Function is defined above the function main: it is not necessary to prototype the function : Function Prototype « Function « C Tutorial

1. C Tutorial
2. Function
3. Function Prototype

#include <stdio.h>

void f1(int *k)
{
    *k = *k + 10;
}

main ( ){
    int i;
    i = 0;
    printf (" The value of i before call %d \n", i);
    f1 (&i);
    printf (" The value of i after call %d \n", i);
}

The value of i before call 0
       The value of i after call 10