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com.trolltech.qt.core
Class QSemaphore

java.lang.Object
  extended by com.trolltech.qt.QSignalEmitter
      extended by com.trolltech.qt.QtJambiObject
          extended by com.trolltech.qt.core.QSemaphore
All Implemented Interfaces:
QtJambiInterface

public class QSemaphore
extends QtJambiObject

The QSemaphore class provides a general counting semaphore.

A semaphore is a generalization of a mutex. While a mutex can only be locked once, it's possible to acquire a semaphore multiple times. Semaphores are typically used to protect a certain number of identical resources.

Semaphores support two fundamental operations, acquire and release:

There's also a tryAcquire function that returns immediately if it cannot acquire the resources, and an available function that returns the number of available resources at any time.

Example:

    QSemaphore sem(5);      // sem.available() == 5

    sem.acquire(3);         // sem.available() == 2
    sem.acquire(2);         // sem.available() == 0
    sem.release(5);         // sem.available() == 5
    sem.release(5);         // sem.available() == 10

    sem.tryAcquire(1);      // sem.available() == 9, returns true
    sem.tryAcquire(250);    // sem.available() == 9, returns false

A typical application of semaphores is for controlling access to a circular buffer shared by a producer thread and a consumer thread. The Semaphores example shows how to use QSemaphore to solve that problem.

A non-computing example of a semaphore would be dining at a restaurant. A semaphore is initialized with the number of chairs in the restaurant. As people arrive, they want a seat. As seats are filled, available is decremented. As people leave, the available is incremented, allowing more people to enter. If a party of 10 people want to be seated, but there are only 9 seats, those 10 people will wait, but a party of 4 people would be seated (taking the available seats to 5, making the party of 10 people wait longer).

See Also:
QMutex, QWaitCondition, QThread, Example

Nested Class Summary
 
Nested classes/interfaces inherited from class com.trolltech.qt.QSignalEmitter
QSignalEmitter.Signal0, QSignalEmitter.Signal1<A>, QSignalEmitter.Signal2<A,B>, QSignalEmitter.Signal3<A,B,C>, QSignalEmitter.Signal4<A,B,C,D>, QSignalEmitter.Signal5<A,B,C,D,E>, QSignalEmitter.Signal6<A,B,C,D,E,F>, QSignalEmitter.Signal7<A,B,C,D,E,F,G>, QSignalEmitter.Signal8<A,B,C,D,E,F,G,H>, QSignalEmitter.Signal9<A,B,C,D,E,F,G,H,I>
 
Constructor Summary
QSemaphore()
          Equivalent to QSemaphore(0).
QSemaphore(int n)
          Creates a new semaphore and initializes the number of resources it guards to n (by default, 0).
 
Method Summary
 void acquire()
          Equivalent to acquire(1).
 void acquire(int n)
          Tries to acquire n resources guarded by the semaphore.
 int available()
          Returns the number of resources currently available to the semaphore.
static QSemaphore fromNativePointer(QNativePointer nativePointer)
          This function returns the QSemaphore instance pointed to by nativePointer
 void release()
          Equivalent to release(1).
 void release(int n)
          Releases n resources guarded by the semaphore.
 boolean tryAcquire()
          Equivalent to tryAcquire(1).
 boolean tryAcquire(int n)
          Tries to acquire n resources guarded by the semaphore and returns true on success.
 boolean tryAcquire(int n, int timeout)
          Tries to acquire n resources guarded by the semaphore and returns true on success.
 
Methods inherited from class com.trolltech.qt.QtJambiObject
dispose, disposed, finalize, reassignNativeResources, tr, tr, tr
 
Methods inherited from class com.trolltech.qt.QSignalEmitter
blockSignals, disconnect, disconnect, signalsBlocked, signalSender, thread
 
Methods inherited from class java.lang.Object
clone, equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 
Methods inherited from interface com.trolltech.qt.QtJambiInterface
disableGarbageCollection, nativeId, nativePointer, reenableGarbageCollection, setJavaOwnership
 

Constructor Detail

QSemaphore

public QSemaphore()

Equivalent to QSemaphore(0).


QSemaphore

public QSemaphore(int n)

Creates a new semaphore and initializes the number of resources it guards to n (by default, 0).

See Also:
release, available
Method Detail

acquire

public final void acquire()

Equivalent to acquire(1).


acquire

public final void acquire(int n)

Tries to acquire n resources guarded by the semaphore. If n > available, this call will block until enough resources are available.

See Also:
release, available, tryAcquire

available

public final int available()

Returns the number of resources currently available to the semaphore. This number can never be negative.

See Also:
acquire, release

release

public final void release()

Equivalent to release(1).


release

public final void release(int n)

Releases n resources guarded by the semaphore.

This function can be used to "create" resources as well. For example:

    QSemaphore sem(5);      // a semaphore that guards 5 resources
    sem.acquire(5);         // acquire all 5 resources
    sem.release(5);         // release the 5 resources
    sem.release(10);        // "create" 10 new resources

See Also:
acquire, available

tryAcquire

public final boolean tryAcquire()

Equivalent to tryAcquire(1).


tryAcquire

public final boolean tryAcquire(int n)

Tries to acquire n resources guarded by the semaphore and returns true on success. If available < n, this call immediately returns false without acquiring any resources.

Example:

    QSemaphore sem(5);      // sem.available() == 5
    sem.tryAcquire(250);    // sem.available() == 5, returns false
    sem.tryAcquire(3);      // sem.available() == 2, returns true

See Also:
acquire

tryAcquire

public final boolean tryAcquire(int n,
                                int timeout)

Tries to acquire n resources guarded by the semaphore and returns true on success. If available < n, this call will wait for at most timeout milliseconds for resources to become available.

Note: Passing a negative number as the timeout is equivalent to calling acquire, i.e. this function will wait forever for resources to become available if timeout is negative.

Example:

    QSemaphore sem(5);            // sem.available() == 5
    sem.tryAcquire(250, 1000);    // sem.available() == 5, waits 1000 milliseconds and returns false
    sem.tryAcquire(3, 30000);     // sem.available() == 2, returns true without waiting

See Also:
acquire

fromNativePointer

public static QSemaphore fromNativePointer(QNativePointer nativePointer)
This function returns the QSemaphore instance pointed to by nativePointer

Parameters:
nativePointer - the QNativePointer of which object should be returned.

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