path: root/manual/maint.texi

@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.
* Symbol handling::       How to handle symbols in 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 Symbol handling
@appendixsec Symbol handling in the GNU C Library

@menu
* 64-bit time symbol handling :: How to handle 64-bit time related
                                    symbols in the GNU C Library.
@end menu

@node 64-bit time symbol handling
@appendixsubsec 64-bit time symbol handling in the GNU C Library

With respect to time handling, @glibcadj{} configurations fall in two
classes depending on the value of @code{__TIMESIZE}:

@table @code

@item @code{__TIMESIZE == 32}

These @dfn{dual-time} configurations have both 32-bit and 64-bit time
support.  32-bit time support provides type @code{time_t} and cannot
handle dates beyond @dfn{Y2038}.  64-bit time support provides type
@code{__time64_t} and can handle dates beyond @dfn{Y2038}.

In these configurations, time-related types have two declarations,
a 64-bit one, and a 32-bit one; and time-related functions generally
have two definitions: a 64-bit one, and a 32-bit one which is a wrapper
around the former.  Therefore, for every @code{time_t}-related symbol,
there is a corresponding @code{__time64_t}-related symbol, the name of
which is usually the 32-bit symbol's name with @code{__} (a double
underscore) prepended and @code{64} appended.  For instance, the
64-bit-time counterpart of @code{clock_gettime} is
@code{__clock_gettime64}.

@item @code{__TIMESIZE == 64}

These @dfn{single-time} configurations only have a 64-bit @code{time_t}
and related functions, which can handle dates beyond 2038-01-19
03:14:07 (aka @dfn{Y2038}).

In these configurations, time-related types only have a 64-bit
declaration; and time-related functions only have one 64-bit definition.
However, for every @code{time_t}-related symbol, there is a
corresponding @code{__time64_t}-related macro, the name of which is
derived as in the dual-time configuration case, and which expands to
the symbol's name.  For instance, the macro @code{__clock_gettime64}
expands to @code{clock_gettime}.

These macros are purely internal to @theglibc{} and exist only so that
a single definition of the 64-bit time functions can be used on both
single-time and dual-time configurations, and so that glibc code can
freely call the 64-bit functions internally in all configurations.

@end table

@c The following paragraph should be removed once external interfaces
@c get support for both time sizes.

Note: at this point, 64-bit time support in dual-time configurations is
work-in-progress, so for these configurations, the public API only makes
the 32-bit time support available.  In a later change, the public API
will allow user code to choose the time size for a given compilation
unit.

64-bit variants of time-related types or functions are defined for all
configurations and use 64-bit-time symbol names (for dual-time
configurations) or macros (for single-time configurations).

32-bit variants of time-related types or functions are defined only for
dual-time configurations.

Here is an example with @code{localtime}:

Function @code{localtime} is declared in @file{time/time.h} as
@smallexample
extern struct tm *localtime (const time_t *__timer) __THROW;
libc_hidden_proto (localtime)
@end smallexample

For single-time configurations, @code{__localtime64} is a macro which
evaluates to @code{localtime}; for dual-time configurations,
@code{__localtime64} is a function similar to @code{localtime} except
it uses Y2038-proof types:
@smallexample
#if __TIMESIZE == 64
# define __localtime64 localtime
#else
extern struct tm *__localtime64 (const __time64_t *__timer) __THROW;
libc_hidden_proto (__localtime64)
#endif
@end smallexample

(note: type @code{time_t} is replaced with @code{__time64_t} because
@code{time_t} is not Y2038-proof, but @code{struct tm} is not
replaced because it is already Y2038-proof.)

The 64-bit-time implementation of @code{localtime} is written as follows
and is compiled for both dual-time and single-time configuration classes.

@smallexample
struct tm *
__localtime64 (const __time64_t *t)
@{
  return __tz_convert (*t, 1, &_tmbuf);
@}
libc_hidden_def (__localtime64)
@end smallexample

The 32-bit-time implementation is a wrapper and is only compiled for
dual-time configurations:

@smallexample
#if __TIMESIZE != 64

struct tm *
localtime (const time_t *t)
@{
  __time64_t t64 = *t;
  return __localtime64 (&t64);
@}
libc_hidden_def (localtime)

#endif
@end smallexample

@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 routine