What is stopping data flow with .NET 3.5 asynchronous System.Net.Sockets.Socket?
- by TonyG
I have a .NET 3.5 client/server socket interface using the asynchronous methods. The client connects to the server and the connection should remain open until the app terminates. The protocol consists of the following pattern:
send stx
receive ack
send data1
receive ack
send data2 (repeat 5-6 while more data)
receive ack
send etx
So a single transaction with two datablocks as above would consist of 4 sends from the client. After sending etx the client simply waits for more data to send out, then begins the next transmission with stx. I do not want to break the connection between individual exchanges or after each stx/data/etx payload.
Right now, after connection, the client can send the first stx, and get a single ack, but I can't put more data onto the wire after that. Neither side disconnects, the socket is still intact. The client code is seriously abbreviated as follows - I'm following the pattern commonly available in online code samples.
private void SendReceive(string data) {
// ...
SocketAsyncEventArgs completeArgs;
completeArgs.Completed += new EventHandler<SocketAsyncEventArgs>(OnSend);
clientSocket.SendAsync(completeArgs);
// two AutoResetEvents, one for send, one for receive
if ( !AutoResetEvent.WaitAll(autoSendReceiveEvents , -1) )
Log("failed");
else
Log("success");
// ...
}
private void OnSend( object sender , SocketAsyncEventArgs e ) {
// ...
Socket s = e.UserToken as Socket;
byte[] receiveBuffer = new byte[ 4096 ];
e.SetBuffer(receiveBuffer , 0 , receiveBuffer.Length);
e.Completed += new EventHandler<SocketAsyncEventArgs>(OnReceive);
s.ReceiveAsync(e);
// ...
}
private void OnReceive( object sender , SocketAsyncEventArgs e ) {}
// ...
if ( e.BytesTransferred > 0 ) {
Int32 bytesTransferred = e.BytesTransferred;
String received = Encoding.ASCII.GetString(e.Buffer , e.Offset , bytesTransferred);
dataReceived += received;
}
autoSendReceiveEvents[ SendOperation ].Set(); // could be moved elsewhere
autoSendReceiveEvents[ ReceiveOperation ].Set(); // releases mutexes
}
The code on the server is very similar except that it receives first and then sends a response - the server is not doing anything (that I can tell) to modify the connection after it sends a response. The problem is that the second time I hit SendReceive in the client, the connection is already in a weird state.
Do I need to do something in the client to preserve the SocketAsyncEventArgs, and re-use the same object for the lifetime of the socket/connection? I'm not sure which eventargs object should hang around during the life of the connection or a given exchange.
Do I need to do something, or Not do something in the server to ensure it continues to Receive data?
The server setup and response processing looks like this:
void Start() {
// ...
listenSocket.Bind(...);
listenSocket.Listen(0);
StartAccept(null); // note accept as soon as we start. OK?
mutex.WaitOne();
}
void StartAccept(SocketAsyncEventArgs acceptEventArg) {
if ( acceptEventArg == null )
{
acceptEventArg = new SocketAsyncEventArgs();
acceptEventArg.Completed += new EventHandler<SocketAsyncEventArgs>(OnAcceptCompleted);
}
Boolean willRaiseEvent = this.listenSocket.AcceptAsync(acceptEventArg);
if ( !willRaiseEvent )
ProcessAccept(acceptEventArg);
// ...
}
private void OnAcceptCompleted( object sender , SocketAsyncEventArgs e ) {
ProcessAccept(e);
}
private void ProcessAccept( SocketAsyncEventArgs e ) {
// ...
SocketAsyncEventArgs readEventArgs = new SocketAsyncEventArgs();
readEventArgs.SetBuffer(dataBuffer , 0 , Int16.MaxValue);
readEventArgs.Completed += new EventHandler<SocketAsyncEventArgs>(OnIOCompleted);
readEventArgs.UserToken = e.AcceptSocket;
dataReceived = ""; // note server is degraded for single client/thread use
// As soon as the client is connected, post a receive to the connection.
Boolean willRaiseEvent = e.AcceptSocket.ReceiveAsync(readEventArgs);
if ( !willRaiseEvent )
this.ProcessReceive(readEventArgs);
// Accept the next connection request.
this.StartAccept(e);
}
private void OnIOCompleted( object sender , SocketAsyncEventArgs e ) {
// switch ( e.LastOperation )
case SocketAsyncOperation.Receive:
ProcessReceive(e); // similar to client code
// operate on dataReceived here
case SocketAsyncOperation.Send:
ProcessSend(e); // similar to client code
}
// execute this when a data has been processed into a response (ack, etc)
private SendResponseToClient(string response) {
// create buffer with response
// currentEventArgs has class scope and is re-used
currentEventArgs.SetBuffer(sendBuffer , 0 , sendBuffer.Length);
Boolean willRaiseEvent = currentClient.SendAsync(currentEventArgs);
if ( !willRaiseEvent )
ProcessSend(currentEventArgs);
}
A .NET trace shows the following when sending ABC\r\n:
Socket#7588182::SendAsync()
Socket#7588182::SendAsync(True#1)
Data from Socket#7588182::FinishOperation(SendAsync)
00000000 : 41 42 43 0D 0A
Socket#7588182::ReceiveAsync()
Exiting Socket#7588182::ReceiveAsync() - True#1
And it stops there. It looks just like the first send from the client but the server shows no activity.
I think that could be info overload for now but I'll be happy to provide more details as required.
Thanks!
