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                         <img src="https://www.rust-lang.org/logos/rust-logo-128x128-blk.png" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span>
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 </pre><pre class="rust"><code><span class="comment">// Copyright 2018 Developers of the Rand project.
 //
 // Licensed under the Apache License, Version 2.0 &lt;LICENSE-APACHE or
 // https://www.apache.org/licenses/LICENSE-2.0&gt; or the MIT license
 // &lt;LICENSE-MIT or https://opensource.org/licenses/MIT&gt;, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.

 </span><span class="doccomment">//! Thread-local random number generator

 </span><span class="kw">use </span>core::cell::UnsafeCell;
 <span class="kw">use </span>std::rc::Rc;
 <span class="kw">use </span>std::thread_local;

 <span class="kw">use </span><span class="kw">super</span>::std::Core;
 <span class="kw">use </span><span class="kw">crate</span>::rngs::adapter::ReseedingRng;
 <span class="kw">use </span><span class="kw">crate</span>::rngs::OsRng;
 <span class="kw">use crate</span>::{CryptoRng, Error, RngCore, SeedableRng};

 <span class="comment">// Rationale for using `UnsafeCell` in `ThreadRng`:
 //
 // Previously we used a `RefCell`, with an overhead of ~15%. There will only
 // ever be one mutable reference to the interior of the `UnsafeCell`, because
 // we only have such a reference inside `next_u32`, `next_u64`, etc. Within a
 // single thread (which is the definition of `ThreadRng`), there will only ever
 // be one of these methods active at a time.
 //
 // A possible scenario where there could be multiple mutable references is if
 // `ThreadRng` is used inside `next_u32` and co. But the implementation is
 // completely under our control. We just have to ensure none of them use
 // `ThreadRng` internally, which is nonsensical anyway. We should also never run
 // `ThreadRng` in destructors of its implementation, which is also nonsensical.


 // Number of generated bytes after which to reseed `ThreadRng`.
 // According to benchmarks, reseeding has a noticeable impact with thresholds
 // of 32 kB and less. We choose 64 kB to avoid significant overhead.
 </span><span class="kw">const </span>THREAD_RNG_RESEED_THRESHOLD: u64 = <span class="number">1024 </span>* <span class="number">64</span>;

 <span class="doccomment">/// A reference to the thread-local generator
 ///
 /// An instance can be obtained via [`thread_rng`] or via `ThreadRng::default()`.
 /// This handle is safe to use everywhere (including thread-local destructors),
 /// though it is recommended not to use inside a fork handler.
 /// The handle cannot be passed between threads (is not `Send` or `Sync`).
 ///
 /// `ThreadRng` uses the same PRNG as [`StdRng`] for security and performance
 /// and is automatically seeded from [`OsRng`].
 ///
 /// Unlike `StdRng`, `ThreadRng` uses the  [`ReseedingRng`] wrapper to reseed
 /// the PRNG from fresh entropy every 64 kiB of random data as well as after a
 /// fork on Unix (though not quite immediately; see documentation of
 /// [`ReseedingRng`]).
 /// Note that the reseeding is done as an extra precaution against side-channel
 /// attacks and mis-use (e.g. if somehow weak entropy were supplied initially).
 /// The PRNG algorithms used are assumed to be secure.
 ///
 /// [`ReseedingRng`]: crate::rngs::adapter::ReseedingRng
 /// [`StdRng`]: crate::rngs::StdRng
 </span><span class="attribute">#[cfg_attr(doc_cfg, doc(cfg(all(feature = <span class="string">&quot;std&quot;</span>, feature = <span class="string">&quot;std_rng&quot;</span>))))]
 #[derive(Clone, Debug)]
 </span><span class="kw">pub struct </span>ThreadRng {
     <span class="comment">// Rc is explicitly !Send and !Sync
     </span>rng: Rc&lt;UnsafeCell&lt;ReseedingRng&lt;Core, OsRng&gt;&gt;&gt;,
 }

 <span class="macro">thread_local!</span>(
     <span class="comment">// We require Rc&lt;..&gt; to avoid premature freeing when thread_rng is used
     // within thread-local destructors. See #968.
     </span><span class="kw">static </span>THREAD_RNG_KEY: Rc&lt;UnsafeCell&lt;ReseedingRng&lt;Core, OsRng&gt;&gt;&gt; = {
         <span class="kw">let </span>r = Core::from_rng(OsRng).unwrap_or_else(|err|
                 <span class="macro">panic!</span>(<span class="string">&quot;could not initialize thread_rng: {}&quot;</span>, err));
         <span class="kw">let </span>rng = ReseedingRng::new(r,
                                     THREAD_RNG_RESEED_THRESHOLD,
                                     OsRng);
         Rc::new(UnsafeCell::new(rng))
     }
 );

 <span class="doccomment">/// Retrieve the lazily-initialized thread-local random number generator,
 /// seeded by the system. Intended to be used in method chaining style,
 /// e.g. `thread_rng().gen::&lt;i32&gt;()`, or cached locally, e.g.
 /// `let mut rng = thread_rng();`.  Invoked by the `Default` trait, making
 /// `ThreadRng::default()` equivalent.
 ///
 /// For more information see [`ThreadRng`].
 </span><span class="attribute">#[cfg_attr(doc_cfg, doc(cfg(all(feature = <span class="string">&quot;std&quot;</span>, feature = <span class="string">&quot;std_rng&quot;</span>))))]
 </span><span class="kw">pub fn </span>thread_rng() -&gt; ThreadRng {
     <span class="kw">let </span>rng = THREAD_RNG_KEY.with(|t| t.clone());
     ThreadRng { rng }
 }

 <span class="kw">impl </span>Default <span class="kw">for </span>ThreadRng {
     <span class="kw">fn </span>default() -&gt; ThreadRng {
         <span class="kw">crate</span>::prelude::thread_rng()
     }
 }

 <span class="kw">impl </span>RngCore <span class="kw">for </span>ThreadRng {
     <span class="attribute">#[inline(always)]
     </span><span class="kw">fn </span>next_u32(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; u32 {
         <span class="comment">// SAFETY: We must make sure to stop using `rng` before anyone else
         // creates another mutable reference
         </span><span class="kw">let </span>rng = <span class="kw">unsafe </span>{ <span class="kw-2">&amp;mut *</span><span class="self">self</span>.rng.get() };
         rng.next_u32()
     }

     <span class="attribute">#[inline(always)]
     </span><span class="kw">fn </span>next_u64(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; u64 {
         <span class="comment">// SAFETY: We must make sure to stop using `rng` before anyone else
         // creates another mutable reference
         </span><span class="kw">let </span>rng = <span class="kw">unsafe </span>{ <span class="kw-2">&amp;mut *</span><span class="self">self</span>.rng.get() };
         rng.next_u64()
     }

     <span class="kw">fn </span>fill_bytes(<span class="kw-2">&amp;mut </span><span class="self">self</span>, dest: <span class="kw-2">&amp;mut </span>[u8]) {
         <span class="comment">// SAFETY: We must make sure to stop using `rng` before anyone else
         // creates another mutable reference
         </span><span class="kw">let </span>rng = <span class="kw">unsafe </span>{ <span class="kw-2">&amp;mut *</span><span class="self">self</span>.rng.get() };
         rng.fill_bytes(dest)
     }

     <span class="kw">fn </span>try_fill_bytes(<span class="kw-2">&amp;mut </span><span class="self">self</span>, dest: <span class="kw-2">&amp;mut </span>[u8]) -&gt; <span class="prelude-ty">Result</span>&lt;(), Error&gt; {
         <span class="comment">// SAFETY: We must make sure to stop using `rng` before anyone else
         // creates another mutable reference
         </span><span class="kw">let </span>rng = <span class="kw">unsafe </span>{ <span class="kw-2">&amp;mut *</span><span class="self">self</span>.rng.get() };
         rng.try_fill_bytes(dest)
     }
 }

 <span class="kw">impl </span>CryptoRng <span class="kw">for </span>ThreadRng {}


 <span class="attribute">#[cfg(test)]
 </span><span class="kw">mod </span>test {
     <span class="attribute">#[test]
     </span><span class="kw">fn </span>test_thread_rng() {
         <span class="kw">use </span><span class="kw">crate</span>::Rng;
         <span class="kw">let </span><span class="kw-2">mut </span>r = <span class="kw">crate</span>::thread_rng();
         r.gen::&lt;i32&gt;();
         <span class="macro">assert_eq!</span>(r.gen_range(<span class="number">0</span>..<span class="number">1</span>), <span class="number">0</span>);
     }
 }
 </code></pre></div>
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	</pre><pre class="rust"><code><span class="comment">// Copyright 2018 Developers of the Rand project.
	//
	// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
	// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
	// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
	// option. This file may not be copied, modified, or distributed
	// except according to those terms.

	</span><span class="doccomment">//! Thread-local random number generator

	</span><span class="kw">use </span>core::cell::UnsafeCell;
	<span class="kw">use </span>std::rc::Rc;
	<span class="kw">use </span>std::thread_local;

	<span class="kw">use </span><span class="kw">super</span>::std::Core;
	<span class="kw">use </span><span class="kw">crate</span>::rngs::adapter::ReseedingRng;
	<span class="kw">use </span><span class="kw">crate</span>::rngs::OsRng;
	<span class="kw">use crate</span>::{CryptoRng, Error, RngCore, SeedableRng};

	<span class="comment">// Rationale for using `UnsafeCell` in `ThreadRng`:
	//
	// Previously we used a `RefCell`, with an overhead of ~15%. There will only
	// ever be one mutable reference to the interior of the `UnsafeCell`, because
	// we only have such a reference inside `next_u32`, `next_u64`, etc. Within a
	// single thread (which is the definition of `ThreadRng`), there will only ever
	// be one of these methods active at a time.
	//
	// A possible scenario where there could be multiple mutable references is if
	// `ThreadRng` is used inside `next_u32` and co. But the implementation is
	// completely under our control. We just have to ensure none of them use
	// `ThreadRng` internally, which is nonsensical anyway. We should also never run
	// `ThreadRng` in destructors of its implementation, which is also nonsensical.


	// Number of generated bytes after which to reseed `ThreadRng`.
	// According to benchmarks, reseeding has a noticeable impact with thresholds
	// of 32 kB and less. We choose 64 kB to avoid significant overhead.
	</span><span class="kw">const </span>THREAD_RNG_RESEED_THRESHOLD: u64 = <span class="number">1024 </span>* <span class="number">64</span>;

	<span class="doccomment">/// A reference to the thread-local generator
	///
	/// An instance can be obtained via [`thread_rng`] or via `ThreadRng::default()`.
	/// This handle is safe to use everywhere (including thread-local destructors),
	/// though it is recommended not to use inside a fork handler.
	/// The handle cannot be passed between threads (is not `Send` or `Sync`).
	///
	/// `ThreadRng` uses the same PRNG as [`StdRng`] for security and performance
	/// and is automatically seeded from [`OsRng`].
	///
	/// Unlike `StdRng`, `ThreadRng` uses the [`ReseedingRng`] wrapper to reseed
	/// the PRNG from fresh entropy every 64 kiB of random data as well as after a
	/// fork on Unix (though not quite immediately; see documentation of
	/// [`ReseedingRng`]).
	/// Note that the reseeding is done as an extra precaution against side-channel
	/// attacks and mis-use (e.g. if somehow weak entropy were supplied initially).
	/// The PRNG algorithms used are assumed to be secure.
	///
	/// [`ReseedingRng`]: crate::rngs::adapter::ReseedingRng
	/// [`StdRng`]: crate::rngs::StdRng
	</span><span class="attribute">#[cfg_attr(doc_cfg, doc(cfg(all(feature = <span class="string">"std"</span>, feature = <span class="string">"std_rng"</span>))))]
	#[derive(Clone, Debug)]
	</span><span class="kw">pub struct </span>ThreadRng {
	<span class="comment">// Rc is explicitly !Send and !Sync
	</span>rng: Rc<UnsafeCell<ReseedingRng<Core, OsRng>>>,
	}

	<span class="macro">thread_local!</span>(
	<span class="comment">// We require Rc<..> to avoid premature freeing when thread_rng is used
	// within thread-local destructors. See #968.
	</span><span class="kw">static </span>THREAD_RNG_KEY: Rc<UnsafeCell<ReseedingRng<Core, OsRng>>> = {
	<span class="kw">let </span>r = Core::from_rng(OsRng).unwrap_or_else(\|err\|
	<span class="macro">panic!</span>(<span class="string">"could not initialize thread_rng: {}"</span>, err));
	<span class="kw">let </span>rng = ReseedingRng::new(r,
	THREAD_RNG_RESEED_THRESHOLD,
	OsRng);
	Rc::new(UnsafeCell::new(rng))
	}
	);

	<span class="doccomment">/// Retrieve the lazily-initialized thread-local random number generator,
	/// seeded by the system. Intended to be used in method chaining style,
	/// e.g. `thread_rng().gen::<i32>()`, or cached locally, e.g.
	/// `let mut rng = thread_rng();`. Invoked by the `Default` trait, making
	/// `ThreadRng::default()` equivalent.
	///
	/// For more information see [`ThreadRng`].
	</span><span class="attribute">#[cfg_attr(doc_cfg, doc(cfg(all(feature = <span class="string">"std"</span>, feature = <span class="string">"std_rng"</span>))))]
	</span><span class="kw">pub fn </span>thread_rng() -> ThreadRng {
	<span class="kw">let </span>rng = THREAD_RNG_KEY.with(\|t\| t.clone());
	ThreadRng { rng }
	}

	<span class="kw">impl </span>Default <span class="kw">for </span>ThreadRng {
	<span class="kw">fn </span>default() -> ThreadRng {
	<span class="kw">crate</span>::prelude::thread_rng()
	}
	}

	<span class="kw">impl </span>RngCore <span class="kw">for </span>ThreadRng {
	<span class="attribute">#[inline(always)]
	</span><span class="kw">fn </span>next_u32(<span class="kw-2">&mut </span><span class="self">self</span>) -> u32 {
	<span class="comment">// SAFETY: We must make sure to stop using `rng` before anyone else
	// creates another mutable reference
	</span><span class="kw">let </span>rng = <span class="kw">unsafe </span>{ <span class="kw-2">&mut *</span><span class="self">self</span>.rng.get() };
	rng.next_u32()
	}

	<span class="attribute">#[inline(always)]
	</span><span class="kw">fn </span>next_u64(<span class="kw-2">&mut </span><span class="self">self</span>) -> u64 {
	<span class="comment">// SAFETY: We must make sure to stop using `rng` before anyone else
	// creates another mutable reference
	</span><span class="kw">let </span>rng = <span class="kw">unsafe </span>{ <span class="kw-2">&mut *</span><span class="self">self</span>.rng.get() };
	rng.next_u64()
	}

	<span class="kw">fn </span>fill_bytes(<span class="kw-2">&mut </span><span class="self">self</span>, dest: <span class="kw-2">&mut </span>[u8]) {
	<span class="comment">// SAFETY: We must make sure to stop using `rng` before anyone else
	// creates another mutable reference
	</span><span class="kw">let </span>rng = <span class="kw">unsafe </span>{ <span class="kw-2">&mut *</span><span class="self">self</span>.rng.get() };
	rng.fill_bytes(dest)
	}

	<span class="kw">fn </span>try_fill_bytes(<span class="kw-2">&mut </span><span class="self">self</span>, dest: <span class="kw-2">&mut </span>[u8]) -> <span class="prelude-ty">Result</span><(), Error> {
	<span class="comment">// SAFETY: We must make sure to stop using `rng` before anyone else
	// creates another mutable reference
	</span><span class="kw">let </span>rng = <span class="kw">unsafe </span>{ <span class="kw-2">&mut *</span><span class="self">self</span>.rng.get() };
	rng.try_fill_bytes(dest)
	}
	}

	<span class="kw">impl </span>CryptoRng <span class="kw">for </span>ThreadRng {}


	<span class="attribute">#[cfg(test)]
	</span><span class="kw">mod </span>test {
	<span class="attribute">#[test]
	</span><span class="kw">fn </span>test_thread_rng() {
	<span class="kw">use </span><span class="kw">crate</span>::Rng;
	<span class="kw">let </span><span class="kw-2">mut </span>r = <span class="kw">crate</span>::thread_rng();
	r.gen::<i32>();
	<span class="macro">assert_eq!</span>(r.gen_range(<span class="number">0</span>..<span class="number">1</span>), <span class="number">0</span>);
	}
	}
	</code></pre></div>
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