pass by value and pass by reference in functions

Pass by Value: In this method, the value of each of the actual arguments in the calling function is copied into corresponding formal arguments of the called function. In pass by value, the changes made to formal arguments in the called function have no effect on the values of actual arguments in the calling function.

Example:

#include<stdio.h>
void swap(int x, int y) {
 int t;
 t = x;
 x = y;
 y = t;
} 
int main() {
 int m = 10, n = 20;
 printf("Before executing swap m=%d n=%d\n", m, n);
 swap(m, n);
 printf("After executing swap m=%d n=%d\n", m, n); 
 return 0;

Output:

Before executing swap m = 10 n = 20

After executing swap m = 10 n=20 

Explanation:

In the main function, value of variables m, n are not changed though they are passed to function ‘swap’. Swap function has a copy of m, n and hence it can not manipulate the actual value of arguments passed to it.

Pass by Reference: In this method, the addresses of actual arguments in the calling function are copied into formal arguments of the called function. This means that using these addresses, we would have an access to the actual arguments and hence we would be able to manipulate them. C does not support Call by reference. But it can be simulated using pointers.

Example:

#include<stdio.h>
/* function definition */
void swap(int *x, int *y) {
 int t;
 t = *x; /* assign the value at address x to t */
 *x = *y; /* put the value at y into x */
 *y = t; /* put the value at to y */
} 
int main() {
 int m = 10, n = 20;
 printf("Before executing swap m = %d n = %d\n", m, n);
 swap(&m, &n);
 printf("After executing swap m = %d n = %d\n", m, n);
 return 0;
}

Output:

Before executing swap m=10 n=20

After executing swap m=20 n=10

Explanation:

In the main function, address of variables m, n are sent as arguments to the function ‘swap’. As swap function has the access to address of the arguments, manipulation of passed arguments inside swap function would be directly reflected in the values of m, n.