C allows differences in how negative integers are represented (at least, C89 did---I'm not sure about C99 as I don't have the standard in front of me). There a

C allows differences in how negative integers are represented (at least, C89 did---I'm not sure about C99 as I don't have the standard in front of me). There are three different ways to represent negative numbers in binary, sign magnitude, one's complement, and two's complement (these days, most computers are two's complement). The upshot is that you might end up with two type of zeros (sign magnitude, one's complement) or an unequal range of negative numbers (two's complement has one additional negative number) and taking the absolute value of an integer may not be possible for some values (given a 16-bit integer on a two's complement system, you cannot get the absolute value for -32,768, which is a valid integer on such a system).