The fp_class functions determine the class of the specified IEEE
    floating-point number, returning a constant from the <fp_class.h>
    header file. They never cause an exception, even for signaling
    NaNs (Not-a-Number). These functions implement the recommended
    class(x) function in the appendix of the IEEE 754-1985 standard
    for binary floating-point arithmetic. The constants in <fp_
    class.h> refer to the following classes of values:

    FP_SNAN           Signaling NaN (Not-a-Number)
    FP_QNAN           Quiet NaN
    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)