libevent2.0分析：事件循环的一生

对于一个网络框架而言，其中的事件循环无疑是重要的组成部分，下面，我们就来分析一下libevent的事件循环。

一：关键结构体event_base

libevent中的事件循环信息主要是存在结构体event_base中

PS：这个结构体成员太多，我没有全部列出，只是列出了一部分比较重要的

struct event_base {
	//指定某个eventop结构体，它决定了该event_base使用哪种I/O多路复用技术（注解1）
	const struct eventop *evsel;
	void *evbase;

	//告知后端下次执行事件分发时需要注意的哪些事件
	struct event_changelist changelist;

	//一个eventop，专门用来处理信号事件的
	const struct eventop *evsigsel;
	//存储信号处理的信息
	struct evsig_info sig;

	//虚拟事件的个数
	int virtual_event_count;
	//总事件个数
	int event_count;
	//活跃事件个数
	int event_count_active;

	//处理完当前所有的事件之后退出
	int event_gotterm;
	//立即退出
	int event_break;
	//立即启动一个新的事件循环
	int event_continue;

	//当前运行事件的优先级
	int event_running_priority;

	//是否正在进行事件循环
	int running_loop;

	//活跃事件的链表
	struct event_list *activequeues;
	//活跃事件的个数
	int nactivequeues;

	//要延迟处理的事件队列
	struct deferred_cb_queue defer_queue;

	//IO事件队列（这个命名。。。）
	struct event_io_map io;

	//信号事件队列
	struct event_signal_map sigmap;

	//所有注册事件的队列
	struct event_list eventqueue;

	//管理定时事件的最小堆
	struct min_heap timeheap;
	
	//IO就绪的时候和缓存时间（这里我也不会怎么说明）
	struct timeval event_tv;
	struct timeval tv_cache;

	......
};

二：event_base的创建

创建一个event_base，要使用event_base_new函数，而这个函数主要是调用了event_base_new_with_config，下面对其进行分析

struct event_base *event_base_new_with_config(const struct event_config *cfg)
{
	int i;
	struct event_base *base;
	int should_check_environment;

#ifndef _EVENT_DISABLE_DEBUG_MODE
	event_debug_mode_too_late = 1;
#endif

	//为结构体分配内存
	if ((base = mm_calloc(1, sizeof(struct event_base))) == NULL) {
		event_warn("%s: calloc", __func__);
		return NULL;
	}
	
	//确定程序是否能使用monotonic时间
	detect_monotonic();
	//获取当前系统时间
	gettime(base, &base->event_tv);

	//初始化定时器小根堆
	min_heap_ctor(&base->timeheap);
	//初始化事件队列
	TAILQ_INIT(&base->eventqueue);

	//初始化用于信号事件的pair
	base->sig.ev_signal_pair[0] = -1;
	base->sig.ev_signal_pair[1] = -1;
	//初始化用于线程通信的pair
	base->th_notify_fd[0] = -1;
	base->th_notify_fd[1] = -1;
	
	//初始化deferred_queue
	event_deferred_cb_queue_init(&base->defer_queue);
	base->defer_queue.notify_fn = notify_base_cbq_callback;
	base->defer_queue.notify_arg = base;

	//设置标识位
	if (cfg)
		base->flags = cfg->flags;

	//初始化各个队列
	evmap_io_initmap(&base->io);
	evmap_signal_initmap(&base->sigmap);
	event_changelist_init(&base->changelist);

	base->evbase = NULL;

	should_check_environment =
	    !(cfg && (cfg->flags & EVENT_BASE_FLAG_IGNORE_ENV));

	//遍历整个eventops数组，选择最优的IO复用机制（注解1）
	for (i = 0; eventops[i] && !base->evbase; i++) {
		if (cfg != NULL) {
			/* determine if this backend should be avoided */
			
			if (event_config_is_avoided_method(cfg,
				eventops[i]->name))
				continue;
			if ((eventops[i]->features & cfg->require_features)
			    != cfg->require_features)
				continue;
		}

		/* also obey the environment variables */
		if (should_check_environment &&
		    event_is_method_disabled(eventops[i]->name))
			continue;

		//选择之后，对其进行初始化
		base->evsel = eventops[i];
		base->evbase = base->evsel->init(base);
	}

	//选择IO机制失败或对其进行初始化失败，返回NULL
	if (base->evbase == NULL) {
		event_warnx("%s: no event mechanism available",
		    __func__);
		base->evsel = NULL;
		event_base_free(base);
		return NULL;
	}

	if (evutil_getenv("EVENT_SHOW_METHOD"))
		event_msgx("libevent using: %s", base->evsel->name);

	//初始化优先级队列
	if (event_base_priority_init(base, 1) < 0) {
		event_base_free(base);
		return NULL;
	}

	/* prepare for threading */

	//以下都是线程或WINDOWS方面内容的初始化，略过
	..............

	return (base);
}

三：开始事件循环

libevent中，开始事件要使用event_base_dispatch函数，这个函数只是event_base_loop的马甲

int event_base_loop(struct event_base *base, int flags)
{
	const struct eventop *evsel = base->evsel;
	struct timeval tv;
	struct timeval *tv_p;
	int res, done, retval = 0;

	/* Grab the lock.  We will release it inside evsel.dispatch, and again
	 * as we invoke user callbacks. */
	EVBASE_ACQUIRE_LOCK(base, th_base_lock);

	//如果running_loop非零，证明该事件循环已启动，直接返回-1
	if (base->running_loop) {
		event_warnx("%s: reentrant invocation.  Only one event_base_loop"
		    " can run on each event_base at once.", __func__);
		EVBASE_RELEASE_LOCK(base, th_base_lock);
		return -1;
	}

	base->running_loop = 1;

	//清空时间缓存
	clear_time_cache(base);

	//如果加入了信号事件，就初始化与信号相关的全局变量
	if (base->sig.ev_signal_added && base->sig.ev_n_signals_added)
		evsig_set_base(base);

	done = 0;

#ifndef _EVENT_DISABLE_THREAD_SUPPORT
	base->th_owner_id = EVTHREAD_GET_ID();
#endif

	base->event_gotterm = base->event_break = 0;

	while (!done) {
		base->event_continue = 0;

		/* Terminate the loop if we have been asked to */
		if (base->event_gotterm) {
			break;
		}

		if (base->event_break) {
			break;
		}

		//更新时间缓存
		timeout_correct(base, &tv);

		tv_p = &tv;
		//如果有活动event或者指定标识EVLOOP_NONBLOCK，清空时间tv。
		//否则计算一下要等待下一个定时事件发生的时间。
		if (!N_ACTIVE_CALLBACKS(base) && !(flags & EVLOOP_NONBLOCK)) {
			timeout_next(base, &tv_p);
		} else {
			/*
			 * if we have active events, we just poll new events
			 * without waiting.
			 */
			evutil_timerclear(&tv);
		}

		/* If we have no events, we just exit */
		if (!event_haveevents(base) && !N_ACTIVE_CALLBACKS(base)) {
			event_debug(("%s: no events registered.", __func__));
			retval = 1;
			goto done;
		}

		/* update last old time */
		gettime(base, &base->event_tv);

		clear_time_cache(base);

		//调用后端的处理函数（注1）
		res = evsel->dispatch(base, tv_p);

		if (res == -1) {
			event_debug(("%s: dispatch returned unsuccessfully.",
				__func__));
			retval = -1;
			goto done;
		}

		//更新时间缓存
		update_time_cache(base);

		//处理已过期的事件
		//将这些事件从各个队列中取出，加入active队列，并在ev_res中设置EV_TIMEOUT标识
		timeout_process(base);

		//关键的事件处理函数event_process_active，以后的文章会详细介绍
		//如果没有事件了，就结束循环
		if (N_ACTIVE_CALLBACKS(base)) {
			int n = event_process_active(base);
			if ((flags & EVLOOP_ONCE)
			    && N_ACTIVE_CALLBACKS(base) == 0
			    && n != 0)
				done = 1;
		} else if (flags & EVLOOP_NONBLOCK)
			done = 1;
	}
	event_debug(("%s: asked to terminate loop.", __func__));

done:

	clear_time_cache(base);
	base->running_loop = 0;

	EVBASE_RELEASE_LOCK(base, th_base_lock);

	return (retval);
}

四：结束事件循环

libevent中，我们使用event_base_loopexit函数结束一个事件循环，这个函数其实只是注册了一个只运行一次的定时器（event_loopexit_cb），而event_loopexit_cb只是简单地将标志位（event_gotterm）设置为1，请看代码：

int event_base_loopexit(struct event_base *event_base, const struct timeval *tv)
{
	return (event_base_once(event_base, -1, EV_TIMEOUT, event_loopexit_cb,
		    event_base, tv));
}

/** Callback: used to implement event_base_loopexit by telling the event_base
 * that it's time to exit its loop. */
static void
event_loopexit_cb(evutil_socket_t fd, short what, void *arg)
{
	struct event_base *base = arg;
	base->event_gotterm = 1;
}

五：event_base的销毁

这个函数比较简单，代码里的注释也比较详细，就不多作解释了

void event_base_free(struct event_base *base)
{
	int i, n_deleted=0;
	struct event *ev;
	/* XXXX grab the lock? If there is contention when one thread frees
	 * the base, then the contending thread will be very sad soon. */

	/* event_base_free(NULL) is how to free the current_base if we
	 * made it with event_init and forgot to hold a reference to it. */
	if (base == NULL && current_base)
		base = current_base;
	/* If we're freeing current_base, there won't be a current_base. */
	if (base == current_base)
		current_base = NULL;
	/* Don't actually free NULL. */
	if (base == NULL) {
		event_warnx("%s: no base to free", __func__);
		return;
	}
	/* XXX(niels) - check for internal events first */

#ifdef WIN32
	event_base_stop_iocp(base);
#endif

	/* threading fds if we have them */
	if (base->th_notify_fd[0] != -1) {
		event_del(&base->th_notify);
		EVUTIL_CLOSESOCKET(base->th_notify_fd[0]);
		if (base->th_notify_fd[1] != -1)
			EVUTIL_CLOSESOCKET(base->th_notify_fd[1]);
		base->th_notify_fd[0] = -1;
		base->th_notify_fd[1] = -1;
		event_debug_unassign(&base->th_notify);
	}

	/* Delete all non-internal events. */
	for (ev = TAILQ_FIRST(&base->eventqueue); ev; ) {
		struct event *next = TAILQ_NEXT(ev, ev_next);
		if (!(ev->ev_flags & EVLIST_INTERNAL)) {
			event_del(ev);
			++n_deleted;
		}
		ev = next;
	}
	while ((ev = min_heap_top(&base->timeheap)) != NULL) {
		event_del(ev);
		++n_deleted;
	}
	for (i = 0; i < base->n_common_timeouts; ++i) {
		struct common_timeout_list *ctl =
		    base->common_timeout_queues[i];
		event_del(&ctl->timeout_event); /* Internal; doesn't count */
		event_debug_unassign(&ctl->timeout_event);
		for (ev = TAILQ_FIRST(&ctl->events); ev; ) {
			struct event *next = TAILQ_NEXT(ev,
			    ev_timeout_pos.ev_next_with_common_timeout);
			if (!(ev->ev_flags & EVLIST_INTERNAL)) {
				event_del(ev);
				++n_deleted;
			}
			ev = next;
		}
		mm_free(ctl);
	}
	if (base->common_timeout_queues)
		mm_free(base->common_timeout_queues);

	for (i = 0; i < base->nactivequeues; ++i) {
		for (ev = TAILQ_FIRST(&base->activequeues[i]); ev; ) {
			struct event *next = TAILQ_NEXT(ev, ev_active_next);
			if (!(ev->ev_flags & EVLIST_INTERNAL)) {
				event_del(ev);
				++n_deleted;
			}
			ev = next;
		}
	}

	if (n_deleted)
		event_debug(("%s: %d events were still set in base",
			__func__, n_deleted));

	if (base->evsel != NULL && base->evsel->dealloc != NULL)
		base->evsel->dealloc(base);

	for (i = 0; i < base->nactivequeues; ++i)
		EVUTIL_ASSERT(TAILQ_EMPTY(&base->activequeues[i]));

	EVUTIL_ASSERT(min_heap_empty(&base->timeheap));
	min_heap_dtor(&base->timeheap);

	mm_free(base->activequeues);

	EVUTIL_ASSERT(TAILQ_EMPTY(&base->eventqueue));

	evmap_io_clear(&base->io);
	evmap_signal_clear(&base->sigmap);
	event_changelist_freemem(&base->changelist);

	EVTHREAD_FREE_LOCK(base->th_base_lock, EVTHREAD_LOCKTYPE_RECURSIVE);
	EVTHREAD_FREE_COND(base->current_event_cond);

	mm_free(base);
}

六：注解

注解1：请见：http://blog.csdn.net/small_qch/article/details/19487929