<!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_bytes` fn in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, write_bytes"><title>std::intrinsics::write_bytes - 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'>intrinsics</a></p><script>window.sidebarCurrent = {name: 'write_bytes', 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 1.0.0'>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/intrinsics.rs.html#1556-1558' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='../index.html'>std</a>::<wbr><a href='index.html'>intrinsics</a>::<wbr><a class="fn" href=''>write_bytes</a></span></h1><pre class='rust fn'>pub unsafe fn write_bytes<T>(dst: <a class="primitive" href="../primitive.pointer.html">*mut T</a>, val: <a class="primitive" href="../primitive.u8.html">u8</a>, count: <a class="primitive" href="../primitive.usize.html">usize</a>)</pre><div class='docblock'><p>Sets <code>count * size_of::<T>()</code> bytes of memory starting at <code>dst</code> to
<code>val</code>.</p>
<p><code>write_bytes</code> is similar to C's <a href="https://en.cppreference.com/w/c/string/byte/memset"><code>memset</code></a>, but sets <code>count * size_of::<T>()</code> bytes to <code>val</code>.</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 of <code>count * size_of::<T>()</code> bytes.</p>
</li>
<li>
<p><code>dst</code> must be properly aligned.</p>
</li>
</ul>
<p>Additionally, the caller must ensure that writing <code>count * size_of::<T>()</code> bytes to the given region of memory results in a valid
value of <code>T</code>. Using a region of memory typed as a <code>T</code> that contains an
invalid value of <code>T</code> is undefined behavior.</p>
<p>Note that even if the effectively copied size (<code>count * size_of::<T>()</code>) is
<code>0</code>, the pointer must be non-NULL and properly aligned.</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">use</span> <span class="ident">std</span>::<span class="ident">ptr</span>;
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">vec</span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="number">0u32</span>; <span class="number">4</span>];
<span class="kw">unsafe</span> {
<span class="kw">let</span> <span class="ident">vec_ptr</span> <span class="op">=</span> <span class="ident">vec</span>.<span class="ident">as_mut_ptr</span>();
<span class="ident">ptr</span>::<span class="ident">write_bytes</span>(<span class="ident">vec_ptr</span>, <span class="number">0xfe</span>, <span class="number">2</span>);
}
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">vec</span>, [<span class="number">0xfefefefe</span>, <span class="number">0xfefefefe</span>, <span class="number">0</span>, <span class="number">0</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%3Aptr%3B%0A%0Alet%20mut%20vec%20%3D%20vec!%5B0u32%3B%204%5D%3B%0Aunsafe%20%7B%0A%20%20%20%20let%20vec_ptr%20%3D%20vec.as_mut_ptr()%3B%0A%20%20%20%20ptr%3A%3Awrite_bytes(vec_ptr%2C%200xfe%2C%202)%3B%0A%7D%0Aassert_eq!(vec%2C%20%5B0xfefefefe%2C%200xfefefefe%2C%200%2C%200%5D)%3B%0A%7D">Run</a></pre></div>
<p>Creating an invalid value:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">ptr</span>;
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">v</span> <span class="op">=</span> <span class="ident">Box</span>::<span class="ident">new</span>(<span class="number">0i32</span>);
<span class="kw">unsafe</span> {
<span class="comment">// Leaks the previously held value by overwriting the `Box<T>` with</span>
<span class="comment">// a null pointer.</span>
<span class="ident">ptr</span>::<span class="ident">write_bytes</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">v</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">Box</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span>, <span class="number">0</span>, <span class="number">1</span>);
}
<span class="comment">// At this point, using or dropping `v` results in undefined behavior.</span>
<span class="comment">// drop(v); // ERROR</span>
<span class="comment">// Even leaking `v` "uses" it, and hence is undefined behavior.</span>
<span class="comment">// mem::forget(v); // ERROR</span>
<span class="comment">// In fact, `v` is invalid according to basic type layout invariants, so *any*</span>
<span class="comment">// operation touching it is undefined behavior.</span>
<span class="comment">// let v2 = v; // ERROR</span>
<span class="kw">unsafe</span> {
<span class="comment">// Let us instead put in a valid value</span>
<span class="ident">ptr</span>::<span class="ident">write</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">v</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">Box</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span>, <span class="ident">Box</span>::<span class="ident">new</span>(<span class="number">42i32</span>));
}
<span class="comment">// Now the box is fine</span>
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="kw-2">*</span><span class="ident">v</span>, <span class="number">42</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%3Aptr%3B%0A%0Alet%20mut%20v%20%3D%20Box%3A%3Anew(0i32)%3B%0A%0Aunsafe%20%7B%0A%20%20%20%20%2F%2F%20Leaks%20the%20previously%20held%20value%20by%20overwriting%20the%20%60Box%3CT%3E%60%20with%0A%20%20%20%20%2F%2F%20a%20null%20pointer.%0A%20%20%20%20ptr%3A%3Awrite_bytes(%26mut%20v%20as%20*mut%20Box%3Ci32%3E%2C%200%2C%201)%3B%0A%7D%0A%0A%2F%2F%20At%20this%20point%2C%20using%20or%20dropping%20%60v%60%20results%20in%20undefined%20behavior.%0A%2F%2F%20drop(v)%3B%20%2F%2F%20ERROR%0A%0A%2F%2F%20Even%20leaking%20%60v%60%20%22uses%22%20it%2C%20and%20hence%20is%20undefined%20behavior.%0A%2F%2F%20mem%3A%3Aforget(v)%3B%20%2F%2F%20ERROR%0A%0A%2F%2F%20In%20fact%2C%20%60v%60%20is%20invalid%20according%20to%20basic%20type%20layout%20invariants%2C%20so%20*any*%0A%2F%2F%20operation%20touching%20it%20is%20undefined%20behavior.%0A%2F%2F%20let%20v2%20%3D%20v%3B%20%2F%2F%20ERROR%0A%0Aunsafe%20%7B%0A%20%20%20%20%2F%2F%20Let%20us%20instead%20put%20in%20a%20valid%20value%0A%20%20%20%20ptr%3A%3Awrite(%26mut%20v%20as%20*mut%20Box%3Ci32%3E%2C%20Box%3A%3Anew(42i32))%3B%0A%7D%0A%0A%2F%2F%20Now%20the%20box%20is%20fine%0Aassert_eq!(*v%2C%2042)%3B%0A%7D">Run</a></pre></div>
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