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<h1>
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<span class="m-breadcrumb"><a href="Cookbook.html">Cookbook</a> »</span>
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Request Cancellation
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<h3>Contents</h3>
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<ul>
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<li><a href="#CancelARunningTaskflow">Cancel Execution of Taskflows</a></li>
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<li><a href="#UnderstandTheLimitationsOfCancellation">Understand the Limitations of Cancellation</a></li>
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<p>This chapters discusses how to cancel submitted tasks.</p><section id="CancelARunningTaskflow"><h2><a href="#CancelARunningTaskflow">Cancel Execution of Taskflows</a></h2><p>When you submit a taskflow to an executor (e.g., <a href="classtf_1_1Executor.html#a8d08f0cb79e7b3780087975d13368a96" class="m-doc">tf::<wbr />Executor::<wbr />run</a>), the executor returns a <a href="classtf_1_1Future.html" class="m-doc">tf::<wbr />Future</a> object that will hold the result of the execution. <a href="classtf_1_1Future.html" class="m-doc">tf::<wbr />Future</a> is a derived class from <a href="http://en.cppreference.com/w/cpp/thread/future.html" class="m-doc-external">std::<wbr />future</a>. In addition to base methods of <a href="http://en.cppreference.com/w/cpp/thread/future.html" class="m-doc-external">std::<wbr />future</a>, you can call <a href="classtf_1_1Future.html#a3bf5f104864ab2590b6409712d3a469b" class="m-doc">tf::<wbr />Future::<wbr />cancel</a> to cancel the execution of a running taskflow. The following example cancels a submission of a taskflow that contains 1000 tasks each running one second.</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="n">executor</span><span class="p">;</span>
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<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>
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<span class="k">for</span><span class="p">(</span><span class="kt">int</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"><</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>
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<span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){</span><span class="w"> </span>
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<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">this_thread</span><span class="o">::</span><span class="n">sleep_for</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">chrono</span><span class="o">::</span><span class="n">seconds</span><span class="p">(</span><span class="mi">1</span><span class="p">));</span>
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<span class="w"> </span><span class="p">});</span>
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<span class="p">}</span>
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<span class="c1">// submit the taskflow</span>
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<span class="n">tf</span><span class="o">::</span><span class="n">Future</span><span class="o"><</span><span class="kt">void</span><span class="o">></span><span class="w"> </span><span class="n">fu</span><span class="w"> </span><span class="o">=</span><span class="w"> </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>
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<span class="c1">// request to cancel the above submitted execution</span>
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<span class="n">fu</span><span class="p">.</span><span class="n">cancel</span><span class="p">();</span>
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<span class="c1">// wait until the cancellation completes</span>
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<span class="n">fu</span><span class="p">.</span><span class="n">get</span><span class="p">();</span></pre><aside class="m-note m-info"><h4>Note</h4><p><a href="classtf_1_1Future.html#a3bf5f104864ab2590b6409712d3a469b" class="m-doc">tf::<wbr />Future::<wbr />cancel</a> is <em>non-deterministic</em> and <em>out-of-order</em>.</p></aside><p>When you request a cancellation, the executor will stop scheduling the rest tasks of the taskflow. Tasks that are already running will continue to finish, but their successor tasks will not be scheduled to run. A cancellation is considered complete when all these running tasks finish. To wait for a cancellation to complete, you may explicitly call <code>tf::Future::get</code>.</p><aside class="m-note m-warning"><h4>Attention</h4><p>It is your responsibility to ensure that the taskflow remains alive before the cancellation completes.</p></aside><p>For instance, the following code results in undefined behavior:</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="n">executor</span><span class="p">;</span>
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<span class="p">{</span>
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<span class="w"> </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>
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<span class="w"> </span>
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<span class="w"> </span><span class="k">for</span><span class="p">(</span><span class="kt">int</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"><</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>
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<span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){});</span>
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<span class="w"> </span><span class="p">}</span>
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<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">Future</span><span class="w"> </span><span class="n">fu</span><span class="w"> </span><span class="o">=</span><span class="w"> </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>
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<span class="w"> </span><span class="n">fu</span><span class="p">.</span><span class="n">cancel</span><span class="p">();</span><span class="w"> </span><span class="c1">// there can still be task running after cancellation</span>
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<span class="p">}</span><span class="w"> </span><span class="c1">// destroying taskflow here can result in undefined behavior</span></pre><p>The undefined behavior problem exists because <a href="classtf_1_1Future.html#a3bf5f104864ab2590b6409712d3a469b" class="m-doc">tf::<wbr />Future::<wbr />cancel</a> does not guarantee an immediate cancellation. To fix the problem, call <code>get</code> to ensure the cancellation completes before the end of the scope destroys the taskflow.</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="n">executor</span><span class="p">;</span>
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<span class="p">{</span>
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<span class="w"> </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>
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<span class="w"> </span>
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<span class="w"> </span><span class="k">for</span><span class="p">(</span><span class="kt">int</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"><</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>
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<span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){});</span>
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<span class="w"> </span><span class="p">}</span>
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<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">Future</span><span class="w"> </span><span class="n">fu</span><span class="w"> </span><span class="o">=</span><span class="w"> </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>
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<span class="w"> </span><span class="n">fu</span><span class="p">.</span><span class="n">cancel</span><span class="p">();</span><span class="w"> </span><span class="c1">// there can still be task running after cancellation</span>
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<span class="w"> </span><span class="n">fu</span><span class="p">.</span><span class="n">get</span><span class="p">();</span><span class="w"> </span><span class="c1">// waits until the cancellation completes</span>
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<span class="p">}</span></pre></section><section id="UnderstandTheLimitationsOfCancellation"><h2><a href="#UnderstandTheLimitationsOfCancellation">Understand the Limitations of Cancellation</a></h2><p>Canceling the execution of a running taskflow has the following limitations:</p><ul><li>Cancellation is non-preemptive. A running task will not be cancelled until it finishes.</li><li>Cancelling a taskflow with tasks acquiring and/or releasing <a href="classtf_1_1Semaphore.html" class="m-doc">tf::<wbr />Semaphore</a> results is currently not supported.</li></ul><p>We may overcome these limitations in the future releases.</p></section>
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<p>Taskflow handbook is part of the <a href="https://taskflow.github.io">Taskflow project</a>, copyright © <a href="https://tsung-wei-huang.github.io/">Dr. Tsung-Wei Huang</a>, 2018–2024.<br />Generated by <a href="https://doxygen.org/">Doxygen</a> 1.9.1 and <a href="https://mcss.mosra.cz/">m.css</a>.</p>
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