1. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type.
2. The recommended way of working with threads in Qt has changed since the documentation was written. It is usually better not to add signals, let alone slots, to QThread. Instead, create a QObject (derived) instance, and call moveToThread on it to move it to the thread. Put your signals and slots in this worker object instead.

QThread inherits QObject. It emits signals to indicate that the thread started or finished executing, and provides a few slots as well.

Use QThread, reimplement run and emit signals as needed. Connect the signals to the GUI thread's slots using queued signal/slot connections. One call: 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. Qt provides the signals and slots framework which allows you to do just that and is thread-safe, allowing safe communication directly from running threads to your GUI frontend. Signals allow you to.emit values, which are then picked up elsewhere in your code by slot functions which have been linked with.connect.

More interesting is that QObjects 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.

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:

• 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 (this) as the parent of an object created in the thread (since the QThread object itself was created in another thread).
• Event driven objects may only be used in a single thread. Specifically, this applies to the timer mechanism and the network module. For example, you cannot start a timer or connect a socket in a thread that is not the object's thread.
• 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 run() implementation.

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 Mandelbrot and the Blocking Fortune Client example.

Per-Thread Event Loop

Qt Thread Signal Slot Examples

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 exit(int) function and a 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 Signals and Slots Across Threads section below.

A QObject instance is said to 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 delete on a QObject from a thread other than the one that 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 DeferredDelete event will be posted, which the event loop of the object's thread will eventually pick up. By default, the thread that owns a QObject is the thread that 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 exec(), the QTimer will never emit its timeout() signal. Calling 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 postEvent()) can only be used to dispatch events to objects living in the thread from which the function is called.

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.

Qt Signal Slot Example

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 -- 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.

Signals and Slots Across Threads

Qt supports these signal-slot connection types:

• 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.'
• 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.
• 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.
• 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.

• 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.

The connection type can be specified by passing an additional argument to 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 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 Blocking Fortune Client example uses a separate thread for communicating with a TCP server asynchronously.

© 2016 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.

This blog is part of a series of blogs explaining the internals of signals and slots.

In this article, we will explore the mechanisms powering the Qt queued connections.

Summary from Part 1

In the first part, we saw that signalsare just simple functions, whose body is generated by moc. They are just calling QMetaObject::activate, with an array of pointers to arguments on the stack.Here is the code of a signal, as generated by moc: (from part 1)

QMetaObject::activatewill then look in internal data structures to find out what are the slots connected to that signal.As seen in part 1, for each slot, the following code will be executed:

So in this blog post we will see what exactly happens in queued_activateand other parts that were skipped for the BlockingQueuedConnection

Qt Event Loop

A QueuedConnection will post an event to the event loop to eventually be handled.

When posting an event (in QCoreApplication::postEvent),the event will be pushed in a per-thread queue(QThreadData::postEventList).The event queued is protected by a mutex, so there is no race conditions when threadspush events to another thread's event queue.

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

The event will be deleted right after being processed in the thread that processes it.

An event posted using a QueuedConnection is a QMetaCallEvent. When processed, that event will call the slot the same way we call them for direct connections.All the information (slot to call, parameter values, ...) are stored inside the event.

Copying the parameters

The argv coming from the signal is an array of pointers to the arguments. The problem is that these pointers point to the stack of the signal where the arguments are. Once the signal returns, they will not be valid anymore. So we'll have to copy the parameter values of the function on the heap. In order to do that, we just ask QMetaType. We have seen in the QMetaType article that QMetaType::create has the ability to copy any type knowing it's QMetaType ID and a pointer to the type.

To know the QMetaType ID of a particular parameter, we will look in the QMetaObject, which contains the name of all the types. We will then be able to look up the particular type in the QMetaType database.

queued_activate

We can now put it all together and read through the code ofqueued_activate, which is called by QMetaObject::activate to prepare a Qt::QueuedConnection slot call.The code showed here has been slightly simplified and commented:

Upon reception of this event, QObject::event will set the sender and call QMetaCallEvent::placeMetaCall. That later function will dispatch just the same way asQMetaObject::activate would do it for direct connections, as seen in Part 1

BlockingQueuedConnection

BlockingQueuedConnection is a mix between DirectConnection and QueuedConnection. Like with aDirectConnection, the arguments can stay on the stack since the stack is on the thread thatis blocked. No need to copy the arguments.Like with a QueuedConnection, an event is posted to the other thread's event loop. The event also containsa pointer to a QSemaphore. The thread that delivers the event will release thesemaphore right after the slot has been called. Meanwhile, the thread that called the signal will acquirethe semaphore in order to wait until the event is processed.

It is the destructor of QMetaCallEvent which will release the semaphore. This is good becausethe event will be deleted right after it is delivered (i.e. the slot has been called) but also whenthe event is not delivered (e.g. because the receiving object was deleted).

A BlockingQueuedConnection can be useful to do thread communication when you want to invoke afunction in another thread and wait for the answer before it is finished. However, it must be donewith care.

The dangers of BlockingQueuedConnection

You must be careful in order to avoid deadlocks.

Obviously, if you connect two objects using BlockingQueuedConnection living on the same thread,you will deadlock immediately. You are sending an event to the sender's own thread and then are locking thethread waiting for the event to be processed. Since the thread is blocked, the event will never beprocessed and the thread will be blocked forever. Qt detects this at run time and prints a warning,but does not attempt to fix the problem for you.It has been suggested that Qt could then just do a normal DirectConnection if both objects are inthe same thread. But we choose not to because BlockingQueuedConnection is something that can only beused if you know what you are doing: You must know from which thread to what other thread theevent will be sent.

The real danger is that you must keep your design such that if in your application, you do aBlockingQueuedConnection from thread A to thread B, thread B must never wait for thread A, or you willhave a deadlock again.

When emitting the signal or calling QMetaObject::invokeMethod(), you must not have any mutex lockedthat thread B might also try locking.

A problem will typically appear when you need to terminate a thread using a BlockingQueuedConnection, for example in thispseudo code:

You cannot just call wait here because the child thread might have already emitted, or is about to emitthe signal that will wait for the parent thread, which won't go back to its event loop. All the thread cleanup information transfer must only happen withevents posted between threads, without using wait(). A better way to do it would be:

The downside is that MyOperation::cleanup() is now called asynchronously, which may complicate the design.

Conclusion

This article should conclude the series. I hope these articles have demystified signals and slots,and that knowing a bit how this works under the hood will help you make better use of them in yourapplications.