BUG: Incorrect Code When Increment Pointer Based on a Pointer

• Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51 - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0,

       4.1, 4.2, 5.0
  

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

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] );
   }