Sometimes you want to build Libtool libraries that should not be installed. These are called libtool convenience libraries and are typically used to encapsulate many sublibraries, later gathered into one big installed library.
Libtool convenience libraries are declared by directory-less variables
such as noinst_LTLIBRARIES, check_LTLIBRARIES, or even
EXTRA_LTLIBRARIES. Unlike installed Libtool libraries they do
not need an -rpath flag at link time (this is in fact the only
difference).
Convenience libraries listed in noinst_LTLIBRARIES are always
built. Those listed in check_LTLIBRARIES are built only upon
‘make check’. Finally, libraries listed in
EXTRA_LTLIBRARIES are never built explicitly: Automake outputs
rules to build them, but if the library does not appear as a Makefile
dependency anywhere it won’t be built (this is why
EXTRA_LTLIBRARIES is used for conditional compilation).
Here is a sample setup merging Libtool convenience libraries from subdirectories into one main libtop.la library.
# -- Top-level Makefile.am -- SUBDIRS = sub1 sub2 ... lib_LTLIBRARIES = libtop.la libtop_la_SOURCES = libtop_la_LIBADD = \ sub1/libsub1.la \ sub2/libsub2.la \ ... # -- sub1/Makefile.am -- noinst_LTLIBRARIES = libsub1.la libsub1_la_SOURCES = ... # -- sub2/Makefile.am -- # showing nested convenience libraries SUBDIRS = sub2.1 sub2.2 ... noinst_LTLIBRARIES = libsub2.la libsub2_la_SOURCES = libsub2_la_LIBADD = \ sub21/libsub21.la \ sub22/libsub22.la \ ...
When using such a setup, beware that automake will assume
libtop.la is to be linked with the C linker. This is because
libtop_la_SOURCES is empty, so automake picks C as
default language. If libtop_la_SOURCES was not empty,
automake would select the linker as explained in How the Linker is Chosen.
If one of the sublibraries contains non-C source, it is important that
the appropriate linker be chosen. One way to achieve this is to
pretend that there is such a non-C file among the sources of the
library, thus forcing automake to select the appropriate
linker. Here is the top-level Makefile of our example updated
to force C++ linking.
SUBDIRS = sub1 sub2 ... lib_LTLIBRARIES = libtop.la libtop_la_SOURCES = # Dummy C++ source to cause C++ linking. nodist_EXTRA_libtop_la_SOURCES = dummy.cxx libtop_la_LIBADD = \ sub1/libsub1.la \ sub2/libsub2.la \ ...
‘EXTRA_*_SOURCES’ variables are used to keep track of source
files that might be compiled (this is mostly useful when doing
conditional compilation using AC_SUBST; see Libtool Libraries with Conditional Sources), and the nodist_ prefix means the listed
sources are not to be distributed (see Program and Library Variables). In effect the file dummy.cxx does not need to
exist in the source tree. Of course if you have some real source file
to list in libtop_la_SOURCES there is no point in cheating with
nodist_EXTRA_libtop_la_SOURCES.