In simulation of a lan-messenger in c# I am using the loopback address just for starters to check the functionality of the code.However after sending a few messages, I am getting a socket exception while using the end-connect method and then a socket exception while sending the data. Is there any reason behind this exception.
private void button1_Click(object sender, EventArgs e)
{
HideCaret(this.textBox1.Handle);
StringBuilder sb = new StringBuilder(this.textBox1.Text);
str = this.textBox2.Text;
Socket newsock = new Socket(AddressFamily.InterNetwork,socketType.Stream, ProtocolType.Tcp);
IPEndPoint ip = new IPEndPoint(localaddress, 5555);
newsock.BeginConnect(ip, new AsyncCallback(connected), newsock);
if (str != "")
{
sb.AppendLine(this.textBox2.Text);
this.textBox1.Text = sb.ToString();
this.textBox2.Text = "\0";
send = Encoding.ASCII.GetBytes(str);
try
{
newsock.BeginSend(send, 0, send.Length, SocketFlags.None, new AsyncCallback(sent), newsock);
}
catch (ArgumentException)
{
MessageBox.Show("arguments incorrect in begin-send call", "Error", MessageBoxButtons.OK);
}
catch (SocketException)
{
MessageBox.Show("error in accessing socket while sending", "Error", MessageBoxButtons.OK);
}
catch (ObjectDisposedException)
{
MessageBox.Show("socket closed while sending", "Error", MessageBoxButtons.OK);
}
catch (Exception)
{
MessageBox.Show("error while sending", "Error", MessageBoxButtons.OK);
}
}
}
Please help
It's possibly because you're not waiting for the socket to be connected before sending data. You have two options:
Send the data asynchronously from your connected callback method.
Send the data synchronously by blocking this thread until connected.
You accomplish #1 by combining the data to send and the socket into a new class, then passing that as state in your call to BeginConnect. Then move your send code to the callback method.
You accomplish #2 by setting up a System.Threading.ManualResetEvent and calling WaitOne after BeginConnect. Then call EndConnect in your connected callback method, followed by a Set on your ManualResetEvent. Then proceed to send as you have above.
You'll also probably learn more if you find out what kind of SocketException you're getting.
From the MSDN Documentation:
If you receive a SocketException,
use the SocketException.ErrorCode
property to obtain the specific error
code. After you have obtained this
code, refer to the Windows Sockets
version 2 API error code documentation
in the MSDN library for a detailed
description of the error.
Do you have a separate thread reading from the socket? If not, your socket buffers are probably filling up.
You can also use .Net Socket.Connect method it will block your code until connection. So, you don't need to use ManualResetEvent and calling WaitOne. And you can further use other socket asynchronous methods too.
Related
I feel, that I am misunderstanding something about async sockets in .Net. The situation is as follows : I have 1 async socket client and 1 async socket server. They communicate without any visible problems, but when I close listener and disconnect clients, the "OnConnectRequest" which is bound to "BeginAccept" as a callback, still gets called at least once. The "BeginReceive", "OnConnectRequest", "Disconnect" and "Dispose" methods are :
public void BeginReceive()
{
_listener.Bind(_endpoint);
_listener.Listen(_maxConnections);
try
{
_listener.BeginAccept(new AsyncCallback(OnConnectRequest), _listener);
}
catch (SocketException se)
{
OnListeningError(this, new Exception("Server cannot accept connections due to network shutdown or some fatal failure", se));
}
}
protected void OnConnectRequest(IAsyncResult ar)
{
Socket listener = (Socket)ar.AsyncState;
Socket client = listener.EndAccept(ar);
var remoteEndpoint = client.RemoteEndPoint;
IDuplexStateObject state = new DuplexStateObject();
state.WorkSocket = client;
if (_clients.Count <= _maxConnections)
{
lock (_clients)
{
_clients.Add(state);
}
OnConnected(this, state);
}
else
{
//denying connection
client.Close();
AcceptingError(this, null, new Exception(string.Format("Maximal connection count reached, connection attempt {0} has been denied", (remoteEndpoint != null) ? remoteEndpoint.ToString() : null)));
}
//accept connections from other clients
try
{
listener.BeginAccept(new AsyncCallback(OnConnectRequest), listener);
}
catch (SocketException se)
{
if (se.SocketErrorCode == SocketError.TooManyOpenSockets)
{
OnListeningError(this, new Exception("Maximal connection count reached, not possible to create any more connections"));
}
else
{
OnListeningError(this, new Exception("Server cannot accept connections due to network shutdown or some fatal failure"));
}
}
}
public void Disconnect(IStateObject state)
{
if (state.WorkSocket == null)
{
//OnDisconnectError(this, state.ClientInfo,
// new Exception("No underlying work socket found for client. Already disconnected, disposing connection..."));
OnDisconnected(this, state.ClientInfo);
return;
}
try
{
if (state.WorkSocket.Connected)
{
state.WorkSocket.Shutdown(SocketShutdown.Both);
}
state.WorkSocket.Close();
}
catch (SocketException se)
{
OnDisconnectError(this, state.ClientInfo, se);
}
OnDisconnected(this, state.ClientInfo);
lock (_clients)
{
_clients.Remove(state);
}
}
public void Dispose()
{
_listener.Close();
//keys are cloned before disconnecting
foreach (var client in _clients.ToList())
{
Disconnect(client);
}
}
What I am doing is calling "Dispose" to closes listener and shut down all client sockets. The client is then still active, and it tries to reconnect, but what I expected to happen was server being completely unavailable on corresponding IP and port. What I see instead is "OnConnectRequest" callback being called, which crashes because of attempt to use already disposed socket. Can you please explain, what is wrong here, and how graceful shutdown of a listener and all accepted connections should look like ?
No, this is correct -- the callback you specify in a Begin... operation will always be called, even if you close the socket (if you close the socket, it will be called because of that). You should be catching the ObjectDisposedException you get on the EndAccept and then return without further action. Closing/disposing a socket/listener is the only way to cancel an asynchronous operation on it. (EndAccept can also produce SocketException, which should be handled normally.)
Using a flag you maintain yourself to check if the listener is still available is asking for trouble, because you're introducing shared state that needs to be synchronized (volatile reads and the like). You can easily introduce race conditions that way. The listener already maintains such a flag for you internally, which it uses to throw ObjectDisposedException, so I'd just use that. It's true that under normal circumstances catching ObjectDisposedException is a possible sign of a coding error (since you're supposed to know when an object is disposed), but with asynchronous code it's pretty standard.
I feel, that I am misunderstanding something about async sockets in .Net. The situation is as follows : I have 1 async socket client and 1 async socket server. They communicate without any visible problems, but when I close listener and disconnect clients, the "OnConnectRequest" which is bound to "BeginAccept" as a callback, still gets called at least once. The "BeginReceive", "OnConnectRequest", "Disconnect" and "Dispose" methods are :
public void BeginReceive()
{
_listener.Bind(_endpoint);
_listener.Listen(_maxConnections);
try
{
_listener.BeginAccept(new AsyncCallback(OnConnectRequest), _listener);
}
catch (SocketException se)
{
OnListeningError(this, new Exception("Server cannot accept connections due to network shutdown or some fatal failure", se));
}
}
protected void OnConnectRequest(IAsyncResult ar)
{
Socket listener = (Socket)ar.AsyncState;
Socket client = listener.EndAccept(ar);
var remoteEndpoint = client.RemoteEndPoint;
IDuplexStateObject state = new DuplexStateObject();
state.WorkSocket = client;
if (_clients.Count <= _maxConnections)
{
lock (_clients)
{
_clients.Add(state);
}
OnConnected(this, state);
}
else
{
//denying connection
client.Close();
AcceptingError(this, null, new Exception(string.Format("Maximal connection count reached, connection attempt {0} has been denied", (remoteEndpoint != null) ? remoteEndpoint.ToString() : null)));
}
//accept connections from other clients
try
{
listener.BeginAccept(new AsyncCallback(OnConnectRequest), listener);
}
catch (SocketException se)
{
if (se.SocketErrorCode == SocketError.TooManyOpenSockets)
{
OnListeningError(this, new Exception("Maximal connection count reached, not possible to create any more connections"));
}
else
{
OnListeningError(this, new Exception("Server cannot accept connections due to network shutdown or some fatal failure"));
}
}
}
public void Disconnect(IStateObject state)
{
if (state.WorkSocket == null)
{
//OnDisconnectError(this, state.ClientInfo,
// new Exception("No underlying work socket found for client. Already disconnected, disposing connection..."));
OnDisconnected(this, state.ClientInfo);
return;
}
try
{
if (state.WorkSocket.Connected)
{
state.WorkSocket.Shutdown(SocketShutdown.Both);
}
state.WorkSocket.Close();
}
catch (SocketException se)
{
OnDisconnectError(this, state.ClientInfo, se);
}
OnDisconnected(this, state.ClientInfo);
lock (_clients)
{
_clients.Remove(state);
}
}
public void Dispose()
{
_listener.Close();
//keys are cloned before disconnecting
foreach (var client in _clients.ToList())
{
Disconnect(client);
}
}
What I am doing is calling "Dispose" to closes listener and shut down all client sockets. The client is then still active, and it tries to reconnect, but what I expected to happen was server being completely unavailable on corresponding IP and port. What I see instead is "OnConnectRequest" callback being called, which crashes because of attempt to use already disposed socket. Can you please explain, what is wrong here, and how graceful shutdown of a listener and all accepted connections should look like ?
No, this is correct -- the callback you specify in a Begin... operation will always be called, even if you close the socket (if you close the socket, it will be called because of that). You should be catching the ObjectDisposedException you get on the EndAccept and then return without further action. Closing/disposing a socket/listener is the only way to cancel an asynchronous operation on it. (EndAccept can also produce SocketException, which should be handled normally.)
Using a flag you maintain yourself to check if the listener is still available is asking for trouble, because you're introducing shared state that needs to be synchronized (volatile reads and the like). You can easily introduce race conditions that way. The listener already maintains such a flag for you internally, which it uses to throw ObjectDisposedException, so I'd just use that. It's true that under normal circumstances catching ObjectDisposedException is a possible sign of a coding error (since you're supposed to know when an object is disposed), but with asynchronous code it's pretty standard.
I create a TCP listener by using the code below:
TCPListener = new TcpListener(IPAddress.Any, 1234);
I start to listen TCP devices by using the code below:
TCPListener.Start();
But here, i don't control if the port is in use. When the port is in use, program gives an exception: "Only one usage of each socket address (protocol/network address/port) is normally permitted.".
How do i handle this exception? I want to warn user that the port is in use.
Put a try/catch block around TCPListener.Start(); and catch SocketException. Also if you are opening multiple connections from your program, then its better if you keep track of your connections in a list and before opening a connection see if you already have a connection opened
It's not a good idea to get an exception to check whether the port is in use or not. Use the IPGlobalProperties object to get to an array of TcpConnectionInformation objects, which you can then interrogate about endpoint IP and port.
int port = 1234; //<--- This is your value
bool isAvailable = true;
// Evaluate current system tcp connections. This is the same information provided
// by the netstat command line application, just in .Net strongly-typed object
// form. We will look through the list, and if our port we would like to use
// in our TcpClient is occupied, we will set isAvailable to false.
IPGlobalProperties ipGlobalProperties = IPGlobalProperties.GetIPGlobalProperties();
TcpConnectionInformation[] tcpConnInfoArray = ipGlobalProperties.GetActiveTcpConnections();
foreach (TcpConnectionInformation tcpi in tcpConnInfoArray)
{
if (tcpi.LocalEndPoint.Port==port)
{
isAvailable = false;
break;
}
}
// At this point, if isAvailable is true, we can proceed accordingly.
For details please read this.
For handling the exception you will use try/catch as habib suggested
try
{
TCPListener.Start();
}
catch(SocketException ex)
{
...
}
Catch it and display your own error message.
Check the exception type and use this type in catch clause.
try
{
TCPListener.Start();
}
catch(SocketException)
{
// Your handling goes here
}
Put it in a try catch block.
try {
TCPListener = new TcpListener(IPAddress.Any, 1234);
TCPListener.Start();
} catch (SocketException e) {
// Error handling routine
Console.WriteLine( e.ToString());
}
Use try-catch blocks and catch the SocketException.
try
{
//Code here
}
catch (SocketException ex)
{
//Handle exception here
}
Well, considering that you're talking about exceptional situation, just handle that exception with suitable try/catch block, and inform a user about a fact.
I was told several times that async is better or that I should use async instead of sync sockets and as such started learning it, but am already having difficult.
I've got the basic feel of how the callback works and how to establish a connection.
I am using this msdn code as reference!
A few problems I am having with the code:
Currently that code will connect to the server, send a text, read the response and exit. How do I do so I can keep receiving the data until either the server disconnects me and/or I end it by myself ? I am not much sure on how I should do it, if I would need to wrap it on a thread with while or simple call that Receive again once the ReceiveCallback is done.
Another things I've noticed is when it connects, the socket is assigned to client but the code itself is always reassigning the client socket which I don't understand very well compared to the sync socket we have a main socket that we are always consulting etc.
I am not sure on how old the reference I am using is but would appreciate if you could help me with examples of what I have pointed out as it is easier for me to understand.
UPDATE:
private void SetupRecieveCallback(Socket sock)
{
new Thread(
delegate()
{
while (isReceiving)
{
_receiveQueue.Reset();
try
{
AsyncCallback recieveData = new AsyncCallback(OnRecievedData);
sock.BeginReceive(m_byBuff, 0, m_byBuff.Length, SocketFlags.None, recieveData, sock);
}
catch (Exception ex)
{
_logger.Error("Setup Recieve Callback failed! " + ex.Message);
}
_receiveQueue.WaitOne();
}
}
).Start();
/*
// The original code
try
{
AsyncCallback recieveData = new AsyncCallback(OnRecievedData);
sock.BeginReceive(m_byBuff, 0, m_byBuff.Length, SocketFlags.None, recieveData, sock);
}
catch (Exception ex)
{
_logger.Error("Setup Recieve Callback failed! " + ex.Message);
}
*/
}
Simply call BeginReceive() again in the callback to keep receiving. When the server breaks the connection then your callback will be called and EndReceive() throws an ObjectDisposedException. That's how you know to stop calling BeginReceive().
Second question is harder to decode (ask only one). I'm guessing you are puzzled about this statement:
private static void ConnectCallback(IAsyncResult ar) {
try {
// Retrieve the socket from the state object.
Socket client = (Socket) ar.AsyncState;
// etc..
No reassigning the socket is happening here. The code simply retrieves a reference to the original socket. Which is a useful technique, it allows this callback to be used by more than one connection. The ar.AsyncState value got to be the socket by this statement:
client.BeginConnect( remoteEP,
new AsyncCallback(ConnectCallback), client);
Note how client is passed to the AsyncCallback constructor. The exact same client that's retrieved in the callback. Any object can be passed.
I have a Socket Client and I was wondering what would be the correct approach to read the incoming data from it.
Currently I am using the follow function:
private void _ReadResponsePackets()
{
while (_socket.Connected)
{
try
{
byte[] bytes = new byte[1500];
_socket.Receive(bytes);
if (bytes.Length > 0)
LoginserverPackets.Enqueue(bytes);
}
catch (Exception ex)
{
_log.Write(ErrorType.ERROR, "[LOGINCLIENT] " + ex.ToString(), true);
}
}
}
That is called by it is own thread:
Thread _readDataThread = new Thread(_ReadResponsePackets);
_readDataThread.Start();
As you can see I am merelly reading and pilling up the packets received on a queue list to be further threaded but my question here is:
Should I use a Thread.Sleep in my
read function or leave it as it is ?
Would it impact more on the
performance or memory usage as it is
or using Thread.Sleep ?
Any other toughts ?
Please check the following link, the socket will be in blocking mode, so you don't need to use Thread.Sleep. Listener