D. Library Maintenance

D.1 Adding New Functions

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

The name of the subdirectory, for example ‘stdio’. This variable must be defined.

headers

The names of the header files in this section of the library, such as ‘stdio.h’.

routines

aux

The names of the modules (source files) in this section of the library. These should be simple names, such as ‘strlen’ (rather than complete file names, such as ‘strlen.c’). Use 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

The names of test programs for this section of the library. These should be simple names, such as ‘tester’ (rather than complete file names, such as ‘tester.c’). ‘make tests’ will build and run all the test programs. If a test program needs input, put the test data in a file called ‘test-program.input’; it will be given to the test program on its standard input. If a test program wants to be run with arguments, put the arguments (all on a single line) in a file called ‘test-program.args’. Test programs should exit with zero status when the test passes, and nonzero status when the test indicates a bug in the library or error in building.

others

The names of “other” programs associated with this section of the library. These are programs which are not tests per se, but are other small programs included with the library. They are built by ‘make others’.

install-lib

install-data

install

Files to be installed by ‘make install’. Files listed in ‘install-lib’ are installed in the directory specified by ‘libdir’ in ‘configparms’ or ‘Makeconfig’ (see section Installing the GNU C Library). Files listed in 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

Other files from this subdirectory which should be put into a distribution tar file. You need not list here the makefile itself or the source and header files listed in the other standard variables. Only define distribute if there are files used in an unusual way that should go into the distribution.

generated

Files which are generated by ‘Makefile’ in this subdirectory. These files will be removed by ‘make clean’, and they will never go into a distribution.

extra-objs

Extra object files which are built by ‘Makefile’ in this subdirectory. This should be a list of file names like ‘foo.o’; the files will actually be found in whatever directory object files are being built in. These files will be removed by ‘make clean’. This variable is used for secondary object files needed to build others or tests.

D.2 Porting the GNU C Library

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’:

‘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.

‘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 (autoconf.info)Introduction section `Introduction' in Autoconf: Generating Automatic Configuration Scripts, 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.

D.2.1 Layout of the ‘sysdeps’ Directory Hierarchy

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.

‘generic’

As described above (see section Porting the GNU C Library), this is the subdirectory that every configuration implicitly uses after all others.

‘ieee754’

This directory is for code using the IEEE 754 floating-point format, where the C type 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’.

‘libm-ieee754’

This directory contains an implementation of a mathematical library usable on platforms which use IEEE 754 conformant floating-point arithmetic.

‘libm-i387’

This is a special case. Ideally the code should be in ‘sysdeps/i386/fpu’ but for various reasons it is kept aside.

‘posix’

This directory contains implementations of things in the library in terms of POSIX.1 functions. This includes some of the POSIX.1 functions themselves. Of course, POSIX.1 cannot be completely implemented in terms of itself, so a configuration using just ‘posix’ cannot be complete.

‘unix’

This is the directory for Unix-like things. See section Porting the GNU C Library to Unix Systems. ‘unix’ implies ‘posix’. There are some special-purpose subdirectories of ‘unix’:

‘unix/common’

This directory is for things common to both BSD and System V release 4. Both ‘unix/bsd’ and ‘unix/sysv/sysv4’ imply ‘unix/common’.

‘unix/inet’

This directory is for socket and related functions on Unix systems. ‘unix/inet/Subdirs’ enables the ‘inet’ top-level subdirectory. ‘unix/common’ implies ‘unix/inet’.

‘mach’

This is the directory for things based on the Mach microkernel from CMU (including the GNU operating system). Other basic operating systems (VMS, for example) would have their own directories at the top level of the ‘sysdeps’ hierarchy, parallel to ‘unix’ and ‘mach’.

D.2.2 Porting the GNU C Library to Unix Systems

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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