Complying with standard C

First example: using a cast

In the following code, assume that an unsigned char is smaller than an int.

   int f(void)
   {
       int i = -2;
       unsigned char uc = 1;
   
       return (i + uc) < 17;
   }

NOTE: In the ANSI C compiler, the code above causes the compiler to issue the following warning:

line 6: warning: semantics of "<" change in ANSI C;
           use explicit cast

The result of the addition has type int (value preserving) or unsigned int (unsigned preserving), but the bit pattern does not change between these two. A two's-complement machine has:

     i:  111...110  (-2)
   + uc: 000...001  ( 1)
    ===================
         111...111  (-1 or UINT_MAX)

This bit representation corresponds to -1 for int and UINT_MAX for unsigned int. Thus, if the result has type int, a signed comparison is used and the less-than test is true; if the result has type unsigned int, an unsigned comparison is used and the less-than test is false.

The addition of a cast serves to specify which of the two behaviors is desired:

   value preserving:
   
     (i + (int)uc) < 17
   

   unsigned preserving:
   

     (i + (unsigned int)uc) < 17

Because this expression can be viewed as ambiguous (since differing compilers chose different meanings for the same code), the addition of a cast is as much to help the reader as it is to eliminate the warning message.