FIX: Using CMultiLock Class Can Cause a Memory Leak

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1

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.

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.

MORE INFORMATION

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);
   //....

   }