libevent中事件优先级实现过程

首先它是通过指向struct event_list的指针的指针来实现的
在struct event_base结构中体现如下：

/* active event management */
struct event_list **activequeues;
int nactivequeues;

其中nactivequeues为活动事件队列链表数，对应的优先级有0到nactivequeues-1级
默认是1

相应的初始化部分在如下代码：

int
event_base_priority_init(struct event_base *base, int npriorities)
{
	int i;

	if (base->event_count_active)
		return (-1);

	if (base->nactivequeues && npriorities != base->nactivequeues) {
		for (i = 0; i < base->nactivequeues; ++i) {
			free(base->activequeues[i]);
		}
		free(base->activequeues);
	}

	/* Allocate our priority queues */
	base->nactivequeues = npriorities;
	base->activequeues = (struct event_list **)calloc(base->nactivequeues,
	    npriorities * sizeof(struct event_list *));
	if (base->activequeues == NULL)
		event_err(1, "%s: calloc", __func__);

	for (i = 0; i < base->nactivequeues; ++i) {
		base->activequeues[i] = malloc(sizeof(struct event_list));
		if (base->activequeues[i] == NULL)
			event_err(1, "%s: malloc", __func__);
		TAILQ_INIT(base->activequeues[i]);
	}

	return (0);
}

具体体现在活动事件处理函数中如下：

static void
event_process_active(struct event_base *base)
{
	struct event *ev;
	struct event_list *activeq = NULL;
	int i;
	short ncalls;

	for (i = 0; i < base->nactivequeues; ++i) {
		if (TAILQ_FIRST(base->activequeues[i]) != NULL) {
			activeq = base->activequeues[i];
			break;
		}
	}

	assert(activeq != NULL);

	for (ev = TAILQ_FIRST(activeq); ev; ev = TAILQ_FIRST(activeq)) {
		if (ev->ev_events & EV_PERSIST)
			event_queue_remove(base, ev, EVLIST_ACTIVE);
		else
			event_del(ev);
		
		/* Allows deletes to work */
		ncalls = ev->ev_ncalls;
		ev->ev_pncalls = &ncalls;
		while (ncalls) {
			ncalls--;
			ev->ev_ncalls = ncalls;
			(*ev->ev_callback)((int)ev->ev_fd, ev->ev_res, ev->ev_arg);
			if (event_gotsig || base->event_break)
				return;
		}
	}
}

优先级值越小越早处理