C++ Makefile, Static Library


All of our previous examples have used only shared libraries. While shared libraries are very useful, static libraries also have their use. Link-time optimization can be done much more aggressively. Any functions that are not called can be pruned by the compiler, whereas they cannot be pruned from a shared library. Here, we will improve the makefile to compile either shared or static libraries.

First, let’s add variables to indicate whether we should build a static or a shared library.


If BUILD_SHARED is non-zero, we will build a shared library. If BUILD_STATIC is non-zero, we will build a static library. If both are non-zero, we will make both libraries. We will link the executables against whichever library’s value is greater.

Now, we define each library that we want to make.

ifneq ($(BUILD_SHARED),0)
    SHARED_LIBRARY_OUTPUT = $(patsubst %,lib/%.so,$(LIBRARY_FOLDERS))

ifneq ($(BUILD_STATIC),0)
    STATIC_LIBRARY_OUTPUT = $(patsubst %,lib/%.a,$(LIBRARY_FOLDERS))

Next, we want to define a different rule for building the object files. When compiling a shared library, we want the -fPIC flag, whereas we don’t want it when compiling a static library. The object files to go into a static library will end in .o, and object files to go into a shared library will end in .os.

build/$(BUILD)/build/%.os: %.cc
	mkdir -p $(@D)
	$(CXX) -c -fPIC $(CPPFLAGS) $(CXXFLAGS) $< -o $@

build/$(BUILD)/build/%.o: %.cc
	mkdir -p $(@D)
	$(CXX) -c $(CPPFLAGS) $(CXXFLAGS) $< -o $@

Now, we add a rule for each of the libraries. We will add a variable AR to allow the user to select which archiver to use. In addition, we define library_os_files in terms of library_o_files.

AR = ar
library_os_files  = $(addsuffix s,$(call library_o_files,$(1)))

build/$(BUILD)/lib/lib%.a: $$(call library_o_files,%)
	mkdir -p $(@D)
	$(AR) rcs $@ $^

build/$(BUILD)/lib/lib%.so: $$(call library_os_files,%)
	mkdir -p $(@D)
	$(CXX) $(LDFLAGS) $^ -shared $(SHARED_LDLIBS) -o $@

Finally, we want to have a rule to compile the executables. We will make two versions of the rule, depending on whether we are linking against the shared or the static libraries.

ifeq ($(shell test $(BUILD_SHARED) -gt $(BUILD_STATIC); echo $$?),0)
build/$(BUILD)/bin/%: build/$(BUILD)/build/%.o $(O_FILES) | $(SHARED_LIBRARY_OUTPUT)
	mkdir -p $(@D)
	$(CXX) $(ALL_LDFLAGS) $^ $(ALL_LDLIBS) -o $@
build/$(BUILD)/bin/%: build/$(BUILD)/build/%.o $(O_FILES) $(STATIC_LIBRARY_OUTPUT)
	mkdir -p $(@D)
	$(CXX) $(ALL_LDFLAGS) $^ $(ALL_LDLIBS) -o $@

Now, we can compile the code to either a shared or a static library, depending on what is needed for a particular project. The full makefile can be found on github.