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.
NAME_MAX. It is defined in
`dirent.h'.
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.
obase that the bc utility is
guaranteed to support.
bc utility
is guaranteed to support.
scale that the 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;
}
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