Qt Signal Thread
Qt Thread Example
Qt Virtual Slot
Qt Public Slots
Qt Signal And Slots
Qt Thread Pool

This is the sequel of my previous article explaining the implementation details of the signals and slots. In the Part 1, we have seen the general principle and how it works with the old syntax.In this blog post, we will see the implementation details behind the new function pointer based syntax in Qt5.
I wanted to cite this mailing list question from me about models and views on different threads in Qt (along with the ensuing answers). The qt-interest mailing list entries from 2009 seem to have all but disappeared from the web, but I found this one in an Internet Archive cache off of 'gmane'.

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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.

Thread Safety and Reentrancy. Using Thread Pools. Qt Concurrent: High level Threading API. Managing Processes. Inter Process Communication. Qt is the platform of choice for thousands of software projects, both open source and commercial, including the. Making multithreading easy. 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.

Earlier versions of Qt offered an option to build the library without thread support. Since Qt 4.0, threads are always enabled.

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.

Topics:

The Threading Classes

Qt includes the following thread classes:

QThread provides the means to start a new thread.
QThreadStorage provides per-thread data storage.
QMutex provides a mutual exclusion lock, or mutex.
QMutexLocker is a convenience class that automatically locks and unlocks a QMutex.
QReadWriteLock provides a lock that allows simultaneous read access.
QReadLocker and QWriteLocker are convenience classes that automatically lock and unlock a QReadWriteLock.
QSemaphore provides an integer semaphore (a generalization of a mutex).
QWaitCondition provides a way for threads to go to sleep until woken up by 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.

Creating a Thread

To create a thread, subclass QThread and reimplement its run() function. For example:

Then, create an instance of the thread object and call QThread::start(). The code that appears in the 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 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.

In addition, you must create the QApplication (or QCoreApplication) object before you can create a QThread.

Synchronizing Threads

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 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 Semaphores example shows a typical application of semaphores: synchronizing access to a circular buffer between a producer and a consumer.

Qt Signal Thread

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 wakeOne() or wakeAll(). Use wakeOne() to wake one randomly selected event or wakeAll() to wake them all. The 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.

Reentrancy and Thread-Safety

Throughout the Qt documentation, the terms reentrant and thread-safe are used to specify how a function can be used in multithreaded applications:

A reentrant function can be called simultaneously by multiple threads provided that each invocation of the function references unique data.
A thread-safe function can be called simultaneously by multiple threads when each invocation references shared data. All access to the shared data is serialized.

By extension, a class is said to be reentrant if each and every one of its functions can be called simultaneously by multiple threads on different instances of the class. Similarly, the class is said to be thread-safe if the functions can be called by different threads on the same instance.

Classes in the documentation will be documented as thread-safe only if they are intended to be used by multiple threads. How to distribute poker chips 4 players.

Note that the terminology in this domain isn't entirely standardized. POSIX uses a somewhat different definition of reentrancy and thread-safety for its C APIs. When dealing with an object-oriented C++ class library such as Qt, the definitions must be adapted.

Most C++ classes are inherently reentrant, since they typically only reference member data. Any thread can call such a member function on an instance of the class, as long as no other thread is calling a member function on the same instance. For example, the Counter class below is reentrant:

The class isn't thread-safe, because if multiple threads try to modify the data member n, the result is undefined. This is because C++'s ++ and -- operators aren't necessarily atomic. Indeed, they usually expand to three machine instructions:

Load the variable's value in a register.
Increment or decrement the register's value.
Store the register's value back into main memory.

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!

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 p

Qt Slot Thread-safety