Is there a non-transitive version of target-specific variables?

Question

I've been using target-specific variables for a while to pass object-specific flags. For example, if I need to link an executable with a certain library, I might do:

bin/foo: LDFLAGS += -lbar

And the bin/% rule will pick up -lbar. This works great for system libraries, but there's a problem when the libraries I'm linking against are also part of the project. In this case I'll have a dependency like

bin/foo: lib/libbar.so

and the target-specific rule I applied to bin/foo is also passed when building lib/libbar.so! This doesn't work since libbar.so can't link to itself. As the manual says,

There is one more special feature of target-specific variables: when you define a target-specific variable that variable value is also in effect for all prerequisites of this target, and all their prerequisites, etc. (unless those prerequisites override that variable with their own target-specific variable value).

What can I do to add that flag only for bin/foo but not its prerequisites? The quote above suggests something like

lib/libbar.so: LDFLAGS :=

but that's not ideal since my real project has other LDFLAGS set globally that I want to keep.

Here's a reproducer:

Makefile

bin/%:
  @mkdir -p $(@D)
  gcc $(LDFLAGS) $(filter %.o,$^) -o $@

lib/%:
  @mkdir -p $(@D)
  gcc $(LDFLAGS) -shared $(filter %.o,$^) -o $@

obj/%.o: src/%.c
  @mkdir -p $(@D)
  gcc $(CFLAGS) -c $< -o $@

bin/foo: obj/foo.o lib/libbar.so
bin/foo: LDFLAGS := -Wl,-rpath='$$ORIGIN/../lib' -Llib -lbar

lib/libbar.so: obj/bar.o

clean:
  rm -rf bin lib obj

.PHONY: clean

src/foo.c

int bar(void);

int main() {
  return bar();
}

src/bar.c

int bar() {
  return 42;
}

This is broken:

$ make bin/foo
gcc  -c src/foo.c -o obj/foo.o
gcc  -c src/bar.c -o obj/bar.o
gcc -Wl,-rpath='$ORIGIN/../lib' -Llib -lbar -shared obj/bar.o -o lib/libbar.so
/bin/ld: lib/libbar.so: file not recognized: file truncated
collect2: error: ld returned 1 exit status
make: *** [Makefile:7: lib/libbar.so] Error 1

But if I build the lib first it works:

$ make lib/libbar.so
gcc  -c src/bar.c -o obj/bar.o
gcc  -shared obj/bar.o -o lib/libbar.so
$ make bin/foo
gcc  -c src/foo.c -o obj/foo.o
gcc -Wl,-rpath='$ORIGIN/../lib' -Llib -lbar obj/foo.o -o bin/foo
$ ./bin/foo; echo $?
42

MadScientist MadScientist · Accepted Answer · 2018-08-07T20:51:22

You could use constructed variable names instead of target-specific variables. It's a different syntax but allows for specific target settings.

Something like:

LDFLAGS += $($@_FLAGS)

bin/foo_FLAGS = -lbar

More details can be found here (and related articles)

The short answer is there's no target-specific-without-inheritance feature available. You'll have to use something different if you don't want inheritance.

Is there a non-transitive version of target-specific variables?

2 Answers