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C++ Makefile, Library-Specific Flags

Tags: C++ , Make   (2015-08-24)

In the previous post, we made a makefile capable of compiling multiple shared libraries, and multiple executables. Now, modifying the makefile so that each library can have specific compilation flags.

Makefiles allow for variables to be target-specific variables to be specified. For example, if a specific library needs to be compiled under C++03, while the rest of the project requires C++11, we could overwrite the CXXFLAGS variable for those specific targets.

build/libLibrary03/%: override CXXFLAGS += -std=c++03

This shouldn’t be in the makefile, though, because I want it to remain as general as possible. Instead, let’s define a few variables inside an include file.

CPPFLAGS_EXTRA =
CXXFLAGS_EXTRA = -std=c++03
LDFLAGS_EXTRA  =
SHARED_LDLIBS  =

Now, we include that in our makefile and set the target-specific variables.

-include libLibrary03/Makefile.inc
build/libLibrary03/%.o: override CPPFLAGS := $(CPPFLAGS) $(CPPFLAGS_EXTRA)
build/libLibrary03/%.o: override CXXFLAGS := $(CXXFLAGS) $(CXXFLAGS_EXTRA)
lib/libLibrary03.so:  override LDFLAGS  := $(LDFLAGS)  $(LDFLAGS_EXTRA)
lib/libLibrary03.so:  override SHARED_LDLIBS := $(SHARED_LDLIBS)

Note how we can define target-specific variables for each a single target file, or for a pattern rule. This lets us easily add the CXXFLAGS to any .o file present in the library. However, this is still specific to one library. To make it more general, let’s define a template that will be used for any library.

define library_variables
CPPFLAGS_EXTRA =
CXXFLAGS_EXTRA =
LDFLAGS_EXTRA  =
SHARED_LDLIBS  =
-include $(1)/Makefile.inc
build/$(1)/%.o: override CPPFLAGS := $$(CPPFLAGS) $$(CPPFLAGS_EXTRA)
build/$(1)/%.o: override CXXFLAGS := $$(CXXFLAGS) $$(CXXFLAGS_EXTRA)
lib/$(1).so:  override LDFLAGS  := $$(LDFLAGS)  $$(LDFLAGS_EXTRA)
lib/$(1).so:  override SHARED_LDLIBS := $$(SHARED_LDLIBS)
endef

Now, we evaluate the template for each library that we are going to make.

$(foreach lib,$(LIBRARY_FOLDERS),$(eval $(call library_variables,$(lib))))

With this, we can define additional flags for each library, without needing to edit the makefile itself. As before, the full makefile can be found on github.