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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 the first branch completes, cancelling the remaining branches."><meta name="keywords" content="rust, rustlang, rust-lang, select"><title>select 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 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">&#9776;</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">Macro <a href="index.html">tokio</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/tokio/macros/select.rs.html#401-623">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">&#x2212;</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="comment">// The index of the future to poll first (in bias mode), or the RNG
// expression to use to pick a future to poll first.
</span>start=<span class="macro-nonterminal">$start</span>:expr;
<span class="comment">// One `_` for each branch in the `select!` macro. Passing this to
// `count!` converts $skip to an integer.
</span>( $(<span class="macro-nonterminal">$count</span>:tt)* )
<span class="comment">// Normalized select branches. `( $skip )` is a set of `_` characters.
// There is one `_` for each select branch **before** this one. Given
// that all input futures are stored in a tuple, $skip is useful for
// generating a pattern to reference the future for the current branch.
// $skip is also used as an argument to `count!`, returning the index of
// the current select branch.
</span>$( ( $(<span class="macro-nonterminal">$skip</span>:tt)* ) <span class="macro-nonterminal">$bind</span>:pat = <span class="macro-nonterminal">$fut</span>:expr, <span class="kw">if </span><span class="macro-nonterminal">$c</span>:expr =&gt; <span class="macro-nonterminal">$handle</span>:expr, )+
<span class="comment">// Fallback expression used when all select branches have been disabled.
</span>; <span class="macro-nonterminal">$</span><span class="kw">else</span>:<span class="macro-nonterminal">expr
</span>}) =&gt; { ... };
(@ { start=<span class="macro-nonterminal">$start</span>:expr; $(<span class="macro-nonterminal">$t</span>:tt)* } ) =&gt; { ... };
(@ { start=<span class="macro-nonterminal">$start</span>:expr; $(<span class="macro-nonterminal">$t</span>:tt)* } <span class="kw">else </span>=&gt; <span class="macro-nonterminal">$</span><span class="kw">else</span>:<span class="macro-nonterminal">expr </span>$(,)<span class="question-mark">?</span>) =&gt; { ... };
(@ { start=<span class="macro-nonterminal">$start</span>:expr; ( $(<span class="macro-nonterminal">$s</span>:tt)* ) $(<span class="macro-nonterminal">$t</span>:tt)* } <span class="macro-nonterminal">$p</span>:pat = <span class="macro-nonterminal">$f</span>:expr, <span class="kw">if </span><span class="macro-nonterminal">$c</span>:expr =&gt; <span class="macro-nonterminal">$h</span>:block, $(<span class="macro-nonterminal">$r</span>:tt)* ) =&gt; { ... };
(@ { start=<span class="macro-nonterminal">$start</span>:expr; ( $(<span class="macro-nonterminal">$s</span>:tt)* ) $(<span class="macro-nonterminal">$t</span>:tt)* } <span class="macro-nonterminal">$p</span>:pat = <span class="macro-nonterminal">$f</span>:expr =&gt; <span class="macro-nonterminal">$h</span>:block, $(<span class="macro-nonterminal">$r</span>:tt)* ) =&gt; { ... };
(@ { start=<span class="macro-nonterminal">$start</span>:expr; ( $(<span class="macro-nonterminal">$s</span>:tt)* ) $(<span class="macro-nonterminal">$t</span>:tt)* } <span class="macro-nonterminal">$p</span>:pat = <span class="macro-nonterminal">$f</span>:expr, <span class="kw">if </span><span class="macro-nonterminal">$c</span>:expr =&gt; <span class="macro-nonterminal">$h</span>:block $(<span class="macro-nonterminal">$r</span>:tt)* ) =&gt; { ... };
(@ { start=<span class="macro-nonterminal">$start</span>:expr; ( $(<span class="macro-nonterminal">$s</span>:tt)* ) $(<span class="macro-nonterminal">$t</span>:tt)* } <span class="macro-nonterminal">$p</span>:pat = <span class="macro-nonterminal">$f</span>:expr =&gt; <span class="macro-nonterminal">$h</span>:block $(<span class="macro-nonterminal">$r</span>:tt)* ) =&gt; { ... };
(@ { start=<span class="macro-nonterminal">$start</span>:expr; ( $(<span class="macro-nonterminal">$s</span>:tt)* ) $(<span class="macro-nonterminal">$t</span>:tt)* } <span class="macro-nonterminal">$p</span>:pat = <span class="macro-nonterminal">$f</span>:expr, <span class="kw">if </span><span class="macro-nonterminal">$c</span>:expr =&gt; <span class="macro-nonterminal">$h</span>:expr ) =&gt; { ... };
(@ { start=<span class="macro-nonterminal">$start</span>:expr; ( $(<span class="macro-nonterminal">$s</span>:tt)* ) $(<span class="macro-nonterminal">$t</span>:tt)* } <span class="macro-nonterminal">$p</span>:pat = <span class="macro-nonterminal">$f</span>:expr =&gt; <span class="macro-nonterminal">$h</span>:expr ) =&gt; { ... };
(@ { start=<span class="macro-nonterminal">$start</span>:expr; ( $(<span class="macro-nonterminal">$s</span>:tt)* ) $(<span class="macro-nonterminal">$t</span>:tt)* } <span class="macro-nonterminal">$p</span>:pat = <span class="macro-nonterminal">$f</span>:expr, <span class="kw">if </span><span class="macro-nonterminal">$c</span>:expr =&gt; <span class="macro-nonterminal">$h</span>:expr, $(<span class="macro-nonterminal">$r</span>:tt)* ) =&gt; { ... };
(@ { start=<span class="macro-nonterminal">$start</span>:expr; ( $(<span class="macro-nonterminal">$s</span>:tt)* ) $(<span class="macro-nonterminal">$t</span>:tt)* } <span class="macro-nonterminal">$p</span>:pat = <span class="macro-nonterminal">$f</span>:expr =&gt; <span class="macro-nonterminal">$h</span>:expr, $(<span class="macro-nonterminal">$r</span>:tt)* ) =&gt; { ... };
(biased; <span class="macro-nonterminal">$p</span>:pat = $(<span class="macro-nonterminal">$t</span>:tt)* ) =&gt; { ... };
( <span class="macro-nonterminal">$p</span>:pat = $(<span class="macro-nonterminal">$t</span>:tt)* ) =&gt; { ... };
() =&gt; { ... };
}</code></pre></div>
</div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Waits on multiple concurrent branches, returning when the <strong>first</strong> branch
completes, cancelling the remaining branches.</p>
<p>The <code>select!</code> macro must be used inside of async functions, closures, and
blocks.</p>
<p>The <code>select!</code> macro accepts one or more branches with the following pattern:</p>
<div class="example-wrap"><pre class="language-text"><code>&lt;pattern&gt; = &lt;async expression&gt; (, if &lt;precondition&gt;)? =&gt; &lt;handler&gt;,</code></pre></div>
<p>Additionally, the <code>select!</code> macro may include a single, optional <code>else</code>
branch, which evaluates if none of the other branches match their patterns:</p>
<div class="example-wrap"><pre class="language-text"><code>else =&gt; &lt;expression&gt;</code></pre></div>
<p>The macro aggregates all <code>&lt;async expression&gt;</code> expressions and runs them
concurrently on the <strong>current</strong> task. Once the <strong>first</strong> expression
completes with a value that matches its <code>&lt;pattern&gt;</code>, the <code>select!</code> macro
returns the result of evaluating the completed branch’s <code>&lt;handler&gt;</code>
expression.</p>
<p>Additionally, each branch may include an optional <code>if</code> precondition. If the
precondition returns <code>false</code>, then the branch is disabled. The provided
<code>&lt;async expression&gt;</code> is still evaluated but the resulting future is never
polled. This capability is useful when using <code>select!</code> within a loop.</p>
<p>The complete lifecycle of a <code>select!</code> expression is as follows:</p>
<ol>
<li>Evaluate all provided <code>&lt;precondition&gt;</code> expressions. If the precondition
returns <code>false</code>, disable the branch for the remainder of the current call
to <code>select!</code>. Re-entering <code>select!</code> due to a loop clears the “disabled”
state.</li>
<li>Aggregate the <code>&lt;async expression&gt;</code>s from each branch, including the
disabled ones. If the branch is disabled, <code>&lt;async expression&gt;</code> is still
evaluated, but the resulting future is not polled.</li>
<li>Concurrently await on the results for all remaining <code>&lt;async expression&gt;</code>s.</li>
<li>Once an <code>&lt;async expression&gt;</code> returns a value, attempt to apply the value
to the provided <code>&lt;pattern&gt;</code>, if the pattern matches, evaluate <code>&lt;handler&gt;</code>
and return. If the pattern <strong>does not</strong> match, disable the current branch
and for the remainder of the current call to <code>select!</code>. Continue from step 3.</li>
<li>If <strong>all</strong> branches are disabled, evaluate the <code>else</code> expression. If no
else branch is provided, panic.</li>
</ol>
<h2 id="runtime-characteristics"><a href="#runtime-characteristics">Runtime characteristics</a></h2>
<p>By running all async expressions on the current task, the expressions are
able to run <strong>concurrently</strong> but not in <strong>parallel</strong>. This means all
expressions are run on the same thread and if one branch blocks the thread,
all other expressions will be unable to continue. If parallelism is
required, spawn each async expression using <a href="task/fn.spawn.html"><code>tokio::spawn</code></a> and pass the
join handle to <code>select!</code>.</p>
<h2 id="fairness"><a href="#fairness">Fairness</a></h2>
<p>By default, <code>select!</code> randomly picks a branch to check first. This provides
some level of fairness when calling <code>select!</code> in a loop with branches that
are always ready.</p>
<p>This behavior can be overridden by adding <code>biased;</code> to the beginning of the
macro usage. See the examples for details. This will cause <code>select</code> to poll
the futures in the order they appear from top to bottom. There are a few
reasons you may want this:</p>
<ul>
<li>The random number generation of <code>tokio::select!</code> has a non-zero CPU cost</li>
<li>Your futures may interact in a way where known polling order is significant</li>
</ul>
<p>But there is an important caveat to this mode. It becomes your responsibility
to ensure that the polling order of your futures is fair. If for example you
are selecting between a stream and a shutdown future, and the stream has a
huge volume of messages and zero or nearly zero time between them, you should
place the shutdown future earlier in the <code>select!</code> list to ensure that it is
always polled, and will not be ignored due to the stream being constantly
ready.</p>
<h2 id="panics"><a href="#panics">Panics</a></h2>
<p>The <code>select!</code> macro panics if all branches are disabled <strong>and</strong> there is no
provided <code>else</code> branch. A branch is disabled when the provided <code>if</code>
precondition returns <code>false</code> <strong>or</strong> when the pattern does not match the
result of <code>&lt;async expression&gt;</code>.</p>
<h2 id="cancellation-safety"><a href="#cancellation-safety">Cancellation safety</a></h2>
<p>When using <code>select!</code> in a loop to receive messages from multiple sources,
you should make sure that the receive call is cancellation safe to avoid
losing messages. This section goes through various common methods and
describes whether they are cancel safe. The lists in this section are not
exhaustive.</p>
<p>The following methods are cancellation safe:</p>
<ul>
<li><a href="sync/mpsc/struct.Receiver.html#method.recv"><code>tokio::sync::mpsc::Receiver::recv</code></a></li>
<li><a href="sync/mpsc/struct.UnboundedReceiver.html#method.recv"><code>tokio::sync::mpsc::UnboundedReceiver::recv</code></a></li>
<li><a href="sync/broadcast/struct.Receiver.html#method.recv"><code>tokio::sync::broadcast::Receiver::recv</code></a></li>
<li><a href="sync/watch/struct.Receiver.html#method.changed"><code>tokio::sync::watch::Receiver::changed</code></a></li>
<li><a href="net/struct.TcpListener.html#method.accept"><code>tokio::net::TcpListener::accept</code></a></li>
<li><a href="net/struct.UnixListener.html#method.accept"><code>tokio::net::UnixListener::accept</code></a></li>
<li><a href="crate::signal::unix::Signal::recv"><code>tokio::signal::unix::Signal::recv</code></a></li>
<li><a href="io/trait.AsyncReadExt.html#method.read"><code>tokio::io::AsyncReadExt::read</code></a> on any <code>AsyncRead</code></li>
<li><a href="io/trait.AsyncReadExt.html#method.read_buf"><code>tokio::io::AsyncReadExt::read_buf</code></a> on any <code>AsyncRead</code></li>
<li><a href="io/trait.AsyncWriteExt.html#method.write"><code>tokio::io::AsyncWriteExt::write</code></a> on any <code>AsyncWrite</code></li>
<li><a href="io/trait.AsyncWriteExt.html#method.write_buf"><code>tokio::io::AsyncWriteExt::write_buf</code></a> on any <code>AsyncWrite</code></li>
<li><a href="https://docs.rs/tokio-stream/0.1/tokio_stream/trait.StreamExt.html#method.next"><code>tokio_stream::StreamExt::next</code></a> on any <code>Stream</code></li>
<li><a href="https://docs.rs/futures/0.3/futures/stream/trait.StreamExt.html#method.next"><code>futures::stream::StreamExt::next</code></a> on any <code>Stream</code></li>
</ul>
<p>The following methods are not cancellation safe and can lead to loss of data:</p>
<ul>
<li><a href="io/trait.AsyncReadExt.html#method.read_exact"><code>tokio::io::AsyncReadExt::read_exact</code></a></li>
<li><a href="io/trait.AsyncReadExt.html#method.read_to_end"><code>tokio::io::AsyncReadExt::read_to_end</code></a></li>
<li><a href="io/trait.AsyncReadExt.html#method.read_to_string"><code>tokio::io::AsyncReadExt::read_to_string</code></a></li>
<li><a href="io/trait.AsyncWriteExt.html#method.write_all"><code>tokio::io::AsyncWriteExt::write_all</code></a></li>
</ul>
<p>The following methods are not cancellation safe because they use a queue for
fairness and cancellation makes you lose your place in the queue:</p>
<ul>
<li><a href="sync/struct.Mutex.html#method.lock"><code>tokio::sync::Mutex::lock</code></a></li>
<li><a href="sync/struct.RwLock.html#method.read"><code>tokio::sync::RwLock::read</code></a></li>
<li><a href="sync/struct.RwLock.html#method.write"><code>tokio::sync::RwLock::write</code></a></li>
<li><a href="sync/struct.Semaphore.html#method.acquire"><code>tokio::sync::Semaphore::acquire</code></a></li>
<li><a href="sync/struct.Notify.html#method.notified"><code>tokio::sync::Notify::notified</code></a></li>
</ul>
<p>To determine whether your own methods are cancellation safe, look for the
location of uses of <code>.await</code>. This is because when an asynchronous method is
cancelled, that always happens at an <code>.await</code>. If your function behaves
correctly even if it is restarted while waiting at an <code>.await</code>, then it is
cancellation safe.</p>
<p>Cancellation safety can be defined in the following way: If you have a
future that has not yet completed, then it must be a no-op to drop that
future and recreate it. This definition is motivated by the situation where
a <code>select!</code> is used in a loop. Without this guarantee, you would lose your
progress when another branch completes and you restart the <code>select!</code> by
going around the loop.</p>
<p>Be aware that cancelling something that is not cancellation safe is not
necessarily wrong. For example, if you are cancelling a task because the
application is shutting down, then you probably don’t care that partially
read data is lost.</p>
<h2 id="examples"><a href="#examples">Examples</a></h2>
<p>Basic select with two branches.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">async fn </span>do_stuff_async() {
<span class="comment">// async work
</span>}
<span class="kw">async fn </span>more_async_work() {
<span class="comment">// more here
</span>}
<span class="attribute">#[tokio::main]
</span><span class="kw">async fn </span>main() {
<span class="macro">tokio::select! </span>{
<span class="kw">_ </span>= do_stuff_async() =&gt; {
<span class="macro">println!</span>(<span class="string">&quot;do_stuff_async() completed first&quot;</span>)
}
<span class="kw">_ </span>= more_async_work() =&gt; {
<span class="macro">println!</span>(<span class="string">&quot;more_async_work() completed first&quot;</span>)
}
};
}</code></pre></div>
<p>Basic stream selecting.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio_stream::{<span class="self">self </span><span class="kw">as </span>stream, StreamExt};
<span class="attribute">#[tokio::main]
</span><span class="kw">async fn </span>main() {
<span class="kw">let </span><span class="kw-2">mut </span>stream1 = stream::iter(<span class="macro">vec!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]);
<span class="kw">let </span><span class="kw-2">mut </span>stream2 = stream::iter(<span class="macro">vec!</span>[<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>]);
<span class="kw">let </span>next = <span class="macro">tokio::select! </span>{
v = stream1.next() =&gt; v.unwrap(),
v = stream2.next() =&gt; v.unwrap(),
};
<span class="macro">assert!</span>(next == <span class="number">1 </span>|| next == <span class="number">4</span>);
}</code></pre></div>
<p>Collect the contents of two streams. In this example, we rely on pattern
matching and the fact that <code>stream::iter</code> is “fused”, i.e. once the stream
is complete, all calls to <code>next()</code> return <code>None</code>.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio_stream::{<span class="self">self </span><span class="kw">as </span>stream, StreamExt};
<span class="attribute">#[tokio::main]
</span><span class="kw">async fn </span>main() {
<span class="kw">let </span><span class="kw-2">mut </span>stream1 = stream::iter(<span class="macro">vec!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]);
<span class="kw">let </span><span class="kw-2">mut </span>stream2 = stream::iter(<span class="macro">vec!</span>[<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>]);
<span class="kw">let </span><span class="kw-2">mut </span>values = <span class="macro">vec!</span>[];
<span class="kw">loop </span>{
<span class="macro">tokio::select! </span>{
<span class="prelude-val">Some</span>(v) = stream1.next() =&gt; values.push(v),
<span class="prelude-val">Some</span>(v) = stream2.next() =&gt; values.push(v),
<span class="kw">else </span>=&gt; <span class="kw">break</span>,
}
}
values.sort();
<span class="macro">assert_eq!</span>(<span class="kw-2">&amp;</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>], <span class="kw-2">&amp;</span>values[..]);
}</code></pre></div>
<p>Using the same future in multiple <code>select!</code> expressions can be done by passing
a reference to the future. Doing so requires the future to be <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html"><code>Unpin</code></a>. A
future can be made <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html"><code>Unpin</code></a> by either using <a href="https://doc.rust-lang.org/nightly/alloc/boxed/struct.Box.html#method.pin"><code>Box::pin</code></a> or stack pinning.</p>
<p>Here, a stream is consumed for at most 1 second.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio_stream::{<span class="self">self </span><span class="kw">as </span>stream, StreamExt};
<span class="kw">use </span>tokio::time::{<span class="self">self</span>, Duration};
<span class="attribute">#[tokio::main]
</span><span class="kw">async fn </span>main() {
<span class="kw">let </span><span class="kw-2">mut </span>stream = stream::iter(<span class="macro">vec!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]);
<span class="kw">let </span>sleep = time::sleep(Duration::from_secs(<span class="number">1</span>));
<span class="macro">tokio::pin!</span>(sleep);
<span class="kw">loop </span>{
<span class="macro">tokio::select! </span>{
maybe_v = stream.next() =&gt; {
<span class="kw">if let </span><span class="prelude-val">Some</span>(v) = maybe_v {
<span class="macro">println!</span>(<span class="string">&quot;got = {}&quot;</span>, v);
} <span class="kw">else </span>{
<span class="kw">break</span>;
}
}
<span class="kw">_ </span>= <span class="kw-2">&amp;mut </span>sleep =&gt; {
<span class="macro">println!</span>(<span class="string">&quot;timeout&quot;</span>);
<span class="kw">break</span>;
}
}
}
}</code></pre></div>
<p>Joining two values using <code>select!</code>.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::sync::oneshot;
<span class="attribute">#[tokio::main]
</span><span class="kw">async fn </span>main() {
<span class="kw">let </span>(tx1, <span class="kw-2">mut </span>rx1) = oneshot::channel();
<span class="kw">let </span>(tx2, <span class="kw-2">mut </span>rx2) = oneshot::channel();
tokio::spawn(<span class="kw">async move </span>{
tx1.send(<span class="string">&quot;first&quot;</span>).unwrap();
});
tokio::spawn(<span class="kw">async move </span>{
tx2.send(<span class="string">&quot;second&quot;</span>).unwrap();
});
<span class="kw">let </span><span class="kw-2">mut </span>a = <span class="prelude-val">None</span>;
<span class="kw">let </span><span class="kw-2">mut </span>b = <span class="prelude-val">None</span>;
<span class="kw">while </span>a.is_none() || b.is_none() {
<span class="macro">tokio::select! </span>{
v1 = (<span class="kw-2">&amp;mut </span>rx1), <span class="kw">if </span>a.is_none() =&gt; a = <span class="prelude-val">Some</span>(v1.unwrap()),
v2 = (<span class="kw-2">&amp;mut </span>rx2), <span class="kw">if </span>b.is_none() =&gt; b = <span class="prelude-val">Some</span>(v2.unwrap()),
}
}
<span class="kw">let </span>res = (a.unwrap(), b.unwrap());
<span class="macro">assert_eq!</span>(res.<span class="number">0</span>, <span class="string">&quot;first&quot;</span>);
<span class="macro">assert_eq!</span>(res.<span class="number">1</span>, <span class="string">&quot;second&quot;</span>);
}</code></pre></div>
<p>Using the <code>biased;</code> mode to control polling order.</p>
<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="kw">let </span><span class="kw-2">mut </span>count = <span class="number">0u8</span>;
<span class="kw">loop </span>{
<span class="macro">tokio::select! </span>{
<span class="comment">// If you run this example without `biased;`, the polling order is
// pseudo-random, and the assertions on the value of count will
// (probably) fail.
</span>biased;
<span class="kw">_ </span>= <span class="kw">async </span>{}, <span class="kw">if </span>count &lt; <span class="number">1 </span>=&gt; {
count += <span class="number">1</span>;
<span class="macro">assert_eq!</span>(count, <span class="number">1</span>);
}
<span class="kw">_ </span>= <span class="kw">async </span>{}, <span class="kw">if </span>count &lt; <span class="number">2 </span>=&gt; {
count += <span class="number">1</span>;
<span class="macro">assert_eq!</span>(count, <span class="number">2</span>);
}
<span class="kw">_ </span>= <span class="kw">async </span>{}, <span class="kw">if </span>count &lt; <span class="number">3 </span>=&gt; {
count += <span class="number">1</span>;
<span class="macro">assert_eq!</span>(count, <span class="number">3</span>);
}
<span class="kw">_ </span>= <span class="kw">async </span>{}, <span class="kw">if </span>count &lt; <span class="number">4 </span>=&gt; {
count += <span class="number">1</span>;
<span class="macro">assert_eq!</span>(count, <span class="number">4</span>);
}
<span class="kw">else </span>=&gt; {
<span class="kw">break</span>;
}
};
}
}</code></pre></div>
<h3 id="avoid-racy-if-preconditions"><a href="#avoid-racy-if-preconditions">Avoid racy <code>if</code> preconditions</a></h3>
<p>Given that <code>if</code> preconditions are used to disable <code>select!</code> branches, some
caution must be used to avoid missing values.</p>
<p>For example, here is <strong>incorrect</strong> usage of <code>sleep</code> with <code>if</code>. The objective
is to repeatedly run an asynchronous task for up to 50 milliseconds.
However, there is a potential for the <code>sleep</code> completion to be missed.</p>
<div class="example-wrap should_panic"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::time::{<span class="self">self</span>, Duration};
<span class="kw">async fn </span>some_async_work() {
<span class="comment">// do work
</span>}
<span class="attribute">#[tokio::main]
</span><span class="kw">async fn </span>main() {
<span class="kw">let </span>sleep = time::sleep(Duration::from_millis(<span class="number">50</span>));
<span class="macro">tokio::pin!</span>(sleep);
<span class="kw">while </span>!sleep.is_elapsed() {
<span class="macro">tokio::select! </span>{
<span class="kw">_ </span>= <span class="kw-2">&amp;mut </span>sleep, <span class="kw">if </span>!sleep.is_elapsed() =&gt; {
<span class="macro">println!</span>(<span class="string">&quot;operation timed out&quot;</span>);
}
<span class="kw">_ </span>= some_async_work() =&gt; {
<span class="macro">println!</span>(<span class="string">&quot;operation completed&quot;</span>);
}
}
}
<span class="macro">panic!</span>(<span class="string">&quot;This example shows how not to do it!&quot;</span>);
}</code></pre></div>
<p>In the above example, <code>sleep.is_elapsed()</code> may return <code>true</code> even if
<code>sleep.poll()</code> never returned <code>Ready</code>. This opens up a potential race
condition where <code>sleep</code> expires between the <code>while !sleep.is_elapsed()</code>
check and the call to <code>select!</code> resulting in the <code>some_async_work()</code> call to
run uninterrupted despite the sleep having elapsed.</p>
<p>One way to write the above example without the race would be:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::time::{<span class="self">self</span>, Duration};
<span class="kw">async fn </span>some_async_work() {
<span class="comment">// do work
</span>}
<span class="attribute">#[tokio::main]
</span><span class="kw">async fn </span>main() {
<span class="kw">let </span>sleep = time::sleep(Duration::from_millis(<span class="number">50</span>));
<span class="macro">tokio::pin!</span>(sleep);
<span class="kw">loop </span>{
<span class="macro">tokio::select! </span>{
<span class="kw">_ </span>= <span class="kw-2">&amp;mut </span>sleep =&gt; {
<span class="macro">println!</span>(<span class="string">&quot;operation timed out&quot;</span>);
<span class="kw">break</span>;
}
<span class="kw">_ </span>= some_async_work() =&gt; {
<span class="macro">println!</span>(<span class="string">&quot;operation completed&quot;</span>);
}
}
}
}</code></pre></div>
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