Pattern rule depending on another pattern rule

Question

consider the following fragment of my makefile:

gui_backend: $(BUILDDIR)/gui_backend

$(BUILDDIR)/gui_backend: $(BUILDDIR)/gui_backend.o $(BUILDDIR)/config.pb.o
    $(CXX) $(LDFLAGS) -o $@ $^

$(BUILDDIR)/%.o: $(SRCDIR)/%.cc $(SRCDIR)/%.h | $(BUILDDIR)
    $(CXX) $(CXXFLAGS) -c -o $@ $<

$(SRCDIR)/%.pb.cc: $(PROTODIR)/%.proto
    $(PROTOC) $(PROTOOPTIONS) --cpp_out=$(SRCDIR) $<

now if I call to make gui_backend, I get

make: *** No rule to make target `build/config.pb.o', needed by `build/gui_backend'.

Why doesn't make pick up on the $(BUILDDIR)/%.o pattern here? If I state things explicitly by adding the rule

$(BUILDDIR)/config.pb.o: $(SRCDIR)/config.pb.cc $(SRCDIR)/config.pb.h | $(BUILDDIR)
    $(CXX) $(CXXFLAGS) -c -o $@ $<

it seems to work. However, I would like to keep the makefile as concise as possible.

Running make with -d gives me the following (complete output here):

Considering target file `build/config.pb.o'.
       File `build/config.pb.o' does not exist.
       Looking for an implicit rule for `build/config.pb.o'.
       Trying pattern rule with stem `config.pb'.
       Trying implicit prerequisite `src/config.pb.cc'.
       Trying pattern rule with stem `config.pb'.
       Trying implicit prerequisite `build/config.pb.c'.
       Trying pattern rule with stem `config.pb'.
       Trying implicit prerequisite `build/config.pb.cc'.
       Trying pattern rule with stem `config.pb'.
       [...]
       Trying implicit prerequisite `build/SCCS/s.config.pb.o'.
       Trying pattern rule with stem `config.pb'.
       Trying implicit prerequisite `src/config.pb.cc'.
       Looking for a rule with intermediate file `src/config.pb.cc'.
        Avoiding implicit rule recursion.
        Trying pattern rule with stem `config'.
        Trying implicit prerequisite `src/proto/config.proto'.
        [...]

I'm running GNU Make 3.81, btw.

Also, I just noticed that if I run make src/config.pb.cc and then make build/config.pb.o manually, it works.

Why doesn't this work?

Answer 1

Your example looks correct to me. The only difference between the pattern rule and the explicit rule I see is that the explicit rule doesn't have the order-only prerequisite. What version of GNU make are you running? Are you sure it supports order-only prerequisites? If it doesn't then the pattern rule will not match as it's trying to find a way to build a target like | and can't.

Whenever you hit issues like this the simplest thing to do is run make -d. The output is voluminous but it is also very enlightening: find the section where it's trying to build config.pb.o and see what patterns it's trying and why it decides to give up on that one.