Makefile to build shared library

Question

I've been building a C++11 library, and the number of header/source files has grown to the point where compiling programs invoking it, entails passing 20+ .cpp files to g++. I've been reading up on shared libraries and it seems to be the best solution.

However, as headers/source change frequently, I'm hoping to create a makefile that would automatically generate all the .so files from the headers and source.

To better demonstrate what I'm trying to do, I'll take one of my sub-libraries, Chrono and show how I would do this manually.

I first create the object files like so,

$ g++ -std=c++11 -fPIC -g -c -Wall ../src/Chrono/cpp/DateTime.cpp
$ g++ -std=c++11 -fPIC -g -c -Wall ../src/Chrono/cpp/Schedule.cpp
$ g++ -std=c++11 -fPIC -g -c -Wall ../src/Chrono/cpp/Duration.cpp
$ g++ -std=c++11 -fPIC -g -c -Wall ../src/Chrono/cpp/DayCount.cpp

So that I now have DateTime.o, Schedule.o, Duration.o, and DayCount.o in the current directory. I then create the .so file,

$ g++ -shared -Wl,-soname,libChrono.so.1 -o libChrono.so.1.0.1 DateTime.o Schedule.o Duration.o DayCount.o -lc

I then go,

$ rm ./*.o && ldconfig -n ./

So that my working directory now contains, libChrono.so.1.0.1 and the symlink libChrono.so.1.

There are quite a few subdirectories I need to do this for, so you can see that this quickly grows inefficient whenever changes to headers/source are made. I would be grateful if anyone can help me design a makefile that accomplishes all this simply by invoking make.

Thanks!

UPDATE:

Based on goldilock's advice and some digging, I managed to bang together:

CXX=g++
CFLAGS=-std=c++11 
TARGET=./lib/libChrono.so.1.0.1
CHRONODIR=./src/Chrono
CHRONOSRC=$(wildcard $(CHRONODIR)/cpp/*.cpp)
CHRONOOBJ=$(join $(addsuffix ../obj/,  $(dir $(CHRONOSRC))), $(notdir (CHRONOSRC:.cpp=.o)))

all: $(TARGET)
    @true

clean:
    @-rm -f $(TARGET) $(CHRONOOBJ)

./lib/libChrono.so.1.0.1: $(CHRONOOBJ)
    @echo "======================="
    @echo "Creating library file $@"
    @echo "======================="
    @$(CXX) -shared -Wl,-soname,$(join $(basename $@), .1) -o $@ $^ -l
    @echo "-- $@ file created --"

$(CHRONODIR)/cpp/../obj/%.o : $(CHRONOSRC)
    @mkdir -p $(dir $@)
    @echo "============="
    @echo "Compiling $<"
    @$(CXX) $(CFLAGS) -fPIC -g -Wall -c $< -o $@

4 .o files are produced in lib/ but I get multiple definition complaints from ld. Before I was compiling the object files separately, but this unwinds CHRONOOBJ on one line. Any ideas?

Answer 1

Fortunately you included the origin of your problem:

I've been building a C++11 library, and the number of header/source files has grown to the point where compiling programs invoking it, entails passing 20+ .cpp files to g++.

Because this reveals a potential XY problem. The straightforward solution to this is to put object files into an archive (aka. a static library) and use that.

GNU make has an implicit rule for creating C++ .o files. It amounts to this:

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

Meaning, if you make DateTime.o in a directory with a makefile that doesn't redefine this, it will make DateTime.o. You may want to add things to $(CXXFLAGS) however, e.g.:

CXXFLAGS += -Wall -Wextra --std=c++11

If you intend to stick with the shared lib route, -fPIC can go there too. That one line could be your entire makefile.

However, you also want to put these together, so you must first declare all the objects and a rule for combining them:

OBJS = DateTime.o Schedule.o Duration.o

libChrono.a: $(OBJS)
    ar crvs $@ $^

This last line (see man ar) creates the archive (libChrono.a) containing all the objects in $(OBJS). You can then use this with whatever program by placing it in the same directory (or a directory in the library path) and linking -lChrono. Only the necessary parts will be extracted and compiled in. This saves you having to maintain a shared lib in a system directory.

If you still think you need a shared lib, $@ and $^ are automatic variables; you can use similar methodology to create a .so, something along the lines of:

SO_FLAGS = -shared

libChrono.so.1.0.1: $(OBJS)
    $(CXX) $(SO_FLAGS) -Wl,-soname,libChrono.so.1 -o $@ $^ -lc

If that is your first rule, make will take care of everything: building first the objects and then the library. Notice this one has excluded your normal $(CXXFLAGS) to duplicate exactly the compiler line from the question.