HOWTO: Using getsockopt() and setsockopt() in Visual
Basic |
The information in this article applies to:
- Microsoft Visual Basic Learning Edition for Windows, version
5.0
- Microsoft Visual Basic Professional Edition for Windows,
version 5.0
- Microsoft Visual Basic Enterprise Edition 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
For additional information, click the
article number below to view the article in the Microsoft Knowledge
Base:
Q110219
LONG: How to Call Windows API from VB 3.0--General Guidelines
Q106553
HOWTO: Write C DLLs and Call Them from Visual Basic
Additional query words: