Discussion about "Threads, Events and QObjects" article
-
[quote author="Gerolf Reinwardt" date="1292312695"]Hi Peppe,
As I can't send you emails via the system, I try to give some feedback to the article this way:
[/quote]I think "private messages" were somehow disabled, hope I've enabled them now...
[quote]
First of all: Congratulations, a very good article. I also thought about writing something about this, but now you have :-))
[/quote]Thank you so much :)
[quote]
I just have some small extensions / add-ons:- In the chapter Events and the event loop you state, events are always asynchronous, thats true, if you don't send them via QCoreApplication::sendEvent(...)
[/quote]
That's true, but I don't know how to bring that in. All in all, the article (as of now!) is not an in-depth review of Qt's event system; along with sendEvent, many many things are missing (event filters, QCoreApplication::notify, how to create and send custom events, etc...)
[quote]
- Please mare a bigger warning on using QCoreApplication::processEvents(). I saw many applications crashing, because they used it and were surprised that the quit was executed although they are currently processing some events. And that happened esecially if tsendPostedEvents was called in a library...
[/quote]
What do you mean? Right now there's a simple example by an hypotetic recursion into a slot. Do you think I should stressing on it even more?
[quote]
- Regrading the dialogs, they spin a local event loop :-)
[/quote]
I know, in fact there's a note there.
[quote]
- Threads and Objects:
** please state that the thread afinity depends on the running thread, that creates the object. If I create an object inside the QThread constructor, it depends on the creator's thread. Often seen problems here :-)
[/quote]
Good point. Will do.
[quote]
** you could add that QObject::moveToThread() is a push, not a poll, which could make it a bit clearer, what is meant by "... we must use it from the thread the object is living in...". Some of my colleges understood push and poll better..
[/quote]Good point again; the Qt docs use a good lexicon :)
- In the chapter Events and the event loop you state, events are always asynchronous, thats true, if you don't send them via QCoreApplication::sendEvent(...)
-
bq. What do you mean? Right now there’s a simple example by an hypotetic recursion into a slot. Do you think I should stressing on it even more?
I meant that it can not only fore recursion, it can also fore a stutdown or delete of an object, that you currently work on. Think of deleteLater, which is executed by the event loop. This can happen when you call @QApplication::processEvents@. And then, perhaps, an object, where yiou think it exists, is away. And I saw suxgh tghings, especially not recursions but unexpected object deletion which result in a crash. And then all you get is:
bq. It crashes, I don't know why, but here the memory is freed.....And you are the happy person to find the bug, they introduced by spinning the event loop out of scope...
So I would make a bigger note with many exclamation marks and warnings... :-))
-
[quote author="Gerolf Reinwardt" date="1292356379"]bq. What do you mean? Right now there’s a simple example by an hypotetic recursion into a slot. Do you think I should stressing on it even more?
I meant that it can not only fore recursion, it can also fore a stutdown or delete of an object, that you currently work on. Think of deleteLater, which is executed by the event loop. This can happen when you call @QApplication::processEvents@. And then, perhaps, an object, where yiou think it exists, is away. And I saw suxgh tghings, especially not recursions but unexpected object deletion which result in a crash. And then all you get is:
bq. It crashes, I don't know why, but here the memory is freed.....And you are the happy person to find the bug, they introduced by spinning the event loop out of scope...
So I would make a bigger note with many exclamation marks and warnings... :-))[/quote]
Ok, I got it now :-)
Well, I've integrated your suggestions here and there. Give it a read if you want to :)
[quote author="chetankjain" date="1292393650"]moving this to Wiki forum, thats the right place to initiate discussions on wiki articles[/quote]
Ops! You're right, sorry about that.
-
bq. how to start, stop, join a thread under (at least) one major operating system;
Is joining threads such a common pattern?
I've always imagined threads as autonomous agents. This I was able to produce programmatically and at some point they were finished and forgotten. Whether or not the program code orphaned never seemed to matter to me. -
[quote author="Wolf P." date="1292845577"]bq. how to start, stop, join a thread under (at least) one major operating system;
Is joining threads such a common pattern?
I've always imagined threads as autonomous agents. This I was able to produce programmatically and at some point they were finished and forgotten. Whether or not the program code orphaned never seemed to matter to me.[/quote]It's a pattern, that's all. For instance, a possible use case is telling a worker thread to finish, then actually wait for it to end (by joining it), then deallocate some resources used by it.
-
[quote author="Wolf P." date="1292847084"]Gerolf, I see. The somewhat outdated framework I worked with, provided only the forking.
Do you know a good real-world example?Peppe, as I see, to join means simply to wait?[/quote]
I was pretty sure it was standard lexicon when it comes to threading: it means "block the calling thread until the target thread terminates"; and yes, it's what QThread::wait() does. See for instance:
http://www.kernel.org/doc/man-pages/online/pages/man3/pthread_join.3.html
http://download.oracle.com/javase/6/docs/api/java/lang/Thread.html#join()
http://perldoc.perl.org/threads.html#DESCRIPTIONNow that you're telling me, perhaps should I change that term?
-
-
Sorry for this naive comment. Finally I found that this is Qt terminology: http://doc.qt.nokia.com/latest/threads-reentrancy.html
-
It is not just Qt terminology. It's general programming terminology and something everyone who does at least the slightest bit of multi-threading should know about.
http://en.wikipedia.org/wiki/Reentrant_(subroutine)
http://en.wikipedia.org/wiki/Thread_safety