Monitors

In addition to the mutex type PRLock, NSPR provides a special type, PRMonitor, for use in Java programming. This chapter describes the NSPR API for creation and manipulation of a mutex of type PRMonitor.

With a mutex of type PRLock, a single thread may enter the monitor only once before it exits, and the mutex can have multiple associated condition variables.

With a mutex of type PRMonitor, a single thread may re-enter a monitor as many times as it sees fit. The first time the thread enters a monitor, it acquires the monitor's lock and the thread's entry count is incremented to 1. Each subsequent time the thread successfully enters the same monitor, the thread's entry count is incremented again, and each time the thread exits the monitor, the thread's entry count is decremented. When the entry count for a thread reaches zero, the thread releases the monitor's lock, and other threads that were blocked while trying to enter the monitor will be rescheduled.

A call to PR_Wait temporarily returns the entry count to zero. When the calling thread resumes, it has the same entry count it had before the wait operation.

Unlike a mutex of type PRLock, a mutex of type PRMonitor has a single, implicitly associated condition variable that may be used to facilitate synchronization of threads with the change in state of monitored data.

For an introduction to NSPR thread synchronization, including locks and condition variables, see Chapter 1 "Introduction to NSPR"

Monitor Type

With the exception of PR_NewMonitor, which creates a new monitor object, all monitor functions require a pointer to an opaque object of type PRMonitor.

Monitor Functions

All monitor functions are thread-safe. However, this safety does not extend to protecting the monitor object from deletion.

  • PR_NewMonitor creates a new monitor.
  • PR_DestroyMonitor destroys a monitor object.
  • PR_EnterMonitor enters the lock associated with a specified monitor.
  • PR_ExitMonitor decrements the entry count associated with a specified monitor.
  • PR_Wait waits for a notify on a specified monitor's condition variable.
  • PR_Notify notifies a thread waiting on a specified monitor's condition variable.
  • PR_NotifyAll notifies all threads waiting on a specified monitor's condition variable.

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