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POSegfault accessing classes across threads
 

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    <p>I'm a bit stumped on an issue I'm having with threading and C++. I'm writing a DSP plugin for Windows Media Player, and I want to send the data I intercept to a separate thread where I'll send it out on the network. I'm using a simple producer-consumer queue like the one explained <a href="http://msmvps.com/blogs/vandooren/archive/2007/01/05/creating-a-thread-safe-producer-consumer-queue-in-c-without-using-locks.aspx" rel="nofollow">here</a> </p> <p>The program is crashing on the isFull() function which just compares two integers:</p> <pre><code>bool ThreadSafeQueue::isFull() { if (inCount == outCount) //CRASH! return true; else return false; } </code></pre> <p>The thread that's doing the dequeuing:</p> <pre><code>void WMPPlugin::NetworkThread (LPVOID pParam) { ThreadSafeQueue* dataQueue = (ThreadSafeQueue*)(pParam); while (!networkThreadDone) { Sleep(2); /// so we don't hog the processor or make a race condition if (!dataQueue-&gt;isFull()) short s = dataQueue-&gt;dequeue(); if (networkThreadDone) // variable set in another process so we know to exit break; } } </code></pre> <p>The constructor of the class that's creating the consumer thread:</p> <pre><code>WMPPlugin::WMPPlugin() { // etc etc dataQueue = new ThreadSafeQueue(); _beginthread(WMPPlugin::NetworkThread, 0, dataQueue); } </code></pre> <p>inCount and outCount are just integers and they're only read here, not written. I was under the impression this meant they were thread safe. The part that writes them aren't included, but each variable is only written to by one thread, never by both. I've done my best to not include code that I don't feel is the issue, but I can include more if necessary. Thanks in advance for any help.</p>
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    1. CO"*inCount and outCount are just integers and they're only read here, not written.*" Yes, but presumably they're written _elsewhere_, and without a memory barrier apparently...
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      1. USildjarn
    2. COHow sure are you that it's crashing at the comparison? If they're both integers there's no reason why comparing them should crash the application with a segmentation fault. A segmentation fault occurs when your program tries to read from or write to invalid memory. The attempt to dereference `dataQueue` in `dataQueue->isFull()` seems like a more likely culprit.
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      1. USRobert Allan Hennigan Leahy
 

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