C# socket read buffer error with multiple clients connecting to servers

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I write an Windows C# application which can create up to 16 threads. Each thread creates a socket for a remote device. Each thread send commands to read the device status. (every 300 ms) It is OK when I create one or two threads to read the status. But when I create 10 threads to read device status, I will get wrong data in the socket receive buffer.

Please refer to the following for my socket driver code:

C#

class SocketClient {

    private IPAddress ipAddress;
    private IPEndPoint remoteEP;
    private Socket mSocket;
    private SocketAsyncEventArgs e = new SocketAsyncEventArgs();
    private System.Timers.Timer timer_connection;
    private static byte[] response = new byte[1024];
    private Boolean waittingConnectDone;
    private Boolean boolConnected;
    public SocketClient() {
    }

    private byte[] acknowledge = null;

    public Boolean Connect(String IP, int Port) {
        Boolean bRet = true;
        ipAddress = IPAddress.Parse(IP);
        remoteEP = new IPEndPoint(ipAddress, Port);
        mSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        //mSocket.ReceiveTimeout = GlobalVar.ethernet_timeout;
        mSocket.ReceiveTimeout = 500;

        try {
            waittingConnectDone = false;
            e.RemoteEndPoint = remoteEP;
            e.UserToken = mSocket;
            e.Completed += new EventHandler<socketasynceventargs>(e_Completed);
            mSocket.ConnectAsync(e);

            if (timer_connection != null) {
                timer_connection.Dispose();
            } else {
                timer_connection = new System.Timers.Timer();
            }
            timer_connection.Interval = 2000;
            timer_connection.Elapsed += new ElapsedEventHandler(timer_connection_Tick);
            timer_connection.Start();
            while (true) {
                if (waittingConnectDone)
                    break;
                Application.DoEvents();
            }
            bRet = boolConnected;

            //sender.Connect(remoteEP);
        } catch {
            Debug.WriteLine("### Ethernet ### Connection Error!");
            bRet = false;
        }
        return bRet;
    }

    private void e_Completed(object sender, SocketAsyncEventArgs e) {
        boolConnected = true;
        waittingConnectDone = true;
    }

    private void timer_connection_Tick(object sender, EventArgs e) {
        if (!mSocket.Connected) {
            timer_connection.Stop();
            boolConnected = false;
            waittingConnectDone = true;
        }
    }

    public void Disconnect() {
        try {
            mSocket.Shutdown(SocketShutdown.Both);
            mSocket.Close();
        } catch {
        }
    }

Each Thread use following code to read the device status:

C#

private byte[] acknowledge = null;
private static byte[] response = new byte[1024];

public byte[] sendCommand(byte[] Cmp_TxData) {
    try {
        bytesSent = mSocket.Send(Cmp_TxData);
        bytesRec = mSocket.Receive(response);

        acknowledge = new byte[bytesRec];
        Array.Copy(response, 0, acknowledge, 0, bytesRec);
    }
    catch
    {
        acknowledge = null;
    }

    return acknowledge;
}

And the buffer data error is something like following:

TX --> 00-03-01-F4-00-03-44-14 
RX <-- 00-00-00-00-00-00-00-00-00-00-00

Sometimes I read correct data, but sometimes the data are all 0!

Is there anything wrong with my socket driver?

Really appreciate your help.

Posted 23-Feb-15 8:00am

Yu-Long Fang

Updated 23-Feb-15 18:46pm

StM0n

v2

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Comments

StM0n 24-Feb-15 0:51am

I'm not quite sure, but <<private static byte[] response = new byte[1024];>> shouldn't be private... is there a reason therefore?

Yu-Long Fang 24-Feb-15 3:55am

I set it to public, but still with error. Thank you for your information anyway.

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Richard MacCutchan · Accepted Answer · 2015-02-23T22:21:00

Solution 1

C#

private static byte[] response = new byte[1024];

That means only one buffer for all threads. The buffer should be created locally within the thread, not as a static entity. You also call Receive straight after Send, but you do not check whether you received any data, or whether there is still some data being transmitted, see https://msdn.microsoft.com/en-us/library/8s4y8aff(v=vs.110).aspx[^].

Posted 23-Feb-15 22:21pm

Richard MacCutchan

Comments

Yu-Long Fang 24-Feb-15 5:53am

I modify code as following, but still with the same error!

bool goOn = true;
int recvd = 0;
byte[] temp_buf = new byte[1024];
int expected_read_len = ethernet_get_read_len(Cmp_TxData);

try {
bytesSent = mSocket.Send(Cmp_TxData);
}
catch {
return null;
}

while (goOn)
{
try
{
recvd = mSocket.Receive(response);
}
catch
{
//something bad happened (timeout, connection reset, etc). Deal with it
return null;
}

if (recvd > 0)
{
// do something with the bytes, e. g. pass them on to a List<byte> etc.
Array.Copy(response, 0, temp_buf, bytesRec, recvd);
bytesRec = bytesRec + recvd;

if (expected_read_len != 0) {
if (bytesRec == expected_read_len)
{
goOn = false;
// other side closed, do something useful with data received
acknowledge = new byte[bytesRec];
Array.Copy(response, 0, acknowledge, 0, bytesRec);
}
}
}
else
{
// other side closed, do something useful with data received
// It's the way that sockets work. 0 can only be received if the socket is down.
goOn = false;
acknowledge = new byte[bytesRec];
Array.Copy(response, 0, acknowledge, 0, bytesRec);
}
}

Richard MacCutchan 24-Feb-15 6:45am

Nothing obvious shows up, so I can only suggest adding some debug code to try and trace what is happening at each stage.

Yu-Long Fang 24-Feb-15 7:38am

I set the buffer as local not static and it works! Thank you!