学步园

    之前的文章中介绍过libevent事件处理的整体流程，本文主要从事件的注册和删除等方面来分析libevent的源码。

    1).event_add()函数分析：

int
event_add(struct event *ev, const struct timeval *tv)
{
	struct event_base *base = ev->ev_base;
	const struct eventop *evsel = base->evsel;
	void *evbase = base->evbase;
	int res = 0;

	event_debug((
		 "event_add: event: %p, %s%s%scall %p",
		 ev,
		 ev->ev_events & EV_READ ? "EV_READ " : " ",
		 ev->ev_events & EV_WRITE ? "EV_WRITE " : " ",
		 tv ? "EV_TIMEOUT " : " ",
		 ev->ev_callback));

	assert(!(ev->ev_flags & ~EVLIST_ALL));

	/*
	 * prepare for timeout insertion further below, if we get a
	 * failure on any step, we should not change any state.
	 */
	if (tv != NULL && !(ev->ev_flags & EVLIST_TIMEOUT)) {
		if (min_heap_reserve(&base->timeheap,
			1 + min_heap_size(&base->timeheap)) == -1)
			return (-1);  /* ENOMEM == errno */
	}

	if ((ev->ev_events & (EV_READ|EV_WRITE|EV_SIGNAL)) &&
	    !(ev->ev_flags & (EVLIST_INSERTED|EVLIST_ACTIVE))) {
		res = evsel->add(evbase, ev);
		if (res != -1)
			event_queue_insert(base, ev, EVLIST_INSERTED);
	}

	/* 
	 * we should change the timout state only if the previous event
	 * addition succeeded.
	 */
	if (res != -1 && tv != NULL) {
		struct timeval now;

		/* 
		 * we already reserved memory above for the case where we
		 * are not replacing an exisiting timeout.
		 */
		if (ev->ev_flags & EVLIST_TIMEOUT)
			event_queue_remove(base, ev, EVLIST_TIMEOUT);

		/* Check if it is active due to a timeout.  Rescheduling
		 * this timeout before the callback can be executed
		 * removes it from the active list. */
		if ((ev->ev_flags & EVLIST_ACTIVE) &&
		    (ev->ev_res & EV_TIMEOUT)) {
			/* See if we are just active executing this
			 * event in a loop
			 */
			if (ev->ev_ncalls && ev->ev_pncalls) {
				/* Abort loop */
				*ev->ev_pncalls = 0;
			}
			
			event_queue_remove(base, ev, EVLIST_ACTIVE);
		}

		gettime(base, &now);
		evutil_timeradd(&now, tv, &ev->ev_timeout);

		event_debug((
			 "event_add: timeout in %ld seconds, call %p",
			 tv->tv_sec, ev->ev_callback));

		event_queue_insert(base, ev, EVLIST_TIMEOUT);
	}

	return (res);
}

    该函数主要根据事件的类型，将事件添加到相应的注册项中去。

    a).首先如果判断是timer定时器事件，则增加最小堆的大小；

    b).对于IO事件和signal事件，先调用系统的evsel->add()添加系统的监听事件，再将该事件插入到注册的事件队列中；

    c).对于定时器事件，先清除过期的事件，然后将event插入到定时器最小堆中去；

    通过上面的分析我们可以看到，event_add做的事情很简单，就是将event添加到event_base的监听事件队列中去，同时注册系统的监听的事件；

    2).event_del()函数的分析：

int
event_del(struct event *ev)
{
	struct event_base *base;
	const struct eventop *evsel;
	void *evbase;

	event_debug(("event_del: %p, callback %p",
		 ev, ev->ev_callback));

	/* An event without a base has not been added */
	if (ev->ev_base == NULL)
		return (-1);

	base = ev->ev_base;
	evsel = base->evsel;
	evbase = base->evbase;

	assert(!(ev->ev_flags & ~EVLIST_ALL));

	/* See if we are just active executing this event in a loop */
	if (ev->ev_ncalls && ev->ev_pncalls) {
		/* Abort loop */
		*ev->ev_pncalls = 0;
	}

	if (ev->ev_flags & EVLIST_TIMEOUT)
		event_queue_remove(base, ev, EVLIST_TIMEOUT);

	if (ev->ev_flags & EVLIST_ACTIVE)
		event_queue_remove(base, ev, EVLIST_ACTIVE);

	if (ev->ev_flags & EVLIST_INSERTED) {
		event_queue_remove(base, ev, EVLIST_INSERTED);
		return (evsel->del(evbase, ev));
	}

	return (0);
}

    该函数的作用是将event从event_base监听的列表中删除；

    a).首先清空该事件还需要被call的次数，然后从相应的队列中删除事件；如果是注册监听的事件，则还需要删除系统的监听事件；

    3).event_set()函数分析：

void
event_set(struct event *ev, int fd, short events,
	  void (*callback)(int, short, void *), void *arg)
{
	/* Take the current base - caller needs to set the real base later */
	ev->ev_base = current_base;

	ev->ev_callback = callback;
	ev->ev_arg = arg;
	ev->ev_fd = fd;
	ev->ev_events = events;
	ev->ev_res = 0;
	ev->ev_flags = EVLIST_INIT;
	ev->ev_ncalls = 0;
	ev->ev_pncalls = NULL;

	min_heap_elem_init(ev);

	/* by default, we put new events into the middle priority */
	if(current_base)
		ev->ev_pri = current_base->nactivequeues/2;
}

    该函数主要是设置事件的回调函数、回调函数的参数、事件监听的fd、超时timeout；