Multiple Threads and MFC _USRDLLs

The information in this article applies to:

• Microsoft Foundation Classes (MFC) version 3.0, included with Microsoft Visual C++, 32-bit Edition, version 2.0 4.0 4.1

SUMMARY

While the Microsoft Foundations Classes versions 3.0 and later that ship with Visual C++ versions 2.0 and later do support multiple threads, only one external thread may be attached to a _USRDLL.

MORE INFORMATION

_USRDLLs are MFC DLLs that are built as stand-alone DLLs. They can be used with non-MFC applications. They are not closely tied to the application that calls them and contain their own CWinApp derived class.

During the initialization of a _USRDLL, MFC allocates per-thread local data using Thread Local Storage (TLS). Because this is done only during the DLL_PROCESS_ATTACH, only data for the attaching thread is allocated. MFC _USRDLLs do not handle the DLL_THREAD_ATTACH case where additional threads are created, or existing threads other than the one that originally attached to the DLL.

Because most MFC 3.0 objects end up using TLS, accessing most MFC objects from a secondary thread created outside the _USRDLL will cause access violations. This is because each thread that uses MFC must be started with AfxBeginThread so that this TLS data can be initialized.

This is a change from MFC version 2.0 that shipped with Visual C++ version 1.0 for Windows NT, where no TLS data was used by MFC. It was possible in MFC version 2.0 to access _USRDLLs with multiple threads, as long as only one thread accessed the DLL at a time. (This could be accomplished by using EnterCriticalSection() or a semaphore.)

It is still possible to access the _USRDLL from multiple threads created in the application if only one thread accesses the _USRDLL at a time and each thread loads and unloads the _USRDLL. TLS data will be created for the current thread each time the DLL is loaded.

Threads created inside the _USRDLL by creating CWinThread derived objects will correctly create TLS data for those threads and work correctly.

_AFXDLL, which is closely tied to the executable, does not have these restrictions.

REFERENCES

For more information on _USRDLLs and _AFXDLLs, please see MFC technotes in Books Online : TN011 "Using MFC as Part of a DLL", and TN033 "DLL version of MFC".