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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="API documentation for the Rust `select` macro in crate `crossbeam`."><meta name="keywords" content="rust, rustlang, rust-lang, select"><title>crossbeam::channel::select - Rust</title><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="../../dark.css"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script src="../../storage.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="shortcut icon" href="../../favicon.ico"><style type="text/css">#crate-search{background-image:url("../../down-arrow.svg");}</style></head><body class="rustdoc macro"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">&#9776;</div><a href='../../crossbeam/index.html'><div class='logo-container'><img src='../../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='../index.html'>crossbeam</a>::<wbr><a href='index.html'>channel</a></p><script>window.sidebarCurrent = {name: 'select', ty: 'macro', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form js-only"><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><a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><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'>&#x2212;</span>]</a></span><a class='srclink' href='../../src/crossbeam_channel/select_macro.rs.html#1195-1201' title='goto source code'>[src]</a></span><span class='in-band'>Macro <a href='../index.html'>crossbeam</a>::<wbr><a href='index.html'>channel</a>::<wbr><a class="macro" href=''>select</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><div class="example-wrap"><pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">select</span> {
($ ($ <span class="ident">tokens</span> : <span class="ident">tt</span>) <span class="kw-2">*</span>) <span class="op">=</span><span class="op">&gt;</span> { ... };
}</pre></div>
</div><div class='docblock'><p>Selects from a set of channel operations.</p>
<p>This macro allows you to define a set of channel operations, wait until any one of them becomes
ready, and finally execute it. If multiple operations are ready at the same time, a random one
among them is selected.</p>
<p>It is also possible to define a <code>default</code> case that gets executed if none of the operations are
ready, either right away or for a certain duration of time.</p>
<p>An operation is considered to be ready if it doesn't have to block. Note that it is ready even
when it will simply return an error because the channel is disconnected.</p>
<p>The <code>select</code> macro is a convenience wrapper around <a href="struct.Select.html"><code>Select</code></a>. However, it cannot select over a
dynamically created list of channel operations.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>Block until a send or a receive operation is selected:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">thread</span>;
<span class="kw">use</span> <span class="ident">crossbeam_channel</span>::<span class="ident">unbounded</span>;
<span class="kw">let</span> (<span class="ident">s1</span>, <span class="ident">r1</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
<span class="kw">let</span> (<span class="ident">s2</span>, <span class="ident">r2</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
<span class="ident">s1</span>.<span class="ident">send</span>(<span class="number">10</span>).<span class="ident">unwrap</span>();
<span class="comment">// Since both operations are initially ready, a random one will be executed.</span>
<span class="macro">select</span><span class="macro">!</span> {
<span class="ident">recv</span>(<span class="ident">r1</span>) <span class="op">-</span><span class="op">&gt;</span> <span class="ident">msg</span> <span class="op">=</span><span class="op">&gt;</span> <span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">msg</span>, <span class="prelude-val">Ok</span>(<span class="number">10</span>)),
<span class="ident">send</span>(<span class="ident">s2</span>, <span class="number">20</span>) <span class="op">-</span><span class="op">&gt;</span> <span class="ident">res</span> <span class="op">=</span><span class="op">&gt;</span> {
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">res</span>, <span class="prelude-val">Ok</span>(()));
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">r2</span>.<span class="ident">recv</span>(), <span class="prelude-val">Ok</span>(<span class="number">20</span>));
}
}</pre></div>
<p>Select from a set of operations without blocking:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">thread</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">time</span>::<span class="ident">Duration</span>;
<span class="kw">use</span> <span class="ident">crossbeam_channel</span>::<span class="ident">unbounded</span>;
<span class="kw">let</span> (<span class="ident">s1</span>, <span class="ident">r1</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
<span class="kw">let</span> (<span class="ident">s2</span>, <span class="ident">r2</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
<span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
<span class="ident">thread</span>::<span class="ident">sleep</span>(<span class="ident">Duration</span>::<span class="ident">from_secs</span>(<span class="number">1</span>));
<span class="ident">s1</span>.<span class="ident">send</span>(<span class="number">10</span>).<span class="ident">unwrap</span>();
});
<span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
<span class="ident">thread</span>::<span class="ident">sleep</span>(<span class="ident">Duration</span>::<span class="ident">from_millis</span>(<span class="number">500</span>));
<span class="ident">s2</span>.<span class="ident">send</span>(<span class="number">20</span>).<span class="ident">unwrap</span>();
});
<span class="comment">// None of the operations are initially ready.</span>
<span class="macro">select</span><span class="macro">!</span> {
<span class="ident">recv</span>(<span class="ident">r1</span>) <span class="op">-</span><span class="op">&gt;</span> <span class="ident">msg</span> <span class="op">=</span><span class="op">&gt;</span> <span class="macro">panic</span><span class="macro">!</span>(),
<span class="ident">recv</span>(<span class="ident">r2</span>) <span class="op">-</span><span class="op">&gt;</span> <span class="ident">msg</span> <span class="op">=</span><span class="op">&gt;</span> <span class="macro">panic</span><span class="macro">!</span>(),
<span class="ident">default</span> <span class="op">=</span><span class="op">&gt;</span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">&quot;not ready&quot;</span>),
}</pre></div>
<p>Select over a set of operations with a timeout:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">thread</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">time</span>::<span class="ident">Duration</span>;
<span class="kw">use</span> <span class="ident">crossbeam_channel</span>::<span class="ident">unbounded</span>;
<span class="kw">let</span> (<span class="ident">s1</span>, <span class="ident">r1</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
<span class="kw">let</span> (<span class="ident">s2</span>, <span class="ident">r2</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
<span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
<span class="ident">thread</span>::<span class="ident">sleep</span>(<span class="ident">Duration</span>::<span class="ident">from_secs</span>(<span class="number">1</span>));
<span class="ident">s1</span>.<span class="ident">send</span>(<span class="number">10</span>).<span class="ident">unwrap</span>();
});
<span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
<span class="ident">thread</span>::<span class="ident">sleep</span>(<span class="ident">Duration</span>::<span class="ident">from_millis</span>(<span class="number">500</span>));
<span class="ident">s2</span>.<span class="ident">send</span>(<span class="number">20</span>).<span class="ident">unwrap</span>();
});
<span class="comment">// None of the two operations will become ready within 100 milliseconds.</span>
<span class="macro">select</span><span class="macro">!</span> {
<span class="ident">recv</span>(<span class="ident">r1</span>) <span class="op">-</span><span class="op">&gt;</span> <span class="ident">msg</span> <span class="op">=</span><span class="op">&gt;</span> <span class="macro">panic</span><span class="macro">!</span>(),
<span class="ident">recv</span>(<span class="ident">r2</span>) <span class="op">-</span><span class="op">&gt;</span> <span class="ident">msg</span> <span class="op">=</span><span class="op">&gt;</span> <span class="macro">panic</span><span class="macro">!</span>(),
<span class="ident">default</span>(<span class="ident">Duration</span>::<span class="ident">from_millis</span>(<span class="number">100</span>)) <span class="op">=</span><span class="op">&gt;</span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">&quot;timed out&quot;</span>),
}</pre></div>
<p>Optionally add a receive operation to <code>select!</code> using <a href="fn.never.html"><code>never</code></a>:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">thread</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">time</span>::<span class="ident">Duration</span>;
<span class="kw">use</span> <span class="ident">crossbeam_channel</span>::{<span class="ident">never</span>, <span class="ident">unbounded</span>};
<span class="kw">let</span> (<span class="ident">s1</span>, <span class="ident">r1</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
<span class="kw">let</span> (<span class="ident">s2</span>, <span class="ident">r2</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
<span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
<span class="ident">thread</span>::<span class="ident">sleep</span>(<span class="ident">Duration</span>::<span class="ident">from_secs</span>(<span class="number">1</span>));
<span class="ident">s1</span>.<span class="ident">send</span>(<span class="number">10</span>).<span class="ident">unwrap</span>();
});
<span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
<span class="ident">thread</span>::<span class="ident">sleep</span>(<span class="ident">Duration</span>::<span class="ident">from_millis</span>(<span class="number">500</span>));
<span class="ident">s2</span>.<span class="ident">send</span>(<span class="number">20</span>).<span class="ident">unwrap</span>();
});
<span class="comment">// This receiver can be a `Some` or a `None`.</span>
<span class="kw">let</span> <span class="ident">r2</span> <span class="op">=</span> <span class="prelude-val">Some</span>(<span class="kw-2">&amp;</span><span class="ident">r2</span>);
<span class="comment">// None of the two operations will become ready within 100 milliseconds.</span>
<span class="macro">select</span><span class="macro">!</span> {
<span class="ident">recv</span>(<span class="ident">r1</span>) <span class="op">-</span><span class="op">&gt;</span> <span class="ident">msg</span> <span class="op">=</span><span class="op">&gt;</span> <span class="macro">panic</span><span class="macro">!</span>(),
<span class="ident">recv</span>(<span class="ident">r2</span>.<span class="ident">unwrap_or</span>(<span class="kw-2">&amp;</span><span class="ident">never</span>())) <span class="op">-</span><span class="op">&gt;</span> <span class="ident">msg</span> <span class="op">=</span><span class="op">&gt;</span> <span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">msg</span>, <span class="prelude-val">Ok</span>(<span class="number">20</span>)),
}</pre></div>
<p>To optionally add a timeout to <code>select!</code>, see the <a href="fn.never.html#examples">example</a> for <a href="fn.never.html"><code>never</code></a>.</p>
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