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24.1 Extending Automake Rules

With some minor exceptions (for example _PROGRAMS variables, TESTS, or XFAIL_TESTS) being rewritten to append ‘$(EXEEXT)’), the contents of a Makefile.am is copied to Makefile.in verbatim.

These copying semantics mean that many problems can be worked around by simply adding some make variables and rules to Makefile.am. Automake will ignore these additions.

Since a Makefile.in is built from data gathered from three different places (Makefile.am, configure.ac, and automake itself), it is possible to have conflicting definitions of rules or variables. When building Makefile.in the following priorities are respected by automake to ensure the user always has the last word:

These overriding semantics make it possible to fine tune some default settings of Automake, or replace some of its rules. Overriding Automake rules is often inadvisable, particularly in the topmost directory of a package with subdirectories. The -Woverride option (see automake Invocation) comes in handy to catch overridden definitions.

Note that Automake does not make any distinction between rules with commands and rules that only specify dependencies. So it is not possible to append new dependencies to an automake-defined target without redefining the entire rule.

However, various useful targets have a ‘-local’ version you can specify in your Makefile.am. Automake will supplement the standard target with these user-supplied targets.

The targets that support a local version are all, info, dvi, ps, pdf, html, check, install-data, install-dvi, install-exec, install-html, install-info, install-pdf, install-ps, uninstall, installdirs, installcheck and the various clean targets (mostlyclean, clean, distclean, and maintainer-clean).

Note that there are no uninstall-exec-local or uninstall-data-local targets; just use uninstall-local. It doesn't make sense to uninstall just data or just executables.

For instance, here is one way to erase a subdirectory during ‘make clean’ (see Clean). 

     clean-local:
             -rm -rf testSubDir

You may be tempted to use install-data-local to install a file to some hard-coded location, but you should avoid this (see Hard-Coded Install Paths).

With the -local targets, there is no particular guarantee of execution order; typically, they are run early, but with parallel make, there is no way to be sure of that.

In contrast, some rules also have a way to run another rule, called a hook; hooks are always executed after the main rule's work is done. The hook is named after the principal target, with ‘-hook’ appended. The targets allowing hooks are install-data, install-exec, uninstall, dist, and distcheck.

For instance, here is how to create a hard link to an installed program: 

     install-exec-hook:
             ln $(DESTDIR)$(bindir)/program$(EXEEXT) \
                $(DESTDIR)$(bindir)/proglink$(EXEEXT)

Although cheaper and more portable than symbolic links, hard links will not work everywhere (for instance, OS/2 does not have ln). Ideally you should fall back to ‘cp -p’ when ln does not work. An easy way, if symbolic links are acceptable to you, is to add AC_PROG_LN_S to configure.ac (see Particular Program Checks) and use ‘$(LN_S)’ in Makefile.am.

For instance, here is how you could install a versioned copy of a program using ‘$(LN_S)’: 

     install-exec-hook:
             cd $(DESTDIR)$(bindir) && \
               mv -f prog$(EXEEXT) prog-$(VERSION)$(EXEEXT) && \
               $(LN_S) prog-$(VERSION)$(EXEEXT) prog$(EXEEXT)

Note that we rename the program so that a new version will erase the symbolic link, not the real binary. Also we cd into the destination directory in order to create relative links.

When writing install-exec-hook or install-data-hook, please bear in mind that the exec/data distinction is based on the installation directory, not on the primary used (see The Two Parts of Install). So a foo_SCRIPTS will be installed by install-data, and a barexec_SCRIPTS will be installed by install-exec. You should define your hooks consequently.