Makefile目标需要其他目标的依赖性

Question

I have three c files and their header files:

1. main/program.{c,h}
2. lib/lib1.{c,h}
3. lib/lib2.{c,h}

program depends on lib1.o , which depends on lib2.o . program does not directly depend on lib2.o .

program.main() calls lib1.func1() , which calls lib2.func2() .

I have two Makefile s; one in the main folder, and another in lib.

lib/Makefile:

all: lib1.o lib2.o

lib1.o: lib2.o lib2.h lib1.h

lib2.o: lib2.h

main/Makefile:

VPATH=../lib

all: program

program: program.o lib1.o lib1.h
    $(CC) $(CFLAGS) -o $@ $^

When I run make -C main all , I get an undefined reference to 'func2' error. Both the lib object files compiled fine.

Do I need to make lib2 a dependency of program even though it doesn't directly call it?

Answer 1

Yes. lib1.o contains a reference to a function that only exists in lib2.o. If you don't tell the linker for main to also link against lib2.o it will never find it. The compiler doesn't resolves the reference from lib1 to lib2 at compile time, it just checks the header to make sure it is being called correctly.

You could have you lib directory Makefile build a .a (man ar) which includes both .o files, then link against the .a file from your main build.