How do I make Makefile to recompile only changed files?

Question

I have been struggling a bit to get make to compile only the files that have been edited. However I didn\'t have much success and all the files get recompiled. Can someone e

Answer 1

Not sure if this is causing your specific problem but the two lines:

a_functions.c: a.h
main.c: main.h

are definitely wrong, because there's generally no command to re-create a C file based on a header it includes.

C files don't depend on their header files, the objects created by those C files do.

For example, a main.c of:

#include 
#include 
int main (void) { return 0; }

would be in the makefile as something like:

main.o: main.c hdr1.h hdr2.h
    gcc -c -o main.o main.c

Change:

a_functions.o: a_functions.c
a_functions.c: a.h
main.o: main.c
main.c: main.h

to:

a_functions.o: a_functions.c a.h
main.o: main.c main.h

(assuming that a_functions.c includes a.h and main.c includes main.h) and try again.

If that assumption above is wrong, you'll have to tell us what C files include what headers so we can tell you the correct rules.

---

If your contention is that the makefile is still building everything even after those changes, you need look at two things.

The first is the output from ls -l on all relevant files so that you can see what dates and times they have.

The second is the actual output from make. The output of make -d will be especially helpful since it shows what files and dates make is using to figure out what to do.

---

In terms of investigation, make seems to work fine as per the following transcript:

=====
pax$ cat qq.h
#define QQ 1
=====
pax$ cat qq.c
#include "qq.h"
int main(void) { return 0; }
=====
pax$ cat qq.mk
qq: qq.o
        gcc -o qq qq.o

qq.o: qq.c qq.h
        gcc -c -o qq.o qq.c
=====
pax$ touch qq.c qq.h
=====
pax$ make -f qq.mk
gcc -c -o qq.o qq.c
gcc -o qq qq.o
=====
pax$ make -f qq.mk
make: `qq' is up to date.