c++ - 使用 Windows USB 虚拟 Com 端口识别断开连接事件

Question

我正在开发一个使用虚拟串行端口设备驱动程序与 USB 设备通信的应用程序。我们遇到过这样一种情况，如果在串口句柄打开时设备被拔掉(或死机)，那么在串口句柄关闭后重新连接的唯一方法是拔掉设备，然后再插回。

如果我能够足够快地检测到故障，则有潜在的解决方法。问题在于，在这些条件下，以下函数调用不会报告错误:

• ClearCommError
• GetCommModemStatus
• ReadFile

根据我的经验，当设备被拔掉时唯一返回错误的函数是 WriteFile 。可以理解，我真的不想为了测试端口连接是否仍然有效而编写无意义的数据。

我的问题是是否有一些方法可以用来确定端口连接是否仍然有效。

如果对我正在做的事情有任何疑问，下面的代码片段显示了我的端口轮询线程正在做什么:

// set up the communications timeouts
COMMTIMEOUTS timeouts;
if (!GetCommTimeouts(port_handle, &timeouts))
   throw OsException(my_strings[strid_get_comm_timeouts_failed].c_str());

timeouts.ReadIntervalTimeout = MAXDWORD;
timeouts.ReadTotalTimeoutMultiplier = MAXDWORD;
timeouts.ReadTotalTimeoutConstant = 10;
timeouts.WriteTotalTimeoutMultiplier = 0;
timeouts.WriteTotalTimeoutConstant = 10000;
if(!SetCommTimeouts(port_handle, &timeouts))
   throw OsException(my_strings[strid_set_comm_timeouts_failed].c_str());

on_open();

// we need to set a base for the carrier detect signal.  This will be used to determine
// when the signal "changes" while the loop executes
bool carrier_detect_set = false;
uint4 modem_status = 0;

if(!GetCommModemStatus(port_handle, &modem_status))
   throw OsException(my_strings[strid_get_modem_status_failed].c_str());
if(modem_status & MS_RLSD_ON)
   carrier_detect_set = true;

 // we are now ready to enter the main service loop for this thread.
 OVERLAPPED io_control;
 memset(&io_control, 0, sizeof(io_control));
 while(!should_close)
 {
    // we need to check to see if any comm errors have occurred
    uint4 comm_errors = 0;
    if(!ClearCommError(port_handle, &comm_errors, 0))
       throw OsException(my_strings[strid_clear_comm_errors_failed].c_str());
    if(comm_errors != 0)
       on_comm_errors(comm_errors);

    // we also need to determine if the carrier detect line has changed
    modem_status = 0;
    if(!GetCommModemStatus(port_handle, &modem_status))
       throw OsException(my_strings[strid_get_modem_status_failed].c_str());
    if(carrier_detect_set && (modem_status & MS_RLSD_ON) == 0)
       on_carrier_detect_change(false);
    else if(!carrier_detect_set && (modem_status & MS_RLSD_ON) != 0)
       on_carrier_detect_change(true);

    // we will now execute any command that might be waiting
    command_handle command;
    commands_protector.lock();
    while(!commands.empty())
    {
       command = commands.front();
       commands.pop_front();
       commands_protector.unlock();
       command->execute(this, port_handle, false);
       commands_protector.lock();
    }
    commands_protector.unlock();

    // now we will try to write anything that is pending in the write queue
    fill_write_buffer(tx_queue);
    while(!tx_queue.empty() && !should_close)
    {
       uint4 bytes_avail = tx_queue.copy(tx_buff, sizeof(tx_buff));
       uint4 bytes_written = 0;

       rcd = WriteFile(port_handle, tx_buff, bytes_avail, &bytes_written, &io_control);
       if(!rcd || bytes_written == 0)
          throw Csi::OsException(my_strings[strid_write_failed].c_str());
       if(rcd)
       {
          SetLastError(0);
          if(bytes_written)
          {
             tx_queue.pop(bytes_written);
             on_low_level_write(tx_buff, bytes_written);
          }
          if(bytes_written < bytes_avail)
             throw OsException(my_strings[strid_write_timed_out].c_str());
       }
    }

    // we will now poll to see if there is any data available to be written
    uint4 bytes_read = 0;

    rcd = ReadFile(port_handle, rx_buff, sizeof(rx_buff), &bytes_read, &io_control);
    if(rcd && bytes_read)
       on_low_level_read(rx_buff, bytes_read);
    else if(!rcd)
       throw OsException(my_strings[strid_read_failed].c_str());
 }

我在使用重叠 I/O 时也遇到了同样的问题。

Answer 1

我可以通过添加以下代码来检测端口故障:

while(!should_close)
{
  // we will attempt to get and then set the comm state each time the loop repeats.
  // This will hopefully allow use to verify that the port is still valid.
  if(tx_queue.empty())
  {
      DCB dcb;
      init_dcb(&dcb);
      if(!GetCommState(port_handle, &dcb))
         throw OsException(my_strings[strid_get_modem_status_failed].c_str());
      if(!SetCommState(port_handle, &dcb))
         throw OsException(my_strings[strid_get_modem_status_failed].c_str());
 }

我担心在每个轮询循环中执行此测试的开销。我可能会尝试将其与 RegisterDeviceNotification 结合使用正如 MSalters 所建议的那样。

红利阅读

• MSDN:Registering for Device Notification ^🕗