OpenVMS usage:mask_longword
    type:         longword (unsigned)
    access:       write only
    mechanism:    by value

    The status return value is an unsigned longword bit mask
    containing the condition codes raised by the function. CVT$FTOF
    returns CVT$K_NORMAL; otherwise, it sets one or more recoverable
    and unrecoverable conditions.

    Return status values are in the $CVTDEF module in the system
    symbol libraries.