Home · Overviews · Examples 

QSemaphore Class Reference
[com.trolltech.qt.core module]

The QSemaphore class provides a general counting semaphore. More...


Detailed Description

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, and Semaphores Example.


Copyright © 2008 Trolltech Trademarks
Qt Jambi 4.3.5_01