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 explain me why?

My files are:

main.c a_functions.c 

where main.c includes main.h and a_functions.c includes a.h

Here is my makefile:

CC=gcc CFLAGS=-Wall -I. -c EXEC_FILE=program1   all: program  a_functions.o: a_functions.c a_functions.c: a.h main.o: main.c main.c: main.h  objects: a_functions.c main.c     $(CC) a_functions.c main.c $(CFLAGS)  program: a_functions.o main.o     $(CC) a_functions.o main.o -o $(EXEC_FILE) 

Changing the makefile as per suggestions seems to have the same problem::

all: program  a_functions.o: a_functions.c a.h     gcc a_functions.c -c  main.o: main.c main.h     gcc main.c -c  program: a_functions.o main.o     gcc a_functions.o main.o -o program1 

Answer 1

The specific problem you're talking about -- Make rebuilds program1 (by relinking the objects) even when nothing has changed -- is in this rule:

program: a_functions.o main.o     gcc a_functions.o main.o -o program1 

The target of this rule is program, and Make assumes that it is a file. But since there is no such file, every time you run Make, Make thinks that this file needs to be rebuilt, and executes the rule. I suggest this:

program1: a_functions.o main.o     gcc a_functions.o main.o -o program1 

Or better, this:

program1: a_functions.o main.o     gcc $^ -o $@ 

Or better still this:

$(EXEC_FILE): a_functions.o main.o     $(CC) $^ -o $@ 

(And don't forget to change the all rule to match.)

A few other points:

1. As @paxdiablo pointed out,

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

2. It doesn't make sense to link these objects together unless something in one (probably main.o) calls something in the other (probably a_functions.o), so I would expect to see a dependency like this:

   main.o: a.h 

   So I suspect that you have some misplaced declarations.

3. You declare an objects rule, but never refer to it. So you never actually use it; Make uses the default rule for %.o: %.c. I suggest this:

   OBJECTS = a_functions.o main.o $(OBJECTS): %.o: %.c     $(CC) $< $(CFLAGS) -o $@ 

   (In which case you can change $(EXEC_FILE): a_functions.o main.o to $(EXEC_FILE): $(OBJECTS).) Or just this:

   %.o: %.c     $(CC) $< $(CFLAGS) -o $@ 

Answer 2

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 <hdr1.h> #include <hdr2.h> 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.

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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.

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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.