How do you implement a Makefile that remembers the last build target?

Question

Let\'s say you have a Makefile with two pseudo-targets, \'all\' and \'debug\'. The \'debug\' target is meant to build the same project as \'all\', except with some different

Answer 1

Put the build products into different directory trees (whilst keeping one copy of the source of course). That way you are always just a short compile from an up-to-date build, be it debug or release (or even others). No possibility of confusion either.

EDIT

Sketch of the above.

src := 1.c 2.c 3.c
bare-objs := ${src:%.c=%.o}
release-objs := ${bare-objs:%=Release/%}
debug-objs := ${bare-objs:%=Debug/%}

Release/prog: ${release-objs}
Debug/prog: ${debug-objs}

${release-objs}: Release/%.o: %.c # You gotta lurve static pattern rules
    gcc -c $< -o $@

${debug-objs}: Debug/%.o: %.c
    gcc -c $< -o $@

Release/prog Debug/prog:
    gcc $^ -o $@

.PHONY: all
all: Release/prog ; echo $@ Success

.PHONY: debug
debug: Debug/prog ; echo $@ Success

(Disclaimer: not tested, nor even run through make.)

There you go. It's even -j safe so you can do make -j5 all debug. There is a lot of obvious boiler plate just crying out for tidying up.