cWinsock TCP Programming Guide

📖 Table of Contents

Overview
TCP Client Programming
TCP Server Programming
Advanced Features
Common Issues

Overview

TCP (Transmission Control Protocol) is a connection-oriented, reliable transmission protocol, suitable for application scenarios requiring data integrity and ordering guarantee.

TCP Characteristics

✅ Connection-oriented: Need to establish connection first
✅ Reliable transmission: Guarantees data arrival, ordering, and integrity
✅ Flow control: Avoids network congestion
✅ Congestion control: Automatically adjusts transmission rate
❌ Higher overhead: More protocol overhead compared to UDP

Use Cases

File transfer
Chat applications
Remote control
Database connection
Web services

TCP Client Programming

Basic Flow

1. Create cWinsock object
2. Set protocol to TCP
3. Connect to server
4. Wait for Connect event
5. Send/receive data
6. Close connection

Complete Example

' Declare client object
Private WithEvents m_oClient As cWinsock

' Connect button
Private Sub cmdConnect_Click()
    On Error GoTo EH
    
    If m_oClient Is Nothing Then
        Set m_oClient = New cWinsock
    End If
    
    ' Set protocol
    m_oClient.Protocol = sckTCPProtocol
    
    ' Connect to server
    m_oClient.Connect txtHost.Text, CLng(txtPort.Text)
    
    ' Update UI
    cmdConnect.Enabled = False
    cmdDisconnect.Enabled = False
    lblStatus.Caption = "Connecting..."
    
    Exit Sub
    
EH:
    Debug.Print "Connection error: " & Err.Description
    lblStatus.Caption = "Connection failed"
End Sub

' Connection success event
Private Sub m_oClient_Connect(Client As cWinsock)
    Debug.Print "Connected to " & Client.RemoteHostIP & ":" & Client.RemotePort
    
    ' Update UI
    cmdConnect.Enabled = False
    cmdDisconnect.Enabled = True
    lblStatus.Caption = "Connected"
End Sub

' Disconnect button
Private Sub cmdDisconnect_Click()
    If Not m_oClient Is Nothing Then
        m_oClient.Close_
    End If
    
    ' Update UI
    cmdConnect.Enabled = True
    cmdDisconnect.Enabled = False
    lblStatus.Caption = "Disconnected"
End Sub

' Send data button
Private Sub cmdSend_Click()
    On Error GoTo EH
    
    If Not m_oClient Is Nothing And m_oClient.State = sckConnected Then
        m_oClient.SendData txtSend.Text
        Debug.Print "Sent: " & txtSend.Text
        
        ' Clear input box
        txtSend.Text = ""
    Else
        MsgBox "Not connected", vbExclamation
    End If
    
    Exit Sub
    
EH:
    Debug.Print "Send error: " & Err.Description
End Sub

' Data arrival event
Private Sub m_oClient_DataArrival(Client As cWinsock, ByVal bytesTotal As Long)
    Dim sData As String
    
    ' Receive data
    Client.GetData sData
    
    Debug.Print "Received data (" & bytesTotal & " bytes): " & sData
    
    ' Display on UI
    txtReceive.SelStart = Len(txtReceive.Text)
    txtReceive.SelText = sData & vbCrLf
    txtReceive.SelStart = Len(txtReceive.Text)
End Sub

' Connection closed event
Private Sub m_oClient_CloseEvent(Client As cWinsock)
    Debug.Print "Connection closed"
    
    ' Update UI
    cmdConnect.Enabled = True
    cmdDisconnect.Enabled = False
    lblStatus.Caption = "Connection closed"
End Sub

' Error event
Private Sub m_oClient_Error(Client As cWinsock, ByVal Number As Long, Description As String, ByVal Scode As Long)
    Debug.Print "Error [" & Number & "]: " & Description
    
    ' Update UI
    lblStatus.Caption = "Error: " & Description
End Sub

' Form unload
Private Sub Form_Unload(Cancel As Integer)
    On Error Resume Next
    m_oClient.Close_
End Sub

TCP Server Programming

Basic Flow

1. Create cWinsock object
2. Set protocol to TCP
3. Listen on port
4. Wait for ConnectionRequest event
5. Accept or reject connection
6. Process client data via DataArrival
7. Clean up when client disconnects

Complete Example

' Declare server object
Private WithEvents m_oServer As cWinsock

' Start server button
Private Sub cmdStart_Click()
    On Error GoTo EH
    
    If m_oServer Is Nothing Then
        Set m_oServer = New cWinsock
    End If
    
    ' Set protocol
    m_oServer.Protocol = sckTCPProtocol
    
    ' Start listening
    m_oServer.Listen CLng(txtPort.Text)
    
    Debug.Print "Server started, listening on port: " & m_oServer.LocalPort
    
    ' Update UI
    cmdStart.Enabled = False
    cmdStop.Enabled = True
    lblStatus.Caption = "Listening..."
    lblClientCount.Caption = "0"
    
    Exit Sub
    
EH:
    Debug.Print "Failed to start server: " & Err.Description
    MsgBox "Cannot start server: " & Err.Description, vbExclamation
End Sub

' Stop server button
Private Sub cmdStop_Click()
    On Error Resume Next
    
    If Not m_oServer Is Nothing Then
        m_oServer.Close_
    End If
    
    Debug.Print "Server stopped"
    
    ' Update UI
    cmdStart.Enabled = True
    cmdStop.Enabled = False
    lblStatus.Caption = "Stopped"
    lstClients.Clear
End Sub

' New connection request event
Private Sub m_oServer_ConnectionRequest(Client As cWinsock, ByRef DisConnect As Boolean)
    Debug.Print "New client connection: " & Client.RemoteHostIP & ":" & Client.RemotePort
    
    ' Check connection count limit
    If m_oServer.ClientCount >= 100 Then
        Debug.Print "Maximum connections reached, rejecting connection"
        DisConnect = True
        Exit Sub
    End If
    
    ' IP blacklist check
    If IsInBlacklist(Client.RemoteHostIP) Then
        Debug.Print "IP in blacklist, rejecting connection: " & Client.RemoteHostIP
        DisConnect = True
        Exit Sub
    End If
    
    ' Accept connection (DisConnect = False)
    Debug.Print "Accept connection: " & Client.Tag
    
    ' Update client list
    UpdateClientList
End Sub

' Data arrival event (all client data triggered through this event)
Private Sub m_oServer_DataArrival(Client As cWinsock, ByVal bytesTotal As Long)
    Dim sData As String
    
    ' Receive data
    Client.GetData sData
    
    Debug.Print "Data from " & Client.Tag & " (" & Client.RemoteHostIP & "): " & sData
    
    ' Display in log
    LogMessage Client.Tag & ": " & sData
    
    ' Echo to client
    Client.SendData "Echo: " & sData
End Sub

' Connection closed event
Private Sub m_oServer_CloseEvent(Client As cWinsock)
    Debug.Print "Client " & Client.Tag & " disconnected"
    
    ' Update client list
    UpdateClientList
End Sub

' Error event
Private Sub m_oServer_Error(Client As cWinsock, ByVal Number As Long, Description As String, ByVal Scode As Long)
    Debug.Print "Error [" & Number & "]: " & Description
    
    If Client.IsServer Then
        ' Server error
        LogMessage "Server error: " & Description
    Else
        ' Client error
        LogMessage "Client " & Client.Tag & " error: " & Description
    End If
End Sub

' Update client list
Private Sub UpdateClientList()
    lstClients.Clear
    lblClientCount.Caption = m_oServer.ClientCount
    
    Dim oClient As cWinsock
    For Each oClient In m_oServer.Clients
        lstClients.AddItem oClient.Tag & " - " & oClient.RemoteHostIP & ":" & oClient.RemotePort
    Next
End Sub

' Add log
Private Sub LogMessage(ByVal sMsg As String)
    txtLog.SelStart = Len(txtLog.Text)
    txtLog.SelText = Format$(Now, "hh:mm:ss") & " - " & sMsg & vbCrLf
    txtLog.SelStart = Len(txtLog.Text)
End Sub

' Form unload
Private Sub Form_Unload(Cancel As Integer)
    On Error Resume Next
    m_oServer.Close_
End Sub

' Blacklist check
Private Function IsInBlacklist(ByVal sIP As String) As Boolean
    ' Load blacklist from config file or database
    ' Simplified demonstration here
    IsInBlacklist = False
End Function

Advanced Features

🔄 Heartbeat Detection

' Server-side heartbeat detection
Private Const HEARTBEAT_INTERVAL As Long = 30 ' 30 seconds

Private Sub tmrHeartbeat_Timer()
    Dim oClient As cWinsock
    Dim tSession As tSessionData
    
    For Each oClient In m_oServer.Clients
        tSession = oClient.UserData
        
        ' Check if timeout
        If DateDiff("s", tSession.LastActivity, Now) > HEARTBEAT_INTERVAL Then
            Debug.Print "Client " & oClient.Tag & " heartbeat timeout"
            oClient.Close_
        Else
            ' Send heartbeat request
            oClient.SendData "PING"
        End If
    Next
End Sub

' Client response to heartbeat
Private Sub m_oClient_DataArrival(Client As cWinsock, ByVal bytesTotal As Long)
    Dim sData As String
    Client.GetData sData
    
    If sData = "PING" Then
        ' Respond to heartbeat
        Client.SendData "PONG"
    ElseIf sData = "PONG" Then
        Debug.Print "Received heartbeat response"
    Else
        ' Process other data
        ProcessData sData
    End If
End Sub

📦 Protocol Encapsulation

' Define protocol header
Private Type tPacketHeader
    Magic As Long       ' Magic number: &H12345678
    Length As Long      ' Data length
    Type As Long       ' Data type: 1=text, 2=binary
    Checksum As Long   ' Checksum
End Type

' Send structured data
Private Sub SendPacket(ByVal oSocket As cWinsock, ByVal eType As Long, ByVal baData() As Byte)
    Dim tHeader As tPacketHeader
    Dim baPacket() As Byte
    Dim lOffset As Long
    
    ' Fill header
    tHeader.Magic = &H12345678
    tHeader.Length = UBound(baData) + 1
    tHeader.Type = eType
    tHeader.Checksum = CalculateChecksum(baData)
    
    ' Construct complete packet
    ReDim baPacket(0 To Len(tHeader) + tHeader.Length - 1) As Byte
    
    ' Copy header
    CopyMemory baPacket(0), tHeader, Len(tHeader)
    
    ' Copy data body
    lOffset = Len(tHeader)
    CopyMemory baPacket(lOffset), baData(0), tHeader.Length
    
    ' Send
    oSocket.SendData baPacket
End Sub

' Receive structured data
Private Sub m_oServer_DataArrival(Client As cWinsock, ByVal bytesTotal As Long)
    Dim tHeader As tPacketHeader
    Dim baData() As Byte
    
    ' Read header first
    Client.GetData tHeader
    
    ' Verify magic number
    If tHeader.Magic <> &H12345678 Then
        Debug.Print "Invalid packet"
        Exit Sub
    End If
    
    ' Verify checksum
    ReDim baData(0 To tHeader.Length - 1) As Byte
    Client.GetData baData
    
    If CalculateChecksum(baData) <> tHeader.Checksum Then
        Debug.Print "Checksum error"
        Exit Sub
    End If
    
    ' Process data by type
    Select Case tHeader.Type
        Case 1 ' Text
            Dim sText As String
            sText = BytesToString(baData)
            Debug.Print "Text data: " & sText
            
        Case 2 ' Binary
            Debug.Print "Binary data: " & tHeader.Length & " bytes"
            
    End Select
End Sub

🔄 Auto Reconnect

' Auto reconnect client
Private WithEvents m_oClient As cWinsock
Private m_bAutoReconnect As Boolean
Private m_lReconnectInterval As Long

Private Sub StartClient()
    Set m_oClient = New cWinsock
    m_oClient.Protocol = sckTCPProtocol
    m_bAutoReconnect = True
    m_lReconnectInterval = 5 ' 5 seconds
    
    ConnectToServer
End Sub

Private Sub ConnectToServer()
    On Error GoTo EH
    
    m_oClient.Connect "127.0.0.1", 8080
    Debug.Print "Connecting..."
    
    Exit Sub
    
EH:
    Debug.Print "Connection failed: " & Err.Description
    
    If m_bAutoReconnect Then
        Debug.Print "Reconnecting in " & m_lReconnectInterval & " seconds..."
        tmrReconnect.Interval = m_lReconnectInterval * 1000
        tmrReconnect.Enabled = True
    End If
End Sub

Private Sub m_oClient_CloseEvent(Client As cWinsock)
    Debug.Print "Connection closed"
    
    If m_bAutoReconnect Then
        Debug.Print "Reconnecting in " & m_lReconnectInterval & " seconds..."
        tmrReconnect.Interval = m_lReconnectInterval * 1000
        tmrReconnect.Enabled = True
    End If
End Sub

Private Sub tmrReconnect_Timer()
    tmrReconnect.Enabled = False
    ConnectToServer
End Sub

🚦 Flow Control

' File transfer with flow control
Private m_oClient As cWinsock
Private m_lChunkSize As Long
Private m_bSending As Boolean

Private Sub SendFile(ByVal sFilePath As String)
    Dim iFileNum As Integer
    Dim baChunk() As Byte
    Dim lFileSize As Long
    Dim lSent As Long
    
    iFileNum = FreeFile
    Open sFilePath For Binary As #iFileNum
    lFileSize = LOF(iFileNum)
    
    m_lChunkSize = 8192 ' 8KB per chunk
    lSent = 0
    m_bSending = True
    
    Do While lSent < lFileSize And m_bSending
        ' Read data chunk
        ReDim baChunk(0 To m_lChunkSize - 1) As Byte
        Get #iFileNum, , baChunk
        
        ' Send
        m_oClient.SendData baChunk
        lSent = lSent + m_lChunkSize
        
        ' Update progress
        UpdateProgress lSent, lFileSize
        
        ' Wait for send completion
        Do While m_bSending
            DoEvents
            If Not m_oClient.State = sckConnected Then Exit Do
        Loop
        
        If Not m_oClient.State = sckConnected Then Exit Do
    Loop
    
    Close #iFileNum
    
    Debug.Print "File transfer completed"
End Sub

Private Sub m_oClient_SendComplete(Client As cWinsock)
    m_bSending = False
End Sub

Common Issues

❓ Issue 1: Connection Refused

Symptom: Error 10061 - Connection refused

Cause:

Server not started
Port blocked by firewall
Wrong IP address or port

Solution:

' Check if server is running
If m_oServer.State <> sckListening Then
    MsgBox "Server not started", vbExclamation
    Exit Sub
End If

' Check port
If Not IsPortOpen(txtPort.Text) Then
    MsgBox "Port " & txtPort.Text & " not open", vbExclamation
End If

❓ Issue 2: Connection Timeout

Symptom: Error 10060 - Connection timeout

Cause:

Network unreachable
Server slow to respond
Firewall blocking

Solution:

' Add retry mechanism
Private Function ConnectWithRetry(ByVal sHost As String, ByVal lPort As Long, ByVal lRetries As Long) As Boolean
    Dim i As Long
    
    For i = 1 To lRetries
        On Error Resume Next
        m_oClient.Connect sHost, lPort
        
        If Err.Number = 0 Then
            ConnectWithRetry = True
            Exit Function
        End If
        
        Debug.Print "Retry " & i & "/" & lRetries & " failed: " & Err.Description
        
        ' Wait before retry
        Sleep 2000
    Next
    
    ConnectWithRetry = False
End Function

❓ Issue 3: Data Loss

Symptom: Sent data not received by peer

Cause:

Network issues
Buffer overflow
Peer not reading correctly

Solution:

' Confirm peer received
Private Sub SendWithAck(ByVal sData As String)
    Dim sAck As String
    
    ' Send data
    m_oClient.SendData "DATA:" & sData
    
    ' Wait for acknowledgment
    sAck = WaitForAck(5000) ' 5 second timeout
    
    If sAck = "ACK" Then
        Debug.Print "Peer confirmed receipt"
    Else
        Debug.Print "No acknowledgment, resending"
        m_oClient.SendData "DATA:" & sData
    End If
End Sub

Last Updated: 2026-01-09

cWinsock TCP Programming Guide ​

📖 Table of Contents ​

Overview ​

TCP Characteristics ​

Use Cases ​

TCP Client Programming ​

Basic Flow ​

Complete Example ​

TCP Server Programming ​

Basic Flow ​

Complete Example ​

Advanced Features ​

🔄 Heartbeat Detection ​

📦 Protocol Encapsulation ​

🔄 Auto Reconnect ​

🚦 Flow Control ​

Common Issues ​

❓ Issue 1: Connection Refused ​

❓ Issue 2: Connection Timeout ​

❓ Issue 3: Data Loss ​

cWinsock TCP Programming Guide

📖 Table of Contents

Overview

TCP Characteristics

Use Cases

TCP Client Programming

Basic Flow

Complete Example

TCP Server Programming

Basic Flow

Complete Example

Advanced Features

🔄 Heartbeat Detection

📦 Protocol Encapsulation

🔄 Auto Reconnect

🚦 Flow Control

Common Issues

❓ Issue 1: Connection Refused

❓ Issue 2: Connection Timeout

❓ Issue 3: Data Loss