6.4.3 First Example: The Use of 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;
}

The code above causes the compiler to issue the following warning when you use the -xtransition option:

line 6: warning: semantics of "<" change in ISO 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. On a two’s-complement machine, we have:

    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

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