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.
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 falseA 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).
See also
PySide.QtCore.QMutex PySide.QtCore.QWaitCondition PySide.QtCore.QThread Semaphores Example
Parameters: | n – PySide.QtCore.int |
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Creates a new semaphore and initializes the number of resources it guards to n (by default, 0).
Parameters: | n – PySide.QtCore.int |
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Tries to acquire n resources guarded by the semaphore. If n > PySide.QtCore.QSemaphore.available() , this call will block until enough resources are available.
Return type: | PySide.QtCore.int |
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Returns the number of resources currently available to the semaphore. This number can never be negative.
Parameters: | n – PySide.QtCore.int |
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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
Parameters: | n – PySide.QtCore.int |
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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.
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
Parameters: |
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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 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.
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