Queue

A queue stores elements in first-in-first-out order, each slot is exactly elesz bytes, enqueue adds at the back, dequeue removes from the front, peek reads the front without removing, initialization, destructor behavior, and error conventions follow the same pattern as deq_init and related deque APIs.

Header

#include <queue.h>

Struct

struct queue {
  struct deque deq;
};

deq holds the queue state, including element size and the optional destructor given to queue_init.

Macros

queue_empty

queue_empty(queue)

Returns non-zero if the queue contains no elements.

Parameters

• queue — pointer to the queue

---

queue_size

queue_size(queue)

Returns the current element count.

Parameters

• queue — pointer to the queue

---

Functions

queue_init

int queue_init(struct queue *queue, size_t elesz, void (*destroy)(void *));

Initializes an empty queue with element size elesz and an optional destructor. Must be called before any other function. Returns 0 on success, -1 on error.

Parameters

• queue — pointer to an uninitialized queue struct
• elesz — byte size of each element, must be non-zero
• destroy — called on each element when it is discarded, or NULL for no-op

---

queue_fini

void queue_fini(struct queue *queue);

Destroys all elements and frees the buffer. No-op if queue is NULL. Does not free the struct queue itself.

Parameters

• queue — pointer to the queue

---

queue_enq

int queue_enq(struct queue *queue, void *ele);

Copies elesz bytes from ele into a new slot at the back, growing the buffer if needed. Returns 0 on success, -1 on error.

Parameters

• queue — pointer to the queue
• ele — pointer to the value to copy, must not be NULL

---

queue_deq

int queue_deq(struct queue *queue, void *dest);

Removes the front element. If dest is non-NULL, copies the element there and skips destroy. If dest is NULL and destroy is set, calls destroy on the element. Returns 0 on success, -1 if the queue is empty or queue is NULL.

Parameters

• queue — pointer to the queue
• dest — destination buffer of at least elesz bytes to receive the element, or NULL to invoke destroy

---

queue_peek

void *queue_peek(struct queue *queue);

Returns a pointer to the front element, or NULL if the queue is empty or queue is NULL.

Parameters

• queue — pointer to the queue

---

queue_clear

void queue_clear(struct queue *queue);

Removes all elements and resets size to zero, calling destroy on each when set. Does not free the buffer or the struct queue itself. No-op if queue is NULL.

Parameters

• queue — pointer to the queue

---

Example

#include <queue.h>

int main(void)
{
  struct queue q;
  int x, y;

  if (queue_init(&q, sizeof(int), NULL) != 0)
    return 1;

  x = 10;
  queue_enq(&q, &x);
  x = 20;
  queue_enq(&q, &x);

  if (!queue_empty(&q))
    y = *(int *)queue_peek(&q); /* oldest value 10 */

  queue_deq(&q, &y); /* y == 10 */
  queue_deq(&q, &y); /* y == 20 */

  queue_clear(&q);
  queue_fini(&q);
  return 0;
}