Discussion about "Threads, Events and QObjects" article

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... :-))

? Offline

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

Wolf P.

[quote author="Franzk" date="1292858262"]...and something everyone who does at least the slightest bit of multi-threading should know about.[/quote] thx for the WP references :)

BTW: I did some multithreaded coding without problems, and without thinking about reentrance.

Discussion about &quot;Threads, Events and QObjects&quot; article

Discussion about "Threads, Events and QObjects" article