class QSystemSemaphore¶

The QSystemSemaphore class provides a general counting system semaphore. More…

Synopsis¶

Methods¶

def __init__()
def acquire()
def error()
def errorString()
def key()
def nativeIpcKey()
def release()
def setKey()
def setNativeKey()

Static functions¶

def isKeyTypeSupported()
def legacyNativeKey()
def platformSafeKey()

Note

This documentation may contain snippets that were automatically translated from C++ to Python. We always welcome contributions to the snippet translation. If you see an issue with the translation, you can also let us know by creating a ticket on https:/bugreports.qt.io/projects/PYSIDE

Detailed Description¶

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

A system semaphore is a generalization of QSemaphore . Typically, a semaphore is used to protect a certain number of identical resources.

Like its lighter counterpart, a QSystemSemaphore can be accessed from multiple threads . Unlike QSemaphore , a 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 .

Semaphores support two fundamental operations, 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.

System semaphores are identified by a key, represented by QNativeIpcKey . A key can be created in a cross-platform manner by using platformSafeKey(). A system semaphore is created by the QSystemSemaphore constructor when passed an access mode parameter of AccessMode::Create. Once it is created, other processes may attach to the same semaphore using the same key and an access mode parameter of AccessMode::Open.

Example: Create a system semaphore

sem = QSystemSemaphore(QSystemSemaphore.platformSafeKey("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

For details on the key types, platform-specific limitations, and interoperability with older or non-Qt applications, see the Native IPC Keys documentation. That includes important information for sandboxed applications on Apple platforms, including all apps obtained via the Apple App Store.

See also

Inter-Process Communication QSharedMemory QSemaphore

class 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.

Constant

Description

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.

Constant	Description
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.

class SystemSemaphoreError¶

Constant	Description
QSystemSemaphore.NoError	No error occurred.
QSystemSemaphore.PermissionDenied	The operation failed because the caller didn’t have the required permissions.
QSystemSemaphore.KeyError	The operation failed because of an invalid key.
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	Something else happened and it was bad.

__init__(key[, initialValue=0[, arg__3=QSystemSemaphore.AccessMode.Open]])¶

Parameters:

key – QNativeIpcKey
initialValue – int
arg__3 – AccessMode

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

In Unix, if the mode is Open 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 Create 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.

The 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 Create , the resource count will not be reset to initialValue but will retain the initial value it had been given by the crashed process.

See also

acquire() key()

__init__(key[, initialValue=0[, mode=QSystemSemaphore.AccessMode.Open]])

Parameters:

key – str
initialValue – int
mode – AccessMode

Requests a system semaphore identified by the legacy key key.

acquire()¶

Return type:: 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.

See also

release()

error()¶

Return type:: SystemSemaphoreError

Returns a value indicating whether an error occurred, and, if so, which error it was.

See also

errorString()

errorString()¶

Return type:: str

Returns a text description of the last error that occurred. If error() returns an error value , call this function to get a text string that describes the error.

See also

error()

static isKeyTypeSupported(type)¶

Parameters:: type – Type
Return type:: bool

key()¶

Return type:: str

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

See also

setKey()

static legacyNativeKey(key[, type=QNativeIpcKey.legacyDefaultTypeForOs()])¶

Parameters:

key – str
type – Type

Return type:

QNativeIpcKey

nativeIpcKey()¶

Return type:: QNativeIpcKey

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

You can use the native key to access system semaphores that have not been created by Qt, or to grant access to non-Qt applications. See Native IPC Keys for more information.

See also

setNativeKey()

static platformSafeKey(key[, type=QNativeIpcKey.DefaultTypeForOs])¶

Parameters:

key – str
type – Type

Return type:

QNativeIpcKey

release([n=1])¶

Parameters:: n – int
Return type:: bool

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

Releases 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(QSystemSemaphore.platformSafeKey("market"), 5, QSystemSemaphore.Create)
for (int i = 0 i < 5; ++i) # acquire all 5 resources
    sem.acquire()
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 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.

See also

acquire()

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

Parameters:

key – str
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. The initialValue and mode parameters are as defined for the constructor.

See also

QSystemSemaphore() key()

setNativeKey(key[, initialValue=0[, arg__3=QSystemSemaphore.AccessMode.Open]])¶

Parameters:

key – QNativeIpcKey
initialValue – int
arg__3 – AccessMode

This function is useful if the native key was shared from another process. See Native IPC Keys for more information.

See also

QSystemSemaphore() nativeIpcKey()

setNativeKey(key[, initialValue=0[, mode=QSystemSemaphore.AccessMode.Open[, type=QNativeIpcKey.legacyDefaultTypeForOs()]]])

Parameters:

key – str
initialValue – int
mode – AccessMode
type – Type