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# 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`:

- acquire(n) tries to acquire n resources. If there aren't that many resources available, the call will block until this is the case.
- release(n) releases n resources.

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.