| <!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 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class="sidebar-menu-toggle">☰</button><a class="sidebar-logo" href="../../tokio/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../../tokio/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a><div class="sidebar-elems"><h2><a href="index.html">In tokio::task</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">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></h1><span class="out-of-band"><a class="srclink" href="../../src/tokio/task/blocking.rs.html#204-210">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="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">ⓘ<span class="notable-traits-tooltiptext"><span class="docblock"><span class="notable">Notable traits for <a class="struct" href="struct.JoinHandle.html" title="struct tokio::task::JoinHandle">JoinHandle</a><T></span><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="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="associatedtype">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="struct.JoinError.html" title="struct tokio::task::JoinError">JoinError</a>>;</span></code></span></span></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></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> |
| <p>In general, issuing a blocking call or performing a lot of compute in a |
| future without yielding is problematic, as it may prevent the executor from |
| driving other futures forward. This function runs the provided closure on a |
| thread dedicated to blocking operations. See the <a href="../index.html#cpu-bound-tasks-and-blocking-code">CPU-bound tasks and |
| blocking code</a> section for more information.</p> |
| <p>Tokio will spawn more blocking threads when they are requested through this |
| function until the upper limit configured on the <a href="../runtime/struct.Builder.html"><code>Builder</code></a> is reached. |
| After reaching the upper limit, the tasks are put in a queue. |
| The thread limit is very large by default, because <code>spawn_blocking</code> is often |
| used for various kinds of IO operations that cannot be performed |
| asynchronously. When you run CPU-bound code using <code>spawn_blocking</code>, you |
| should keep this large upper limit in mind. When running many CPU-bound |
| computations, a semaphore or some other synchronization primitive should be |
| used to limit the number of computation executed in parallel. Specialized |
| CPU-bound executors, such as <a href="https://docs.rs/rayon">rayon</a>, may also be a good fit.</p> |
| <p>This function is intended for non-async operations that eventually finish on |
| their own. If you want to spawn an ordinary thread, you should use |
| <a href="https://doc.rust-lang.org/nightly/std/thread/fn.spawn.html"><code>thread::spawn</code></a> instead.</p> |
| <p>Closures spawned using <code>spawn_blocking</code> cannot be cancelled abruptly; there |
| is no standard low level API to cause a thread to stop running. However, |
| a useful pattern is to pass some form of “cancellation token” into |
| the thread. This could be an <a href="https://doc.rust-lang.org/nightly/core/sync/atomic/struct.AtomicBool.html"><code>AtomicBool</code></a> that the task checks periodically. |
| Another approach is to have the thread primarily read or write from a channel, |
| and to exit when the channel closes; assuming the other side of the channel is dropped |
| when cancellation occurs, this will cause the blocking task thread to exit |
| soon after as well.</p> |
| <p>When you shut down the executor, it will wait indefinitely for all blocking operations to |
| 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 |
| certain timeout. Be aware that this will still not cancel the tasks — they |
| are simply allowed to keep running after the method returns. It is possible |
| for a blocking task to be cancelled if it has not yet started running, but this |
| is not guaranteed.</p> |
| <p>Note that if you are using the single threaded runtime, this function will |
| still spawn additional threads for blocking operations. The current-thread |
| scheduler’s single thread is only used for asynchronous code.</p> |
| <h2 id="related-apis-and-patterns-for-bridging-asynchronous-and-blocking-code"><a href="#related-apis-and-patterns-for-bridging-asynchronous-and-blocking-code">Related APIs and patterns for bridging asynchronous and blocking code</a></h2> |
| <p>In simple cases, it is sufficient to have the closure accept input |
| parameters at creation time and return a single value (or struct/tuple, etc.).</p> |
| <p>For more complex situations in which it is desirable to stream data to or from |
| the synchronous context, the <a href="../sync/mpsc/index.html"><code>mpsc channel</code></a> has <code>blocking_send</code> and |
| <code>blocking_recv</code> methods for use in non-async code such as the thread created |
| by <code>spawn_blocking</code>.</p> |
| <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 |
| is operating on byte streams. For example, you might use an asynchronous |
| HTTP client such as <a href="https://docs.rs/hyper">hyper</a> to fetch data, but perform complex parsing |
| of the payload body using a library written for synchronous I/O.</p> |
| <p>Finally, see also <a href="https://tokio.rs/tokio/topics/bridging">Bridging with sync code</a> for discussions |
| around the opposite case of using Tokio as part of a larger synchronous |
| codebase.</p> |
| <h2 id="examples"><a href="#examples">Examples</a></h2> |
| <p>Pass an input value and receive result of computation:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::task; |
| |
| <span class="comment">// Initial input |
| </span><span class="kw">let </span><span class="kw-2">mut </span>v = <span class="string">"Hello, "</span>.to_string(); |
| <span class="kw">let </span>res = task::spawn_blocking(<span class="kw">move </span>|| { |
| <span class="comment">// Stand-in for compute-heavy work or using synchronous APIs |
| </span>v.push_str(<span class="string">"world"</span>); |
| <span class="comment">// Pass ownership of the value back to the asynchronous context |
| </span>v |
| }).<span class="kw">await</span><span class="question-mark">?</span>; |
| |
| <span class="comment">// `res` is the value returned from the thread |
| </span><span class="macro">assert_eq!</span>(res.as_str(), <span class="string">"Hello, world"</span>);</code></pre></div> |
| <p>Use a channel:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::task; |
| <span class="kw">use </span>tokio::sync::mpsc; |
| |
| <span class="kw">let </span>(tx, <span class="kw-2">mut </span>rx) = mpsc::channel(<span class="number">2</span>); |
| <span class="kw">let </span>start = <span class="number">5</span>; |
| <span class="kw">let </span>worker = task::spawn_blocking(<span class="kw">move </span>|| { |
| <span class="kw">for </span>x <span class="kw">in </span><span class="number">0</span>..<span class="number">10 </span>{ |
| <span class="comment">// Stand in for complex computation |
| </span>tx.blocking_send(start + x).unwrap(); |
| } |
| }); |
| |
| <span class="kw">let </span><span class="kw-2">mut </span>acc = <span class="number">0</span>; |
| <span class="kw">while let </span><span class="prelude-val">Some</span>(v) = rx.recv().<span class="kw">await </span>{ |
| acc += v; |
| } |
| <span class="macro">assert_eq!</span>(acc, <span class="number">95</span>); |
| worker.<span class="kw">await</span>.unwrap();</code></pre></div> |
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