Call by Value and Call by Reference

The call by value method of passing arguments to a function copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.

The call by reference method of passing arguments to a function copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. It means the changes made to the parameter affect the passed argument.

Swap two numbers using call by Value

/* function definition to swap the values */
void swap(int x, int y) {

   int temp;

   temp = x; /* save the value of x */
   x = y;    /* put y into x */
   y = temp; /* put temp into y */
  
   return;
}

#include <stdio.h>
 
/* function declaration */
void swap(int x, int y);
 
int main () {

   /* local variable definition */
   int a = 100;
   int b = 200;
 
   printf("Before swap, value of a : %d\n", a );
   printf("Before swap, value of b : %d\n", b );
 
   /* calling a function to swap the values */
   swap(a, b);
 
   printf("After swap, value of a : %d\n", a );
   printf("After swap, value of b : %d\n", b );
 
   return 0;
}