CopperSpice API  1.9.1
QSemaphore Class Reference

The QSemaphore class provides a general counting semaphore. More...

Public Methods

 QSemaphore (int n=0)
 ~QSemaphore ()
void acquire (int n=1)
int available () const
void release (int n=1)
bool tryAcquire (int n, int timeout)
bool tryAcquire (int n=1)

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 is 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 are not that many resources available, the call will block until this is the case.
  • release(n) releases n resources.

There's also a tryAcquire() method that returns immediately if it can not 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.

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

Constructor & Destructor 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(), available()
QSemaphore::~QSemaphore ( )

Destroys the semaphore.

Destroying a semaphore that is in use may result in undefined behavior.

Method Documentation

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(), tryAcquire()
int QSemaphore::available ( ) const

Returns the number of resources currently available to the semaphore. This number can never be negative.

See also
acquire(), release()
void QSemaphore::release ( int  n = 1)

Releases n resources guarded by the semaphore.

This function can be used to "create" resources as well.

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(), available()
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
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