How to use getsockopt and setsockopt in Visual Basic

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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).

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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.

' 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_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
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"
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_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.

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
' 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.

' 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)
lBufferLen = LenB(linger)
lResult = getsockopt(lSocket, lLevel, lOption, linger, lBufferLen)
lBuffer = linger.l_onoff
End If

If (lResult = SOCKET_ERROR) Then
GetSocketOption = Err.LastDllError
GetSocketOption = lBuffer
End If
End Function
Private Sub Form_Load()
Winsock1.Bind 8377 ' Set up socket enough to get nonzero socket. handle
End Sub


  • Platform SDK documentation for Windows Sockets Version 2 API
  • Visual Basic documentation for Visual Basic and the Winsock Control
For additional information, click the article number below to view the article in the Microsoft Knowledge Base:

110219 LONG: How to Call Windows API from VB 3.0--General Guidelines
106553 HOWTO: Write C DLLs and Call Them from Visual Basic

ID do Artigo: 237688 - Última Revisão: 21 de nov de 2006 - Revisão: 1