Knowledge Base

PRB: Access Violation in GetAmbientDisplayName

Article ID: 190149

Article Last Modified on 9/18/2003

Microsoft ActiveX Template Library 3.0, when used with:
- Microsoft Visual C++ 6.0 Enterprise Edition
- Microsoft Visual C++ 6.0 Professional Edition
- Microsoft Visual C++ 6.0 Standard Edition
- Microsoft Visual C++ .NET 2003 Standard Edition
- Microsoft Visual C++ .NET 2002 Standard Edition

This article was previously published under Q190149

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.

Keywords: kbprb KB190149