Compile multiple **changed** source files at once in GNU make

Question

I know there have been several questions with similar titles but none seem to provide an answer to what I need (correct me if I'm wrong).

Consider this makefile:

SOURCES=file1.cpp file2.cpp file3.cpp
OBJECTS=$(SOURCES:.cpp=.o)
EXECUTABLE=myprog

all: $(SOURCES) $(EXECUTABLE)

$(EXECUTABLE): $(OBJECTS)
    $(CXX) -o $@ $(OBJECTS)

file1.o: file1.cpp file1.h
file2.o: file2.cpp file2.h file1.h
file3.o: file3.cpp

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

If I change file1.h, the following is run:

g++ -c -o file1.o file1.cpp
g++ -c -o file2.o file2.cpp
g++ -o myprog file1.o file2.o file3.o 

What I would like to have is:

g++ -c file1.cpp file2.cpp
g++ -o myprog file1.o file2.o file3.o 

(I know I can't specify object output directory with GCC, but this I can live with; it should be possible to work around with some cd commands.)

In nmake, this is done with a double-colon inference rule (so-called called "batch-mode rule"). Basically, it groups the inference rules (e.g. ".obj.cpp:") for multiple targets and invokes the compiler for all dependencies instead of once per file. The $< variable gets the list of dependencies instead of just the first one.

Right now we're using parallel building (make -j) but it has its own issues, and VC++ compiler works much better in one-invocation mode so I'd prefer using that.

Answer 1

I don't see why you want this effect, but here's how to get it (in GNUMake):

SOURCES=file1.cpp file2.cpp file3.cpp
OBJECTS=$(SOURCES:.cpp=.o)
EXECUTABLE=myprog

all: $(EXECUTABLE)

$(EXECUTABLE): $(SOURCES)
    $(CXX) $(CXXFLAGS) -c $?
    $(CXX) -o $@ $(OBJECTS)

EDIT:
I'm surprised that that solution works -- there's something wrong with my idea of what Make does -- but I don't think it'll work in your case, with header dependencies, without the following kludge. (There are one or two other approaches that might work, if this doesn't pan out.)

SOURCES=file1.cpp file2.cpp file3.cpp
OBJECTS=$(SOURCES:.cpp=.o)
EXECUTABLE=myprog

all: $(EXECUTABLE)

$(EXECUTABLE): $(OBJECTS)
    $(CXX) -o $@ $(OBJECTS)

file1.cpp: file1.h
file2.cpp: file2.h file1.h

$(SOURCES):
    @touch $@

$(OBJECTS): $(SOURCES)
    $(CXX) $(CXXFLAGS) -c $?
    @touch $(OBJECTS)

Answer 2

You can make GNUmake do what you want by collecting the files to be rebuilt in the build rule and then actually building them when you link:

SOURCES=file1.cpp file2.cpp file3.cpp
OBJECTS=$(SOURCES:.cpp=.o)
EXECUTABLE=myprog

all: $(EXECUTABLE)

.PHONY: build_list

build_list:
        -rm -f build_list

$(OBJECTS): %.o: %.cpp | build_list
        echo $< >> build_list

$(EXECUTABLE): build_list $(OBJECTS)
        if [ -r build_list ]; then $(CXX) $(CXXFLAGS) -c `cat build_list`; fi
        $(CXX) -o $@ $(OBJECTS)

file1.o: file1.h
file2.o: file2.h file1.h