The functionality of what was called (and I agree confusingly so)
"sign()" here is supplied by "signbit()" in C99.  That standard isn't
free, but the relevant part is incorporated in the free Open Group
standards:

http://www.opengroup.org/onlinepubs/000095399/functions/signbit.html

The 754 standard says NaNs have sign bits, but assigns no meaning to
them.  In particular, the value of the sign bit of a NaN resulting from
a string->double routine applied to the string "nan" isn't defined by
754, or (AFAICT) by C99 either.