A MsgRd.T is a reader which presents the abstraction of a stream of messages. A message is a (possibly zero length) sequence of bytes terminated by an end of message marker. The reader is initially positioned to the start of the first message. When the end of message marker is encountered, it is represented by EndOfFile on the reader. The nextMsg method can be used to advance to the next message in the stream. This method waits for the next message and returns TRUE when it becomes available. A return value of FALSE indicates that there are (and will be) no further messages. The reader's current position is set to zero on return from nextMsg, and the reader no longer reports EndOfFile (unless of course the next message is zero length).

If nextMsg is invoked when the reader is not at EndOfFile, the remaining bytes in the current message will be skipped.

Calling Rd.Close on a MsgRd.T will release all associated resources, and trigger checked runtime errors on further attempts to read from the closed reader.

INTERFACE MsgRd;

IMPORT Thread, Rd;

TYPE
  T = Rd.T OBJECT METHODS
    nextMsg() : BOOLEAN RAISES {Rd.Failure, Thread.Alerted};
  END;

The nextMsg method advances to the next message. A return value of TRUE indicates the presence of a new message. FALSE indicates that no next message is present, and that the end of the stream of messages has been reached.

\begin{tabular}{ll} & \\ len(rd) & is the length of the next message \\ src(rd) & is the contents of the next message \\ cur(rd) & is zero \\ avail(rd) & is unspecified \\ closed(rd) & is unchanged \\ seekable(rd) & is unchanged \\ intermittent(rd) & is unchanged \\ & \\ \end{tabular}

Message reader buffers must be word-aligned in memory. More precisely, if byte i in the data stream is stored in the buffer at memory address j, then i and j must be equal modulo the machine word size.

END MsgRd.