DOCUMENT:Q168932  13-FEB-2002  [visualc]
TITLE   :BUG: Multiple Based Conversion Results May Be Incorrect
PRODUCT :Microsoft C Compiler
PROD/VER::2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
OPER/SYS:
KEYWORDS:kbtool kbCompiler kbVC500bug kbVC600bug

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The information in this article applies to:

 - Microsoft Visual C++, versions 2.0, 2.1, 2.2, 4.0, 4.1 
 - Microsoft Visual C++, 32-bit Enterprise Edition, versions 4.2, 5.0, 6.0 
 - Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 5.0, 6.0 
 - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
 - Microsoft Visual C++.NET (2002) 
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SYMPTOMS
========

When you perform multiple conversions on pointers that are declared with the
__based keyword, that have the same base, and are compiled as C++, the
conversion results may be incorrect.

RESOLUTION
==========

To work around this problem, limit __based type conversions to one. The sample
code below demonstrates how to implement some workarounds.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

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

Steps to Reproduce Behavior
---------------------------

   /* Compiler options: Use .cpp extension or /Tp switch */ 
   #include <stdio.h>

   void *pBase   = (void*)0x00010000;
   void *pGlobal = (void*)0x00010000;

   void __based(pBase)* SomeFunction()
   {
       static void* pv = (void*)0x00010000;
       return( void __based(pBase)* )pv;
   }

   void main()
   {
       int __based(pBase)* test1;

       /* The following conversion always fails.  A possible
          solution is to have SomeFunction() simply return a
          void*.
       */ 
       test1 = ( int __based(pBase)* )SomeFunction();
       printf(  "Conversion 1 = %08X, should be 00000000\n",
                (unsigned long)test1 );

       /* This conversion fails on some versions.  In this
          example, the double conversion isn't necessary.
       */ 
       test1 = ( int __based(pBase)* )( (void __based(pBase)*)
                                             ((void*)0x00010000) );
       printf(  "Conversion 2 = %08X, should be 00000000\n",
                (unsigned long)test1 );

       /* This conversion always works.
       */ 
       test1 = ( int __based(pBase)* )( ( void __based(pBase)* )
                                                         pGlobal );
       printf(  "Conversion 3 = %08X, should be 00000000\n",
                (unsigned long)test1 );
   }

Output:

Conversion 1 = FFFF0000, should be 00000000
Conversion 2 = 00010000, should be 00000000
Conversion 3 = 00000000, should be 00000000

Additional query words:

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Keywords          : kbtool kbCompiler kbVC500bug kbVC600bug 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbVC220 kbVC410 kbVC420 kbVC500 kbVC600 kbVC200 kbVC210 kbVC32bitSearch kbVCNET kbVC500Search
Version           : :2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
Issue type        : kbbug

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