path: root/doc/src/tutorials/threads.qdoc

/****************************************************************************
**
** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** GNU Free Documentation License
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file.
**
** Other Usage
** Alternatively, this file may be used in accordance with the terms
** and conditions contained in a signed written agreement between you
** and Nokia.
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \page thread-basics.html
    \ingroup tutorials
    \startpage {index.html}{Qt Reference Documentation}

    \title Threading Basics
    \brief An introduction to threads

    \section1 What Are Threads?

    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. 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 \mdash 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. 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. 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.

    \section2 GUI Thread and Worker Thread

    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 QPixmap, don'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.

    \section2 Simultaneous Access to Data

    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.

    \image threadvisual-example.png "Thread visualization"

    If two threads have a pointer to the same object, it is possible that both
    threads will access that object at the same time and this can potentially
    destroy the object's integrity. It's easy to imagine the many things that
    can go wrong when two methods of the same object are executed
    simultaneously.

    Sometimes it is necessary to access one object from different threads;
    for example, when objects living in different threads need to communicate.
    Since threads use the same address space, it is easier and faster for
    threads to exchange data than it is for processes. Data does not have to be
    serialized and copied. Passing pointers is possible, but there must be a
    strict coordination of what thread touches which object. Simultaneous
    execution of operations on one object must be prevented. There are several
    ways of achieving this and some of them are described below.

    So what can be done safely? All objects created in a thread can be used
    safely within that thread provided that other threads don't have references
    to them and objects don't have implicit coupling with other threads. Such
    implicit coupling may happen when data is shared between instances as with
    static members, singletons or global data. Familiarize yourself with the
    concept of \l{Reentrancy and Thread-Safety}{thread safe and reentrant}
    classes and functions.

    \section1 Using Threads

    There are basically two use cases for threads:

    \list
    \o Make processing faster by making use of multicore processors.
    \o Keep the GUI thread or other time critical threads responsive by
       offloading long lasting processing or blocking calls to other threads.
    \endlist

    \section2 When to Use Alternatives to Threads

    Developers need to be very careful with threads. It is easy to start other
    threads, but very hard to ensure that all shared data remains consistent.
    Problems are often hard to find because they may only show up once in a
    while or only on specific hardware configurations. Before creating threads
    to solve certain problems, possible alternatives should be considered.

    \table
    \header
        \o Alternative
        \o Comment
    \row
        \o QEventLoop::processEvents()
        \o Calling QEventLoop::processEvents() repeatedly during a
           time-consuming calculation prevents GUI blocking. However, this
           solution doesn't scale well because the call to processEvents() may
           occur too often, or not often enough, depending on hardware.
    \row
        \o QTimer
        \o Background processing can sometimes be done conveniently using a
           timer to schedule execution of a slot at some point in the future.
           A timer with an interval of 0 will time out as soon as there are no
           more events to process.
    \row
        \o QSocketNotifier QNetworkAccessManager QIODevice::readyRead()
        \o This is an alternative to having one or multiple threads, each with
           a blocking read on a slow network connection. As long as the
           calculation in response to a chunk of network data can be executed
           quickly, this reactive design is better than synchronous waiting in
           threads. Reactive design is less error prone and energy efficient
           than threading. In many cases there are also performance benefits.
    \endtable

    In general, it is recommended to only use safe and tested paths and to
    avoid introducing ad-hoc threading concepts. QtConcurrent provides an easy
    interface for distributing work to all of the processor's cores. The
    threading code is completely hidden in the QtConcurrent framework, so you
    don't have to take care of the details. However, QtConcurrent can't be used
    when communication with the running thread is needed, and it shouldn't be
    used to handle blocking operations.

    \section2 Which Qt Thread Technology Should You Use?

    Sometimes you want to do more than just running a method in the context of
    another thread. You may want to have an object which lives in another
    thread that provides a service to the GUI thread. Maybe you want another
    thread to stay alive forever to poll hardware ports and send a signal to
    the GUI thread when something noteworthy has happened. Qt provides
    different solutions for developing threaded applications. The right
    solution depends on the purpose of the new thread as well as on the
    thread's lifetime.

    \table
    \header
        \o Lifetime of thread
        \o Development task
        \o Solution
    \row
        \o One call
        \o Run one method within another thread and quit the thread when the
           method is finished.
        \o Qt provides different solutions:
           \list
              \o Write a function and run it with QtConcurrent::run()
              \o Derive a class from QRunnable and run it in the global thread
                 pool with QThreadPool::globalInstance()->start()
              \o Derive a class from QThread, reimplement the QThread::run()
                 method and use QThread::start() to run it.
           \endlist

    \row
        \o One call
        \o Operations are to be performed on all items of a container.
           Processing should be performed using all available cores. A common
           example is to produce thumbnails from a list of images.
        \o QtConcurrent provides the \l{QtConcurrent::}{map()} function for
           applying operations on every container element,
           \l{QtConcurrent::}{filter()} for selecting container elements, and
           the option of specifying a reduce function for combining the
           remaining elements.
    \row
        \o One call
        \o A long running operation has to be put in another thread. During the
           course of processing, status information should be sent to the GUI
           thread.
        \o Use QThread, reimplement run and emit signals as needed. Connect the
           signals to the GUI thread's slots using queued signal/slot
           connections.

    \row
        \o Permanent
        \o Have an object living in another thread and let it perform different
           tasks upon request.
           This means communication to and from the worker thread is required.
        \o Derive a class from QObject and implement the necessary slots and
           signals, move the object to a thread with a running event loop and
           communicate with the object over queued signal/slot connections.
    \row
        \o Permanent
        \o Have an object living in another thread, let the object perform
           repeated tasks such as polling a port and enable communication with
           the GUI thread.
        \o Same as above but also use a timer in the worker thread to implement
           polling. However, the best solution for polling is to avoid it
           completely. Sometimes using QSocketNotifier is an alternative.
    \endtable


    \section1 Qt Thread Basics

    QThread is a very convenient cross platform abstraction of native platform
    threads. Starting a thread is very simple. Let us look at a short piece of
    code that generates another thread which says hello in that thread and then
    exits.

    \snippet examples/tutorials/threads/hellothread/hellothread.h 1

    We derive a class from QThread and reimplement the \l{QThread::}{run()}
    method.

    \snippet examples/tutorials/threads/hellothread/hellothread.cpp 1

    The run method contains the code that will be run in a separate thread. In
    this example, a message containing the thread ID will be printed.
    QThread::start() will call the method in another thread.

    \snippet examples/tutorials/threads/hellothread/main.cpp 1

    To start the thread, our thread object needs to be instantiated. The
    \l{QThread::}{start()} method creates a new thread and calls the
    reimplemented \l{QThread::}{run()} method in this new thread. Right after
    \l{QThread::}{start()} is called, two program counters walk through the
    program code. The main function starts with only the GUI thread running and
    it should terminate with only the GUI thread running. Exiting the program
    when another thread is still busy is a programming error, and therefore,
    wait is called which blocks the calling thread until the
    \l{QThread::}{run()} method has completed.

    This is the result of running the code:

    \badcode
    hello from GUI thread  3079423696
    hello from worker thread  3076111216
    \endcode


    \section2 QObject and Threads

    A QObject is said to have a \e{thread affinity} or, in other words, that it
    lives in a certain thread. This means that, at creation time, QObject saves
    a pointer to the current thread. This information becomes relevant when an
    event is posted with \l{QCoreApplication::}{postEvent()}. The event will be
    put in the corresponding thread's event loop. If the thread where the
    QObject lives doesn't have an event loop, the event will never be delivered.

    To start an event loop, \l{QThread::}{exec()} must be called inside
    \l{QThread::}{run()}. Thread affinity can be changed using
    \l{QObject::}{moveToThread()}.

    As mentioned above, developers must always be careful when calling objects'
    methods from other threads. Thread affinity does not change this situation.
    Qt documentation marks several methods as thread-safe.
    \l{QCoreApplication::}{postEvent()} is a noteworthy example. A thread-safe
    method may be called from different threads simultaneously.

    In cases where there is usually no concurrent access to methods, calling
    non-thread-safe methods of objects in other threads may work thousands
    of times before a concurrent access occurs, causing unexpected behavior.
    Writing test code does not entirely ensure thread correctness, but it is
    still important.
    On Linux, Valgrind and Helgrind can help detect threading errors.

    The anatomy of QThread is quite interesting:

    \list
    \o QThread does not live in the new thread where \l{QThread::}{run()} is
       executed. It lives in the old thread.
    \o Most QThread methods are the thread's control interface and are meant to
       be called from the old thread. Do not move this interface to the newly
       created thread using \l{QObject::}{moveToThread()}; i.e., calling
       \l{QObject::moveToThread()}{moveToThread(this)} is regarded as bad
       practice.
    \o \l{QThread::}{exec()} and the static methods
       \l{QThread::}{usleep()}, \l{QThread::}{msleep()},
       \l{QThread::}{sleep()} are meant to be called from the newly created
       thread.
    \o Additional members defined in the QThread subclass are
       accessible by both threads. The developer is responsible for
       coordinating access. A typical strategy is to set the members before
       \l{QThread::}{start()} is called. Once the worker thread is running,
       the main thread should not touch the additional members anymore. After
       the worker has terminated, the main thread can access the additional
       members again. This is a convenient strategy for passing parameters to a
       thread before it is started as well as for collecting the result once it
       has terminated.
    \endlist

    A QObject's parent must always be in the same thread. This has a surprising
    consequence for objects generated within the \l{QThread::}{run()} method:

    \code
    void HelloThread::run()
    {
         QObject *object1 = new QObject(this);  //error, parent must be in the same thread
         QObject object2;  // OK
         QSharedPointer <QObject> object3(new QObject); // OK
    }
    \endcode

    \section2 Using a Mutex to Protect the Integrity of Data

    A mutex is an object that has \l{QMutex::}{lock()} and \l{QMutex::}{unlock()}
    methods and remembers if it is already locked. A mutex is designed to be
    called from multiple threads. \l{QMutex::}{lock()} returns immediately if
    the mutex is not locked. The next call from another thread will find the
    mutex in a locked state and then \l{QMutex::}{lock()} will block the thread
    until the other thread calls \l{QMutex::}{unlock()}. This functionality can
    make sure that a code section will be executed by only one thread at a time.

    The following line sketches how a mutex can be used to make a method
    thread-safe:

    \code
    void Worker::work()
    {
        this->mutex.lock();  // first thread can pass, other threads will be blocked here
        doWork();
        this->mutex.unlock();
    }
    \endcode

    What happens if one thread does not unlock a mutex? The result can be a
    frozen application. In the example above, an exception might be thrown and
    \c{mutex.unlock()} will never be reached. To prevent problems like this,
    QMutexLocker should be used.

    \code
    void Worker::work()
    {
        QMutexLocker locker(&mutex);  // Locks the mutex and unlocks when locker exits the scope
        doWork();
    }
    \endcode

    This looks easy, but mutexes introduce a new class of problems: deadlocks.
    A deadlock happens when a thread waits for a mutex to become unlocked, but
    the mutex remains locked because the owning thread is waiting for the first
    thread to unlock it. The result is a frozen application. Mutexes can be
    used to make a method thread safe. Most Qt methods aren't thread safe
    because there is always a performance penalty when using mutexes.

    It isn't always possible to lock and unlock a mutex in a method. Sometimes
    the need to lock spans several calls. For example, modifying a container
    with an iterator requires a sequence of several calls which should not be
    interrupted by other threads. In such a scenario, locking can be achieved
    with a mutex that is kept outside of the object to be manipulated. With an
    external mutex, the duration of locking can be adjusted to the needs of the
    operation. One disadvantage is that external mutexes aid locking, but do
    not enforce it because users of the object may forget to use it.

    \section2 Using the Event Loop to Prevent Data Corruption

    The event loops of Qt are a very valuable tool for inter-thread
    communication. Every thread may have its own event loop. A safe way of
    calling a slot in another thread is by placing that call in another
    thread's event loop. This ensures that the target object finishes the
    method that is currently running before another method is started.

    So how is it possible to put a method invocation in an event loop? Qt has
    two ways of doing this. One way is via queued signal-slot connections; the
    other way is to post an event with QCoreApplication::postEvent(). A queued
    signal-slot connection is a signal slot connection that is executed
    asynchronously. The internal implementation is based on posted events. The
    arguments of the signal are put into the event loop and the signal method
    returns immediately.

    The connected slot will be executed at a time which depends on what else is
    in the event loop.

    Communication via the event loop eliminates the deadlock problem we face
    when using mutexes. This is why we recommend using the event loop rather
    than locking an object using a mutex.

    \section2 Dealing with Asynchronous Execution

    One way to obtain a worker thread's result is by waiting for the thread
    to terminate. In many cases, however, a blocking wait isn't acceptable. The
    alternative to a blocking wait are asynchronous result deliveries with
    either posted events or queued signals and slots. This generates a certain
    overhead because an operation's result does not appear on the next source
    line, but in a slot located somewhere else in the source file. Qt
    developers are used to working with this kind of asynchronous behavior
    because it is much similar to the kind of event-driven programming used in
    GUI applications.

    \section1 Examples

    This tutorial comes with examples for Qt's three basic ways of working with
    threads. Two more examples show how to communicate with a running thread
    and how a QObject can be placed in another thread, providing service to the
    main thread.

    \list
    \o Using QThread as shown \l{Qt thread basics}{above}
    \o \l{Example 1: Using the Thread Pool}{Using the global QThreadPool}
    \o \l{Example 2: Using QtConcurrent}{Using QtConcurrent}
    \o \l{Example 3: Clock}{Communication with the GUI thread}
    \o \l{Example 4: A Permanent Thread}{A permanent QObject in another thread
       provides service to the main thread}
    \endlist

    The following examples can all be compiled and run independently. The source can
    be found in the examples directory: examples/tutorials/threads/

    \section2 Example 1: Using the Thread Pool

    Creating and destroying threads frequently can be expensive. To avoid the
    cost of thread creation, a thread pool can be used. A thread pool is a
    place where threads can be parked and fetched. We can write the same
    "hello thread" program as \l{Qt Thread Basics}{above} using the global
    thread pool. We derive a class from QRunnable. The code we want to run in
    another thread needs to be placed in the reimplemented QRunnable::run()
    method.

    \snippet examples/tutorials/threads/hellothreadpool/hellothreadpool.cpp  1

    We instantiate Work in main(), locate the global thread pool and use the
    QThreadPool::start() method. Now the thread pool runs our worker in another
    thread. Using the thread pool has a performance advantage because threads
    are not destroyed after they have finished running. They are kept in a pool
    and wait to be used again later.

    \section2 Example 2: Using QtConcurrent

    \snippet examples/tutorials/threads/helloconcurrent/helloconcurrent.cpp  1

    We write a global function hello() to implement the work. QtConcurrent::run()
    is used to run the function in another thread. The result is a QFuture.
    QFuture provides a method called \l{QFuture::}{waitForFinished()}, which
    blocks until the calculation is completed. The real power of QtConcurrent
    becomes visible when data can be made available in a container. QtConcurrent
    provides several functions that are able to process itemized data on all
    available cores simultaneously. The use of QtConcurrent is very similar to
    applying an STL algorithm to an STL container.
    \l{examples-threadandconcurrent.html}{QtConcurrent Map} is a very short and
    clear example about how a container of images can be scaled on all available
    cores. The image scaling example uses the blocking variants of the functions
    used. For every blocking function there is also a non-blocking, asynchronous
    counterpart. Getting results asynchronously is implemented with QFuture and
    QFutureWatcher.

    \section2 Example 3: Clock

    \image thread_clock.png "clock"

    We want to produce a clock application. The application has a GUI and a
    worker thread. The worker thread checks every 10 milliseconds what time it
    is. If the formatted time has changed, the result will be sent to the GUI
    thread where it is displayed.

    Of course, this is an overly complicated way of designing a clock and,
    actually, a separate thread is unnecessary. We would be better off placing
    the timer in the main thread because the calculation made in the timer slot
    is very short-lived. This example is purely for instructional use and shows
    how to communicate from a worker thread to a GUI thread. Note that
    communication in this direction is easy. We only need to add a signal
    to QThread and make a queued signal/slot connection to the main thread.
    Communication from the GUI to the worker thread is shown in the next
    example.

    \snippet examples/tutorials/threads/clock/main.cpp  1

    We've connected the \c clockThread with the label. The connection must be a
    queued signal-slot connection because we want to put the call in the event
    loop.

    \snippet examples/tutorials/threads/clock/clockthread.h  1

    We have derived a class from QThread and declared the \c sendTime() signal.

    \snippet examples/tutorials/threads/clock/clockthread.cpp  1

    The trickiest part of this example is that the timer is connected to its
    slot via a direct connection. A default connection would produce a queued
    signal-slot connection because the connected objects live in different
    threads; remember that QThread does not live in the thread it creates.

    Still it is safe to access ClockThread::timerHit() from the worker thread
    because ClockThread::timerHit() is private and only touches local variables
    and a private member that isn't touched by public methods.
    QDateTime::currentDateTime() isn't marked as thread-safe in Qt
    documentation, however we can get away with using it in this small
    example because we know that the QDateTime::currentDateTime() static
    method isn't used in any other threads.

    \section2 Example 4: A Permanent Thread

    This example shows how it is possible to have a QObject in a worker thread
    that accepts requests from the GUI thread, does polling using a timer and
    continuously reports results back to the GUI thread. The actual work
    including the polling must be implemented in a class derived from QObject.
    We have called this class \c WorkerObject in the code shown below. The
    thread-specific code is hidden in a class called \c Thread, derived from
    QThread.
    \c Thread has two additional public members. The \c launchWorker() member
    takes the worker object and moves it to another thread with a started event
    loop.
    The call blocks for a very short moment until the thread creation operation
    is completed, allowing the worker object to be used again on the next line.
    The \c Thread class's code is short but somewhat involved, so we only show
    how to use the class.

    \snippet examples/tutorials/threads/movedobject/main.cpp  1

    QMetaObject::invokeMethod() calls a slot via the event loop. The worker
    object's methods should not be called directly after the object has been
    moved to another thread. We let the worker thread do some work and polling,
    and use a timer to shut the application down after 3 seconds. Shutting the
    worker down needs some care. We call \c{Thread::stop()} to exit the event
    loop. We wait for the thread to terminate and, after this has occurred, we
    delete the worker.

    \section1 Digging Deeper

    Threading is a very complicated subject. Qt offers more classes for
    threading than we have presented in this tutorial. The following materials
    can help you go into the subject in more depth:

    \list
    \o Good video tutorials about threads with Qt can be found in the material
       from the \l{Training Day at Qt Developer Days 2009}.
    \o The \l{Thread Support in Qt} document is a good starting point into
       the reference documentation.
    \o Qt comes with several additional examples for
       \l{Threading and Concurrent Programming Examples}{QThread and QtConcurrent}.
    \o Several good books describe how to work with Qt threads. The most
       extensive coverage can be found in \e{Advanced Qt Programming} by Mark
       Summerfield, Prentice Hall - roughly 70 of 500 pages cover QThread and
       QtConcurrent.
    \endlist
*/