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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><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-ac92e1bbe349e143.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="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.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]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../tokio/index.html">tokio</a><span class="version">1.33.0</span></h2></div><div class="sidebar-elems"></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../tokio/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Macro <a href="index.html">tokio</a>::<wbr><a class="macro" href="#">join</a><button id="copy-path" title="Copy item path to clipboard"><img src="../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../src/tokio/macros/join.rs.html#57-166">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><span class="macro">macro_rules! </span>join {
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(@ {
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<span class="comment">// One `_` for each branch in the `join!` macro. This is not used once
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// normalization is complete.
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</span>( $(<span class="macro-nonterminal">$count</span>:tt)* )
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<span class="comment">// The expression `0+1+1+ ... +1` equal to the number of branches.
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</span>( $(<span class="macro-nonterminal">$total</span>:tt)* )
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<span class="comment">// Normalized join! branches
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</span>$( ( $(<span class="macro-nonterminal">$skip</span>:tt)* ) <span class="macro-nonterminal">$e</span>:expr, )*
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}) => { ... };
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(@ { ( $(<span class="macro-nonterminal">$s</span>:tt)* ) ( $(<span class="macro-nonterminal">$n</span>:tt)* ) $(<span class="macro-nonterminal">$t</span>:tt)* } <span class="macro-nonterminal">$e</span>:expr, $(<span class="macro-nonterminal">$r</span>:tt)* ) => { ... };
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( $(<span class="macro-nonterminal">$e</span>:expr),+ $(,)<span class="question-mark">?</span>) => { ... };
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() => { ... };
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}</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 href="#notes">Notes</a></h2>
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<p>The supplied futures are stored inline and does not require allocating a
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<code>Vec</code>.</p>
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<h4 id="runtime-characteristics"><a href="#runtime-characteristics">Runtime characteristics</a></h4>
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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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<h2 id="examples"><a href="#examples">Examples</a></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="attr">#[tokio::main]
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</span><span class="kw">async fn </span>main() {
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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
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</span>}</code></pre></div>
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</div></details></section></div></main></body></html> |