Discussion about "Threads, Events and QObjects" article
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It is common in Boost threadding for example, that's where I know it from. If it is a general term... ???
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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?
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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"
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[quote author="Volker" date="1292850647"] "how to start, stop, wait for a thread [/quote]
This sounds very familiar to me (FYI: Win32/VCL). -
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 :)
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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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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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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 -
[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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To be honest, the little problem is that there might be some confusion due to literature and/or other toolkits. That's why I specified that in the article I follow the Qt conventions; anyway, I added a link to http://doc.qt.nokia.com/latest/threads-reentrancy.html, just to make it even more clear :-)
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It's good. The basics are the same across libraries though (bad library if it diverges...).
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After reading "Reentrancy and Thread-Safety":http://doc.qt.nokia.com/latest/threads-reentrancy.html , I think the term reentrance is not the best choice, because re-entering (in a sense of entering it twice) isn't really possible. (My problem seems to be that I'm familiar with the non-reentrance of MS-DOS.)
Classes that can be safely used by different threads at different times, I would name just safe. To be honest, I would not discuss it at all, but rather mark those that cannot be used from different threads at different times, maybe as "tread-local" or so.
Am I completely wrong here?
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Hi Wolf,
Thread-local is normally used for members/memory. So there is the "ThreadLocalStorage":http://en.wikipedia.org/wiki/Thread-local_storage for example. "Reentrant":http://en.wikipedia.org/wiki/Reentrant_(subroutine) and "thread-safety":http://en.wikipedia.org/wiki/Thread_safety are general terms (from my understanding) which are widely used. So I would stay with the used terms. -
[quote author="Wolf P." date="1292922748"]
Classes that can be safely used by different threads at different times, I would name just safe. To be honest, I would not discuss it at all, but rather mark those that cannot be used from different threads at different times, maybe as "tread-local" or so.Am I completely wrong here?[/quote]
There are three possible cases:
- Classes/methods/objects/functions/data structures which (...whose instances) can be used at the same time from multiple threads, without the need of serializing cuncurrent accesses. That's what thread-safe means.
- Classes/methods/objects/functions/data structures which (...whose instances) cannot be used at the same time from multiple threads, therefore all accesses must be externally serialized. That's what reentrant means. Notice that
** Thread-safe implies reentrant
** Taking a reentrant class and forcibly serializing all possible accesses with a mutex makes it thread-safe - Classes/methods/objects/functions/data structures which (...whose instances) cannot be used from multiple threads at all. There isn't a specific name for this case (we usually say "not thread-safe nor reentrant"). For instance, QWidget and all of its subclasses are usable only from the main thread.
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Ok. The term reentrant (in the given context) is now clear to me.
But please note the following example: when you call the Win32 function SendMessage (sending to another process) and get reply-blocked, your process can be re-entered by SendMessage calls from other processes. So, for me, reentrance (in general) has also something to do with recursion.
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I added a toc to the page. (and to the "wiki syntax help":http://developer.qt.nokia.com/wiki/WikiSyntax too)
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Unfortunately, the term "reentrancy" is not really clearly defined in computer world.
Michael Suess complains in his "blog entry:"http://www.thinkingparallel.com/2007/06/08/the-most-overused-word-in-parallel-programming-reentrancy/ about the situation. Reading the comments, it seems that there are at least two definitions of reentrancy in the context of single threading (regarding recursive function calls) and in the context of multi threading. This may confuse the people with a single threading background (DOS!) when heading over to multithreaded programming.
Anyways, the definitions are out in the wild and as long as we are in Qt context, we should use the terms defined by the Trolls to avoid further confusion. Otherwise we would need another round of BabelFishing for these kinds of things, but I doubt there's any T-Shirts to win :-)
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[quote author="Volker" date="1292932902"]but I doubt there's any T-Shirts to win :-)[/quote]Huh, imagine T-shirts stating something about your re-entrancy...
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[quote author="Franzk" date="1292935405"][quote author="Volker" date="1292932902"]but I doubt there's any T-Shirts to win :-)[/quote]Huh, imagine T-shirts stating something about your re-entrancy...
[/quote]What a about
"I'm a male - I'm not thread safe!"
[Edit - ok, a bit offtopic now :-) Volker]
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"I am NOT re-entrant"