2.live555源码分析----服务端doEventLoop()函数分析

    上一篇博客说道，live555服务端main函数做的最后一件事就是调用如下代码陷入死循环：

env->taskScheduler().doEventLoop(); // does not return

   那么这个doEventLoop是什么样的呢？如下：

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

   就是不停地调用SingleStep（）这个函数，SingleStep函数中代码比较多，我下面仅截取关键代码，首先是使用selet陷入阻塞，等待事件发生：

int selectResult = select(fMaxNumSockets, &readSet, &writeSet, &exceptionSet, &tv_timeToDelay);

   返回值后之后会对所有的socket进行遍历，找到是哪个socket发生了事件：

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 >= 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
  }

    当找到相应的的socket的时候，调用的是(*handler->handlerProc)这个函数，这个函数是什么呢？在上一篇博客里讲过，其实这个函数就是我们用turnOnBackgroundReadHandling（）和相应socket绑定的函数，那么相对于一个server socket，绑定的就是incomingConnectionHandlerRTSP，主要功能是接受连接，创建新会话。如果是一个处理单个客户端的socket，那它绑定的就是incomingRequestHandler()，也就是负责从socket里读出数据，然后使用函数handleRequstBytes（）对数据进行处理。

  handler类型是HandlerDescriptor，这个类的定义是：

class HandlerDescriptor {
  HandlerDescriptor(HandlerDescriptor* nextHandler);
  virtual ~HandlerDescriptor();

public:
  int socketNum;
  int conditionSet;
  TaskScheduler::BackgroundHandlerProc* handlerProc;
  void* clientData;

private:
  // Descriptors are linked together in a doubly-linked list:
  friend class HandlerSet;
  friend class HandlerIterator;
  HandlerDescriptor* fNextHandler;
  HandlerDescriptor* fPrevHandler;
};

    这个类存储了select监控的socket的状况和相关参数。其中：

TaskScheduler::BackgroundHandlerProc* handlerProc；
 就是当这个socket发生事件时所需要调用的函数。