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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="Spawns a new asynchronous task, returning a `JoinHandle` for it."><meta name="keywords" content="rust, rustlang, rust-lang, spawn"><title>spawn in tokio - 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></h2><div id="sidebar-vars" data-name="spawn" 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 class="fn" href="#">spawn</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/spawn.rs.html#125-137" title="goto source code">[src]</a></span></h1><div class="docblock item-decl"><pre class="rust fn"><code>pub fn spawn<T>(future: T) -> <a class="struct" href="task/struct.JoinHandle.html" title="struct tokio::task::JoinHandle">JoinHandle</a><T::<a class="associatedtype" href="https://doc.rust-lang.org/1.59.0/core/future/future/trait.Future.html#associatedtype.Output" title="type core::future::future::Future::Output">Output</a>><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="task/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="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/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="task/struct.JoinError.html" title="struct tokio::task::JoinError">JoinError</a>>;</span></code></span></div></span></span> <span class="where fmt-newline">where<br> 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> + <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> T::<a class="associatedtype" href="https://doc.rust-lang.org/1.59.0/core/future/future/trait.Future.html#associatedtype.Output" title="type core::future::future::Future::Output">Output</a>: <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>Spawns a new asynchronous task, returning a
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<a href="task/struct.JoinHandle.html"><code>JoinHandle</code></a> for it.</p>
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<p>Spawning a task enables the task to execute concurrently to other tasks. The
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spawned task may execute on the current thread, or it may be sent to a
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different thread to be executed. The specifics depend on the current
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<a href="runtime/struct.Runtime.html"><code>Runtime</code></a> configuration.</p>
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<p>There is no guarantee that a spawned task will execute to completion.
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When a runtime is shutdown, all outstanding tasks are dropped,
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regardless of the lifecycle of that task.</p>
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<p>This function must be called from the context of a Tokio runtime. Tasks running on
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the Tokio runtime are always inside its context, but you can also enter the context
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using the <a href="runtime/struct.Runtime.html#method.enter"><code>Runtime::enter</code></a> method.</p>
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<h2 id="examples" class="section-header"><a href="#examples">Examples</a></h2>
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<p>In this example, a server is started and <code>spawn</code> is used to start a new task
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that processes each received connection.</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::net</span>::{<span class="ident">TcpListener</span>, <span class="ident">TcpStream</span>};
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<span class="kw">use</span> <span class="ident">std::io</span>;
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">process</span>(<span class="ident">socket</span>: <span class="ident">TcpStream</span>) {
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<span class="comment">// ...</span>
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}
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<span class="attribute">#[<span class="ident">tokio::main</span>]</span>
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">main</span>() -> <span class="ident">io::Result</span><span class="op"><</span>()<span class="op">></span> {
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<span class="kw">let</span> <span class="ident">listener</span> <span class="op">=</span> <span class="ident">TcpListener::bind</span>(<span class="string">"127.0.0.1:8080"</span>).<span class="kw">await</span><span class="question-mark">?</span>;
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<span class="kw">loop</span> {
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<span class="kw">let</span> (<span class="ident">socket</span>, <span class="kw">_</span>) <span class="op">=</span> <span class="ident">listener</span>.<span class="ident">accept</span>().<span class="kw">await</span><span class="question-mark">?</span>;
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<span class="ident">tokio::spawn</span>(<span class="kw">async</span> <span class="kw">move</span> {
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<span class="comment">// Process each socket concurrently.</span>
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<span class="ident">process</span>(<span class="ident">socket</span>).<span class="kw">await</span>
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});
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}
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}</code></pre></div>
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<h2 id="panics" class="section-header"><a href="#panics">Panics</a></h2>
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<p>Panics if called from <strong>outside</strong> of the Tokio runtime.</p>
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<h2 id="using-send-values-from-a-task" class="section-header"><a href="#using-send-values-from-a-task">Using <code>!Send</code> values from a task</a></h2>
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<p>The task supplied to <code>spawn</code> must implement <code>Send</code>. However, it is
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possible to <strong>use</strong> <code>!Send</code> values from the task as long as they only
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exist between calls to <code>.await</code>.</p>
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<p>For example, this will work:</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">std::rc::Rc</span>;
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<span class="kw">fn</span> <span class="ident">use_rc</span>(<span class="ident">rc</span>: <span class="ident">Rc</span><span class="op"><</span>()<span class="op">></span>) {
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<span class="comment">// Do stuff w/ rc</span>
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}
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<span class="attribute">#[<span class="ident">tokio::main</span>]</span>
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="ident">tokio::spawn</span>(<span class="kw">async</span> {
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<span class="comment">// Force the `Rc` to stay in a scope with no `.await`</span>
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{
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<span class="kw">let</span> <span class="ident">rc</span> <span class="op">=</span> <span class="ident">Rc::new</span>(());
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<span class="ident">use_rc</span>(<span class="ident">rc</span>.<span class="ident">clone</span>());
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}
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<span class="ident">task::yield_now</span>().<span class="kw">await</span>;
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}).<span class="kw">await</span>.<span class="ident">unwrap</span>();
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}</code></pre></div>
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<p>This will <strong>not</strong> work:</p>
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<div class='information'><div class='tooltip compile_fail'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail"><code><span class="kw">use</span> <span class="ident">tokio::task</span>;
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<span class="kw">use</span> <span class="ident">std::rc::Rc</span>;
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<span class="kw">fn</span> <span class="ident">use_rc</span>(<span class="ident">rc</span>: <span class="ident">Rc</span><span class="op"><</span>()<span class="op">></span>) {
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<span class="comment">// Do stuff w/ rc</span>
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}
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<span class="attribute">#[<span class="ident">tokio::main</span>]</span>
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="ident">tokio::spawn</span>(<span class="kw">async</span> {
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<span class="kw">let</span> <span class="ident">rc</span> <span class="op">=</span> <span class="ident">Rc::new</span>(());
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<span class="ident">task::yield_now</span>().<span class="kw">await</span>;
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<span class="ident">use_rc</span>(<span class="ident">rc</span>.<span class="ident">clone</span>());
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}).<span class="kw">await</span>.<span class="ident">unwrap</span>();
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}</code></pre></div>
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<p>Holding on to a <code>!Send</code> value across calls to <code>.await</code> will result in
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an unfriendly compile error message similar to:</p>
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<div class="example-wrap"><pre class="language-text"><code>`[... some type ...]` cannot be sent between threads safely</code></pre></div>
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<p>or:</p>
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<div class="example-wrap"><pre class="language-text"><code>error[E0391]: cycle detected when processing `main`</code></pre></div></div></details></section><section id="search" class="content hidden"></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="tokio" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.59.0 (9d1b2106e 2022-02-23)" ></div>
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