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<!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 `OsRng` struct in crate `rand`."><meta name="keywords" content="rust, rustlang, rust-lang, OsRng"><title>rand::rngs::OsRng - 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://www.rust-lang.org/favicon.ico"></head><body class="rustdoc struct"><!--[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='../../rand/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk.png' alt='logo' width='100'></a><p class='location'>Struct OsRng</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#methods">Methods</a><div class="sidebar-links"><a href="#method.new">new</a></div><a class="sidebar-title" href="#implementations">Trait Implementations</a><div class="sidebar-links"><a href="#impl-Clone">Clone</a><a href="#impl-Debug">Debug</a><a href="#impl-CryptoRng">CryptoRng</a><a href="#impl-RngCore">RngCore</a></div><a class="sidebar-title" href="#synthetic-implementations">Auto Trait Implementations</a><div class="sidebar-links"><a href="#impl-Send">Send</a><a href="#impl-Sync">Sync</a></div></div><p class='location'><a href='../index.html'>rand</a>::<wbr><a href='index.html'>rngs</a></p><script>window.sidebarCurrent = {name: 'OsRng', ty: 'struct', 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'>Struct <a href='../index.html'>rand</a>::<wbr><a href='index.html'>rngs</a>::<wbr><a class="struct" href=''>OsRng</a></span><span class='out-of-band'><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/rand/rngs/os.rs.html#118' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class='rust struct'>pub struct OsRng(_);</pre></div><div class='docblock'><p>A random number generator that retrieves randomness straight from the
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operating system.</p>
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<p>This is the preferred external source of entropy for most applications.
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Commonly it is used to initialize a user-space RNG, which can then be used
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to generate random values with much less overhead than <code>OsRng</code>.</p>
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<p>You may prefer to use <a href="struct.EntropyRng.html"><code>EntropyRng</code></a> instead of <code>OsRng</code>. It is unlikely, but
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not entirely theoretical, for <code>OsRng</code> to fail. In such cases <a href="struct.EntropyRng.html"><code>EntropyRng</code></a>
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falls back on a good alternative entropy source.</p>
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<p><code>OsRng::new()</code> is guaranteed to be very cheap (after the first successful
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call), and will never consume more than one file handle per process.</p>
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<h1 id="platform-sources" class="section-header"><a href="#platform-sources">Platform sources</a></h1><table><thead><tr><th> OS </th><th> interface</th></tr></thead><tbody>
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<tr><td> Linux, Android </td><td> <a href="http://man7.org/linux/man-pages/man2/getrandom.2.html"><code>getrandom</code></a> system call if available, otherwise <a href="http://man7.org/linux/man-pages/man4/urandom.4.html"><code>/dev/urandom</code></a> after reading from <code>/dev/random</code> once</td></tr>
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<tr><td> Windows </td><td> <a href="https://msdn.microsoft.com/en-us/library/windows/desktop/aa387694.aspx"><code>RtlGenRandom</code></a></td></tr>
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<tr><td> macOS, iOS </td><td> <a href="https://developer.apple.com/documentation/security/1399291-secrandomcopybytes?language=objc"><code>SecRandomCopyBytes</code></a></td></tr>
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<tr><td> FreeBSD </td><td> <a href="https://www.freebsd.org/cgi/man.cgi?query=random&sektion=4"><code>kern.arandom</code></a></td></tr>
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<tr><td> OpenBSD, Bitrig </td><td> <a href="https://man.openbsd.org/getentropy.2"><code>getentropy</code></a></td></tr>
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<tr><td> NetBSD </td><td> <a href="http://netbsd.gw.com/cgi-bin/man-cgi?random+4+NetBSD-current"><code>/dev/urandom</code></a> after reading from <code>/dev/random</code> once</td></tr>
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<tr><td> Dragonfly BSD </td><td> <a href="https://leaf.dragonflybsd.org/cgi/web-man?command=random&section=4"><code>/dev/random</code></a></td></tr>
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<tr><td> Solaris, illumos </td><td> <a href="https://docs.oracle.com/cd/E88353_01/html/E37841/getrandom-2.html"><code>getrandom</code></a> system call if available, otherwise <a href="https://docs.oracle.com/cd/E86824_01/html/E54777/random-7d.html"><code>/dev/random</code></a></td></tr>
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<tr><td> Fuchsia OS </td><td> <a href="https://fuchsia.googlesource.com/zircon/+/HEAD/docs/syscalls/cprng_draw.md"><code>cprng_draw</code></a></td></tr>
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<tr><td> Redox </td><td> <a href="https://github.com/redox-os/randd/blob/master/src/main.rs"><code>rand:</code></a></td></tr>
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<tr><td> CloudABI </td><td> <a href="https://github.com/NuxiNL/cloudabi/blob/v0.20/cloudabi.txt#L1826"><code>random_get</code></a></td></tr>
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<tr><td> Haiku </td><td> <code>/dev/random</code> (identical to <code>/dev/urandom</code>)</td></tr>
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<tr><td> Web browsers </td><td> <a href="https://www.w3.org/TR/WebCryptoAPI/#Crypto-method-getRandomValues"><code>Crypto.getRandomValues</code></a> (see <a href="https://www.w3.org/TR/WebCryptoAPI/#Crypto-method-getRandomValues">Support for WebAssembly and ams.js</a>)</td></tr>
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<tr><td> Node.js </td><td> <a href="https://nodejs.org/api/crypto.html#crypto_crypto_randombytes_size_callback"><code>crypto.randomBytes</code></a> (see <a href="#support-for-webassembly-and-amsjs">Support for WebAssembly and ams.js</a>)</td></tr>
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</tbody></table>
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<p>Rand doesn't have a blanket implementation for all Unix-like operating
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systems that reads from <code>/dev/urandom</code>. This ensures all supported operating
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systems are using the recommended interface and respect maximum buffer
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sizes.</p>
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<h2 id="support-for-webassembly-and-amsjs" class="section-header"><a href="#support-for-webassembly-and-amsjs">Support for WebAssembly and ams.js</a></h2>
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<p>The three Emscripten targets <code>asmjs-unknown-emscripten</code>,
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<code>wasm32-unknown-emscripten</code> and <code>wasm32-experimental-emscripten</code> use
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Emscripten's emulation of <code>/dev/random</code> on web browsers and Node.js.
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Unfortunately it falls back to the insecure <code>Math.random()</code> if a browser
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doesn't support <a href="https://github.com/redox-os/randd/blob/master/src/main.rs"><code>Crypto.getRandomValues</code></a>.</p>
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<p>The bare Wasm target <code>wasm32-unknown-unknown</code> tries to call the javascript
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methods directly, using <code>stdweb</code> in combination with <code>cargo-web</code>.
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<code>wasm-bindgen</code> is not yet supported.</p>
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<h2 id="early-boot" class="section-header"><a href="#early-boot">Early boot</a></h2>
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<p>It is possible that early in the boot process the OS hasn't had enough time
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yet to collect entropy to securely seed its RNG, especially on virtual
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machines.</p>
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<p>Some operating systems always block the thread until the RNG is securely
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seeded. This can take anywhere from a few seconds to more than a minute.
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Others make a best effort to use a seed from before the shutdown and don't
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document much.</p>
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<p>A few, Linux, NetBSD and Solaris, offer a choice between blocking, and
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getting an error. With <code>try_fill_bytes</code> we choose to get the error
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(<a href="../enum.ErrorKind.html#variant.NotReady"><code>ErrorKind::NotReady</code></a>), while the other methods use a blocking interface.</p>
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<p>On Linux (when the <code>genrandom</code> system call is not available) and on NetBSD
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reading from <code>/dev/urandom</code> never blocks, even when the OS hasn't collected
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enough entropy yet. As a countermeasure we try to do a single read from
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<code>/dev/random</code> until we know the OS RNG is initialized (and store this in a
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global static).</p>
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<h1 id="panics" class="section-header"><a href="#panics">Panics</a></h1>
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<p><code>OsRng</code> is extremely unlikely to fail if <code>OsRng::new()</code>, and one read from
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it, where succesfull. But in case it does fail, only <a href="../trait.RngCore.html#method.tymethod.try_fill_bytes"><code>try_fill_bytes</code></a> is
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able to report the cause. Depending on the error the other <a href="../trait.RngCore.html"><code>RngCore</code></a>
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methods will retry several times, and panic in case the error remains.</p>
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</div><h2 id='methods' class='small-section-header'>Methods<a href='#methods' class='anchor'></a></h2><h3 id='impl' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl <a class="struct" href="../../rand/rngs/struct.OsRng.html" title="struct rand::rngs::OsRng">OsRng</a></code><a href='#impl' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#126-131' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.new' class="method"><span id='new.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.new' class='fnname'>new</a>() -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><<a class="struct" href="../../rand/rngs/struct.OsRng.html" title="struct rand::rngs::OsRng">OsRng</a>, <a class="struct" href="../../rand/struct.Error.html" title="struct rand::Error">Error</a>></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#128-130' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Create a new <code>OsRng</code>.</p>
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</div></div><h2 id='implementations' class='small-section-header'>Trait Implementations<a href='#implementations' class='anchor'></a></h2><div id='implementations-list'><h3 id='impl-Clone' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl <a class="trait" href="https://doc.rust-lang.org/nightly/core/clone/trait.Clone.html" title="trait core::clone::Clone">Clone</a> for <a class="struct" href="../../rand/rngs/struct.OsRng.html" title="struct rand::rngs::OsRng">OsRng</a></code><a href='#impl-Clone' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#117' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.clone' class="method"><span id='clone.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='https://doc.rust-lang.org/nightly/core/clone/trait.Clone.html#tymethod.clone' class='fnname'>clone</a>(&self) -> <a class="struct" href="../../rand/rngs/struct.OsRng.html" title="struct rand::rngs::OsRng">OsRng</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#117' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Returns a copy of the value. <a href="https://doc.rust-lang.org/nightly/core/clone/trait.Clone.html#tymethod.clone">Read more</a></p>
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</div><h4 id='method.clone_from' class="method"><span id='clone_from.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='https://doc.rust-lang.org/nightly/core/clone/trait.Clone.html#method.clone_from' class='fnname'>clone_from</a>(&mut self, source: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&</a>Self)</code></span></td><td><span class='out-of-band'><div class='ghost'></div><div class='since' title='Stable since Rust version 1.0.0'>1.0.0</div><a class='srclink' href='https://doc.rust-lang.org/nightly/src/core/clone.rs.html#130-132' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Performs copy-assignment from <code>source</code>. <a href="https://doc.rust-lang.org/nightly/core/clone/trait.Clone.html#method.clone_from">Read more</a></p>
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</div></div><h3 id='impl-Debug' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a> for <a class="struct" href="../../rand/rngs/struct.OsRng.html" title="struct rand::rngs::OsRng">OsRng</a></code><a href='#impl-Debug' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#120-124' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.fmt' class="method"><span id='fmt.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html#tymethod.fmt' class='fnname'>fmt</a>(&self, f: &mut <a class="struct" href="https://doc.rust-lang.org/nightly/core/fmt/struct.Formatter.html" title="struct core::fmt::Formatter">Formatter</a>) -> <a class="type" href="https://doc.rust-lang.org/nightly/core/fmt/type.Result.html" title="type core::fmt::Result">Result</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#121-123' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Formats the value using the given formatter. <a href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html#tymethod.fmt">Read more</a></p>
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</div></div><h3 id='impl-CryptoRng' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl <a class="trait" href="../../rand/trait.CryptoRng.html" title="trait rand::CryptoRng">CryptoRng</a> for <a class="struct" href="../../rand/rngs/struct.OsRng.html" title="struct rand::rngs::OsRng">OsRng</a></code><a href='#impl-CryptoRng' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#133' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'></div><h3 id='impl-RngCore' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl <a class="trait" href="../../rand/trait.RngCore.html" title="trait rand::RngCore">RngCore</a> for <a class="struct" href="../../rand/rngs/struct.OsRng.html" title="struct rand::rngs::OsRng">OsRng</a></code><a href='#impl-RngCore' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#135-217' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.next_u32' class="method"><span id='next_u32.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='../../rand/trait.RngCore.html#tymethod.next_u32' class='fnname'>next_u32</a>(&mut self) -> <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.u32.html">u32</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#136-138' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Return the next random <code>u32</code>. <a href="../../rand/trait.RngCore.html#tymethod.next_u32">Read more</a></p>
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</div><h4 id='method.next_u64' class="method"><span id='next_u64.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='../../rand/trait.RngCore.html#tymethod.next_u64' class='fnname'>next_u64</a>(&mut self) -> <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.u64.html">u64</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#140-142' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Return the next random <code>u64</code>. <a href="../../rand/trait.RngCore.html#tymethod.next_u64">Read more</a></p>
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</div><h4 id='method.fill_bytes' class="method"><span id='fill_bytes.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='../../rand/trait.RngCore.html#tymethod.fill_bytes' class='fnname'>fill_bytes</a>(&mut self, dest: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.slice.html">&mut [</a><a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.u8.html">u8</a><a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.slice.html">]</a>)</code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#144-194' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Fill <code>dest</code> with random data. <a href="../../rand/trait.RngCore.html#tymethod.fill_bytes">Read more</a></p>
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</div><h4 id='method.try_fill_bytes' class="method"><span id='try_fill_bytes.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='../../rand/trait.RngCore.html#tymethod.try_fill_bytes' class='fnname'>try_fill_bytes</a>(&mut self, dest: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.slice.html">&mut [</a><a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.u8.html">u8</a><a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.slice.html">]</a>) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.unit.html">()</a>, <a class="struct" href="../../rand/struct.Error.html" title="struct rand::Error">Error</a>></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/rand/rngs/os.rs.html#196-216' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Fill <code>dest</code> entirely with random data. <a href="../../rand/trait.RngCore.html#tymethod.try_fill_bytes">Read more</a></p>
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