Re: Makefile - how to filter out x.c files that do not have corresponding x.h in a rule that has %.h as prerequisite

[Paul D. Smith]

%% hzmonte@hotmail.com writes:

  h> I found out there is a thread "optional prerequisite" in
  h> help-make@gnu.org on Aug 31, 2005 that says it cannot be done.
  h> I ended up doing this:
  h> $(NO_H_OBJ) =prog3.o prog4.o
  h> $(NO_H_OBJ): %.o %.c
  h>       $(CC) -c $< -o $@
  h> $(filter-out $(NO_H_OBJ), $(OBJ)): %.o: %.c %.h
  h>       $(CC) -c $< -o $@
  h> Still, is there a better way?

It's much easier to leave the %.h off the pattern, and add the header as
a prerequisite to those targets that DO have it.  If you put the %.h in
the pattern then there's no way around it: every single target that
matches the pattern will have to have a .h file; that's what that _means_.


Why not do something like this:

    %.o : %.c
            $(CC) -c $< -o $@

    foo.o: foo.h
    bar.o: bar.h
    baz.o: baz.h

etc.  If you want it to be a little more automatic you could do
something like this:

    %.o : %.c
            $(CC) -c $< -o $@

    __have_h := $(basename $(wildcard $(patsubst %.o,%.h,$(OBJ))))

    $(foreach B,$(__have_h),$(eval $B.o: $B.h))

Note this only works in newer versions of GNU make.

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