Q122543: BUG: Incorrect Code When Increment Pointer Based on a Pointer

Article: Q122543
Product(s): Microsoft C Compiler
Version(s): 1.0,2.0,2.1,4.0,4.1,5.0,6.0
Operating System(s): 
Keyword(s): kbCodeGen kbVC500bug
Last Modified: 17-FEB-2002

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), included with:
   - Microsoft C/C++ for MS-DOS 
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 4.1 
   - 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, versions 5.0, 6.0 
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SYMPTOMS
========

Incorrect code is generated when an assignment is made in the same statement
where a pointer based on a pointer is incremented.

For example, executing the following code:

     char buf[100];
     void *pvbuf=buf;
     unsigned int _based(pvbuf) *Test;

     *(char _based(pvbuf) *)(((char *)Test)++) = '6'

in a Win32-based application causes an application error; executing the same code
in a 16-bit Windows-based application causes this error:

  run-time error R6001 -null pointer assignment

CAUSE
=====

The compiler generates an unnecessary sub command, as shown here:

  mov  bx,WORD PTR _Test
  sub  bx,WORD PTR _pvbuf    ; Not needed
  mov  si,WORD PTR _pvbuf
  mov  BYTE PTR [bx][si],5
  inc  WORD PTR _Test

RESOLUTION
==========

Split the statement into multiple lines. For example,

  *(char _based(pvbuf) *)(((char *)Test)) = '5';
     ((char*)Test)++;

STATUS
======

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

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

The following sample demonstrates both the problem and its solution.

Sample Code
-----------

  /* Compile options needed: none
  */ 

  #include <stdio.h>
  #include <conio.h>
  char buf[100];
  void *pvbuf=buf;
  unsigned int _based(pvbuf) *Test;

  void good()
  {
     *(char _based(pvbuf) *)(((char *)Test)) = '5';
     ((char*)Test)++;
     *(char _based(pvbuf) *)(((char *)Test)) = '5';
  }

  void bad()
  {
     *(char _based(pvbuf) *)(((char *)Test)++) = 6;
  }

  void main(void)
  {
     good();
     printf( "Should print out: 5 5 \n%c %c\n",buf[0],buf[1] );
     getch();
     bad();
     printf( "Should print out: 5 6 \n%c %c\n",buf[0],buf[1] );
  }

Additional query words: kbVC400bug 8.00 8.00c 9.00 10.00 10.10 10.20

======================================================================
Keywords          : kbCodeGen kbVC500bug 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : :1.0,2.0,2.1,4.0,4.1,5.0,6.0
Issue type        : kbbug

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