The difference between signed and unsigned when doing `mod'

[Dave Thompson]

If signed int on your platform can hold all the values of an unsigned
short, 'a' and 'b' will be promoted to signed int for the subtraction.
Equivalent to: <snip>
Right.

The expression inside the parentheses evaluates to (int)-7. One of
the two possible correct values for -7 % 8 allowed prior to C99 is -7.

One of two correct values in C89/90 (and C95) and the only one in C99.
Prior to C89/90, I'm not sure if % was specified that precisely.

When you assign (int)-7 to an unsigned short, the behavior of unsigned
types with out of range values occurs. (int)-7 is converted to
USHRT_MAX - 7. For a 16-bit unsigned short, this is 0xffff - 7, which
equals 0xfff9.

(USHRT_MAX + 1) - 7, in this case 0x10000 - 7. In mathematically
correct arithmetic, not necessarily C or machine arithmetic.

<snip rest>
- David.Thompson1 at worldnet.att.net