Monitors Supplement

Notes

• We will discuss two changes to the buffer.cpp code
  • Both are using a design pattern or reusable solution to a commonly needed behavior
  • A monitor is a solution to mutual exclusion, preventing multiple objects from using a mutex/semaphore/spin-lock/... at the same time.
  • A singleton is a solution to the need for one instance of a unique object, accessible everywhere.
• I have implemented the solution to bounded buffer problem with monitors.
• See bufferMon.cpp.
• class BufferMonitor is a straight forward implementation
  • It cleans up the global variables
    • All semaphores related to the problem are contained in it (except printing.)
    • In addition, the global counts are held in this as well.
    • And the Queue
  • GetMessage and PostMessage methods are now part of this
  • A global instance of this needs to be declared and care must be taken to assure that only one instance exists.
    • But that will be cleaned up soon.
• The PrintMonitor also collects the code/data associated with the unique print problem
  • But this has been implemented as a singleton
  • This assures that only one instance of this monitor is available.
    • This should have been done with the BufferMonitor as well, but I did not want to get too complex.
  • This pattern relies on private constructors and static classes members
  • Since the constructor is private, it can only be called from a class member
  • Since Get is static,
    • it can be called without an existing member of the class
    • But since there is a static instance contained in this member, it will be constructed
    • And since this is a member function, the static member can be constructed.