Q45237: INFO: Unsigned Characters and Arithmetic Operators

Article: Q45237
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbLangC kbVC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbV
Last Modified: 29-NOV-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft C for MS-DOS 
- Microsoft C/C++ for MS-DOS 
- Microsoft Visual C++, versions 1.0, 1.5, 1.51, 1.52, 2.0, 2.1, 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
-------------------------------------------------------------------------------

SUMMARY
=======

In the sample program below, two unsigned character variables are compared. The
conditional always evaluates to true, even when x and y are complements. When
examining the assembly code produced, it appears as though the compiler is
generating code to compare two unsigned integers, not unsigned characters.

Sample Code
-----------

  /* Compile options needed: none
  */ 

  #include <stdio.h>

  void main (void)
  {
     unsigned char x, y;
     x = 0;
     y = 255;

     if (x != (~y))
         printf ("y is not a complement of x\n");
     else
         printf ("y is a complement of x\n");
  }

The code generated in this case is correct. Operands of unsigned character type
are promoted to unsigned integer type when using arithmetic operators.

For this sample code to work as intended, cast the "~y" as unsigned character, as
follows:

  if (x != (unsigned char)(~y))
      printf ("y is not a complement of x\n");
  else
      printf ("y is a complement of x\n");

Additional query words:

======================================================================
Keywords          : kbcode kbLangC kbVC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC600 kbVC151 kbVC200 kbVC210 kbVC32bitSearch kbVC152 kbVC500Search
Version           : :1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Issue type        : kbinfo

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.