Article ID: 122543 - Last Review: July 5, 2005 - Revision: 1.1

BUG: Incorrect Code When Increment Pointer Based on a Pointer

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This article was previously published under Q122543

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

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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

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RESOLUTION

Split the statement into multiple lines. For example,

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

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STATUS

Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.

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MORE INFORMATION

The following sample demonstrates both the problem and its solution.

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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] );
}

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