Instantly detect client disconnection from server socket

Question

How can I detect that a client has disconnected from my server?

I have the following code in my AcceptCallBack method

static Socket hand         


        

Answer 1

Someone mentioned keepAlive capability of TCP Socket. Here it is nicely described:

http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/overview.html

I'm using it this way: after the socket is connected, I'm calling this function, which sets keepAlive on. The keepAliveTime parameter specifies the timeout, in milliseconds, with no activity until the first keep-alive packet is sent. The keepAliveInterval parameter specifies the interval, in milliseconds, between when successive keep-alive packets are sent if no acknowledgement is received.

    void SetKeepAlive(bool on, uint keepAliveTime, uint keepAliveInterval)
    {
        int size = Marshal.SizeOf(new uint());

        var inOptionValues = new byte[size * 3];

        BitConverter.GetBytes((uint)(on ? 1 : 0)).CopyTo(inOptionValues, 0);
        BitConverter.GetBytes((uint)keepAliveTime).CopyTo(inOptionValues, size);
        BitConverter.GetBytes((uint)keepAliveInterval).CopyTo(inOptionValues, size * 2);

        socket.IOControl(IOControlCode.KeepAliveValues, inOptionValues, null);
    }

I'm also using asynchronous reading:

socket.BeginReceive(packet.dataBuffer, 0, 128,
                    SocketFlags.None, new AsyncCallback(OnDataReceived), packet);

And in callback, here is caught timeout SocketException, which raises when socket doesn't get ACK signal after keep-alive packet.

public void OnDataReceived(IAsyncResult asyn)
{
    try
    {
        SocketPacket theSockId = (SocketPacket)asyn.AsyncState;

        int iRx = socket.EndReceive(asyn);
    }
    catch (SocketException ex)
    {
        SocketExceptionCaught(ex);
    }
}

This way, I'm able to safely detect disconnection between TCP client and server.