msgbuf

    OpenVMS usage:char_string
    type:         character-coded text string
    access:       read only
    mechanism:    by descriptor-fixed-length string descriptor
    User buffer specifying the operation to be performed and the
    information needed to perform that operation. The msgbuf argument
    is the address of a character string descriptor pointing to the
    buffer.

    The format and contents of the buffer vary with the requested
    operation; however, the first byte in any buffer is the request
    code, which specifies the operation to be performed. The $OPCMSG
    macro defines the symbolic names for these request codes.

    The following table shows each operation that $SNDOPR performs
    and the request code that specifies that operation:

    Request Code   Corresponding Operation

    OPC$_RQ_       Sends a user cancellation request to specified
    CANCEL         operator terminals. You use this request code
                   to notify one or more operators that a previous
                   request is to be canceled. To specify OPC$_RQ_
                   CANCEL, you must also specify the chan argument.

    OPC$_RQ_LOGI   Initializes the operator log file.

    OPC$_RQ_       Sends an operator reply to a user who has made a
    REPLY          request. Operators use this request code to report
                   the status of a user request. The format of the
                   message buffer for this request is the format
                   of the reply found in the user's mailbox after
                   the call to $SNDOPR completes. All functions of
                   $SNDOPR that deliver a reply to a mailbox do so in
                   the format described for this request code.

    OPC$_RQ_RQST   Sends a user request to operator terminals. This
                   request code is used to make an operator request.
                   If you specify a reply to the request (by using
                   the chan argument), the operator request is kept
                   active until the operator responds.

    OPC$_RQ_       Reports the status of an operator terminal.
    STATUS         Operators use this request to display the operator
                   classes for which the specified terminal is
                   enabled and a list of outstanding requests.

    OPC$_RQ_       Enables an operator terminal. You use this request
    TERME          to enable a specified terminal to receive operator
                   messages.

    Refer to the VSI OpenVMS System Services Reference Manual to
    view the diagrams depicting the message buffer for each of these
    request codes.

 chan

    OpenVMS usage:channel
    type:         word (unsigned)
    access:       read only
    mechanism:    by value
    Channel assigned to the mailbox to which the reply is to be sent.
    The chan argument is a longword value containing the number of
    the channel. If you do not specify chan or specify it as the
    value 0 (the default), no reply is sent.

    If a reply from the operator is desired, you must specify the
    chan argument.