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Discussion about "Threads, Events and QObjects" article

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  • G Offline
    G Offline
    giesbert
    wrote on last edited by
    #3

    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... :-))

    Nokia Certified Qt Specialist.
    Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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      Guest
      wrote on last edited by
      #4

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

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        dangelog
        wrote on last edited by
        #5

        [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.

        Software Engineer
        KDAB (UK) Ltd., a KDAB Group company

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        • G Offline
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          giesbert
          wrote on last edited by
          #6

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

          Nokia Certified Qt Specialist.
          Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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          • W Offline
            W Offline
            Wolf P.
            wrote on last edited by
            #7

            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.

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            • G Offline
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              giesbert
              wrote on last edited by
              #8

              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 :-))

              Nokia Certified Qt Specialist.
              Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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              • D Offline
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                dangelog
                wrote on last edited by
                #9

                [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.

                Software Engineer
                KDAB (UK) Ltd., a KDAB Group company

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                • W Offline
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                  Wolf P.
                  wrote on last edited by
                  #10

                  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?

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                  • G Offline
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                    giesbert
                    wrote on last edited by
                    #11

                    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.

                    Nokia Certified Qt Specialist.
                    Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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                    • D Offline
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                      dangelog
                      wrote on last edited by
                      #12

                      [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?

                      Software Engineer
                      KDAB (UK) Ltd., a KDAB Group company

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                      • G Offline
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                        goetz
                        wrote on last edited by
                        #13

                        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.

                        http://www.catb.org/~esr/faqs/smart-questions.html

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                        • G Offline
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                          giesbert
                          wrote on last edited by
                          #14

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

                          Nokia Certified Qt Specialist.
                          Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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                            dangelog
                            wrote on last edited by
                            #15

                            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?

                            Software Engineer
                            KDAB (UK) Ltd., a KDAB Group company

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                            • G Offline
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                              goetz
                              wrote on last edited by
                              #16

                              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"

                              http://www.catb.org/~esr/faqs/smart-questions.html

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                                Wolf P.
                                wrote on last edited by
                                #17

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

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                                • D Offline
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                                  dangelog
                                  wrote on last edited by
                                  #18

                                  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 :)

                                  Software Engineer
                                  KDAB (UK) Ltd., a KDAB Group company

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                                  • W Offline
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                                    Wolf P.
                                    wrote on last edited by
                                    #19

                                    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.

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                                    • W Offline
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                                      Wolf P.
                                      wrote on last edited by
                                      #20

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

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                                      • F Offline
                                        F Offline
                                        Franzk
                                        wrote on last edited by
                                        #21

                                        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

                                        "Horse sense is the thing a horse has which keeps it from betting on people." -- W.C. Fields

                                        http://www.catb.org/~esr/faqs/smart-questions.html

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                                        • W Offline
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                                          Wolf P.
                                          wrote on last edited by
                                          #22

                                          [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.

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