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Please review the following information to ensure ** the GNU Free Documentation License version 1.3 requirements ** will be met: http://www.gnu.org/copyleft/fdl.html. ** $QT_END_LICENSE$ ** ****************************************************************************/ /*! \group thread \title Threading Classes */ /*! \page threads.html \title Thread Support in Qt \ingroup qt-basic-concepts \brief A detailed discussion of thread handling in Qt. \ingroup frameworks-technologies \nextpage Starting Threads with QThread Qt provides thread support in the form of platform-independent threading classes, a thread-safe way of posting events, and signal-slot connections across threads. This makes it easy to develop portable multithreaded Qt applications and take advantage of multiprocessor machines. Multithreaded programming is also a useful paradigm for performing time-consuming operations without freezing the user interface of an application. Earlier versions of Qt offered an option to build the library without thread support. Since Qt 4.0, threads are always enabled. \section1 Topics: \list \o \l{Recommended Reading} \o \l{The Threading Classes} \o \l{Starting Threads with QThread} \o \l{Synchronizing Threads} \o \l{Reentrancy and Thread-Safety} \o \l{Threads and QObjects} \o \l{Concurrent Programming} \o \l{Thread-Support in Qt Modules} \endlist \section1 Recommended Reading This document is intended for an audience that has knowledge of, and experience with, multithreaded applications. If you are new to threading see our Recommended Reading list: \list \o \l{Threads Primer: A Guide to Multithreaded Programming} \o \l{Thread Time: The Multithreaded Programming Guide} \o \l{Pthreads Programming: A POSIX Standard for Better Multiprocessing} \o \l{Win32 Multithreaded Programming} \endlist \section1 The Threading Classes These classes are relevant to threaded applications. \annotatedlist thread \omit \list \o QThread provides the means to start a new thread. \o QThreadStorage provides per-thread data storage. \o QThreadPool manages a pool of threads that run QRunnable objects. \o QRunnable is an abstract class representing a runnable object. \o QMutex provides a mutual exclusion lock, or mutex. \o QMutexLocker is a convenience class that automatically locks and unlocks a QMutex. \o QReadWriteLock provides a lock that allows simultaneous read access. \o QReadLocker and QWriteLocker are convenience classes that automatically lock and unlock a QReadWriteLock. \o QSemaphore provides an integer semaphore (a generalization of a mutex). \o QWaitCondition provides a way for threads to go to sleep until woken up by another thread. \o QAtomicInt provides atomic operations on integers. \o QAtomicPointer provides atomic operations on pointers. \endlist \endomit \note Qt's threading classes are implemented with native threading APIs; e.g., Win32 and pthreads. Therefore, they can be used with threads of the same native API. */ /*! \page threads-starting.html \title Starting Threads with QThread \contentspage Thread Support in Qt \nextpage Synchronizing Threads A QThread instance represents a thread and provides the means to \l{QThread::start()}{start()} a thread, which will then execute the reimplementation of QThread::run(). The \c run() implementation is for a thread what the \c main() entry point is for the application. All code executed in a call stack that starts in the \c run() function is executed by the new thread, and the thread finishes when the function returns. QThread emits signals to indicate that the thread started or finished executing. \section1 Creating a Thread To create a thread, subclass QThread and reimplement its \l{QThread::run()}{run()} function. For example: \snippet doc/src/snippets/threads/threads.h 0 \codeline \snippet doc/src/snippets/threads/threads.cpp 0 \snippet doc/src/snippets/threads/threads.cpp 1 \dots \snippet doc/src/snippets/threads/threads.cpp 2 \section1 Starting a Thread Then, create an instance of the thread object and call QThread::start(). Note that you must create the QApplication (or QCoreApplication) object before you can create a QThread. The function will return immediately and the main thread will continue. The code that appears in the \l{QThread::run()}{run()} reimplementation will then be executed in a separate thread. Creating threads is explained in more detail in the QThread documentation. Note that QCoreApplication::exec() must always be called from the main thread (the thread that executes \c{main()}), not from a QThread. In GUI applications, the main thread is also called the GUI thread because it's the only thread that is allowed to perform GUI-related operations. */ /*! \page threads-synchronizing.html \title Synchronizing Threads \previouspage Starting Threads with QThread \contentspage Thread Support in Qt \nextpage Reentrancy and Thread-Safety The QMutex, QReadWriteLock, QSemaphore, and QWaitCondition classes provide means to synchronize threads. While the main idea with threads is that they should be as concurrent as possible, there are points where threads must stop and wait for other threads. For example, if two threads try to access the same global variable simultaneously, the results are usually undefined. QMutex provides a mutually exclusive lock, or mutex. At most one thread can hold the mutex at any time. If a thread tries to acquire the mutex while the mutex is already locked, the thread will be put to sleep until the thread that currently holds the mutex unlocks it. Mutexes are often used to protect accesses to shared data (i.e., data that can be accessed from multiple threads simultaneously). In the \l{Reentrancy and Thread-Safety} section below, we will use it to make a class thread-safe. QReadWriteLock is similar to QMutex, except that it distinguishes between "read" and "write" access to shared data and allows multiple readers to access the data simultaneously. Using QReadWriteLock instead of QMutex when it is possible can make multithreaded programs more concurrent. QSemaphore is a generalization of QMutex that protects a certain number of identical resources. In contrast, a mutex protects exactly one resource. The \l{threads/semaphores}{Semaphores} example shows a typical application of semaphores: synchronizing access to a circular buffer between a producer and a consumer. QWaitCondition allows a thread to wake up other threads when some condition has been met. One or many threads can block waiting for a QWaitCondition to set a condition with \l{QWaitCondition::wakeOne()}{wakeOne()} or \l{QWaitCondition::wakeAll()}{wakeAll()}. Use \l{QWaitCondition::wakeOne()}{wakeOne()} to wake one randomly selected event or \l{QWaitCondition::wakeAll()}{wakeAll()} to wake them all. The \l{threads/waitconditions}{Wait Conditions} example shows how to solve the producer-consumer problem using QWaitCondition instead of QSemaphore. Note that Qt's synchronization classes rely on the use of properly aligned pointers. For instance, you cannot use packed classes with MSVC. */ /*! \page threads-reentrancy.html \title Reentrancy and Thread-Safety \keyword reentrant \keyword thread-safe \previouspage Synchronizing Threads \contentspage Thread Support in Qt \nextpage Threads and QObjects Throughout the documentation, the terms \e{reentrant} and \e{thread-safe} are used to mark classes and functions to indicate how they can be used in multithread applications: \list \o A \e thread-safe function can be called simultaneously from multiple threads, even when the invocations use shared data, because all references to the shared data are serialized. \o A \e reentrant function can also be called simultaneously from multiple threads, but only if each invocation uses its own data. \endlist Hence, a \e{thread-safe} function is always \e{reentrant}, but a \e{reentrant} function is not always \e{thread-safe}. By extension, a class is said to be \e{reentrant} if its member functions can be called safely from multiple threads, as long as each thread uses a \e{different} instance of the class. The class is \e{thread-safe} if its member functions can be called safely from multiple threads, even if all the threads use the \e{same} instance of the class. \note Qt classes are only documented as \e{thread-safe} if they are intended to be used by multiple threads. If a function is not marked as thread-safe or reentrant, it should not be used from different threads. If a class is not marked as thread-safe or reentrant then a specific instance of that class should not be accessed from different threads. \section1 Reentrancy C++ classes are often reentrant, simply because they only access their own member data. Any thread can call a member function on an instance of a reentrant class, as long as no other thread can call a member function on the \e{same} instance of the class at the same time. For example, the \c Counter class below is reentrant: \snippet doc/src/snippets/threads/threads.cpp 3 \snippet doc/src/snippets/threads/threads.cpp 4 The class isn't thread-safe, because if multiple threads try to modify the data member \c n, the result is undefined. This is because the \c ++ and \c -- operators aren't always atomic. Indeed, they usually expand to three machine instructions: \list 1 \o Load the variable's value in a register. \o Increment or decrement the register's value. \o Store the register's value back into main memory. \endlist If thread A and thread B load the variable's old value simultaneously, increment their register, and store it back, they end up overwriting each other, and the variable is incremented only once! \section1 Thread-Safety Clearly, the access must be serialized: Thread A must perform steps 1, 2, 3 without interruption (atomically) before thread B can perform the same steps; or vice versa. An easy way to make the class thread-safe is to protect all access to the data members with a QMutex: \snippet doc/src/snippets/threads/threads.cpp 5 \snippet doc/src/snippets/threads/threads.cpp 6 The QMutexLocker class automatically locks the mutex in its constructor and unlocks it when the destructor is invoked, at the end of the function. Locking the mutex ensures that access from different threads will be serialized. The \c mutex data member is declared with the \c mutable qualifier because we need to lock and unlock the mutex in \c value(), which is a const function. \section1 Notes on Qt Classes Many Qt classes are \e{reentrant}, but they are not made \e{thread-safe}, because making them thread-safe would incur the extra overhead of repeatedly locking and unlocking a QMutex. For example, QString is reentrant but not thread-safe. You can safely access \e{different} instances of QString from multiple threads simultaneously, but you can't safely access the \e{same} instance of QString from multiple threads simultaneously (unless you protect the accesses yourself with a QMutex). Some Qt classes and functions are thread-safe. These are mainly the thread-related classes (e.g. QMutex) and fundamental functions (e.g. QCoreApplication::postEvent()). \note Terminology in the multithreading domain isn't entirely standardized. POSIX uses definitions of reentrant and thread-safe that are somewhat different for its C APIs. When using other object-oriented C++ class libraries with Qt, be sure the definitions are understood. */ /*! \page threads-qobject.html \title Threads and QObjects \previouspage Reentrancy and Thread Safety \contentspage Thread Support in Qt \nextpage Concurrent Programming 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 \l{QObject}s 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. \section1 QObject Reentrancy QObject is reentrant. Most of its non-GUI subclasses, such as QTimer, QTcpSocket, QUdpSocket, QFtp, 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: \list \o \e{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 (\c this) as the parent of an object created in the thread (since the QThread object itself was created in another thread). \o \e{Event driven objects may only be used in a single thread.} Specifically, this applies to the \l{timers.html}{timer mechanism} and the \l{QtNetwork}{network module}. For example, you cannot start a timer or connect a socket in a thread that is not the \l{QObject::thread()}{object's thread}. \o \e{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 \l{QThread::run()}{run()} implementation. \endlist 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 \l{threads/mandelbrot}{Mandelbrot} and the \l{network/blockingfortuneclient}{Blocking Fortune Client} example. \section1 Per-Thread Event Loop Each thread can have its own event loop. The initial thread starts its event loops using QCoreApplication::exec(); other threads can start an event loop using QThread::exec(). Like QCoreApplication, QThread provides an \l{QThread::exit()}{exit(int)} function and a \l{QThread::quit()}{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 \l{Signals and Slots Across Threads} section below. \image threadsandobjects.png Threads, objects, and event loops A QObject instance is said to \e 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(). Note that for QObjects that are created before QApplication, QObject::thread() returns zero. This means that the main thread will only handle posted events for these objects; other event processing is not done at all for objects with no thread. Use the QObject::moveToThread() function to change the thread affinity for an object and its children (the object cannot be moved if it has a parent). Calling \c delete on a QObject from a thread other than the one that \e 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 \l{QEvent::DeferredDelete}{DeferredDelete} event will be posted, which the event loop of the object's thread will eventually pick up. By default, the thread that \e owns a QObject is the thread that \e 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 \l{QThread::exec()}{exec()}, the QTimer will never emit its \l{QTimer::timeout()}{timeout()} signal. Calling \l{QObject::deleteLater()}{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 \l{QCoreApplication::postEvent()}{postEvent()}) can only be used to dispatch events to objects living in the thread from which the function is called. \section1 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 -- \e 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. \section1 Signals and Slots Across Threads Qt supports these signal-slot connection types: \list \o \l{Qt::AutoConnection}{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." \o \l{Qt::DirectConnection}{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. \o \l{Qt::QueuedConnection}{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. \o \l{Qt::BlockingQueuedConnection}{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. \o \l{Qt::UniqueConnection}{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. \endlist The connection type can be specified by passing an additional argument to \l{QObject::connect()}{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 \l{threads/mandelbrot}{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 \l{network/blockingfortuneclient}{Blocking Fortune Client} example uses a separate thread for communicating with a TCP server asynchronously. */ /*! \page threads-qtconcurrent.html \title Concurrent Programming \previouspage Threads and QObjects \contentspage Thread Support in Qt \nextpage Thread-Support in Qt Modules \target qtconcurrent intro The QtConcurrent namespace provides high-level APIs that make it possible to write multi-threaded programs without using low-level threading primitives such as mutexes, read-write locks, wait conditions, or semaphores. Programs written with QtConcurrent automatically adjust the number of threads used according to the number of processor cores available. This means that applications written today will continue to scale when deployed on multi-core systems in the future. QtConcurrent includes functional programming style APIs for parallel list processing, including a MapReduce and FilterReduce implementation for shared-memory (non-distributed) systems, and classes for managing asynchronous computations in GUI applications: \list \o QtConcurrent::map() applies a function to every item in a container, modifying the items in-place. \o QtConcurrent::mapped() is like map(), except that it returns a new container with the modifications. \o QtConcurrent::mappedReduced() is like mapped(), except that the modified results are reduced or folded into a single result. \o QtConcurrent::filter() removes all items from a container based on the result of a filter function. \o QtConcurrent::filtered() is like filter(), except that it returns a new container with the filtered results. \o QtConcurrent::filteredReduced() is like filtered(), except that the filtered results are reduced or folded into a single result. \o QtConcurrent::run() runs a function in another thread. \o QFuture represents the result of an asynchronous computation. \o QFutureIterator allows iterating through results available via QFuture. \o QFutureWatcher allows monitoring a QFuture using signals-and-slots. \o QFutureSynchronizer is a convenience class that automatically synchronizes several QFutures. \endlist Qt Concurrent supports several STL-compatible container and iterator types, but works best with Qt containers that have random-access iterators, such as QList or QVector. The map and filter functions accept both containers and begin/end iterators. STL Iterator support overview: \table \header \o Iterator Type \o Example classes \o Support status \row \o Input Iterator \o \o Not Supported \row \o Output Iterator \o \o Not Supported \row \o Forward Iterator \o std::slist \o Supported \row \o Bidirectional Iterator \o QLinkedList, std::list \o Supported \row \o Random Access Iterator \o QList, QVector, std::vector \o Supported and Recommended \endtable Random access iterators can be faster in cases where Qt Concurrent is iterating over a large number of lightweight items, since they allow skipping to any point in the container. In addition, using random access iterators allows Qt Concurrent to provide progress information trough QFuture::progressValue() and QFutureWatcher:: progressValueChanged(). The non in-place modifying functions such as mapped() and filtered() makes a copy of the container when called. If you are using STL containers this copy operation might take some time, in this case we recommend specifying the begin and end iterators for the container instead. */ /*! \page threads-modules.html \title Thread-Support in Qt Modules \previouspage Concurrent Programming \contentspage Thread Support in Qt \section1 Threads and the SQL Module A connection can only be used from within the thread that created it. Moving connections between threads or creating queries from a different thread is not supported. In addition, the third party libraries used by the QSqlDrivers can impose further restrictions on using the SQL Module in a multithreaded program. Consult the manual of your database client for more information \section1 Painting in Threads QPainter can be used in a thread to paint onto QImage, QPrinter, and QPicture paint devices. Painting onto QPixmaps and QWidgets is \e not supported. On Mac OS X the automatic progress dialog will not be displayed if you are printing from outside the GUI thread. Any number of threads can paint at any given time, however only one thread at a time can paint on a given paint device. In other words, two threads can paint at the same time if each paints onto separate QImages, but the two threads cannot paint onto the same QImage at the same time. Note that on X11 systems without FontConfig support, Qt cannot render text outside of the GUI thread. You can use the QFontDatabase::supportsThreadedFontRendering() function to detect whether or not font rendering can be used outside the GUI thread. \section1 Threads and Rich Text Processing The QTextDocument, QTextCursor, and \link richtext.html all related classes\endlink are reentrant. Note that a QTextDocument instance created in the GUI thread may contain QPixmap image resources. Use QTextDocument::clone() to create a copy of the document, and pass the copy to another thread for further processing (such as printing). \section1 Threads and the SVG module The QSvgGenerator and QSvgRenderer classes in the QtSvg module are reentrant. \section1 Threads and Implicitly Shared Classes Qt uses an optimization called \l{implicit sharing} for many of its value class, notably QImage and QString. Beginning with Qt 4, implicit shared classes can safely be copied across threads, like any other value classes. They are fully \l{Reentrancy and Thread-Safety}{reentrant}. The implicit sharing is really \e implicit. In many people's minds, implicit sharing and multithreading are incompatible concepts, because of the way the reference counting is typically done. Qt, however, uses atomic reference counting to ensure the integrity of the shared data, avoiding potential corruption of the reference counter. Note that atomic reference counting does not guarantee \l{Reentrancy and Thread-Safety}{thread-safety}. Proper locking should be used when sharing an instance of an implicitly shared class between threads. This is the same requirement placed on all \l{Reentrancy and Thread-Safety}{reentrant} classes, shared or not. Atomic reference counting does, however, guarantee that a thread working on its own, local instance of an implicitly shared class is safe. We recommend using \l{Signals and Slots Across Threads}{signals and slots} to pass data between threads, as this can be done without the need for any explicit locking. To sum it up, implicitly shared classes in Qt 4 are really \e implicitly shared. Even in multithreaded applications, you can safely use them as if they were plain, non-shared, reentrant value-based classes. */