8.3.3 Building Libtool Libraries Conditionally

Like conditional programs (see section Conditional compilation of programs), there are two main ways to build conditional libraries: using Automake conditionals or using Autoconf AC_SUBSTitutions.

The important implementation detail you have to be aware of is that the place where a library will be installed matters to libtool: it needs to be indicated at link-time using the ‘-rpath’ option.

For libraries whose destination directory is known when Automake runs, Automake will automatically supply the appropriate ‘-rpath’ option to libtool. This is the case for libraries listed explicitly in some installable _LTLIBRARIES variables such as lib_LTLIBRARIES.

However, for libraries determined at configure time (and thus mentioned in EXTRA_LTLIBRARIES), Automake does not know the final installation directory. For such libraries you must add the ‘-rpath’ option to the appropriate _LDFLAGS variable by hand.

The examples below illustrate the differences between these two methods.

Here is an example where WANTEDLIBS is an AC_SUBSTed variable set at ‘./configure’-time to either ‘libfoo.la’, ‘libbar.la’, both, or none. Although ‘$(WANTEDLIBS)’ appears in the lib_LTLIBRARIES, Automake cannot guess it relates to ‘libfoo.la’ or ‘libbar.la’ at the time it creates the link rule for these two libraries. Therefore the ‘-rpath’ argument must be explicitly supplied.

EXTRA_LTLIBRARIES = libfoo.la libbar.la
lib_LTLIBRARIES = $(WANTEDLIBS)
libfoo_la_SOURCES = foo.c …
libfoo_la_LDFLAGS = -rpath '$(libdir)'
libbar_la_SOURCES = bar.c …
libbar_la_LDFLAGS = -rpath '$(libdir)'

Here is how the same ‘Makefile.am’ would look using Automake conditionals named WANT_LIBFOO and WANT_LIBBAR. Now Automake is able to compute the ‘-rpath’ setting itself, because it’s clear that both libraries will end up in ‘$(libdir)’ if they are installed.

lib_LTLIBRARIES =
if WANT_LIBFOO
lib_LTLIBRARIES += libfoo.la
endif
if WANT_LIBBAR
lib_LTLIBRARIES += libbar.la
endif
libfoo_la_SOURCES = foo.c …
libbar_la_SOURCES = bar.c …

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