Socket read freezes

Question

I'm working on small program for learning purpose.

My program is simple Telnet console.

Using visual studio 2010 Express I created pretty cool UI now I'm trying to establish communication with remote server(Creston control processor). I copied and pasted class from this Microsoft article and when I execute the class the program freezes. I'm not sure how to properly describe what happens but in basic word all controls(including close button) stop working.

Here code for my class:(I added few debug lines);

public class StateObject
{
    // Client socket.
    public Socket workSocket = null;
    // Size of receive buffer.
    public const int BufferSize = 256;
    // Receive buffer.
    public byte[] buffer = new byte[BufferSize];
    // Received data string.
    public StringBuilder sb = new StringBuilder();
    public string TempString = string.Empty;
    public int TotalBytesRead = 0;
    public char[] charBuffer = new char[1000];
}
public class AsynchronousClient
{
    // The port number for the remote device.
    private const int port = 23;

    // ManualResetEvent instances signal completion.
    private static ManualResetEvent connectDone =
        new ManualResetEvent(false);
    private static ManualResetEvent sendDone =
        new ManualResetEvent(false);
    private static ManualResetEvent receiveDone =
        new ManualResetEvent(false);

    // The response from the remote device.
    private static String response = String.Empty;
    public AsynchronousClient()
    {

    }
    public void New()
    {
        StartClient();
    }
    private static void StartClient()
    {
        // Connect to a remote device.
        try
        {
            // Establish the remote endpoint for the socket.
            // The name of the 
            // remote device is "host.contoso.com".
            IPEndPoint remoteEP = new IPEndPoint(IPAddress.Parse("10.106.6.60"), port);

            // Create a TCP/IP socket.
            Socket client = new Socket(AddressFamily.InterNetwork,
                SocketType.Stream, ProtocolType.Tcp);

            // Connect to the remote endpoint.
            client.BeginConnect(remoteEP,
                new AsyncCallback(ConnectCallback), client);
            connectDone.WaitOne();
            Console.WriteLine("StartSend");
            // Send test data to the remote device.
            //Send(client, "hostname\n");
            //sendDone.WaitOne();
            Console.WriteLine("WaitForResponse");
            // Receive the response from the remote device.
            Receive(client);
            receiveDone.WaitOne();

            // Write the response to the console.
            Console.WriteLine("Read From Socket : {0}", response);
            Console.WriteLine("ReleaseSocket");
            // Release the socket.
            client.Shutdown(SocketShutdown.Both);
            client.Disconnect(true);

        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void ConnectCallback(IAsyncResult ar)
    {
        try
        {
            // Retrieve the socket from the state object.
            Socket client = (Socket)ar.AsyncState;

            // Complete the connection.
            client.EndConnect(ar);

            Console.WriteLine("Socket connected to {0}",
                client.RemoteEndPoint.ToString());

            // Signal that the connection has been made.
            connectDone.Set();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void Receive(Socket client)
    {
        try
        {
            // Create the state object.
            Console.WriteLine("Receive");
            StateObject state = new StateObject();
            state.workSocket = client;

            // Begin receiving the data from the remote device.
            client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReceiveCallback), state);
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void ReceiveCallback(IAsyncResult ar)
    {
        try
        {
            // Retrieve the state object and the client socket 
            // from the asynchronous state object.
            Console.WriteLine("Start Read");
            StateObject state = (StateObject)ar.AsyncState;
            Socket client = state.workSocket;

            // Read data from the remote device.
            int bytesRead = client.EndReceive(ar);
            Console.WriteLine("Bytes read: {0}", bytesRead);
            if (bytesRead > 0)
            {
                // There might be more data, so store the data received so far.
                string tsTempString = Encoding.ASCII.GetString(state.buffer, 0, bytesRead);
                state.TempString += tsTempString;
                Console.WriteLine("String {0}",  tsTempString);
                state.sb.Append(Encoding.ASCII.GetString(state.buffer, 0, bytesRead));

                // Get the rest of the data.
                client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReceiveCallback), state);
            }
            else
            {
                // All the data has arrived; put it in response.
                if (state.sb.Length > 1)
                {
                    response = state.sb.ToString();
                }
                // Signal that all bytes have been received.
                receiveDone.Set();
            }
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void Send(Socket client, String data)
    {
        Console.WriteLine("Send");
        // Convert the string data to byte data using ASCII encoding.
        byte[] byteData = Encoding.ASCII.GetBytes(data);

        // Begin sending the data to the remote device.
        client.BeginSend(byteData, 0, byteData.Length, 0,
            new AsyncCallback(SendCallback), client);
    }

    private static void SendCallback(IAsyncResult ar)
    {
        try
        {
            Console.WriteLine("SendCallBack");
            // Retrieve the socket from the state object.
            Socket client = (Socket)ar.AsyncState;

            // Complete sending the data to the remote device.
            int bytesSent = client.EndSend(ar);
            Console.WriteLine("Sent {0} bytes to server.", bytesSent);

            // Signal that all bytes have been sent.
            sendDone.Set();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

}

When connecting to the server it responds with following lines:

CP3 Console
Warning: Another console session is open

CP3>

When I run the program i get following output:

Socket connected to 10.106.6.60:23
StartSend
WaitForResponse
Receive
Start Read
Bytes read: 13
String CP3 Console

Start Read
Bytes read: 43
String Warning: Another console session is open 

Start Read
Bytes read: 6
String 
CP3>
The thread '<No Name>' (0x1d80) has exited with code 0 (0x0).
The thread '<No Name>' (0x24bc) has exited with code 0 (0x0).

I tried different methods of reading stream but no successes. The program still hangs up at the same spot.

It seems as after reading last byte ">" the program does not re-executes method "ReceiveCallback" to exit out of the loop.

I get feeling it has something with the way "ReceiveCallback" is being called but I could not figure out what client.beginReceive() parameters actually do.

Answer 1

To be honest, this example from microsoft is really bad. In general, client.EndReceive never returns 0 except when the connection is closed. So receiveDone.Set will never be called in your example. In a more real-live example, you should check the received data in ReceiveCallback and when a keyword (like "Login: ") occurs, set the receiveDone and start sending (the login data for example) and the start receiving again and so on. You might also want to look at the async/await keyword (.net 4.5) and maybe upgrade to Visual Studio community edition :)

There are tons of examples on codeproject or found via google (like http://blogs.msdn.com/b/pfxteam/archive/2011/12/15/10248293.aspx).

Answer 2

To expand on Roemer's answer: EndReceive will return 0 only if the conversation has terminated. That usually means a TCP packet with the FIN or RST flag has been received, most often because the peer - in your case, the server - closed its endpoint or exited. (There are various other cases; for example, a firewall or intermediate node could generate an RST.)

(Roemer wrote "BeginReceive never returns zero...", but clearly meant EndReceive, since BeginReceive returns the IAsyncResult object.)

If the peer doesn't close its end of the conversation, EndReceive won't return 0, and you'll never enter the else branch of your callback that invokes receiveDone.Set().

It's not entirely clear from your description exactly how the server behaves, but its last send of 6 bytes was clearly a newline followed by the "CP3>" prompt. At that point it's waiting for your client to send something. It's not going to close the connection.

TCP is a bytestream service - it doesn't provide record boundaries. So there's no way for the socket layer to know when the server is "done sending", except when the server closes the connection. You have three choices for your client:

1. Parse the server's output as you receive it and recognize when it's waiting for input (in this case, that means recognizing the "CP3>" prompt)
2. Decide the server's done after some arbitrary time has passed with no further data from the server
3. Don't worry about it, and simply output data from the server as you receive it

That third option is the simplest one. Do your first BeginReceive after making the connection. Then, in your callback method, if the conversation is still open and no errors have occurred, process the data you've received and invoke BeginReceive again.