void exchange (int *, int *);        /* prototype */

   main()                                                           
   {                                                                
       int  x, y;                                                   
       x = 100;                                                     
       y = 200;                                                     
       printf("Before exchange  : x = %d   y = %d\n\n", x, y);      
       exchange(&x,&y);         /* call */
                                        
       printf("After exchange   : x = %d   y = %d\n\n", x, y);      
   }                                                                
   exchange (int *a, int *b)
   {                                                                
       int t;                                                       
       t = *a;    /* Assign the value at address a to t */
          
       *a = *b;   /* put b into a */
                                
       *b = t;    /* put t into b */
                                
   }                                                                
                                                                    
Output
                                                                    
   Before exchange  : x = 100   y = 200                             
   After exchange   : x = 200   y = 100