GENERIC INTERFACE SyncQueue(Elem);

Where Elem.T is a type that is not an open array type, and Elem contains

      CONST Brand = <text-constant>;

IMPORT Thread;

EXCEPTION EndOfFile;

TYPE
  T <: Public;

  Public = Private OBJECT METHODS
    init(sizeHint: CARDINAL := 5): T;

    put(READONLY e: Elem.T);
    close();

    get(): Elem.T RAISES {EndOfFile, Thread.Alerted};
    EOF(): BOOLEAN RAISES {Thread.Alerted};
    size(): CARDINAL;
  END;

  Private <: ROOT;

The put method appends a new element to the end of the queue. The queue is allowed to grow without bound, so the put operation never blocks the calling thread.

The get method removes and returns the element at the head of the queue. If the queue is empty, get blocks the calling thread, until data is again available to be gotten.

Synchronized queues support the notion of an end-of-file marker. On the writing side, the close method (conceptually) appends an EOF mark to the end of the queue. It is a checked runtime error to call put after close has been called. If a get operation encounters the EOF marker, it raises the EndOfFile exception. The presence of the end-of-file may also be tested explicitly, by the EOF method. Note that, if the queue is empty but it has not been closed, the EOF method will block the calling thread.

The size method can be used to implement non-blocking gets. It returns the number of elements that could be gotten without blocking. For the purposes of size, the end-of-file marker counts as one element.

END SyncQueue.