INFO: Unsigned Characters and Arithmetic Operators

• Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax
• Microsoft C/C++ for MS-DOS, version 7.0
• Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
• Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

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");
   }

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");

Keywords          : CLngIss kbcode kbfasttip
Version           : MS-DOS:6.0,6.00a,6.00ax,7.0; WINDOWS:1.0,1.5,1.51,1.52;  WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbinfo