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    <p>All the API functions that you refer to have in common that they send(!) some message to the target window. UpdateWindow is probably the most obvious, because it needs to send WM_PAINT. Notice also that it "sends" the message and doesn't post to the queue (for UpdateWindow, the MSDN documentation calls this out explicitly, for the others it may be less obvious). </p> <p>Also notice that windows have <a href="http://blogs.msdn.com/b/oldnewthing/archive/2005/10/10/479124.aspx" rel="nofollow">thread affinity</a> as alluded to in some of the comments. Among other things this means that messages to that window are only ever received/dispatched on one thread. If you send a message to a window of another thread, the operating system is left with the task to determine when it should dispatch that message (i.e. call the window procedure). This (dispatching incoming sent messages) only happens during certain API calls during which it can be assumed to be safe to invoke the window procedure with a random message. The relevant times are during GetMessage and PeekMessage*.</p> <p>So if your window owning thread (also called UI thread) is pumping messages normally, incoming sent messages are also quickly dispatched. From your question it seems however, that your UI thread is currently busy. If the second thread then invokes one of said functions, then it will block until the first thread provides a chance to have the sent messages dispatched.</p> <p>As others have said, it is usually a good idea to keep user interface code on one dedicated UI thread (although exceptions - as always - prove the rule). And it is definitely necessary (for a good user experience) to have window owning threads be responsive to messages at all times. If your UI thread also has to wait on some synchronization objects, you may find MsgWaitForMultipleObjects helpful.</p> <p>*the list might not be complete.</p>
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    1. POWin32 API deadlocks while using different threads
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    1. USDaniel Flassig
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    1. COThank you very much for the answer. Unfortunately it is not that simple to keep everything in one thread in my case since I am writing plugin for another application, and this other application choose to call the window from the another thread.
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    2. COWhatever the details of your architecture may be, the take-away message is: the window owning thread needs to be pumping messages. Otherwise APIs like UpdateWindow cannot be called. What you could of course do is define some WM_APP specific messages to tell the window to resize itself and post those to the queue (I'm not saying such a design is advisable)
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      1. USDaniel Flassig
 

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