C :: Program Using A Function To Add Two Integers

I compiled a simple program using a function to add two integers, in order to understand the difference of calling by value vs by reference.

Code:

#include <stdio.h>
int add(int a, int b, int sum);
int main()
{
int a, b, sum;
printf("Please enter two integers
");
scanf("%d%d", &a, &b);

[Code]...

I used these two (one, two) pages I found in the net while studying the subject. So here are my questions...

(a) Do I understand correctly that in the above code all a, b and sum are passed by value?
(b) If I wanted only sum to be passed by reference, how the above code would change?
(c) If I wanted all (a, b, sum) to be passed by reference, how the above code would change?