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.

Answer 2

The only clean solution is to incorporate the difference into the target names. E.g. you can define a variable $(DEBUG) and consistently use it in all targets that depend on the compile step.

Answer 3

Keeping variant sets of object files (as in bobbogo's solution) is probably the best way, but if for some reason you don't want to do that, you can use empty files as markers, to indicate which way you last built the executable:

%-marker:
        @rm -f $(OBJECTS) *-marker
        @touch $@

debug: GCCFLAGS += -ggdb

debug: SOMEOTHERFLAG = WHATEVER

all debug: % : %-marker
        @echo making $@
        @$(MAKE) -S GCCFLAGS='$(GCCFLAGS)' SOMEOTHERFLAG='$(SOMEOTHERFLAG)' main

There are other variants on this idea; you could have a small file containing the flag settings, which the makefile would build and include. That would be clever, but not really any cleaner than this.