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The QSemaphore class provides a general counting semaphore. More...
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).
Creates a new semaphore and initializes the number of resources it guards to n (by default, 0).
See also release() and available().
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().
Returns the number of resources currently available to the semaphore. This number can never be negative.
See also acquire() and release().
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() and available().
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().
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().
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