x                  The tangent of the argument.
    HUGE_VAL           x is a singular point ( . . . -3pi/2, -pi/2,
                       pi/2 . . . ).
    NaN                x is NaN; errno is set to EDOM.
    0                  x is Infinity; errno is set to EDOM.
    HUGE_VAL           Overflow occurred; errno is set to ERANGE.
    0                  Underflow occurred; errno is set to ERANGE.