| <!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="Interface to the operating system’s random number generator."><meta name="keywords" content="rust, rustlang, rust-lang, getrandom"><title>getrandom - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><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="../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../dark.css" disabled><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script src="../crates.js"></script><script defer src="../main.js"></script> |
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| </a><h2 class="location">Crate getrandom</h2><div class="block version"><div class="narrow-helper"></div><p>Version 0.2.3</p></div><div class="sidebar-elems"><a id="all-types" href="all.html"><p>See all getrandom's items</p></a><div class="block items"><ul><li><a href="#structs">Structs</a></li><li><a href="#functions">Functions</a></li></ul></div><div id="sidebar-vars" data-name="getrandom" data-ty="mod" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../getrandom/index.html"><img src="https://www.rust-lang.org/logos/rust-logo-128x128-blk.png" alt="logo"></a><nav class="sub"><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="18" height="18" alt="Pick another theme!" src="../brush.svg"></button><div id="theme-choices" role="menu"></div></div><form class="search-form"><div class="search-container"><div> |
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| <details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Interface to the operating system’s random number generator.</p> |
| <h2 id="supported-targets" class="section-header"><a href="#supported-targets">Supported targets</a></h2><div><table><thead><tr><th>Target</th><th>Target Triple</th><th>Implementation</th></tr></thead><tbody> |
| <tr><td>Linux, Android</td><td><code>*‑linux‑*</code></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 successfully polling <code>/dev/random</code></td></tr> |
| <tr><td>Windows</td><td><code>*‑windows‑*</code></td><td><a href="https://docs.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom"><code>BCryptGenRandom</code></a></td></tr> |
| <tr><td>macOS</td><td><code>*‑apple‑darwin</code></td><td><a href="https://www.unix.com/man-page/mojave/2/getentropy/"><code>getentropy()</code></a> if available, otherwise <a href="https://www.unix.com/man-page/mojave/4/random/"><code>/dev/random</code></a> (identical to <code>/dev/urandom</code>)</td></tr> |
| <tr><td>iOS</td><td><code>*‑apple‑ios</code></td><td><a href="https://developer.apple.com/documentation/security/1399291-secrandomcopybytes?language=objc"><code>SecRandomCopyBytes</code></a></td></tr> |
| <tr><td>FreeBSD</td><td><code>*‑freebsd</code></td><td><a href="https://www.freebsd.org/cgi/man.cgi?query=getrandom&manpath=FreeBSD+12.0-stable"><code>getrandom()</code></a> if available, otherwise <a href="https://www.freebsd.org/cgi/man.cgi?query=random&sektion=4"><code>kern.arandom</code></a></td></tr> |
| <tr><td>OpenBSD</td><td><code>*‑openbsd</code></td><td><a href="https://man.openbsd.org/getentropy.2"><code>getentropy</code></a></td></tr> |
| <tr><td>NetBSD</td><td><code>*‑netbsd</code></td><td><a href="https://man.netbsd.org/sysctl.7"><code>kern.arandom</code></a></td></tr> |
| <tr><td>Dragonfly BSD</td><td><code>*‑dragonfly</code></td><td><a href="https://leaf.dragonflybsd.org/cgi/web-man?command=getrandom"><code>getrandom()</code></a> if available, otherwise <a href="https://leaf.dragonflybsd.org/cgi/web-man?command=random&section=4"><code>/dev/random</code></a></td></tr> |
| <tr><td>Solaris, illumos</td><td><code>*‑solaris</code>, <code>*‑illumos</code></td><td><a href="https://docs.oracle.com/cd/E88353_01/html/E37841/getrandom-2.html"><code>getrandom()</code></a> if available, otherwise <a href="https://docs.oracle.com/cd/E86824_01/html/E54777/random-7d.html"><code>/dev/random</code></a></td></tr> |
| <tr><td>Fuchsia OS</td><td><code>*‑fuchsia</code></td><td><a href="https://fuchsia.dev/fuchsia-src/zircon/syscalls/cprng_draw"><code>cprng_draw</code></a></td></tr> |
| <tr><td>Redox</td><td><code>*‑redox</code></td><td><a href="https://github.com/redox-os/randd/blob/master/src/main.rs"><code>rand:</code></a></td></tr> |
| <tr><td>Haiku</td><td><code>*‑haiku</code></td><td><code>/dev/random</code> (identical to <code>/dev/urandom</code>)</td></tr> |
| <tr><td>SGX</td><td><code>x86_64‑*‑sgx</code></td><td><a href="https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide">RDRAND</a></td></tr> |
| <tr><td>VxWorks</td><td><code>*‑wrs‑vxworks‑*</code></td><td><code>randABytes</code> after checking entropy pool initialization with <code>randSecure</code></td></tr> |
| <tr><td>Emscripten</td><td><code>*‑emscripten</code></td><td><code>/dev/random</code> (identical to <code>/dev/urandom</code>)</td></tr> |
| <tr><td>WASI</td><td><code>wasm32‑wasi</code></td><td><a href="https://github.com/WebAssembly/WASI/blob/main/phases/snapshot/docs.md#-random_getbuf-pointeru8-buf_len-size---errno"><code>random_get</code></a></td></tr> |
| <tr><td>Web Browser</td><td><code>wasm32‑*‑unknown</code></td><td><a href="https://www.w3.org/TR/WebCryptoAPI/#Crypto-method-getRandomValues"><code>Crypto.getRandomValues()</code></a>, see <a href="#webassembly-support">WebAssembly support</a></td></tr> |
| <tr><td>Node.js</td><td><code>wasm32‑*‑unknown</code></td><td><a href="https://nodejs.org/api/crypto.html#crypto_crypto_randombytes_size_callback"><code>crypto.randomBytes</code></a>, see <a href="#webassembly-support">WebAssembly support</a></td></tr> |
| </tbody></table> |
| </div> |
| <p>There is no blanket implementation on <code>unix</code> targets that reads from |
| <code>/dev/urandom</code>. This ensures all supported targets are using the recommended |
| interface and respect maximum buffer sizes.</p> |
| <p>Pull Requests that add support for new targets to <code>getrandom</code> are always welcome.</p> |
| <h3 id="unsupported-targets" class="section-header"><a href="#unsupported-targets">Unsupported targets</a></h3> |
| <p>By default, <code>getrandom</code> will not compile on unsupported targets, but certain |
| features allow a user to select a “fallback” implementation if no supported |
| implementation exists.</p> |
| <p>All of the below mechanisms only affect unsupported |
| targets. Supported targets will <em>always</em> use their supported implementations. |
| This prevents a crate from overriding a secure source of randomness |
| (either accidentally or intentionally).</p> |
| <h4 id="rdrand-on-x86" class="section-header"><a href="#rdrand-on-x86">RDRAND on x86</a></h4> |
| <p><em>If the <code>"rdrand"</code> Cargo feature is enabled</em>, <code>getrandom</code> will fallback to using |
| the <a href="https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide"><code>RDRAND</code></a> instruction to get randomness on <code>no_std</code> <code>x86</code>/<code>x86_64</code> |
| targets. This feature has no effect on other CPU architectures.</p> |
| <h4 id="webassembly-support" class="section-header"><a href="#webassembly-support">WebAssembly support</a></h4> |
| <p>This crate fully supports the |
| <a href="https://github.com/CraneStation/wasi"><code>wasm32-wasi</code></a> and |
| <a href="https://www.hellorust.com/setup/emscripten/"><code>wasm32-unknown-emscripten</code></a> |
| targets. However, the <code>wasm32-unknown-unknown</code> target (i.e. the target used |
| by <code>wasm-pack</code>) is not automatically |
| supported since, from the target name alone, we cannot deduce which |
| JavaScript interface is in use (or if JavaScript is available at all).</p> |
| <p>Instead, <em>if the <code>"js"</code> Cargo feature is enabled</em>, this crate will assume |
| that you are building for an environment containing JavaScript, and will |
| call the appropriate methods. Both web browser (main window and Web Workers) |
| and Node.js environments are supported, invoking the methods |
| <a href="#supported-targets">described above</a> using the |
| <a href="https://github.com/rust-lang/rust-bindgen">wasm-bindgen</a> toolchain.</p> |
| <p>This feature has no effect on targets other than <code>wasm32-unknown-unknown</code>.</p> |
| <h4 id="custom-implementations" class="section-header"><a href="#custom-implementations">Custom implementations</a></h4> |
| <p>The [<code>register_custom_getrandom!</code>] macro allows a user to mark their own |
| function as the backing implementation for <a href="fn.getrandom.html" title="getrandom"><code>getrandom</code></a>. See the macro’s |
| documentation for more information about writing and registering your own |
| custom implementations.</p> |
| <p>Note that registering a custom implementation only has an effect on targets |
| that would otherwise not compile. Any supported targets (including those |
| using <code>"rdrand"</code> and <code>"js"</code> Cargo features) continue using their normal |
| implementations even if a function is registered.</p> |
| <h4 id="indirect-dependencies" class="section-header"><a href="#indirect-dependencies">Indirect Dependencies</a></h4> |
| <p>If <code>getrandom</code> is not a direct dependency of your crate, you can still |
| enable any of the above fallback behaviors by enabling the relevant |
| feature in your root crate’s <code>Cargo.toml</code>:</p> |
| <div class="example-wrap"><pre class="language-toml"><code>[dependencies] |
| getrandom = { version = "0.2", features = ["js"] }</code></pre></div><h3 id="early-boot" class="section-header"><a href="#early-boot">Early boot</a></h3> |
| <p>Sometimes, early in the boot process, the OS has not collected enough |
| entropy to securely seed its RNG. This is especially common on virtual |
| machines, where standard “random” events are hard to come by.</p> |
| <p>Some operating system interfaces always block until the RNG is securely |
| seeded. This can take anywhere from a few seconds to more than a minute. |
| A few (Linux, NetBSD and Solaris) offer a choice between blocking and |
| getting an error; in these cases, we always choose to block.</p> |
| <p>On Linux (when the <code>getrandom</code> system call is not available), reading from |
| <code>/dev/urandom</code> never blocks, even when the OS hasn’t collected enough |
| entropy yet. To avoid returning low-entropy bytes, we first poll |
| <code>/dev/random</code> and only switch to <code>/dev/urandom</code> once this has succeeded.</p> |
| <h3 id="error-handling" class="section-header"><a href="#error-handling">Error handling</a></h3> |
| <p>We always choose failure over returning insecure “random” bytes. In general, |
| on supported platforms, failure is highly unlikely, though not impossible. |
| If an error does occur, then it is likely that it will occur on every call to |
| <code>getrandom</code>, hence after the first successful call one can be reasonably |
| confident that no errors will occur.</p> |
| </div></details><h2 id="structs" class="small-section-header"><a href="#structs">Structs</a></h2> |
| <div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Error.html" title="getrandom::Error struct">Error</a></div><div class="item-right docblock-short"><p>A small and <code>no_std</code> compatible error type</p> |
| </div></div></div><h2 id="functions" class="small-section-header"><a href="#functions">Functions</a></h2> |
| <div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.getrandom.html" title="getrandom::getrandom fn">getrandom</a></div><div class="item-right docblock-short"><p>Fill <code>dest</code> with random bytes from the system’s preferred random number |
| source.</p> |
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