              
QSemaphore Class ReferenceThe QSemaphore class provides a general counting semaphore. More... #include <QSemaphore>Note: All functions in this class are thread-safe. Public Functions
Detailed DescriptionThe 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. Member Function Documentation
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