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:
The basic variables that a subdirectory makefile defines are:
routinesfor modules that define functions in the library, and
auxfor auxiliary modules containing things like data definitions. But the values of
auxare just concatenated, so there really is no practical difference.
install-dataare installed in the directory specified by `datadir' in `configparms' or `Makeconfig'. Files listed in
installare installed in the directory specified by `bindir' in `configparms' or `Makeconfig'.
distributeif there are files used in an unusual way that should go into the distribution.
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:
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
ENOSYS (Function not implemented).
See section Error Reporting. If you define a stub function, you must place
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
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':
makeconditional 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
makevariable `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'.
m4macro `GLIBC_PROVIDES'. This macro does several
AC_PROVIDEcalls 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.
floatis IEEE 754 single-precision format, and
doubleis 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'.
socketand 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).
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