Q198009: PRB: Calling String Conversion Macros in Catch Block Causes AV

Article: Q198009
Product(s): Microsoft C Compiler
Version(s): 2.1,3.0,5.0,6.0
Operating System(s): 
Keyword(s): kbATL kbATL210 kbExceptHandSEH kbVC500 kbVC600 kbATL300 kbGrpDSMFCATL kbExceptHandCPP k
Last Modified: 11-JUN-2002

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The information in this article applies to:

- The Microsoft Active Template Library (ATL), versions 2.1, 3.0, used with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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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.
(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

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Keywords          : kbATL kbATL210 kbExceptHandSEH kbVC500 kbVC600 kbATL300 kbGrpDSMFCATL kbExceptHandCPP kbExceptHandling 
Technology        : kbVCsearch kbAudDeveloper kbATLsearch
Version           : :2.1,3.0,5.0,6.0
Issue type        : kbprb

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