make include directive and dependency generation with -MM

Question

I want a build rule to be triggered by an include directive if the target of the include is out of date or doesn\'t exist.

Currently the makefile looks like this:

Answer 1

You are relying on an implicit rule to compile your .cpp files. You have to redefine it to use the -MM and -MF flags that will create the dependency file.

%.o: %.cpp
    $(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@ -MM -MF $@.d

Then, you have to include these dependencies files in the Makefile, using -include that will not error when the dependencies files do not exist yet (on the first time, or after a clean).

program_DEPS := $(program_OBJS:.o=.o.d)
-include $(program_DEPS)

And remember to add the rm command for the dependencies files in the clean rule.