Mozilla

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

Syntax

#include <prcvar.h>

typedef struct PRCondVar PRCondVar;

Description

Condition Variable Functions

PR_NewCondVar

Syntax

#include <prcvar.h>

PRCondVar* PR_NewCondVar(PRLock *lock);

Parameter

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

Returns

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

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

PR_DestroyCondVar

Syntax

#include <prcvar.h>

void PR_DestroyCondVar(PRCondVar *cvar);

Parameter

cvar	A pointer to the condition variable object to be destroyed.

Description

PR_WaitCondVar

Syntax

#include <prcvar.h>

PRStatus PR_WaitCondVar(
   PRCondVar *cvar,
   PRIntervalTime timeout);

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

• 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

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

Syntax

#include <prcvar.h>

PRStatus PR_NotifyCondVar(PRCondVar *cvar);

Parameter

cvar	The condition variable to notify.

Returns

• If successful, PR_SUCCESS.

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

Description

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

Syntax

#include <prcvar.h>

PRStatus PR_NotifyAllCondVar(PRCondVar *cvar);

Returns

• If successful, PR_SUCCESS.

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

Description

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.