automake-1.16: LIBOBJS
8.6 Special handling for ‘LIBOBJS’ and ‘ALLOCA’
===============================================
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_LIBOBJ’,
DONTPRINTYET ‘AC_REPLACE_FUNCS’ (⇒Generic Function Checks (autoconf)Generic
Functions.), or ‘AC_FUNC_ALLOCA’ (*noteParticular Function Checks:
DONTPRINTYET ‘AC_REPLACE_FUNCS’ (⇒Generic Function Checks (autoconf)Generic
Functions.), or ‘AC_FUNC_ALLOCA’ (⇒Particular Function Checks
(autoconf)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 ‘$(ALLOCA)’
might be empty, and building an empty library is not portable. You
should ensure that there is always something to put in ‘libcompat.a’.
Most projects will also add some utility functions in that directory,
and list them in ‘libcompat_a_SOURCES’, so in practice ‘libcompat.a’
cannot be empty.
Finally here is how this library could be used from the ‘src/’
directory.
# src/Makefile.am
# Link all programs in this directory with libcompat.a
LDADD = ../lib/libcompat.a
bin_PROGRAMS = tool1 tool2 ...
tool1_SOURCES = ...
tool2_SOURCES = ...
When option ‘subdir-objects’ is not used, as in the above example,
the variables ‘$(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’): ‘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. ⇒
‘AC_LIBOBJ’ vs. ‘LIBOBJS’ (autoconf)AC_LIBOBJ vs LIBOBJS.
Info Catalog
automake-1.16: Default _SOURCES
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automake-1.16: Program Variables