<!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_unaligned` fn in crate `core`."><meta name="keywords" content="rust, rustlang, rust-lang, write_unaligned"><title>core::ptr::write_unaligned - 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: 'write_unaligned', 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 1.17.0'>1.17.0</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#800-805' 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=''>write_unaligned</a></span></h1><pre class='rust fn'>pub unsafe fn write_unaligned<T>(dst: *mut T, src: T)</pre><div class='docblock'><p>Overwrites a memory location with the given value without reading or
dropping the old value.</p>
<p>Unlike <a href="./fn.write.html"><code>write</code></a>, the pointer may be unaligned.</p>
<p><code>write_unaligned</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>
<p>This is appropriate for initializing uninitialized memory, or overwriting
memory that has previously been read with <a href="./fn.read_unaligned.html"><code>read_unaligned</code></a>.</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><code>dst</code> must be <a href="../ptr/index.html#safety">valid</a> for writes.</li>
</ul>
<p>Note that even if <code>T</code> has size <code>0</code>, the pointer must be non-NULL.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>Access fields in a packed struct:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::{<span class="ident">mem</span>, <span class="ident">ptr</span>};
<span class="attribute">#[<span class="ident">repr</span>(<span class="ident">packed</span>, <span class="ident">C</span>)]</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Default</span>)]</span>
<span class="kw">struct</span> <span class="ident">Packed</span> {
<span class="ident">_padding</span>: <span class="ident">u8</span>,
<span class="ident">unaligned</span>: <span class="ident">u32</span>,
}
<span class="kw">let</span> <span class="ident">v</span> <span class="op">=</span> <span class="number">0x01020304</span>;
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">x</span>: <span class="ident">Packed</span> <span class="op">=</span> <span class="kw">unsafe</span> { <span class="ident">mem</span>::<span class="ident">zeroed</span>() };
<span class="kw">unsafe</span> {
<span class="comment">// Take a reference to a 32-bit integer which is not aligned.</span>
<span class="kw">let</span> <span class="ident">unaligned</span> <span class="op">=</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">x</span>.<span class="ident">unaligned</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">u32</span>;
<span class="comment">// Dereferencing normally will emit an aligned store instruction,</span>
<span class="comment">// causing undefined behavior because the pointer is not aligned.</span>
<span class="comment">// *unaligned = v; // ERROR</span>
<span class="comment">// Instead, use `write_unaligned` to write improperly aligned values.</span>
<span class="ident">ptr</span>::<span class="ident">write_unaligned</span>(<span class="ident">unaligned</span>, <span class="ident">v</span>);
}
<span class="comment">// Accessing unaligned values directly is safe.</span>
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">x</span>.<span class="ident">unaligned</span> <span class="op">==</span> <span class="ident">v</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%3A%7Bmem%2C%20ptr%7D%3B%0A%0A%23%5Brepr(packed%2C%20C)%5D%0A%23%5Bderive(Default)%5D%0Astruct%20Packed%20%7B%0A%20%20%20%20_padding%3A%20u8%2C%0A%20%20%20%20unaligned%3A%20u32%2C%0A%7D%0A%0Alet%20v%20%3D%200x01020304%3B%0Alet%20mut%20x%3A%20Packed%20%3D%20unsafe%20%7B%20mem%3A%3Azeroed()%20%7D%3B%0A%0Aunsafe%20%7B%0A%20%20%20%20%2F%2F%20Take%20a%20reference%20to%20a%2032-bit%20integer%20which%20is%20not%20aligned.%0A%20%20%20%20let%20unaligned%20%3D%20%26mut%20x.unaligned%20as%20*mut%20u32%3B%0A%0A%20%20%20%20%2F%2F%20Dereferencing%20normally%20will%20emit%20an%20aligned%20store%20instruction%2C%0A%20%20%20%20%2F%2F%20causing%20undefined%20behavior%20because%20the%20pointer%20is%20not%20aligned.%0A%20%20%20%20%2F%2F%20*unaligned%20%3D%20v%3B%20%2F%2F%20ERROR%0A%0A%20%20%20%20%2F%2F%20Instead%2C%20use%20%60write_unaligned%60%20to%20write%20improperly%20aligned%20values.%0A%20%20%20%20ptr%3A%3Awrite_unaligned(unaligned%2C%20v)%3B%0A%7D%0A%0A%2F%2F%20Accessing%20unaligned%20values%20directly%20is%20safe.%0Aassert!(x.unaligned%20%3D%3D%20v)%3B%0A%7D">Run</a></pre></div>
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