How To Debug MFC Module and Thread State Problems (189485)

SUMMARY

MFC maintains three different types of data: process specific data, module specific data, and thread specific data. Because there can be more than one MFC module or thread in a process, MFC maintains state variables to keep track of the current module and thread instance data.

Many ASSERTs can occur when objects are created in the context of one module or thread state, and used or destructed in another. Following are a few common ASSERTs in Visual C++, version 5.0, Service Pack 3:

   AFXWIN1.INL line 19,  ASSERT(afxCurrentInstanceHandle != NULL)
   AFXWIN1.INL line 22,  ASSERT(afxCurrentResourceHandle != NULL)
   WINCORE.CPP line 871, ASSERT(pMap != NULL)
   WINCORE.CPP line 874, ASSERT((p = pMap->LookupPermanent(m_hWnd)) != NULL
                            || (p = pMap->LookupTemporary(m_hWnd)) != NULL)
   WINCORE.CPP line 876, ASSERT((CWnd*)p == this)

MORE INFORMATION

Each MFC module state is tied to an initialization of the shared MFC DLL. The application, as well as each regular MFC DLL and MFC ActiveX control, maintains a separate initialization of MFC. MFC extension DLLs use the module state of the calling application or DLL.

To correctly switch between module states, MFC requires that each entry point contain an AFX_MANAGE_STATE macro that sets and restores the current module state. If an entry point is missing this macro, the current module state can be corrupted. This can result in an ASSERT, general protection fault, or invalid resource being loaded from somewhere else in the process, including inside another MFC DLL.

While thread state is stored in Thread Local Storage (TLS) and is always correct for the running thread, it's still possible to see problems when MFC objects are passed between threads. MFC objects that use Windows or Graphics Device Interface (GDI) handles are stored in thread specific handle maps. When these objects are passed between threads, problems occur.

Below are two macros that you can use to find these problems. You can place them in the stdafx.h file, and use them anywhere in the application or DLL to compare module or thread states:

   #ifdef _DEBUG
   #define MODULE_TRACE()  TRACE("%s(%d) : Module State nInst = 0x%X\n", \ 
         __FILE__, __LINE__, AfxGetModuleState()->m_hCurrentInstanceHandle)
   #define THREAD_TRACE()  TRACE("%s(%d) : Thread State Address = 0x%X\n",\ 
         __FILE__, __LINE__, AfxGetThreadState())
   #else
   #define MODULE_TRACE()
   #define THREAD_TRACE()
   #endif  //_DEBUG

Each of these macros prints out a message in the output window, identifying the current module or thread state. Double-clicking on the message takes you to the source line where they appear.

NOTE: you may need to run the MFC Tracer utility from the DevStudio Tools menu, and turn off support for "Multiple application debugging," as this prepends the module name to the debug string sent to the output window.

MODULE_TRACE() is most useful when you place it in every CWinApp::InitInstance() in the process, as well as where the object that is causing the ASSERTs is being created and destroyed. THREAD_TRACE() works like MODULE_TRACE(), but you should also place it in the CWinThread::InitInstance() of secondary threads.

MODULE_TRACE() returns the same value in every function in the same DLL or .exe file. THREAD_TRACE() returns the same value in every method for a CWnd, or CGdiObject-derived class. If not, there is a problem. You can call the macros more often to help isolate the problem.

Once the problem has been isolated, see the following references on how to fix or resolve the problem.

REFERENCES

For more information on MFC Module State, please see:

Visual C++ Online Documentation: Microsoft Foundation Class Reference; MFC Technical Notes; TN058: MFC Module State Implementation.

For more information on MFC thread handle maps, please see:

Visual C++ Online Documentation: Visual C++ Programmer's Guide; Adding Program Functionality; Details; Multithreading Topics; click on:

Programming tips for MFC multithreaded programming

For more information on AFX_MANAGE_STATE, search on AFX_MANAGE_STATE in the VC++ 5.0 Online Documentation.

For additional information, please see the following articles in the Microsoft Knowledge Base:

127074 How to Use AFX_MANAGE_STATE in an OLE Control

140850 How To Converting DLLTRACE to Use MFC in Shared Library