<!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 `park` fn in crate `alacritty`."><meta name="keywords" content="rust, rustlang, rust-lang, park"><title>alacritty::util::thread::park - 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 fn"><!--[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">☰</div><a href='../../../alacritty/index.html'><img src='../../../rust-logo.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='../../index.html'>alacritty</a>::<wbr><a href='../index.html'>util</a>::<wbr><a href='index.html'>thread</a></p><script>window.sidebarCurrent = {name: 'park', ty: 'fn', 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 class='since' title='Stable since Rust version 1.0.0'>1.0.0</span><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='https://doc.rust-lang.org/nightly/src/std/thread/mod.rs.html#881-914' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='../../index.html'>alacritty</a>::<wbr><a href='../index.html'>util</a>::<wbr><a href='index.html'>thread</a>::<wbr><a class="fn" href=''>park</a></span></h1><pre class='rust fn'>pub fn park()</pre><div class='docblock'><p>Blocks unless or until the current thread's token is made available.</p>
<p>A call to <code>park</code> does not guarantee that the thread will remain parked
forever, and callers should be prepared for this possibility.</p>
<h1 id="park-and-unpark" class="section-header"><a href="#park-and-unpark">park and unpark</a></h1>
<p>Every thread is equipped with some basic low-level blocking support, via the
<a href="../../std/thread/fn.park.html"><code>thread::park</code></a> function and <a href="../../std/thread/struct.Thread.html#method.unpark"><code>thread::Thread::unpark</code></a>
method. <a href="../../std/thread/fn.park.html"><code>park</code></a> blocks the current thread, which can then be resumed from
another thread by calling the <a href="../../std/thread/struct.Thread.html#method.unpark"><code>unpark</code></a> method on the blocked thread's
handle.</p>
<p>Conceptually, each <a href="../../std/thread/struct.Thread.html"><code>Thread</code></a> handle has an associated token, which is
initially not present:</p>
<ul>
<li>
<p>The <a href="../../std/thread/fn.park.html"><code>thread::park</code></a> function blocks the current thread unless or
until the token is available for its thread handle, at which point it
atomically consumes the token. It may also return <em>spuriously</em>, without
consuming the token. <a href="../../std/thread/fn.park_timeout.html"><code>thread::park_timeout</code></a> does the same, but allows
specifying a maximum time to block the thread for.</p>
</li>
<li>
<p>The <a href="../../std/thread/struct.Thread.html#method.unpark"><code>unpark</code></a> method on a <a href="../../std/thread/struct.Thread.html"><code>Thread</code></a> atomically makes the token available
if it wasn't already. Because the token is initially absent, <a href="../../std/thread/struct.Thread.html#method.unpark"><code>unpark</code></a>
followed by <a href="../../std/thread/fn.park.html"><code>park</code></a> will result in the second call returning immediately.</p>
</li>
</ul>
<p>In other words, each <a href="../../std/thread/struct.Thread.html"><code>Thread</code></a> acts a bit like a spinlock that can be
locked and unlocked using <code>park</code> and <code>unpark</code>.</p>
<p>Notice that being unblocked does not imply any synchronization with someone
that unparked this thread, it could also be spurious.
For example, it would be a valid, but inefficient, implementation to make both <a href="../../std/thread/fn.park.html"><code>park</code></a> and
<a href="../../std/thread/struct.Thread.html#method.unpark"><code>unpark</code></a> return immediately without doing anything.</p>
<p>The API is typically used by acquiring a handle to the current thread,
placing that handle in a shared data structure so that other threads can
find it, and then <code>park</code>ing in a loop. When some desired condition is met, another
thread calls <a href="../../std/thread/struct.Thread.html#method.unpark"><code>unpark</code></a> on the handle.</p>
<p>The motivation for this design is twofold:</p>
<ul>
<li>
<p>It avoids the need to allocate mutexes and condvars when building new
synchronization primitives; the threads already provide basic
blocking/signaling.</p>
</li>
<li>
<p>It can be implemented very efficiently on many platforms.</p>
</li>
</ul>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<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">sync</span>::{<span class="ident">Arc</span>, <span class="ident">atomic</span>::{<span class="ident">Ordering</span>, <span class="ident">AtomicBool</span>}};
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">time</span>::<span class="ident">Duration</span>;
<span class="kw">let</span> <span class="ident">flag</span> <span class="op">=</span> <span class="ident">Arc</span>::<span class="ident">new</span>(<span class="ident">AtomicBool</span>::<span class="ident">new</span>(<span class="bool-val">false</span>));
<span class="kw">let</span> <span class="ident">flag2</span> <span class="op">=</span> <span class="ident">Arc</span>::<span class="ident">clone</span>(<span class="kw-2">&</span><span class="ident">flag</span>);
<span class="kw">let</span> <span class="ident">parked_thread</span> <span class="op">=</span> <span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span> <span class="op">||</span> {
<span class="comment">// We want to wait until the flag is set. We *could* just spin, but using</span>
<span class="comment">// park/unpark is more efficient.</span>
<span class="kw">while</span> <span class="op">!</span><span class="ident">flag2</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Acquire</span>) {
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"Parking thread"</span>);
<span class="ident">thread</span>::<span class="ident">park</span>();
<span class="comment">// We *could* get here spuriously, i.e., way before the 10ms below are over!</span>
<span class="comment">// But that is no problem, we are in a loop until the flag is set anyway.</span>
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"Thread unparked"</span>);
}
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"Flag received"</span>);
});
<span class="comment">// Let some time pass for the thread to be spawned.</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">10</span>));
<span class="comment">// Set the flag, and let the thread wake up.</span>
<span class="comment">// There is no race condition here, if `unpark`</span>
<span class="comment">// happens first, `park` will return immediately.</span>
<span class="comment">// Hence there is no risk of a deadlock.</span>
<span class="ident">flag</span>.<span class="ident">store</span>(<span class="bool-val">true</span>, <span class="ident">Ordering</span>::<span class="ident">Release</span>);
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"Unpark the thread"</span>);
<span class="ident">parked_thread</span>.<span class="ident">thread</span>().<span class="ident">unpark</span>();
<span class="ident">parked_thread</span>.<span class="ident">join</span>().<span class="ident">unwrap</span>();</pre></div>
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