A declaration before a later definition

The earlier functions appeared above main. In a longer file, you might want the definition below it. Put a prototype first so the compiler knows the return and parameter types when it reaches the call. The prototype ends in a semicolon; the later definition supplies the body.

#include <stdio.h>

int add_minutes(int start, int extra); /* declaration */

int main(void) {
    printf("End: %d\n", add_minutes(10, 5));
    return 0;
}

int add_minutes(int start, int extra) { /* definition */
    return start + extra;
}

Here main passes two integers to add_minutes, then printf displays the returned value. Try changing the second argument to 8. The prototype does not create another function; its return and parameter types must agree with the definition.