These routines determine the class of IEEE floating-point values.
    They return one of the constants in the file <fp_class.h> and
    never cause an exception, even for signaling NaNs. These routines
    implement the recommended function class(x) in the appendix of
    the IEEE Std 754. The constants in <fp_class.h> refer to the
    following classes of values:

    Constant           Class

    FP_SNAN            Signaling NaN (Not-a-Number)
    FP_QNAN            Quiet NaN (Not-a-Number)
    FP_POS_INF         +Infinity
    FP_NEG_INF         -Infinity
    FP_POS_NORM        Positive normalized
    FP_NEG_NORM        Negative normalized
    FP_POS_DENORM      Positive denormalized
    FP_NEG_DENORM      Negative denormalized
    FP_POS_ZERO        +0.0 (positive zero)
    FP_NEG_ZERO        -0.0 (negative zero)

                           Entry-Point Names

    Generic                                       Compaq
    Function   Data Type                          Tru64 UNIX
    Name       Required    OpenVMS Alpha          Alpha

    fp_class   S_FLOAT     math$fp_class_s        fp_classf
               T_FLOAT     math$fp_class_t        fp_class
               X_FLOAT     math$fp_class_x        fp_classl
               F_FLOAT     math$fp_class_f
               G_FLOAT     math$fp_class_g