<!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 `read_volatile` fn in crate `core`."><meta name="keywords" content="rust, rustlang, rust-lang, read_volatile"><title>core::ptr::read_volatile - Rust</title><link rel="stylesheet" type="text/css" href="../../normalize1.35.0.css"><link rel="stylesheet" type="text/css" href="../../rustdoc1.35.0.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../dark1.35.0.css"><link rel="stylesheet" type="text/css" href="../../light1.35.0.css" id="themeStyle"><script src="../../storage1.35.0.js"></script><noscript><link rel="stylesheet" href="../../noscript1.35.0.css"></noscript><link rel="shortcut icon" href="../../favicon1.35.0.ico"><style type="text/css">#crate-search{background-image:url("../../down-arrow1.35.0.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='../../core/index.html'><img src='../../rust-logo1.35.0.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='../index.html'>core</a>::<wbr><a href='index.html'>ptr</a></p><script>window.sidebarCurrent = {name: 'read_volatile', 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="../../brush1.35.0.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme1.35.0.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="../../wheel1.35.0.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 1.9.0'>1.9.0</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/ptr.rs.html#874-876' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='../index.html'>core</a>::<wbr><a href='index.html'>ptr</a>::<wbr><a class="fn" href=''>read_volatile</a></span></h1><pre class='rust fn'>pub unsafe fn read_volatile<T>(src: *const T) -> T</pre><div class='docblock'><p>Performs a volatile read of the value from <code>src</code> without moving it. This
leaves the memory in <code>src</code> unchanged.</p>
<p>Volatile operations are intended to act on I/O memory, and are guaranteed
to not be elided or reordered by the compiler across other volatile
operations.</p>
<p>Memory accessed with <code>read_volatile</code> or <a href="./fn.write_volatile.html"><code>write_volatile</code></a> should not be
accessed with non-volatile operations.</p>
<h1 id="notes" class="section-header"><a href="#notes">Notes</a></h1>
<p>Rust does not currently have a rigorously and formally defined memory model,
so the precise semantics of what "volatile" means here is subject to change
over time. That being said, the semantics will almost always end up pretty
similar to <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf">C11's definition of volatile</a>.</p>
<p>The compiler shouldn't change the relative order or number of volatile
memory operations. However, volatile memory operations on zero-sized types
(e.g., if a zero-sized type is passed to <code>read_volatile</code>) are noops
and may be ignored.</p>
<h1 id="safety" class="section-header"><a href="#safety">Safety</a></h1>
<p>Behavior is undefined if any of the following conditions are violated:</p>
<ul>
<li>
<p><code>src</code> must be <a href="../ptr/index.html#safety">valid</a> for reads.</p>
</li>
<li>
<p><code>src</code> must be properly aligned.</p>
</li>
</ul>
<p>Like <a href="./fn.read.html"><code>read</code></a>, <code>read_unaligned</code> creates a bitwise copy of <code>T</code>, regardless of
whether <code>T</code> is <a href="../marker/trait.Copy.html"><code>Copy</code></a>. If <code>T</code> is not <a href="../marker/trait.Copy.html"><code>Copy</code></a>, using both the returned
value and the value at <code>*src</code> can <a href="./fn.read.html#ownership-of-the-returned-value">violate memory safety</a>.
However, storing non-<a href="../marker/trait.Copy.html"><code>Copy</code></a> types in volatile memory is almost certainly
incorrect.</p>
<p>Note that even if <code>T</code> has size <code>0</code>, the pointer must be non-NULL and properly aligned.</p>
<p>Just like in C, whether an operation is volatile has no bearing whatsoever
on questions involving concurrent access from multiple threads. Volatile
accesses behave exactly like non-atomic accesses in that regard. In particular,
a race between a <code>read_volatile</code> and any write operation to the same location
is undefined behavior.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>Basic usage:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="number">12</span>;
<span class="kw">let</span> <span class="ident">y</span> <span class="op">=</span> <span class="kw-2">&</span><span class="ident">x</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw">const</span> <span class="ident">i32</span>;
<span class="kw">unsafe</span> {
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">std</span>::<span class="ident">ptr</span>::<span class="ident">read_volatile</span>(<span class="ident">y</span>), <span class="number">12</span>);
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20x%20%3D%2012%3B%0Alet%20y%20%3D%20%26x%20as%20*const%20i32%3B%0A%0Aunsafe%20%7B%0A%20%20%20%20assert_eq!(std%3A%3Aptr%3A%3Aread_volatile(y)%2C%2012)%3B%0A%7D%0A%7D">Run</a></pre></div>
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