Synopsis¶

Methods¶

def __init__()
def acquire()
def available()
def release()
def tryAcquire()

Note

This documentation may contain snippets that were automatically translated from C++ to Python. We always welcome contributions to the snippet translation. If you see an issue with the translation, you can also let us know by creating a ticket on https:/bugreports.qt.io/projects/PYSIDE

Detailed Description¶

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

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 Producer and Consumer using 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 Producer and Consumer using Semaphores

__init__([n=0])¶

Parameters:: n – int

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

See also

release() available()

acquire([n=1])¶

Parameters:: n – int

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()

available()¶

Return type:: int

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

See also

acquire() release()

release([n=1])¶

Parameters:: n – int

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

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 resources()

QSemaphoreReleaser is a RAII wrapper around this function.

See also

acquire() available() QSemaphoreReleaser

tryAcquire([n=1])¶

Parameters:: n – int
Return type:: bool

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

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