Nd the index of the signal and of the slot Keep in an internal map which signal is connected to what slots When emitting a signal, QMetaObject::activate is called. It calls qt metacall (generated by moc) with the slot index which call the actual slot. Connecting in Qt 5. There are several ways to connect a signal in Qt 5. Qt 5 continues to support the old string-based syntax for connecting signals and slots defined in a QObject or any class that inherits from QObject (including QWidget). Connect( sender, SIGNAL( valueChanged( QString, QString ) ), receiver, SLOT( updateValue( QString ) ) ).
- @sierdzio said in Cannot connect signal and slot from different thread.: Also, remember to use Qt::QueuedConnection for your inter-thread connections - then you don't have to worry about locking any mutexes and such. Thank you sierdzio for your help!
- The connection mechanism uses a vector indexed by signals. But all the slots waste space in the vector and there are usually more slots than signals in an object. So from Qt 4.6, a new internal signal index which only includes the signal index is used. While developing with Qt, you only need to know about the absolute method index.
While the purpose of threads is to allow code to run in parallel, there are times where threads must stop and wait for other threads. For example, if two threads try to write to the same variable simultaneously, the result is undefined. The principle of forcing threads to wait for one another is called mutual exclusion. It is a common technique for protecting shared resources such as data.
Qt provides low-level primitives as well as high-level mechanisms for synchronizing threads.
Low-Level Synchronization Primitives
QMutex is the basic class for enforcing mutual exclusion. A thread locks a mutex in order to gain access to a shared resource. If a second thread tries to lock the mutex while it is already locked, the second thread will be put to sleep until the first thread completes its task and unlocks the mutex.
QReadWriteLock is similar to QMutex, except that it distinguishes between 'read' and 'write' access. When a piece of data is not being written to, it is safe for multiple threads to read from it simultaneously. A QMutex forces multiple readers to take turns to read shared data, but a QReadWriteLock allows simultaneous reading, thus improving parallelism.
QSemaphore is a generalization of QMutex that protects a certain number of identical resources. In contrast, a QMutex 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 Slot Thread Contexto
QWaitCondition synchronizes threads not by enforcing mutual exclusion but by providing a condition variable. While the other primitives make threads wait until a resource is unlocked, QWaitCondition makes threads wait until a particular condition has been met. To allow the waiting threads to proceed, call wakeOne() to wake one randomly selected thread or wakeAll() to wake them all simultaneously. The Wait Conditions Example shows how to solve the producer-consumer problem using QWaitCondition instead of QSemaphore.
Note: Qt's synchronization classes rely on the use of properly aligned pointers. For instance, you cannot use packed classes with MSVC.
Qt Signals And Slots Tutorial
These synchronization classes can be used to make a method thread safe. However, doing so incurs a performance penalty, which is why most Qt methods are not made thread safe.
Risks
If a thread locks a resource but does not unlock it, the application may freeze because the resource will become permanently unavailable to other threads. This can happen, for example, if an exception is thrown and forces the current function to return without releasing its lock.
Another similar scenario is a deadlock. For example, suppose that thread A is waiting for thread B to unlock a resource. If thread B is also waiting for thread A to unlock a different resource, then both threads will end up waiting forever, so the application will freeze.
Convenience classes
QMutexLocker, QReadLocker and QWriteLocker are convenience classes that make it easier to use QMutex and QReadWriteLock. They lock a resource when they are constructed, and automatically unlock it when they are destroyed. They are designed to simplify code that use QMutex and QReadWriteLock, thus reducing the chances that a resource becomes permanently locked by accident.
High-Level Event Queues
Qt's event system is very useful for inter-thread communication. Every thread may have its own event loop. To call a slot (or any invokable method) in another thread, place that call in the target thread's event loop. This lets the target thread finish its current task before the slot starts running, while the original thread continues running in parallel.
To place an invocation in an event loop, make a queued signal-slot connection. Whenever the signal is emitted, its arguments will be recorded by the event system. The thread that the signal receiver lives in will then run the slot. Alternatively, call QMetaObject::invokeMethod() to achieve the same effect without signals. In both cases, a queued connection must be used because a direct connection bypasses the event system and runs the method immediately in the current thread.
There is no risk of deadlocks when using the event system for thread synchronization, unlike using low-level primitives. However, the event system does not enforce mutual exclusion. If invokable methods access shared data, they must still be protected with low-level primitives.
Having said that, Qt's event system, along with implicitly shared data structures, offers an alternative to traditional thread locking. If signals and slots are used exclusively and no variables are shared between threads, a multithreaded program can do without low-level primitives altogether.
See also QThread::exec() and Threads and QObjects.
© 2020 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.
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.
Unfortunately, documentation do not specify that ‘lives' stands for and no examples is available. I have tried the following code:
main.h:
main.cpp:
Qt Signal Slot Thread
Output is:
MySlot() is never called :(. What I'm doing wrong?
There are quite a few problems with your code :
- like said by Evan the emit keyword is missing
- all your objects live in the main thread, only the code in the run methods live in other threads, which means that the MySlot slot would be called in the main thread and I'm not sure that's what you want
- your slot will never be called since the main event loop will never been launched : your two calls to wait() will only timeout after a very long time (and you'll probably kill your application before that happens) and I don't think that's what you want either, anyway they really have no use in your code.
This code would most likely work (though I have not tested it) and I think it does what you want it to do :
- @sierdzio said in Cannot connect signal and slot from different thread.: Also, remember to use Qt::QueuedConnection for your inter-thread connections - then you don't have to worry about locking any mutexes and such. Thank you sierdzio for your help!
- The connection mechanism uses a vector indexed by signals. But all the slots waste space in the vector and there are usually more slots than signals in an object. So from Qt 4.6, a new internal signal index which only includes the signal index is used. While developing with Qt, you only need to know about the absolute method index.
While the purpose of threads is to allow code to run in parallel, there are times where threads must stop and wait for other threads. For example, if two threads try to write to the same variable simultaneously, the result is undefined. The principle of forcing threads to wait for one another is called mutual exclusion. It is a common technique for protecting shared resources such as data.
Qt provides low-level primitives as well as high-level mechanisms for synchronizing threads.
Low-Level Synchronization Primitives
QMutex is the basic class for enforcing mutual exclusion. A thread locks a mutex in order to gain access to a shared resource. If a second thread tries to lock the mutex while it is already locked, the second thread will be put to sleep until the first thread completes its task and unlocks the mutex.
QReadWriteLock is similar to QMutex, except that it distinguishes between 'read' and 'write' access. When a piece of data is not being written to, it is safe for multiple threads to read from it simultaneously. A QMutex forces multiple readers to take turns to read shared data, but a QReadWriteLock allows simultaneous reading, thus improving parallelism.
QSemaphore is a generalization of QMutex that protects a certain number of identical resources. In contrast, a QMutex 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 Slot Thread Contexto
QWaitCondition synchronizes threads not by enforcing mutual exclusion but by providing a condition variable. While the other primitives make threads wait until a resource is unlocked, QWaitCondition makes threads wait until a particular condition has been met. To allow the waiting threads to proceed, call wakeOne() to wake one randomly selected thread or wakeAll() to wake them all simultaneously. The Wait Conditions Example shows how to solve the producer-consumer problem using QWaitCondition instead of QSemaphore.
Note: Qt's synchronization classes rely on the use of properly aligned pointers. For instance, you cannot use packed classes with MSVC.
Qt Signals And Slots Tutorial
These synchronization classes can be used to make a method thread safe. However, doing so incurs a performance penalty, which is why most Qt methods are not made thread safe.
Risks
If a thread locks a resource but does not unlock it, the application may freeze because the resource will become permanently unavailable to other threads. This can happen, for example, if an exception is thrown and forces the current function to return without releasing its lock.
Another similar scenario is a deadlock. For example, suppose that thread A is waiting for thread B to unlock a resource. If thread B is also waiting for thread A to unlock a different resource, then both threads will end up waiting forever, so the application will freeze.
Convenience classes
QMutexLocker, QReadLocker and QWriteLocker are convenience classes that make it easier to use QMutex and QReadWriteLock. They lock a resource when they are constructed, and automatically unlock it when they are destroyed. They are designed to simplify code that use QMutex and QReadWriteLock, thus reducing the chances that a resource becomes permanently locked by accident.
High-Level Event Queues
Qt's event system is very useful for inter-thread communication. Every thread may have its own event loop. To call a slot (or any invokable method) in another thread, place that call in the target thread's event loop. This lets the target thread finish its current task before the slot starts running, while the original thread continues running in parallel.
To place an invocation in an event loop, make a queued signal-slot connection. Whenever the signal is emitted, its arguments will be recorded by the event system. The thread that the signal receiver lives in will then run the slot. Alternatively, call QMetaObject::invokeMethod() to achieve the same effect without signals. In both cases, a queued connection must be used because a direct connection bypasses the event system and runs the method immediately in the current thread.
There is no risk of deadlocks when using the event system for thread synchronization, unlike using low-level primitives. However, the event system does not enforce mutual exclusion. If invokable methods access shared data, they must still be protected with low-level primitives.
Having said that, Qt's event system, along with implicitly shared data structures, offers an alternative to traditional thread locking. If signals and slots are used exclusively and no variables are shared between threads, a multithreaded program can do without low-level primitives altogether.
See also QThread::exec() and Threads and QObjects.
© 2020 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.
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.
Unfortunately, documentation do not specify that ‘lives' stands for and no examples is available. I have tried the following code:
main.h:
main.cpp:
Qt Signal Slot Thread
Output is:
MySlot() is never called :(. What I'm doing wrong?
There are quite a few problems with your code :
- like said by Evan the emit keyword is missing
- all your objects live in the main thread, only the code in the run methods live in other threads, which means that the MySlot slot would be called in the main thread and I'm not sure that's what you want
- your slot will never be called since the main event loop will never been launched : your two calls to wait() will only timeout after a very long time (and you'll probably kill your application before that happens) and I don't think that's what you want either, anyway they really have no use in your code.
This code would most likely work (though I have not tested it) and I think it does what you want it to do :
Qt Signal Slot Example
Now MyObject will live in thread2 (thanks to moveToThread).
MySignal should be sent from thread1 (thought I'm not sure on that one, it might be sent from main thread, it doesn't really matter).
No event loop is needed in thread1 since emitting a signal doesn't need an event loop. An event loop is needed in thread2 (lanched by exec()) to receive the signal.
MySlot will be called in thread2.
Do not subclass QThread for Qt 4.4+
While Aiua's answer is good, I want to point out some issues with QThread and Qt 4.6 or 4.7.
This article sums it up: http://blog.qt.io/blog/2010/06/17/youre-doing-it-wrong/
Lack of Documentation on Qt's part
Unfortunately the problem stems from a lack of updates to documentation. Prior to Qt 4.4 QThread had no default run() implementation, which meant that you had to subclass QThread in order to use it.
Qt Connect Signal Slot
If you're using Qt 4.6 or 4.7 then you almost certainly should not subclass QThread.
Use moveToThread
The key to getting slots to execute in a worker thread is to use the moveToThread method as Aiua pointed out.
Qt Signal Slot Performance
Tags: qt
