A MsgWr.T is a writer 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 writer is initially positioned to the start of the first message. The nextMsg method can be used to end the current message, and position the writer at the start of the next message. The writer's current position is reset to zero on return from nextMsg.

Invoking Wr.Flush on a MsgWr.T will flush the current buffer to the abstract writer target, but will not end the current message.

Calling Wr.Close on a MsgWr.T will release all associated resources and trigger checked runtime errors on further attempts to write to the closed writer. It also flushes and terminates the current message. This means that a zero-length message will be sent at close time if no data has been written into the current message (e.g. directly after nextMsg or object initialization).

INTERFACE MsgWr;

IMPORT Thread, Wr;

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

nextMsg ends the current message, and starts the next.

\begin{tabular}{ll} & \\ len(wr) & is zero \\ c(wr) & is empty \\ cur(wr) & is zero \\ target(wr) & is empty \\ closed(wr) & is unchanged \\ seekable(wr) & is unchanged \\ buffered(wr) & is unchanged \\ & \\ \end{tabular}

Message writer 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 MsgWr.