We are trying to write a portable shared library that makes use of some Qt classes for convenience (mainly QTimer and QTcpSocket); no GUI stuff, though. The according signal/slot connections appear to require some Qt event loop, so we "prime" a QCoreApplication as outlined in this answer. Accordingly, we set up a worker object that does the heavy lifting and move it to a QThread.
The problem we run into now is that the queued connections between the QThread's owner object (within the main thread) and the worker object within the QThread seem to never get handled on Linux systems, at least as long as the program that implements our library does not provide any further Qt event loop of its own in the main thread. This is not very helpful, since the data passed from the worker to the main thread should be passed further using some callback functions, which now never get called, though.
My question is thus: is there a way to get an event loop to work in the library main thread without locking it or the host program up (which seems to be the case when just putting a QCoreApplication::exec() or similar there)? Or will we have to set up a different inter-thread communication scheme (independent from Qt) in order to deal with these data transfers?
Since we do not know if the host software is going to run on a QApplication or not, ideally I'd also have a check for that before setting up a main thread event loop. Is a simple if(qApp != nullptr) enough for that?
P.S.: A few things I tried but which did not work for me, either:
Settings up a QEventLoop in a std::thread launched from the main thread (probably not working because still not in the main thread)
Setting up a QEventLoop in the main thread class and triggering its processEvents() function periodically using a QTimer (probably not working due to the missing event loop for the QTimer::timeout signal in the main function)
Starting the QCoreApplication in a std::thread (gives a run-time warning on Windows that QCoreApplication should be started in the main thread)
In Qt parlance, a callback is called Qt::DirectConnection. But of course those callbacks will run on your worker thread. But that’d be the case with any other library that uses callbacks, so Qt is not a problem here, and neither is your code: the basic idea has this property.
If the host application is not using an event loop (any event loop, not necessarily Qt’s), then there’s nothing you can do other than polling – see below.
If the host application runs an X11 event loop, then you need to ensure that your copy of Qt is using the same underlying event loop as the host application. Usually, this would be the glib’s event loop, and then it should work automagically. Otherwise, you’ll need to pass to the user the file descriptor of the synchronization primitive used by Qt’s event loop, and the user will need to integrate it into their event loop. You’ll face the same problem whether you use Qt or not: rolling your own communication method won’t fix it, since you still need a waitable primitive that will interoperate with whatever event loop the user is using.
The user can of course poll for callbacks whenever they feel like it: expose a mainPoll() method that forwards to QCoreApplication::processEvents().
Despite accepting another answer (which I deem more correct), I'd still like to mention a workaround that worked surprisingly well: We actually managed to get around the event loop/thread problems on most systems by connecting the worker thread signals with lambda functions in constructor of the class that sets up the worker.
Now, I doubt that this behaviour is properly thread-safe, and having relatively lengthy lambda functions declared in connect function calls is certainly not good style. But in case anyone else ends up struggling with this issue, this may be a short-term solution or (temporary) workaround.
Related
From Qt documentation on Performance Considerations And Suggestions I got the following:
use asynchronous, event-driven programming wherever possible
I'm not sure what that means, so would like to ask. Does it mean I should use signal/slots whenever possible (because they are asynchronous?)?
Qt signals/slots are not necessarily asynchronous. From https://doc.qt.io/qt-5/threads-qobject.html:
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.
A signal subscribed to by a slot with a Direct Connection will be essentially a method call that you can "hook up" at runtime.
Also, yes, you should probably use "asynchronous, event-driven programming" "whenever possible" for a sane definition of "whenever possible".
Obviously, don't replace all method calls between your objects with signals and slots. And when you do use signals and slots, don't always make them asynchronous (Queued) - sometimes you will want the objects subscribed to your signals to finish their "reactions" to your signals before the emitting function proceeds.
In general, when you don't really care if the subscribers of your signals get their slots invoked immedtiatelly or later, just connect them up without specifying a connection type, and Qt will use Auto Connection, which will do the right thing (thread-wise). When you do care, just specify the type of connection you want.
If you feel confused by this at first, a reasonable thing to do might also be to make all connections Queued by default - you won't really notice any performance difference, and this might prevent you from accidentally writing code that depends on the slots executing "directly", when that was not your intent.
The suggestion in your link is mainly meant for any events that get generated on your main thread, most likely by UI elements - buttons, etc. The main idea is that you want to process any input events as quickly as possible, to keep the main thread free for accepting any later events and rendering your UI, and, if the events cause any significant work to be done, move that work to another thread, and have your main thread wait for a completion signal, so that your main thread remains "responsive". If you want your UI to immedtially react to any events, for example, by initiating a "loading spinner" or displaying a progress bar, you can, of course, do that directly. This, of course, also applies to any other threads that might need to remain responsive and handle other events while a larger calculation is happening in the background.
When should moveToThread be preferred over subclassing QThread?
This link shows that both methods work. On what basis should I decide what to use from those two?
I would focus on the differences between the two methods. There isn't a general answer that fits all use cases, so it's good to understand exactly what they are to choose the best that fits your case.
Using moveToThread()
moveToThread() is used to control the object's thread affinity, which basically means setting the thread (or better the Qt event loop) from which the object will emit signals and its slots will be executed.
As shown in the documentation you linked, this can be used to run code on a different thread, basically creating a dummy worker, writing the code to run in a public slot (in the example the doWork() slot) and then using moveToThread to move it to a different event loop.
Then, a signal connected to that slot is fired. Since the object that emits the signal (the Controller in the example) lives in a different thread, and the signal is connected to our doWork method with a queued connection, the doWork method will be executed in the worker thread.
The key here is that you are creating a new event loop, run by the worker thread. Hence, once the doWork slot has started, the whole event loop will be busy until it exits, and this means that incoming signals will be queued.
Subclassing QThread()
The other method described in Qt's documentation is subclassing QThread. In this case, one overrides the default implementation of the QThread::run() method, which creates an event loop, to run something else.
There's nothing wrong with this approach itself, although there are several catches.
First of all, it is very easy to write unsafe code, because the run() method is the only one in that class that will be actually run on another thread.
If as an example, you have a member variable that you initialize in the constructor and then use in the run() method, your member is initialized in the thread of the caller and then used in the new thread.
Same story for any public method that could be called either from the caller or inside run().
Also slots would be executed from the caller's thread, (unless you do something really weird as moveToThread(this)) leading to extra confusion.
So, it is possible, but you really are on your own with this approach and you must pay extra attention.
Other approaches
There are of course alternatives to both approaches, depending on what you need. If you just need to run some code in background while your GUI thread is running you may consider using QtConcurrent::run().
However, keep in mind that QtConcurrent will use the global QThreadPool. If the whole pool is busy (meaning there aren't available threads in the pool), your code will not run immediately.
Another alternative, if you are at the least on C++11, is to use a lower level API such as std::thread.
As a starting point: use neither. In most cases, you have a unit of work that you wish to run asynchronously. Use QtConcurrent::run for that.
If you have an object that reacts to events and/or uses timers, it's a QObject that should be non-blocking and go in a thread, perhaps shared with other objects.
Such an object can also wrap blocking APIs.
Subclassing QThread is never necessary in practice. It's like subclassing QFile. QThread is a thread handle. It wraps a system resource. Overloading it is a bit silly.
Simple answer is ALWAYS.
When you move object to thread:
it is easy to write test for code
it is easy to refactor code (you can use thread but you don't have to).
you do not mix functionality of thread with business logic
there is no problem with object lifetime
When you subclass QThread
it is harder to write test
object clean up process can get very confusing leading to strange errors.
There is full description of the problem from Qt blog: You’re doing it wrong….
QtConcurrent::run is also very handy.
Please remember that by default slots are trying to jump between treads when signal is send from other thread object is assigned to. For details see documentation of Qt::ConnectionType.
QThread is low level thread abstraction, first look at high level API QtConcurrent module and QRunnable
If nothing of these is suitable for you, then read this old article, it tells how you should use QThread. Think about thread and task performed in this thread as a separate objects, don't mix them together.
So, if you need to write come custom, specific or extended thread wrapper then you should subclass QThread.
If you have QObject derived class with signals and slots, then use moveToThread on it.
In other cases use QtConcurrent, QRunnable and QThreadPoll.
I am implementing a function in library which takes a while (up to a minute). It initialize a device. Now generally any long function should run in its own thread and report to main thread when it completes but I am not sure since this function is in library.
My dilemma is this, even if I implement this in a separate thread, another thread in the application has to wait on it. If so why not let the application run this function in that thread anyways?
I could pass queue or mailbox to the library function but I would prefer a simpler mechanism where the library can be used in VB, VC, C# or other windows platforms.
Alternatively I could pass HWND of the window and the library function can post message to it when it completes instead of signaling any event. That seems like most practical approach if I have to implement the function in its own thread. Is this reasonable?
Currently my function prototype is:
void InitDevice(HANDLE hWait)
When initialization is complete than I signal bWait. This works fine but I am not convinced I should use thread anyways when another secondary thread will have to wait on InitDevice. Should I pass HWNDinstead? That way the message will be posted to the primary thread and it will make better sense with multithreading.
In general, when I write library code, I normally try to stay away from creating threads unless it's really necessary. By creating a thread, you're forcing a particular threading model on the application. Perhaps they wish to use it from a very simplistic command-line tool where a single thread is fine. Or they could use it from a GUI tool where things must be multi-threaded.
So, instead, just give the library user understanding that a function call is a long-term blocking call, some callback mechanism to monitor the progress, and finally a way to immediately halt the operation which could be used by a multi-threaded application.
What you do want to claim is being thread safe. Use mutexes to protect data items if there are other functions they can call to affect the operation of the blocking function.
I have a UI program in which i used QFuture to run a function in a different class that was heavy on processor etc. and caused my UI to crash, after using QFuture to thread this out it works fine, however I have another function that will need to be called right after however this one is within the same class as where I'm calling it from.
I have used the same code to do the second one but it just shows errors about Timers which I'm not even using, also I don't know if its right that I'm using this in the arguments:
future = new QFuture<void>;
*future = QtConcurrent::run(this, &GUI::test);
test being a local function, is it something to do with the fact that I can't have two concurrent::run calls or something, I've tried looking at the documentation which hasn't helped unfortunately
basically when this is called the UI locks up and I get some strange warnings, and I thought the point of threading it was that the UI wouldn't crash
Here's my best guess on this.
I'm guessing that you have the following situation. (Note: understanding the situation you're in is very useful when trying to come up with a solution for a problem. Often it turns out that the OP is going about something in a fundamentally wrong way and should be doing something entirely different.)
You have a GUI. You have an operation that takes a long time, which you want to run in a worker thread. And at the end of the long operation, you need to call a function on the GUI thread, probably to update the GUI or something similar.
So you launch the background task using QtConcurrent::run(&someObject, &SomeObject::task). Then you need some way to be notified, on the GUI thread, when the task is completed.
The correct way to do this is to attach a QtFutureWatcher to the future returned from run() and connect a slot on your GUI class to its finished signal. Qt's signal mechanism will ensure that the slot is called on the right thread.
I'm having trouble with Qt signals.
I don't understand how DirectConnection and QueuedConnection works?
I'd be thankful if someone will explain when to use which of these (sample code would be appreciated).
You won't see much of a difference unless you're working with objects having different thread affinities. Let's say you have QObjects A and B and they're both attached to different threads. A has a signal called somethingChanged() and B has a slot called handleChange().
If you use a direct connection
connect( A, SIGNAL(somethingChanged()), B, SLOT(handleChange()), Qt::DirectConnection );
the method handleChange() will actually run in the A's thread. Basically, it's as if emitting the signal calls the slot method "directly". If B::handleChange() isn't thread-safe, this can cause some (difficult to locate) bugs. At the very least, you're missing out on the benefits of the extra thread.
If you change the connection method to Qt::QueuedConnection (or, in this case, let Qt decide which method to use), things get more interesting. Assuming B's thread is running an event loop, emitting the signal will post an event to B's event loop. The event loop queues the event, and eventually invokes the slot method whenever control returns to it (it being the event loop). This makes it pretty easy to deal with communication between/among threads in Qt (again, assuming your threads are running their own local event loops). You don't have to worry about locks, etc. because the event loop serializes the slot invocations.
Note: If you don't know how to change a QObject's thread affinity, look into QObject::moveToThread. That should get you started.
Edit
I should clarify my opening sentence. It does make a difference if you specify a queued connection - even for two objects on the same thread. The event is still posted to the thread's event loop. So, the method call is still asynchronous, meaning it can be delayed in unpredictable ways (depending on any other events the loop may need to process). However, if you don't specify a connection method, the direct method is automatically used for connections between objects on the same thread (at least it is in Qt 4.8).
in addition to Jacob Robbins answer:
the statement "You won't see much of a difference unless you're working with objects having different thread affinities" is wrong;
emitting a signal to a direct connection within the same thread will execute the slot immediately, just like a simple function call.
emitting a signal to a queued connection within the same thread will enqueue the call into the threads event loop, thus the execution will always happen delayed.
QObject based class has a queued connection to itself
Jacob's answer is awesome. I'd just like to add a comparative example to Embedded Programming.
Coming from an embedded RTOS/ISR background, it was helpful to see the similarities in Qt's DirectConnection to Preemptive behavior of the ISRs and Qt's QueuedConnection to Queued Messages in an RTOS between tasks.
Side note: Coming from an Embedded background, it's difficult for me to not define the behavior in the programming. I never leave the argument as Auto, but that is just a personal opinion. I prefer everything to be explicitly written, and yes that gets difficult at times!