Qt slot thread
Jan 24, · How To Really, Truly Use QThreads; thread, SLOT(quit())); Than there The documentation of Qt thread managment has been significantly enhanced. the second one calls the method inside the current thread, so the emit is done properly. If you want to make signals go from one thread to another (the main program. Qt ; Threads and 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.
"How to use QThread in the right way (Part 1)"
Since the thread is blocked, the event will never be processed and the thread will be blocked forever. Finally, we can find both usages in the documentation of QThread. Thanks to a mechanism called queued connections, it is safe to connect signals and slots across different threads. Per-Thread Event Loop Each thread can have its own event loop. Instead, allocate such resources in the main function slot such as process in this case as when that is called the object will be on the new thread instance and thus it will own the resource.
Like with a DirectConnection, the arguments can stay on the stack since the stack is on the thread that is 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 contains a pointer to a QSemaphore. 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. Dead lock detected while activating a BlockingQueuedConnection: This is good because the event will be deleted right after it is delivered i. A BlockingQueuedConnection can be useful to do thread communication when you want to invoke a function in another thread and wait for the answer before it is finished. However, it must be done with 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 the thread waiting for the event to be processed.
Since the thread is blocked, the event will never be processed 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 in the same thread. But we choose not to because BlockingQueuedConnection is something that can only be used if you know what you are doing: You must know from which thread to what other thread the event will be sent.
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This is rather intuitive and easy to used. Hughes, one of the Qt core developers, recommend that use worker objects by moving them to the thread using QObject:: 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. The run implementation is for a thread what the main entry point is for the application. Usage To run some code in a new thread, subclass QThread and reimplement its run function.
Consider that the former will be executed in main thread while the latter is executed in worker thread, mutex or other facility is needed. Usage 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:: 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 From main thread: 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 ; and some other add following line to the thread constructor. As onTimeout which is a member of our Thread object, get called from the creating thread too. Both of them are bad uasge?! Usage As none of the member of QThread object are designed to be called from the worker thread.
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