Q168932: BUG: Multiple Based Conversion Results May Be Incorrect

Article: Q168932
Product(s): Microsoft C Compiler
Version(s): 2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
Operating System(s): 
Keyword(s): kbtool kbCompiler kbVC500bug kbVC600bug
Last Modified: 13-FEB-2002

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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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