Knowledge Base

An access violation occurs when you try to call one of the string conversion macros in Visual C++

Article ID: 198009

Article Last Modified on 6/2/2005

Microsoft ActiveX Template Library 3.0, when used with:
- Microsoft Visual C++ 5.0 Enterprise Edition
- Microsoft Visual C++ 6.0 Enterprise Edition
- Microsoft Visual C++ 5.0 Professional Edition
- Microsoft Visual C++ 6.0 Professional Edition
- Microsoft Visual C++ 6.0 Standard Edition
Microsoft ActiveX Template Library 2.1, when used with:
- Microsoft Visual C++ 5.0 Enterprise Edition
- Microsoft Visual C++ 6.0 Enterprise Edition
- Microsoft Visual C++ 5.0 Professional Edition
- Microsoft Visual C++ 6.0 Professional Edition
- Microsoft Visual C++ 6.0 Standard Edition

This article was previously published under Q198009

SYMPTOMS

Calling one of the string conversion macros, such as OLE2T or T2OLE, inside a catch block of a try/catch exception handler causes an access violation.

CAUSE

Some of the string conversion macros will call _alloca(). However, _alloca() cannot be used inside an exception handler.

RESOLUTION

Use MultiByteToWideChar() or WideCharToMultiByte() rather than string conversion macros inside an exception handler.

STATUS

This behavior is by design.

MORE INFORMATION

Exception Handler (EH) routines that run on x86-class processors operate in their own memory "frame:" that is, they perform their tasks in memory space that is not based on the current location of the stack pointer of the enclosing function. Therefore, explicitly calling _alloca in any of the following scenarios results in program failure during the return to the calling EH routine:

Windows NT SEH exception filter expression: __except ( alloca() )
Windows NT SEH final exception handler: __finally { alloca() }
C++ EH catch clause expression

Steps to Reproduce Behavior

   try
   {
      throw (1);
   }
   catch (...)
   {
      USES_CONVERSION;
      LPTSTR str = OLE2T(L"Some String");
   }

REFERENCES

Visual C++ Online Documentation for _alloca()

(c) Microsoft Corporation 1998, All Rights Reserved. Contributions by Samson Tanrena, Microsoft Corporation.

Additional query words: A2W A2CW T2OLE OLE2W A2OLE A2T T2W W2A W2CA OLE2T W2OLE OLE2A T2A W2T

Keywords: kbtshoot kbprb kbexcepthandling KB198009