What's wrong w/this makefile? (.SUFFIXES:)

[Howard Johnson]

The problem with Mr. Bergan's makefile stems from the evaluation of the
.SUFFIXES dependencies.  His goal was to have a preprocessor perform some
transformation on his source code and produce C code (presumably "cat" is
for illustration).  The solution:

	.SUFFIXES:	.qc

	mtest:	mtest.o
		$(CC) -o $@ mtest.o

	.qc.o:
		cat $< > $*.c
		$(CC) $(CFLAGS) -c $*.c

	.qc.c:		# optional rule so "make mtest.c" works
		cat $< > $*.c

The problem with Mr. Bergan's first example is discussed below:

>	.SUFFIXES: .qc
>
>	.qc.c:
>		cat $< >mtest.c
>
>	mtest: mtest.o
>		cc -o mtest mtest.o

First, "make mtest" is invoked.  Unless a .SUFFIXES: dependency with nothing
after the colon is given (to clear default dependencies), the .qc suffix is
appended to a default list producing something like:

	.SUFFIXES: .o .c .e .r .f .y .yr .ye .l .s .qc

"mtest" depends on "mtest.o".  Since there are no explicit dependencies for
mtest.o, the .SUFFIXES list is scanned.  Note that the first suffix
which has an implicit dependency for .o is .c.  Several other implicit
dependency rules are found, but none of them have the inferred prerequisites
either.  (This includes the .qc prerequisite, since there is no .qc.o rule.)
Hence, *make* gives up, complaining that (most likely) the prerequisite
mtest.c file is missing.

Mr. Bergan's quick fix (including the explicit "mtest.o: mtest.c" dependency)
works because it forces make to match the .SUFFIX dependencies for a .qc.c
rule.

After wading through a few 10-page makefiles, one soon has motivation to
learn about details such as this.
-- 
	Howard Johnson		Cyb Systems, Austin, TX
		cyb-eng!howard at ut-sally.ARPA or
..!{gatech,harvard,ihnp4,nbires,seismo,ucb-vax}!ut-sally!cyb-eng!howard