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)
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.


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().


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.


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.


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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