QSystemSemaphore ¶

QSystemSemaphore class provides a general counting system semaphore. 更多 …

概要 ¶

函数 ¶

def acquire ()
def error ()
def errorString ()
def key ()
def release ([n=1])
def setKey (key[, initialValue=0[, mode=Open]])

详细描述 ¶

信号量是互斥的一般化。互斥只可以锁定一次，而信号量可以多次获取。通常，信号量用于保护一定数量的恒等资源。

像其更轻搭档 QSemaphore ， QSystemSemaphore can be accessed from multiple threads 。不像 QSemaphore ， QSystemSemaphore can also be accessed from multiple processes . This means QSystemSemaphore is a much heavier class, so if your application doesn’t need to access your semaphores across multiple processes, you will probably want to use QSemaphore .

信号量支持 2 基础操作 acquire() and release() :

acquire() tries to acquire one resource. If there isn’t a resource available, the call blocks until a resource becomes available. Then the resource is acquired and the call returns.

release() releases one resource so it can be acquired by another process. The function can also be called with a parameter n > 1, which releases n resources.

A system semaphore is created with a string key that other processes can use to use the same semaphore.

范例：创建系统信号量
sem = QSystemSemaphore("market", 3, QSystemSemaphore.Create)
# resources available == 3
sem.acquire()               # resources available == 2
sem.acquire()               # resources available == 1
sem.acquire()               # resources available == 0
sem.release()               # resources available == 1
sem.release(2)              # resources available == 3
											
A typical application of system semaphores is for controlling access to a circular buffer shared by a producer process and a consumer processes.

特定平台行为 ¶

当使用此类时，要意识到以下平台差异：

Windows: QSystemSemaphore does not own its underlying system semaphore. Windows owns it. This means that when all instances of QSystemSemaphore for a particular key have been destroyed, either by having their destructors called, or because one or more processes crash, Windows removes the underlying system semaphore.

Unix:

QSystemSemaphore owns the underlying system semaphore in Unix systems. This means that the last process having an instance of QSystemSemaphore for a particular key must remove the underlying system semaphore in its destructor. If the last process crashes without running the QSystemSemaphore destructor, Unix does not automatically remove the underlying system semaphore, and the semaphore survives the crash. A subsequent process that constructs a QSystemSemaphore with the same key will then be given the existing system semaphore. In that case, if the QSystemSemaphore constructor has specified its access mode as 打开 , its initial resource count will not be reset to the one provided but remain set to the value it received in the crashed process. To protect against this, the first process to create a semaphore for a particular key (usually a server), must pass its access mode as 创建 , which will force Unix to reset the resource count in the underlying system semaphore.

When a process using QSystemSemaphore terminates for any reason, Unix automatically reverses the effect of all acquire operations that were not released. Thus if the process acquires a resource and then exits without releasing it, Unix will release that resource.

另请参阅

QSharedMemory QSemaphore

class QSystemSemaphore ( key [ , initialValue=0 [ , mode=Open ] ] ) ¶

param initialValue

int

param key

unicode

param mode

AccessMode

Requests a system semaphore for the specified key 。参数 initialValue and mode are used according to the following rules, which are system dependent.

In Unix, if the mode is 打开 and the system already has a semaphore identified by key , that semaphore is used, and the semaphore’s resource count is not changed, i.e., initialValue is ignored. But if the system does not already have a semaphore identified by key , it creates a new semaphore for that key and sets its resource count to initialValue .

In Unix, if the mode is 创建 and the system already has a semaphore identified by key , that semaphore is used, and its resource count is set to initialValue . If the system does not already have a semaphore identified by key , it creates a new semaphore for that key and sets its resource count to initialValue .

In Windows, mode is ignored, and the system always tries to create a semaphore for the specified key . If the system does not already have a semaphore identified as key , it creates the semaphore and sets its resource count to initialValue . But if the system already has a semaphore identified as key it uses that semaphore and ignores initialValue .

mode parameter is only used in Unix systems to handle the case where a semaphore survives a process crash. In that case, the next process to allocate a semaphore with the same key will get the semaphore that survived the crash, and unless mode is 创建 , the resource count will not be reset to initialValue but will retain the initial value it had been given by the crashed process.

另请参阅

acquire() key()

PySide2.QtCore.QSystemSemaphore. AccessMode ¶

This enum is used by the constructor and setKey() . Its purpose is to enable handling the problem in Unix implementations of semaphores that survive a crash. In Unix, when a semaphore survives a crash, we need a way to force it to reset its resource count, when the system reuses the semaphore. In Windows, where semaphores can’t survive a crash, this enum has no effect.

常量	描述
QSystemSemaphore.Open	If the semaphore already exists, its initial resource count is not reset. If the semaphore does not already exist, it is created and its initial resource count set.
QSystemSemaphore.Create	`QSystemSemaphore` takes ownership of the semaphore and sets its resource count to the requested value, regardless of whether the semaphore already exists by having survived a crash. This value should be passed to the constructor, when the first semaphore for a particular key is constructed and you know that if the semaphore already exists it could only be because of a crash. In Windows, where a semaphore can’t survive a crash, Create and Open have the same behavior.

常量

描述

QSystemSemaphore.Open

If the semaphore already exists, its initial resource count is not reset. If the semaphore does not already exist, it is created and its initial resource count set.

QSystemSemaphore.Create

QSystemSemaphore takes ownership of the semaphore and sets its resource count to the requested value, regardless of whether the semaphore already exists by having survived a crash. This value should be passed to the constructor, when the first semaphore for a particular key is constructed and you know that if the semaphore already exists it could only be because of a crash. In Windows, where a semaphore can’t survive a crash, Create and Open have the same behavior.

PySide2.QtCore.QSystemSemaphore. SystemSemaphoreError ¶

常量	描述
QSystemSemaphore.NoError	没有发生错误。
QSystemSemaphore.PermissionDenied	The operation failed because the caller didn’t have the required permissions.
QSystemSemaphore.KeyError	操作失败，因为键无效。
QSystemSemaphore.AlreadyExists	The operation failed because a system semaphore with the specified key already existed.
QSystemSemaphore.NotFound	The operation failed because a system semaphore with the specified key could not be found.
QSystemSemaphore.OutOfResources	The operation failed because there was not enough memory available to fill the request.
QSystemSemaphore.UnknownError	发生其它事情且很糟糕。

PySide2.QtCore.QSystemSemaphore. acquire ( ) ¶

返回类型: bool

Acquires one of the resources guarded by this semaphore, if there is one available, and returns true . If all the resources guarded by this semaphore have already been acquired, the call blocks until one of them is released by another process or thread having a semaphore with the same key.

If false is returned, a system error has occurred. Call error() to get a value of SystemSemaphoreError that indicates which error occurred.

另请参阅

release()

PySide2.QtCore.QSystemSemaphore. error ( ) ¶

返回类型: SystemSemaphoreError

返回指示是否发生错误的值，且若如此，指示错误是什么。

另请参阅

errorString()

PySide2.QtCore.QSystemSemaphore. errorString ( ) ¶

返回类型: unicode

返回最后发生错误的文本描述。若 error() 返回 error value ，调用此函数以获取描述错误的文本字符串。

另请参阅

error()

PySide2.QtCore.QSystemSemaphore. key ( ) ¶

返回类型: unicode

Returns the key assigned to this system semaphore. The key is the name by which the semaphore can be accessed from other processes.

另请参阅

setKey()

PySide2.QtCore.QSystemSemaphore. release ( [ n=1 ] ) ¶

参数: n – int
返回类型: bool

发行 n resources guarded by the semaphore. Returns true unless there is a system error.

Example: Create a system semaphore having five resources; acquire them all and then release them all.

												sem = QSystemSemaphore("market", 5, QSystemSemaphore.Create)
sem.acquire(5)           # acquire all 5 resources
sem.release(5)           # release the 5 resources
												
											

This function can also “create” resources. For example, immediately following the sequence of statements above, suppose we add the statement:

												sem.release(10)          # "create" 10 new resources
												
											

Ten new resources are now guarded by the semaphore, in addition to the five that already existed. You would not normally use this function to create more resources.

另请参阅

acquire()

PySide2.QtCore.QSystemSemaphore. setKey ( key [ , initialValue=0 [ , mode=Open ] ] ) ¶

参数

key – unicode
initialValue – int
mode – AccessMode

This function works the same as the constructor. It reconstructs this QSystemSemaphore object. If the new key is different from the old key, calling this function is like calling the destructor of the semaphore with the old key, then calling the constructor to create a new semaphore with the new key 。 initialValue and mode parameters are as defined for the constructor.

另请参阅

QSystemSemaphore() key()

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QSystemSemaphore ¶

概要 ¶

函数 ¶

详细描述 ¶

特定平台行为 ¶