Table Of Contents

Previous topic


Next topic





Detailed Description

The PySide.QtCore.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, PySide.QtCore.QSemaphore.acquire() and PySide.QtCore.QSemaphore.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 PySide.QtCore.QSemaphore.tryAcquire() function that returns immediately if it cannot acquire the resources, and an PySide.QtCore.QSemaphore.available() function that returns the number of available resources at any time.


sem = QSemaphore(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 PySide.QtCore.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, PySide.QtCore.QSemaphore.available() is decremented. As people leave, the PySide.QtCore.QSemaphore.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).

class PySide.QtCore.QSemaphore([n=0])

Creates a new semaphore and initializes the number of resources it guards to n (by default, 0).


Tries to acquire n resources guarded by the semaphore. If n > PySide.QtCore.QSemaphore.available() , this call will block until enough resources are available.


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


Releases n resources guarded by the semaphore.

This function can be used to “create” resources as well. For example:

sem = QSemaphore(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
Return type:PySide.QtCore.bool

Tries to acquire n resources guarded by the semaphore and returns true on success. If PySide.QtCore.QSemaphore.available() < n , this call immediately returns false without acquiring any resources.


sem = QSemaphore(5)      # sem.available() == 5
sem.tryAcquire(250)      # sem.available() == 5, returns false
sem.tryAcquire(3)        # sem.available() == 2, returns true
PySide.QtCore.QSemaphore.tryAcquire(n, timeout)
Return type:


Tries to acquire n resources guarded by the semaphore and returns true on success. If PySide.QtCore.QSemaphore.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 PySide.QtCore.QSemaphore.acquire() , i.e. this function will wait forever for resources to become available if timeout is negative.


sem = QSemaphore(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