VMS Help RMS, $TRUNCATE, Returns  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    OpenVMS usage:cond_value
    type:         longword
    access:       write only
    mechanism:    by value

    The value is returned in symbolic offset RAB$L_STS. Symbolic
    offset RAB$L_STV may contain additional status information.