KB122543 - BUG: Incorrect Code When Increment Pointer Based on a Pointer

Article ID: 122543

Article Last Modified on 7/5/2005

Microsoft Visual C++ 1.0 Professional Edition
Microsoft Visual C++ 2.0 Professional Edition
Microsoft Visual C++ 2.1
Microsoft Visual C++ 4.0 Standard Edition
Microsoft Visual C++ 4.1 Subscription
Microsoft Visual C++ 5.0 Enterprise Edition
Microsoft Visual C++ 6.0 Enterprise Edition
Microsoft Visual C++ 5.0 Professional Edition
Microsoft Visual C++ 6.0 Professional Edition
Microsoft Visual C++ 5.0 Learning Edition
Microsoft Visual C++ 6.0 Standard Edition

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

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 that this is a bug in the Microsoft products that are 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: kbbug kbcodegen KB122543