Qt documentation states that signals and slots can be direct, queued and auto. It also stated that if object that owns slot 'lives' in a thread different from object that owns signal, emitting such signal will be like posting message - signal emit will return instantly and slot method will be called in target thread's event loop. The Gambling Control Division qt slots and signals threads has a list of FAQs along with a chat bot (see lower right corner logo). Persons, firms, or organizations that sell, lease, qt slots and signals threads or otherwise distribute bingo supplies (e. It's a wrapper around a thread object. This wrapper provides the signals, slots and methods to easily use the thread object within a Qt project. To use it, prepare a QObject subclass with all your desired functionality in it.
Hello all !
I have some difficulties to find an appropriate way to solve a problem of communication between objects in differents threads (although I already read the Signals/slots accross threads). Here is my problem:
I have an object a of class A living in a thread T1.
I have an object b of class B living in a thread T2.
I have an object b of class B living in a thread T2.
Both objects a and b can emit signals and have slots to manage the signals emitted by the object living in the other thread (Object a has slots to manage signals from object b, and object b have slots to manage signals from object a).
Once I have moved the object to their respective threads, I connect the signals/slots using Qt::connect(..., Qt::QueuedConnection), but once the signals is emitted, the slot is never executed.
I don't use Qt::DirectConnection (because it's quite the same that invoking directly the method, and I don't want this), and I would like to avoid to manage an event dispatcher or an event loop in the parent thread.
Does someone know how to use correctly the Qt threads in order to make the communication between signals/slots between objects living in different threads (none living in the main thread) ?
Thank you a lot for your answers !
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 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
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.
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.
Qt Signal Slot With 2 Arguments
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.
Qt Signals And Slots Across Threads
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.
Qt Signals And Slots Tutorial
Similarly, the Blocking Fortune Client example uses a separate thread for communicating with a TCP server asynchronously.
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