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POProblem with waveOutWrite and waveOutGetPosition deadlock
 

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  1. POProblem with waveOutWrite and waveOutGetPosition deadlock
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    <p>I'm working on an app that plays audio continuously using the <code>waveOut...</code> API from <code>winmm.dll</code>. The app uses "leapfrog" buffers, which are basically a bunch of arrays of samples that you dump into the audio queue. Windows plays them seamlessly in sequence, and as each buffer completes Windows calls a callback function. Inside this function, I load the next set of samples into the buffer, process them however, and then dump the buffer back into the audio queue. In this way, the audio plays indefinitely.</p> <p>For animation purposes, I'm trying to incorporate <code>waveOutGetPosition</code> into the application (since the "buffer done" callbacks are irregular enough to cause jerky animation). <code>waveOutGetPosition</code> returns the current position of playback, so it's hyper-precise.</p> <p>The problem is that in my application, making calls to <code>waveOutGetPosition</code> eventually causes the application to lock up - the sound stops and the call never returns. I've boiled things down to a simple app that demonstrates the problem. You can run the app here:</p> <p><a href="http://www.musigenesis.com/SO/waveOut%20demo.exe" rel="nofollow noreferrer">http://www.musigenesis.com/SO/waveOut%20demo.exe</a></p> <p>If you just hear a tiny bit of piano over and over, it's working. It's just meant to demonstrate the problem. The source code for this project is here (all the meat is in LeapFrogPlayer.cs):</p> <p><a href="http://www.musigenesis.com/SO/WaveOutDemo.zip" rel="nofollow noreferrer">http://www.musigenesis.com/SO/WaveOutDemo.zip</a></p> <p>The first button runs the app in leapfrog mode without making the calls to <code>waveOutGetPosition</code>. If you click this, the app will play forever without breaking (the X button will close it and shut it off). The second button starts the leapfrogger and also starts a forms timer that calls the <code>waveOutGetPosition</code> and displays the current position. Click this and the app will run for a short while and then lock up. On my laptop, it usually locks up in 15-30 seconds; at most it's taken a minute.</p> <p>I have no idea how to fix this, so any help or suggestions would be most welcome. I've found very few posts on this issue, but it seems that there is a potential deadlock, either from multiple calls to <code>waveOutGetPosition</code> or from calls to that and <code>waveOutWrite</code> that occur at the same time. It's possible that I'm calling this too frequently for the system to handle.</p> <p><strong>Edit</strong>: forgot to mention, I'm running this on Windows Vista. This might not happen at all on other OSes.</p> <p><strong>Edit 2</strong>: I've found little about this problem online, except for these (unanswered) posts:</p> <p><a href="http://social.msdn.microsoft.com/Forums/en-US/windowsgeneraldevelopmentissues/thread/c6a1e80e-4a18-47e7-af11-56a89f638ad7" rel="nofollow noreferrer">http://social.msdn.microsoft.com/Forums/en-US/windowsgeneraldevelopmentissues/thread/c6a1e80e-4a18-47e7-af11-56a89f638ad7</a></p> <p><strong>Edit 3</strong>: Well, I'm now able to reproduce this problem at will. If I call <code>waveOutGetPosition</code> immediately after <code>waveOutWrite</code> (in the following line of code) the application hangs every time. It also hangs in an especially bad way - it seems to lock up my whole OS for awhile, not just the app itself. So it appears that <code>waveOutGetPosition</code> deadlocks if it occurs at <em>nearly</em> the same time as <code>waveOutWrite</code>, not just literally at the same time, which might explain why the locks aren't working for me. Yeesh.</p>
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