You are currently viewing a snapshot of www.mozilla.org taken on April 21, 2008. Most of this content is highly out of date (some pages haven't been updated since the project began in 1998) and exists for historical purposes only. If there are any pages on this archive site that you think should be added back to www.mozilla.org, please file a bug.



NSPR Reference
Previous     Contents     Next     


Chapter 6   Condition Variables

This chapter describes the API for creating and destroying condition variables, notifying condition variables of changes in monitored data, and making a thread wait on such notification.

Condition Variable Type
Condition Variable Functions

Conditions are closely associated with a single monitor, which typically consists of a mutex, one or more condition variables, and the monitored data. The association between a condition and a monitor is established when a condition variable is created, and the association persists for its life. In addition, a static association exists between the condition and some data within the monitor. This data is what will be manipulated by the program under the protection of the monitor.

A call to PR_WaitCondVar causes a thread to block until a specified condition variable receives notification of a change of state in its associated monitored data. Other threads may notify the condition variable when changes occur.

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

For reference information on NSPR locks, see Chapter 5 "Locks"

NSPR provides a special type, PRMonitor, for use with Java. Unlike a mutex of type PRLock, which can have multiple associated condition variables of type PRCondVar, a mutex of type PRMonitor has a single, implicitly associated condition variable. For information about PRMonitor, see Chapter 7 "Monitors"

Condition Variable Type

PRCondVar

Structure for a condition variable.


Syntax
#include <prcvar.h>

typedef struct PRCondVar PRCondVar;


Description
An NSPR condition variable is an opaque object identified by a pointer.

Condition Variable Functions

PR_NewCondVar
PR_DestroyCondVar
PR_WaitCondVar
PR_NotifyCondVar
PR_NotifyAllCondVar

PR_NewCondVar

Creates a new condition variable.


Syntax
#include <prcvar.h>

PRCondVar* PR_NewCondVar(PRLock *lock);


Parameter
PR_NewCondVar has one parameter:

lock

The identity of the mutex that protects the monitored data, including this condition variable.


Returns
The function returns one of the following values:

  • If successful, a pointer to the new condition variable object.

  • If unsuccessful (for example, if system resources are unavailable), NULL.

PR_DestroyCondVar

Destroys a condition variable.


Syntax
#include <prcvar.h>

void PR_DestroyCondVar(PRCondVar *cvar);


Parameter
PR_DestroyCondVar has one parameter:

cvar

A pointer to the condition variable object to be destroyed.


Description
Before calling PR_DestroyCondVar, the caller is responsible for ensuring that the condition variable is no longer in use.

PR_WaitCondVar

Waits on a condition.


Syntax
#include <prcvar.h>

PRStatus PR_WaitCondVar(
   PRCondVar *cvar,
   PRIntervalTime timeout);


Parameters
PR_WaitCondVar has the following parameters:

cvar

The condition variable on which to wait.

timeout

The value PR_INTERVAL_NO_TIMEOUT requires that a condition be notified (or the thread interrupted) before it will resume from the wait. The value PR_INTERVAL_NO_WAIT causes the thread to release the lock, possibly causing a rescheduling within the runtime, then immediately attempt to reacquire the lock and resume.


Returns
The function returns one of the following values:

  • If successful, PR_SUCCESS.

  • If unsuccessful (for example, if the caller has not locked the lock associated with the condition variable or the thread was interrupted with PR_Interrupt), PR_FAILURE. The details can be determined with PR_GetError.


Description
Before the call to PR_WaitCondVar, the lock associated with the condition variable must be held by the calling thread. After a call to PR_WaitCondVar, the lock is released and the thread is blocked in a "waiting on condition" state until another thread notifies the condition or a caller-specified amount of time expires.

When the condition variable is notified, a thread waiting on that condition moves from the "waiting on condition" state to the "ready" state. When scheduled, the thread attemps to reacquire the lock that it held when PR_WaitCondVar was called.

Any value other than PR_INTERVAL_NO_TIMEOUT or PR_INTERVAL_NO_WAIT for the timeout parameter will cause the thread to be rescheduled due to either explicit notification or the expiration of the specified interval. The latter must be determined by treating time as one part of the monitored data being protected by the lock and tested explicitly for an expired interval.

PR_NotifyCondVar

Notifies a condition variable of a change in its associated monitored data.


Syntax
#include <prcvar.h>

PRStatus PR_NotifyCondVar(PRCondVar *cvar);


Parameter
PR_NotifyCondVar has one parameter:

cvar

The condition variable to notify.


Returns
The function returns one of the following values:

  • If successful, PR_SUCCESS.

  • If unsuccessful (for example, if the caller has not locked the lock associated with the condition variable), PR_FAILURE.


Description
The calling thread must hold the lock that protects the condition, as well as the invariants that are tightly bound to the condition.

Notification of a condition variable signals a change of state in some monitored data. When the notification occurs, the runtime promotes a thread that is waiting on the condition variable to a ready state. If more than one thread is waiting, the selection of which thread gets promoted cannot be predicted. This implies that all threads waiting on a single condition variable must have the same semantics. If no thread is waiting on the condition variable, the notify operation is a no-op.

PR_NotifyAllCondVar

Notifies all of the threads waiting on a specified condition variable.


Syntax
#include <prcvar.h>

PRStatus PR_NotifyAllCondVar(PRCondVar *cvar);


Returns
The function returns one of the following values:

  • If successful, PR_SUCCESS.

  • If unsuccessful (for example, if the caller has not locked the lock associated with the condition variable), PR_FAILURE


Description
The calling thread must hold the lock that protects the condition, as well as the invariants that are tightly bound to the condition.

A call to PR_NotifyAllCondVar causes all of the threads waiting on the specified condition variable to be promoted to a ready state. If no threads are waiting, the operation is no-op.


Previous     Contents     Next     

Last Updated May 18, 2001