modf() splits a floating-point number x into a fractional part f
    and an integer part i such that |f| < 1.0 and (f + i) = x. Both f
    and i have the same sign as x. modf() returns f and stores i into
    the location pointed to by n.

                           Entry-Point Names

    Generic
    Function   Data Type   OpenVMS        Tru64 UNIX
    Name       Required

    modf       S_FLOAT     math$modf_s    modff
               T_FLOAT     math$modf_t    modf
               X_FLOAT     math$modf_x    modfl
               F_FLOAT     math$modf_f
               G_FLOAT     math$modf_g