diff options
author | Bruno Haible <bruno@clisp.org> | 2017-05-10 19:19:51 +0200 |
---|---|---|
committer | Bruno Haible <bruno@clisp.org> | 2017-05-10 19:19:51 +0200 |
commit | 67d14683778cb3fd8dbfda5a03f5fe407f57389a (patch) | |
tree | 19fc3d2bf61f2514a25a47aee028b0402986004b /doc/transversal.texi | |
parent | 3f67783fb1ff94c6a3732cfbc4850048bcabc4ff (diff) | |
download | gnulib-67d14683778cb3fd8dbfda5a03f5fe407f57389a.tar.gz |
Prepare for reordering sections in the manual.
* doc/gnulib.texi: Move several sections to separate files. Include
these files.
* doc/out-of-memory.texi: New file, extracted from doc/gnulib.texi.
* doc/obsolete.texi: Likewise.
* doc/extra-tests.texi: Likewise.
* doc/transversal.texi: Likewise.
* doc/namespace.texi: Likewise.
* doc/check-version.texi: Likewise.
* doc/windows-sockets.texi: Likewise.
* doc/windows-libtool.texi: Likewise.
* doc/licenses-texi.texi: Likewise.
* doc/build-automation.texi: Likewise.
* doc/c-locale.texi: Likewise.
Diffstat (limited to 'doc/transversal.texi')
-rw-r--r-- | doc/transversal.texi | 69 |
1 files changed, 69 insertions, 0 deletions
diff --git a/doc/transversal.texi b/doc/transversal.texi new file mode 100644 index 0000000000..6c814a9a8f --- /dev/null +++ b/doc/transversal.texi @@ -0,0 +1,69 @@ +@node Modules that modify the way other modules work +@section Modules that modify the way other modules work + +The normal way to design modules is that each module has its own code, +and the module dependencies provide the facilities on which this code +can rely. But sometimes it is necessary to use more advanced +techniques. For example: +@itemize +@item +You may want to have optional module dependencies: Let module A use +facilities provided by module B, if module B is present, but without +requiring that module B is present. +@item +A module can indicate support for particular behaviours. For example, +Gnulib has a module @samp{sigpipe} that requests POSIX compatible +SIGPIPE behaviour from all other modules -- something that is not +enabled by default. Or consider the @samp{nonblocking} module, that is +an indicator that all I/O functions should handle non-blocking file +descriptors -- something that, equally, is not enabled by default. +@item +A module can indicate to other modules that they can rely on certain +guarantees, and thus omit specific code. For example, when Gnulib's +@samp{malloc-gnu} module is present, you can omit code that test +@code{n} against zero when you call @code{malloc (n)}. +@end itemize + +Be aware that these advanced techniques likely cause breakage in the +situation of multiple @code{gnulib-tool} invocations in the scope of a +single @code{configure} file. This is because the question ``is module +B present?'' does not have a unique answer in such situations. +@code{gnulib-tool} has support for these techniques in the situation of +@code{--create-testdir --single-configure}, which basically has two +@code{gnulib-tool} invocations, one for a set of modules that end up in +@code{gllib}, and one for the set of modules that end up in +@code{gltests}. But you should be aware that this does not cover the +general situation. + +Which technique to use, depends on the answer to the question: ``If my +module occurs among the modules of @code{gltests}, should it have an +effect on the modules in @code{gllib}?'' + +If the answer is ``no'', your module description should invoke the +Autoconf macro @code{gl_MODULE_INDICATOR}. This Autoconf macro takes +one argument: the name of your module. The effect of +@code{gl_MODULE_INDICATOR([@var{my-module}])} is to define, in +@code{config.h}, a C macro @code{GNULIB_@var{MY_MODULE}} that indicates +whether your macro is considered to be present. This works even when +your macro is used in @code{gltests}: @code{GNULIB_@var{MY_MODULE}} +will then evaluate to 1 in @code{gltests} but to 0 in @code{gllib}. + +If the answer is ``yes'', you have two techniques available. The first +one is to invoke a similar Autoconf macro, named +@code{gl_MODULE_INDICATOR_FOR_TESTS}. It works similarly. However, +when your macro is used in @code{gltests}, @code{GNULIB_@var{MY_MODULE}} +will evaluate to 1 both in @code{gltests} and in @code{gllib}. + +The second one is to define a shell variable in the @code{configure} +file that tells whether your module is present, through use of +@code{m4_divert_text}. The Autoconf macros of a dependency module will +initialize this shell variable, through +@samp{m4_divert_text([DEFAULTS], [@var{my_shell_var}=no])}. The +Autoconf macros of your module will override this value, through +@samp{m4_divert_text([INIT_PREPARE], [@var{my_shell_var}=yes])}. Then +you can use @code{@var{my_shell_var}} in the Autoconf macros of both +modules. You can find more details about this technique in the Gnulib +module @code{getopt-gnu}. + +Reminder: These techniques are advanced. They have the potential to +cause lots of headaches if you apply them incorrectly. |