Question

I’m trying to make a module that takes .cpp and .swg files as input and creates a .so file using SWIG. The trouble is, I don’t know much about makefiles, and I’m not quite sure exactly what I’m doing wrong. Here is my makefile:

CXX = g++
SWIG = swig
SWIGFLAGS = -c++ -python 
CXXFLAGS = -c -fpic -Wall #for debugging purposes
LDFLAGS = -shared

file_processor.so: %*.o
    $(CXX) $(LDFLAGS) $^

%.o: %.cxx %.cpp
    $(CXX) $(CXXFLAGS) $? -o $@

%.cxx: %.swg
    $(SWIG) $(SWIGFLAGS) $<

When I run this, make says:

make: *** No rule to make target `%*.o', needed by `file_processor.so'.  Stop.

What exactly am I doing wrong? Can anyone suggest a better way to accomplish what I’m trying to do?

Answer 1

1) The default rule:

_file_processor.so: %*.o
    ...

As written, this requires a prerequisite called “%*.o”, which Make can neither find nor build. I think you meant this:

_file_processor.so: *.o
    ...

but then Make would pull in all existing .o files — and not worry if any were missing. I recommend:

OBJS = file_processor.o file_processor_wrap.o

_file_processor.so: $(OBJS)
    ...

2) The object rule:

%.o: %.cxx %.cpp
    $(CXX) $(CXXFLAGS) $? -o $@

This rule won’t apply unless both prerequisites exist, which doesn’t seem to be what you have in mind. You’ll have to split it into two rules:

%.o: %.cxx
    $(CXX) $(CXXFLAGS) $< -o $@

%.o: %.cpp
    $(CXX) $(CXXFLAGS) $< -o $@

(Pattern rules don’t work quite the same way as ordinary rules.)

3) Small correction:

swig -c++ -python file_processor.swg

Did you mean file_processor_wrap.swg? The flow doesn’t make sense otherwise, since you’d be building file_processor.cxx but never using it.