Like conditional programs (see Conditional compilation of programs), there are
two main ways to build conditional libraries: using Automake
conditionals or using Autoconf
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
_LTLIBRARIES variables such as
However, for libraries determined at configure time (and thus
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
The examples below illustrate the differences between these two methods.
Here is an example where
WANTEDLIBS is an
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 '$(libdir)'
Here is how the same Makefile.am would look using Automake
Automake is able to compute the -rpath setting itself, because
it’s clear that both libraries will end up in ‘$(libdir)’ if they
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 …