| <!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="Selects from a set of channel operations."><meta name="keywords" content="rust, rustlang, rust-lang, select"><title>select in crossbeam_channel - 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 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><a class="sidebar-logo" href="../crossbeam_channel/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="../crossbeam_channel/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 crossbeam_channel</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">Macro <a href="index.html">crossbeam_channel</a>::<wbr><a class="macro" href="#">select</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 class="srclink" href="../src/crossbeam_channel/select_macro.rs.html#1110-1116">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><div class="example-wrap"><pre class="rust macro"><code><span class="macro">macro_rules! </span>select { |
| ($(<span class="macro-nonterminal">$tokens</span>:tt)<span class="kw-2">*</span>) => { ... }; |
| }</code></pre></div> |
| </div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><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> |
| <h2 id="examples"><a href="#examples">Examples</a></h2> |
| <p>Block until a send or a receive operation is selected:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>crossbeam_channel::{select, unbounded}; |
| |
| <span class="kw">let </span>(s1, r1) = unbounded(); |
| <span class="kw">let </span>(s2, r2) = unbounded(); |
| s1.send(<span class="number">10</span>).unwrap(); |
| |
| <span class="comment">// Since both operations are initially ready, a random one will be executed. |
| </span><span class="macro">select! </span>{ |
| recv(r1) -> msg => <span class="macro">assert_eq!</span>(msg, <span class="prelude-val">Ok</span>(<span class="number">10</span>)), |
| send(s2, <span class="number">20</span>) -> res => { |
| <span class="macro">assert_eq!</span>(res, <span class="prelude-val">Ok</span>(())); |
| <span class="macro">assert_eq!</span>(r2.recv(), <span class="prelude-val">Ok</span>(<span class="number">20</span>)); |
| } |
| }</code></pre></div> |
| <p>Select from a set of operations without blocking:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::thread; |
| <span class="kw">use </span>std::time::Duration; |
| <span class="kw">use </span>crossbeam_channel::{select, unbounded}; |
| |
| <span class="kw">let </span>(s1, r1) = unbounded(); |
| <span class="kw">let </span>(s2, r2) = unbounded(); |
| |
| thread::spawn(<span class="kw">move </span>|| { |
| thread::sleep(Duration::from_secs(<span class="number">1</span>)); |
| s1.send(<span class="number">10</span>).unwrap(); |
| }); |
| thread::spawn(<span class="kw">move </span>|| { |
| thread::sleep(Duration::from_millis(<span class="number">500</span>)); |
| s2.send(<span class="number">20</span>).unwrap(); |
| }); |
| |
| <span class="comment">// None of the operations are initially ready. |
| </span><span class="macro">select! </span>{ |
| recv(r1) -> msg => <span class="macro">panic!</span>(), |
| recv(r2) -> msg => <span class="macro">panic!</span>(), |
| default => <span class="macro">println!</span>(<span class="string">"not ready"</span>), |
| }</code></pre></div> |
| <p>Select over a set of operations with a timeout:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::thread; |
| <span class="kw">use </span>std::time::Duration; |
| <span class="kw">use </span>crossbeam_channel::{select, unbounded}; |
| |
| <span class="kw">let </span>(s1, r1) = unbounded(); |
| <span class="kw">let </span>(s2, r2) = unbounded(); |
| |
| thread::spawn(<span class="kw">move </span>|| { |
| thread::sleep(Duration::from_secs(<span class="number">1</span>)); |
| s1.send(<span class="number">10</span>).unwrap(); |
| }); |
| thread::spawn(<span class="kw">move </span>|| { |
| thread::sleep(Duration::from_millis(<span class="number">500</span>)); |
| s2.send(<span class="number">20</span>).unwrap(); |
| }); |
| |
| <span class="comment">// None of the two operations will become ready within 100 milliseconds. |
| </span><span class="macro">select! </span>{ |
| recv(r1) -> msg => <span class="macro">panic!</span>(), |
| recv(r2) -> msg => <span class="macro">panic!</span>(), |
| default(Duration::from_millis(<span class="number">100</span>)) => <span class="macro">println!</span>(<span class="string">"timed out"</span>), |
| }</code></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"><code><span class="kw">use </span>std::thread; |
| <span class="kw">use </span>std::time::Duration; |
| <span class="kw">use </span>crossbeam_channel::{select, never, unbounded}; |
| |
| <span class="kw">let </span>(s1, r1) = unbounded(); |
| <span class="kw">let </span>(s2, r2) = unbounded(); |
| |
| thread::spawn(<span class="kw">move </span>|| { |
| thread::sleep(Duration::from_secs(<span class="number">1</span>)); |
| s1.send(<span class="number">10</span>).unwrap(); |
| }); |
| thread::spawn(<span class="kw">move </span>|| { |
| thread::sleep(Duration::from_millis(<span class="number">500</span>)); |
| s2.send(<span class="number">20</span>).unwrap(); |
| }); |
| |
| <span class="comment">// This receiver can be a `Some` or a `None`. |
| </span><span class="kw">let </span>r2 = <span class="prelude-val">Some</span>(<span class="kw-2">&</span>r2); |
| |
| <span class="comment">// None of the two operations will become ready within 100 milliseconds. |
| </span><span class="macro">select! </span>{ |
| recv(r1) -> msg => <span class="macro">panic!</span>(), |
| recv(r2.unwrap_or(<span class="kw-2">&</span>never())) -> msg => <span class="macro">assert_eq!</span>(msg, <span class="prelude-val">Ok</span>(<span class="number">20</span>)), |
| }</code></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> |
| </div></details></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="crossbeam_channel" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
