<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_mj.html#X7B55EA0E8672087C"><span class="RefLink">9.1-1</span></a>), written with <codeclass="func">SyncWrite</code> (<a href="chap9_mj.html#X85E910BF7BBF3270"><span class="RefLink">9.1-2</span></a>) and read with <code class="func">SyncRead</code> (<a href="chap9_mj.html#X7B42B29B8441B4F5"><span class="RefLink">9.1-3</span></a>).</p>
<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>
<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>
<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_mj.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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