Functions and how the call stack works

Why functions exist

Not to look tidy. Two real reasons.

First, a function lets you name an idea. is_prime(n) says what it does. Fourteen lines of loop in the middle of main does not.

Second, a function is tested once and trusted everywhere. If your factorial is right, every call is right. Copy-pasted code has to be fixed in every copy, and you will miss one.

Declaration, definition, call

#include <stdio.h>

int add(int a, int b);          /* declaration (prototype) */

int main(void)
{
    int sum = add(3, 4);        /* call */
    printf("%d\n", sum);        /* 7 */
    return 0;
}

int add(int a, int b)           /* definition */
{
    return a + b;
}