C Makefile for a source tree

Question

Need help in writing a Makefile for below source tree. I have tried out simple examples for Makefile and those worked fine. But not able to figure out how do I write a Makefile for below kind of source tree.

I am organizing my code as below

root_dir:

    Makefile

    component1:
        dir1:
            file1.c file1.h 
        dir2:
            file2.c file2.h
        dir3:
            file3.c file3.h

    component2:
        dir4:
            file4.c file4.h
        dir5:
            file5.c file5.h

    common:
        debug.c debug.h
        utils.c utils.h
        main.c main.h

Here, main.c uses some functions declared in debug.h, utils.h, file1.h and file4.h. These file1.c and file4.c use the debug.h and utils.h. I have started writing the Makefile rules as below.

CC=gcc
CFlags=-c -g3 -Wall

myexec: main.o
        $(CC) main.o -o myexec

main.o: common/main.c common/main.h common/utils.h
        $(CC) $(CFLAGS) common/main.c

This Makefile gives "undefined reference" error messages for the functions declared in utils.h. It can crib about the functions declared in debug.h, file1.h or file4.h, but shouldn't have given the error message for functions from utils.h

Please help me in finding out what is wrong with the Makefile here.

Beta · Accepted Answer

First let's fix the main.o rule:

main.o: common/main.c common/main.h common/utils.h
    $(CC) $(CFLAGS) common/main.c

If, as you say main.c uses some functions declared in debug.h, utils.h, file1.h and file4.h, then those headers should be prerequisites of the rule (so that if you modify a header, Make will rebuild main.o):

main.o: common/main.c common/main.h common/debug.h common/utils.h component1/dir1/file1.h component2/dir4/file4.h
    $(CC) $(CFLAGS) common/main.c

Now look at the myexec rule:

myexec: main.o
    $(CC) main.o -o myexec

It's no surprise that you get "undefined reference" errors; the functions declared in debug.h, utils.h, file1.h and file4.h are defined in (I presume) debug.c, utils.c, file1.c and file4.c, which this makefile never mentions. Do not proceed until you understand this.

The way to handle this is tho have the myexec rule link all of the relevant object files:

myexec: main.o debug.o utils.o file1.o file4.o
    $(CC) main.o debug.o utils.o file1.o file4.o -o myexec

Naturally you must have rules for debug.o, utils.o, file1.o and file4.o, similar to the one for main.o.

Once that is done, you have a makefile that works but is needlessly long and redundant. "Effective but crude". It can be made much shorter and more elegant, but this answer is getting long; just get it working, then we can work on making it cleaner.

C Makefile for a source tree

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