VMS Help RTL Routines, LIB$, LIB$LEN, Argument  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 source-string

    OpenVMS usage:char_string
    type:         character string
    access:       read only
    mechanism:    by descriptor

    Source string whose length is returned by LIB$LEN. The source-
    string argument contains the address of a descriptor pointing to
    this source string.