第一个 Makefile 问题：没有制定目标的规则

Question

I'm attempting to create my first ever Makefile and looking for the most simple, understandable version--even if it isn't "good practice." I'll deal with that at a later time. :)

Basically, I just want to run make in the command line and have it execute the several g++ commands so I don't have to. For example, instead of entering the following (at top level dir) in the command line...

$g++ -c ./src/RaspPi4Main.cpp -o ./src/RaspPi4Main.o

$g++ -c ./src/funcI2cCom/funcI2cCom.cpp -o ./src/funcI2cCom/funcI2cCom.o

$g++ -o ./bin/RaspPi4Main ./src/RaspPi4Main.o ./src/funcI2cCom/funcI2cCom.o -lwiringPi

...I'd like something similar to my attempt at a Makefile:

RaspPi4Main: RaspPi4Main.o funcI2cCom.o (and all header files? or not needed if added below?)
    g++ -o ./bin/RaspPi4Main ./src/RaspPi4Main.o  ./src/funcI2cCom/funcI2cCom.o -lwiringPi

RaspPi4Main.o: RaspPi4Main.cpp
    g++ -c ./src/RaspPi4Main.cpp -o ./src/RaspPi4Main.o

funcI2cCom.o: funcI2cCom.cpp funcI2cCom.h
    g++ -c ./src/funcI2cCom/funcI2cCom.cpp -o ./src/funcI2cCom/funcI2cCom.o

Hopefully, I'm close? Thanks!

Answer 1

As a first step, you need to make sure your recipes produce their target and that inputs are either files or targets of other rules:

bin/RaspPi4Main: src/RaspPi4Main.o src/funcI2cCom/funcI2cCom.o 
    g++ -o bin/RaspPi4Main src/RaspPi4Main.o src/funcI2cCom/funcI2cCom.o -lwiringPi

src/RaspPi4Main.o: src/RaspPi4Main.cpp
    g++ -c src/RaspPi4Main.cpp -o src/RaspPi4Main.o

src/funcI2cCom/funcI2cCom.o: src/funcI2cCom/funcI2cCom.cpp src/funcI2cCom/funcI2cCom.h
    g++ -c src/funcI2cCom/funcI2cCom.cpp -o src/funcI2cCom/funcI2cCom.o

Note that this is quite verbose. You can shorten it quite significantly by making use of automatic variables . Concretely, $@ is the name of the target, $< is the name of the first dependency, and $^ are all dependencies:

bin/RaspPi4Main: src/RaspPi4Main.o src/funcI2cCom/funcI2cCom.o 
    g++ -o $@ $^ -lwiringPi

src/RaspPi4Main.o: src/RaspPi4Main.cpp
    g++ -c $< -o $@

src/funcI2cCom/funcI2cCom.o: src/funcI2cCom/funcI2cCom.cpp src/funcI2cCom/funcI2cCom.h
    g++ -c $< -o $@

Note that there's quite some repetition. Luckily, Make already knows how to compile a .cpp file into a .o file through implicit rules . Thus you can omit the recipes for these rules:

bin/RaspPi4Main: src/RaspPi4Main.o src/funcI2cCom/funcI2cCom.o 
    g++ -o $@ $^ -lwiringPi

src/RaspPi4Main.o: src/RaspPi4Main.cpp
src/funcI2cCom/funcI2cCom.o: src/funcI2cCom/funcI2cCom.cpp src/funcI2cCom/funcI2cCom.h