Emphasize the drawbacks of the --symlink option.

[gnulib.git] / doc / gnulib-tool.texi

@node Invoking gnulib-tool
@chapter Invoking gnulib-tool

@c Copyright (C) 2005, 2006 Free Software Foundation, Inc.

@c Permission is granted to copy, distribute and/or modify this document
@c under the terms of the GNU Free Documentation License, Version 1.2 or
@c any later version published by the Free Software Foundation; with no
@c Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
@c Texts.  A copy of the license is included in the ``GNU Free
@c Documentation License'' file as part of this distribution.

@pindex gnulib-tool
@cindex invoking @command{gnulib-tool}

The @command{gnulib-tool} command is the recommended way to import
Gnulib modules.  It is possible to borrow Gnulib modules in a package
without using @command{gnulib-tool}, relying only on the
meta-information stored in the @file{modules/*} files, but with a
growing number of modules this becomes tedious.  @command{gnulib-tool}
simplifies the management of source files, @file{Makefile.am}s and
@file{configure.ac} in packages incorporating Gnulib modules.

Run @samp{gnulib-tool --help} for information.  To get familiar with
@command{gnulib-tool} without affecting your sources, you can also try
some commands with the option @samp{--dry-run}; then
@code{gnulib-tool} will only report which actions it would perform in
a real run without changing anything.

@menu
* Initial import::              First import of Gnulib modules.
* Modified imports::            Changing the import specification.
* Simple update::               Tracking Gnulib development.
* CVS Issues::                  Integration with CVS.
@end menu


@node Initial import
@section Initial import
@cindex initial import

Gnulib assumes your project uses Autoconf and Automake.  Invoking
@samp{gnulib-tool --import} will copy source files, create a
@file{Makefile.am} to build them, generate a file @file{gnulib-comp.m4} with
Autoconf M4 macro declarations used by @file{configure.ac}, and generate
a file @file{gnulib-cache.m4} containing the cached specification of how
Gnulib is used.

Our example will be a library that uses Autoconf, Automake and
Libtool.  It calls @code{strdup}, and you wish to use gnulib to make
the package portable to C89 (which doesn't have @code{strdup}).

@example
~/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
Copying file m4/gnulib-tool.m4
Copying file m4/onceonly_2_57.m4
Copying file lib/strdup.c
Copying file lib/strdup.h
Copying file m4/strdup.m4
Creating lib/Makefile.am
Creating m4/gnulib-cache.m4
Creating m4/gnulib-comp.m4
Finished.

You may need to add #include directives for the following .h files.
  #include "strdup.h"

Don't forget to
  - add "lib/Makefile" to AC_CONFIG_FILES in ./configure.ac,
  - mention "lib" in SUBDIRS in Makefile.am,
  - mention "-I m4" in ACLOCAL_AMFLAGS in Makefile.am,
  - invoke gl_EARLY in ./configure.ac, right after AC_PROG_CC,
  - invoke gl_INIT in ./configure.ac.
~/src/libfoo$
@end example

By default, the source code is copied into @file{lib/} and the M4
macros in @file{m4/}.  You can override these paths by using
@code{--source-base=DIRECTORY} and @code{--m4-base=DIRECTORY}.  Some
modules also provide other files necessary for building. These files
are copied into the directory specified by @samp{AC_CONFIG_AUX_DIR} in
@file{configure.ac} or by the @code{--aux-dir=DIRECTORY} option.  If
neither is specified, the current directory is assumed.

@code{gnulib-tool} can make symbolic links instead of copying the
source files.  The option to specify for this is @samp{--symlink}, or
@samp{-s} for short.  This can be useful to save a few kilobytes of disk
space.  But it is likely to introduce bugs when @code{gnulib} is updated;
it is more reliable to use @samp{gnulib-tool --update} (see below)
to update to newer versions of @code{gnulib}.  Furthermore it requires
extra effort to create self-contained tarballs, and it may disturb some
mechanism the maintainer applies to the sources.  For these reasons,
this option is generally discouraged.

@code{gnulib-tool} will overwrite any pre-existing files, in
particular @file{Makefile.am}.  Unfortunately, separating the
generated @file{Makefile.am} content (for building the gnulib library)
into a separate file, say @file{gnulib.mk}, that could be included
by your handwritten @file{Makefile.am} is not possible, due to how
variable assignments are handled by Automake.

Consequently, it is a good idea to choose directories that are not
already used by your projects, to separate gnulib imported files from
your own files.  This approach is also useful if you want to avoid
conflicts between other tools (e.g., @code{gettextize} that also copy
M4 files into your package.  Simon Josefsson successfully uses a source
base of @file{gl/}, and a M4 base of @file{gl/m4/}, in several
packages.

After the @samp{--import} option on the command line comes the list of
Gnulib modules that you want to incorporate in your package.  The names
of the modules coincide with the filenames in Gnulib's @file{modules/}
directory.

Some Gnulib modules depend on other Gnulib modules.  @code{gnulib-tool}
will automatically add the needed modules as well; you need not list
them explicitly.  @code{gnulib-tool} will also memorize which dependent
modules it has added, so that when someday a dependency is dropped, the
implicitly added module is dropped as well (unless you have explicitly
requested that module).

If you want to cut a dependency, i.e., not add a module although one of
your requested modules depends on it, you may use the option
@samp{--avoid=@var{module}} to do so.  Multiple uses of this option are
possible.  Of course, you will then need to implement the same interface
as the removed module.

A few manual steps are required to finish the initial import.
@code{gnulib-tool} printed a summary of these steps.

First, you must ensure Autoconf can find the macro definitions in
@file{gnulib-comp.m4}.  Use the @code{ACLOCAL_AMFLAGS} specifier in
your top-level @file{Makefile.am} file, as in:

@example
ACLOCAL_AMFLAGS = -I m4
@end example

You are now ready to call the M4 macros in @code{gnulib-comp.m4} from
@file{configure.ac}.  The macro @code{gl_EARLY} must be called as soon
as possible after verifying that the C compiler is working.
Typically, this is immediately after @code{AC_PROG_CC}, as in:

@example
...
AC_PROG_CC
gl_EARLY
...
@end example

The core part of the gnulib checks are done by the macro
@code{gl_INIT}.  Place it further down in the file, typically where
you normally check for header files or functions.  It must come after
other checks which may affect the compiler invocation, such as
@code{AC_MINIX}.  For example:

@example
...
# For gnulib.
gl_INIT
...
@end example

@code{gl_INIT} will in turn call the macros related with the
gnulib functions, be it specific gnulib macros, like @code{gl_FUNC_ALLOCA}
or autoconf or automake macros like @code{AC_FUNC_ALLOCA} or
@code{AM_FUNC_GETLINE}.  So there is no need to call those macros yourself
when you use the corresponding gnulib modules.

You must also make sure that the gnulib library is built.  Add the
@code{Makefile} in the gnulib source base directory to
@code{AC_CONFIG_FILES}, as in:

@example
AC_CONFIG_FILES(... lib/Makefile ...)
@end example

You must also make sure that @code{make} will recurse into the gnulib
directory.  To achieve this, add the gnulib source base directory to a
@code{SUBDIRS} Makefile.am statement, as in:

@example
SUBDIRS = lib
@end example

or if you, more likely, already have a few entries in @code{SUBDIRS},
you can add something like:

@example
SUBDIRS += lib
@end example

Finally, you have to add compiler and linker flags in the appropriate
source directories, so that you can make use of the gnulib library.
Since some modules (@samp{getopt}, for example) may copy files into
the build directory, @file{top_builddir/lib} is needed as well
as @file{top_srcdir/lib}.  For example:

@example
...
AM_CPPFLAGS = -I$(top_srcdir)/lib -I$(top_builddir)/lib
...
LDADD = lib/libgnu.a
...
@end example

Don't forget to @code{#include} the various header files.  In this
example, you would need to make sure that @samp{#include "strdup.h"}
is evaluated when compiling all source code files, that want to make
use of @code{strdup}.

When an include file is provided by Gnulib
you shouldn't try to include the corresponding system header files
yourself, but let the gnulib header file do it.  The ordering
of the definition for some symbols may be significant; the Gnulib
header files take care of that.

For example, to use the @code{time_r} gnulib module you should
use include header file provided by the gnulib, and so
@samp{#include "time_r.h"}, but you shouldn't explicitly
@samp{#include <time.h>} as it is already done in @file{time_r.h}
before the redefinition of some symbols.

A final word of warning: Gnulib currently assumes it will be
responsible for @emph{all} functions that end up in the Autoconf
@code{@@LIBOBJS@@} variables (and/or @code{@@LTLIBOBJS@@} if using
Libtool), e.g., those specified in @code{AC_REPLACE_FUNCS} in your
@file{configure.ac}.  Therefore, if you have any functions which are
not covered by Gnulib which need that treatment, you have to
essentially reimplement AC_REPLACE_FUNCS using different names; for an
example, see the Findutils sources.  Perhaps this will be improved in
the future.


@node Modified imports
@section Modified imports

You can at any moment decide to use Gnulib differently than the last time.

If you only want to use more Gnulib modules, simply invoke
@command{gnulib-tool --import @var{new-modules}}.  @code{gnulib-tool}
remembers which modules were used last time.  The list of modules that
you pass after @samp{--import} is @emph{added} to the previous list of
modules.

For most changes, such as added or removed modules, or even different
choices of @samp{--lib}, @samp{--source-base} or @samp{--aux-dir}, there
are two ways to perform the change.

The standard way is to modify manually the file @file{gnulib-cache.m4}
in the M4 macros directory, then launch @samp{gnulib-tool --import}.

The other way is to call @command{gnulib-tool} again, with the changed
command-line options.  Note that this doesn't let you remove modules,
because as you just learned, the list of modules is always cumulated.
Also this way is often impractical, because you don't remember the way
you invoked @code{gnulib-tool} last time.

The only change for which this doesn't work is a change of the
@samp{--m4-base} directory.  Because, when you pass a different value of
@samp{--m4-base}, @code{gnulib-tool} will not find the previous
@file{gnulib-cache.m4} file any more... A possible solution is to manually
copy the @file{gnulib-cache.m4} into the new M4 macro directory.

In the @file{gnulib-cache.m4}, the macros have the following meaning:
@table @code
@item gl_MODULES
The argument is a space separated list of the requested modules, not including
dependencies.

@item gl_AVOID
The argument is a space separated list of modules that should not be used,
even if they occur as dependencies.  Corresponds to the @samp{--avoid}
command line argument.

@item gl_SOURCE_BASE
The argument is the relative file name of the directory containing the gnulib
source files (mostly *.c and *.h files).  Corresponds to the
@samp{--source-base} command line argument.

@item gl_M4_BASE
The argument is the relative file name of the directory containing the gnulib
M4 macros (*.m4 files).  Corresponds to the @samp{--m4-base} command line
argument.

@item gl_TESTS_BASE
The argument is the relative file name of the directory containing the gnulib
unit test files.  Corresponds to the @samp{--tests-base} command line argument.

@item gl_LIB
The argument is the name of the library to be created.  Corresponds to the
@samp{--lib} command line argument.

@item gl_LGPL
The presence of this macro corresponds to the @samp{--lgpl} command line
argument.  It takes no arguments.

@item gl_LIBTOOL
The presence of this macro corresponds to the @samp{--libtool} command line
argument and to the absence of the @samp{--no-libtool} command line argument.
It takes no arguments.

@item gl_MACRO_PREFIX
The argument is the prefix to use for macros in the @file{gnulib-comp.m4}
file.  Corresponds to the @samp{--macro-prefix} command line argument.
@end table

@node Simple update
@section Simple update

When you want to update to a more recent version of Gnulib, without
changing the list of modules or other parameters, a simple call
does it:

@smallexample
$ gnulib-tool --import
@end smallexample

This will create, update or remove files, as needed.

@node CVS Issues
@section CVS Issues

All files created by @code{gnulib-tool}, except @file{gnulib-cache.m4},
should be treated like generated source files, like for example a
@file{parser.c} file is generated from @file{parser.y}.

In projects which commit all source files, whether generated or not, into
CVS, the @code{gnulib-tool} generated files should all be committed.

In projects which customarily omit from the CVS all files that generated
from other source files, all these files and directories would not be
added into CVS.  The only file that must be added to CVS is
@file{gnulib-cache.m4} in the M4 macros directory.  Also, the script for
restoring files not in CVS, customarily called @file{autogen.sh} or
@file{bootstrap.sh}, will typically contain the statement for restoring
the omitted files:

@smallexample
$ gnulib-tool --update
@end smallexample

The @samp{--update} option operates much like the @samp{--import} option,
but it does not offer the possibility to change the way Gnulib is used.
Also it does not report in the ChangeLogs the files that it had to add
because they were missing.