QSystemSemaphore¶
The
QSystemSemaphore
class provides a general counting system semaphore. More…
Synopsis¶
Functions¶
Detailed Description¶
A semaphore is a generalization of a mutex. While a mutex can be locked only once, a semaphore can be acquired multiple times. Typically, a semaphore is used to protect a certain number of identical resources.
Like its lighter counterpart
QSemaphore
, aQSystemSemaphore
can be accessed from multiplethreads
. UnlikeQSemaphore
, aQSystemSemaphore
can also be accessed from multipleprocesses
. This meansQSystemSemaphore
is a much heavier class, so if your application doesn’t need to access your semaphores across multiple processes, you will probably want to useQSemaphore
.Semaphores support two fundamental operations,
acquire()
andrelease()
:
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.
Example: Create a system semaphore
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 == 3A typical application of system semaphores is for controlling access to a circular buffer shared by a producer process and a consumer processes.
Platform-Specific Behavior¶
When using this class, be aware of the following platform differences:
Windows:
QSystemSemaphore
does not own its underlying system semaphore. Windows owns it. This means that when all instances ofQSystemSemaphore
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 ofQSystemSemaphore
for a particular key must remove the underlying system semaphore in its destructor. If the last process crashes without running theQSystemSemaphore
destructor, Unix does not automatically remove the underlying system semaphore, and the semaphore survives the crash. A subsequent process that constructs aQSystemSemaphore
with the same key will then be given the existing system semaphore. In that case, if theQSystemSemaphore
constructor has specified itsaccess mode
asOpen
, 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 itsaccess mode
asCreate
, 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.See also
QSharedMemory
QSemaphore
- class PySide2.QtCore.QSystemSemaphore(key[, initialValue=0[, mode=Open]])¶
- param initialValue:
int
- param key:
str
- param mode:
Requests a system semaphore for the specified
key
. The parametersinitialValue
andmode
are used according to the following rules, which are system dependent.In Unix, if the
mode
isOpen
and the system already has a semaphore identified bykey
, 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 bykey
, it creates a new semaphore for that key and sets its resource count toinitialValue
.In Unix, if the
mode
isCreate
and the system already has a semaphore identified bykey
, that semaphore is used, and its resource count is set toinitialValue
. If the system does not already have a semaphore identified bykey
, it creates a new semaphore for that key and sets its resource count toinitialValue
.In Windows,
mode
is ignored, and the system always tries to create a semaphore for the specifiedkey
. If the system does not already have a semaphore identified askey
, it creates the semaphore and sets its resource count toinitialValue
. But if the system already has a semaphore identified askey
it uses that semaphore and ignoresinitialValue
.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 samekey
will get the semaphore that survived the crash, and unlessmode
isCreate
, the resource count will not be reset toinitialValue
but will retain the initial value it had been given by the crashed process.
- 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.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.
- PySide2.QtCore.QSystemSemaphore.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.
- PySide2.QtCore.QSystemSemaphore.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 ofSystemSemaphoreError
that indicates which error occurred.See also
- PySide2.QtCore.QSystemSemaphore.error()¶
- Return type:
Returns a value indicating whether an error occurred, and, if so, which error it was.
See also
- PySide2.QtCore.QSystemSemaphore.errorString()¶
- Return type:
str
Returns a text description of the last error that occurred. If
error()
returns anerror value
, call this function to get a text string that describes the error.See also
- PySide2.QtCore.QSystemSemaphore.key()¶
- Return type:
str
Returns the key assigned to this system semaphore. The key is the name by which the semaphore can be accessed from other processes.
See also
- PySide2.QtCore.QSystemSemaphore.release([n=1])¶
- Parameters:
n – int
- Return type:
bool
Releases
n
resources guarded by the semaphore. Returnstrue
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.
See also
- PySide2.QtCore.QSystemSemaphore.setKey(key[, initialValue=0[, mode=Open]])¶
- Parameters:
key – str
initialValue – int
mode –
AccessMode
This function works the same as the constructor. It reconstructs this
QSystemSemaphore
object. If the newkey
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 newkey
. TheinitialValue
andmode
parameters are as defined for the constructor.See also
QSystemSemaphore()
key()
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