Signal Slot Event Loop
The signal/slot mechanism is totally independent of any GUI event loop. The emit will return when all slots have returned. If several slots are connected to one signal, the slots will be executed one after the other, in an arbitrary order, when the signal is emitted. Signals and Slot Contd.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, we need to use insure synchronization explicitly.For Queued connections or for the AUTO connection when receiving thread lives on a different thread.
- Signal Slot Event Loop Tutorial
- Signal Slot Event Loop Software
- Qt Signal Slot Event Loop
- Signal Slot Event Loop Tool
- Signal Slot Event Loop Python
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
Signal Slot Event Loop Tutorial
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.
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This page was used to describe the new signal and slot syntax during its development. The feature is now released with Qt 5.
- Differences between String-Based and Functor-Based Connections (Official documentation)
- Introduction (Woboq blog)
- Implementation Details (Woboq blog)
Note: This is in addition to the old string-based syntax which remains valid.
- 1Connecting in Qt 5
- 2Disconnecting in Qt 5
- 4Error reporting
- 5Open questions
Connecting in Qt 5
There are several ways to connect a signal in Qt 5.
Old syntax
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)
Signal Slot Event Loop Software
New: connecting to QObject member
Here's Qt 5's new way to connect two QObjects and pass non-string objects:
Pros
- Compile time check of the existence of the signals and slot, of the types, or if the Q_OBJECT is missing.
- Argument can be by typedefs or with different namespace specifier, and it works.
- Possibility to automatically cast the types if there is implicit conversion (e.g. from QString to QVariant)
- It is possible to connect to any member function of QObject, not only slots.
Cons
- More complicated syntax? (you need to specify the type of your object)
- Very complicated syntax in cases of overloads? (see below)
- Default arguments in slot is not supported anymore.
New: connecting to simple function
The new syntax can even connect to functions, not just QObjects:
Pros
- Can be used with std::bind:
- Can be used with C++11 lambda expressions:
Cons
Qt Signal Slot Event Loop
- There is no automatic disconnection when the 'receiver' is destroyed because it's a functor with no QObject. However, since 5.2 there is an overload which adds a 'context object'. When that object is destroyed, the connection is broken (the context is also used for the thread affinity: the lambda will be called in the thread of the event loop of the object used as context).
Disconnecting in Qt 5
As you might expect, there are some changes in how connections can be terminated in Qt 5, too.
Old way
You can disconnect in the old way (using SIGNAL, SLOT) but only if
- You connected using the old way, or
- If you want to disconnect all the slots from a given signal using wild card character
Symetric to the function pointer one
Only works if you connected with the symmetric call, with function pointers (Or you can also use 0 for wild card)In particular, does not work with static function, functors or lambda functions.
New way using QMetaObject::Connection
Works in all cases, including lambda functions or functors.
Signal Slot Event Loop Tool
Asynchronous made easier
With C++11 it is possible to keep the code inline
Here's a QDialog without re-entering the eventloop, and keeping the code where it belongs:
Another example using QHttpServer : http://pastebin.com/pfbTMqUm
Error reporting
Tested with GCC.
Fortunately, IDEs like Qt Creator simplifies the function naming
Missing Q_OBJECT in class definition
Type mismatch
Open questions
Default arguments in slot
If you have code like this:
The old method allows you to connect that slot to a signal that does not have arguments.But I cannot know with template code if a function has default arguments or not.So this feature is disabled.
There was an implementation that falls back to the old method if there are more arguments in the slot than in the signal.This however is quite inconsistent, since the old method does not perform type-checking or type conversion. It was removed from the patch that has been merged.
Overload
As you might see in the example above, connecting to QAbstractSocket::error is not really beautiful since error has an overload, and taking the address of an overloaded function requires explicit casting, e.g. a connection that previously was made as follows:
connect(mySpinBox, SIGNAL(valueChanged(int)), mySlider, SLOT(setValue(int));
cannot be simply converted to:
...because QSpinBox has two signals named valueChanged() with different arguments. Instead, the new code needs to be:
Unfortunately, using an explicit cast here allows several types of errors to slip past the compiler. Adding a temporary variable assignment preserves these compile-time checks:
Some macro could help (with C++11 or typeof extensions). A template based solution was introduced in Qt 5.7: qOverload
The best thing is probably to recommend not to overload signals or slots …
… but we have been adding overloads in past minor releases of Qt because taking the address of a function was not a use case we support. But now this would be impossible without breaking the source compatibility.
Disconnect
Signal Slot Event Loop Python
Should QMetaObject::Connection have a disconnect() function?
The other problem is that there is no automatic disconnection for some object in the closure if we use the syntax that takes a closure.One could add a list of objects in the disconnection, or a new function like QMetaObject::Connection::require
Callbacks
Function such as QHostInfo::lookupHost or QTimer::singleShot or QFileDialog::open take a QObject receiver and char* slot.This does not work for the new method.If one wants to do callback C++ way, one should use std::functionBut we cannot use STL types in our ABI, so a QFunction should be done to copy std::function.In any case, this is irrelevant for QObject connections.