BUG: Incorrect Code When Increment Pointer Based on a Pointer

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

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' 

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
Version : winnt:6.0
Platform : winnt
Issue type : kbbug