BUG: Multiple Based Conversion Results May Be Incorrect

 Last reviewed: July 14, 1997
Article ID: Q168932
The information in this article applies to:
  • Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0, 4.1, 4.2, 5.0

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. We are researching this bug and will post new information here in the Microsoft Knowledge Base as it becomes available.

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

Keywords : CLIss kbtool vcbuglist500
Version : 2.0 2.1 2.2 4.0 4.1 4.2 5.0
Platform : NT WINDOWS
Issue type : kbbug

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Last reviewed: July 14, 1997
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