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16 Rebuilding Makefiles

Automake generates rules to automatically rebuild ‘Makefile’s, ‘configure’, and other derived files like ‘Makefile.in’.

If you are using AM_MAINTAINER_MODE in ‘configure.ac’, then these automatic rebuilding rules are only enabled in maintainer mode.

Sometimes it is convenient to supplement the rebuild rules for ‘configure’ or ‘config.status’ with additional dependencies. The variables CONFIGURE_DEPENDENCIES and CONFIG_STATUS_DEPENDENCIES can be used to list these extra dependencies. These variables should be defined in all ‘Makefile’s of the tree (because these two rebuild rules are output in all them), so it is safer and easier to AC_SUBST them from ‘configure.ac’. For instance, the following statement will cause ‘configure’ to be rerun each time ‘version.sh’ is changed.

AC_SUBST([CONFIG_STATUS_DEPENDENCIES], ['$(top_srcdir)/version.sh'])

Note the ‘$(top_srcdir)/’ in the file name. Since this variable is to be used in all ‘Makefile’s, its value must be sensible at any level in the build hierarchy.

Beware not to mistake CONFIGURE_DEPENDENCIES for CONFIG_STATUS_DEPENDENCIES.

CONFIGURE_DEPENDENCIES adds dependencies to the ‘configure’ rule, whose effect is to run autoconf. This variable should be seldom used, because automake already tracks m4_included files. However it can be useful when playing tricky games with m4_esyscmd or similar non-recommendable macros with side effects. Be also aware that interactions of this variable with the autom4te cache in The Autoconf Manual are quite problematic and can cause subtle breakage, so you might want to disable the cache if you want to use CONFIGURE_DEPENDENCIES.

CONFIG_STATUS_DEPENDENCIES adds dependencies to the ‘config.status’ rule, whose effect is to run ‘configure’. This variable should therefore carry any non-standard source that may be read as a side effect of running configure, like ‘version.sh’ in the example above.

Speaking of ‘version.sh’ scripts, we recommend against them today. They are mainly used when the version of a package is updated automatically by a script (e.g., in daily builds). Here is what some old-style ‘configure.ac’s may look like:

AC_INIT
. $srcdir/version.sh
AM_INIT_AUTOMAKE([name], $VERSION_NUMBER)
…

Here, ‘version.sh’ is a shell fragment that sets VERSION_NUMBER. The problem with this example is that automake cannot track dependencies (listing ‘version.sh’ in CONFIG_STATUS_DEPENDENCIES, and distributing this file is up to the user), and that it uses the obsolete form of AC_INIT and AM_INIT_AUTOMAKE. Upgrading to the new syntax is not straightforward, because shell variables are not allowed in AC_INIT’s arguments. We recommend that ‘version.sh’ be replaced by an M4 file that is included by ‘configure.ac’:

m4_include([version.m4])
AC_INIT([name], VERSION_NUMBER)
AM_INIT_AUTOMAKE
…

Here ‘version.m4’ could contain something like ‘m4_define([VERSION_NUMBER], [1.2])’. The advantage of this second form is that automake will take care of the dependencies when defining the rebuild rule, and will also distribute the file automatically. An inconvenience is that autoconf will now be rerun each time the version number is bumped, when only ‘configure’ had to be rerun in the previous setup.


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