| <!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="Marks async function to be executed by the selected runtime. This macro helps set up a `Runtime` without requiring the user to use Runtime or Builder directly."><meta name="keywords" content="rust, rustlang, rust-lang, main"><title>main 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" href="../normalize.css"><link rel="stylesheet" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../ayu.css" disabled><link rel="stylesheet" href="../dark.css" disabled><link rel="stylesheet" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script defer src="sidebar-items.js"></script><script defer src="../main.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.svg"></head><body class="rustdoc attr"><!--[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><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</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">Attribute Macro <a href="index.html">tokio</a>::<wbr><a class="attr" href="#">main</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 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 attr"><code>#[main]</code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Marks async function to be executed by the selected runtime. This macro |
| helps set up a <code>Runtime</code> without requiring the user to use |
| <a href="../tokio/runtime/struct.Runtime.html">Runtime</a> or |
| <a href="../tokio/runtime/struct.Builder.html">Builder</a> directly.</p> |
| <p>Note: This macro is designed to be simplistic and targets applications that |
| do not require a complex setup. If the provided functionality is not |
| sufficient, you may be interested in using |
| <a href="../tokio/runtime/struct.Builder.html">Builder</a>, which provides a more |
| powerful interface.</p> |
| <p>Note: This macro can be used on any function and not just the <code>main</code> |
| function. Using it on a non-main function makes the function behave as if it |
| was synchronous by starting a new runtime each time it is called. If the |
| function is called often, it is preferable to create the runtime using the |
| runtime builder so the runtime can be reused across calls.</p> |
| <h2 id="multi-threaded-runtime"><a href="#multi-threaded-runtime">Multi-threaded runtime</a></h2> |
| <p>To use the multi-threaded runtime, the macro can be configured using</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[tokio::main(flavor = <span class="string">"multi_thread"</span>, worker_threads = <span class="number">10</span>)]</span></code></pre></div> |
| <p>The <code>worker_threads</code> option configures the number of worker threads, and |
| defaults to the number of cpus on the system. This is the default flavor.</p> |
| <p>Note: The multi-threaded runtime requires the <code>rt-multi-thread</code> feature |
| flag.</p> |
| <h2 id="current-thread-runtime"><a href="#current-thread-runtime">Current thread runtime</a></h2> |
| <p>To use the single-threaded runtime known as the <code>current_thread</code> runtime, |
| the macro can be configured using</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[tokio::main(flavor = <span class="string">"current_thread"</span>)]</span></code></pre></div> |
| <h3 id="function-arguments"><a href="#function-arguments">Function arguments:</a></h3> |
| <p>Arguments are allowed for any functions aside from <code>main</code> which is special</p> |
| <h3 id="usage"><a href="#usage">Usage</a></h3><h4 id="using-the-multi-thread-runtime"><a href="#using-the-multi-thread-runtime">Using the multi-thread runtime</a></h4> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[tokio::main] |
| </span><span class="kw">async fn </span>main() { |
| <span class="macro">println!</span>(<span class="string">"Hello world"</span>); |
| }</code></pre></div> |
| <p>Equivalent code not using <code>#[tokio::main]</code></p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">fn </span>main() { |
| tokio::runtime::Builder::new_multi_thread() |
| .enable_all() |
| .build() |
| .unwrap() |
| .block_on(<span class="kw">async </span>{ |
| <span class="macro">println!</span>(<span class="string">"Hello world"</span>); |
| }) |
| }</code></pre></div> |
| <h4 id="using-current-thread-runtime"><a href="#using-current-thread-runtime">Using current thread runtime</a></h4> |
| <p>The basic scheduler is single-threaded.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[tokio::main(flavor = <span class="string">"current_thread"</span>)] |
| </span><span class="kw">async fn </span>main() { |
| <span class="macro">println!</span>(<span class="string">"Hello world"</span>); |
| }</code></pre></div> |
| <p>Equivalent code not using <code>#[tokio::main]</code></p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">fn </span>main() { |
| tokio::runtime::Builder::new_current_thread() |
| .enable_all() |
| .build() |
| .unwrap() |
| .block_on(<span class="kw">async </span>{ |
| <span class="macro">println!</span>(<span class="string">"Hello world"</span>); |
| }) |
| }</code></pre></div> |
| <h4 id="set-number-of-worker-threads"><a href="#set-number-of-worker-threads">Set number of worker threads</a></h4> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[tokio::main(worker_threads = <span class="number">2</span>)] |
| </span><span class="kw">async fn </span>main() { |
| <span class="macro">println!</span>(<span class="string">"Hello world"</span>); |
| }</code></pre></div> |
| <p>Equivalent code not using <code>#[tokio::main]</code></p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">fn </span>main() { |
| tokio::runtime::Builder::new_multi_thread() |
| .worker_threads(<span class="number">2</span>) |
| .enable_all() |
| .build() |
| .unwrap() |
| .block_on(<span class="kw">async </span>{ |
| <span class="macro">println!</span>(<span class="string">"Hello world"</span>); |
| }) |
| }</code></pre></div> |
| <h4 id="configure-the-runtime-to-start-with-time-paused"><a href="#configure-the-runtime-to-start-with-time-paused">Configure the runtime to start with time paused</a></h4> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[tokio::main(flavor = <span class="string">"current_thread"</span>, start_paused = <span class="bool-val">true</span>)] |
| </span><span class="kw">async fn </span>main() { |
| <span class="macro">println!</span>(<span class="string">"Hello world"</span>); |
| }</code></pre></div> |
| <p>Equivalent code not using <code>#[tokio::main]</code></p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">fn </span>main() { |
| tokio::runtime::Builder::new_current_thread() |
| .enable_all() |
| .start_paused(<span class="bool-val">true</span>) |
| .build() |
| .unwrap() |
| .block_on(<span class="kw">async </span>{ |
| <span class="macro">println!</span>(<span class="string">"Hello world"</span>); |
| }) |
| }</code></pre></div> |
| <p>Note that <code>start_paused</code> requires the <code>test-util</code> feature to be enabled.</p> |
| <h4 id="rename-package"><a href="#rename-package">Rename package</a></h4> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio <span class="kw">as </span>tokio1; |
| |
| <span class="attribute">#[tokio1::main(<span class="kw">crate </span>= <span class="string">"tokio1"</span>)] |
| </span><span class="kw">async fn </span>main() { |
| <span class="macro">println!</span>(<span class="string">"Hello world"</span>); |
| }</code></pre></div> |
| <p>Equivalent code not using <code>#[tokio::main]</code></p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio <span class="kw">as </span>tokio1; |
| |
| <span class="kw">fn </span>main() { |
| tokio1::runtime::Builder::new_multi_thread() |
| .enable_all() |
| .build() |
| .unwrap() |
| .block_on(<span class="kw">async </span>{ |
| <span class="macro">println!</span>(<span class="string">"Hello world"</span>); |
| }) |
| }</code></pre></div> |
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