# thread.rs - source
Source of the Rust file `/home/key/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.8.5/src/rngs/thread.rs`.
Keywords: rust, rustlang, rust-lang

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    // 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.

    //! Thread-local random number generator

    use core::cell::UnsafeCell;
    use std::rc::Rc;
    use std::thread_local;

    use super::std::Core;
    use crate::rngs::adapter::ReseedingRng;
    use crate::rngs::OsRng;
    use crate::{CryptoRng, Error, RngCore, SeedableRng};

    // 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.
    const THREAD_RNG_RESEED_THRESHOLD: u64 = 1024 * 64;

    /// 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
    #[cfg_attr(doc_cfg, doc(cfg(all(feature = "std", feature = "std_rng"))))]
    #[derive(Clone, Debug)]
    pub struct ThreadRng {
        // Rc is explicitly !Send and !Sync
        rng: Rc<UnsafeCell<ReseedingRng<Core, OsRng>>>,
    }

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

    /// 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`].
    #[cfg_attr(doc_cfg, doc(cfg(all(feature = "std", feature = "std_rng"))))]
    pub fn thread_rng() -> ThreadRng {
        let rng = THREAD_RNG_KEY.with(|t| t.clone());
        ThreadRng { rng }
    }

    impl Default for ThreadRng {
        fn default() -> ThreadRng {
            crate::prelude::thread_rng()
        }
    }

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

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

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

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

    impl CryptoRng for ThreadRng {}

    #[cfg(test)]
    mod test {
        #[test]
        fn test_thread_rng() {
            use crate::Rng;
            let mut r = crate::thread_rng();
            r.gen::<i32>();
            assert_eq!(r.gen_range(0..1), 0);
        }
    }