HOWTO: Using getsockopt() and setsockopt() in Visual Basic

• Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, version 5.0
• Microsoft Win32 Application Programming Interface (API)

SUMMARY

This article demonstrates how to use the Windows Sockets (Winsock) getsockopt and setsockopt functions in Visual Basic to read and set options on a socket. A specific parameter that is valid for the socket's protocol type can be read or set by passing a socket handle to these functions along with an option level and option name. Either a socket obtained by way of the Winsock Socket function or even the Winsock Control's socket handle (exposed by way of the SocketHandle property) can be used (as shown here).

MORE INFORMATION

As a general rule, when calling C functions in Visual Basic, the relevant constants, structures (types), and functional prototypes must be redefined or declared. This is also the case with the Winsock API. This example shows how to set up the functions and constants, read all the available socket options, and then set the two relevant socket options for a specific protocol.

The sample application contains two buttons, Command1 and Command2, which read and set values, respectively. The sample also contains a Winsock Control, Winsock1, to supply a socket handle. Please note that the Winsock Control must be in a bind state to be able to set the options on the socket. In other words, the Winsock1.SocketHandle property must have a value greater than zero. That is handled in the Form_Load event.

Some socket options require structures to be passed to get or set the information for a socket. SO_LINGER is one such option and is illustrated in the sample code that follows.

If an option is not supported for a specific socket or type of socket, the error 10042 (WSAENOPROTOOPT) is returned.

Option Explicit

' Error returned by Winsock API.
Const SOCKET_ERROR = -1

' Level number for (get/set)sockopt() to apply to socket itself.
Const SOL_SOCKET = 65535      ' Options for socket level.
Const IPPROTO_TCP = 6         ' Protocol constant for TCP.

' option flags per socket
Const SO_DEBUG = &H1&         ' Turn on debugging info recording
Const SO_ACCEPTCONN = &H2&    ' Socket has had listen() - READ-ONLY.
Const SO_REUSEADDR = &H4&     ' Allow local address reuse.
Const SO_KEEPALIVE = &H8&     ' Keep connections alive.
Const SO_DONTROUTE = &H10&    ' Just use interface addresses.
Const SO_BROADCAST = &H20&    ' Permit sending of broadcast msgs.
Const SO_USELOOPBACK = &H40&  ' Bypass hardware when possible.
Const SO_LINGER = &H80&       ' Linger on close if data present.
Const SO_OOBINLINE = &H100&   ' Leave received OOB data in line.

Const SO_DONTLINGER = Not SO_LINGER
Const SO_EXCLUSIVEADDRUSE = Not SO_REUSEADDR ' Disallow local address reuse.

' Additional options.
Const SO_SNDBUF = &H1001&     ' Send buffer size.
Const SO_RCVBUF = &H1002&     ' Receive buffer size.
Const SO_ERROR = &H1007&      ' Get error status and clear.
Const SO_TYPE = &H1008&       ' Get socket type - READ-ONLY.

' TCP Options
Const TCP_NODELAY = &H1&      ' Turn off Nagel Algorithm.

' linger structure
Private Type LINGER_STRUCT
  l_onoff As Integer          ' Is linger on or off?
  l_linger As Integer         ' Linger timeout in seconds.
End Type
 
' Winsock API declares
Private Declare Function setsockopt Lib "wsock32.dll" (ByVal s As Long, ByVal level As Long, ByVal optname As Long, optval As Any, ByVal optlen As Long) As Long
Private Declare Function getsockopt Lib "wsock32.dll" (ByVal s As Long, ByVal level As Long, ByVal optname As Long, optval As Any, optlen As Long) As Long

Private Sub Command1_Click()
  ' Read all the options and present in a message box.
  Dim socket As Long
  
  socket = Winsock1.SocketHandle
  
  If socket = 0 Then
    MsgBox "No Socket"
  Else
    MsgBox "Socket Options:" & vbCrLf & _
           "    SO_DEBUG: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_DEBUG)) & vbCrLf & _
           "    SO_ACCEPTCONN: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_ACCEPTCONN)) & vbCrLf & _
           "    SO_REUSEADDR: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_REUSEADDR)) & vbCrLf & _
           "    SO_KEEPALIVE: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_KEEPALIVE)) & vbCrLf & _
           "    SO_DONTROUTE: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_DONTROUTE)) & vbCrLf & _
           "    SO_BROADCAST: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_BROADCAST)) & vbCrLf & _
           "    SO_USELOOPBACK: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_USELOOPBACK)) & vbCrLf & _
           "    SO_LINGER: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_LINGER)) & vbCrLf & _
           "    SO_OOBINLINE: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_OOBINLINE)) & vbCrLf & _
           "    SO_DONTLINGER: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_DONTLINGER)) & vbCrLf & _
           "    SO_EXCLUSIVEADDRUSE: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_EXCLUSIVEADDRUSE)) & vbCrLf & _
           "    SO_SNDBUF: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_SNDBUF)) & vbCrLf & _
           "    SO_RCVBUF: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_RCVBUF)) & vbCrLf & _
           "    SO_ERROR: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_ERROR)) & vbCrLf & _
           "    SO_TYPE: " & CStr(GetSocketOption(socket, SOL_SOCKET, SO_TYPE)) & vbCrLf & vbCrLf & _
           "TCP Options:" & vbCrLf & _
           "    TCP_NODELAY: " & CStr(GetSocketOption(socket, IPPROTO_TCP, TCP_NODELAY))
    End If
End Sub
Private Sub Command2_Click()
  Dim lResult As Long     ' Results of 1st option.
  Dim lResult2 As Long    ' Results of 2nd option.
  Dim linger As LINGER_STRUCT
  
  If (Winsock1.Protocol = sckTCPProtocol) Then
    ' Change two options valid for TCP Sockets.
    lResult = setsockopt(Winsock1.SocketHandle, IPPROTO_TCP, TCP_NODELAY, 1, 4)
    If (lResult = SOCKET_ERROR) Then
      MsgBox "Error setting TCP_NODELAY option: " & CStr(Err.LastDllError)
    End If
    
    ' Set up the linger structure.
    linger.l_onoff = 1
    linger.l_linger = 5
    lResult2 = setsockopt(Winsock1.SocketHandle, SOL_SOCKET, SO_LINGER, linger, LenB(linger))
    If (lResult2 = SOCKET_ERROR) Then
      MsgBox "Error setting SO_LINGER option: " & CStr(Err.LastDllError)
    End If
  Else
    ' Change two options valid for UDP sockets.
    lResult = setsockopt(Winsock1.SocketHandle, SOL_SOCKET, SO_REUSEADDR, 1, 4)
    If (lResult = SOCKET_ERROR) Then
      MsgBox "Error setting SO_REUSEADDR option: " & CStr(Err.LastDllError)
    End If

    lResult2 = setsockopt(Winsock1.SocketHandle, SOL_SOCKET, SO_BROADCAST, 0, 4)
    If (lResult2 = SOCKET_ERROR) Then
      MsgBox "Error setting SO_BROADCAST option: " & CStr(Err.LastDllError)
    End If
  End If
  
  If (lResult = 0) And (lResult2 = 0) Then
    MsgBox "Options Set"
  End If
End Sub
Public Function GetSocketOption(lSocket As Long, lLevel As Long, lOption As Long) As Long
  Dim lResult As Long       ' Result of API call.
  Dim lBuffer As Long       ' Buffer to get value into.
  Dim lBufferLen As Long    ' len of buffer.
  Dim linger As LINGER_STRUCT
  
  ' Linger requires a structure so we will get that option differently.
  If (lOption <> SO_LINGER) And (lOption <> SO_DONTLINGER) Then
    lBufferLen = LenB(lBuffer)
    lResult = getsockopt(lSocket, lLevel, lOption, lBuffer, lBufferLen)
  Else
    lBufferLen = LenB(linger)
    lResult = getsockopt(lSocket, lLevel, lOption, linger, lBufferLen)
    lBuffer = linger.l_onoff
  End If
  
  If (lResult = SOCKET_ERROR) Then
    GetSocketOption = Err.LastDllError
  Else
    GetSocketOption = lBuffer
  End If
End Function
Private Sub Form_Load()
  Winsock1.Bind 8377      ' Set up socket enough to get nonzero socket. handle
End Sub 

REFERENCES

• Platform SDK documentation for Windows Sockets Version 2 API



• Visual Basic documentation for Visual Basic and the Winsock Control

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Additional query words:

Keywords : kbnetwork kbAPI kbCtrl kbSDKPlatform kbSDKWin32 kbVBp kbVBp600 kbWinsock kbDSupport kbGrpNet
Version : WINDOWS:5.0; winnt:
Platform : WINDOWS winnt
Issue type : kbhowto