The ‘$(LIBOBJS)’ and ‘$(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_REPLACE_FUNCS (see Generic Function Checks in The Autoconf Manual), or
AC_FUNC_ALLOCA (see Particular
Function Checks in The Autoconf Manual). Many other Autoconf
Using these variables is very similar to doing conditional compilation
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
# 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
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) $(ALLOCA)
The library can have any name, of course, and anyway it is not going
to be installed: it just holds the replacement versions of the missing
or broken functions so we can later link them in. Many projects
also include extra functions, specific to the project, in that
library: they are simply added on the
There is a small trap here, though: ‘$(LIBOBJS)’ and
‘$(ALLOCA)’ might be empty, and building an empty library is not
portable. You should ensure that there is always something to put in
libcompat.a. Most projects will also add some utility
functions in that directory, and list them in
libcompat_a_SOURCES, so in practice libcompat.a cannot
Finally here is how this library could be used from the src/ directory.
# src/Makefile.am # Link all programs in this directory with libcompat.a LDADD = ../lib/libcompat.a bin_PROGRAMS = tool1 tool2 … tool1_SOURCES = … tool2_SOURCES = …
When option subdir-objects is not used, as in the above
example, the variables ‘$(LIBOBJS)’ or ‘$(ALLOCA)’ can only
be used in the directory where their sources lie. E.g., here it would
be wrong to use ‘$(LIBOBJS)’ or ‘$(ALLOCA)’ in
src/Makefile.am. However if both subdir-objects and
AC_CONFIG_LIBOBJ_DIR are used, it is OK to use these variables
in other directories. For instance src/Makefile.am could be
changed as follows.
# src/Makefile.am AUTOMAKE_OPTIONS = subdir-objects 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):
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
LIBOBJS in The Autoconf Manual.