@node Maintenance, Platform, Installation, Top

@c %MENU% How to enhance and port the GNU C Library

@appendix Library Maintenance

@menu

* Source Layout::         How to add new functions or header files

                             to the GNU C Library.

* Porting::               How to port the GNU C Library to

                             a new machine or operating system.

@end menu

@node Source Layout

@appendixsec Adding New Functions

The process of building the library is driven by the makefiles, which

make heavy use of special features of GNU @code{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 @file{string} subdirectory has all the string-manipulation

functions, @file{math} has all the mathematical functions, etc.

Each subdirectory contains a simple makefile, called @file{Makefile},

which defines a few @code{make} variables and then includes the global

makefile @file{Rules} with a line like:

@smallexample

include ../Rules

@end smallexample

@noindent

The basic variables that a subdirectory makefile defines are:

@table @code

@item subdir

The name of the subdirectory, for example @file{stdio}.

This variable @strong{must} be defined.

@item headers

The names of the header files in this section of the library,

such as @file{stdio.h}.

@item routines

@itemx aux

The names of the modules (source files) in this section of the library.

These should be simple names, such as @samp{strlen} (rather than

complete file names, such as @file{strlen.c}).  Use @code{routines} for

modules that define functions in the library, and @code{aux} for

auxiliary modules containing things like data definitions.  But the

values of @code{routines} and @code{aux} are just concatenated, so there

really is no practical difference.@refill

@item tests

The names of test programs for this section of the library.  These

should be simple names, such as @samp{tester} (rather than complete file

names, such as @file{tester.c}).  @w{@samp{make tests}} will build and

run all the test programs.  If a test program needs input, put the test

data in a file called @file{@var{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 @file{@var{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.

@item 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

@w{@samp{make others}}.@refill

@item install-lib

@itemx install-data

@itemx install

Files to be installed by @w{@samp{make install}}.  Files listed in

@samp{install-lib} are installed in the directory specified by

@samp{libdir} in @file{configparms} or @file{Makeconfig}

(@pxref{Installation}).  Files listed in @code{install-data} are

installed in the directory specified by @samp{datadir} in

@file{configparms} or @file{Makeconfig}.  Files listed in @code{install}

are installed in the directory specified by @samp{bindir} in

@file{configparms} or @file{Makeconfig}.@refill

@item 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 @code{distribute} if there are files used in an unusual way

that should go into the distribution.

@item generated

Files which are generated by @file{Makefile} in this subdirectory.

These files will be removed by @w{@samp{make clean}}, and they will

never go into a distribution.

@item extra-objs

Extra object files which are built by @file{Makefile} in this

subdirectory.  This should be a list of file names like @file{foo.o};

the files will actually be found in whatever directory object files are

being built in.  These files will be removed by @w{@samp{make clean}}.

This variable is used for secondary object files needed to build

@code{others} or @code{tests}.

@end table

@menu

* Platform: Adding Platform-specific.             Adding platform-specific

                                         features.

@end menu

@node Adding Platform-specific

@appendixsubsec Platform-specific types, macros and functions

It's sometimes necessary to provide nonstandard, platform-specific

features to developers.  The C library is traditionally the

lowest library layer, so it makes sense for it to provide these

low-level features.  However, including these features in the C

library may be a disadvantage if another package provides them

as well as there will be two conflicting versions of them.  Also,

the features won't be available to projects that do not use

@theglibc{} but use other GNU tools, like GCC.

The current guidelines are:

@itemize @bullet

@item

If the header file provides features that only make sense on a particular

machine architecture and have nothing to do with an operating system, then

the features should ultimately be provided as GCC built-in functions.  Until

then, @theglibc{} may provide them in the header file.  When the GCC built-in

functions become available, those provided in the header file should be made

conditionally available prior to the GCC version in which the built-in

function was made available.

@item

If the header file provides features that are specific to an operating system,

both GCC and @theglibc{} could provide it, but @theglibc{} is preferred

as it already has a lot of information about the operating system.

@item

If the header file provides features that are specific to an operating system

but used by @theglibc{}, then @theglibc{} should provide them.

@end itemize

The general solution for providing low-level features is to export them as

follows:

@itemize @bullet

@item

A nonstandard, low-level header file that defines macros and inline

functions should be called @file{sys/platform/@var{name}.h}.

@item

Each header file's name should include the platform name, to avoid

users thinking there is anything in common between the different

header files for different platforms.  For example, a

@file{sys/platform/@var{arch}.h} name such as

@file{sys/platform/ppc.h} is better than @file{sys/platform.h}.

@item

A platform-specific header file provided by @theglibc{} should coordinate

with GCC such that compiler built-in versions of the functions and macros are

preferred if available.  This means that user programs will only ever need to

include @file{sys/platform/@var{arch}.h}, keeping the same names of types,

macros, and functions for convenience and portability.

@item

Each included symbol must have the prefix @code{__@var{arch}_}, such as

@code{__ppc_get_timebase}.

@end itemize

The easiest way to provide a header file is to add it to the

@code{sysdep_headers} variable.  For example, the combination of

Linux-specific header files on PowerPC could be provided like this:

@smallexample

sysdep_headers += sys/platform/ppc.h

@end smallexample

Then ensure that you have added a @file{sys/platform/ppc.h}

header file in the machine-specific directory, e.g.,

@file{sysdeps/powerpc/sys/platform/ppc.h}.

@node Porting

@appendixsec Porting @theglibc{}

@Theglibc{} 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 @file{sysdeps} under the top-level

library source directory.  This directory contains a hierarchy of

subdirectories (@pxref{Hierarchy Conventions}).

Each subdirectory of @file{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 @file{unix/bsd/vax} is equivalent to specifying the

list @file{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 @file{Implies} exists in a

subdirectory, it lists other subdirectories of @file{sysdeps} which are

appended to the list, appearing after the subdirectory containing the

@file{Implies} file.  Lines in an @file{Implies} file that begin with a

@samp{#} character are ignored as comments.  For example,

@file{unix/bsd/Implies} contains:@refill

@smallexample

# BSD has Internet-related things.

unix/inet

@end smallexample

@noindent

and @file{unix/Implies} contains:

@need 300

@smallexample

posix

@end smallexample

@noindent

So the final list is @file{unix/bsd/vax unix/bsd unix/inet unix posix}.

@file{sysdeps} has a ``special'' subdirectory called @file{generic}.  It

is always implicitly appended to the list of subdirectories, so you

needn't put it in an @file{Implies} file, and you should not create any

subdirectories under it intended to be new specific categories.

@file{generic} serves two purposes.  First, the makefiles do not bother

to look for a system-dependent version of a file that's not in

@file{generic}.  This means that any system-dependent source file must

have an analogue in @file{generic}, even if the routines defined by that

file are not implemented on other platforms.  Second, the @file{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 @file{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

@dfn{stub} function is a function which cannot be implemented on a

particular machine or operating system.  Stub functions always return an

error, and set @code{errno} to @code{ENOSYS} (Function not implemented).

@xref{Error Reporting}.  If you define a stub function, you must place

the statement @code{stub_warning(@var{function})}, where @var{function}

is the name of your function, after its definition.  This causes the

function to be listed in the installed @code{<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

@file{generic} directory), only in the system-dependent @file{Makefile}

in the specific system's subdirectory.

If you come across a file that is in one of the main source directories

(@file{string}, @file{stdio}, etc.), and you want to write a machine- or

operating system-dependent version of it, move the file into

@file{sysdeps/generic} and write your new implementation in the

appropriate system-specific subdirectory.  Note that if a file is to be

system-dependent, it @strong{must not} appear in one of the main source

directories.@refill

There are a few special files that may exist in each subdirectory of

@file{sysdeps}:

@comment Blank lines after items make the table look better.

@table @file

@item Makefile

A makefile for this machine or operating system, or class of machine or

operating system.  This file is included by the library makefile

@file{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 @code{make}

conditional directives based on the variable @samp{subdir} (see above) to

select different sets of variables and rules for different sections of

the library.  It can also set the @code{make} variable

@samp{sysdep-routines}, to specify extra modules to be included in the

library.  You should use @samp{sysdep-routines} rather than adding

modules to @samp{routines} because the latter is used in determining

what to distribute for each subdirectory of the main source tree.@refill

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 @samp{sysdep-routines}

rather than simply setting it:

@smallexample

sysdep-routines := $(sysdep-routines) foo bar

@end smallexample

@need 1000

@item 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 @file{stdio} and

@file{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 @file{sysdeps} implements.  For example,

@file{sysdeps/unix/inet/Subdirs} contains @file{inet}; the @file{inet}

directory contains various network-oriented operations which only make

sense to put in the library on systems that support the Internet.@refill

@item configure

This file is a shell script fragment to be run at configuration time.

The top-level @file{configure} script uses the shell @code{.} command to

read the @file{configure} file in each system-dependent directory

chosen, in order.  The @file{configure} files are often generated from

@file{configure.ac} files using Autoconf.

A system-dependent @file{configure} script will usually add things to

the shell variables @samp{DEFS} and @samp{config_vars}; see the

top-level @file{configure} script for details.  The script can check for

@w{@samp{--with-@var{package}}} options that were passed to the

top-level @file{configure}.  For an option

@w{@samp{--with-@var{package}=@var{value}}} @file{configure} sets the

shell variable @w{@samp{with_@var{package}}} (with any dashes in

@var{package} converted to underscores) to @var{value}; if the option is

just @w{@samp{--with-@var{package}}} (no argument), then it sets

@w{@samp{with_@var{package}}} to @samp{yes}.

@item configure.ac

This file is an Autoconf input fragment to be processed into the file

@file{configure} in this subdirectory.  @xref{Introduction,,,

autoconf.info, Autoconf: Generating Automatic Configuration Scripts},

for a description of Autoconf.  You should write either @file{configure}

or @file{configure.ac}, but not both.  The first line of

@file{configure.ac} should invoke the @code{m4} macro

@samp{GLIBC_PROVIDES}.  This macro does several @code{AC_PROVIDE} calls

for Autoconf macros which are used by the top-level @file{configure}

script; without this, those macros might be invoked again unnecessarily

by Autoconf.

@end table

That is the general system for how system-dependencies are isolated.

@iftex

The next section explains how to decide what directories in

@file{sysdeps} to use.  @ref{Porting to Unix}, has some tips on porting

the library to Unix variants.

@end iftex

@menu

* Hierarchy Conventions::       The layout of the @file{sysdeps} hierarchy.

* Porting to Unix::             Porting the library to an average

                                   Unix-like system.

@end menu

@node Hierarchy Conventions

@appendixsubsec Layout of the @file{sysdeps} Directory Hierarchy

A GNU configuration name has three parts: the CPU type, the

manufacturer's name, and the operating system.  @file{configure} uses

these to pick the list of system-dependent directories to look for.  If

the @samp{--nfp} option is @emph{not} passed to @file{configure}, the

directory @file{@var{machine}/fpu} is also used.  The operating system

often has a @dfn{base operating system}; for example, if the operating

system is @samp{Linux}, the base operating system is @samp{unix/sysv}.

The algorithm used to pick the list of directories is simple:

@file{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 @w{@samp{i686-linux-gnu}}

results in @file{unix/sysv/linux/i386/i686}.  @file{configure} then

tries removing each element of the list in turn, so

@file{unix/sysv/linux} and @file{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 @file{irix6.2} and @file{irix6.3} directories,

@file{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 @w{@samp{i686-linux-gnu}}:

@smallexample

sysdeps/i386/elf

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

@end smallexample

Different machine architectures are conventionally subdirectories at the

top level of the @file{sysdeps} directory tree.  For example,

@w{@file{sysdeps/sparc}} and @w{@file{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

@w{@file{sysdeps/m68k/68020}}.  Code which is specific to the

floating-point coprocessor used with a particular machine should go in

@w{@file{sysdeps/@var{machine}/fpu}}.

There are a few directories at the top level of the @file{sysdeps}

hierarchy that are not for particular machine architectures.

@table @file

@item generic

As described above (@pxref{Porting}), this is the subdirectory

that every configuration implicitly uses after all others.

@item ieee754

This directory is for code using the IEEE 754 floating-point format,

where the C type @code{float} is IEEE 754 single-precision format, and

@code{double} is IEEE 754 double-precision format.  Usually this

directory is referred to in the @file{Implies} file in a machine

architecture-specific directory, such as @file{m68k/Implies}.

@item libm-ieee754

This directory contains an implementation of a mathematical library

usable on platforms which use @w{IEEE 754} conformant floating-point

arithmetic.

@item libm-i387

This is a special case.  Ideally the code should be in

@file{sysdeps/i386/fpu} but for various reasons it is kept aside.

@item posix

This directory contains implementations of things in the library in

terms of @sc{POSIX.1} functions.  This includes some of the @sc{POSIX.1}

functions themselves.  Of course, @sc{POSIX.1} cannot be completely

implemented in terms of itself, so a configuration using just

@file{posix} cannot be complete.

@item unix

This is the directory for Unix-like things.  @xref{Porting to Unix}.

@file{unix} implies @file{posix}.  There are some special-purpose

subdirectories of @file{unix}:

@table @file

@item unix/common

This directory is for things common to both BSD and System V release 4.

Both @file{unix/bsd} and @file{unix/sysv/sysv4} imply @file{unix/common}.

@item unix/inet

This directory is for @code{socket} and related functions on Unix systems.

@file{unix/inet/Subdirs} enables the @file{inet} top-level subdirectory.

@file{unix/common} implies @file{unix/inet}.

@end table

@item mach

This is the directory for things based on the Mach microkernel from CMU

(including @gnuhurdsystems{}).  Other basic operating systems

(VMS, for example) would have their own directories at the top level of

the @file{sysdeps} hierarchy, parallel to @file{unix} and @file{mach}.

@end table

@node Porting to Unix

@appendixsubsec Porting @theglibc{} 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 @file{unix}, at the top

level of the @file{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 @file{syscalls.list}.  There are several

such files, one in @file{sysdeps/unix} and others in its subdirectories.

Some special system calls are implemented in files that are named with a

suffix of @samp{.S}; for example, @file{_exit.S}.  Files ending in

@samp{.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

@file{sysdep.h}.  The @file{sysdep.h} file in @file{sysdeps/unix}

partially defines them; a @file{sysdep.h} file in another directory must

finish defining them for the particular machine and operating system

variant.  See @file{sysdeps/unix/sysdep.h} and the machine-specific

@file{sysdep.h} implementations to see what these macros are and what

they should do.@refill

The system-specific makefile for the @file{unix} directory

(@file{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 @emph{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 @file{ioctls.h}, @file{errnos.h}, @file{sys/param.h}, and

@file{errlist.c} (for the @file{stdio} section of the library).

@ignore

@c This section might be a good idea if it is finished,

@c but there's no point including it as it stands. --rms

@c @appendixsec Compatibility with Traditional C

@c ??? This section is really short now.  Want to keep it? --roland

@c It's not anymore true.  glibc 2.1 cannot be used with K&R compilers.

@c --drepper

Although @theglibc{} implements the @w{ISO C} library facilities, you

@emph{can} use @theglibc{} with traditional, ``pre-ISO'' C

compilers.  However, you need to be careful because the content and

organization of the @glibcadj{} header files differs from that of

traditional C implementations.  This means you may need to make changes

to your program in order to get it to compile.

@end ignore