live555学习笔记3－消息循环

三 消息循环

看服端的主体：live555MediaServer.cpp中的main()函数，可见其创建一个RTSPServer类实例后，即进入一个函数env->taskScheduler().doEventLoop()中，看名字很明显是一个消息循坏，执行到里面后不停地转圈，生名不息，转圈不止。那么在这个人生的圈圈中如何实现RTSP服务和RTP传输呢？别想那么远了，还是先看这个圈圈中实现了什么功能吧。

void BasicTaskScheduler0::doEventLoop(char* watchVariable) {
	// Repeatedly loop, handling readble sockets and timed events:
	while (1) {
		if (watchVariable != NULL && *watchVariable != 0)
			break;
		SingleStep();
	}
}

BasicTaskScheduler0从TaskScheduler派生，所以还是一个任务调度对象，所以依然说明任务调度对象是整个程序的发动机。

循环中每次走一步：SingleStep()。这走一步中都做些什么呢？

总结为以下四步：

１为所有需要操作的socket执行select。

２找出第一个应执行的socket任务(handler)并执行之。

３找到第一个应响应的事件，并执行之。

４找到第一个应执行的延迟任务并执行之。

可见，每一步中只执行三个任务队列中的一项。下面详细分析函数SingleStep()：

//循坏中主要执行的函数
void BasicTaskScheduler::SingleStep(unsigned maxDelayTime) {
	fd_set readSet = fReadSet; // make a copy for this select() call
	fd_set writeSet = fWriteSet; // ditto
	fd_set exceptionSet = fExceptionSet; // ditto

	//计算select socket们时的超时时间。
	DelayInterval const& timeToDelay = fDelayQueue.timeToNextAlarm();
	struct timeval tv_timeToDelay;
	tv_timeToDelay.tv_sec = timeToDelay.seconds();
	tv_timeToDelay.tv_usec = timeToDelay.useconds();
	// Very large "tv_sec" values cause select() to fail.
	// Don't make it any larger than 1 million seconds (11.5 days)
	const long MAX_TV_SEC = MILLION;
	if (tv_timeToDelay.tv_sec > MAX_TV_SEC) {
		tv_timeToDelay.tv_sec = MAX_TV_SEC;
	}
	// Also check our "maxDelayTime" parameter (if it's > 0):
	if (maxDelayTime > 0
			&& (tv_timeToDelay.tv_sec > (long) maxDelayTime / MILLION
					|| (tv_timeToDelay.tv_sec == (long) maxDelayTime / MILLION
							&& tv_timeToDelay.tv_usec
									> (long) maxDelayTime % MILLION))) {
		tv_timeToDelay.tv_sec = maxDelayTime / MILLION;
		tv_timeToDelay.tv_usec = maxDelayTime % MILLION;
	}

	//先执行socket的select操作，以确定哪些socket任务（handler）需要执行。
	int selectResult = select(fMaxNumSockets,
			&readSet, &writeSet,&exceptionSet,
			&tv_timeToDelay);

	if (selectResult < 0) {
//#if defined(__WIN32__) || defined(_WIN32)
		int err = WSAGetLastError();
		// For some unknown reason, select() in Windoze sometimes fails with WSAEINVAL if
		// it was called with no entries set in "readSet".  If this happens, ignore it:
		if (err == WSAEINVAL && readSet.fd_count == 0) {
			err = EINTR;
			// To stop this from happening again, create a dummy socket:
			int dummySocketNum = socket(AF_INET, SOCK_DGRAM, 0);
			FD_SET((unsigned) dummySocketNum, &fReadSet);
		}
		if (err != EINTR) {
//#else
//		if (errno != EINTR && errno != EAGAIN) {
//#endif
			// Unexpected error - treat this as fatal:
//#if !defined(_WIN32_WCE)
//			perror("BasicTaskScheduler::SingleStep(): select() fails");
//#endif
			internalError();
		}
	}

	// Call the handler function for one readable socket:
	HandlerIterator iter(*fHandlers);
	HandlerDescriptor* handler;
	// To ensure forward progress through the handlers, begin past the last
	// socket number that we handled:
	if (fLastHandledSocketNum >= 0) {
		//找到上次执行的socket handler的下一个
		while ((handler = iter.next()) != NULL) {
			if (handler->socketNum == fLastHandledSocketNum)
				break;
		}
		if (handler == NULL) {
			fLastHandledSocketNum = -1;
			iter.reset(); // start from the beginning instead
		}
	}

	//从找到的handler开始，找一个可以执行的handler，不论其状态是可读，可写，还是出错，执行之。
	while ((handler = iter.next()) != NULL) {
		int sock = handler->socketNum; // alias
		int resultConditionSet = 0;
		if (FD_ISSET(sock, &readSet)
				&& FD_ISSET(sock, &fReadSet)/*sanity check*/)
			resultConditionSet |= SOCKET_READABLE;
		if (FD_ISSET(sock, &writeSet)
				&& FD_ISSET(sock, &fWriteSet)/*sanity check*/)
			resultConditionSet |= SOCKET_WRITABLE;
		if (FD_ISSET(sock, &exceptionSet)
				&& FD_ISSET(sock, &fExceptionSet)/*sanity check*/)
			resultConditionSet |= SOCKET_EXCEPTION;
		if ((resultConditionSet & handler->conditionSet)
				!= 0 && handler->handlerProc != NULL) {
			fLastHandledSocketNum = sock;
			// Note: we set "fLastHandledSocketNum" before calling the handler,
			// in case the handler calls "doEventLoop()" reentrantly.
			(*handler->handlerProc)(handler->clientData, resultConditionSet);
			break;
		}
	}

	//如果寻找完了依然没有执行任何handle，则从头再找。
	if (handler == NULL && fLastHandledSocketNum >= 0) {
		// We didn't call a handler, but we didn't get to check all of them,
		// so try again from the beginning:
		iter.reset();
		while ((handler = iter.next()) != NULL) {
			int sock = handler->socketNum; // alias
			int resultConditionSet = 0;
			if (FD_ISSET(sock, &readSet)&& FD_ISSET(sock, &fReadSet)/*sanity check*/)
				resultConditionSet |= SOCKET_READABLE;
			if (FD_ISSET(sock, &writeSet)&& FD_ISSET(sock, &fWriteSet)/*sanity check*/)
				resultConditionSet |= SOCKET_WRITABLE;
			if (FD_ISSET(sock, &exceptionSet)	&& FD_ISSET(sock, &fExceptionSet)/*sanity check*/)
				resultConditionSet |= SOCKET_EXCEPTION;
			if ((resultConditionSet & handler->conditionSet)
					!= 0 && handler->handlerProc != NULL) {
				fLastHandledSocketNum = sock;
				// Note: we set "fLastHandledSocketNum" before calling the handler,
				// in case the handler calls "doEventLoop()" reentrantly.
				(*handler->handlerProc)(handler->clientData, resultConditionSet);
				break;
			}
		}

		//依然没有找到可执行的handler。
		if (handler == NULL)
			fLastHandledSocketNum = -1; //because we didn't call a handler
	}

	//响应事件
	// Also handle any newly-triggered event
	// (Note that we do this *after* calling a socket handler,
	// in case the triggered event handler modifies The set of readable sockets.)
	if (fTriggersAwaitingHandling != 0) {
		if (fTriggersAwaitingHandling == fLastUsedTriggerMask) {
			// Common-case optimization for a single event trigger:
			fTriggersAwaitingHandling = 0;
			if (fTriggeredEventHandlers[fLastUsedTriggerNum] != NULL) {
				//执行一个事件处理函数
				(*fTriggeredEventHandlers[fLastUsedTriggerNum])(fTriggeredEventClientDatas[fLastUsedTriggerNum]);
			}
		} else {
			// Look for an event trigger that needs handling
			// (making sure that we make forward progress through all possible triggers):
			unsigned i = fLastUsedTriggerNum;
			EventTriggerId mask = fLastUsedTriggerMask;

			do {
				i = (i + 1) % MAX_NUM_EVENT_TRIGGERS;
				mask >>= 1;
				if (mask == 0)
					mask = 0x80000000;

				if ((fTriggersAwaitingHandling & mask) != 0) {
					//执行一个事件响应
					fTriggersAwaitingHandling &= ~mask;
					if (fTriggeredEventHandlers[i] != NULL) {
						(*fTriggeredEventHandlers[i])(fTriggeredEventClientDatas[i]);
					}

					fLastUsedTriggerMask = mask;
					fLastUsedTriggerNum = i;
					break;
				}
			} while (i != fLastUsedTriggerNum);
		}
	}

	//执行一个最迫切的延迟任务。
	// Also handle any delayed event that may have come due.
	fDelayQueue.handleAlarm();
}