QSemaphore Class
The QSemaphore class provides a general counting semaphore. More...
| Header: | #include <QSemaphore> |
| qmake: | QT += core |
Note: All functions in this class are thread-safe.
Public Functions
| QSemaphore(int n = 0) | |
| ~QSemaphore() | |
| void | acquire(int n = 1) |
| int | available() const |
| void | release(int n = 1) |
| bool | tryAcquire(int n = 1) |
| bool | tryAcquire(int n, int timeout) |
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.
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().
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.
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().