1. Qt Signal Slot Example
2. Qt Thread Safe Signal Slot Machines
3. Qt Thread Safe Signal Slot Booster
4. Qt Thread Safe Signal Slot Machine
5. Qt Connect Signal Slot

In this case if you emit a signal from one thread, and catching it in another one (e.g. In main GUI thread) - Qt will put a slot's call to the message queue and will make all calls sequentially. Read this for further info - http://qt-project.org/doc/qt-4.8/threads-qobject.html#signals-and-slots-across-threads.

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2. Qt - SigSlot - Boost Libraries Qt was the original signal/slots implementation, but it Sigslot and Boost on the other hand are pure ISO C, but both have some disadvantages. None of these are thread-safe and it can be somewhat inconvenient manually Qt documentation states that signals and slots can be direct, queued and auto.
3. 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 ConnectionThe slot is invoked immediately, when the signal is emitted.
4. The thread that delivers the event will release the semaphore right after the slot has been called. Meanwhile, the thread that called the signal will acquire the semaphore in order to wait until the event is processed.

how in BOOST send a signal in a thread and have the corresponding slot executed in another thread?

In Qt for instance if you emit a signal in a thread other that the GUI thread, the signal is enqueued and executed later in the GUI thread, is there a way to do that with boost?

thanks

For an event loop use boost::asio::io_service. You can post tasks inside this object and have another thread execute them, in a thread safe way:

Messaging and Signaling in C++, But as this blog post is more on signaling then system events. Qt signal/slot implementation is thread safe, so that you can use it to send messages important, as anything UI related should run in the main thread of Qt, anything that I use this in a different program to have one widget for editing flag like Almost all classes provided by Boost.Signals2 are thread safe and can be used in multithreaded applications. For example, objects of type boost::signals2::signal and boost::signals2::connection can be accessed from different threads. On the other hand, boost::signals2::shared_connection_block is not thread safe.

Not directly, because boost does not provide an event loop.

To have a signal handled in another thread, that another thread needs to be checking the queue of handlers it should run and execute them (which usually means some kind of event-loop). Boost does not provide one, so you'll need to get it from elsewhere or write it.

If you have an event-loop, that does not provide signals, (or implement some simple solution with queues) you should be able to (ab)use boost.signals2 (not boost.signals, because that version is not thread-safe) by overriding the operator+= to wrap each handler in something, that will queue it for execution in the other thread. You might even be able to implement it for signals with return values (which is not supported by Qt, but is supported by boost), but you'll have to be careful to avoid dead-lock.

[PDF] Boost.Signals2, Signals2 library is an implementation of a managed signals and slots system. This documentation describes a thread-safe variant of the original Boost. so we put 'Hello' into a group that must be executed before the group possible to set up tracking in a post-constructor which is called after the object has been created​ To have a signal handled in another thread, that another thread needs to be checking the queue of handlers it should run and execute them (which usually means some kind of event-loop). Boost does not provide one, so you'll need to get it from elsewhere or write it.

Signals & Slots, Signals and slots are made possible by Qt's meta-object system. to one signal, the slots will be executed one after the other, in the order they have valueChanged() , and it has a slot which other objects can send signals to. The context object provides information about in which thread the receiver should be executed. Special behavior for C++: If a thread is sent a signal using pthread_kill() and that thread does not handle the signal, then destructors for local objects may not be executed. Usage notes. The SIGTHSTOP and SIGTHCONT signals can be issued by this function. pthread_kill() is the only function that can issue SIGTHSTOP or SIGTHCONT. Returned value

Chila's answer is correct, but it's missing one important thing:A boost::thread object will only call the function its passed once. Since the boost::io_service has no work to do until the signal is emitted, the thread will finish immediately. To counter this there is a boost::asio::io_service::work class.Before you call the run() method of the io_service you should create a work object and pass it the io_service:

Note: At the time of writing (boost 1.67) this method is already deprecated and you are supposed to use io_context::executor_work_guard (basically same functionality as io_service::work). I was not able to compile when using the new method though, and the work solution is still working in boost 1.67.

Qt Signal Slot Example

Slots, It also implements a few conditional (event) related classes. Qt - SigSlot - Boost Libraries Qt was the original signal/slots implementation, but it Sigslot and Boost on the other hand are pure ISO C++, but both have some disadvantages. None of these are thread-safe and it can be somewhat inconvenient manually 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.

For some reason, the assignment operator of boost::asio::executor_work_guard<boost::asio::io_context::executor_type> is deleted, but you still can construct it.

Here's my version of the code that posts some movable Event object and processes it on the thread running io_context::run():

Qt Thread Safe Signal Slot Machines

It requires C++14 and was tested with VS2017 and GCC 6.4 with thread & memory sanitizers.

Observer pattern with Stl, boost and qt, A comparison between the Qt signal and slot mechanism and some Each slot is a potential callback ○ Adds more run-time introspection, The Synapse library ○ Another signals/slot like library ○ Submitted to Very similar to boost::​signals2 ○ Have the ability to transfer control between threads 29; 30. So, when the thread is created from the create_thread method it will call the io_service::run method and it passes the io_service object as an argument. Typically one io_service object can be used with multiple socket objects.

QThreads: Are You Using Them Wrong?, Show related SlideShares at end The Basics of QThread QThread manages one thread of execution ○ The Signal Slot Connections and Threads ○ Qt::​DirectConnection have same thread affinity: Direct ○ If objects have different thread It implies you want to send cross-thread signals to yourself. 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.

What do I do if a slot is not invoked?, A practical checklist to debug your signal/slot connections that an event loop is running in the thread the receiver has affinity with;; that all the arguments Using this signal is very easy – it just acts like a flag, but you can wait for it as well as read it. The following unit test (also in GitHub) shows how the signal makes it easy for threads to set gates on each other. Note that the final signal in this example could have been done with thread.join(), but wasn’t for the purposes of the test.

[Boost-users] Signals2 benchmark, I want to test it against boost::signals2 to get an idea of how well it performs. Suppose one thread disconnects a slot while another fires a signal Each Signal-type has its own corresponding vector of slots defined within the Emitter. the copy being made instead of every slot being called and executed. POSIX requires that signal is thread-safe, and specifies a list of async-signal-safe library functions that may be called from any signal handler. Signal handlers are expected to have C linkage and, in general, only use the features from the common subset of C and C++. It is implementation-defined if a function with C++ linkage can be used as a

Comments

• THX for this very helpfull sample ! Since boost::signal is deprecated I have to use boost::signals2::signal<>.

A short history

Long long ago, subclass QThread and reimplement its run() function is the only recommended way of using QThread. This is rather intuitive and easy to used. But when SLOTS and Qt event loop are used in the worker thread, some users do it wrong. So Bradley T. Hughes, one of the Qt core developers, recommend that use worker objects by moving them to the thread using QObject::moveToThread . Unfortunately, some users went on a crusade against the former usage. So Olivier Goffart, one of the former Qt core developers, tell the subclass users: You were not doing so wrong. Finally, we can find both usages in the documentation of QThread.

QThread::run() is the thread entry point

From the Qt Documentation, we can see that

A QThread instance represents a thread and provides the means to start() a thread, which will then execute the reimplementation of QThread::run(). The run() implementation is for a thread what the main() entry point is for the application.

As QThread::run() is the thread entry point, it is rather intuitive to use the Usage 1.

Usage 1-0

To run some code in a new thread, subclass QThread and reimplement its run() function.

For example

The output more or less look like:

Usage 1-1

As QThread::run() is the thread entry point, so it easy to undersand that, all the codes that are not get called in the run() function directly won't be executed in the worker thread.

In the following example, the member variable m_stop will be accessed by both stop() and run(). Consider that the former will be executed in main thread while the latter is executed in worker thread, mutex or other facility is needed.

The output is more or less like

You can see that the Thread::stop() is executed in the main thread.

Usage 1-2 (Wrong Usage)

Though above examples are easy to understand, but it's not so intuitive when event system(or queued-connection) is introduced in worker thread.

For example, what should we do if we want to do something periodly in the worker thread?

• Create a QTimer in the Thread::run()
• Connect the timeout signal to the slot of Thread

At first glance, the code seems fine. When the thread starts executing, we setup a QTimer thats going to run in the current thread's event queue. We connect the onTimeout() to the timeout signal. Then we except it works in the worker thread?

But, the result of the example is

Qt Thread Safe Signal Slot Booster

Oh, No!!! They get called in the main thread instead of the work thread.

Very interesting, isn't it? (We will discuss what happened behined this in next blog)

How to solve this problem

In order to make the this SLOT works in the worker thread, some one pass the Qt::DirectConnection to the connect() function,

and some other add following line to the thread constructor.

Both of them work as expected. But ...

The second usage is wrong,

Even though this seems to work, it’s confusing, and not how QThread was designed to be used(all of the functions in QThread were written and intended to be called from the creating thread, not the thread that QThread starts)

In fact, according to above statements, the first workaround is wrong too. As onTimeout() which is a member of our Thread object, get called from the creating thread too.

Both of them are bad uasge?! what should we do?

Usage 1-3

As none of the member of QThread object are designed to be called from the worker thread. So we must create an independent worker object if we want to use SLOTS.

The result of the application is

Problem solved now!

Qt Thread Safe Signal Slot Machine

Though this works perfect, but you may have notice that, when event loop QThread::exec() is used in the worker thread, the code in the QThread::run() seems has nothing to do with QThread itself.

So can we move the object creation out of the QThread::run(), and at the same time, the slots of they will still be called by the QThread::run()?

Usage 2-0

If we only want to make use of QThread::exec(), which has been called by QThread::run() by default, there will be no need to subclass the QThread any more.

• Create a Worker object
• Do signal and slot connections
• Move the Worker object to a sub-thread
• Start thread

The result is:

As expected, the slot doesn't run in the main thread.

In this example, both of the QTimer and Worker are moved to the sub-thread. In fact, moving QTimer to sub-thread is not required.

Simply remove the line timer.moveToThread(&t); from above example will work as expected too.

The difference is that:

In last example,

• The signal timeout() is emitted from sub-thread
• As timer and worker live in the same thread, their connection type is direct connection.
• The slot get called in the same thead in which signal get emitted.

While in this example,

• The signal timeout() emitted from main thread,
• As timer and worker live in different threads, their connection type is queued connection.
• The slot get called in its living thread, which is the sub-thread.

Thanks to a mechanism called queued connections, it is safe to connect signals and slots across different threads. If all the across threads communication are done though queued connections, the usual multithreading precautions such as QMutex will no longer need to be taken.

In short

• Subclass QThread and reimplement its run() function is intuitive and there are still many perfectly valid reasons to subclass QThread, but when event loop is used in worker thread, it's not easy to do it in the right way.
• Use worker objects by moving them to the thread is easy to use when event loop exists, as it has hidden the details of event loop and queued connection.

Reference

Qt Connect Signal Slot

• http://blog.qt.digia.com/blog/2010/06/17/youre-doing-it-wrong/
• http://woboq.com/blog/qthread-you-were-not-doing-so-wrong.html
• http://ilearnstuff.blogspot.com/2012/08/when-qthread-isnt-thread.html