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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="Runs the provided closure on a thread where blocking is acceptable."><meta name="keywords" content="rust, rustlang, rust-lang, spawn_blocking"><title>spawn_blocking in tokio::task - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><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="../../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../../dark.css" disabled><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script src="../../crates.js"></script><script defer src="../../main.js"></script>
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</a><div class="sidebar-elems"><h2 class="location">Other items in<br><a href="../index.html">tokio</a>::<wbr><a href="index.html">task</a></h2><div id="sidebar-vars" data-name="spawn_blocking" data-ty="fn" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../../tokio/index.html"><img class="rust-logo" src="../../rust-logo.png" alt="logo"></a><nav class="sub"><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="18" height="18" alt="Pick another theme!" src="../../brush.svg"></button><div id="theme-choices" role="menu"></div></div><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" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" id="help-button" title="help">?</button><a id="settings-menu" href="../../settings.html" title="settings"><img width="18" height="18" alt="Change settings" src="../../wheel.svg"></a></div></form></nav></div><section id="main-content" class="content"><h1 class="fqn"><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><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../../clipboard.svg" width="19" height="18" alt="Copy item path"></button></span><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#192-198" title="goto source code">[src]</a></span></h1><div class="docblock item-decl"><pre class="rust fn"><code>pub fn spawn_blocking<F, R>(f: F) -> <a class="struct" href="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"><div class="notable">Notable traits for <a class="struct" href="struct.JoinHandle.html" title="struct tokio::task::JoinHandle">JoinHandle</a><T></div><code class="content"><span class="where fmt-newline">impl<T> <a class="trait" href="https://doc.rust-lang.org/1.59.0/core/future/future/trait.Future.html" title="trait core::future::future::Future">Future</a> for <a class="struct" href="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/1.59.0/core/future/future/trait.Future.html#associatedtype.Output" class="associatedtype">Output</a> = <a class="enum" href="https://doc.rust-lang.org/1.59.0/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, <a class="struct" href="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/1.59.0/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> R + <a class="trait" href="https://doc.rust-lang.org/1.59.0/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + 'static,<br> R: <a class="trait" href="https://doc.rust-lang.org/1.59.0/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + 'static, </span></code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><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="../runtime/struct.Builder.html"><code>Builder</code></a> is reached.
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After reaching the upper limit, the tasks are put in a queue.
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The thread limit is very large by default, because <code>spawn_blocking</code> is often
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used for various kinds of IO operations that cannot be performed
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asynchronously. When you run CPU-bound code using <code>spawn_blocking</code>, you
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should keep this large upper limit in mind. When running many CPU-bound
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computations, a semaphore or some other synchronization primitive should be
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used to limit the number of computation executed in parallel. Specialized
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CPU-bound 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/1.59.0/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="../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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<h2 id="related-apis-and-patterns-for-bridging-asynchronous-and-blocking-code" class="section-header"><a href="#related-apis-and-patterns-for-bridging-asynchronous-and-blocking-code">Related APIs and patterns for bridging asynchronous and blocking code</a></h2>
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<p>In simple cases, it is sufficient to have the closure accept input
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parameters at creation time and return a single value (or struct/tuple, etc.).</p>
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<p>For more complex situations in which it is desirable to stream data to or from
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the synchronous context, the <a href="crate::sync::mpsc"><code>mpsc channel</code></a> has <code>blocking_send</code> and
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<code>blocking_recv</code> methods for use in non-async code such as the thread created
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by <code>spawn_blocking</code>.</p>
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<p>Another option is <a href="https://docs.rs/tokio-util/0.6/tokio_util/io/struct.SyncIoBridge.html"><code>SyncIoBridge</code></a> for cases where the synchronous context
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is operating on byte streams. For example, you might use an asynchronous
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HTTP client such as <a href="https://docs.rs/hyper">hyper</a> to fetch data, but perform complex parsing
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of the payload body using a library written for synchronous I/O.</p>
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<p>Finally, see also <a href="https://tokio.rs/tokio/topics/bridging">Bridging with sync code</a> for discussions
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around the opposite case of using Tokio as part of a larger synchronous
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codebase.</p>
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<h2 id="examples" class="section-header"><a href="#examples">Examples</a></h2>
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<p>Pass an input value and receive result of computation:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">tokio::task</span>;
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<span class="comment">// Initial input</span>
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">v</span> <span class="op">=</span> <span class="string">"Hello, "</span>.<span class="ident">to_string</span>();
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<span class="kw">let</span> <span class="ident">res</span> <span class="op">=</span> <span class="ident">task::spawn_blocking</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
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<span class="comment">// Stand-in for compute-heavy work or using synchronous APIs</span>
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<span class="ident">v</span>.<span class="ident">push_str</span>(<span class="string">"world"</span>);
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<span class="comment">// Pass ownership of the value back to the asynchronous context</span>
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<span class="ident">v</span>
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}).<span class="kw">await</span><span class="question-mark">?</span>;
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<span class="comment">// `res` is the value returned from the thread</span>
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<span class="macro">assert_eq!</span>(<span class="ident">res</span>.<span class="ident">as_str</span>(), <span class="string">"Hello, world"</span>);</code></pre></div>
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<p>Use a channel:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">tokio::task</span>;
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<span class="kw">use</span> <span class="ident">tokio::sync::mpsc</span>;
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<span class="kw">let</span> (<span class="ident">tx</span>, <span class="kw-2">mut</span> <span class="ident">rx</span>) <span class="op">=</span> <span class="ident">mpsc::channel</span>(<span class="number">2</span>);
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<span class="kw">let</span> <span class="ident">start</span> <span class="op">=</span> <span class="number">5</span>;
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<span class="kw">let</span> <span class="ident">worker</span> <span class="op">=</span> <span class="ident">task::spawn_blocking</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
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<span class="kw">for</span> <span class="ident">x</span> <span class="kw">in</span> <span class="number">0</span>..<span class="number">10</span> {
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<span class="comment">// Stand in for complex computation</span>
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<span class="ident">tx</span>.<span class="ident">blocking_send</span>(<span class="ident">start</span> <span class="op">+</span> <span class="ident">x</span>).<span class="ident">unwrap</span>();
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}
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});
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">acc</span> <span class="op">=</span> <span class="number">0</span>;
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<span class="kw">while</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">v</span>) <span class="op">=</span> <span class="ident">rx</span>.<span class="ident">recv</span>().<span class="kw">await</span> {
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<span class="ident">acc</span> <span class="op">+</span><span class="op">=</span> <span class="ident">v</span>;
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}
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<span class="macro">assert_eq!</span>(<span class="ident">acc</span>, <span class="number">95</span>);
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<span class="ident">worker</span>.<span class="kw">await</span>.<span class="ident">unwrap</span>();</code></pre></div>
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