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 Hierarchy Conventions).
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 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
Use this when there are completely new sets of functions and header
files that should go into the library for the system this subdirectory
of sysdeps implements. For example,
sysdeps/unix/inet/Subdirs contains inet; the inet
directory contains various network-oriented operations which only make
sense to put in the library on systems that support the Internet.
. 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.