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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 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 $(INTLLIBS) rmt_LDADD = cpio_SOURCES = … pax_SOURCES = … mt_SOURCES = … rmt_SOURCES = …
prog_LDADD is inappropriate for passing program-specific
linker flags (except for ‘-l’, ‘-L’, ‘-dlopen’ and
‘-dlpreopen’). So, use the prog_LDFLAGS variable for
this purpose.
It is also occasionally useful to have a program depend on some other
target that is not actually part of that program. This can be done
using either the prog_DEPENDENCIES or the
EXTRA_prog_DEPENDENCIES variable. Each program depends on
the contents both variables, but no further interpretation is done.
Since these dependencies are associated to the link rule used to
create the programs they should normally list files used by the link
command. That is ‘*.$(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 section Built 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 compilation of sources shows a situation where _DEPENDENCIES
may be used.
The EXTRA_prog_DEPENDENCIES may be useful for cases where
you merely want to augment the automake-generated
prog_DEPENDENCIES rather than replacing it.
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.
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