RosettaCodeData/Task/Queue-Definition/C/queue-definition-2.c

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

typedef struct node_t node_t, *node, *queue;
struct node_t { int val; node prev, next; };

#define HEAD(q) q->prev
#define TAIL(q) q->next
queue q_new()
{
    node q = malloc(sizeof(node_t));
    q->next = q->prev = 0;
    return q;
}

int empty(queue q)
{
    return !HEAD(q);
}

void enqueue(queue q, int n)
{
    node nd = malloc(sizeof(node_t));
    nd->val = n;
    if (!HEAD(q)) HEAD(q) = nd;
    nd->prev = TAIL(q);
    if (nd->prev) nd->prev->next = nd;
    TAIL(q) = nd;
    nd->next = 0;
}

int dequeue(queue q, int *val)
{
    node tmp = HEAD(q);
    if (!tmp) return 0;
    *val = tmp->val;

    HEAD(q) = tmp->next;
    if (TAIL(q) == tmp) TAIL(q) = 0;
    free(tmp);

    return 1;
}