#include <stdio.h>
        #include <stdlib.h>
        #include <assert.h>
 
        /* pray that a long is the size of a pointer */
        typedef unsigned long pointer;
 
        typedef struct
        {
          pointer next_prev;
          int payload;
        } link;
 
        link* add_data(int payload, link* list)
        {
          link *new_link = malloc(sizeof(link));
          assert(new_link);
          new_link->next_prev = (pointer)list;
          new_link->payload = payload;
          if (list != NULL)
            list->next_prev ^= (pointer)new_link;
          return new_link;
        }
 
        void walk_list(link *list)
        {
          pointer prev = 0;
          while (list != NULL)
          {
            pointer next = prev ^ list->next_prev;
            printf("%d ", list->payload);
            prev = (pointer)list;
            list = (link*)next;
          }
          printf("\n");
        }
 
        int main(void)
        {
          /* create a list */
          link *l2 = add_data(2, NULL);
          /* add something to the front ... */
          link *l1 = add_data(1, l2);
          /* add something to the back ... */
          link *l3 = add_data(3, l2);
 
          /* walk from front to back */
          walk_list(l1);
          /* walk from back to front */
          walk_list(l3);
 
          return 0;
        }

123 XOR 456 = 435 (butterfly radix)

435 XOR 123 = 456
435 XOR 456 = 123