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<h1>
<span class="m-breadcrumb"><a href="namespacetf.html">tf</a>::<wbr/></span>Semaphore <span class="m-thin">class</span>
<div class="m-doc-include m-code m-inverted m-text-right"><span class="cp">#include</span> <a class="cpf" href="semaphore_8hpp.html">&lt;taskflow/core/semaphore.hpp&gt;</a></div>
</h1>
<p>class to create a semophore object for building a concurrency constraint</p>
<nav class="m-block m-default">
<h3>Contents</h3>
<ul>
<li>
Reference
<ul>
<li><a href="#typeless-methods">Constructors, destructors, conversion operators</a></li>
<li><a href="#pub-methods">Public functions</a></li>
</ul>
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<p>A semaphore creates a constraint that limits the maximum concurrency, i.e., the number of workers, in a set of tasks. You can let a task acquire/release one or multiple semaphores before/after executing its work. A task can acquire and release a semaphore, or just acquire or just release it. A <a href="classtf_1_1Semaphore.html" class="m-doc">tf::<wbr />Semaphore</a> object starts with an initial count. As long as that count is above 0, tasks can acquire the semaphore and do their work. If the count is 0 or less, a task trying to acquire the semaphore will not run but goes to a waiting list of that semaphore. When the semaphore is released by another task, it reschedules all tasks on that waiting list.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="nf">executor</span><span class="p">(</span><span class="mi">8</span><span class="p">);</span><span class="w"> </span><span class="c1">// create an executor of 8 workers</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Semaphore</span><span class="w"> </span><span class="nf">semaphore</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span><span class="w"> </span><span class="c1">// create a semaphore with initial count 1</span>
<span class="k">for</span><span class="p">(</span><span class="kt">size_t</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">&lt;</span><span class="mi">1000</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([</span><span class="o">&amp;</span><span class="p">](</span><span class="n">tf</span><span class="o">::</span><span class="n">Runtime</span><span class="o">&amp;</span><span class="w"> </span><span class="n">rt</span><span class="p">){</span><span class="w"> </span>
<span class="w"> </span><span class="n">rt</span><span class="p">.</span><span class="n">acquire</span><span class="p">(</span><span class="n">semaphore</span><span class="p">);</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;critical section here (one worker at any time)</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span>
<span class="w"> </span><span class="n">critical_section</span><span class="p">();</span>
<span class="w"> </span><span class="n">rt</span><span class="p">.</span><span class="n">release</span><span class="p">(</span><span class="n">semaphore</span><span class="p">);</span>
<span class="w"> </span><span class="p">});</span>
<span class="p">}</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">wait</span><span class="p">();</span></pre><p>The above example creates a taskflow of 1000 independent tasks while only one worker will run <code>critical_section</code> at any time due to the semaphore constraint. This arrangement limits the parallelism of <code>critical_section</code> to just one.</p><aside class="m-note m-info"><h4>Note</h4><p>Taskflow use a non-blocking algorithm to implement the acquisition of semaphores and thus is deadlock-free.</p></aside>
<section id="typeless-methods">
<h2><a href="#typeless-methods">Constructors, destructors, conversion operators</a></h2>
<dl class="m-doc">
<dt>
<span class="m-doc-wrap-bumper"><a href="#a9f019c0fdcd9b1bbf1d7f46760878d2a" class="m-doc">Semaphore</a>(</span><span class="m-doc-wrap">)</span>
</dt>
<dd>constructs a default semaphore with count equal to zero</dd>
<dt>
<span class="m-doc-wrap-bumper"><a href="#aafbb7c98f5e510f2ba053b3e9fd2cb25" class="m-doc">Semaphore</a>(</span><span class="m-doc-wrap">size_t count) <span class="m-label m-flat m-info">explicit</span> </span>
</dt>
<dd>constructs a semaphore with the given count</dd>
</dl>
</section>
<section id="pub-methods">
<h2><a href="#pub-methods">Public functions</a></h2>
<dl class="m-doc">
<dt>
<span class="m-doc-wrap-bumper">auto <a href="#aa3d4f0f1930f5d5ec6238a9d1460b06b" class="m-doc">count</a>(</span><span class="m-doc-wrap">std::memory_order memory_order = std::memory_order_seq_cst) const -&gt; size_t</span>
</dt>
<dd>queries the current value of the associated counter</dd>
<dt>
<span class="m-doc-wrap-bumper">auto <a href="#a2bbcb0be1e7e767d5911ad79d63b59f0" class="m-doc">try_acquire</a>(</span><span class="m-doc-wrap">) -&gt; bool</span>
</dt>
<dd>tries to atomically decrement the internal counter by <code>1</code> if it is greater than <code>0</code></dd>
<dt>
<span class="m-doc-wrap-bumper">void <a href="#a495348aa6280a4d2756a42fbf23fbe20" class="m-doc">release</a>(</span><span class="m-doc-wrap">size_t n = 1)</span>
</dt>
<dd>atomically increment the internal counter by <code>n</code></dd>
<dt>
<span class="m-doc-wrap-bumper">void <a href="#a6d0481c1a39eddc6497aad21695c0e8e" class="m-doc">reset</a>(</span><span class="m-doc-wrap">size_t count,
std::memory_order memory_order = std::memory_order_seq_cst)</span>
</dt>
<dd>resets the semaphore to the given count</dd>
</dl>
</section>
<section>
<h2>Function documentation</h2>
<section class="m-doc-details" id="a9f019c0fdcd9b1bbf1d7f46760878d2a"><div>
<h3>
<span class="m-doc-wrap-bumper"> tf::<wbr />Semaphore::<wbr /></span><span class="m-doc-wrap"><span class="m-doc-wrap-bumper"><a href="#a9f019c0fdcd9b1bbf1d7f46760878d2a" class="m-doc-self">Semaphore</a>(</span><span class="m-doc-wrap">)</span></span>
</h3>
<p>constructs a default semaphore with count equal to zero</p>
<p>Application can use <a href="#a6d0481c1a39eddc6497aad21695c0e8e" class="m-doc">tf::<wbr />Semaphore::<wbr />reset</a> to reset the counter of the semaphore later.</p>
</div></section>
<section class="m-doc-details" id="aafbb7c98f5e510f2ba053b3e9fd2cb25"><div>
<h3>
<span class="m-doc-wrap-bumper"> tf::<wbr />Semaphore::<wbr /></span><span class="m-doc-wrap"><span class="m-doc-wrap-bumper"><a href="#aafbb7c98f5e510f2ba053b3e9fd2cb25" class="m-doc-self">Semaphore</a>(</span><span class="m-doc-wrap">size_t count) <span class="m-label m-info">explicit</span> </span></span>
</h3>
<p>constructs a semaphore with the given count</p>
<p>A semaphore creates a constraint that limits the maximum concurrency, i.e., the number of workers, in a set of tasks.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Semaphore</span><span class="w"> </span><span class="nf">semaphore</span><span class="p">(</span><span class="mi">4</span><span class="p">);</span><span class="w"> </span><span class="c1">// a semaphore initialized with 4</span></pre>
</div></section>
<section class="m-doc-details" id="aa3d4f0f1930f5d5ec6238a9d1460b06b"><div>
<h3>
<span class="m-doc-wrap-bumper">size_t tf::<wbr />Semaphore::<wbr /></span><span class="m-doc-wrap"><span class="m-doc-wrap-bumper"><a href="#aa3d4f0f1930f5d5ec6238a9d1460b06b" class="m-doc-self">count</a>(</span><span class="m-doc-wrap">std::memory_order memory_order = std::memory_order_seq_cst) const</span></span>
</h3>
<p>queries the current value of the associated counter</p>
<table class="m-table m-fullwidth m-flat">
<thead>
<tr><th colspan="2">Parameters</th></tr>
</thead>
<tbody>
<tr>
<td style="width: 1%">memory_order</td>
<td>the memory order of this load (default std::memory_order_seq_cst)</td>
</tr>
</tbody>
</table>
<p>Queries the current value of the associated counter.</p>
</div></section>
<section class="m-doc-details" id="a2bbcb0be1e7e767d5911ad79d63b59f0"><div>
<h3>
<span class="m-doc-wrap-bumper">bool tf::<wbr />Semaphore::<wbr /></span><span class="m-doc-wrap"><span class="m-doc-wrap-bumper"><a href="#a2bbcb0be1e7e767d5911ad79d63b59f0" class="m-doc-self">try_acquire</a>(</span><span class="m-doc-wrap">)</span></span>
</h3>
<p>tries to atomically decrement the internal counter by <code>1</code> if it is greater than <code>0</code></p>
<table class="m-table m-fullwidth m-flat">
<tfoot>
<tr>
<th style="width: 1%">Returns</th>
<td><code>true</code> if it decremented the internal counter, otherwise <code>false</code></td>
</tr>
</tfoot>
</table>
<p>Tries to atomically decrement the internal counter by <code>1</code>. If the operation succeeds, returns <code>true</code>, otherwise <code>false</code>.</p>
</div></section>
<section class="m-doc-details" id="a495348aa6280a4d2756a42fbf23fbe20"><div>
<h3>
<span class="m-doc-wrap-bumper">void tf::<wbr />Semaphore::<wbr /></span><span class="m-doc-wrap"><span class="m-doc-wrap-bumper"><a href="#a495348aa6280a4d2756a42fbf23fbe20" class="m-doc-self">release</a>(</span><span class="m-doc-wrap">size_t n = 1)</span></span>
</h3>
<p>atomically increment the internal counter by <code>n</code></p>
<table class="m-table m-fullwidth m-flat">
<thead>
<tr><th colspan="2">Parameters</th></tr>
</thead>
<tbody>
<tr>
<td style="width: 1%">n</td>
<td>the value by which the internal counter will be incremented</td>
</tr>
</tbody>
</table>
<p>The release operation always succeeds as it simply increments the counter of this semaphore.</p>
</div></section>
<section class="m-doc-details" id="a6d0481c1a39eddc6497aad21695c0e8e"><div>
<h3>
<span class="m-doc-wrap-bumper">void tf::<wbr />Semaphore::<wbr /></span><span class="m-doc-wrap"><span class="m-doc-wrap-bumper"><a href="#a6d0481c1a39eddc6497aad21695c0e8e" class="m-doc-self">reset</a>(</span><span class="m-doc-wrap">size_t count,
std::memory_order memory_order = std::memory_order_seq_cst)</span></span>
</h3>
<p>resets the semaphore to the given count</p>
<table class="m-table m-fullwidth m-flat">
<thead>
<tr><th colspan="2">Parameters</th></tr>
</thead>
<tbody>
<tr>
<td style="width: 1%">count</td>
<td>the new count value</td>
</tr>
<tr>
<td>memory_order</td>
<td>memory order to which this operation will be applied (default std::memory_order_seq_cst)</td>
</tr>
</tbody>
</table>
<aside class="m-note m-info"><h4>Note</h4><p>Calling <a href="#a6d0481c1a39eddc6497aad21695c0e8e" class="m-doc">tf::<wbr />Semaphore::<wbr />reset</a> will immediately change the underlying counter to the given <code>count</code> value, regardless other threads acquiring or releasing the semaphore.</p></aside>
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