DEFINITION MODULE TermFile;
  
  (* Access to the terminal device *)
  
  (* Channels opened by this module are connected to a single terminal device; typed
     characters are distributed between channels according to the sequence of read requests.
  *)
  
IMPORT IOChan, ChanConsts;
  
TYPE
  ChanId = IOChan.ChanId;
  FlagSet = ChanConsts.FlagSet;
  OpenResults = ChanConsts.OpenResults;
  
  (* Accepted singleton values of FlagSet *)
  
CONST
  read = FlagSet{ChanConsts.readFlag};   (* input operations are requested/available *)
  write = FlagSet{ChanConsts.writeFlag}; (* output operations are requested/available *)
  text = FlagSet{ChanConsts.textFlag};   (* text operations are requested/available *)
  raw = FlagSet{ChanConsts.rawFlag};     (* raw operations are requested/available *)
  echo = FlagSet{ChanConsts.echoFlag};   (* echoing by interactive device on reading of
                                            characters from input stream requested/applies
                                         *)
  
PROCEDURE Open (VAR cid: ChanId; flags: FlagSet; VAR res: OpenResults);
  (* Attempts to obtain and open a channel connected to the terminal.
     Without the raw flag, text is implied.
     Without the echo flag, line mode is requested, otherwise single character mode is
     requested.
     If successful, assigns to cid the identity of the opened channel, and assigns the value
     opened to res.
     If a channel cannot be opened as required, the value of res indicates the reason, and
     cid identifies the invalid channel.
  *)
  
PROCEDURE IsTermFile (cid: ChanId): BOOLEAN;
  (* Tests if the channel identified by cid is open to the terminal. *)
  
PROCEDURE Close (VAR cid: ChanId);
  (* If the channel identified by cid is not open to the terminal, the exception wrongDevice
     is raised; otherwise closes the channel and assigns the value identifying the invalid
     channel to cid.
  *)
  
END TermFile.