This manual is for GNU Gnulib (updated $Date: 2004/11/06 17:38:57 $), which is a library of common routines intended to be shared at the source level.
Copyright © 2004 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”(a) The FSF's Back-Cover Text is: “You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.”
This is not a real manual. It's just a place to store random notes until someone (you?) gets around to actually writing a manual.
Getting started:
Where to put comments describing functions: Because of risk of divergence, we prefer to keep most function describing comments in only one place: just above the actual function definition. Some people prefer to put that documentation in the .h file. In any case, it should appear in just one place unless you can ensure that the multiple copies will always remain identical.
It is a tradition to use CPP tricks to avoid parsing the same header file more than once, which might cause warnings. The trick is to wrap the content of the header file (say, foo.h) in a block, as in:
#ifndef FOO_H
# define FOO_H
...
body of header file goes here
...
#endif /* FOO_H */
Whether to use FOO_H or _FOO_H is a matter of taste and
style. The C89 and C99 standards reserve all identifiers that begin with an
underscore and either an uppercase letter or another underscore, for
any use. Thus, in theory, an application might not safely assume that
_FOO_H has not already been defined by a library. On the other
hand, using FOO_H will likely lead the higher risk of
collisions with other symbols (e.g., KEY_H, XK_H, BPF_H,
which are CPP macro constants, or COFF_LONG_H, which is a CPP
macro function). Your preference may depend on whether you consider
the header file under discussion as part of the application (which has
its own namespace for CPP symbols) or a supporting library (that
shouldn't interfere with the application's CPP symbol namespace).
Adapting C header files for use in C++ applications can use another CPP trick, as in:
# ifdef __cplusplus
extern "C"
{
# endif
...
body of header file goes here
...
# ifdef __cplusplus
}
# endif
The idea here is that __cplusplus is defined only by C++
implementations, which will wrap the header file in an `extern "C"'
block. Again, whether to use this trick is a matter of taste and
style. While the above can be seen as harmless, it could be argued
that the header file is written in C, and any C++ application using it
should explicitly use the `extern "C"' block itself. Your
preference might depend on whether you consider the API exported by
your header file as something available for C programs only, or for C
and C++ programs alike.
The ctime function need not be reentrant, and consequently is
not required to be thread safe. Implementations of ctime
typically write the time stamp into static buffer. If two threads
call ctime at roughly the same time, you might end up with the
wrong date in one of the threads, or some undefined string. There is
a re-entrant interface ctime_r, that take a pre-allocated
buffer and length of the buffer, and return NULL on errors.
The input buffer should be at least 26 bytes in size. The output
string is locale-independent. However, years can have more than 4
digits if time_t is sufficiently wide, so the length of the
required output buffer is not easy to determine. Increasing the
buffer size when ctime_r return NULL is not necessarily
sufficient. The NULL return value could mean some other error
condition, which will not go away by increasing the buffer size.
A more flexible function is strftime. However, note that it is
locale dependent.
The inet_ntoa function need not be reentrant, and consequently
is not required to be thread safe. Implementations of
inet_ntoa typically write the time stamp into static buffer.
If two threads call inet_ntoa at roughly the same time, you
might end up with the wrong date in one of the threads, or some
undefined string. Further, inet_ntoa is specific for
IPv4 addresses.
A protocol independent function is inet_ntop.
The GSS API does not have a standard error code for the out of memory error condition. Instead of adding a non-standard error code, this library has chosen to adopt a different strategy. Out of memory handling happens in rare situations, but performing the out of memory error handling after almost all API function invocations pollute your source code and might make it harder to spot more serious problems. The strategy chosen improve code readability and robustness.
For most applications, aborting the application with an error message when the out of memory situation occur is the best that can be wished for. This is how the library behaves by default.
However, we realize that some applications may not want to have the
GSS library abort execution in any situation. The GSS library support
a hook to let the application regain control and perform its own
cleanups when an out of memory situation has occured. The application
can define a function (having a void prototype, i.e., no return
value and no parameters) and set the library variable
xalloc_fail_func to that function. The variable should be
declared as follows.
extern void (*xalloc_fail_func) (void);
The GSS library will invoke this function if an out of memory error occurs. Note that after this the GSS library is in an undefined state, so you must unload or restart the application to continue call GSS library functions. The hook is only intended to allow the application to log the situation in a special way. Of course, care must be taken to not allocate more memory, as that will likely also fail.
Run `gnulib-tool --help', and use the source. gnulib-tool is the way to import Gnulib modules.
Gnulib assumes your project uses Autoconf and Automake. Invoking `gnulib-tool --import' will copy source files, create a Makefile.am to build them, and generate a gnulib.m4 with Autoconf M4 macro declarations used by configure.ac.
Our example will be a library that uses Autoconf, Automake and
Libtool. It calls strdup, and you wish to use gnulib to make
the package portable to C89 (which doesn't have strdup).
~/src/libfoo$ gnulib-tool --import strdup
Module list with included dependencies:
strdup
File list:
lib/strdup.c
lib/strdup.h
m4/onceonly_2_57.m4
m4/strdup.m4
Creating ./lib/Makefile.am...
Creating ./m4/gnulib.m4...
Finished.
Don't forget to add "lib/Makefile"
to AC_CONFIG_FILES in "./configure.ac" and to mention
"lib" in SUBDIRS in some Makefile.am.
~/src/libfoo$
By default, the source code is copied into lib/ and the M4
macros in m4/. You can override these paths by using
--source-base=DIRECTORY and --m4-base=DIRECTORY, or by
adding `gl_SOURCE_BASE(DIRECTORY)' and
`gl_M4_BASE(DIRECTORY)' to your configure.ac.
gnulib-tool will overwrite any pre-existing files, in
particular Makefile.am. Unfortunately, separating the
generated Makefile.am content (for building the gnulib library)
into a separate file, say gnulib.mk, that could be included
by your handwritten Makefile.am is not possible, due to how
variable assignments are handled by Automake.
Consequently, it can be a good idea to chose directories that are not
already used by your projects, to separate gnulib imported files from
your own files. This approach can also be useful if you want to avoid
conflicts between other tools (e.g., getextize that also copy
M4 files into your package. Simon Josefsson successfully uses a source
base of gl/, and a M4 base of gl/m4/, in several
packages.
A few manual steps are required to finish the initial import.
First, you need to make sure Autoconf can find the macro definitions
in gnulib.m4. Use the ACLOCAL_AMFLAGS specifier in your
top-level Makefile.am file, as in:
ACLOCAL_AMFLAGS = -I m4
Naturally, replace m4 with the value from --m4-base or
gl_M4_BASE. If the M4 base is gl/m4 you would use:
ACLOCAL_AMFLAGS = -I gl/m4
You are now ready to call the M4 macros in gnulib.m4 from
configure.ac. The macro gl_EARLY must be called as soon
as possible after verifying that the C compiler is working.
Typically, this is immediately after AC_PROG_CC, as in:
...
AC_PROG_CC
gl_EARLY
...
The core part of the gnulib checks are done by the macro
gl_INIT. Place it further down in the file, typically where
you normally check for header files or functions. Or in a separate
section with other gnulib statements, such as gl_SOURCE_BASE.
For example:
...
# For gnulib.
gl_INIT
...
You must also make sure that the gnulib library is built. Add the
Makefile in the gnulib source base directory to
AC_CONFIG_FILES, as in:
AC_CONFIG_FILES(... lib/Makefile ...)
If your gnulib source base is gl, you would use:
AC_CONFIG_FILES(... gl/Makefile ...)
You must also make sure that make work in the gnulib directory.
Add the gnulib source base directory to a SUBDIRS Makefile.am
statement, as in:
SUBDIRS = lib
or if you, more likely, already have a few entries in SUBDIRS,
you can add something like:
SUBDIRS += lib
If you are using a gnulib source base of gl, you would use:
SUBDIRS += gl
Finally, you have add C flags and LD flags, so that you can make use of the gnulib library. For example:
...
AM_CPPFLAGS = -I$(top_srcdir)/lib
...
LIBADD = lib/libgnu.la
...
Don't forget to #include the various header files. In this
example, you would need to make sure that `#include <strdup.h>'
is evaluated when compiling all source code files, that want to make
use of strdup.
From time to time, you may want to invoke `gnulib-tool --import' to update the files in your package. Once you have set up your package for gnulib, this step is quite simple. For example:
~/src/libfoo$ gnulib-tool --import --source-base gl --m4-base gl/m4 strdup
Module list with included dependencies:
strdup
File list:
lib/strdup.c
lib/strdup.h
m4/onceonly_2_57.m4
m4/strdup.m4
Creating ./lib/Makefile.am...
Creating ./m4/gnulib.m4...
Finished.
Don't forget to add "lib/Makefile"
to AC_CONFIG_FILES in "./configure.ac" and to mention
"lib" in SUBDIRS in some Makefile.am.
~/src/libfoo$
If you don't recall how you invoked the tool last time, the commands used (and the operations it resulted in) are placed in comments within the generated Makefile.am and gnulib.m4, as in:
...
# Invoked as: gnulib-tool --import strdup
# Reproduce by: gnulib-tool --import --dir=. --lib=libgnu --source-base=lib --m4-base=m4 --libtool strdup
...
Invoking `gnulib-tool --import' with the proper parameters (e.g., `--m4-base gl/m4') and list of modules (e.g., `strdup snprintf getline minmax') can be tedious. To simplify this procedure, you may put the command line parameters in your configure.ac. For example:
...
AC_PROG_CC
gl_EARLY
...
# For gnulib.
gl_SOURCE_BASE(gl)
gl_M4_BASE(gl/m4)
gl_LIB(libgl)
gl_MODULES(getopt progname strdup dummy exit error getpass-gnu getaddrinfo)
gl_INIT
...
This illustrate all macros defined in gnulib.m4. With the above, importing new files are as simple as running `gnulib-tool --import' with no additional parameters.
The macros gl_EARLY, gl_INIT, gl_SOURCE_BASE, and
gl_M4_BASE have been discussed earlier. The gl_LIB
macro can be used if you wish to change the library name (by default
libgnu.a or libgnu.la if you use libtool). The
gl_MODULES macro is used to specify which modules to import.
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ctime: ctimegnulib-tool: Invoking gnulib-toolinet_ntoa: inet_ntoa