Question

I have a sending application that uses TCP to send files. Sometimes these files contain one message, and other times the file may contain multiple messages. Unfortunately, I do not have access to the Sending application’s code.

I am working on editing legacy code to receive these messages. I have managed to get the legacy application to accept a file when there is a single message sent. However, since I disconnect the socket after receiving a single message, the Sender gives a disconnect error.

I wrote a small process to help determine whether there was another message. If it worked, I was going to incorporate it into the code, but I had mixed results:

Dim check(1) As Byte
If (handler.Receive(check, SocketFlags.Peek) > 0) Then
   Dim bytesRec As Integer
   ReDim bytes(1024)
   bytesRec = handler.Receive(bytes)
End If

If there is another message being sent, this will detect it. However, if the file only has a single message, it locks up on Receive until I send another file, and then it is accepted.

Is there a way to tell if there is another message pending that will not lock up if the stream is empty?

I won’t post all of the code for accepting the message, as it is a legacy rat’s nest, but the general idea is below:

s2 = CType(ar.AsyncState, Socket)
handler = s2.EndAccept(ar)
bytes = New Byte(1024) {}
Dim bytesRec As Integer = handler.Receive(bytes)
' Send Ack/Nak.  
numAckBytesSent = handler.Send(myByte) 

Thank you in advance for any assistance.

Answer 1

Socket.Select can be used as a quick way of polling a socket for readability. Pass in a timeout of 0 seconds, and the socket in question in the readability list, and it will simply check and report back immediately.

Two other options might be to set Socket.ReceiveTimeout on your socket, or make the socket non-blocking using Socket.Blocking, so that you can find out (as part of the Receive call) whether there is incoming data. These look a bit inconvenient to do in .NET, though, as they throw exceptions rather than simply returning a value, which might make the code a little longer.