Include threads docs from qtdoc, rename old threads to threads-basics.

Change-Id: Ie603582809e61c2e46566a46cfc81fead4168aad
Reviewed-by: Jerome Pasion <jerome.pasion@nokia.com>

1 files changed, 627 insertions, 494 deletions

diff --git a/doc/src/core/threads.qdoc b/doc/src/core/threads.qdoc
index 09707f5dfc..cab50c6fe2 100644
--- a/doc/src/core/threads.qdoc
+++ b/doc/src/core/threads.qdoc
@@ -26,547 +26,680 @@
 ****************************************************************************/
 
 /*!
-    \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:
+    \group thread
+    \title Threading Classes
+*/
 
-    \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
+/*!
+    \page threads.html
+    \title Thread Support in Qt
+    \ingroup qt-basic-concepts
+    \brief A detailed discussion of thread handling in Qt.
 
-    \section2 When to Use Alternatives to Threads
+    \ingroup frameworks-technologies
 
-    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.
+    \nextpage Starting Threads with QThread
 
-    \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
+    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.
 
-    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.
+    Earlier versions of Qt offered an option to build the library
+    without thread support. Since Qt 4.0, threads are always enabled.
 
-    \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
+    \section1 Topics:
 
-    \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.
+    \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
 
-    \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 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:
 
-    \section1 Qt Thread Basics
+    \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
 
-    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.
+    \section1 The Threading Classes
 
-    \snippet examples/tutorials/threads/hellothread/hellothread.h 1
+    These classes are relevant to threaded applications.
 
-    We derive a class from QThread and reimplement the \l{QThread::}{run()}
-    method.
+    \annotatedlist thread
 
-    \snippet examples/tutorials/threads/hellothread/hellothread.cpp 1
+\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
 
-    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.
+    \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.
+*/
 
-    \snippet examples/tutorials/threads/hellothread/main.cpp 1
+/*!
+    \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.
+*/
 
-    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.
+/*!
+    \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.
+*/
 
-    This is the result of running the code:
+/*!
+    \page threads-reentrancy.html
+    \title Reentrancy and Thread-Safety
 
-    \badcode
-    hello from GUI thread  3079423696
-    hello from worker thread  3076111216
-    \endcode
+    \keyword reentrant
+    \keyword thread-safe
 
+    \previouspage Synchronizing Threads
+    \contentspage Thread Support in Qt
+    \nextpage Threads and QObjects
 
-    \section2 QObject and Threads
+    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:
 
-    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.
+    \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
 
-    To start an event loop, \l{QThread::}{exec()} must be called inside
-    \l{QThread::}{run()}. Thread affinity can be changed using
-    \l{QObject::}{moveToThread()}.
+    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
 
-    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.
+    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.
+*/
 
-    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.
+/*!
+    \page threads-qobject.html
+    \title Threads and QObjects
 
-    The anatomy of QThread is quite interesting:
+    \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 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.
+    \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
 
-    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.
+    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 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}
+
+    \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 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.
+    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.
+*/
 
-    \snippet examples/tutorials/threads/clock/main.cpp  1
+/*!
+    \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:
 
-    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.
+    \list
 
-    \snippet examples/tutorials/threads/clock/clockthread.h  1
+    \o QtConcurrent::map() applies a function to every item in a container,
+    modifying the items in-place.
 
-    We have derived a class from QThread and declared the \c sendTime() signal.
+    \o QtConcurrent::mapped() is like map(), except that it returns a new
+    container with the modifications.
 
-    \snippet examples/tutorials/threads/clock/clockthread.cpp  1
+    \o QtConcurrent::mappedReduced() is like mapped(), except that the
+    modified results are reduced or folded into a single result.
 
-    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.
+    \o QtConcurrent::filter() removes all items from a container based on the
+    result of a filter function.
 
-    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.
+    \o QtConcurrent::filtered() is like filter(), except that it returns a new
+    container with the filtered results.
 
-    \section2 Example 4: A Permanent Thread
+    \o QtConcurrent::filteredReduced() is like filtered(), except that the
+    filtered results are reduced or folded into a single result.
 
-    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.
+    \o QtConcurrent::run() runs a function in another thread.
 
-    \snippet examples/tutorials/threads/movedobject/main.cpp  1
+    \o QFuture represents the result of an asynchronous computation.
 
-    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.
+    \o QFutureIterator allows iterating through results available via QFuture.
 
-    \section1 Digging Deeper
+    \o QFutureWatcher allows monitoring a QFuture using signals-and-slots.
 
-    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:
+    \o QFutureSynchronizer is a convenience class that automatically
+    synchronizes several QFutures.
 
-    \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
+
+    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.
 */