c - Makefile include header

Question

I am new to Linux programming I tried to compile a simple test construction. But I'm getting an error when compiling. Adding inc.c as well (in the app: line) doesn't work. How should I include the file correct?

Makefile:

app: main.c inc.h
    cc -o app main.c

Terminal:

make
cc -o app main.c
/tmp/ccGgdRNy.o: In function `main':
main.c:(.text+0x14): undefined reference to `test'
collect2: error: ld returned 1 exit status
make: *** [app] Error 1

main.c:

#include <stdio.h>
#include "inc.h"

int main()
{
    printf("Kijken of deze **** werkt:
");
    test();

    getchar();
    return 0;
}

inc.h

#ifndef INCLUDE_H

#define INCLUDE_H

void test();

#endif

inc.c

#include <stdio.h>

void test()
{
    printf("Blijkbaar wel!");
}

Answer 1

You must link against the compilation unit inc.o which you obtain by compiling inc.c.

In general that means that you must supply all object files that contain functions that are used in main.c (transitively). You can compile these with implicit rules of make, no need to specify extra rules.

You could say:

app: main.c inc.o inc.h
    cc -o app inc.o main.c

And make will know on its own how to compile inc.o from inc.c although it will not take inc.h into account when determining whether inc.o must be rebuilt. For that you would have to specify your own rules.