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gobyexample/public/non-blocking-channel-operations
Oleg Butuzov 646c8b39da bugfix (mmcgrana/gobyexample#294) sha1 sum calculator 
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<title>Go by Example: Non-Blocking Channel Operations</title>
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<div class="example" id="non-blocking-channel-operations">
<h2><a href="./">Go by Example</a>: Non-Blocking Channel Operations</h2>
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<p>Basic sends and receives on channels are blocking.
However, we can use <code>select</code> with a <code>default</code> clause to
implement <em>non-blocking</em> sends, receives, and even
non-blocking multi-way <code>select</code>s.</p>
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<a href="http://play.golang.org/p/TFv6-7OVNVq"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
</pre></div>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">messages</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</span><span class="p">)</span>
<span class="nx">signals</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">bool</span><span class="p">)</span>
</pre></div>
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<p>Here&rsquo;s a non-blocking receive. If a value is
available on <code>messages</code> then <code>select</code> will take
the <code>&lt;-messages</code> <code>case</code> with that value. If not
it will immediately take the <code>default</code> case.</p>
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<div class="highlight"><pre> <span class="k">select</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">msg</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">messages</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;received message&quot;</span><span class="p">,</span> <span class="nx">msg</span><span class="p">)</span>
<span class="k">default</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;no message received&quot;</span><span class="p">)</span>
<span class="p">}</span>
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<p>A non-blocking send works similarly. Here <code>msg</code>
cannot be sent to the <code>messages</code> channel, because
the channel has no buffer and there is no receiver.
Therefore the <code>default</code> case is selected.</p>
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<div class="highlight"><pre> <span class="nx">msg</span> <span class="o">:=</span> <span class="s">&quot;hi&quot;</span>
<span class="k">select</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">messages</span> <span class="o">&lt;-</span> <span class="nx">msg</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;sent message&quot;</span><span class="p">,</span> <span class="nx">msg</span><span class="p">)</span>
<span class="k">default</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;no message sent&quot;</span><span class="p">)</span>
<span class="p">}</span>
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<p>We can use multiple <code>case</code>s above the <code>default</code>
clause to implement a multi-way non-blocking
select. Here we attempt non-blocking receives
on both <code>messages</code> and <code>signals</code>.</p>
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<div class="highlight"><pre> <span class="k">select</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">msg</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">messages</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;received message&quot;</span><span class="p">,</span> <span class="nx">msg</span><span class="p">)</span>
<span class="k">case</span> <span class="nx">sig</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">signals</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;received signal&quot;</span><span class="p">,</span> <span class="nx">sig</span><span class="p">)</span>
<span class="k">default</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;no activity&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
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<div class="highlight"><pre><span class="gp">$</span> go run non-blocking-channel-operations.go
<span class="go">no message received</span>
<span class="go">no message sent</span>
<span class="go">no activity</span>
</pre></div>
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</table>
<p class="next">
Next example: <a href="closing-channels">Closing Channels</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/non-blocking-channel-operations">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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