live555学习笔记【5】---消息循环

如果你真的有认真阅读live555的源码，你会发现，无论是它的服务器还是客户端，最终都会走到一个env->taskScheduler().doEventLoop()函数内，看名字应该就可以看出来，这是个消息循环处理函数。实际上，无论是客户端还是服务器，我们可以通过watchVariable变量来控制服务器或客户端的正常退出，否则它们永远都不会退出。

void BasicTaskScheduler0::doEventLoop(char volatile* watchVariable) {
  // Repeatedly loop, handling readble sockets and timed events:
  while (1) {
    //将传入参数watchVariable 在外部置位非0，则循环结束
    if (watchVariable != NULL && *watchVariable != 0) break;
    SingleStep();
  }
}

下面我们来看看void BasicTaskScheduler::SingleStep(unsigned maxDelayTime)函数里到底做了哪些工作！这里我先总结一下，如何再认真阅读一下他的源码：

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()：
 

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

  //防止等待时间过长，导致select出错
  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;
  }

  int selectResult = select(fMaxNumSockets, &readSet, &writeSet, &exceptionSet, &tv_timeToDelay);
  //select出错
  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:
      if (fDummySocketNum >= 0) closeSocket(fDummySocketNum);
      fDummySocketNum = socket(AF_INET, SOCK_DGRAM, 0);
      FD_SET((unsigned)fDummySocketNum, &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");
	// Because this failure is often "Bad file descriptor" - which is caused by an invalid socket number (i.e., a socket number
	// that had already been closed) being used in "select()" - we print out the sockets that were being used in "select()",
	// to assist in debugging:
	fprintf(stderr, "socket numbers used in the select() call:");
	for (int i = 0; i < 10000; ++i) {
	  if (FD_ISSET(i, &fReadSet) || FD_ISSET(i, &fWriteSet) || FD_ISSET(i, &fExceptionSet)) {
	    fprintf(stderr, " %d(", i);
	    if (FD_ISSET(i, &fReadSet)) fprintf(stderr, "r");
	    if (FD_ISSET(i, &fWriteSet)) fprintf(stderr, "w");
	    if (FD_ISSET(i, &fExceptionSet)) fprintf(stderr, "e");
	    fprintf(stderr, ")");
	  }
	}
	fprintf(stderr, "\n");
#endif
	internalError();
      }
  }
  //找到上次执行成功的socket节点
  // 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) {
    while ((handler = iter.next()) != NULL) {
      if (handler->socketNum == fLastHandledSocketNum) break;
    }
    if (handler == NULL) {
      fLastHandledSocketNum = -1;
      iter.reset(); // start from the beginning instead
    }
  }
  //判断socket触发的事件是否和套接字属性一致，如果一致，则执行
  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;
    }
  }
  //没有找到可执行的套接字，则从头开始遍历
  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;
      }
    }
    if (handler == NULL) fLastHandledSocketNum = -1;//because we didn't call a handler
  }

  // 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 &=~ fLastUsedTriggerMask;
      if (fTriggeredEventHandlers[fLastUsedTriggerNum] != NULL) {
	(*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) {
	  fTriggersAwaitingHandling &=~ mask;
	  if (fTriggeredEventHandlers[i] != NULL) {
	    (*fTriggeredEventHandlers[i])(fTriggeredEventClientDatas[i]);
	  }

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

  //执行DelayQueue事件
  // Also handle any delayed event that may have come due.
  fDelayQueue.handleAlarm();
}