FIX: Compiler Generates Bad Char Pointer Code with /Ot

MS-DOS                 | OS/2       | WINDOWS

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE), included with:

      - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
      - Microsoft C for OS/2, versions 6.0 and 6.0a
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, version 1.0
  

SYMPTOMS

The compiler may treat an unsigned character pointer as an integer pointer when compiling using the /Ot option.

RESOLUTION

There are several ways to work around this problem:

• In the sample code in the "MORE INFORMATION" section, below, remove the register modifier on the integer variable or remove the unsigned modifier on the character-string variable.

  -or-

• Disable optimization by either compiling using the /Od option or by using #pragma optimize to disable optimization on a per-function basis.

  -or-

• If you are using the C/C++ Compiler, version 7.0 or later, using the /f option (fast compile) causes the compiler to generate correct code.

STATUS

Microsoft has confirmed this to be a problem with the C Compiler for MS- DOS, versions 6.0, 6.0a, 6.0ax, 7.0, and 8.0. This problem was corrected in the C/C++ compiler for MS-DOS, version 8.0c, included with Visual C++ for MS-DOS, version 1.5.

This problem does not occur with the 32-bit C/C++ Compiler, version 8.0, included with Visual C++ 32-bit Edition, version 1.0.

MORE INFORMATION

Using the /Fc option, you can generate an assembly listing of the source code. This is an excerpt from the assembly listing of the sample code below, which shows where the error occurs:

   ;|***    while(ch==*str);
   ; Line 8
                $FC180:
    *** 00000f  8b 5e fe        mov   bx,WORD PTR [bp-2]      ;str
    *** 000012  8b 37           mov   si,WORD PTR [bx]
    *** 000014  81 e6 ff 00     and   si,255  ;00ffH
    *** 000018  75 f5           jne   $FC180

   ;    while(ch==*str);
   ; Line 8
                L00180:
    *** 000010  8b 5e fc         mov     bx,WORD PTR -4[bp]
    *** 000013  8a 07            mov     al,BYTE PTR [bx]
    *** 000015  25 ff 00         and     ax,OFFSET 255
    *** 000018  8b f0            mov     si,ax
    *** 00001a  8b c6            mov     ax,si
    *** 00001c  3d 00 00         cmp     ax,OFFSET 0
    *** 00001f  75 03 e9 00 00   je      L00181
    *** 000024  e9 e9 ff         jmp     L00180

Sample Code

/* Compile options needed: /Ot (/Fc to generate assembly listing)
*/

   #include<stdio.h>

   void main(void)
   {
      register int ch;
      unsigned char *str="abc";

      while(ch==*str);
   }