DOCUMENT:Q173619  09-SEP-1997  [vbwin]
TITLE   :PRB: Winsock Control Generates Error 10048 - Address in Use
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:WINDOWS:5.0
OPER/SYS:WINDOWS
KEYWORDS:vb5all kberrmsg

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The information in this article applies to:

 - Microsoft Visual Basic Enterprise Edition for Windows, version 5.0
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SYMPTOMS
========

The Winsock control will get an error after a Connect method is called if
the same local port has been used within the past four minutes.

CAUSE
=====

The Windows socket control does not have the ability to set the socket
option SO_REUSEADDR. This option allows the socket to be bound to an
address that is already in use.

When you close the handle to a socket, some additional negotiation goes on
between the client and the server. The socket will wait for up to two times
the maximum time that windows would wait to receive an acknowledgement from
the other end of the socket that closed the port. By default, this option
is set to two minutes. Therefore, Windows may wait up to four minutes
before the port is actually released.

This makes that specific port unavailable until it is actually released.

RESOLUTION
==========

The only workaround is to not use a specific local port. If you set the
LocalPort property to Zero, Winsock will pick a random local port for you
and use it until a Close method is called on the WinSock Control.

STATUS
======

This behavior is by design.

MORE INFORMATION
================

Setting the LocalPort property of the socket to Zero, will cause Windows to
pick a random port.

To do this, place the following code just before the line that does the
connect:

   Winsock1.LocalPort = 0
   Winsock1.Connect

Steps to Reproduce Behavior
---------------------------

The following sample demonstrates how to reproduce the issue and how to
work around it. In the code "myserver" indicates any Windows NT Server
running Simple TCP/IP Services, which includes an Echo server on Port 7.

1. Start a new project in Visual Basic. Form1 is created by default.

2. Add the Windows Socket Control to Form1.

3. Add the following code to the form:

      Option Explicit
      Private Sub Form_Load()
        Dim res As Long
        Winsock1.LocalPort = 2500
        Winsock1.RemoteHost = "myserver"
        Winsock1.RemotePort = 7
        Winsock1.Connect
      End Sub

      Private Sub Winsock1_Error(ByVal Number As Integer, _
        Description As String, ByVal Scode As Long, _
        ByVal Source As String, ByVal HelpFile As String, _
        ByVal HelpContext As Long, CancelDisplay As Boolean)

        MsgBox Description, vbOKOnly, "Winsock Error: " & Number
        CancelDisplay = True
      End Sub

4. Run the program.

5. Exit the program.

6. Run the program again. Note that this causes a dialog box to appear
    with the error message: Address in use.

To correct the code, change the line:

   Winsock1.LocalPort = 2500

to:

   Winsock1.LocalPort = 0

and perform steps 4 through 6 several times. You should not get any errors.
======================================================================
Keywords          : vb5all kberrmsg
Version           : WINDOWS:5.0
Platform          : WINDOWS
Issue type        : kbprb

=============================================================================

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