Only one usage of each socket address

Question

This is the first time I have used Windows Form application to do a socket server / client.

I have ran into a problem with my Socket.bind(IPEndpoint) under my public void Listen(). I get the error

Only one usage of each socket address (protocol/network address/port) is normally permitted

Any ideas how I could go about fixing this?

public Form1()
{
    CheckForIllegalCrossThreadCalls = false;
    InitializeComponent();
}

Socket sck;
Socket acc;
IPEndPoint ipe;
List<Socket> lstClient = new List<Socket>();
Thread handleClient;
string myIP = "";

public void IP()
{
    IPHostEntry host = Dns.GetHostEntry(Dns.GetHostName());
    foreach (IPAddress address in host.AddressList)
    {
        if (address.AddressFamily.ToString() == "InterNetwork")
        {
            myIP = address.ToString();
        }
    }
    ipe = new IPEndPoint(IPAddress.Parse(myIP), 2001);
    sck = new Socket(AddressFamily.InterNetwork, SocketType.Stream, 
        ProtocolType.IP);
}

private void btnListen_Click(object sender, EventArgs e)
{
    this.Form1_Load(sender, e);    
}

private void rtbMain_KeyPress(object sender, KeyPressEventArgs e)
{
    e.Handled = true;
}

private void Form1_Load(object sender, EventArgs e)
{
    IP();
    handleClient = new Thread(new ThreadStart(Listen));
    handleClient.IsBackground = true;
    handleClient.Start();
}
public void Listen()
{
    sck.Bind(ipe); 
    sck.Listen(3);
    while(true)
    {
        Socket acc=sck.Accept();
        lstClient.Add(acc);

        Thread clientProcess = new Thread(myThreadClient);
        clientProcess.IsBackground = true;
        clientProcess.Start(acc);

        rtbMain.SelectionFont = new Font("Arial", 18, FontStyle.Bold);
        rtbMain.SelectionColor = Color.Green;
        rtbMain.AppendText("accept connections from" + 
            acc.RemoteEndPoint.ToString());
        rtbMain.ScrollToCaret();
    }
}

private void myThreadClient(object obj)
{
    Socket clientACC = (Socket)obj;
    while(true)
    {
        byte[] sizeBuf = new byte[1024];
        int rec = clientACC.Receive(sizeBuf);
        foreach (Socket acc in lstClient)
        {
            acc.Send(sizeBuf,sizeBuf.Length,SocketFlags.None);
        }
    }
}

So are you trying to bind more than one socket to the same Protocol/Address/Port? — gunr2171
Basically I want my server to listen for when a client/multiple clients connect to it, basically a chat server / client set up over an IP address if that makes sense? — user3047041

Sriram Sakthivel Sriram Sakthivel · Accepted Answer · 2013-11-30T17:35:52

This could happen if a socket is already listening in same port in your application any other process.

Just make sure no apps us it, and make sure you don't run multiple instances of your same application(verify it with taskmanager).

Never use CheckForIllegalCrossThreadCalls = false
Have meaningful method names.

Your IP method doesn't makes sense to name it as IP. I'd rather name it as GetIPAndCreateSocket or something like that.

Method name should tell us what that method is going to do.

Only one usage of each socket address

1 Answers