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non-blocking-channel-operations
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example: Non-Blocking Channel Operations</title>
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</head>
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<body>
<div class="example" id="non-blocking-channel-operations">
<h2><a href="./">Go by Example</a>: Non-Blocking Channel Operations</h2>
<table>
<tr>
<td class="docs">
<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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<td class="code empty leading">
</td>
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<td class="docs">
</td>
<td class="code leading">
<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" />
<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
</pre>
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<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kn">import</span> <span class="s">"fmt"</span>
</pre>
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<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">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>
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<tr>
<td class="docs">
<p>Here’s a non-blocking receive. If a value is
available on <code>messages</code> then <code>select</code> will take
the <code><-messages</code> <code>case</code> with that value. If not
it will immediately take the <code>default</code> case.</p>
</td>
<td class="code leading">
<pre class="chroma">
<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"><-</span><span class="nx">messages</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"received message"</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="nf">Println</span><span class="p">(</span><span class="s">"no message received"</span><span class="p">)</span>
<span class="p">}</span>
</pre>
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<td class="docs">
<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>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">msg</span> <span class="o">:=</span> <span class="s">"hi"</span>
<span class="k">select</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">messages</span> <span class="o"><-</span> <span class="nx">msg</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"sent message"</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="nf">Println</span><span class="p">(</span><span class="s">"no message sent"</span><span class="p">)</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<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>
</td>
<td class="code">
<pre class="chroma">
<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"><-</span><span class="nx">messages</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"received message"</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"><-</span><span class="nx">signals</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"received signal"</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="nf">Println</span><span class="p">(</span><span class="s">"no activity"</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
</td>
<td class="code">
<pre class="chroma"><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>
</td>
</tr>
</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> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
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