Wsrtjtyk

I'm trying to create a Makefile where make only recompiles necessary .cpp files.

By default, it works well if I edit .cpp, however, when I edit .h files, it just ignores the changes.

So I've read of -MD flag and its friend -MP (used to avoid bugs when typing make).

However, I can't seem to make it working, if I use -MMD, it works perfectly but I rely on system includes too as I'm writing a library too that is evolving along with the project. Thus, if I update the libary header and reinstall the library, typing make in the main project should recompiles files that includes the changed header.

When using -MMD flag, it - as expected - does not recompile the project, however, using -MD flag, it does recompile everything. In fact, -MD recompiles everything every time, even when nothing changed.

Here is a minimal project structure that reproduce the issue:

./Makefile:

all: build



re: clean build



build: build_lib install_lib build_client



build_lib:

    $(MAKE) -C lib

    $(MAKE) install -C lib



build_client:

    $(MAKE) -C client



install_lib:

    $(MAKE) install -C lib





.PHONY: clean

clean: clean_lib clean_client



clean_lib:

    $(MAKE) clean -C lib



clean_client:

    $(MAKE) clean -C client

./client/Makefile:

CC = g++

INC = -I../lib

CXXFLAGS = -Wall $(INC) -g -MD -MP

EXEC_NAME = ../test



src = $(shell find $(SOURCEDIR) -name '*.cpp')

obj = $(src:.cpp=.o)



LIBRARIES = -ltest_lib

LDFLAGS = -rdynamic $(LIBRARIES)



all: $(EXEC_NAME)



re: clean $(EXEC_NAME)



$(EXEC_NAME): $(obj)

    $(CC) -o $@ $^ $(LDFLAGS)

-include $(obj:.o=.d)



.PHONY: clean

clean:

    rm -f $(obj) $(EXEC_NAME)

./lib/Makefile:

.PHONY : clean



CXXFLAGS= -fPIC -g -Itest_lib/include -MMD -MP

LDFLAGS= -shared



SOURCES = $(shell find $(SOURCEDIR) -name '*.cpp')

HEADERS = $(shell find $(SOURCEDIR) -name '*.h')

OBJECTS=$(SOURCES:.cpp=.o)



TARGET=libtest_lib.so

INC_FOLDER=test_lib/include

CUR_DIR = $(shell pwd)



all: $(TARGET)



install:

    sudo rm -rf /usr/local/lib/libtest_lib.so && sudo ln -s $(CUR_DIR)/$(TARGET) /usr/local/lib/libtest_lib.so

    sudo rm -rf /usr/local/include/test_lib && sudo cp -r $(INC_FOLDER) /usr/local/include/test_lib



clean:

    rm -f $(OBJECTS) $(TARGET)



$(TARGET) : $(OBJECTS)

    $(CC) $(CFLAGS) $(OBJECTS) -o $@ $(LDFLAGS)

-include $(OBJECTS:.o=.d)

./client/main.cpp:

#include "bar.h"



int main()

{

    Bar b;



    b.sayHello();

    b.sayBye();



    return 0;

}

./client/bar.h

#ifndef __BAR__

#define __BAR__



#include <test_lib/foo.h>

#include <iostream>



struct Bar : public Foo

{

    Bar() {};

    ~Bar() {};



    void sayBye() const {

        std::cout << "Bye " << name << "..." << std::endl;

    };

};



#endif

./lib/test_lib/include/foo.h

#ifndef __FOO__

#define __FOO__



struct Foo

{

    const char *name;



    Foo(const char *name = "world");

    ~Foo();



    void sayHello() const;

};



#endif

./lib/test_lib/src

#include "foo.h"

#include <iostream>



Foo::Foo(const char *name) : name(name) {}

Foo::~Foo() {}



void Foo::sayHello() const

{

    std::cout << "Hello " << name << " !" << std::endl;

}

Problem was that I copied the library headers all the time in /usr/local so the files got newer and the client make then thought every header of the library had changed.

A simple fix to this problem was to replace in the library Makefile the following line:

sudo rm -rf /usr/local/include/test_lib && sudo cp -r $(INC_FOLDER) /usr/local/include/test_lib

by

sudo rm -rf /usr/local/include/test_lib && sudo ln -s $(CUR_DIR)/$(INC_FOLDER) /usr/local/include/test_lib

On a side note, provided example is missing the removal of .d files on the clean rule.