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  1. PO
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    <p>This might be a long shot, but here is another possibility [the former answer may be more likely] (something I just ran across, but had known before): The use of Synchronize may be causing the deadlock. Here is a blog about why this happens: <a href="http://www.finalbuilder.com/Resources/Blogs/PostId/675/Delphi-Workaround-for-TThread-SynchronizeWaitFor-.aspx" rel="nofollow">Delphi-Workaround-for-TThread-SynchronizeWaitFor-.aspx</a></p> <p>The pertinent point from the article:</p> <blockquote> <p>Thread A calls Synchronize(MethodA)</p> <p>Thread B calls Synchronize(MethodB)</p> <p>Then, inside the context of the Main Thread: </p> <p>Main thread calls CheckSynchronize() while processing messages</p> <p>CheckSynchronize is implemented to batch-process all waiting calls(*). So it picks up the queue of waiting calls (containing MethodA and MethodB) and loops through them one by one. </p> <p>MethodA executes in the main thread's context. Assume MethodA calls ThreadB.WaitFor </p> <p>WaitFor calls CheckSynchronize to process any waiting calls to Synchronize </p> <p>In theory, this should then process ThreadB's Synchronize(MethodB), allowing Thread B to complete. However, MethodB is already a possession of the first CheckSynchronize call, so it never gets called.</p> <p>DEADLOCK!</p> </blockquote> <p><a href="http://qc.embarcadero.com/wc/qcmain.aspx?d=22267" rel="nofollow">Embarcadero QC article</a> describing the problem in more detail.</p> <p>While I don't see any ProcessMessages calls in the above code, or for that matter, a WaitFor that would be called during a Synchronize, it could still be a problem that at the point a synchronize is called, another thread calls the synchronize as well - but the main thread has already synchronized and is blocking. </p> <p>This didn't click with me at first, because I tend to avoid Synchronize calls like the plague and usually design UI updates from threads using other methods like message passing and thread safe lists with message notification instead of synchronize calls.</p>
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    1. POMultithreaded file upload synchronization
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    1. USSilverKnight
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    1. COThanks again for getting into so much details on this. And sorry for the delayed replies, I'm traveling these days... What you described here also occurred to me and I thought Synchronize was the problem. I used it because there is no SendMessage/PostMessage on OSX or at least I don't know if there is an alternative. So Synchronize was an easy solution at that moment. Some time ago I rewrote a lot of the code and I don't have this freeze any more, but I don't know where the problem was. Could have been related to the Indy TCP components I used, because they were not very stable on the OSX...
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    2. CONot a problem. I was looking into something else and came across this post that didn't have an answer. I used it as a learning exercise to see what the monitor class would do (I've never used it). I'm always interested in different techniques that could improve my threaded code (mostly in lowering CPU usage, but also in different communication methods). It has been an interesting dive into the class, and hopefully someone else will benefit from the discussion as well.
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