<!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="../../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'>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="../../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'>ptr</a>::<wbr><a class="fn" href=''>read_volatile</a></span><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#452-454' title='goto source code'>[src]</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>
<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 no-ops
and may be ignored.</p>
<h1 id="safety" class="section-header"><a href="#safety">Safety</a></h1>
<p>Beyond accepting a raw pointer, this is unsafe because it semantically
moves the value out of <code>src</code> without preventing further usage of <code>src</code>.
If <code>T</code> is not <code>Copy</code>, then care must be taken to ensure that the value at
<code>src</code> is not used before the data is overwritten again (e.g. with <code>write</code>,
<code>write_bytes</code>, or <code>copy</code>). Note that <code>*src = foo</code> counts as a use
because it will attempt to drop the value previously at <code>*src</code>.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>Basic usage:</p>
<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>
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