Microsoft KB Archive/173619

= PRB: Winsock Control Generates Error 10048 - Address in Use =

Article ID: 173619

Article Last Modified on 4/28/2000

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APPLIES TO


 * Microsoft Visual Basic 5.0 Enterprise Edition

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This article was previously published under Q173619



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 2000 or Windows NT Server running Simple TCP/IP Services, which includes an Echo server on Port 7.  Start a new project in Visual Basic. Form1 is created by default. Add the Windows Socket Control to Form1.  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

 Run the program. Exit the program.</li>  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. </li></ol>

Keywords: kberrmsg kbwinsock kbapi kbnetwork kbprb kbctrl KB173619

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