struct elem {
　char *data; /* 可変長のデータへのポインタ */
　struct elem *next;
};

typedef struct {
　struct elem *head;
　struct elem *tail;
} queue;

int queue_init(queue *q)
{
　struct elem *dummy_head = malloc(sizeof(struct elem));
　if (dummy_head == NULL) {
　　return -1;
　}
　dummy_head->next = NULL;
　q->head = q->tail = dummy_head;

　return 0;
}

int is_empty(queue *q)
{
　return q->head == q->tail;
}

int enq(queue *q, char *item)
{
　struct elem *new_elem = malloc(sizeof(struct elem));
　if (new_elem == NULL) {
　　return -1;
　}
　new_elem->data = item;
　new_elem->next = NULL;
　
　q->tail->next = new_elem;
　q->tail = new_elem;

　return 0;
}

char *deq(queue *q)
{
　struct elem *first;
　char *value;

　if (is_empty(q)) {
　　return NULL;
　}
　first = q->head->next;
　value = first->data;
　q->head->next = first->next;
　free(first);

　return value;
}