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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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<!DOCTYPE html>
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<meta charset="utf-8">
<title>Go by Example: Closing Channels</title>
<link rel=stylesheet href="site.css">
</head>
<body>
<div class="example" id="closing-channels">
<h2><a href="./">Go by Example</a>: Closing Channels</h2>
<table>
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<td class="docs">
<p><em>Closing</em> a channel indicates that no more values
will be sent on it. This can be useful to communicate
completion to the channel&rsquo;s receivers.</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<a href="http://play.golang.org/p/mkz69rVMHs6">
<img title="Run code" class="run" src="data:image/png;base64,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" />
</a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<td class="docs">
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<td class="code leading">
<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
</pre></div>
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<td class="docs">
<p>In this example we&rsquo;ll use a <code>jobs</code> channel to
communicate work to be done from the <code>main()</code> goroutine
to a worker goroutine. When we have no more jobs for
the worker we&rsquo;ll <code>close</code> the <code>jobs</code> channel.</p>
</td>
<td class="code leading">
<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">jobs</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">int</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span>
<span class="nx">done</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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</tr>
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<td class="docs">
<p>Here&rsquo;s the worker goroutine. It repeatedly receives
from <code>jobs</code> with <code>j, more := &lt;-jobs</code>. In this
special 2-value form of receive, the <code>more</code> value
will be <code>false</code> if <code>jobs</code> has been <code>close</code>d and all
values in the channel have already been received.
We use this to notify on <code>done</code> when we&rsquo;ve worked
all our jobs.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<span class="k">for</span> <span class="p">{</span>
<span class="nx">j</span><span class="p">,</span> <span class="nx">more</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">jobs</span>
<span class="k">if</span> <span class="nx">more</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 job&quot;</span><span class="p">,</span> <span class="nx">j</span><span class="p">)</span>
<span class="p">}</span> <span class="k">else</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 all jobs&quot;</span><span class="p">)</span>
<span class="nx">done</span> <span class="o">&lt;-</span> <span class="kc">true</span>
<span class="k">return</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}()</span>
</pre></div>
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<td class="docs">
<p>This sends 3 jobs to the worker over the <code>jobs</code>
channel, then closes it.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="k">for</span> <span class="nx">j</span> <span class="o">:=</span> <span class="mi">1</span><span class="p">;</span> <span class="nx">j</span> <span class="o">&lt;=</span> <span class="mi">3</span><span class="p">;</span> <span class="nx">j</span><span class="o">++</span> <span class="p">{</span>
<span class="nx">jobs</span> <span class="o">&lt;-</span> <span class="nx">j</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;sent job&quot;</span><span class="p">,</span> <span class="nx">j</span><span class="p">)</span>
<span class="p">}</span>
<span class="nb">close</span><span class="p">(</span><span class="nx">jobs</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 all jobs&quot;</span><span class="p">)</span>
</pre></div>
</td>
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<td class="docs">
<p>We await the worker using the
<a href="channel-synchronization">synchronization</a> approach
we saw earlier.</p>
</td>
<td class="code">
<div class="highlight"><pre> <span class="o">&lt;-</span><span class="nx">done</span>
<span class="p">}</span>
</pre></div>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> go run closing-channels.go
<span class="go">sent job 1</span>
<span class="go">received job 1</span>
<span class="go">sent job 2</span>
<span class="go">received job 2</span>
<span class="go">sent job 3</span>
<span class="go">received job 3</span>
<span class="go">sent all jobs</span>
<span class="go">received all jobs</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>The idea of closed channels leads naturally to our next
example: <code>range</code> over channels.</p>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="range-over-channels">Range over 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/closing-channels">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
</body>
</html>
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