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Reentrancy and Thread-Safety ◦ Thread-Support in Qt Modules

A call to inmain blocks the calling thread until the Qt event loop can process our message, execute the specified method and return the result. For situations where you don’t wait to wait for the result, or you wish to do some other processing while waiting for the result, QtUtils provides the inmainlater function. This works in the same way as inmain, but returns a reference to a Python. Slots, and functions or methods, are not assigned to any thread. A thread, in simple terms, is a function that is the root of the call stack and any function can be called from one thread or another. The function can't know beforehand what thread it will be called from, as it's up to you to actually call it.

QThread inherits QObject. It emits signals to indicate that the thread started or finished executing, and provides a few slots as well.

More interesting is that QObjects can be used in multiple threads, emit signals that invoke slots in other threads, and post events to objects that 'live' in other threads. This is possible because each thread is allowed to have its own event loop.

QObject Reentrancy

QObject is reentrant. Most of its non-GUI subclasses, such as QTimer, QTcpSocket, QUdpSocket and QProcess, are also reentrant, making it possible to use these classes from multiple threads simultaneously. Note that these classes are designed to be created and used from within a single thread; creating an object in one thread and calling its functions from another thread is not guaranteed to work. There are three constraints to be aware of:

• The child of a QObject must always be created in the thread where the parent was created. This implies, among other things, that you should never pass the QThread object (this) as the parent of an object created in the thread (since the QThread object itself was created in another thread).
• Event driven objects may only be used in a single thread. Specifically, this applies to the timer mechanism and the network module. For example, you cannot start a timer or connect a socket in a thread that is not the object's thread.
• You must ensure that all objects created in a thread are deleted before you delete the QThread. This can be done easily by creating the objects on the stack in your run() implementation.

Although QObject is reentrant, the GUI classes, notably QWidget and all its subclasses, are not reentrant. They can only be used from the main thread. As noted earlier, QCoreApplication::exec() must also be called from that thread.

In practice, the impossibility of using GUI classes in other threads than the main thread can easily be worked around by putting time-consuming operations in a separate worker thread and displaying the results on screen in the main thread when the worker thread is finished. This is the approach used for implementing the Mandelbrot Example and the Blocking Fortune Client Example.

In general, creating QObjects before the QApplication is not supported and can lead to weird crashes on exit, depending on the platform. This means static instances of QObject are also not supported. A properly structured single or multi-threaded application should make the QApplication be the first created, and last destroyed QObject.

Per-Thread Event Loop

Each thread can have its own event loop. The initial thread starts its event loop using QCoreApplication::exec(), or for single-dialog GUI applications, sometimes QDialog::exec(). Other threads can start an event loop using QThread::exec(). Like QCoreApplication, QThread provides an exit(int) function and a quit() slot.

An event loop in a thread makes it possible for the thread to use certain non-GUI Qt classes that require the presence of an event loop (such as QTimer, QTcpSocket, and QProcess). It also makes it possible to connect signals from any threads to slots of a specific thread. This is explained in more detail in the Signals and Slots Across Threads section below.

A QObject instance is said to live in the thread in which it is created. Events to that object are dispatched by that thread's event loop. The thread in which a QObject lives is available using QObject::thread().

The QObject::moveToThread() function changes the thread affinity for an object and its children (the object cannot be moved if it has a parent).

Calling delete on a QObject from a thread other than the one that owns the object (or accessing the object in other ways) is unsafe, unless you guarantee that the object isn't processing events at that moment. Use QObject::deleteLater() instead, and a DeferredDelete event will be posted, which the event loop of the object's thread will eventually pick up. By default, the thread that owns a QObject is the thread that creates the QObject, but not after QObject::moveToThread() has been called.

If no event loop is running, events won't be delivered to the object. For example, if you create a QTimer object in a thread but never call exec(), the QTimer will never emit its timeout() signal. Calling deleteLater() won't work either. (These restrictions apply to the main thread as well.)

You can manually post events to any object in any thread at any time using the thread-safe function QCoreApplication::postEvent(). The events will automatically be dispatched by the event loop of the thread where the object was created.

Event filters are supported in all threads, with the restriction that the monitoring object must live in the same thread as the monitored object. Similarly, QCoreApplication::sendEvent() (unlike postEvent()) can only be used to dispatch events to objects living in the thread from which the function is called.

Accessing QObject Subclasses from Other Threads

QObject and all of its subclasses are not thread-safe. This includes the entire event delivery system. It is important to keep in mind that the event loop may be delivering events to your QObject subclass while you are accessing the object from another thread.

If you are calling a function on an QObject subclass that doesn't live in the current thread and the object might receive events, you must protect all access to your QObject subclass's internal data with a mutex; otherwise, you may experience crashes or other undesired behavior.

Like other objects, QThread objects live in the thread where the object was created -- not in the thread that is created when QThread::run() is called. It is generally unsafe to provide slots in your QThread subclass, unless you protect the member variables with a mutex.

On the other hand, you can safely emit signals from your QThread::run() implementation, because signal emission is thread-safe.

Signals and Slots Across Threads

Qt supports these signal-slot connection types:

• Auto Connection (default) If the signal is emitted in the thread which the receiving object has affinity then the behavior is the same as the Direct Connection. Otherwise, the behavior is the same as the Queued Connection.'
• Direct Connection The slot is invoked immediately, when the signal is emitted. The slot is executed in the emitter's thread, which is not necessarily the receiver's thread.
• Queued Connection The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.
• Blocking Queued Connection The slot is invoked as for the Queued Connection, except the current thread blocks until the slot returns.

  Note: Using this type to connect objects in the same thread will cause deadlock.

• Unique Connection The behavior is the same as the Auto Connection, but the connection is made only if it does not duplicate an existing connection. i.e., if the same signal is already connected to the same slot for the same pair of objects, then the connection is not made and connect() returns false.

The connection type can be specified by passing an additional argument to connect(). Be aware that using direct connections when the sender and receiver live in different threads is unsafe if an event loop is running in the receiver's thread, for the same reason that calling any function on an object living in another thread is unsafe.

QObject::connect() itself is thread-safe.

The Mandelbrot Example uses a queued connection to communicate between a worker thread and the main thread. To avoid freezing the main thread's event loop (and, as a consequence, the application's user interface), all the Mandelbrot fractal computation is done in a separate worker thread. The thread emits a signal when it is done rendering the fractal.

Similarly, the Blocking Fortune Client Example uses a separate thread for communicating with a TCP server asynchronously.

Reentrancy and Thread-Safety ◦ Thread-Support in Qt Modules

The QThread class provides a platform-independent way to managethreads. More...

Inherits QObject.

Types

• enum Priority { IdlePriority, LowestPriority, LowPriority, NormalPriority, ..., InheritPriority }

Methods

• int exec_ (self)
• bool isFinished (self)
• Priority priority (self)
• run (self)
• setStackSize (self, int stackSize)
• start (self, Priority priority = QThread.InheritPriority)
• bool wait (self, int msecs = ULONG_MAX)

Static Methods

• int currentThreadId ()
• msleep (int)
• sleep (int)
• yieldCurrentThread ()

Qt Signals

• void started ()

Detailed Description

The QThread class provides a platform-independent way to managethreads.

A QThread object manages one thread of control within theprogram. QThreads begin executing in run(). By default, run() starts the event loop by callingexec_() and runs a Qt event loopinside the thread.

You can use worker objects by moving them to the thread usingQObject.moveToThread().

The code inside the Worker's slot would then execute in aseparate thread. However, you are free to connect the Worker'sslots to any signal, from any object, in any thread. It is safe toconnect signals and slots across different threads, thanks to amechanism called queuedconnections.

Another way to make code run in a separate thread, is tosubclass QThread and reimplement run(). For example:

In that example, the thread will exit after the run function hasreturned. There will not be any event loop running in the threadunless you call exec_().

It is important to remember that a QThread instance lives in the old thread thatinstantiated it, not in the new thread that calls run(). This means that all of QThread'squeued slots will execute in the old thread. Thus, a developer whowishes to invoke slots in the new thread must use the worker-objectapproach; new slots should not be implemented directly into asubclassed QThread.

When subclassing QThread, keep in mind that the constructorexecutes in the old thread while run() executes in the new thread. If amember variable is accessed from both functions, then the variableis accessed from two different threads. Check that it is safe to doso.

Note: Care must be taken when interacting with objectsacross different threads. See Synchronizing Threads fordetails.

Managing threads

QThread will notifiy you via a signal when the thread isstarted(), finished(), and terminated(), or you can use isFinished() and isRunning() to query the state of thethread.

You can stop the thread by calling exit() or quit(). In extreme cases, you may want toforcibly terminate() anexecuting thread. However, doing so is dangerous and discouraged.Please read the documentation for terminate() and setTerminationEnabled()for detailed information.

From Qt 4.8 onwards, it is possible to deallocate objects thatlive in a thread that has just ended, by connecting the finished() signal to QObject.deleteLater().

Use wait() to block the callingthread, until the other thread has finished execution (or until aspecified time has passed).

The static functions currentThreadId() and currentThread() return identifiersfor the currently executing thread. The former returns a platformspecific ID for the thread; the latter returns a QThreadpointer.

To choose the name that your thread will be given (as identifiedby the command ps -L on Linux, for example), you can callsetObjectName() beforestarting the thread. If you don't call setObjectName(), the name givento your thread will be the class name of the runtime type of yourthread object (for example, 'RenderThread' in the case ofthe Mandelbrot Example, asthat is the name of the QThread subclass). Note that this iscurrently not available with release builds on Windows.

QThread also provides static, platform independent sleepfunctions: sleep(), msleep(), and usleep() allow full second, millisecond,and microsecond resolution respectively.

Note:wait() and thesleep() functions should beunnecessary in general, since Qt is an event-driven framework.Instead of wait(), considerlistening for the finished()signal. Instead of the sleep()functions, consider using QTimer.

{Mandelbrot Example}, {Semaphores Example}, {Wait ConditionsExample}

Type Documentation

QThread.Priority

Method Documentation

QThread.__init__ (self, QObjectparent = None)

The parent argument, if not None, causes self to be owned by Qt instead of PyQt.

Constructs a new QThread to manage anew thread. The parent takes ownership of the QThread. The thread does not begin executinguntil start() is called.

See alsostart().

QThread QThread.currentThread ()

Returns a pointer to a QThread whichmanages the currently executing thread.

int QThread.currentThreadId ()

int QThread.exec_ (self)

Enters the event loop and waits until exit() is called, returning the value thatwas passed to exit(). The valuereturned is 0 if exit() is calledvia quit().

Qt Slot Function Thread Tool

This function is meant to be called from within run(). It is necessary to call this functionto start event handling.

See alsoquit() andexit().

QThread.exit (self, int returnCode = 0)

After calling this function, the thread leaves the event loopand returns from the call to QEventLoop.exec(). The QEventLoop.exec() function returnsreturnCode.

By convention, a returnCode of 0 means success, anynon-zero value indicates an error.

Note that unlike the C library function of the same name, thisfunction does return to the caller -- it is event processingthat stops.

No QEventLoops will be started anymore in this thread untilQThread.exec() has been calledagain. If the eventloop in QThread.exec() is not running then thenext call to QThread.exec() willalso return immediately.

See alsoquit() andQEventLoop.

int QThread.idealThreadCount ()

bool QThread.isFinished (self)

See alsoisRunning().

bool QThread.isRunning (self)

See alsoisFinished().

QThread.msleep (int)

See alsosleep() andusleep().

Priority QThread.priority (self)

Returns the priority for a running thread. If the thread is notrunning, this function returns InheritPriority.

This function was introduced in Qt 4.1.

See alsoPriority, setPriority(), and start().

QThread.quit (self)

See alsoexit() andQEventLoop.

QThread.run (self)

The starting point for the thread. After calling start(), the newly created thread callsthis function. The default implementation simply calls exec_().

You can reimplement this function to facilitate advanced threadmanagement. Returning from this method will end the execution ofthe thread.

See alsostart() andwait().

QThread.setPriority (self, Prioritypriority)

This function sets the priority for a running thread. Ifthe thread is not running, this function does nothing and returnsimmediately. Use start() to starta thread with a specific priority.

The priority argument can be any value in theQThread.Priority enum except forInheritPriorty.

The effect of the priority parameter is dependent on theoperating system's scheduling policy. In particular, thepriority will be ignored on systems that do not supportthread priorities (such as on Linux, seehttp://linux.die.net/man/2/sched_setscheduler for moredetails).

This function was introduced in Qt 4.1.

See alsoPriority, priority(), and start().

QThread.setStackSize (self, int stackSize)

See alsostackSize().

QThread.setTerminationEnabled (bool enabled = True)

Enables or disables termination of the current thread based onthe enabled parameter. The thread must have been started byQThread.

When enabled is false, termination is disabled. Futurecalls to QThread.terminate()will return immediately without effect. Instead, the termination isdeferred until termination is enabled.

When enabled is true, termination is enabled. Futurecalls to QThread.terminate()will terminate the thread normally. If termination has beendeferred (i.e. QThread.terminate() was called withtermination disabled), this function will terminate the callingthread immediately. Note that this function will not returnin this case.

See alsoterminate().

QThread.sleep (int)

See alsomsleep() andusleep().

int QThread.stackSize (self)

Returns the maximum stack size for the thread (if set withsetStackSize()); otherwisereturns zero.

See alsosetStackSize().

QThread.start (self, Prioritypriority = QThread.InheritPriority)

This method is also a Qt slot with the C++ signature void start(QThread::Priority = QThread.InheritPriority).

Begins execution of the thread by calling run(). The operating system will schedulethe thread according to the priority parameter. If thethread is already running, this function does nothing.

The effect of the priority parameter is dependent on theoperating system's scheduling policy. In particular, thepriority will be ignored on systems that do not supportthread priorities (such as on Linux, seehttp://linux.die.net/man/2/sched_setscheduler for moredetails).

Qt Start Thread

See alsorun() andterminate().

QThread.terminate (self)

Terminates the execution of the thread. The thread may or maynot be terminated immediately, depending on the operating system'sscheduling policies. Listen for the terminated() signal, or use QThread.wait() after terminate(), to besure.

When the thread is terminated, all threads waiting for thethread to finish will be woken up.

Warning: This function is dangerous and its use isdiscouraged. The thread can be terminated at any point in its codepath. Threads can be terminated while modifying data. There is nochance for the thread to clean up after itself, unlock any heldmutexes, etc. In short, use this function only if absolutelynecessary.

Termination can be explicitly enabled or disabled by callingQThread.setTerminationEnabled().Calling this function while termination is disabled results in thetermination being deferred, until termination is re-enabled. Seethe documentation of QThread.setTerminationEnabled()for more information.

See alsosetTerminationEnabled().

QThread.usleep (int)

See alsosleep() andmsleep().

bool QThread.wait (self, int msecs = ULONG_MAX)

• The thread associated with this QThread object has finished execution (i.e. whenit returns from run()). Thisfunction will return true if the thread has finished. It alsoreturns true if the thread has not been started yet.
• time milliseconds has elapsed. If time isULONG_MAX (the default), then the wait will never timeout (thethread must return from run()). Thisfunction will return false if the wait timed out.

This provides similar functionality to the POSIXpthread_join() function.

See alsosleep() andterminate().

QThread.yieldCurrentThread ()

Qt Signal Documentation

Qt Slot Function Thread Gages

void finished ()

See alsostarted() andterminated().

void started ()

See alsofinished()and terminated().

void terminated ()

See alsostarted() andfinished().