INFO: Bitwise Complement Operator Appears to Fail on Comparison

• The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax - Microsoft C for OS/2, versions 6.0, 6.0a - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5 - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 4.0, 5.0

SYMPTOMS

The bitwise complement operator (~) appears to work incorrectly when an application uses it to compare unsigned characters.

CAUSE

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.

RESOLUTION

Cast the complemented operand to an unsigned character. This prevents the compiler from performing the arithmetic conversions.

MORE INFORMATION

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");
   }

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.

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

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