FIX: CSingleLock: Bogus Assert in CSingleLock::CSingleLock

4.00       | 4.00

The information in this article applies to:

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

SYMPTOMS

Passing the address of a CCriticalSection object to the constructor of a CSingleLock object causes an assertion in the mtex.cpp file at line 97. This line is in the constructor for CSingleLock.

CAUSE

A Critical Section object cannot be passed to ::WaitForSingleObject. This is because Critical Section objects do not have a corresponding kernel object. Because CSingleLock::Lock calls CSyncObject::Lock, which calls ::WaitForSingleObject, the ASSERT tries to catch the case when a Critical Section object is incorrectly passed to ::WaitForSingleObject.

However, CSyncObject::Lock is virtual and CCriticalSection::Lock is overridden to make the right call, ::EnterCriticalSection. As a consequence, ::WaitForSingleObject is never called with a Critical Section object. Hence the ASSERT statement is bogus.

RESOLUTION

The assert can be safely ignored. ASSERT statements are only evaluated in debug builds, so this is not a problem with release builds. To get rid of the assert, remove the assert from the MFC source and rebuild the debug version of the MFC library. For details on rebuilding the MFC libraries, please see the Readme.txt file located in the <msdev>\Mfc\Src directory.

The CCriticalSection class does not necessarily have to work with CSingleLock, so you can avoid this assertion by avoiding the CSingleLock class entirely. This would involve making a call to CCriticalSection::Lock directly. See the Help file for CCriticalSection for more information on this approach.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ 4.1.

MORE INFORMATION

Because the call to CMultiLock::Lock always calls ::WaitForMultipleObjects or ::MsgWaitForMultipleObjects, you must make sure that none of the synchronization objects passed to CMultiLock constructor are of type CCriticalSection. This means that a similar assertion in the constructor for CMultiLock in the mtex.cpp file at line 163 is valid.

REFERENCES

CCriticalSection in the Help file.