extern'ing functions

[chrisname]

Is using extern a valid and reliable way to have a function in one file access another? Or would I be much better off to make a header file which contains this?

For example:

main.c

#include <stdio.h>

extern int difference(int, int);

int main(void) {
    printf("Difference between 5 and 0 is %d.\n", difference(0, 5));
    
    return 0;
}

difference.c:

int difference(int x, int y) {
    return (x >= y) ? x - y : y - x;
}

It would be compiled with gcc -o differences main.c difference.c.

Would this be a valid way of using difference()? Or would I be better off prototyping it in a header? I'd rather use externs if possible, as it keeps the amount of files down...

[firedraco]

I would put it in a header, so then you don't have to go to every program that uses that .c and change the extern(s).

[jsmith]

Headers are better. Only one place to change the prototype then if the function changes.

[chrisname]

Oh, I didn't think of that! Thanks :)