INFO: Bitwise Complement Operator Appears to Fail on Comparison (31510)

SUMMARY

The bitwise complement operator (~) appears to work incorrectly when an application uses it to compare unsigned characters. Before the compiler uses the bitwise complement operator, it performs the "usual arithmetic conversions" which are described in detail in the "C Language Reference" manual. Cast the complemented operand to an unsigned character. This prevents the compiler from performing the arithmetic conversions. The following code example returns the value "failed" even though it appears that values "j" and "~i" should be the same.

Sample Code

/*
 * Compile options needed: none
 */ 

#include <stdio.h>

main()
{
   unsigned char i = 4;
   unsigned char j = ~i;

   if (j == ~i)
      printf("passed\n");
   else
      printf("failed\n");
}

The compiler performs the following four steps to evaluate the expression "if (j == ~i)":

1. The compiler converts the operand "i" to an unsigned integer.
2. The compiler complements the bits of this unsigned integer. On systems that use 16-bit integers, the high byte becomes 0xFF; on systems that use 32-bit integers, the three high bytes become 0xFFFFFF.
3. The compiler converts the operand "j" to an unsigned integer. On systems that use 16-bit integers, the high byte becomes 0x00; on systems that use 32-bit integers, the three high bytes become 0x000000.
4. The compiler compares the two operands.

The comparison fails because the high bytes of the operands differ.

To work around this situation, modify the comparison as follows:

   if (j == (unsigned char)~i)