Class SharedQueue

Inheritance:

Object  »
  SharedQueue

A SharedQueue provides synchronized communication of arbitrary objects between Processes. An object is sent by sending the message nextPut: and received by sending the message next. If no object has been put in the queue when a next message is sent, the process requesting the object will be blocked until one is sent.

The protocol of a SharedQueue resembles that of a ReadWriteStream. To avoid the use of inherited code that is not thread-safe, the class SharedQueue is defined as a direct subclass of Object.

Instance Variables:

• contentsArray - Array
  Together with the instance variables readPosition and writePosition this instance variable implements a first-in first-out buffer. The elements with indices in the range

  readPosition to: writePosition - 1

  
  are occupied with data, all other array elements are nil.

• readPosition - Integer
  For

  readPosition < writePosition

  
  this variable keeps the index of the next data item to be delivered on request.
  Equality of readPosition and writePosition indicates that the queue is empty.

• writePosition - Integer
  This variable keeps the index of the array element that will take the next element that is written into the queue.

• accessProtect - Semaphore
  This semaphore is used to implement exclusive access to the contents of the queue. It ensures that at most one process accesses the instance variables contentsArray, readPosition and writePosition.

• readSynch - Semaphore
  This semaphore counts the number of elements in the queue. It is used to block requestors when the queue is empty.

Class Methods:

• new
  answer a new empty shared queue with space for 10 items. The queue will grow when it turns out that space for more than 10 elements is needed.

• new: initialSize
  answer a new empty shared queue with space for initialSize items. The queue will grow when it turns out that space for more elements is needed.

Instance Methods:

• nextPut: item
  add the object item to the queue.

• next
  remove an item from the queue. The removed item is answered as the message result. If the queue is currently empty, the sender of the message will be suspendend until an item becomes available.

• peek
  answer but do not dequeue the item that will be fetched next from the queue. For an empty queue, this message will immediately answer nil.