VMS Help System Services, $SNDERR, Argument  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 msgbuf

    OpenVMS usage:char_string
    type:         character-coded text string
    access:       read only
    mechanism:    by descriptor-fixed-length string descriptor
    Message to be written to the error log file. The msgbuf argument
    is the address of a character string descriptor pointing to the
    message text.