Doubly-linked list example using a single pointer per node

xlist.c

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

typedef struct {
	int value;
	long nextprev;
} node_t;

/* allocate and initialize a new node */
node_t *new_node(int value)
{
	node_t *n = malloc(sizeof(node_t));
	assert(n != NULL);
	n->value = value;
	n->nextprev = 0;
	return n;
}

/* insert node at the head of the list and return the new head */
node_t *insert_at_head(node_t *head, node_t *n)
{
	if (head == NULL) {
		n->nextprev = 0;
	} else {
		n->nextprev = (long) head;
		head->nextprev ^= (long) n;
	}
	return n;
}

/* print each value in the list */
void print_values(node_t *n)
{
	node_t *prev = NULL;
	while (n) {
		printf(" %d", n->value);
		node_t *tmp = n;
		n = (node_t *) (n->nextprev ^ (long) prev);
		prev = tmp;
	}
}

int main(int argc, char *argv[])
{
	node_t *head = NULL;
	node_t *tail = NULL;

	/* create the first node, we always insert at the head so this node becomes the tail */
	head = tail = insert_at_head(head, new_node(4));

	/* insert a few more nodes */
	head = insert_at_head(head, new_node(3));
	head = insert_at_head(head, new_node(2));
	head = insert_at_head(head, new_node(1));

	/* iterate forward */
	printf("Forward:");
	print_values(head);
	printf("\n");

	/* iterate_backward */
	printf("Backward:");
	print_values(tail);
	printf("\n");

	return 0;
}