Problems in compiling C files

Question

I have these three files which I'm trying to link together. It's a didactic example, I tried different combinations compiling the three files but I have no clue what' s wrong. Even if something with this trivial code is wrong.

What I'm trying to understand is how to separate in different files the declaration of a function in order to include it from an header file in the main one. Or at least this is what I figured out. My teacher set the example in order to have an .h file and two .c files: Here they are...

foo.c

int foo(int x){
return x==42 ? 1 : 0;
}

foo.h

extern int foo(int);

use-foo.c

#include <stdio.h>
#include "foo.h"
int main(){
printf("foo(42) == %d \n", foo(42));
getchar();
return 0;
}

I hope someone can guide my through this, I don't need a different solution , please help me with these 3 only files because the aim of the teacher was this.

Answer 1

You can compile it with the command :

gcc -o use-foo use-foo.c foo.c

Then you'll be able to run it with the command :

./use-foo

Just be sure that all your files are in the same directory, that you call gcc from that directory and that gcc is installed on your system.

A couple more advices on including files, and cross-compiling : All your header files (like foo.h) should have include guards, like that

#ifndef __FOO__
#define __FOO__

int foo (int);

#endif

that'll prevent the compiler from adding your header file more than once in your code.

Another advice would be to include foo.h in your foo.c code as well.