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Q96312: SAMPLE: Windows Application to Unload DLLs from Memory

Article: Q96312
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1
Operating System(s): 
Keyword(s): kbfile kbsample kb16bitonly kbOSWin310
Last Modified: 04-DEC-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 3.1 
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SUMMARY
=======

During development, dynamic-link libraries (DLLs) can sometimes be left in
memory with no applications using them. To unload DLLs that have been left in
memory, developers must exit and then restart Windows, which can be very
inconvenient. DLL UNLOADER is a sample Windows-based application that lets
developers select a DLL, show information about it, and unload it from the
system if desired; this eliminates the need to restart Windows.

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

The following file is available for download from the Microsoft Download
Center:

  Unloader.exe
  (http://download.microsoft.com/download/platformsdk/sample80/3.1/W31/EN-US/UNLOADER.EXE)

For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft used the most current virus detection software available on the date of
posting to scan this file for viruses. Once posted, the file is housed on secure
servers that prevent any unauthorized changes to the file.

There are two ways that a DLL can be left in memory after all applications that
use it have exited:

- The application loads a DLL by calling LoadLibrary() and doesn't call
  FreeLibrary to unload it.

- The application causes a general protection (GP) fault and is terminated by
  Windows.

Either of these occurrences can cause difficulties for developers. For example:

If a DLL is left in memory with no application using it, and is then recompiled
and executed, the new version of the DLL will not be loaded because Windows
thinks the DLL is already loaded (because the module names are the same). If you
try to debug the DLL with CodeView for Windows, the source code window will
display the latest version's source code; however, the old version's code, which
is still in memory, will be executed. This version mismatch causes CodeView to
appear to not be working properly. To resolve this problem, you must either exit
and restart Windows or force Windows to unload the old version of the DLL.

The DLL UNLOADER sample calls ToolHelp to obtain the list of modules currently
loaded in the system. Because this list contains modules that belong to both
DLLs and applications (which are tasks), DLL UNLOADER filters out the modules
that belong to tasks so that they cannot be unloaded accidentally and cause the
system to crash. The list box stores the module name and handle for each module
that doesn't belong to a task. When the user selects a module, its handle is
used to obtain information about it and/or unload it.

To unload a DLL, DLL UNLOADER first calls GetModuleUsage() to retrieve its usage
count and then repeatedly calls FreeLibrary() until the usage count drops to
zero; then Windows unloads the DLL. To obtain information about a DLL, DLL
UNLOADER calls ModuleFindHandle() and displays the information in a dialog box.

Finally, DLL UNLOADER creates an .INI file in the directory where its executable
file resides to remember its last position on the screen before it exits.

Additional query words: 3.10 softlib UNLOADER.EXE kbFile

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Keywords          : kbfile kbsample kb16bitonly kbOSWin310 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310
Version           : WINDOWS:3.1

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