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 compile switches (like -ggdb, for example). Since the targets use different compile switches, you obviously need to rebuild the entire project if you switch between the two. But GNUmake doesn't naturally recognize this.

So if you type make all you'll get

Building ...
...

Then if you type make debug, you get

make: Nothing to be done for `debug'.

So my question is: how do you implement a clean solution in the Makefile to notice that the last build used a different pseudo-target, or different compile switches, than the one you want currently? If they are different, the Makefile would rebuild everything.

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.