cWinsock TCP Programming Guide

📖 Table of Contents

Overview
TCP Client Programming
TCP Server Programming
Advanced Features
FAQ

Overview

TCP (Transmission Control Protocol) is a connection-oriented, reliable transport protocol, suitable for application scenarios requiring data integrity and sequence guarantees.

TCP Characteristics

✅ Connection-oriented: Requires connection establishment first
✅ Reliable transmission: Guarantees data arrival, order, and integrity
✅ Flow control: Prevents network congestion
✅ Congestion control: Automatically adjusts transmission rate
❌ Higher overhead: More protocol overhead compared to UDP

Applicable Scenarios

File transfer
Chat applications
Remote control
Database connections
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 interface
    txtReceive.SelStart = Len(txtReceive.Text)
    txtReceive.SelText = sData & vbCrLf
    txtReceive.SelStart = Len(txtReceive.Text)
End Sub

' Connection close 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 through DataArrival
7. Cleanup 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 "Unable to 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 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 triggers 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 close 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 demo here
    IsInBlacklist = False
End Function

Advanced Features

🔄 Heartbeat Detection

cWinsock's built-in heartbeat manager, embedded cTimer automatic drive, no manual implementation needed.

Server: Timeout Detection

' Configure heartbeat when starting server
m_oServer.AutoHeartbeat = True
m_oServer.HeartbeatTimeout = 120  ' 2 minutes no activity then timeout

' Timeout event
Private Sub m_oServer_ClientTimeout(Client As cWinsock)
    Debug.Print "Client " & Client.Tag & " timeout, automatically disconnected"
    UpdateClientList
End Sub

Client: Heartbeat Keepalive

' Enable heartbeat after connection
m_oClient.AutoHeartbeat = True
m_oClient.HeartbeatInterval = 50  ' 50 seconds no activity then send heartbeat

' Heartbeat event
Private Sub m_oClient_HeartbeatSent(Client As cWinsock)
    Debug.Print "Heartbeat sent, idle: " & Client.IdleSeconds & " seconds"
End Sub

Custom Heartbeat Packet

' Default heartbeat packet is single byte &H00, can be customized
Dim baHB(0 To 3) As Byte
baHB(0) = &H50  ' P
baHB(1) = &H49  ' I
baHB(2) = &H4E  ' N
baHB(3) = &H47  ' G
m_oClient.HeartbeatData = baHB

Working Principle:

Heartbeat manager embedded cTimer, triggers check every 10 seconds
Server: Checks all clients' IdleSeconds, triggers ClientTimeout and auto-disconnects if timeout
Client: If idle exceeds HeartbeatInterval sends heartbeat packet, intelligently skips when data is sent/received
Each send/receive automatically resets LastActivityTime

📦 Data Packet Protocol

cWinsock has three built-in packet protocols, automatically handles TCP fragmentation/sticky packet issues.

Delimiter Protocol (Suitable for text protocols)

m_oServer.PacketProtocol = ppDelimiter
m_oServer.Delimiter = vbCrLf  ' Newline delimiter, suitable for line-based protocols

' Send automatically appends delimiter
Client.SendData "Hello"  ' Actually sends: "Hello" + vbCrLf

' Receive automatic unpacket
Private Sub m_oServer_MessageArrival(Client As cWinsock, ByVal bytesTotal As Long)
    Debug.Print "Complete message: " & Client.GetDataText()
End Sub

Fixed Length Protocol (Suitable for fixed-length messages)

m_oServer.PacketProtocol = ppFixedLength
m_oServer.FixedLength = 256  ' Each message fixed 256 bytes

Length Header Protocol (Suitable for binary protocols)

m_oServer.PacketProtocol = ppLengthHeader
m_oServer.HeaderBytes = 4            ' 4-byte length header
m_oServer.HeaderEndian = eeLittleEndian  ' Little-endian

' Send automatically adds length header
Client.SendData "Hello"  ' Actually sends: [4-byte length] + "Hello"

' Receive automatically strips length header
Private Sub m_oServer_MessageArrival(Client As cWinsock, ByVal bytesTotal As Long)
    Dim baData() As Byte
    baData = Client.GetDataByteArray()
    ProcessBinaryMessage baData
End Sub

Configuration Inheritance: New clients automatically inherit server's protocol and heartbeat configuration, each client holds independent instance, buffers don't interfere with each other.

🔄 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

' Flow-controlled file transfer
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 complete
        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 complete"
End Sub

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

FAQ

❓ Issue 1: Connection Refused

Symptom: Error 10061 - Connection refused

Causes:

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

Solution:

' Check if server is started
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

Causes:

Network unreachable
Server slow response
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 other party

Causes:

Network issues
Buffer overflow
Other party not reading correctly

Solution:

' Confirm other party 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 "Other party confirmed receipt"
    Else
        Debug.Print "No acknowledgment received, resending"
        m_oClient.SendData "DATA:" & sData
    End If
End Sub

Last Updated: 2026-06-09

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cWinsock TCP Programming Guide

📖 Table of Contents

Overview

TCP Characteristics

Applicable Scenarios

TCP Client Programming

Basic Flow

Complete Example

TCP Server Programming

Basic Flow

Complete Example

Advanced Features

🔄 Heartbeat Detection

📦 Data Packet Protocol

🔄 Auto Reconnect

🚦 Flow Control

FAQ

❓ Issue 1: Connection Refused

❓ Issue 2: Connection Timeout

❓ Issue 3: Data Loss

cWinsock TCP Programming Guide ​

📖 Table of Contents ​

Overview ​

TCP Characteristics ​

Applicable Scenarios ​

TCP Client Programming ​

Basic Flow ​

Complete Example ​

TCP Server Programming ​

Basic Flow ​

Complete Example ​

Advanced Features ​

🔄 Heartbeat Detection ​

📦 Data Packet Protocol ​

🔄 Auto Reconnect ​

🚦 Flow Control ​

FAQ ​

❓ Issue 1: Connection Refused ​

❓ Issue 2: Connection Timeout ​

❓ Issue 3: Data Loss ​

cWinsock TCP Programming Guide

📖 Table of Contents

Overview

TCP Characteristics

Applicable Scenarios

TCP Client Programming

Basic Flow

Complete Example

TCP Server Programming

Basic Flow

Complete Example

Advanced Features

🔄 Heartbeat Detection

📦 Data Packet Protocol

🔄 Auto Reconnect

🚦 Flow Control

FAQ

❓ Issue 1: Connection Refused

❓ Issue 2: Connection Timeout

❓ Issue 3: Data Loss