QtCore.QSemaphore Class Reference

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

Inheritance diagram for QtCore.QSemaphore:

Collaboration diagram for QtCore.QSemaphore:

Public Member Functions
	QSemaphore (int n=0)
virtual void	CreateProxy ()
new void	Acquire (int n=1)
new int	Available ()
new void	Release (int n=1)
new bool	TryAcquire (int n=1)
new bool	TryAcquire (int n, int timeout)
new void	Dispose ()

Protected Member Functions
	QSemaphore (System.Type dummy)

Protected Attributes
SmokeInvocation	interceptor

Properties
virtual System.IntPtr	SmokeObject [get, set]

Detailed Description

The 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, 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 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 QMutex, QWaitCondition, QThread, and Semaphores Example.

Constructor & Destructor Documentation

QtCore.QSemaphore.QSemaphore ( System.Type  dummy )

protected

QtCore.QSemaphore.QSemaphore

(

int

n = 0

)

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

See also release() and available().

Member Function Documentation

new void QtCore.QSemaphore.Acquire

(

int

n = 1

)

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(), and tryAcquire().

new int QtCore.QSemaphore.Available

(

)

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

See also acquire() and release().

virtual void QtCore.QSemaphore.CreateProxy ( )

virtual

new void QtCore.QSemaphore.Dispose

(

)

new void QtCore.QSemaphore.Release

(

int

n = 1

)

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 new resources

See also acquire() and available().

new bool QtCore.QSemaphore.TryAcquire

(

int

n = 1

)

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

See also acquire().

new bool QtCore.QSemaphore.TryAcquire	(	int	n,
		int	timeout
	)

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:

QSemaphore sem(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().

Member Data Documentation

SmokeInvocation QtCore.QSemaphore.interceptor

protected

Property Documentation

virtual System.IntPtr QtCore.QSemaphore.SmokeObject

getset