C over the years

Does anyone know why did the definitions of many functions in the C standard library got changed to return ssize_t instead of int? For instance read and write, both returning -1 in case of error, whilst all declarations of ssize_t I've seen are unsigned. What is this crazyness?

>>2
Yes I'm sorry I didn't mean the standard library; they are identified as important system calls in the ANSI standard, how is a thing like that changed with an unsigned return type??!