INFO: Unsigned Characters and Arithmetic Operators (45237)

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");