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:
Makefile
A makefile for this machine or operating system, or class of machine or
operating system. This file is included by the library makefile
Makerules, which is used by the top-level makefile and the
subdirectory makefiles. It can change the variables set in the
including makefile or add new rules. It can use GNU 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
Subdirs
This file contains the names of new whole subdirectories under the
top-level library source tree that should be included for this system.
These subdirectories are treated just like the system-independent
subdirectories in the library source tree, such as stdio and
math.
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.
Dist
This file contains the names of files (relative to the subdirectory of
sysdeps in which it appears) which should be included in the
distribution. List any new files used by rules in the Makefile
in the same directory, or header files used by the source files in that
directory. You don't need to list files that are implementations
(either C or assembly source) of routines whose names are given in the
machine-independent makefiles in the main source tree.
configure
This file is a shell script fragment to be run at configuration time.
The top-level configure script uses the shell . 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'.
configure.in
This file is an Autoconf input fragment to be processed into the file
configure in this subdirectory. See Introduction,
for a description of Autoconf. You should write either configure
or configure.in, but not both. The first line of
configure.in should invoke the 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.
