FIX: Calling delete or free() in ATL Causes Access Violation

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This article was previously published under Q190531

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SYMPTOMS

When you call the delete operator or free() function in an ATL project, it might cause an access violation if you are running on a multi-processor computer.

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ATL provides an implementation of free() in non-debug builds with _ATL_MIN_CRT defined. The delete operator is also overridden to call free(). On multi-processor computers, the pointer passed in to free() is used to calculate an offset to the heap's handle:

void __cdecl free(void* p)
{
#ifndef _ATL_NO_MP_HEAP
   if (_Module.m_phHeaps == NULL)
#endif
      HeapFree(_Module.m_hHeap, 0, p);
#ifndef _ATL_NO_MP_HEAP
   else
   {
      HANDLE* pHeap = ((HANDLE*)p)-nOffsetBlock;
      HeapFree(*pHeap, 0, pHeap);
   }
#endif
}

There is no code here that checks if "p" is NULL; therefore, so HeapFree() is called with an invalid handle.

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RESOLUTION

The implementation of free() needs to be modified in Atlimpl.cpp. One option to resolve this problem is to just return if the pointer is NULL:

   void __cdecl free(void* p)
   {
      if (p == NULL)  // Add this line.
         return;      // Add this line.
   #ifndef _ATL_NO_MP_HEAP
      if (_Module.m_phHeaps == NULL)
   #endif
         HeapFree(_Module.m_hHeap, 0, p);
   #ifndef _ATL_NO_MP_HEAP
      else
      {
         HANDLE* pHeap = ((HANDLE*)p)-nOffsetBlock;
         HeapFree(*pHeap, 0, pHeap);
      }
   #endif
   }

Another option to resolve this problem is to #define _ATL_NO_MP_HEAP. This prevents the problematic code from being included. However, this does turn off the optimization that ATL adds for multi-processor machines. You have only one heap regardless of the number of processors. Heap access may be less than optimal because access from multiple threads is serialized.

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STATUS

Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article. This bug was corrected in Visual Studio 6.0 Service Pack 3. For more information about Visual Studio service packs, please see the following articles in the Microsoft Knowledge Base:

194022 (http://support.microsoft.com/kb/194022/EN-US/ ) INFO: Visual Studio 6.0 Service Packs, What, Where, Why

194295 (http://support.microsoft.com/kb/194295/EN-US/ ) HOWTO: Tell That Visual Studio 6.0 Service Packs Are Installed

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