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Eli Bendersky baf22e84e7 Move <img> for play.golang.org to separate line in the template 
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Fixes #235
2019-06-04 19:03:37 -07:00

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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/n5ttmOsMrrJ">
<img title="Run code" class="run" src="data:image/png;base64,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" />
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
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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>
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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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<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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