如何为包含头文件的目标编写makefile？

Question

I have two files "create-exercise.cpp" and "exercise.hpp". I want to write a makefile to use gnu++17 and g++ to compile them. exercise.hpp is included in create-exercise.cpp. I only want to get a binary out of create-exercise.cpp. the command I would use is g++ -std=gnu++17 create-exercise.cpp -o create-exercise and it works well. I tried using the following in a make file.

CXXFLAGS=-Wall -std=gnu++17

create-exercise: create-exercise.cpp exercise.hpp

but that generated the following g++ -Wall -std=gnu++17 create-exercise.cpp exercise.hpp -o create-exercise I don't want exercise.hpp to be included in the compilation command. I also tried to use the following instead.

CXXFLAGS=-Wall -std=gnu++17

create-exercise.o: create-exercise.cpp exercise.hpp

That generated g++ -Wall -std=gnu++17 -c -o create-exercise.o create-exercise.cpp . I don't want the -c flag. because when I try to run create-exercise.o that results in permission denied error. I tried the following as well:

CXXFLAGS=-Wall -std=gnu++17

create-exercise.o: create-exercise.cpp exercise.hpp
    $(CXX) $(CXXFLAGS) create-exercise.cpp -o create-exercise.o

but when I edit exercise.hpp make says 'create-exercise.o' is up to date. and doesn't recompile it. what should I do?

Answer 1

The rule to write in your makefile would be:

create-exercise.o: exercise.hpp

This is a rule without a recipe, and it simply adds one more prerequisite ( exercise.hpp ) to the target create-exercise.o , ie, whenever exercise.hpp changes, create-exercise.o should be rebuilt.

However, the problem when providing that rule is that GNU Make assumes create-exercise.o is an object file generated from a C source file when building create-exercise . Therefore, it doesn't link the C++ library but the C library instead, that's why you are having the liking error of undefined reference to std::cout .

GNU Make has the following implicit rule for generating an executable from a C++ source file:

%: %.cpp
#  commands to execute (built-in):
   $(LINK.cpp) $^ $(LOADLIBES) $(LDLIBS) -o $@

You can use that rule's recipe to build create-exercise from creating-exercise.o and still treat creating-exercise.o as an object file that was generated from a C++ source file:

create-exercise: create-exercise.o
    $(LINK.cpp) $^ $(LOADLIBES) $(LDLIBS) -o $@

The whole makefile would be then:

CXXFLAGS = -Wall -std=gnu++17 

create-exercise: create-exercise.o
    $(LINK.cpp) $^ $(LOADLIBES) $(LDLIBS) -o $@

create-exercise.o: exercise.hpp

The target create-exercise also becomes the default goal because it's the first rule in the makefile. It's the target to build if you don't specify any to make .

Answer 2

You can do this:

CXXFLAGS = -Wall -std=gnu++17

create-exercise: create-exercise.o
        $(LINK.cpp) $^ $(LDLIBS) -o $@

create-exercise.o: exercise.hpp

This says: build create-exercise from the object file create-exercise.o , if it's changed. And it says rebuild create-exercise.o if the header file exercise.hpp changes; it's not right to modify or rebuild a source file if a header changes. It's the object file that depends on the header.

Make has a number of built in rules. Among them are rules that know how to link a program but unfortunately that assumes your object files were built from C files, so if you're using C++ you have to create a specific recipe. But you can still use GNU make's default variables.

It also knows how to build a .o file from a .cpp file on its own: you don't need to write that recipe.

Answer 3

I am not sure, but maybe try to include the .hpp file instead of compiling it with the other file. You can do so by using g++ *.cpp -I * .hpp -o output

Try it out and let me know if it works for you.