QSemaphore

The QSemaphore class provides a general counting semaphore. More

Inheritance diagram of PySide2.QtCore.QSemaphore

Synopsis

Functions

Detailed Description

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:

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 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

QSemaphoreReleaser QMutex QWaitCondition QThread Semaphores Example

class QSemaphore([n=0])
param n

int

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

PySide2.QtCore.QSemaphore.acquire([n=1])
Parameters

nint

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

PySide2.QtCore.QSemaphore.available()
Return type

int

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

See also

acquire() release()

PySide2.QtCore.QSemaphore.release([n=1])
Parameters

nint

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

QSemaphoreReleaser is a RAII wrapper around this function.

PySide2.QtCore.QSemaphore.tryAcquire([n=1])
Parameters

nint

Return type

bool

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:

sem = QSemaphore(5)      # sem.available() == 5
sem.tryAcquire(250)      # sem.available() == 5, returns false
sem.tryAcquire(3)        # sem.available() == 2, returns true

See also

acquire()

PySide2.QtCore.QSemaphore.tryAcquire(n, timeout)
Parameters
  • nint

  • timeoutint

Return type

bool

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:

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

See also

acquire()