<!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 `write_volatile` fn in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, write_volatile"><title>std::ptr::write_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='../../std/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'>std</a>::<wbr><a href='index.html'>ptr</a></p><script>window.sidebarCurrent = {name: 'write_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#945-947' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='../index.html'>std</a>::<wbr><a href='index.html'>ptr</a>::<wbr><a class="fn" href=''>write_volatile</a></span></h1><pre class='rust fn'>pub unsafe fn write_volatile<T>(dst: <a class="primitive" href="../primitive.pointer.html">*mut T</a>, src: T)</pre><div class='docblock'><p>Performs a volatile write of a memory location with the given value without reading or dropping the old value.</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 <a href="./fn.read_volatile.html"><code>read_volatile</code></a> or <code>write_volatile</code> should not be accessed with non-volatile operations.</p> <p><code>write_volatile</code> does not drop the contents of <code>dst</code>. This is safe, but it could leak allocations or resources, so care should be taken not to overwrite an object that should be dropped.</p> <p>Additionally, it does not drop <code>src</code>. Semantically, <code>src</code> is moved into the location pointed to by <code>dst</code>.</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>write_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>dst</code> must be <a href="../ptr/index.html#safety">valid</a> for writes.</p> </li> <li> <p><code>dst</code> must be properly aligned.</p> </li> </ul> <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>write_volatile</code> and any other operation (reading or writing) on 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="kw-2">mut</span> <span class="ident">x</span> <span class="op">=</span> <span class="number">0</span>; <span class="kw">let</span> <span class="ident">y</span> <span class="op">=</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">x</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">i32</span>; <span class="kw">let</span> <span class="ident">z</span> <span class="op">=</span> <span class="number">12</span>; <span class="kw">unsafe</span> { <span class="ident">std</span>::<span class="ident">ptr</span>::<span class="ident">write_volatile</span>(<span class="ident">y</span>, <span class="ident">z</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%20mut%20x%20%3D%200%3B%0Alet%20y%20%3D%20%26mut%20x%20as%20*mut%20i32%3B%0Alet%20z%20%3D%2012%3B%0A%0Aunsafe%20%7B%0A%20%20%20%20std%3A%3Aptr%3A%3Awrite_volatile(y%2C%20z)%3B%0A%20%20%20%20assert_eq!(std%3A%3Aptr%3A%3Aread_volatile(y)%2C%2012)%3B%0A%7D%0A%7D">Run</a></pre></div> </div></section><section id="search" class="content hidden"></section><section class="footer"></section><aside id="help" class="hidden"><div><h1 class="hidden">Help</h1><div class="shortcuts"><h2>Keyboard Shortcuts</h2><dl><dt><kbd>?</kbd></dt><dd>Show this help dialog</dd><dt><kbd>S</kbd></dt><dd>Focus the search field</dd><dt><kbd>↑</kbd></dt><dd>Move up in search results</dd><dt><kbd>↓</kbd></dt><dd>Move down in search results</dd><dt><kbd>↹</kbd></dt><dd>Switch tab</dd><dt><kbd>⏎</kbd></dt><dd>Go to active search result</dd><dt><kbd>+</kbd></dt><dd>Expand all sections</dd><dt><kbd>-</kbd></dt><dd>Collapse all sections</dd></dl></div><div class="infos"><h2>Search Tricks</h2><p>Prefix searches with a type followed by a colon (e.g., <code>fn:</code>) to restrict the search to a given type.</p><p>Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>.</p><p>Search functions by type signature (e.g., <code>vec -> usize</code> or <code>* -> vec</code>)</p><p>Search multiple things at once by splitting your query with comma (e.g., <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../../";window.currentCrate = "std";</script><script src="../../aliases.js"></script><script src="../../main1.35.0.js"></script><script defer src="../../search-index.js"></script></body></html>