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, and QThread.
Member Function Documentation
QSemaphore::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 () const
Returns 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. 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().
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.
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().
