Question

I am having a problem with using the ManualResetEvent class with a timeout parameter. This problem occurs specifically on the WinXP embedded platform. The code works perfectly on other windows platforms. I am communicating with a TCP server. In my client code, I connect to the server and spawn a new thread wose job is to continuously monitor the receive socket for data. I send the data in the main thread. The code snippet is attached below :

internal void initSocket()
{
   .....
   .....
   if (socket.Connected)
   {
      Tracing.info("Connected to server");
      ReceiveThread = new Thread(new    ThreadStart(StartReceiving));
      ReceiveThread.Start();
    }
}

/// <summary>
/// Sends a request to Server and waits for its response.
/// </summary>
/// <param name="msg"></param>
/// <param name="timeout">Timeout time, when </param>
/// <returns></returns>
private CdcMessage sendSync(CdcMessage msg, int timeout)
{
    resultMessage = null;

    // store current messageId...
    resultMessagePackageId = msg.MessageId;

    String msgToSend = msg.serialize();

    Tracing.debug("SEND    : >> " + msgToSend);
    socketWriter.WriteLine(msgToSend);

    // Wait for response from read thread...
    resultReceivedEvent = new  ManualResetEvent(false);
    bool bReponseSent = resultReceivedEvent.WaitOne(timeout);

    if (!bReponseSent)
    {
        resultMessage = null;
    }

    return resultMessage;

}

/// <summary>
/// Thread function which continuously checks for the 
/// data from server. It will read the data only if it
/// is available
/// </summary>
public void StartReceiving()
{
    while (Connected)
    {
        try
        {
            Thread.Sleep(100);

            String response = socketReader.ReadLine();

            Tracing.info("Raw data received = " + response);

            resultMessage = CdcMessage.deserialize(response);

            Tracing.info("Deserialized response =  " + resultMessage);

            if (resultMessage == null)
            {
                continue;
            }
            else if (resultMessage.IsHeartbeat)
            {
                Tracing.debug("Heartbeat");
                socketWriter.WriteLine(response);
            }
            else if (!resultMessage.MessageId.Equals(resultMessagePackageId))
            {
                // not the correct package id...reject...
                Tracing.warn("REJECTED: Package-ID: " + resultMessage.MessageId);
                continue;
            }
            else
            {
                resultReceivedEvent.Set();
                Tracing.info("StartReceiving() : Received data");
                Tracing.debug("RECEIVED: >> " + response);
            }
        }
        catch (NullReferenceException nre)
        {
            Tracing.error("StartReceiving(): Socket doesn't exist!", nre);
            close();
            break;
        }
        catch (ObjectDisposedException ode)
        {
            Tracing.error("StartReceiving(): Socket is disposed!", ode);
            close();
            break;
        }
        catch (IOException ex)
        {
            Tracing.error("StartReceiving(): Socket IO-Exception!", ex);
            close();
            break;
        }
    }
}

I have highlighted the important aspects of code. It is observed that WaitOne(timeout) function works without any problems on most of the windows OS’s. But on XP embedded, I observe a problem. The WaitOne almost returns immediately with no data received from the receive thread.

What I did was I made the timeout as INFINITE by passing -1 to WaitOne. In this case, I could solve the problem. But this creates other side effects (e.g. if the server was shutdown, then WaitOne never returns!)

Can someone please help me in solving this issue ?

Answer 1

I’m not sure I understand your code correctly, but the lines

socketWriter.WriteLine(msgToSend);
resultReceivedEvent = new  ManualResetEvent(false);
bool bReponseSent = resultReceivedEvent.WaitOne(timeout);

look strange to me. I think this would be better:

resultReceivedEvent.Reset();
socketWriter.WriteLine(msgToSend);
bool bReponseSent = resultReceivedEvent.WaitOne(timeout);

There may be a potential race condition if an old ManualResetEvent gets set before the new one is created. There doesn’t seem to be a reason to create a new instance of ManualResetEvent here. Just call Reset on the old instance, and make sure you reset it before sending the message.