LIBOBJS and ALLOCAThe ‘$(LIBOBJS)’ and ‘$(ALLOCA)’ variables list object files that should be compiled into the project to provide an implementation for functions that are missing or broken on the host system. They are substituted by configure.
These variables are defined by Autoconf macros such as
AC_LIBOBJ, AC_REPLACE_FUNCS (see Generic Functions), or
AC_FUNC_ALLOCA (see Particular Functions). Many other Autoconf
macros call AC_LIBOBJ or AC_REPLACE_FUNCS to
populate ‘$(LIBOBJS)’.
Using these variables is very similar to doing conditional compilation
using AC_SUBST variables, as described in Conditional Sources. That is, when building a program, ‘$(LIBOBJS)’ and
‘$(ALLOCA)’ should be added to the associated ‘*_LDADD’
variable, or to the ‘*_LIBADD’ variable when building a library.
However there is no need to list the corresponding sources in
‘EXTRA_*_SOURCES’ nor to define ‘*_DEPENDENCIES’. Automake
automatically adds ‘$(LIBOBJS)’ and ‘$(ALLOCA)’ to the
dependencies, and it will discover the list of corresponding source
files automatically (by tracing the invocations of the
AC_LIBSOURCE Autoconf macros). If you have already defined
‘*_DEPENDENCIES’ explicitly for an unrelated reason, then you
either need to add these variables manually, or use
‘EXTRA_*_DEPENDENCIES’ instead of ‘*_DEPENDENCIES’.
These variables are usually used to build a portability library that
is linked with all the programs of the project. We now review a
sample setup. First, configure.ac contains some checks that
affect either LIBOBJS or ALLOCA.
# configure.ac … AC_CONFIG_LIBOBJ_DIR([lib]) … AC_FUNC_MALLOC dnl May add malloc.$(OBJEXT) to LIBOBJS AC_FUNC_MEMCMP dnl May add memcmp.$(OBJEXT) to LIBOBJS AC_REPLACE_FUNCS([strdup]) dnl May add strdup.$(OBJEXT) to LIBOBJS AC_FUNC_ALLOCA dnl May add alloca.$(OBJEXT) to ALLOCA … AC_CONFIG_FILES([ lib/Makefile src/Makefile ]) AC_OUTPUT
The AC_CONFIG_LIBOBJ_DIR tells Autoconf that the source files
of these object files are to be found in the lib/ directory.
Automake can also use this information, otherwise it expects the
source files are to be in the directory where the ‘$(LIBOBJS)’
and ‘$(ALLOCA)’ variables are used.
The lib/ directory should therefore contain malloc.c, memcmp.c, strdup.c, alloca.c. Here is its Makefile.am:
# lib/Makefile.am noinst_LIBRARIES = libcompat.a libcompat_a_SOURCES = libcompat_a_LIBADD = $(LIBOBJS) $(ALLOCA)
The library can have any name, of course, and anyway it is not going
to be installed: it just holds the replacement versions of the missing
or broken functions so we can later link them in. Many projects
also include extra functions, specific to the project, in that
library: they are simply added on the _SOURCES line.
There is a small trap here, though: ‘$(LIBOBJS)’ and
‘$(ALLOCA)’ might be empty, and building an empty library is not
portable. You should ensure that there is always something to put in
libcompat.a. Most projects will also add some utility
functions in that directory, and list them in
libcompat_a_SOURCES, so in practice libcompat.a cannot
be empty.
Finally here is how this library could be used from the src/ directory.
# src/Makefile.am # Link all programs in this directory with libcompat.a LDADD = ../lib/libcompat.a bin_PROGRAMS = tool1 tool2 … tool1_SOURCES = … tool2_SOURCES = …
The variables ‘$(LIBOBJS)’ or ‘$(ALLOCA)’ are typically
used in the directory where their sources lie. However, if
AC_CONFIG_LIBOBJ_DIR is used, it is OK to use these variables
in other directories. For instance src/Makefile.am could be
changed as follows.
# src/Makefile.am LDADD = $(LIBOBJS) $(ALLOCA) bin_PROGRAMS = tool1 tool2 … tool1_SOURCES = … tool2_SOURCES = …
Because ‘$(LIBOBJS)’ and ‘$(ALLOCA)’ contain object
file names that end with ‘.$(OBJEXT)’, they are not suitable for
Libtool libraries (where the expected object extension is .lo):
LTLIBOBJS and LTALLOCA should be used instead.
LTLIBOBJS is defined automatically by Autoconf and should not
be defined by hand (as in the past), however at the time of writing
LTALLOCA still needs to be defined from ALLOCA manually.
See AC_LIBOBJ vs. LIBOBJS in The Autoconf Manual.