HOWTO: Clean Up Temporary MFC Object in _USRDLL DLLs

• The Microsoft Foundation Classes (MFC) included with: - Microsoft C/C++ version 7.0 - Microsoft Visual C++ for Windows, version(s) 1.0, 1.5, 1.51 1.52 - Microsoft Visual C++, 32-bit Edition, version(s) 1.0, 2.0, 2.1, 4.0

SUMMARY

The Microsoft Foundation Class (MFC) Libraries create temporary objects that are used inside of message handler functions. In MFC applications, these temporary objects are automatically cleaned up in the CWinApp::OnIdle() function that is called in between processing messages. However, in MFC dynamic-link libraries (DLLs) built using the _USRDLL model, the OnIdle() function is not automatically called. As a result, temporary objects are not automatically cleaned up. To clean up temporary objects, the DLL must explicitly call OnIdle(1) periodically.

MORE INFORMATION

MFC maintains a set of maps that associate windows' handles with MFC objects. When the FromHandle() function is called, it checks the map to see whether an MFC object exists that is associated with the handle. If there is, FromHandle() returns a pointer to the object; if not, FromHandle() creates a temporary object. These temporary objects are cleaned up when CWinApp::OnIdle() is called with a count of 1.

In an MFC application, the OnIdle() function is called when the message loop is idle. It is called inside of a ::PeekMessage() loop that checks for new messages. The ::PeekMessage() loop is in the function CWinApp::Run(). The OnIdle() function is passed an "lCount" parameter, which indicates how many times OnIdle() has been called since the last message. When OnIdle() is called with an lCount equal to 1, the temporary objects are cleaned up. However, because an MFC _USRDLL DLL does not contain a main message loop, OnIdle() is not automatically called and the temporary objects are not cleaned up. To clean up the temporary objects, the DLL must call OnIdle() explicitly.

In general, it can be difficult to determine the best time to call OnIdle()- -it needs to be called often enough that temporary objects do not accumulate, and it needs to be called when no temporary objects are in use. Below are three strategies for calling OnIdle() in _USRDLL DLLs:

1. Call the OnIdle() function at the beginning of each exported function. This will clean up the temporary objects each time the DLL is entered. This method works as long as the DLL doesn't re-enter itself by calling other functions in the DLL that also clean up temporary objects when they exit. Also, the function in the DLL must not yield to Windows by calling functions such as CWnd::MessageBox(), CDialog::DoModal(), and ::PeekMessage(). These functions allow other applications to process messages. If any of these other applications were called back in to the DLL, it would cause the temporary objects to be cleaned up.

2. Have the DLL export a function to be used only for cleaning up temporary objects. Then have each application that uses the DLL call this function when its main message loop is idle. This will allow functions in the DLL to call other functions in the DLL with out prematurely cleaning up the temporary objects. Note, however, that this approach is not safe if the DLL's functions, such as CWnd::MessageBox(), CDialog::DoModal(), and ::PeekMessage(), yield to Windows. The DLLTRACE sample shipped with Visual C++ demonstrates this approach.

3. Maintain a counter that indicates whether the DLL is in the middle of a function. At the beginning of each exported function, increment the counter by 1; at the end of the function, decrement the counter by 1. If at the end of the function the counter is zero, then it is safe to call OnIdle(). The following code demonstrates one way to implement this approach:

Sample Code

   static DWORD dwEntryCount = 0;

   void LockTemporaryObjects()
   {
        InterlockedIncrement (&dwEntryCount);
   }

   void UnlockTemporaryObjects()
   {
       if (dwEntryCount == 0)
   return;//Keep us from going negative

    if (0 == InterlockedDecrement((&dwEntryCount))
       {
           AfxGetApp()->CWinApp::OnIdle(0);  // Updates UI objects
           AfxGetApp()->CWinApp::OnIdle(1);  // Free's tempory objects
       }
   }

Sample Code

   void PASCAL FAR EXPORT MyDllRoutine()
   {
        TRY
        {
              LockTemporaryObjects();

         // Do work

            UnlockTemporaryObjects();
        }
        CATCH_ALL(e)
        {
            // Good idea to deal with exceptions in _USRDLL anyway
            UnlockTemporaryObjects();
        }
        END_CATCH_ALL
   }

   CMyDialog::WindowProc(UINT message, WPARAM wParam, LPARAM lParam)
     {
        LockTemporaryObjects();
        LRESULT lResult = CDialog::WindowProc( message, wParam, lParam);
        UnlockTemporaryObjects();
        return lResult;
      }

Keywords          : MfcDLL kbhowto
Technology        : kbmfc
Version           : 1.0 1.5 1.51 1.52 2.0 2.1 4.0 7.
Platform          : MS-DOS NT WINDOWS