PRB: Memory Leak in MFC Using ODBC Oracle Driver on Win 95

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1

SYMPTOMS

An MFC ODBC application dynamically linked to an MFC DLL and connecting to an Oracle data source from a Windows 95 machine using the Intersolve 32-bit ODBC driver for Oracle, may display the following error messages in the output window of the debugger (with MFC tracing enabled) when exiting:

   The thread 0xFFFB9919 has exited with code -1 (0xFFFFFFFF).
   Detected memory leaks!
   Dumping objects ->
   {203} normal block at 0x00650D10, 512 bytes long.
   Data: <               > CD CD CD CD CD CD CD CD CD CD CD CD CD CD CD CD
   Object dump complete.

CAUSE

When the application connects to the data source, a thread is created that terminates when the application terminates. This causes DllMain in the MFC DLL to be called with DLL_PROCESS_DETACH from the secondary thread and causes the memory detection code to be executed before the memory is actually freed.

RESOLUTION

The error message refers to memory that is actually being freed, not a memory leak. The message can be safely ignored. This can be verified by setting a breakpoint in _AFX_THREAD_STATE destructor (AFXSTATE.CPP) and stepping through the code. In one of the cases, you should see that the following line is executed for the memory at the address that is supposed to be leaking:

        free(m_pSafetyPoolBuffer);

This behavior is by design. However, the design was changed in Visual C++ version 4.2, so this is no longer a problem.

MORE INFORMATION

To reproduce the problem, set up an Oracle data source on a Windows 95 machine with an Intersolve 32-bit ODBC driver and the proper SQL*Net components. Generate a default ODBC application ( SDI, "Database support, no file support" ). Add the following code to the InitInstance() function:

   CDatabase db;
   db.Open(NULL); // select 32-bit Oracle data source at run-time
   db.Close();
   return FALSE;

   /* Compile options needed: Standard
   */

   ARTICLE-ID: Q140498
   TITLE     : INF: Using the Visigenic Oracle Driver on WinNT & Win95