libevent项目分析（四） -- 事件驱动

上一章分析了一下libevent中I/O多路复用的封装和实现，正如在select_dispatch中看到的，该函数由event_base_loo函数调用，且并不负责驱动具体事件。libevent中驱动事件的核心函数正是event_base_loop函数，该函数定义在event.c文件中。

不过在分析event_base_loop之前，首先再详细分析一下event_base这个核心数据结构

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);

	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;

	// 清除base->tv_cache.tv_sec, 否则下面timeout_correct函数不会重新取系统时间
	clear_time_cache(base);

	// 注册了信号事件，并且监视到有信号事件发生
	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) {
		/* Terminate the loop if we have been asked to */
		if (base->event_gotterm) {
			break;
		}

		if (base->event_break) {
			break;
		}

		// 更新时间，调整event_base中的时间堆timeheap
		timeout_correct(base, &tv);

		// 计算下面I/O多路复用的等待超时时间
		// 如果有激活事件，则计算到timeheap中最近的时间间隔; 否则不等待
		tv_p = &tv;
		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);

		// 调用底层I/O多路复用机制封装，如select_dispatch
		res = evsel->dispatch(base, tv_p);

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

		update_time_cache(base);

		timeout_process(base);

		// 处理Active事件
		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);
}