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

[hzmonte]

For the record:
  mkl> I think it would be really nice
  mkl> that 'make' one day would be enhanced to accomodate optional
  mkl> prerequisites.  As you can see, my situation is not a rare one
in
  mkl> real life.  Thanks.

Paul D. Smith replies:

Without stating an opinion one way or the other on the concept of an
optional prerequisite, the case made in your email is not, to me,
compelling.

Having a rule like this:

    %.o: %.c %.h

is just not appropriate, unless you have some rule in your development
process that forces every .c to have a .h (and I would think that
process would be broken).  That's not what a pattern rule is for... you
should only put files that MUST exist, or the thing won't work at all,
as prerequisite patterns.

Also, it's almost unheard of that a .c file includes nothing other than
a single .h file, so you'll have to define all the other .h files as
prerequisites anyway... what makes that particular .h file so special
that it deserves a pattern?

You should define the pattern rule with just the bare minimum MANDATORY
prerequisites, then use extra prerequisite definitions to add the
optional ones to the appropriate target.

    %.o : %.c
            ...
    foo.o: foo.h
    bar.o: bar.h

But more likely, as above, you'll have to implement some kind of
automatically generated list of header files, in which case that will
all be done for you.

====================================================
Just for curiosity, does one put the system header files, e.g.
stdlib.h, as prerequisites, in real life, as in
foo.o: foo.h stdlib.h
?  I guess not because there would be too many.  But strictly speaking,
we should just in case the system gets changed, shouldn't we?