FIX: Access Violation After Unloading Extension DLL

• The AppWizard included with: - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0,

       4.1, 4.2
  

SYMPTOMS

After you dynamically unload an AppWizard-generated Microsoft Foundation Class (MFC) Extension DLL, an access violation may occur as you try to gain access to Windows resources. The last MFC function on the call stack is usually AfxFindResourceHandle().

NOTE: This occurs only when Extension DLLs are loaded and unloaded explicitly with calls to AfxLoadLibrary() and AfxFreeLibrary() or LoadLibrary() and FreeLibrary(). Most Extension DLLs are loaded implicitly by linking to their import libraries.

CAUSE

When AppWizard generates the program template for an Extension DLL, it is not including a call to AfxTermExtensionModule(). This function removes the DLL from the list of loaded MFC Extension DLLs. Since this list is used when searching for resources, an access violation may occur after an AppWizard-generated Extension DLL is freed.

RESOLUTION

Add the following line to the DLL_PROCESS_DETACH section of an AppWizard- generated Extension DLL's DllMain():

   else if (dwReason == DLL_PROCESS_DETACH)
   {
      // Add the following line
      AfxTermExtensionModule(extensionDLL);

          // TODO: perform other cleanup tasks here
   }

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem has been fixed in Visual C++ 5.0.

MORE INFORMATION

NOTE: Dynamically loading and unloading MFC Extension DLLs was not supported in versions prior to Visual C++ 4.0. Dynamically loading or freeing Extension DLLs built with earlier versions could corrupt static MFC state information.

AfxLoadLibrary() and AfxFreeLibrary() should be called to load and free MFC Extension DLLs. Calling the Win32 API functions LoadLibrary() and FreeLibrary() to load or free MFC Extension DLLs can cause problems in multi-threaded applications.

For more information on AfxTermExtensionModule() see the "TN033: DLL Version of MFC" technote in the MFC documentation.