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<div class="ChapSects"><a href="chap8.html#X83F63F0F7827767E">8 <span class="Heading">Semaphores</span></a>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap8.html#X83F63F0F7827767E">8.1 <span class="Heading">Semaphores</span></a>
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<div class="ContSSBlock">
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap8.html#X7AC5500483465AE3">8.1-1 CreateSemaphore</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap8.html#X834E47327A3FC5A2">8.1-2 WaitSemaphore</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap8.html#X7F0C1F1F8540CF8C">8.1-3 SignalSemaphore</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap8.html#X83D868417F0069F0">8.1-4 <span class="Heading">Simulating locks</span></a>
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<h3>8 <span class="Heading">Semaphores</span></h3>
<p><a id="X83F63F0F7827767E" name="X83F63F0F7827767E"></a></p>
<h4>8.1 <span class="Heading">Semaphores</span></h4>
<p>Semaphores are synchronized counters; they can also be used to simulate locks.</p>
<p><a id="X7AC5500483465AE3" name="X7AC5500483465AE3"></a></p>
<h5>8.1-1 CreateSemaphore</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ CreateSemaphore</code>( [<var class="Arg">value</var>] )</td><td class="tdright">( function )</td></tr></table></div>
<p>The function <code class="func">CreateSemaphore</code> takes an optional argument, which defaults to zero. It is the counter with which the semaphore is initialized.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">sem := CreateSemaphore(1);</span>
<semaphore 0x1108e81c0: count = 1>
</pre></div>
<p><a id="X834E47327A3FC5A2" name="X834E47327A3FC5A2"></a></p>
<h5>8.1-2 WaitSemaphore</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ WaitSemaphore</code>( <var class="Arg">sem</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p><code class="func">WaitSemaphore</code> receives a previously created semaphore as its argument. If the semaphore's counter is greater than zero, it decrements the counter and returns; if the counter is zero, it waits until another thread increases it via <code class="func">SignalSemaphore</code> (<a href="chap8.html#X7F0C1F1F8540CF8C"><span class="RefLink">8.1-3</span></a>), then decrements the counter and returns.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">sem := CreateSemaphore(1);</span>
<semaphore 0x1108e81c0: count = 1>
<span class="GAPprompt">gap></span> <span class="GAPinput">WaitSemaphore(sem);</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">sem;</span>
<semaphore 0x1108e81c0: count = 0>
</pre></div>
<p><a id="X7F0C1F1F8540CF8C" name="X7F0C1F1F8540CF8C"></a></p>
<h5>8.1-3 SignalSemaphore</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ SignalSemaphore</code>( <var class="Arg">sem</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p><code class="func">SignalSemaphore</code> receives a previously created semaphore as its argument. It increments the semaphore's counter and returns.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">sem := CreateSemaphore(1);</span>
<semaphore 0x1108e81c0: count = 1>
<span class="GAPprompt">gap></span> <span class="GAPinput">WaitSemaphore(sem);</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">sem;</span>
<semaphore 0x1108e81c0: count = 0>
<span class="GAPprompt">gap></span> <span class="GAPinput">SignalSemaphore(sem);</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">sem;</span>
<semaphore 0x1108e81c0: count = 1>
</pre></div>
<p><a id="X83D868417F0069F0" name="X83D868417F0069F0"></a></p>
<h5>8.1-4 <span class="Heading">Simulating locks</span></h5>
<p>In order to use semaphores to simulate locks, create a semaphore with an initial value of 1. <code class="func">WaitSemaphore</code> (<a href="chap8.html#X834E47327A3FC5A2"><span class="RefLink">8.1-2</span></a>) is then equivalent to a lock operation, <code class="func">SignalSemaphore</code> (<a href="chap8.html#X7F0C1F1F8540CF8C"><span class="RefLink">8.1-3</span></a>) is equivalent to an unlock operation.</p>
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