ilogb(x) returns the unbiased exponent of x as an integer, (as if
    x were normalized >= 1.0 and < 2.0) except:

       ilogb(NaN) is INT_MIN
       ilogb(inf) is INT_MAX
       logb(0) is INT_MIN

    There are no errors. The sign of x is ignored.

                           Entry-Point Names

    Generic
    Function   Data Type   OpenVMS                Tru64 UNIX
    Name       Required

    ilogb      S_FLOAT     math$ilogb_s           ilogbf
               T_FLOAT     math$ilogb_t           ilogb
               X_FLOAT     math$ilogb_x           ilogbl
               F_FLOAT     math$ilogb_f
               G_FLOAT     math$ilogb_g