PRB: Cannot Create an MFC Thread During DLL Startup

The information in this article applies to:

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

SYMPTOMS

An MFC DLL that creates a new MFC thread during startup hangs when loaded by an application. This includes whenever a thread is created by calling AfxBeginThread or CWinThread::CreateThread inside:

• the InitInstance of a CWinApp-derived object in a Regular DLL.
• a supplied DllMain or RawDllMain function in a Regular DLL.
• a supplied DllMain or RawDllMain function in an Extension DLL.

CAUSE

For Regular DLLs, which have a CWinApp-derived object, the CWinApp::InitInstance override is called from MFC's supplied DllMain when a process is attaching to the DLL. That is, DllMain is entered with a Reason For Call of DLL_PROCESS_ATTACH, and in handling this, MFC calls in to InitInstance before cleaning up and leaving DllMain.

For Extension DLLs, the startup of the DLL is the same, except Extension DLLs do not have a CWinApp-derived object and therefore have no InitInstance.

Whenever new threads are created that use code in the DLL, DllMain is called with a Reason For Call of DLL_THREAD_ATTACH to announce to the DLL that a new thread is attaching to it. If a new thread is created in the InitInstance of an MFC Regular DLL or in the DllMain of any MFC DLL during DLL_PROCESS_ATTACH, this second thread will attempt to re-enter the DllMain, which has not yet been exited from in an effort to announce DLL_THREAD_ATTACH.

DllMain, however, is not re-entrant. That means that the second thread will not start executing until the initial creating thread has finished its work in InitInstance, returned to and left DllMain.

In versions of MFC included with 32-bit Visual C++ versions 2.2 and earlier, MFC allowed threads to be created during startup, and usually DLLs that did this would work acceptably. The second thread would get created without problem but would not start executing until the first thread left DllMain. However, it has never been a good idea to create threads in DllMain during DLL_PROCESS_ATTACH.

As of MFC 4.0, the CWinThread::CreateThread function, which is called by AfxBeginThread, now waits on the created thread to start up and initialize MFC specific data before it returns control to the calling thread. Because the second thread is waiting on the first thread to leave DllMain, both threads crash headlong into deadlock and the application hangs.

Note that this information is equally valid for the RawDllMain function. Moreover, MFC DLLs should not use a RawDllMain function at all.

RESOLUTION

Regular DLLs that create threads should only do so in functions exported from the DLL and called by client applications. Furthermore, no MFC DLL -- neither Extension nor Regular -- should create an MFC thread in the DllMain or RawDllMain function. This ensures that the thread will not be created in the middle of any critical startup code.

The recommended solution for MFC DLLs that need to create a thread when the DLL starts is to add a specific exported initialization function and create the thread in it. Applications that use the DLL would need to call this function sometime during startup, most likely during the application's InitInstance if it uses MFC. Or, if the application is loading the DLL explicitly, the application should call the initialization function immediately after the call to load the library.

The practice of exporting an initialization function for a DLL is not uncommon. Nevertheless, there may be situations where DLLs created with earlier versions of MFC are being ported but the client application cannot be changed to include a call to an initialization function. The alternative to an initialization function is to create the thread in one of the pre- existing exported functions. Any of the DLL's exported functions that require a running thread should be responsible for first checking to see if that thread exists and then creating it if it does not.

STATUS

This behavior is by design.