x                  The converted value.
    +/- HUGE_VALF      If the correct value is outside the range of
                       representable values of the type, +/- HUGE_VALF is
                       returned according to the sign of the value; and
                       errno is set to ERANGE.
    n                  If the result underflows, the function returns a
                       value whose magnitude is no greater than the
                       smallest normalized positive numbe; whether errno
                       acquires the value ERANGE is
                       implementation-defined.
    0                  Indicates the conversion could not be
                       performed.