C + Make

[Chris Torek]

[Although this is not strictly a `C topic', a sufficient number of C
systems come with a `make' program that I decided to leave it here.]

In article <1990Sep11.165709.24875 at sj.ate.slb.com>
mullen at sj.ate.slb.com (Lew Mullen) writes:
>There are several "dependency makers" on the net.  They are
>based on a feature of make, that a target may have more than one
>dependency line, but only one may have commands with it ...

Actually, strinctly speaking this is not quite right.  Unix make (and
any clones that follow it sufficiently well) allow more than one `recipe'
if and only if double-colon rules are used:

	foo::
		@echo foo
	foo::
		@echo bar

`make foo' prints `foo\nbar\n'.

>[This] makes it possible to create a self-editing Makefile,
>which updates it's own dependency "section".

I have two recommendations, both of which were learned through experience.
These are:

 1. Put the dependency-making in a separate program.  The method by which
    dependency extraction is done varies from system to system.  This way
    all the details are in one place (the `mkdep' script or whatever).

 2. Do not make `mkdep' edit the makefile.  Put the generated dependency
    lists in a separate file.  4.3BSD-tahoe and later versions of make
    read a file called `.depend', if it exists and no `-f' options are
    given.  Other makes require a subterfuge: instead of running the
    regular `make' program directly, run a front-end that checks for
    .depend.  If the file exists (and no -f arguments are given), run

	make -f Makefile -f .depend <original args>

    or

	make -f makefile -f .depend <original args>

    (use the same rules your `make' uses to locate makefiles to decide
    which is the `main' makefile).

In Bourne shell, the latter can be written as

	if [ -f .depend ]; then
		if [ -f makefile ]; then
			f="-f makefile -f .depend"
		else
			f="-f Makefile -f .depend"
		fi
		for i do
			case "$i" in -f*) f=;; esac
		done
	else
		f=
	fi
	
	exec /bin/make $f ${1+"$@"} MACHINE=${MACHINE-`machine`}

(the MACHINE= is for trees that hide machine-specific sources in
machine-specific directories: a useful trick).  This is what we
used to use on our non-BSD machines, though now we use pmake.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 405 2750)
Domain:	chris at cs.umd.edu	Path:	uunet!mimsy!chris