int mbox_open(int key)
{
	mbox_t *box;
	if (key < 0 || key >= MAX_MBOX)
		return -1;

	box = Q[key];
	lock_acquire(&box->l);

	if (!l->used) {
		box->count = 0;
		box->head = 0;
		box->tail = 0;
	}
	used++;

	lock_release(&box->l);
}