28 lines
1.6 KiB
HTML
28 lines
1.6 KiB
HTML
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<html>
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<head>
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<!-- __CINERA_INCLUDES__ -->
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</head>
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<body>
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<!-- __CINERA_MENUS__ -->
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<!-- __CINERA_PLAYER__ -->
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<!-- __CINERA_SCRIPT__ -->
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<article id="video-notes">
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<h1><!-- __CINERA_TITLE__ --></h1>
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<p>Semaphores:</p>
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<p>A semaphore is essentially a number that the operating system keeps track of, that can be incremented and decremented.
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When you wait for a semaphore, you're essentially telling the OS to let you know when the semaphore number becomes
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greater than zero. Once it does, then the Wait() call will return and the thread can do something. Calling
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ReleaseSemaphore(), maybe a little counterintuitively, increments the semaphore, allowing any threads waiting on it to
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continue working. (Thus, it <em>releases</em> those threads to do work). It doesn't actually change the state of the semaphore
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other than making the number go up. The semaphore number goes down when a thread has successfully Wait()ed for the
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semaphore. In cases like the one demonstrated on stream, this usually means the semaphore number will go up/down really
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fast and stick close to 0 as most of the time the threads are waiting for the semaphore to increment.</p>
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<p>What this allows you to do is tell several threads at once that some work is ready without having to signal to each one
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individually. As long as each one is waiting on the same semaphore object, they'll all know when there's more work to be
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done.</p>
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</article>
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</body>
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</html>
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