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Q190149: PRB: Access Violation in GetAmbientDisplayName

Article: Q190149
Product(s): Microsoft C Compiler
Version(s): 3.0,6.0
Operating System(s): 
Keyword(s): kbnokeyword kbCOMt kbVC600 kbATL300 kbGrpDSMFCATL
Last Modified: 13-JUN-2002

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The information in this article applies to:

- The Microsoft Active Template Library (ATL) 3.0, used with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
   - Microsoft Visual C++.NET (2002) 
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SYMPTOMS
========

An Access Violation may occur in GetAmbientDisplayName if the BSTR parameter
passed to it is uninitialized. This is most likely to occur if the BSTR was
declared as a local variable.

CAUSE
=====

In Visual C++ 6. and Visual C++ .Net, the ATL implementation of
GetAmbientDisplayName assumes that all non-NULL BSTRs are initialized, and
passes them to ::SysFreeString. This assumption can cause problems because any
uninitialized BSTR declared inside a function could be non-NULL.

RESOLUTION
==========

Set any uninitialized BSTRs to NULL before calling GetAmbientDisplayName.

STATUS
======

This behavior is by design.

MORE INFORMATION
================

The ATL implementation of GetAmbientDisplayName in Visual C++ 5.0 assumes all
BSTRs are uninitialized and does not free memory before using them, which can
cause leaks. Also, no memory is allocated for the BSTR and the value of the BSTR
is set to memory that is then freed, which can cause access violations.

These problems have been corrected in Visual C++ 6.0.

(c) Microsoft Corporation 1998, All Rights Reserved. Contributions by Kelly Marie
Ward, Microsoft Corporation.


Additional query words:

======================================================================
Keywords          : kbnokeyword kbCOMt kbVC600 kbATL300 kbGrpDSMFCATL 
Technology        : kbVCsearch kbAudDeveloper kbATLsearch
Version           : :3.0,6.0
Issue type        : kbprb

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