Qt thread queue

Threads are about doing things in parallel, just like processes. So how do threads differ from processes? While you are making calculations on a spreadsheet, there may also be a media player running on the same desktop playing your favorite song. Here is an example of two processes working in parallel: one running the spreadsheet program; one running a media player. Multitasking is a well known term for this.

Threads Events QObjects

A closer look at the media player reveals that there are again things going on in parallel within one single process. While the media player is sending music to the audio driver, the user interface with all its bells and whistles is being constantly updated. This is what threads are for -- concurrency within one single process. So how is concurrency implemented? Parallel work on single core CPUs is an illusion which is somewhat similar to the illusion of moving images in cinema.

For processes, the illusion is produced by interrupting the processor's work on one process after a very short time. Then the processor moves on to the next process. In order to switch between processes, the current program counter is saved and the next processor's program counter is loaded.

This is not sufficient because the same needs to be done with registers and certain architecture and OS specific data. Just as one CPU can power two or more processes, it is also possible to let the CPU run on two different code segments of one single process.

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When a process starts, it always executes one code segment and therefore the process is said to have one thread. However, the program may decide to start a second thread. Then, two different code sequences are processed simultaneously inside one process.

qt thread queue

Concurrency is achieved on single core CPUs by repeatedly saving program counters and registers then loading the next thread's program counters and registers. No cooperation from the program is required to cycle between the active threads. A thread may be in any state when the switch to the next thread occurs. The current trend in CPU design is to have several cores. A typical single-threaded application can make use of only one core.

However, a program with multiple threads can be assigned to multiple cores, making things happen in a truly concurrent way. As a result, distributing work to more than one thread can make a program run much faster on multicore CPUs because additional cores can be used. As mentioned, each program has one thread when it is started. This thread is called the "main thread" also known as the "GUI thread" in Qt applications. The Qt GUI must run in this thread.

All widgets and several related classes, for example QPixmapdon't work in secondary threads. A secondary thread is commonly referred to as a "worker thread" because it is used to offload processing work from the main thread. Each thread has its own stack, which means each thread has its own call history and local variables.

Unlike processes, threads share the same address space. The following diagram shows how the building blocks of threads are located in memory. Program counter and registers of inactive threads are typically kept in kernel space.

There is a shared copy of the code and a separate stack for each thread.Thread Support in Qt Synchronizing Threads. Qt offers many classes and functions for working with threads. Below are four different approaches that Qt programmers can use to implement multithreaded applications.

QThread is the foundation of all thread control in Qt. Each QThread instance represents and controls one thread. QThread can either be instantiated directly or subclassed.

Instantiating a QThread provides a parallel event loop, allowing QObject slots to be invoked in a secondary thread. Subclassing a QThread allows the application to initialize the new thread before starting its event loop, or to run parallel code without an event loop. See the QThread class reference and the threading examples for demonstrations on how to use QThread. Creating and destroying threads frequently can be expensive. To reduce this overhead, existing threads can be reused for new tasks.

QThreadPool is a collection of reuseable QThreads. When a thread becomes available, the code within QRunnable::run will execute in that thread. Each Qt application has a global thread pool, which is accessible through QThreadPool::globalInstance.

This global thread pool automatically maintains an optimal number of threads based on the number of cores in the CPU. However, a separate QThreadPool can be created and managed explicitly. The Qt Concurrent module provides high-level functions that deal with some common parallel computation patterns: map, filter, and reduce.

Unlike using QThread and QRunnablethese functions never require the use of low-level threading primitives such as mutexes or semaphores. Instead, they return a QFuture object which can be used to retrieve the functions' results when they are ready.

Qt Concurrent 's map, filter and reduce algorithms automatically distribute computation across all available processor cores, so applications written today will continue to scale when deployed later on a system with more cores. This module also provides the QtConcurrent::run function, which can run any function in another thread. However, QtConcurrent::run only supports a subset of features available to the map, filter and reduce functions. The QFuture can be used to retrieve the function's return value and to check if the thread is running.

See the Qt Concurrent module documentation for details on the individual functions. Each WorkerScript instance can have one. When WorkerScript. When the script finishes running, it can send a reply back to the GUI thread which will invoke the WorkerScript. Using a WorkerScript is similar to using a worker QObject that has been moved to another thread.

Data is transferred between threads via signals. See the WorkerScript documentation for details on how to implement the script, and for a list of data types that can be passed between threads. As demonstrated above, Qt provides different solutions for developing threaded applications. The right solution for a given application depends on the purpose of the new thread and the thread's lifetime.In the first partwe saw that signals are just simple functions, whose body is generated by moc.

They are just calling QMetaObject::activatewith an array of pointers to arguments on the stack. Here is the code of a signal, as generated by moc: from part 1.

qt thread queue

QMetaObject::activate will then look in internal data structures to find out what are the slots connected to that signal. As seen in part 1, for each slot, the following code will be executed:. The event queued is protected by a mutex, so there is no race conditions when threads push events to another thread's event queue.

Once the event has been added to the queue, and if the receiver is living in another thread, we notify the event dispatcher of that thread by calling QAbstractEventDispatcher::wakeUp. This will wake up the dispatcher if it was sleeping while waiting for more events. If the receiver is in the same thread, the event will be processed later, as the event loop iterates.

When processed, that event will call the slot the same way we call them for direct connections. All the information slot to call, parameter values, The argv coming from the signal is an array of pointers to the arguments. The problem is that these pointers point to the stack of the signal where the arguments are. Once the signal returns, they will not be valid anymore. So we'll have to copy the parameter values of the function on the heap.

In order to do that, we just ask QMetaType. We will then be able to look up the particular type in the QMetaType database. The code showed here has been slightly simplified and commented:. That later function will dispatch just the same way as QMetaObject::activate would do it for direct connections, as seen in Part 1. Like with a DirectConnectionthe arguments can stay on the stack since the stack is on the thread that is blocked. No need to copy the arguments. Like with a QueuedConnectionan event is posted to the other thread's event loop.

The event also contains a pointer to a QSemaphore. The thread that delivers the event will release the semaphore right after the slot has been called. Meanwhile, the thread that called the signal will acquire the semaphore in order to wait until the event is processed. It is the destructor of QMetaCallEvent which will release the semaphore. This is good because the event will be deleted right after it is delivered i.

A BlockingQueuedConnection can be useful to do thread communication when you want to invoke a function in another thread and wait for the answer before it is finished. However, it must be done with care.QThreadPool manages and recyles individual QThread objects to help reduce thread creation costs in programs that use threads.

Each Qt application has one global QThreadPool object, which can be accessed by calling globalInstance. Then create an object of that class and pass it to QThreadPool::start. QThreadPool deletes the QRunnable automatically by default. Use QRunnable::setAutoDelete to change the auto-deletion flag. If autoDelete is enabled the QRunnable will be deleted when the last thread exits the run function. Calling start multiple times with the same QRunnable when autoDelete is enabled creates a race condition and is not recommended.

Threads that are unused for a certain amount of time will expire. The default expiry timeout is milliseconds 30 seconds. This can be changed using setExpiryTimeout. Setting a negative expiry timeout disables the expiry mechanism. Call maxThreadCount to query the maximum number of threads to be used. If needed, you can change the limit with setMaxThreadCount. The activeThreadCount function returns the number of threads currently doing work. The reserveThread function reserves a thread for external use.

Use releaseThread when your are done with the thread, so that it may be reused.

qt thread queue

Essentially, these functions temporarily increase or reduce the active thread count and are useful when implementing time-consuming operations that are not visible to the QThreadPool. Note that QThreadPool is a low-level class for managing threads, see the Qt Concurrent module for higher level alternatives.

Note: It is possible for this function to return a value that is greater than maxThreadCount. See reserveThread for more details. See also reserveThread and releaseThread. Threads that are unused for expiryTimeout milliseconds are considered to have expired and will exit. Such threads will be restarted as needed. The default expiryTimeout is milliseconds 30 seconds.Nine times out of ten, a quick inspection of their code shows that the biggest problem is the very fact they're using threads in the first place, and they're falling in one of the endless pitfalls of parallel programming.

Moreover, threading support in Qt is a double-edged sword: while it makes it very simple for you to do multithread programming, it adds a certain number of features especially when it comes to interaction with QObjects you must be aware of.

The purpose of this document is not to teach you how to use threads, do proper locking, exploit parallelism, nor write scalable programs; there are many good books about these topics; for instance, take a look to the recommended reading list on this page.

Instead, this small article is meant to be a guide to introduce users to threading in Qt 4, in order to avoid the most common pitfalls and help them to develop code that is at the same time more robust and with a better structure. Not being a general-purpose introduction to threads programming, we expect you to have some previous knowledge about:.

In this document we'll follow the Qt naming conventionswhich are:. Being an event-driven toolkit, events and event delivery play a central role in Qt architecture. In this article we'll not give a comprehensive coverage about this topic; we'll instead focus on some thread-related key concepts see here and here for more information about the Qt event system.

An event in Qt is an object which represents something interesting that happened; the main difference between an event and a signal is that events are targeted to a specific object in our application which decides what to do with that eventwhile signals are emitted "in the wild". From a code point of view, all events are instances of some subclass of QEventand all QObject-derived classes can override the QObject::event virtual method in order to handle events targeted to their instances.

The important thing about events is that they're not delivered as soon as they're generated; they're instead queued up in an event queue and sent sometime later. The dispatcher itself loops around the event queue and sends queued events to their target objects, and therefore it is called the event loop. Conceptually, this is how an event loop looks see the Qt Quarterly article linked above :.

If we think about it, all that can generate events at that point is some external source dispatching for all internal events is now complete and there were no more pending events in the event queue to delivery. Therefore, the event loop can be woken up by:. In a UNIX-like system, window manager activity i.

If we decide to implement cross-thread event posting with an internal socketpair 2all that is left is being woken up by activity on:. All Qt needs to do is converting what select returns into an object of the right QEvent subclass and queue it up in the event queue. This isn't an exhaustive list, but if you have the overall picture, you should be able to guess which classes require a running event loop.

Before discussing why you should never ever block the event looplet's try to figure out what this "blocking" means.

Qt based threading for modern devices

Suppose you have a Button widget which emits a signal when clicked; connected to this signal there's a slot of our Worker object, which does a lot of work.

After you click the button, the stack trace will look like this the stack grows downwards :. In main we started the event loop, as usual, by calling QApplication::exec line 2. The window manager sent us the mouse click, which was picked up by the Qt kernel, converted in a QMouseEvent and sent to our widget's event method line 4 by QApplication::notify not shown here. Since Button didn't override eventthe base class implementation QWidget is called.

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QWidget::event detects the event is actually a mouse click and calls the specialized event handler, that is, Button::mousePressEvent line 5. We overrode this method to emit the Button::clicked signal line 6which invokes the Worker::doWork slot of our worker object line 7. While the worker is busy working, what's the event loop doing? You should've guessed it: nothing! It dispatched the mouse press event and it's blocked waiting for the event handler to return.

We managed to block the event loopwhich means that no event is sent any more, until we return from the doWork slot, up the stack, to the event loop, and let it process pending events. With the event delivery stuck, widgets won't update themselves QPaintEvent objects will sit in the queueno further interaction with widgets is possible for the same reasontimers won't fire and networking communications will slow down and stop.

qt thread queue

Moreover, many window managers will detect that your application is not handling events any more and tell the user that your application isn't responding. That's why is so important to quickly react to events and return to the event loop as soon as possible!It implements a queue data structure for items of a same type. A queue is a first in, first out FIFO structure. Items are added to the tail of the queue using enqueue and retrieved from the head using dequeue.

The head function provides access to the head item without removing it. QQueue inherits from QList. All of QList 's functionality also applies to QQueue.

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For example, you can use isEmpty to test whether the queue is empty, and you can traverse a QQueue using QList 's iterator classes for example, QListIterator. But in addition, QQueue provides three convenience functions that make it easy to implement FIFO semantics: enqueuedequeueand head.

QQueue's value type must be an assignable data type. This covers most data types that are commonly used, but the compiler won't let you, for example, store a QWidget as a value. Removes the head item in the queue and returns it.

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This function assumes that the queue isn't empty. See also headenqueueand isEmpty. See also dequeue and head. See also dequeueenqueueand isEmpty. Documentation contributions included herein are the copyrights of their respective owners. Qt and respective logos are trademarks of The Qt Company Ltd. All other trademarks are property of their respective owners. Contents Public Functions Detailed Description. This is the same as QList::takeFirst. This is the same as QList::append. This is the same as QList::first.

This operation is very fast and never fails. This function was introduced in Qt 4.While the purpose of threads is to allow code to run in parallel, there are times where threads must stop and wait for other threads. For example, if two threads try to write to the same variable simultaneously, the result is undefined. The principle of forcing threads to wait for one another is called mutual exclusion. It is a common technique for protecting shared resources such as data.

QMutex is the basic class for enforcing mutual exclusion. A thread locks a mutex in order to gain access to a shared resource. If a second thread tries to lock the mutex while it is already locked, the second thread will be put to sleep until the first thread completes its task and unlocks the mutex. QReadWriteLock is similar to QMutexexcept that it distinguishes between "read" and "write" access.

When a piece of data is not being written to, it is safe for multiple threads to read from it simultaneously.


A QMutex forces multiple readers to take turns to read shared data, but a QReadWriteLock allows simultaneous reading, thus improving parallelism. QSemaphore is a generalization of QMutex that protects a certain number of identical resources. In contrast, a QMutex protects exactly one resource. The Semaphores Example shows a typical application of semaphores: synchronizing access to a circular buffer between a producer and a consumer.

QWaitCondition synchronizes threads not by enforcing mutual exclusion but by providing a condition variable.

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While the other primitives make threads wait until a resource is unlocked, QWaitCondition makes threads wait until a particular condition has been met. To allow the waiting threads to proceed, call wakeOne to wake one randomly selected thread or wakeAll to wake them all simultaneously.

Note: Qt's synchronization classes rely on the use of properly aligned pointers. For instance, you cannot use packed classes with MSVC. These synchronization classes can be used to make a method thread safe. However, doing so incurs a performance penalty, which is why most Qt methods are not made thread safe.

If a thread locks a resource but does not unlock it, the application may freeze because the resource will become permanently unavailable to other threads. This can happen, for example, if an exception is thrown and forces the current function to return without releasing its lock. Another similar scenario is a deadlock. For example, suppose that thread A is waiting for thread B to unlock a resource. If thread B is also waiting for thread A to unlock a different resource, then both threads will end up waiting forever, so the application will freeze.

They lock a resource when they are constructed, and automatically unlock it when they are destroyed. They are designed to simplify code that use QMutex and QReadWriteLockthus reducing the chances that a resource becomes permanently locked by accident. Qt's event system is very useful for inter-thread communication. Every thread may have its own event loop. To call a slot or any invokable method in another thread, place that call in the target thread's event loop. This lets the target thread finish its current task before the slot starts running, while the original thread continues running in parallel.

To place an invocation in an event loop, make a queued signal-slot connection. Whenever the signal is emitted, its arguments will be recorded by the event system. The thread that the signal receiver lives in will then run the slot. Alternatively, call QMetaObject::invokeMethod to achieve the same effect without signals.

In both cases, a queued connection must be used because a direct connection bypasses the event system and runs the method immediately in the current thread.

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There is no risk of deadlocks when using the event system for thread synchronization, unlike using low-level primitives. However, the event system does not enforce mutual exclusion.

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