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<div class="ChapSects"><a href="chap9.html#X84583568784B622A">9 <span class="Heading">Synchronization variables</span></a>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap9.html#X84583568784B622A">9.1 <span class="Heading">Synchronization variables</span></a>
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<span class="ContSS"><br /><span class="nocss">  </span><a href="chap9.html#X7B55EA0E8672087C">9.1-1 CreateSyncVar</a></span>
<span class="ContSS"><br /><span class="nocss">  </span><a href="chap9.html#X85E910BF7BBF3270">9.1-2 SyncWrite</a></span>
<span class="ContSS"><br /><span class="nocss">  </span><a href="chap9.html#X7B42B29B8441B4F5">9.1-3 SyncRead</a></span>
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<h3>9 <span class="Heading">Synchronization variables</span></h3>

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<h4>9.1 <span class="Heading">Synchronization variables</span></h4>

<p>Synchronization variables (also often called dataflow variables in the literature) are variables that can be written only once; attempts to read the variable block until it has been written to.</p>

<p>Synchronization variables are created with <code class="func">CreateSyncVar</code> (<a href="chap9.html#X7B55EA0E8672087C"><span class="RefLink">9.1-1</span></a>), written with <code class="func">SyncWrite</code> (<a href="chap9.html#X85E910BF7BBF3270"><span class="RefLink">9.1-2</span></a>) and read with <code class="func">SyncRead</code> (<a href="chap9.html#X7B42B29B8441B4F5"><span class="RefLink">9.1-3</span></a>).</p>


<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">sv := CreateSyncVar();;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">RunAsyncTask(function()</span>
<span class="GAPprompt">></span> <span class="GAPinput">     Sleep(10);</span>
<span class="GAPprompt">></span> <span class="GAPinput">     SyncWrite(sv, MakeImmutable([1, 2, 3]));</span>
<span class="GAPprompt">></span> <span class="GAPinput">   end);;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">SyncRead(sv);</span>
[ 1, 2, 3 ]
</pre></div>

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<h5>9.1-1 CreateSyncVar</h5>

<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ CreateSyncVar</code>(  )</td><td class="tdright">( function )</td></tr></table></div>
<p>The function <code class="func">CreateSyncVar</code> takes no arguments. It returns a new synchronization variable. There is no need to deallocate it; the garbage collector will free the memory and all related resources when it is no longer accessible.</p>

<p><a id="X85E910BF7BBF3270" name="X85E910BF7BBF3270"></a></p>

<h5>9.1-2 SyncWrite</h5>

<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ SyncWrite</code>( <var class="Arg">syncvar</var>, <var class="Arg">obj</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p><code class="func">SyncWrite</code> attempts to assign the value <var class="Arg">obj</var> to <var class="Arg">syncvar</var>. If <var class="Arg">syncvar</var> has been previously assigned a value, the call will fail with a runtime error; otherwise, <var class="Arg">obj</var> will be assigned to <var class="Arg">syncvar</var>.</p>

<p>In order to make sure that the recipient can read the result, the <var class="Arg">obj</var> argument should not be a thread-local object; it should be public, read-only, or shared.</p>

<p><a id="X7B42B29B8441B4F5" name="X7B42B29B8441B4F5"></a></p>

<h5>9.1-3 SyncRead</h5>

<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ SyncRead</code>( <var class="Arg">syncvar</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p><code class="func">SyncRead</code> reads the value previously assigned to <var class="Arg">syncvar</var> with <code class="func">SyncWrite</code> (<a href="chap9.html#X85E910BF7BBF3270"><span class="RefLink">9.1-2</span></a>). If no value has been assigned yet, it blocks. It returns the assigned value.</p>


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