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You can’t put a configure substitution (e.g., ‘@FOO@’ or
‘$(FOO)’ where FOO is defined via AC_SUBST) into a
_SOURCES variable. The reason for this is a bit hard to
explain, but suffice to say that it simply won’t work. Automake will
give an error if you try to do this.
Fortunately there are two other ways to achieve the same result. One is
to use configure substitutions in _LDADD variables, the other is
to use an Automake conditional.
_LDADD substitutions_LDADD substitutionsAutomake must know all the source files that could possibly go into a
program, even if not all the files are built in every circumstance. Any
files that are only conditionally built should be listed in the
appropriate EXTRA_ variable. For instance, if
hello-linux.c or hello-generic.c were conditionally included
in hello, the Makefile.am would contain:
bin_PROGRAMS = hello hello_SOURCES = hello-common.c EXTRA_hello_SOURCES = hello-linux.c hello-generic.c hello_LDADD = $(HELLO_SYSTEM) hello_DEPENDENCIES = $(HELLO_SYSTEM)
You can then setup the ‘$(HELLO_SYSTEM)’ substitution from configure.ac:
… case $host in *linux*) HELLO_SYSTEM='hello-linux.$(OBJEXT)' ;; *) HELLO_SYSTEM='hello-generic.$(OBJEXT)' ;; esac AC_SUBST([HELLO_SYSTEM]) …
In this case, the variable HELLO_SYSTEM should be replaced by
either hello-linux.o or hello-generic.o, and added to
both hello_DEPENDENCIES and hello_LDADD in order to be
built and linked in.
An often simpler way to compile source files conditionally is to use Automake conditionals. For instance, you could use this Makefile.am construct to build the same hello example:
bin_PROGRAMS = hello if LINUX hello_SOURCES = hello-linux.c hello-common.c else hello_SOURCES = hello-generic.c hello-common.c endif
In this case, configure.ac should setup the LINUX
conditional using AM_CONDITIONAL (see Conditionals).
When using conditionals like this you don’t need to use the
EXTRA_ variable, because Automake will examine the contents of
each variable to construct the complete list of source files.
If your program uses a lot of files, you will probably prefer a conditional ‘+=’.
bin_PROGRAMS = hello hello_SOURCES = hello-common.c if LINUX hello_SOURCES += hello-linux.c else hello_SOURCES += hello-generic.c endif
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