poll()是边缘触发函数吗？

Question

我负责一台通过TCP连接导出数据的服务器。对于服务器传输的每个数据记录，它要求客户端发回一条简短的"\n“确认消息。我有一个客户声称他发送的确认不是从web服务器读取的。下面是我在套接字上用于I/O的代码：

bool can_send = true;
char tx_buff[1024];
char rx_buff[1024];
struct pollfd poll_descriptor;
int rcd;

poll_descriptor.fd = socket_handle;
poll_descriptor.events = POLLIN | POLLOUT;
poll_descriptor.revents = 0;
while(!should_quit && is_connected)
{
   // if we know that data can be written, we need to do this before we poll the OS for
   // events.  This will prevent the 100 msec latency that would otherwise occur
   fill_write_buffer(write_buffer);
   while(can_send && !should_quit && !write_buffer.empty())
   {
      uint4 tx_len = write_buffer.copy(tx_buff, sizeof(tx_buff));
      rcd = ::send(
         socket_handle,
         tx_buff,
         tx_len,
         0);
      if(rcd == -1 && errno != EINTR)
         throw SocketException("socket write failure");
      write_buffer.pop(rcd);
      if(rcd > 0)
         on_low_level_write(tx_buff, rcd);
      if(rcd < tx_len)
         can_send = false;
   }

   // we will use poll for up to 100 msec to determine whether the socket can be read or
   // written
   if(!can_send)
      poll_descriptor.events = POLLIN | POLLOUT;
   else
      poll_descriptor.events = POLLIN;
   poll(&poll_descriptor, 1, 100);

   // check to see if an error has occurred
   if((poll_descriptor.revents & POLLERR) != 0 ||
      (poll_descriptor.revents & POLLHUP) != 0 ||
      (poll_descriptor.revents & POLLNVAL) != 0)
      throw SocketException("socket hung up or socket error");

   // check to see if anything can be written
   if((poll_descriptor.revents & POLLOUT) != 0)
      can_send = true;

   // check to see if anything can be read
   if((poll_descriptor.revents & POLLIN) != 0)
   {
      ssize_t bytes_read;
      ssize_t total_bytes_read = 0;
      int bytes_remaining = 0;
      do
      {
         bytes_read = ::recv(
            socket_handle,
            rx_buff,
            sizeof(rx_buff),
            0);
         if(bytes_read > 0)
         {
            total_bytes_read += bytes_read;
            on_low_level_read(rx_buff,bytes_read);
         }
         else if(bytes_read == -1)
            throw SocketException("read failure");
         ioctl(
            socket_handle,
            FIONREAD,
            &bytes_remaining);
      }
      while(bytes_remaining != 0);

      // recv() will return 0 if the socket has been closed
      if(total_bytes_read > 0)
         read_event::cpost(this);
      else
      {
         is_connected = false;
         closed_event::cpost(this);
      }
   }
}

我编写这段代码是基于这样的假设: poll()是一个级别触发函数，只要有数据要从套接字中读取，它就会立即解除阻塞。我所读到的一切似乎都支持这一假设。是否有我可能错过的原因导致上面的代码错过了一个读取事件？

Answer 1

它不是边缘触发的。它总是被电平触发。我将不得不阅读您的代码来回答您的实际问题。但这回答了标题中的问题。:-)

我在您的代码中看不到任何清晰的原因，为什么您可能会看到您看到的行为。但是您的问题的范围比您所呈现的代码大得多，我不能假装这是一个完整的问题诊断。

Answer 2

它是电平触发的。轮询时，如果套接字接收缓冲区中有数据，则会触发POLLIN；如果套接字发送缓冲区中有空间(几乎总是有)，则会触发POLLOUT。

Answer 3

根据您自己对问题的评估(即，当您希望能够读取确认消息时，您在poll上被阻止)，那么您最终将获得超时。

如果客户的机器与您的服务器之间的距离超过50ms，那么在收到确认消息之前，您总是会在连接上超时，因为您只等待了100ms。这是因为数据到达客户至少需要50毫秒，确认返回至少需要50毫秒。