--build', configure will get confused.
To build the library and related programs, type make. This will
produce a lot of output, some of which may look like errors from
make but isn't. Look for error messages from make
containing `***'. Those indicate that something is really wrong.
The compilation process takes several hours even on fast hardware. Expect at least two hours for the default configuration on i586 for Linux. For Hurd times are much longer. Except for EGCS 1.1 and GCC 2.95 (and later versions of GCC), all supported versions of GCC have a problem which causes them to take several minutes to compile certain files in the iconvdata directory. Do not panic if the compiler appears to hang.
If you want to run a parallel make, you can't just give make the
`-j' option, because it won't be passed down to the sub-makes.
Instead, edit the generated `Makefile' and uncomment the line
# PARALLELMFLAGS = -j 4
You can change the `4' to some other number as appropriate for
your system. Instead of changing the `Makefile', you could give
this option directly to make and call it as, for example,
make PARALLELMFLAGS=-j4. If you're building in the source
directory, you must use the latter approach since in this case no
new `Makefile' is generated for you to change.
To build and run test programs which exercise some of the library
facilities, type make check. If it does not complete
successfully, do not use the built library, and report a bug after
verifying that the problem is not already known. See section Reporting Bugs,
for instructions on reporting bugs. Note that some of the tests assume
they are not being run by root. We recommend you compile and
test glibc as an unprivileged user.
To format the GNU C Library Reference Manual for printing, type
make dvi. You need a working TeX installation to do this.
The distribution already includes the on-line formatted version of the
manual, as Info files. You can regenerate those with make
info, but it shouldn't be necessary.
The library has a number of special-purpose configuration parameters
which you can find in `Makeconfig'. These can be overwritten with
the file `configparms'. To change them, create a
`configparms' in your build directory and add values as appropriate
for your system. The file is included and parsed by make and has
to follow the conventions for makefiles.
It is easy to configure the GNU C library for cross-compilation by
setting a few variables in `configparms'. Set CC to the
cross-compiler for the target you configured the library for; it is
important to use this same CC value when running
configure, like this: `CC=target-gcc configure
target'. Set BUILD_CC to the compiler to use for for
programs run on the build system as part of compiling the library. You
may need to set AR and RANLIB to cross-compiling versions
of ar and ranlib if the native tools are not configured to
work with object files for the target you configured for.
To install the library and its header files, and the Info files of the
manual, type env LANGUAGE=C LC_ALL=C make install. This will
build things if necessary, before installing them. However, you should
still compile everything first. If you are installing glibc as your
primary C library, we recommend that you shut the system down to
single-user mode first, and reboot afterward. This minimizes the risk
of breaking things when the library changes out from underneath.
If you're upgrading from Linux libc5 or some other C library, you need to replace the `/usr/include' with a fresh directory before installing it. The new `/usr/include' should contain the Linux headers, but nothing else.
You must first build the library (`make'), optionally check it (`make check'), switch the include directories and then install (`make install'). The steps must be done in this order. Not moving the directory before install will result in an unusable mixture of header files from both libraries, but configuring, building, and checking the library requires the ability to compile and run programs against the old library.
If you are upgrading from a previous installation of glibc 2.0 or 2.1, `make install' will do the entire job. You do not need to remove the old includes -- if you want to do so anyway you must then follow the order given above.
You may also need to reconfigure GCC to work with the new library. The easiest way to do that is to figure out the compiler switches to make it work again (`-Wl,--dynamic-linker=/lib/ld-linux.so.2' should work on Linux systems) and use them to recompile gcc. You can also edit the specs file (`/usr/lib/gcc-lib/TARGET/VERSION/specs'), but that is a bit of a black art.
You can install glibc somewhere other than where you configured it to go
by setting the install_root variable on the command line for
`make install'. The value of this variable is prepended to all the
paths for installation. This is useful when setting up a chroot
environment or preparing a binary distribution. The directory should be
specified with an absolute file name.
Glibc 2.2 includes a daemon called nscd, which you
may or may not want to run. nscd caches name service lookups; it
can dramatically improve performance with NIS+, and may help with DNS as
well.
One auxiliary program, `/usr/libexec/pt_chown', is installed setuid
root. This program is invoked by the grantpt function; it
sets the permissions on a pseudoterminal so it can be used by the
calling process. This means programs like xterm and
screen do not have to be setuid to get a pty. (There may be
other reasons why they need privileges.) If you are using a 2.1 or
newer Linux kernel with the devptsfs or devfs filesystems
providing pty slaves, you don't need this program; otherwise you do.
The source for `pt_chown' is in `login/programs/pt_chown.c'.
After installation you might want to configure the timezone and locale
installation of your system. The GNU C library comes with a locale
database which gets configured with localedef. For example, to
set up a German locale with name de_DE, simply issue the command
`localedef -i de_DE -f ISO-8859-1 de_DE'. To configure all locales
that are supported by glibc, you can issue from your build directory the
command `make localedata/install-locales'.
To configure the locally used timezone, you can either set the TZ
environment variable. The script tzselect helps you to select
the right value. As an example for Germany, tzselect would tell you to
use `TZ='Europe/Berlin''. For a system wide installation (the
given paths are for an installation with `--prefix=/usr'), link the
timezone file which is in `/usr/share/zoneinfo' to the file
`/etc/localtime'. For Germany, you might execute `ln -s
/usr/share/zoneinfo/Europe/Berlin /etc/localtime'.
We recommend installing the following GNU tools before attempting to build the GNU C library:
make 3.79 or newer
You need the latest version of GNU make. Modifying the GNU C
Library to work with other make programs would be so difficult that we
recommend you port GNU make instead. Really. We
recommend version GNU make version 3.79. All earlier
versions have severe bugs or lack features.
binutils 2.10.1 or later
You must use GNU binutils (as and ld) if you want to build a shared
library. Even if you don't, we recommend you use them anyway. No one
has tested compilation with non-GNU binutils in a long time.
The quality of binutils releases has varied a bit recently. The bugs
are in obscure features, but glibc uses quite a few of those. 2.10.1
and later releases are known to work. Versions after 2.8.1.0.23 may or
may not work. Older versions definitely don't.
For PPC you might need some patches even on top of the last binutils
version. See the FAQ.
texinfo 3.12f
To correctly translate and install the Texinfo documentation you need
this version of the texinfo package. Earlier versions do not
understand all the tags used in the document, and the installation
mechanism for the info files is not present or works differently.
awk 3.0, or some other POSIX awk
Awk is used in several places to generate files. The scripts should
work with any POSIX-compliant awk implementation; gawk 3.0 and
mawk 1.3 are known to work.
sed 3.02 or newer
Sed is used in several places to generate files. Most scripts work with
any version of sed. The known exception is the script
po2test.sed in the intl subdirectory which is used to
generate msgs.h for the testsuite. This script works correctly
only with GNU sed 3.02. If you like to run the testsuite, you
should definitely upgrade sed.
If you change any of the `configure.in' files you will also need
autoconf 2.12 or higher
and if you change any of the message translation files you will need
gettext 0.10.36 or later
You may also need these packages if you upgrade your source tree using patches, although we try to avoid this.
The GNU C Library currently supports configurations that match the following patterns:
alpha*-*-linux arm-*-linux cris-*-linux hppa-*-linux ix86-*-gnu ix86-*-linux ia64-*-linux m68k-*-linux mips*-*-linux powerpc-*-linux s390-*-linux s390x-*-linux sparc-*-linux sparc64-*-linux
Former releases of this library (version 2.1 and/or 2.0) used to run on the following configurations:
arm-*-linuxaout arm-*-none
Very early releases (version 1.09.1 and perhaps earlier versions) used to run on the following configurations:
alpha-dec-osf1 alpha-*-linuxecoff ix86-*-bsd4.3 ix86-*-isc2.2 ix86-*-isc3.n ix86-*-sco3.2 ix86-*-sco3.2v4 ix86-*-sysv ix86-*-sysv4 ix86-force_cpu386-none ix86-sequent-bsd i960-nindy960-none m68k-hp-bsd4.3 m68k-mvme135-none m68k-mvme136-none m68k-sony-newsos3 m68k-sony-newsos4 m68k-sun-sunos4.n mips-dec-ultrix4.n mips-sgi-irix4.n sparc-sun-solaris2.n sparc-sun-sunos4.n
Since no one has volunteered to test and fix these configurations, they are not supported at the moment. They probably don't compile; they definitely don't work anymore. Porting the library is not hard. If you are interested in doing a port, please contact the glibc maintainers by sending electronic mail to @email{bug-glibc@gnu.org}.
Valid cases of `ix86' include `i386', `i486', `i586', and `i686'. All of those configurations produce a library that can run on this processor and newer processors. The GCC compiler by default generates code that's optimized for the machine it's configured for and will use the instructions available on that machine. For example if your GCC is configured for `i686', gcc will optimize for `i686' and might issue some `i686' specific instructions. To generate code for other models, you have to configure for that model and give GCC the appropriate `-march=' and `-mcpu=' compiler switches via CFLAGS.
If you are installing GNU libc on a Linux system, you need to have the header files from a 2.2 kernel around for reference. You do not need to use the 2.2 kernel, just have its headers where glibc can access at them. The easiest way to do this is to unpack it in a directory such as `/usr/src/linux-2.2.1'. In that directory, run `make config' and accept all the defaults. Then run `make include/linux/version.h'. Finally, configure glibc with the option `--with-headers=/usr/src/linux-2.2.1/include'. Use the most recent kernel you can get your hands on.
An alternate tactic is to unpack the 2.2 kernel and run `make config' as above. Then rename or delete `/usr/include', create a new `/usr/include', and make the usual symbolic links of `/usr/include/linux' and `/usr/include/asm' into the 2.2 kernel sources. You can then configure glibc with no special options. This tactic is recommended if you are upgrading from libc5, since you need to get rid of the old header files anyway.
Note that `/usr/include/net' and `/usr/include/scsi' should not be symlinks into the kernel sources. GNU libc provides its own versions of these files.
Linux expects some components of the libc installation to be in `/lib' and some in `/usr/lib'. This is handled automatically if you configure glibc with `--prefix=/usr'. If you set some other prefix or allow it to default to `/usr/local', then all the components are installed there.
If you are upgrading from libc5, you need to recompile every shared library on your system against the new library for the sake of new code, but keep the old libraries around for old binaries to use. This is complicated and difficult. Consult the Glibc2 HOWTO at @url{http://www.imaxx.net/~thrytis/glibc} for details.
You cannot use nscd with 2.0 kernels, due to bugs in the
kernel-side thread support. nscd happens to hit these bugs
particularly hard, but you might have problems with any threaded
program.
There are probably bugs in the GNU C library. There are certainly errors and omissions in this manual. If you report them, they will get fixed. If you don't, no one will ever know about them and they will remain unfixed for all eternity, if not longer.
It is a good idea to verify that the problem has not already been reported. Bugs are documented in two places: The file `BUGS' describes a number of well known bugs and the bug tracking system has a WWW interface at @url{http://www-gnats.gnu.org:8080/cgi-bin/wwwgnats.pl}. The WWW interface gives you access to open and closed reports. The closed reports normally include a patch or a hint on solving the problem.
To report a bug, first you must find it. Hopefully, this will be the hard part. Once you've found a bug, make sure it's really a bug. A good way to do this is to see if the GNU C library behaves the same way some other C library does. If so, probably you are wrong and the libraries are right (but not necessarily). If not, one of the libraries is probably wrong. It might not be the GNU library. Many historical Unix C libraries permit things that we don't, such as closing a file twice.
If you think you have found some way in which the GNU C library does not conform to the ISO and POSIX standards (see section Standards and Portability), that is definitely a bug. Report it!
Once you're sure you've found a bug, try to narrow it down to the smallest test case that reproduces the problem. In the case of a C library, you really only need to narrow it down to one library function call, if possible. This should not be too difficult.
The final step when you have a simple test case is to report the bug.
Do this using the glibcbug script. It is installed with libc, or
if you haven't installed it, will be in your build directory. Send your
test case, the results you got, the results you expected, and what you
think the problem might be (if you've thought of anything).
glibcbug will insert the configuration information we need to
see, and ship the report off to @email{bugs@gnu.org}. Don't send
a message there directly; it is fed to a program that expects mail to be
formatted in a particular way. Use the script.
If you are not sure how a function should behave, and this manual doesn't tell you, that's a bug in the manual. Report that too! If the function's behavior disagrees with the manual, then either the library or the manual has a bug, so report the disagreement. If you find any errors or omissions in this manual, please report them to the Internet address @email{bug-glibc-manual@gnu.org}. If you refer to specific sections of the manual, please include the section names for easier identification.
The process of building the library is driven by the makefiles, which
make heavy use of special features of GNU make. The makefiles
are very complex, and you probably don't want to try to understand them.
But what they do is fairly straightforward, and only requires that you
define a few variables in the right places.
The library sources are divided into subdirectories, grouped by topic.
The `string' subdirectory has all the string-manipulation functions, `math' has all the mathematical functions, etc.
Each subdirectory contains a simple makefile, called `Makefile',
which defines a few make variables and then includes the global
makefile `Rules' with a line like:
include ../Rules
The basic variables that a subdirectory makefile defines are:
subdir
headers
routines
aux
routines for
modules that define functions in the library, and aux for
auxiliary modules containing things like data definitions. But the
values of routines and aux are just concatenated, so there
really is no practical difference.
tests
others
install-lib
install-data
install
install-data are
installed in the directory specified by `datadir' in
`configparms' or `Makeconfig'. Files listed in install
are installed in the directory specified by `bindir' in
`configparms' or `Makeconfig'.
distribute
distribute if there are files used in an unusual way
that should go into the distribution.
generated
extra-objs
others or tests.
The GNU C library is written to be easily portable to a variety of machines and operating systems. Machine- and operating system-dependent functions are well separated to make it easy to add implementations for new machines or operating systems. This section describes the layout of the library source tree and explains the mechanisms used to select machine-dependent code to use.
All the machine-dependent and operating system-dependent files in the library are in the subdirectory `sysdeps' under the top-level library source directory. This directory contains a hierarchy of subdirectories (see section Layout of the `sysdeps' Directory Hierarchy).
Each subdirectory of `sysdeps' contains source files for a particular machine or operating system, or for a class of machine or operating system (for example, systems by a particular vendor, or all machines that use IEEE 754 floating-point format). A configuration specifies an ordered list of these subdirectories. Each subdirectory implicitly appends its parent directory to the list. For example, specifying the list `unix/bsd/vax' is equivalent to specifying the list `unix/bsd/vax unix/bsd unix'. A subdirectory can also specify that it implies other subdirectories which are not directly above it in the directory hierarchy. If the file `Implies' exists in a subdirectory, it lists other subdirectories of `sysdeps' which are appended to the list, appearing after the subdirectory containing the `Implies' file. Lines in an `Implies' file that begin with a `#' character are ignored as comments. For example, `unix/bsd/Implies' contains:
# BSD has Internet-related things. unix/inet
and `unix/Implies' contains:
posix
So the final list is `unix/bsd/vax unix/bsd unix/inet unix posix'.
`sysdeps' has a "special" subdirectory called `generic'. It is always implicitly appended to the list of subdirectories, so you needn't put it in an `Implies' file, and you should not create any subdirectories under it intended to be new specific categories. `generic' serves two purposes. First, the makefiles do not bother to look for a system-dependent version of a file that's not in `generic'. This means that any system-dependent source file must have an analogue in `generic', even if the routines defined by that file are not implemented on other platforms. Second. the `generic' version of a system-dependent file is used if the makefiles do not find a version specific to the system you're compiling for.
If it is possible to implement the routines in a `generic' file in
machine-independent C, using only other machine-independent functions in
the C library, then you should do so. Otherwise, make them stubs. A
stub function is a function which cannot be implemented on a
particular machine or operating system. Stub functions always return an
error, and set errno to ENOSYS (Function not implemented).
See section Error Reporting. If you define a stub function, you must place
the statement stub_warning(function), where function
is the name of your function, after its definition; also, you must
include the file <stub-tag.h> into your file. This causes the
function to be listed in the installed <gnu/stubs.h>, and
makes GNU ld warn when the function is used.
Some rare functions are only useful on specific systems and aren't defined at all on others; these do not appear anywhere in the system-independent source code or makefiles (including the `generic' directory), only in the system-dependent `Makefile' in the specific system's subdirectory.
If you come across a file that is in one of the main source directories (`string', `stdio', etc.), and you want to write a machine- or operating system-dependent version of it, move the file into `sysdeps/generic' and write your new implementation in the appropriate system-specific subdirectory. Note that if a file is to be system-dependent, it must not appear in one of the main source directories.
There are a few special files that may exist in each subdirectory of `sysdeps':
make
conditional directives based on the variable `subdir' (see above) to
select different sets of variables and rules for different sections of
the library. It can also set the make variable
`sysdep-routines', to specify extra modules to be included in the
library. You should use `sysdep-routines' rather than adding
modules to `routines' because the latter is used in determining
what to distribute for each subdirectory of the main source tree.
Each makefile in a subdirectory in the ordered list of subdirectories to
be searched is included in order. Since several system-dependent
makefiles may be included, each should append to `sysdep-routines'
rather than simply setting it:
sysdep-routines := $(sysdep-routines) foo bar
. command to
read the `configure' file in each system-dependent directory
chosen, in order. The `configure' files are often generated from
`configure.in' files using Autoconf.
A system-dependent `configure' script will usually add things to
the shell variables `DEFS' and `config_vars'; see the
top-level `configure' script for details. The script can check for
`--with-package' options that were passed to the
top-level `configure'. For an option
`--with-package=value' `configure' sets the
shell variable `with_package' (with any dashes in
package converted to underscores) to value; if the option is
just `--with-package' (no argument), then it sets
`with_package' to `yes'.
m4 macro
`GLIBC_PROVIDES'. This macro does several AC_PROVIDE calls
for Autoconf macros which are used by the top-level `configure'
script; without this, those macros might be invoked again unnecessarily
by Autoconf.
That is the general system for how system-dependencies are isolated. The next section explains how to decide what directories in `sysdeps' to use. section Porting the GNU C Library to Unix Systems, has some tips on porting the library to Unix variants.
A GNU configuration name has three parts: the CPU type, the manufacturer's name, and the operating system. `configure' uses these to pick the list of system-dependent directories to look for. If the `--nfp' option is not passed to `configure', the directory `machine/fpu' is also used. The operating system often has a base operating system; for example, if the operating system is `Linux', the base operating system is `unix/sysv'. The algorithm used to pick the list of directories is simple: `configure' makes a list of the base operating system, manufacturer, CPU type, and operating system, in that order. It then concatenates all these together with slashes in between, to produce a directory name; for example, the configuration `i686-linux-gnu' results in `unix/sysv/linux/i386/i686'. `configure' then tries removing each element of the list in turn, so `unix/sysv/linux' and `unix/sysv' are also tried, among others. Since the precise version number of the operating system is often not important, and it would be very inconvenient, for example, to have identical `irix6.2' and `irix6.3' directories, `configure' tries successively less specific operating system names by removing trailing suffixes starting with a period.
As an example, here is the complete list of directories that would be tried for the configuration `i686-linux-gnu' (with the `crypt' and `linuxthreads' add-on):
sysdeps/i386/elf crypt/sysdeps/unix linuxthreads/sysdeps/unix/sysv/linux linuxthreads/sysdeps/pthread linuxthreads/sysdeps/unix/sysv linuxthreads/sysdeps/unix linuxthreads/sysdeps/i386/i686 linuxthreads/sysdeps/i386 linuxthreads/sysdeps/pthread/no-cmpxchg sysdeps/unix/sysv/linux/i386 sysdeps/unix/sysv/linux sysdeps/gnu sysdeps/unix/common sysdeps/unix/mman sysdeps/unix/inet sysdeps/unix/sysv/i386/i686 sysdeps/unix/sysv/i386 sysdeps/unix/sysv sysdeps/unix/i386 sysdeps/unix sysdeps/posix sysdeps/i386/i686 sysdeps/i386/i486 sysdeps/libm-i387/i686 sysdeps/i386/fpu sysdeps/libm-i387 sysdeps/i386 sysdeps/wordsize-32 sysdeps/ieee754 sysdeps/libm-ieee754 sysdeps/generic
Different machine architectures are conventionally subdirectories at the top level of the `sysdeps' directory tree. For example, `sysdeps/sparc' and `sysdeps/m68k'. These contain files specific to those machine architectures, but not specific to any particular operating system. There might be subdirectories for specializations of those architectures, such as `sysdeps/m68k/68020'. Code which is specific to the floating-point coprocessor used with a particular machine should go in `sysdeps/machine/fpu'.
There are a few directories at the top level of the `sysdeps' hierarchy that are not for particular machine architectures.
float is IEEE 754 single-precision format, and
double is IEEE 754 double-precision format. Usually this
directory is referred to in the `Implies' file in a machine
architecture-specific directory, such as `m68k/Implies'.
socket and related functions on Unix systems.
`unix/inet/Subdirs' enables the `inet' top-level subdirectory.
`unix/common' implies `unix/inet'.
Most Unix systems are fundamentally very similar. There are variations between different machines, and variations in what facilities are provided by the kernel. But the interface to the operating system facilities is, for the most part, pretty uniform and simple.
The code for Unix systems is in the directory `unix', at the top level of the `sysdeps' hierarchy. This directory contains subdirectories (and subdirectory trees) for various Unix variants.
The functions which are system calls in most Unix systems are implemented in assembly code, which is generated automatically from specifications in files named `syscalls.list'. There are several such files, one in `sysdeps/unix' and others in its subdirectories. Some special system calls are implemented in files that are named with a suffix of `.S'; for example, `_exit.S'. Files ending in `.S' are run through the C preprocessor before being fed to the assembler.
These files all use a set of macros that should be defined in `sysdep.h'. The `sysdep.h' file in `sysdeps/unix' partially defines them; a `sysdep.h' file in another directory must finish defining them for the particular machine and operating system variant. See `sysdeps/unix/sysdep.h' and the machine-specific `sysdep.h' implementations to see what these macros are and what they should do.
The system-specific makefile for the `unix' directory (`sysdeps/unix/Makefile') gives rules to generate several files from the Unix system you are building the library on (which is assumed to be the target system you are building the library for). All the generated files are put in the directory where the object files are kept; they should not affect the source tree itself. The files generated are `ioctls.h', `errnos.h', `sys/param.h', and `errlist.c' (for the `stdio' section of the library).
The GNU C library was written originally by Roland McGrath, and is currently maintained by Ulrich Drepper. Some parts of the library were contributed or worked on by other people.
getopt function and related code was written by
Richard Stallman, David J. MacKenzie, and Roland McGrath.
qsort was written by Michael J. Haertel.
qsort was written
by Douglas C. Schmidt.
malloc, realloc and
free and related code were written by Michael J. Haertel,
Wolfram Gloger, and Doug Lea.
memcpy,
strlen, etc.) were written by Torbj@"orn Granlund.
mips-dec-ultrix4)
was contributed by Brendan Kehoe and Ian Lance Taylor.
crypt and related functions were
contributed by Michael Glad.
ftw and nftw functions were contributed by Ulrich Drepper.
mktime function was contributed by Paul Eggert.
i386-sequent-bsd) was contributed by Jason Merrill.
alpha-dec-osf1) was
contributed by Brendan Kehoe, using some code written by Roland McGrath.
mips-sgi-irix4) was
contributed by Tom Quinn.
mips-anything-gnu) was contributed by Kazumoto Kojima.
printf and friends
and the floating-point reading function used by scanf,
strtod and friends were written by Ulrich Drepper. The
multi-precision integer functions used in those functions are taken from
GNU MP, which was contributed by Torbj@"orn Granlund.
locale and localedef, were written by Ulrich
Drepper. Ulrich Drepper adapted the support code for message catalogs
(`libintl.h', etc.) from the GNU gettext package, which he
also wrote. He also contributed the catgets support and the
entire suite of multi-byte and wide-character support functions
(`wctype.h', `wchar.h', etc.).
i386-anything-linux) was
contributed by Ulrich Drepper, based in large part on work done in
Hongjiu Lu's Linux version of the GNU C Library.
m68k-anything-linux) was
contributed by Andreas Schwab.
arm-ANYTHING-linuxaout) and ARM
standalone (arm-ANYTHING-none), as well as parts of the
IPv6 support code, were contributed by Philip Blundell.
alpha-anything-linux).
powerpc-anything-linux)
was contributed by Geoffrey Keating.
strstr function.
hsearch and drand48
families of functions; reentrant `..._r' versions of the
random family; System V shared memory and IPC support code; and
several highly-optimized string functions for ix86 processors.
fdlibm-5.1 by Sun
Microsystems, as modified by J.T. Conklin, Ian Lance Taylor,
Ulrich Drepper, Andreas Schwab, and Roland McGrath.
libio library used to implement stdio functions on
some platforms was written by Per Bothner and modified by Ulrich Drepper.
iconv).
Copyright (C) 1991 Regents of the University of California. All rights reserved.Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
- Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
- [This condition was removed.]
- Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
random, srandom,
setstate and initstate, which are also the basis for the
rand and srand functions, were written by Earl T. Cohen
for the University of California at Berkeley and are copyrighted by the
Regents of the University of California. They have undergone minor
changes to fit into the GNU C library and to fit the ISO C standard,
but the functional code is Berkeley's.
Portions Copyright (C) 1993 by Digital Equipment Corporation.
Permission to use, copy, modify, and distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies, and that the name of Digital Equipment Corporation not be used in advertising or publicity pertaining to distribution of the document or software without specific, written prior permission.
THE SOFTWARE IS PROVIDED "AS IS" AND DIGITAL EQUIPMENT CORP. DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL DIGITAL EQUIPMENT CORPORATION BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
Copyright (C) 1984, Sun Microsystems, Inc.Sun RPC is a product of Sun Microsystems, Inc. and is provided for unrestricted use provided that this legend is included on all tape media and as a part of the software program in whole or part. Users may copy or modify Sun RPC without charge, but are not authorized to license or distribute it to anyone else except as part of a product or program developed by the user.
SUN RPC IS PROVIDED AS IS WITH NO WARRANTIES OF ANY KIND INCLUDING THE WARRANTIES OF DESIGN, MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR ARISING FROM A COURSE OF DEALING, USAGE OR TRADE PRACTICE.
Sun RPC is provided with no support and without any obligation on the part of Sun Microsystems, Inc. to assist in its use, correction, modification or enhancement.
SUN MICROSYSTEMS, INC. SHALL HAVE NO LIABILITY WITH RESPECT TO THE INFRINGEMENT OF COPYRIGHTS, TRADE SECRETS OR ANY PATENTS BY SUN RPC OR ANY PART THEREOF.
In no event will Sun Microsystems, Inc. be liable for any lost revenue or profits or other special, indirect and consequential damages, even if Sun has been advised of the possibility of such damages.
Sun Microsystems, Inc. 2550 Garcia Avenue Mountain View, California 94043
Mach Operating System Copyright (C) 1991,1990,1989 Carnegie Mellon University All Rights Reserved.Permission to use, copy, modify and distribute this software and its documentation is hereby granted, provided that both the copyright notice and this permission notice appear in all copies of the software, derivative works or modified versions, and any portions thereof, and that both notices appear in supporting documentation.
CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS" CONDITION. CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
Carnegie Mellon requests users of this software to return to
Software Distribution Coordinator School of Computer Science Carnegie Mellon University Pittsburgh PA 15213-3890or @email{Software.Distribution@CS.CMU.EDU} any improvements or extensions that they make and grant Carnegie Mellon the rights to redistribute these changes.
Copyright (C) 1990, 1993, 1994, 1995, 1996, 1997 Sleepycat Software. All rights reserved.Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
- Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
- Redistributions in any form must be accompanied by information on how to obtain complete source code for the DB software and any accompanying software that uses the DB software. The source code must either be included in the distribution or be available for no more than the cost of distribution plus a nominal fee, and must be freely redistributable under reasonable conditions. For an executable file, complete source code means the source code for all modules it contains. It does not mean source code for modules or files that typically accompany the operating system on which the executable file runs, e.g., standard library modules or system header files.
THIS SOFTWARE IS PROVIDED BY SLEEPYCAT SOFTWARE "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL SLEEPYCAT SOFTWARE BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Portions copyright (C) 1995, 1996 The President and Fellows of Harvard University. All rights reserved.Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
- Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
- All advertising materials mentioning features or use of this software must display the following acknowledgement:
This product includes software developed by Harvard University and its contributors.
- Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY HARVARD AND ITS CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL HARVARD OR ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
For a license to use, redistribute or sell DB software under conditions other than those described above, or to purchase support for this software, please contact Sleepycat Software at
Sleepycat Software 394 E. Riding Dr. Carlisle, MA 01741 USA +1-508-287-4781or @email{db@sleepycat.com}.
Version 2, June 1991
Copyright (C) 1991 Free Software Foundation, Inc. 59 Temple Place -- Suite 330, Boston, MA 02111-1307, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. [This is the first released version of the library GPL. It is numbered 2 because it goes with version 2 of the ordinary GPL.]
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public Licenses are intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users.
This license, the Library General Public License, applies to some specially designated Free Software Foundation software, and to any other libraries whose authors decide to use it. You can use it for your libraries, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the library, or if you modify it.
For example, if you distribute copies of the library, whether gratis or for a fee, you must give the recipients all the rights that we gave you. You must make sure that they, too, receive or can get the source code. If you link a program with the library, you must provide complete object files to the recipients so that they can relink them with the library, after making changes to the library and recompiling it. And you must show them these terms so they know their rights.
Our method of protecting your rights has two steps: (1) copyright the library, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the library.
Also, for each distributor's protection, we want to make certain that everyone understands that there is no warranty for this free library. If the library is modified by someone else and passed on, we want its recipients to know that what they have is not the original version, so that any problems introduced by others will not reflect on the original authors' reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that companies distributing free software will individually obtain patent licenses, thus in effect transforming the program into proprietary software. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.
Most GNU software, including some libraries, is covered by the ordinary GNU General Public License, which was designed for utility programs. This license, the GNU Library General Public License, applies to certain designated libraries. This license is quite different from the ordinary one; be sure to read it in full, and don't assume that anything in it is the same as in the ordinary license.
The reason we have a separate public license for some libraries is that they blur the distinction we usually make between modifying or adding to a program and simply using it. Linking a program with a library, without changing the library, is in some sense simply using the library, and is analogous to running a utility program or application program. However, in a textual and legal sense, the linked executable is a combined work, a derivative of the original library, and the ordinary General Public License treats it as such.
Because of this blurred distinction, using the ordinary General Public License for libraries did not effectively promote software sharing, because most developers did not use the libraries. We concluded that weaker conditions might promote sharing better.
However, unrestricted linking of non-free programs would deprive the users of those programs of all benefit from the free status of the libraries themselves. This Library General Public License is intended to permit developers of non-free programs to use free libraries, while preserving your freedom as a user of such programs to change the free libraries that are incorporated in them. (We have not seen how to achieve this as regards changes in header files, but we have achieved it as regards changes in the actual functions of the Library.) The hope is that this will lead to faster development of free libraries.
The precise terms and conditions for copying, distribution and modification follow. Pay close attention to the difference between a "work based on the library" and a "work that uses the library". The former contains code derived from the library, while the latter only works together with the library.
Note that it is possible for a library to be covered by the ordinary General Public License rather than by this special one.
NO WARRANTY
If you develop a new library, and you want it to be of the greatest possible use to the public, we recommend making it free software that everyone can redistribute and change. You can do so by permitting redistribution under these terms (or, alternatively, under the terms of the ordinary General Public License).
To apply these terms, attach the following notices to the library. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.
one line to give the library's name and an idea of what it does. Copyright (C) year name of author This library is free software; you can redistribute it and/or modify it under the terms of the GNU Library General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.
Also add information on how to contact you by electronic and paper mail.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the library, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the library `Frob' (a library for tweaking knobs) written by James Random Hacker. signature of Ty Coon, 1 April 1990 Ty Coon, President of Vice
That's all there is to it!
_POSIX_OPTION_ORDER environment variable.
_POSIX_SAVED_IDS
malloc)
alloca disadvantages
alloca function
malloc
malloc
malloc)
printf)
scanf)
printf
printf conversions
alloca
malloc
exec functions
printf
fcntl function
select
printf)
scanf)
sigaction
printf
scanf
malloc
HOME environment variable
scanf
LANG environment variable
LC_ALL environment variable
LC_COLLATE environment variable
LC_CTYPE environment variable
LC_MESSAGES environment variable
LC_MONETARY environment variable
LC_NUMERIC environment variable
LC_TIME environment variable
LOGNAME environment variable
main function
malloc function
scanf
scanf)
printf)
NLSPATH environment variable
printf
PATH environment variable
pause function
printf)
setuid programs
sigaction flags
sigaction function
SIGCHLD, handling of
signal function
SIGTTIN, from background job
SIGTTOU, from background job
printf
scanf
TERM environment variable
printf)
scanf)
TZ environment variable
volatile declarations
Additions are welcome. Send appropriate information to @email{bug-glibc-manual@gnu.org
Actually, the terminal-specific functions are implemented with IOCTLs on many platforms.
Now you might ask why this information is duplicated. The answer is that we want to make it possible to link directly with these shared objects.
There is a second explanation: we were too lazy to change the Makefiles to allow the generation of shared objects not starting with `lib' but don't tell this to anybody.
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