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。
[cpp] view plain copyvoid 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()：
[cpp] view plain copy//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(); }新闻热点
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