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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="API documentation for the Rust `spawn_blocking` fn in crate `tokio`."><meta name="keywords" content="rust, rustlang, rust-lang, spawn_blocking"><title>tokio::task::spawn_blocking - Rust</title><link rel="stylesheet" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../../ayu.css" disabled ><script id="default-settings"></script><script src="../../storage.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../../favicon.svg">
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<link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><style type="text/css">#crate-search{background-image:url("../../down-arrow.svg");}</style></head><body class="rustdoc fn"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../tokio/index.html'><div class='logo-container rust-logo'><img src='../../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class="location"><a href="../index.html">tokio</a>::<wbr><a href="index.html">task</a></p><script>window.sidebarCurrent = {name: "spawn_blocking", ty: "fn", relpath: ""};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><span class="help-button">?</span>
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<a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><span class="out-of-band"><span id="render-detail"><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span><a class="srclink" href="../../src/tokio/task/blocking.rs.html#109-115" title="goto source code">[src]</a></span><span class="in-band">Function <a href="../index.html">tokio</a>::<wbr><a href="index.html">task</a>::<wbr><a class="fn" href="">spawn_blocking</a></span></h1><pre class="rust fn">pub fn spawn_blocking<F, R>(f: F) -> <a class="struct" href="../../tokio/task/struct.JoinHandle.html" title="struct tokio::task::JoinHandle">JoinHandle</a><R><span class="notable-traits"><span class="notable-traits-tooltip">ⓘ<div class="notable-traits-tooltiptext"><span class="docblock"><h3 class="notable">Notable traits for <a class="struct" href="../../tokio/task/struct.JoinHandle.html" title="struct tokio::task::JoinHandle">JoinHandle</a><T></h3><code class="content"><span class="where fmt-newline">impl<T> <a class="trait" href="https://doc.rust-lang.org/nightly/core/future/future/trait.Future.html" title="trait core::future::future::Future">Future</a> for <a class="struct" href="../../tokio/task/struct.JoinHandle.html" title="struct tokio::task::JoinHandle">JoinHandle</a><T></span><span class="where fmt-newline"> type <a href="https://doc.rust-lang.org/nightly/core/future/future/trait.Future.html#associatedtype.Output" class="type">Output</a> = <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, <a class="struct" href="../../tokio/task/struct.JoinError.html" title="struct tokio::task::JoinError">JoinError</a>>;</span></code></span></div></span></span> <span class="where fmt-newline">where<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> R + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + 'static,<br> R: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + 'static, </span></pre><div class="docblock"><p>Runs the provided closure on a thread where blocking is acceptable.</p>
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<p>In general, issuing a blocking call or performing a lot of compute in a
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future without yielding is problematic, as it may prevent the executor from
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driving other futures forward. This function runs the provided closure on a
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thread dedicated to blocking operations. See the <a href="../index.html#cpu-bound-tasks-and-blocking-code">CPU-bound tasks and
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blocking code</a> section for more information.</p>
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<p>Tokio will spawn more blocking threads when they are requested through this
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function until the upper limit configured on the <a href="../../tokio/runtime/struct.Builder.html"><code>Builder</code></a> is reached.
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This limit is very large by default, because <code>spawn_blocking</code> is often used
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for various kinds of IO operations that cannot be performed asynchronously.
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When you run CPU-bound code using <code>spawn_blocking</code>, you should keep this
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large upper limit in mind. When running many CPU-bound computations, a
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semaphore or some other synchronization primitive should be used to limit
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the number of computation executed in parallel. Specialized CPU-bound
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executors, such as <a href="https://docs.rs/rayon">rayon</a>, may also be a good fit.</p>
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<p>This function is intended for non-async operations that eventually finish on
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their own. If you want to spawn an ordinary thread, you should use
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<a href="https://doc.rust-lang.org/nightly/std/thread/fn.spawn.html"><code>thread::spawn</code></a> instead.</p>
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<p>Closures spawned using <code>spawn_blocking</code> cannot be cancelled. When you shut
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down the executor, it will wait indefinitely for all blocking operations to
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finish. You can use <a href="../../tokio/runtime/struct.Runtime.html#method.shutdown_timeout"><code>shutdown_timeout</code></a> to stop waiting for them after a
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certain timeout. Be aware that this will still not cancel the tasks — they
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are simply allowed to keep running after the method returns.</p>
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<p>Note that if you are using the single threaded runtime, this function will
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still spawn additional threads for blocking operations. The basic
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scheduler's single thread is only used for asynchronous code.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">tokio</span>::<span class="ident">task</span>;
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<span class="kw">let</span> <span class="ident">res</span> <span class="op">=</span> <span class="ident">task</span>::<span class="ident">spawn_blocking</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
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<span class="comment">// do some compute-heavy work or call synchronous code</span>
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<span class="string">"done computing"</span>
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}).<span class="ident">await</span><span class="question-mark">?</span>;
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">res</span>, <span class="string">"done computing"</span>);</pre></div>
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