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<title>Go by Example: Mutexes</title>
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<h2><a href="./">Go by Example</a>: Mutexes</h2>
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<p>In the previous example we saw how to manage simple
counter state using <a href="atomic-counters">atomic operations</a>.
For more complex state we can use a <a href="https://en.wikipedia.org/wiki/Mutual_exclusion"><em>mutex</em></a>
to safely access data across multiple goroutines.</p>
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<a href="https://go.dev/play/p/JU735qy2UmB"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">package</span> <span class="nx">main</span></span></span></code></pre>
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">import</span> <span class="p">(</span>
</span></span><span class="line"><span class="cl"> <span class="s">&#34;fmt&#34;</span>
</span></span><span class="line"><span class="cl"> <span class="s">&#34;sync&#34;</span>
</span></span><span class="line"><span class="cl"><span class="p">)</span></span></span></code></pre>
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<p>Container holds a map of counters; since we want to
update it concurrently from multiple goroutines, we
add a <code>Mutex</code> to synchronize access.
Note that mutexes must not be copied, so if this
<code>struct</code> is passed around, it should be done by
pointer.</p>
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">type</span> <span class="nx">Container</span> <span class="kd">struct</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl"> <span class="nx">mu</span> <span class="nx">sync</span><span class="p">.</span><span class="nx">Mutex</span>
</span></span><span class="line"><span class="cl"> <span class="nx">counters</span> <span class="kd">map</span><span class="p">[</span><span class="kt">string</span><span class="p">]</span><span class="kt">int</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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<p>Lock the mutex before accessing <code>counters</code>; unlock
it at the end of the function using a <a href="defer">defer</a>
statement.</p>
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="p">(</span><span class="nx">c</span> <span class="o">*</span><span class="nx">Container</span><span class="p">)</span> <span class="nf">inc</span><span class="p">(</span><span class="nx">name</span> <span class="kt">string</span><span class="p">)</span> <span class="p">{</span></span></span></code></pre>
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="nx">c</span><span class="p">.</span><span class="nx">mu</span><span class="p">.</span><span class="nf">Lock</span><span class="p">()</span>
</span></span><span class="line"><span class="cl"> <span class="k">defer</span> <span class="nx">c</span><span class="p">.</span><span class="nx">mu</span><span class="p">.</span><span class="nf">Unlock</span><span class="p">()</span>
</span></span><span class="line"><span class="cl"> <span class="nx">c</span><span class="p">.</span><span class="nx">counters</span><span class="p">[</span><span class="nx">name</span><span class="p">]</span><span class="o">++</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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<p>Note that the zero value of a mutex is usable as-is, so no
initialization is required here.</p>
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl"> <span class="nx">c</span> <span class="o">:=</span> <span class="nx">Container</span><span class="p">{</span></span></span></code></pre>
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="nx">counters</span><span class="p">:</span> <span class="kd">map</span><span class="p">[</span><span class="kt">string</span><span class="p">]</span><span class="kt">int</span><span class="p">{</span><span class="s">&#34;a&#34;</span><span class="p">:</span> <span class="mi">0</span><span class="p">,</span> <span class="s">&#34;b&#34;</span><span class="p">:</span> <span class="mi">0</span><span class="p">},</span>
</span></span><span class="line"><span class="cl"> <span class="p">}</span></span></span></code></pre>
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="kd">var</span> <span class="nx">wg</span> <span class="nx">sync</span><span class="p">.</span><span class="nx">WaitGroup</span></span></span></code></pre>
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<p>This function increments a named counter
in a loop.</p>
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="nx">doIncrement</span> <span class="o">:=</span> <span class="kd">func</span><span class="p">(</span><span class="nx">name</span> <span class="kt">string</span><span class="p">,</span> <span class="nx">n</span> <span class="kt">int</span><span class="p">)</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl"> <span class="k">for</span> <span class="nx">i</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">i</span> <span class="p">&lt;</span> <span class="nx">n</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl"> <span class="nx">c</span><span class="p">.</span><span class="nf">inc</span><span class="p">(</span><span class="nx">name</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"> <span class="p">}</span>
</span></span><span class="line"><span class="cl"> <span class="nx">wg</span><span class="p">.</span><span class="nf">Done</span><span class="p">()</span>
</span></span><span class="line"><span class="cl"> <span class="p">}</span></span></span></code></pre>
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<p>Run several goroutines concurrently; note
that they all access the same <code>Container</code>,
and two of them access the same counter.</p>
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="nx">wg</span><span class="p">.</span><span class="nf">Add</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"> <span class="k">go</span> <span class="nf">doIncrement</span><span class="p">(</span><span class="s">&#34;a&#34;</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"> <span class="k">go</span> <span class="nf">doIncrement</span><span class="p">(</span><span class="s">&#34;a&#34;</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"> <span class="k">go</span> <span class="nf">doIncrement</span><span class="p">(</span><span class="s">&#34;b&#34;</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span></span></span></code></pre>
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<p>Wait for the goroutines to finish</p>
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="nx">wg</span><span class="p">.</span><span class="nf">Wait</span><span class="p">()</span>
</span></span><span class="line"><span class="cl"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nx">c</span><span class="p">.</span><span class="nx">counters</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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<p>Running the program shows that the counters
updated as expected.</p>
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="gp">$</span> go run mutexes.go
</span></span><span class="line"><span class="cl"><span class="go">map[a:20000 b:10000]</span></span></span></code></pre>
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<p>Next we&rsquo;ll look at implementing this same state
management task using only goroutines and channels.</p>
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<p class="next">
Next example: <a href="stateful-goroutines">Stateful Goroutines</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>
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