QSemaphore Class ReferenceThe QSemaphore class provides a general counting semaphore. Далее... #include <QSemaphore> Замечание: Все функции в этом классе потокобезопасны. Открытые функции
Подробное описаниеThe 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. Пример: 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. Описание функций-членовQSemaphore::QSemaphore ( int n = 0 )Creates a new semaphore and initializes the number of resources it guards to n (by default, 0). See also release() and available(). QSemaphore::~QSemaphore ()Destroys the semaphore. Warning: Destroying a semaphore that is in use may result in undefined behavior. void QSemaphore::acquire ( int n = 1 )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(), and tryAcquire(). int QSemaphore::available () constReturns the number of resources currently available to the semaphore. This number can never be negative. See also acquire() and release(). void QSemaphore::release ( int n = 1 )Releases n resources guarded by the semaphore. This function can be used to "create" resources as well. Например: 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() and available(). bool QSemaphore::tryAcquire ( int n = 1 )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. Пример: 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(). bool QSemaphore::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. Пример: 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(). |
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