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I need to create a TCP server that connects to a client on the same machine. Client sends a string formatted as minuted:secondes for example: 10:25. The server receives the time string and starts to count down from it down to 00:00

I got it "working" but it crashes after some time running. I press a label to start the server, run timer1 to receive and update the time strings and timer2 to create the count down. The problem is with timer1 but, I'm not sure how to solve it. If I use: MsgBox(ex.Message) in the Catch block, I get the message:

A non-blocking socket operation could not be completed immediately

I do not know what it means. Here are the Project files, and this is my code:

Imports System.Net
Imports System.Net.Sockets

Public Class Form1

Dim TCPServer As Socket
Dim server As TcpListener
Dim sec As Integer
Dim min As Integer

Private Sub Label3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Label3.Click
    Call TCPclient()
End Sub

Private Sub TCPclient()
    Try
        server = New TcpListener(IPAddress.Parse("127.0.0.1"), 1234)
        server.Start()
        TCPServer = server.AcceptSocket()
        TCPServer.Blocking = False
        Timer1.Start()
    Catch ex As Exception
    End Try
End Sub

Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
    Try
        Dim rcvbytes(TCPServer.ReceiveBufferSize) As Byte
        TCPServer.Receive(rcvbytes)
        Dim mensage = System.Text.Encoding.ASCII.GetString(rcvbytes)
        If mensage <> "0" Then
            sec = mensage.Substring(3, 2)
            min = mensage.Substring(0, 2)
            Timer2.Start()
        End If
    Catch ex As Exception
    End Try
End Sub

Private Sub Timer2_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer2.Tick
    Try
        sec -= 1
        If sec = 0 Then
            min -= 1
            sec = 60
        End If
        Label1.Text = min & " min" & ":" & sec & " sec"

        If min = "-1" And sec = "60" Then
            Label1.Text = "0 min:0 sec"
            Timer2.Stop()
        End If
    Catch ex As Exception
    End Try
End Sub

End Class
Jonathan Barraone
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SirAleXbox
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1 Answers1

1

The issue arises in Timer1_Tick at

 TCPServer.Receive(rcvbytes)

when you try to read but there are no bytes available.
You can avoid this simply checking for no bytes available at the beginning of Timer1_Tick function:

 Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
    Try
        If (TCPServer.Available = 0) Then
            Return
        End If
        Dim rcvbytes(TCPServer.ReceiveBufferSize) As Byte
        ...

Hope this helps

Nicola P
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  • The connection closed after a short while, is this because I’m not giving a reply to the client? – SirAleXbox Dec 27 '22 at 23:03
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    It depends on what you do on client side. The connection is not closed by the server. The server code you provided accept a connection and keep this connection open indefinitely handling all the data received on the socket named TCPServer. If your client disconnects after sending the time string, that socket does not receive data anymore and it is not possible for your code to manage data on new incoming connections. Consider to modify the code to manage this situation. – Nicola P Dec 28 '22 at 11:19
  • Thanks for the help. I have a new code that accepts new clients as they come. Even if they disconnect the can reconnect as a new client. – SirAleXbox Dec 28 '22 at 20:05