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

Synopsis¶
Functions¶
def
acquire
([n=1])def
available
()def
release
([n=1])def
tryAcquire
([n=1])def
tryAcquire
(n, timeout)
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()
andrelease()
:
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 anavailable()
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
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, theavailable()
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
QSemaphore
([n=0])¶ - param n
int
Creates a new semaphore and initializes the number of resources it guards to
n
(by default, 0).See also
-
PySide2.QtCore.QSemaphore.
acquire
([n=1])¶ - Parameters
n –
int
Tries to acquire
n
resources guarded by the semaphore. Ifn
>available()
, this call will block until enough resources are available.See also
-
PySide2.QtCore.QSemaphore.
available
()¶ - Return type
int
Returns the number of resources currently available to the semaphore. This number can never be negative.
-
PySide2.QtCore.QSemaphore.
release
([n=1])¶ - Parameters
n –
int
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.See also
-
PySide2.QtCore.QSemaphore.
tryAcquire
([n=1])¶ - Parameters
n –
int
- Return type
bool
Tries to acquire
n
resources guarded by the semaphore and returnstrue
on success. Ifavailable()
<n
, this call immediately returnsfalse
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
-
PySide2.QtCore.QSemaphore.
tryAcquire
(n, timeout) - Parameters
n –
int
timeout –
int
- Return type
bool
Tries to acquire
n
resources guarded by the semaphore and returnstrue
on success. Ifavailable()
<n
, this call will wait for at mosttimeout
milliseconds for resources to become available.Note: Passing a negative number as the
timeout
is equivalent to callingacquire()
, i.e. this function will wait forever for resources to become available iftimeout
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