PRB: Modifying CWinApp Members Causes Error in DBGHEAP.C

ID: Q154744

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++, 32-bit Editions, versions 4.2, 5.0, 6.0

SYMPTOMS

The error below may occur in an MFC application where one of the following is assigned a value by the programmer:


   CWinApp::m_pszAppName
   CWinApp::m_pszRegistryKey
   CWinApp::m_pszExeName
   CWinApp::m_pszHelpFilePath
   CWinApp::m_pszProfileName

The following error is displayed in a message box:


   Debug Assertion failed
   Program: my.exe
   File: dbgheap.c
   Line: 1017
   Expression:_BLOCK_TYPE_IS_VALID(pHead->nBlockUse)

In Visual C++ 6.0, the following error is displayed in a message box:


   Debug Assertion failed
   Program: my.exe
   File: dbgheap.c
   Line: 1011
   Expression: _CrtIsValidHelpPointer(pUserData)

You may see this error when you upgrade from an earlier version of Visual C++ to Visual C++ versions 4.2 or later.

CAUSE

The CWinApp destructor in MFC included with Visual C++ 4.2 and later now frees the data assigned to the member variables shown above by passing the pointer to the free() function. Doing this prevents memory leaks, which would occur if an MFC regular DLL were dynamically loaded and unloaded.

RESOLUTION

This behavior is by design. If you assign a value to m_pszAppName, m_pszRegistryKey, m_pszExeName, m_pszHelpFilePath, or m_pszProfileName, the data must be dynamically allocated on the heap. You may want to use the _tcsdup() run-time library function to do this.

Also, free the memory associated with the current pointer before assigning a new value. Here is an example:


   // First free the string that was allocated by MFC in the startup
   // of CWinApp. The string is allocated before InitInstance is
   // called.
   free((void*)m_pszProfileName);

   // Change the name of the .INI file--CWinApp destructor will free
   // the memory.
   m_pszProfileName=_tcsdup(_T("d:\\somedir\\myini.ini"));

Additional query words: 4.10 port porting strdup

Keywords : kberrmsg kbMFC kbVC420 kbVC500 kbVC600
Version : WINDOWS NT:4.2,5.0,6.0;
Platform : NT WINDOWS
Issue type : kbprb