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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="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_macros - 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_macros</a></h2><div id="sidebar-vars" data-name="main" data-ty="attr" 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_macros/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">Attribute Macro <a href="index.html">tokio_macros</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></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_macros/lib.rs.html#179-181" title="goto source code">[src]</a></span></h1><div class="docblock 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
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helps set up a <code>Runtime</code> without requiring the user to use
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<a href="../tokio/runtime/struct.Runtime.html">Runtime</a> or
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<a href="../tokio/runtime/struct.Builder.html">Builder</a> directly.</p>
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<p>Note: This macro is designed to be simplistic and targets applications that
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do not require a complex setup. If the provided functionality is not
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sufficient, you may be interested in using
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<a href="../tokio/runtime/struct.Builder.html">Builder</a>, which provides a more
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powerful interface.</p>
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<p>Note: This macro can be used on any function and not just the <code>main</code>
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function. Using it on a non-main function makes the function behave as if it
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was synchronous by starting a new runtime each time it is called. If the
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function is called often, it is preferable to create the runtime using the
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runtime builder so the runtime can be reused across calls.</p>
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<h2 id="multi-threaded-runtime" class="section-header"><a href="#multi-threaded-runtime">Multi-threaded runtime</a></h2>
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<p>To use the multi-threaded runtime, the macro can be configured using</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[<span class="ident">tokio::main</span>(<span class="ident">flavor</span> <span class="op">=</span> <span class="string">"multi_thread"</span>, <span class="ident">worker_threads</span> <span class="op">=</span> <span class="number">10</span>)]</span></code></pre></div>
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<p>The <code>worker_threads</code> option configures the number of worker threads, and
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defaults to the number of cpus on the system. This is the default flavor.</p>
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<p>Note: The multi-threaded runtime requires the <code>rt-multi-thread</code> feature
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flag.</p>
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<h2 id="current-thread-runtime" class="section-header"><a href="#current-thread-runtime">Current thread runtime</a></h2>
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<p>To use the single-threaded runtime known as the <code>current_thread</code> runtime,
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the macro can be configured using</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[<span class="ident">tokio::main</span>(<span class="ident">flavor</span> <span class="op">=</span> <span class="string">"current_thread"</span>)]</span></code></pre></div>
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<h3 id="function-arguments" class="section-header"><a href="#function-arguments">Function arguments:</a></h3>
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<p>Arguments are allowed for any functions aside from <code>main</code> which is special</p>
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<h3 id="usage" class="section-header"><a href="#usage">Usage</a></h3><h4 id="using-the-multi-thread-runtime" class="section-header"><a href="#using-the-multi-thread-runtime">Using the multi-thread runtime</a></h4>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><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="macro">println!</span>(<span class="string">"Hello world"</span>);
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}</code></pre></div>
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<p>Equivalent code not using <code>#[tokio::main]</code></p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="ident">tokio::runtime::Builder::new_multi_thread</span>()
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.<span class="ident">enable_all</span>()
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.<span class="ident">build</span>()
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.<span class="ident">unwrap</span>()
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.<span class="ident">block_on</span>(<span class="kw">async</span> {
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<span class="macro">println!</span>(<span class="string">"Hello world"</span>);
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})
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}</code></pre></div>
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<h4 id="using-current-thread-runtime" class="section-header"><a href="#using-current-thread-runtime">Using current thread runtime</a></h4>
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<p>The basic scheduler is single-threaded.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[<span class="ident">tokio::main</span>(<span class="ident">flavor</span> <span class="op">=</span> <span class="string">"current_thread"</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="macro">println!</span>(<span class="string">"Hello world"</span>);
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}</code></pre></div>
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<p>Equivalent code not using <code>#[tokio::main]</code></p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="ident">tokio::runtime::Builder::new_current_thread</span>()
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.<span class="ident">enable_all</span>()
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.<span class="ident">build</span>()
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.<span class="ident">unwrap</span>()
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.<span class="ident">block_on</span>(<span class="kw">async</span> {
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<span class="macro">println!</span>(<span class="string">"Hello world"</span>);
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})
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}</code></pre></div>
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<h4 id="set-number-of-worker-threads" class="section-header"><a href="#set-number-of-worker-threads">Set number of worker threads</a></h4>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[<span class="ident">tokio::main</span>(<span class="ident">worker_threads</span> <span class="op">=</span> <span class="number">2</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="macro">println!</span>(<span class="string">"Hello world"</span>);
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}</code></pre></div>
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<p>Equivalent code not using <code>#[tokio::main]</code></p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="ident">tokio::runtime::Builder::new_multi_thread</span>()
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.<span class="ident">worker_threads</span>(<span class="number">2</span>)
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.<span class="ident">enable_all</span>()
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.<span class="ident">build</span>()
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.<span class="ident">unwrap</span>()
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.<span class="ident">block_on</span>(<span class="kw">async</span> {
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<span class="macro">println!</span>(<span class="string">"Hello world"</span>);
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})
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}</code></pre></div>
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<h4 id="configure-the-runtime-to-start-with-time-paused" class="section-header"><a href="#configure-the-runtime-to-start-with-time-paused">Configure the runtime to start with time paused</a></h4>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[<span class="ident">tokio::main</span>(<span class="ident">flavor</span> <span class="op">=</span> <span class="string">"current_thread"</span>, <span class="ident">start_paused</span> <span class="op">=</span> <span class="bool-val">true</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="macro">println!</span>(<span class="string">"Hello world"</span>);
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}</code></pre></div>
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<p>Equivalent code not using <code>#[tokio::main]</code></p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="ident">tokio::runtime::Builder::new_current_thread</span>()
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.<span class="ident">enable_all</span>()
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.<span class="ident">start_paused</span>(<span class="bool-val">true</span>)
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.<span class="ident">build</span>()
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.<span class="ident">unwrap</span>()
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.<span class="ident">block_on</span>(<span class="kw">async</span> {
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<span class="macro">println!</span>(<span class="string">"Hello world"</span>);
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})
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}</code></pre></div>
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<p>Note that <code>start_paused</code> requires the <code>test-util</code> feature to be enabled.</p>
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<h4 id="note" class="section-header"><a href="#note">NOTE:</a></h4>
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<p>If you rename the Tokio crate in your dependencies this macro will not work.
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If you must rename the current version of Tokio because you’re also using an
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older version of Tokio, you <em>must</em> make the current version of Tokio
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available as <code>tokio</code> in the module where this macro is expanded.</p>
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