Question

I’ve been using qmake and CMake in the past without any problem to generate my makefiles. However, recently I figured on some of the clusters I run my codes these tools are harder to find/install so I’ve decided writing bare Makefiles. Besides I could also learn a thing or two about Makefiles 😉

My project involves several directories with source/header files inside and most often cross dependencies between them. I’ve learned, through SO, to use -MM flag to automatically generate dependency information. My Makefiles look like this

include $(HEAD_DIR)/common.mk

OBJS_DIR = objs
LIBS_DIR = libs

SRCS  = Matrix.cpp MatrixFull.cpp
OBJS  = $(patsubst %,$(OBJS_DIR)/%,$(SRCS:.cpp=.o))
LIB_0 = $(patsubst %, $(LIBS_DIR)/%,libalgebra.a)

DEPS := $(patsubst %,$(OBJS_DIR)/%,$(SRCS:.cpp=.d))
-include $(DEPS)

.PHONY: all
all:    
    @echo   " ===== Building dependencies in lib/algebra ===== "
    @$(MKDIR) $(OBJS_DIR)
    @$(MAKE)  $(OBJS)
    @$(MKDIR) $(LIBS_DIR)
    @$(MAKE)  $(LIB_0)
    @echo   " ===================== done ===================== "

$(OBJS_DIR)/%.o: %.cpp
    @echo " compiling source file:   $< ..."
    $(CXX) -c $(CXXFLAGS) -I$(INCLUDE_PATH) -MM $< -o $@

$(LIB_0): $($(OBJS_DIR)/%.o)
    @echo " generating library file: $(@F) ..."
    @$(AR) $(AR_FLAGS) $@ $^

.PHONY: clean
clean:
    @$(RM) $(OBJS_DIR) $(LIBS_DIR)

This generally works fine but I’d like to improve on couple of things:
1) When I change a header file and I issue make, somehow the compiler does not pick all and seems to default to DEPS. Is this supposed to happen in this makefile? How can I make all the default rule?
2) How can I make depenency files invisible? I tried DEPS := $(patsubst %,$(OBJS_DIR)/.%,$(SRCS:.cpp=.d)) but that did not help
3) Is there any specific suggestion you could give me if you think this Makefile could be improved in any other way?

EDIT: To be honest, I’m not even sure how make invokes the compiler to generate dependencies :p … I do not see any explicit rule for this. Is it somehow hidden in DEPS := $(patsubst %,$(OBJS_DIR)/%,$(SRCS:.cpp=.d)) ?

Answer 1

1) Move the -include $(DEPS) down below the all rule. The first rule is the default rule (unless you deliberately set some special variable), so if include pulls in a rule above all:, that will be the default.

2) I presume that by “make them invisible” you mean you want .foo.d instead of foo.d. You have to modify the rule that makes them:

$(OBJS_DIR)/%.o: %.cpp
    @echo " compiling source file:   $< ..."
    $(CXX) -c $(CXXFLAGS) -I$(INCLUDE_PATH) -MM $< -o $@
    @mv $(OBJS_DIR)/$*.d $(OBJS_DIR)/.$*.d

and then the variable that finds them:

DEPS := $(patsubst %.cpp,$(OBJS_DIR)/.%.d,$(SRCS))

3) I’d get rid of the recursive calls ($(MAKE) ...), but you should make sure everything else is working right first. And I’m surprised the $(OBJS_DIR)/%.o rule doesn’t go in common.mk. Other than that it looks pretty good.

4(?)) The compiler command in the $(OBJS_DIR)/%.o rule uses the -MM flag, so the compiler generates the dependency files as a side-effect.