<!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 `fence` fn in crate `core`."><meta name="keywords" content="rust, rustlang, rust-lang, fence"><title>core::sync::atomic::fence - 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><link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"></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='../../../core/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='../../index.html'>core</a>::<wbr><a href='../index.html'>sync</a>::<wbr><a href='index.html'>atomic</a></p><script>window.sidebarCurrent = {name: 'fence', 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"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><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='in-band'>Function <a href='../../index.html'>core</a>::<wbr><a href='../index.html'>sync</a>::<wbr><a href='index.html'>atomic</a>::<wbr><a class="fn" href=''>fence</a></span><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='../../../src/core/sync/atomic.rs.html#1983-1994' title='goto source code'>[src]</a></span></h1><pre class='rust fn'>pub fn fence(order: <a class="enum" href="../../../core/sync/atomic/enum.Ordering.html" title="enum core::sync::atomic::Ordering">Ordering</a>)</pre><div class='docblock'><p>An atomic fence.</p>
<p>Depending on the specified order, a fence prevents the compiler and CPU from
reordering certain types of memory operations around it.
That creates synchronizes-with relationships between it and atomic operations
or fences in other threads.</p>
<p>A fence 'A' which has (at least) <a href="enum.Ordering.html#variant.Release"><code>Release</code></a> ordering semantics, synchronizes
with a fence 'B' with (at least) <a href="enum.Ordering.html#variant.Acquire"><code>Acquire</code></a> semantics, if and only if there
exist operations X and Y, both operating on some atomic object 'M' such
that A is sequenced before X, Y is synchronized before B and Y observes
the change to M. This provides a happens-before dependence between A and B.</p>
<pre><code class="language-text"> Thread 1 Thread 2
fence(Release); A --------------
x.store(3, Relaxed); X --------- |
| |
| |
-------------> Y if x.load(Relaxed) == 3 {
|-------> B fence(Acquire);
...
}
</code></pre>
<p>Atomic operations with <a href="enum.Ordering.html#variant.Release"><code>Release</code></a> or <a href="enum.Ordering.html#variant.Acquire"><code>Acquire</code></a> semantics can also synchronize
with a fence.</p>
<p>A fence which has <a href="enum.Ordering.html#variant.SeqCst"><code>SeqCst</code></a> ordering, in addition to having both <a href="enum.Ordering.html#variant.Acquire"><code>Acquire</code></a>
and <a href="enum.Ordering.html#variant.Release"><code>Release</code></a> semantics, participates in the global program order of the
other <a href="enum.Ordering.html#variant.SeqCst"><code>SeqCst</code></a> operations and/or fences.</p>
<p>Accepts <a href="enum.Ordering.html#variant.Acquire"><code>Acquire</code></a>, <a href="enum.Ordering.html#variant.Release"><code>Release</code></a>, <a href="enum.Ordering.html#variant.AcqRel"><code>AcqRel</code></a> and <a href="enum.Ordering.html#variant.SeqCst"><code>SeqCst</code></a> orderings.</p>
<h1 id="panics" class="section-header"><a href="#panics">Panics</a></h1>
<p>Panics if <code>order</code> is <a href="enum.Ordering.html#variant.Relaxed"><code>Relaxed</code></a>.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::<span class="ident">AtomicBool</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::<span class="ident">fence</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::<span class="ident">Ordering</span>;
<span class="comment">// A mutual exclusion primitive based on spinlock.</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Mutex</span> {
<span class="ident">flag</span>: <span class="ident">AtomicBool</span>,
}
<span class="kw">impl</span> <span class="ident">Mutex</span> {
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">new</span>() <span class="op">-></span> <span class="ident">Mutex</span> {
<span class="ident">Mutex</span> {
<span class="ident">flag</span>: <span class="ident">AtomicBool</span>::<span class="ident">new</span>(<span class="bool-val">false</span>),
}
}
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">lock</span>(<span class="kw-2">&</span><span class="self">self</span>) {
<span class="kw">while</span> <span class="op">!</span><span class="self">self</span>.<span class="ident">flag</span>.<span class="ident">compare_and_swap</span>(<span class="bool-val">false</span>, <span class="bool-val">true</span>, <span class="ident">Ordering</span>::<span class="ident">Relaxed</span>) {}
<span class="comment">// This fence synchronizes-with store in `unlock`.</span>
<span class="ident">fence</span>(<span class="ident">Ordering</span>::<span class="ident">Acquire</span>);
}
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">unlock</span>(<span class="kw-2">&</span><span class="self">self</span>) {
<span class="self">self</span>.<span class="ident">flag</span>.<span class="ident">store</span>(<span class="bool-val">false</span>, <span class="ident">Ordering</span>::<span class="ident">Release</span>);
}
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Async%3A%3Aatomic%3A%3AAtomicBool%3B%0Ause%20std%3A%3Async%3A%3Aatomic%3A%3Afence%3B%0Ause%20std%3A%3Async%3A%3Aatomic%3A%3AOrdering%3B%0A%0A%2F%2F%20A%20mutual%20exclusion%20primitive%20based%20on%20spinlock.%0Apub%20struct%20Mutex%20%7B%0A%20%20%20%20flag%3A%20AtomicBool%2C%0A%7D%0A%0Aimpl%20Mutex%20%7B%0A%20%20%20%20pub%20fn%20new()%20-%3E%20Mutex%20%7B%0A%20%20%20%20%20%20%20%20Mutex%20%7B%0A%20%20%20%20%20%20%20%20%20%20%20%20flag%3A%20AtomicBool%3A%3Anew(false)%2C%0A%20%20%20%20%20%20%20%20%7D%0A%20%20%20%20%7D%0A%0A%20%20%20%20pub%20fn%20lock(%26self)%20%7B%0A%20%20%20%20%20%20%20%20while%20!self.flag.compare_and_swap(false%2C%20true%2C%20Ordering%3A%3ARelaxed)%20%7B%7D%0A%20%20%20%20%20%20%20%20%2F%2F%20This%20fence%20synchronizes-with%20store%20in%20%60unlock%60.%0A%20%20%20%20%20%20%20%20fence(Ordering%3A%3AAcquire)%3B%0A%20%20%20%20%7D%0A%0A%20%20%20%20pub%20fn%20unlock(%26self)%20%7B%0A%20%20%20%20%20%20%20%20self.flag.store(false%2C%20Ordering%3A%3ARelease)%3B%0A%20%20%20%20%7D%0A%7D%0A%7D">Run</a></pre>
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