DESCRIPTION
The
signbit() macro determines whether the sign of its argument value
x is negative. An argument represented in a format wider than its semantic type is converted to its semantic type first. The determination is then based on the type of the argument.
IEEE754
The sign is determined for all values, including infinities, zeroes, and NaNs
VAX
The sign is determined for finites, true zeros, and dirty zeroes; for ROPs the sign is reported negative.