8.1.2 Linking the program

If you need to link against libraries that are not found by configure, you can use LDADD to do so. This variable is used to specify additional objects or libraries to link with; it is inappropriate for specifying specific linker flags, you should use AM_LDFLAGS for this purpose. Sometimes, multiple programs are built in one directory but do not share the same link-time requirements. In this case, you can use the prog_LDADD variable (where prog is the name of the program as it appears in some _PROGRAMS variable, and usually written in lowercase) to override the global LDADD. If this variable exists for a given program, then that program is not linked using LDADD. For instance, in GNU cpio, pax, cpio and mt are linked against the library libcpio.a. However, rmt is built in the same directory, and has no such link requirement. Also, mt and rmt are only built on certain architectures. Here is what cpio's src/Makefile.am looks like (abridged):

     bin_PROGRAMS = cpio pax $(MT)
     libexec_PROGRAMS = $(RMT)
     EXTRA_PROGRAMS = mt rmt
     
     LDADD = ../lib/libcpio.a $(INTLLIBS)
     rmt_LDADD =
     
     cpio_SOURCES = ...
     pax_SOURCES = ...
     mt_SOURCES = ...
     rmt_SOURCES = ...

prog_LDADD is inappropriate for passing program-specific linker flags (except for -l, -L, -dlopen and -dlpreopen). So, use the prog_LDFLAGS variable for this purpose.

It is also occasionally useful to have a program depend on some other target that is not actually part of that program. This can be done using the prog_DEPENDENCIES variable. Each program depends on the contents of such a variable, but no further interpretation is done.

Since these dependencies are associated to the link rule used to create the programs they should normally list files used by the link command. That is *.$(OBJEXT), *.a, or *.la files. In rare cases you may need to add other kinds of files such as linker scripts, but listing a source file in _DEPENDENCIES is wrong. If some source file needs to be built before all the components of a program are built, consider using the BUILT_SOURCES variable instead (see Sources).

If prog_DEPENDENCIES is not supplied, it is computed by Automake. The automatically-assigned value is the contents of prog_LDADD, with most configure substitutions, -l, -L, -dlopen and -dlpreopen options removed. The configure substitutions that are left in are only ‘$(LIBOBJS)’ and ‘$(ALLOCA)’; these are left because it is known that they will not cause an invalid value for prog_DEPENDENCIES to be generated.

Conditional Sources shows a situation where _DEPENDENCIES is useful.

We recommend that you avoid using -l options in LDADD or prog_LDADD when referring to libraries built by your package. Instead, write the file name of the library explicitly as in the above cpio example. Use -l only to list third-party libraries. If you follow this rule, the default value of prog_DEPENDENCIES will list all your local libraries and omit the other ones.