QSemaphore Class Reference
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QSemaphore 类提供通用计数信号量。更多...

方法

__init__ ( self , int n = 0)
acquire ( self , int n = 1)
int available ( self )
release ( self , int n = 1)
bool tryAcquire ( self , int n = 1)
bool tryAcquire ( self , int n , int timeout )

详细描述

QSemaphore 类提供通用计数信号量。

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.

信号量支持 2 基础操作 acquire () 和 release ():

acquire( n ) 试着获得 n resources. If there aren't that many resources available, the call will block until this is the case.
release( n ) 释放 n 资源。

还有 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.

范例：

 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 () 是 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).

方法文档编制

QSemaphore.init ( self , int n = 0)

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

另请参阅 release () and available ().

QSemaphore.acquire ( self , int n = 1)

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

另请参阅 release (), available ()，和 tryAcquire ().

int QSemaphore.available ( self )

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

另请参阅 acquire () and release ().

QSemaphore.release ( self , int n = 1)

发行 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

另请参阅 acquire () and available ().

bool QSemaphore.tryAcquire ( self , 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.

范例：

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

另请参阅 acquire ().

bool QSemaphore.tryAcquire ( self , 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.

范例：

 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