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69 lines
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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="Waits on multiple concurrent branches, returning when all branches complete."><title>join in tokio - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Italic-81dc35de.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-MediumItalic-ccf7e434.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2"href="../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../static.files/rustdoc-ca0dd0c4.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="tokio" data-themes="" data-resource-suffix="" data-rustdoc-version="1.93.1 (01f6ddf75 2026-02-11) (Arch Linux rust 1:1.93.1-1)" data-channel="1.93.1" data-search-js="search-9e2438ea.js" data-stringdex-js="stringdex-a3946164.js" data-settings-js="settings-c38705f0.js" ><script src="../static.files/storage-e2aeef58.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-a410ff4d.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-263c88ec.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-eab170b8.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-044be391.svg"></head><body class="rustdoc macro"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><rustdoc-topbar><h2><a href="#">join</a></h2></rustdoc-topbar><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../tokio/index.html">tokio</a><span class="version">1.49.0</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">join</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#notes" title="Notes">Notes</a><ul><li><a href="#runtime-characteristics" title="Runtime characteristics">Runtime characteristics</a></li><li><a href="#fairness" title="Fairness">Fairness</a></li></ul></li><li><a href="#examples" title="Examples">Examples</a></li></ul></section><div id="rustdoc-modnav"><h2 class="in-crate"><a href="index.html">In crate tokio</a></h2></div></div></nav><div class="sidebar-resizer" title="Drag to resize sidebar"></div><main><div class="width-limiter"><section id="main-content" class="content"><div class="main-heading"><div class="rustdoc-breadcrumbs"><a href="index.html">tokio</a></div><h1>Macro <span class="macro">join</span> <button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../src/tokio/macros/join.rs.html#110-112">Source</a> </span></div><pre class="rust item-decl"><code>macro_rules! join {
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($(biased;)? $($future:expr),*) => { ... };
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}</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Waits on multiple concurrent branches, returning when <strong>all</strong> branches
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complete.</p>
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<p>The <code>join!</code> macro must be used inside of async functions, closures, and
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blocks.</p>
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<p>The <code>join!</code> macro takes a list of async expressions and evaluates them
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concurrently on the same task. Each async expression evaluates to a future
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and the futures from each expression are multiplexed on the current task.</p>
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<p>When working with async expressions returning <code>Result</code>, <code>join!</code> will wait
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for <strong>all</strong> branches complete regardless if any complete with <code>Err</code>. Use
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<a href="macro.try_join.html" title="macro tokio::try_join"><code>try_join!</code></a> to return early when <code>Err</code> is encountered.</p>
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<h2 id="notes"><a class="doc-anchor" href="#notes">§</a>Notes</h2>
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<p>The supplied futures are stored inline and do not require allocating a
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<code>Vec</code>.</p>
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<h3 id="runtime-characteristics"><a class="doc-anchor" href="#runtime-characteristics">§</a>Runtime characteristics</h3>
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<p>By running all async expressions on the current task, the expressions are
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able to run <strong>concurrently</strong> but not in <strong>parallel</strong>. This means all
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expressions are run on the same thread and if one branch blocks the thread,
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all other expressions will be unable to continue. If parallelism is
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required, spawn each async expression using <a href="task/fn.spawn.html" title="fn tokio::task::spawn"><code>tokio::spawn</code></a> and pass the
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join handle to <code>join!</code>.</p>
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<h3 id="fairness"><a class="doc-anchor" href="#fairness">§</a>Fairness</h3>
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<p>By default, <code>join!</code>’s generated future rotates which contained
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future is polled first whenever it is woken.</p>
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<p>This behavior can be overridden by adding <code>biased;</code> to the beginning of the
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macro usage. See the examples for details. This will cause <code>join</code> to poll
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the futures in the order they appear from top to bottom.</p>
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<p>You may want this if your futures may interact in a way where known polling order is significant.</p>
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<p>But there is an important caveat to this mode. It becomes your responsibility
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to ensure that the polling order of your futures is fair. If for example you
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are joining a stream and a shutdown future, and the stream has a
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huge volume of messages that takes a long time to finish processing per poll, you should
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place the shutdown future earlier in the <code>join!</code> list to ensure that it is
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always polled, and will not be delayed due to the stream future taking a long time to return
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<code>Poll::Pending</code>.</p>
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<h2 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h2>
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<p>Basic join with two branches</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">async fn </span>do_stuff_async() {
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<span class="comment">// async work
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</span>}
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<span class="kw">async fn </span>more_async_work() {
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<span class="comment">// more here
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</span>}
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<span class="kw">let </span>(first, second) = <span class="macro">tokio::join!</span>(
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do_stuff_async(),
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more_async_work());
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<span class="comment">// do something with the values</span></code></pre></div>
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<p>Using the <code>biased;</code> mode to control polling order.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">async fn </span>do_stuff_async() {
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<span class="comment">// async work
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</span>}
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<span class="kw">async fn </span>more_async_work() {
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<span class="comment">// more here
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</span>}
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<span class="kw">let </span>(first, second) = <span class="macro">tokio::join!</span>(
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biased;
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do_stuff_async(),
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more_async_work()
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);
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<span class="comment">// do something with the values</span></code></pre></div></div></details></section></div></main></body></html> |