<!DOCTYPE html>
<html>
<head>
<meta http-eqiv="content-type" content="text/html;charset=utf-8">
<title>Go by Example: Closing Channels</title>
<link rel=stylesheet href="site.css">
</head>
<script type="text/javascript">
if (window.location.host == "gobyexample.com") {
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-34996217-1']);
_gaq.push(['_trackPageview']);
(function() {
var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
})();
}
</script>
<body>
<div class="example" id="closing-channels">
<h2><a href="./">Go by Example</a>: Closing Channels</h2>
<table>
<tr>
<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’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/eFZ2SeKswH"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kn">import</span> <span class="s">"fmt"</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>In this example we’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’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>
</td>
</tr>
<tr>
<td class="docs">
<p>Here’s the worker goroutine. It repeatedly receives
from <code>jobs</code> with <code>j, more := <-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’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"><-</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">"received job"</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">"received all jobs"</span><span class="p">)</span>
<span class="nx">done</span> <span class="o"><-</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>
</td>
</tr>
<tr>
<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"><=</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"><-</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">"sent job"</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">"sent all jobs"</span><span class="p">)</span>
</pre></div>
</td>
</tr>
<tr>
<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"><-</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://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</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>