Discussion about "Threads, Events and QObjects" article

[dangelog]

[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 :)

[giesbert]

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... :-))

[Guest]

moving this to Wiki forum, thats the right place to initiate discussions on wiki articles

[dangelog]

[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.

[giesbert]

Hi,
thanks again, the article is really good, and I think, the pitfalls are clearer now. So looking fporward to your next article :-))

[Wolf P.]

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.

[giesbert]

Joining threads is common, if you put parts of a bigger thing to threads to do that in paralell and want to wait for all results being finished. I know, it can also be done with QtConcurrent but some programs are older than QtConcurrent :-))

[dangelog]

[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.

[Wolf P.]

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?

[giesbert]

Join means wait for finish, yes.
I have some in my work, but they are not open source :-)
removing objects, which are are internally multithreadded for example. You have to close all threads before removing the object.

[dangelog]

[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#DESCRIPTION

Now that you're telling me, perhaps should I change that term?

[goetz]

I must admit, I did not read the wiki article, but have some experience with QThread. The term "join a thread" was unknown to me, so although this might be a common term in threading in general, it seems it is not used in Qt world.

[giesbert]

It is common in Boost threadding for example, that's where I know it from. If it is a general term... ???

[dangelog]

I can put a footnote and/or a link there, just in case. The point is that you should know what QThread::wait() is for. What do you think?

[goetz]

Knowledge about QThread::wait() is definitely needed.

maybe this wording is a bit more clear:

"how to start, stop, wait for a thread (aka join a thread in boost and others) under (at least) one major operating system"

[Wolf P.]

[quote author="Volker" date="1292850647"] "how to start, stop, wait for a thread [/quote]
This sounds very familiar to me (FYI: Win32/VCL).

[dangelog]

Ok, I changed the sentence to
[quote]
how to start and stop a thread, and wait for it to finish, under (at least) one major operating system;
[/quote]

Thank you all for your feedback :)

[Wolf P.]

Another choice of terminology has me confused: reentrant. Thread-safe I understood, but the definition of reentrancy seemed not clearly demarcated from it. Maybe a slight reworking of the text could help for a better understanding of the difference.

[Wolf P.]

Sorry for this naive comment. Finally I found that this is Qt terminology: http://doc.qt.nokia.com/latest/threads-reentrancy.html

[Franzk]

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