Microsoft KB Archive/151033

= FIX: Using CMultiLock Class Can Cause a Memory Leak =

Article ID: 151033

Article Last Modified on 11/21/2006

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APPLIES TO

 Microsoft Foundation Class Library 4.2, when used with:  Microsoft Visual C++ 4.0 Standard Edition

 Microsoft Visual C++ 4.1 Subscription 

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This article was previously published under Q151033



SYMPTOMS
If the CMultiLock class is used to synchronize on more than eight synchronization objects, a memory leak occurs when using CEvent, CSemaphore, or CMutex as the synchronization objects. The size of the leak equals the number of synchronization objects used multiplied by the memory needed for a BOOL variable.



CAUSE
The CMultiLock class uses two arrays to keep a track of the handles of the synchronization objects and their signaled status. As an optimization, local arrays (class data members) are used if the CMultiLock class is used to synchronize on eight or fewer synchronization objects. If more than eight objects are used, memory is allocated at run time by making a call to new. The CMultiLock destructor frees the memory associated with the handles array but not the memory associated with the signaled status array.



RESOLUTION
Working around the memory leak is difficult, because it is not possible to derive a class from CMultiLock and delete the array in destructor. This behavior occurs because CMultiLock's destructor, called after the derived class's destructor, uses this array to unlock the objects.

One approach is to work with multiple CMultiLock objects with less than nine synchronization objects each. This involves performing sequential waits on these CMultiLock objects. However, this is not the same as working with a single CMultiLock object.

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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++ 32- bit Edition version 4.2.

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Sample Code
The following code reproduces the problem: void SyncFun {  //....         CEvent ev1, ev2, ev3, ev4, ev5, ev6, ev7, ev8, ev9; CSyncObject* syncObjects[]= { &ev1, &ev2, &ev3, &ev4, &ev5, &ev6, &ev7, &ev8, &ev9 }; CMultiLock mlock( syncObjects, 9 ); mlock.Lock(1000); //....  }

Additional query words: kbVC400bug 4.00 4.10 4.20 CSemaphore CmutexCEvent vcfixlist420 MfcThreadIss

Keywords: kbbug kbfix kbnoupdate kbthread kbvc420fix KB151033

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