[][src]Trait rand::RngCore

pub trait RngCore {
    fn next_u32(&mut self) -> u32;
fn next_u64(&mut self) -> u64;
fn fill_bytes(&mut self, dest: &mut [u8]);
fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error>; }

The core of a random number generator.

This trait encapsulates the low-level functionality common to all generators, and is the "back end", to be implemented by generators. End users should normally use the Rng trait from the rand crate, which is automatically implemented for every type implementing RngCore.

Three different methods for generating random data are provided since the optimal implementation of each is dependent on the type of generator. There is no required relationship between the output of each; e.g. many implementations of fill_bytes consume a whole number of u32 or u64 values and drop any remaining unused bytes.

The try_fill_bytes method is a variant of fill_bytes allowing error handling; it is not deemed sufficiently useful to add equivalents for next_u32 or next_u64 since the latter methods are almost always used with algorithmic generators (PRNGs), which are normally infallible.

Algorithmic generators implementing SeedableRng should normally have portable, reproducible output, i.e. fix Endianness when converting values to avoid platform differences, and avoid making any changes which affect output (except by communicating that the release has breaking changes).

Typically implementators will implement only one of the methods available in this trait directly, then use the helper functions from the [impls] module to implement the other methods.

It is recommended that implementations also implement:


A simple example, obviously not generating very random output:

use rand_core::{RngCore, Error, impls};

struct CountingRng(u64);

impl RngCore for CountingRng {
    fn next_u32(&mut self) -> u32 {
        self.next_u64() as u32

    fn next_u64(&mut self) -> u64 {
        self.0 += 1;

    fn fill_bytes(&mut self, dest: &mut [u8]) {
        impls::fill_bytes_via_next(self, dest)

    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {

Required methods

fn next_u32(&mut self) -> u32

Return the next random u32.

RNGs must implement at least one method from this trait directly. In the case this method is not implemented directly, it can be implemented using self.next_u64() as u32 or via [fill_bytes][impls::next_u32_via_fill].

fn next_u64(&mut self) -> u64

Return the next random u64.

RNGs must implement at least one method from this trait directly. In the case this method is not implemented directly, it can be implemented via [next_u32][impls::next_u64_via_u32] or via [fill_bytes][impls::next_u64_via_fill].

fn fill_bytes(&mut self, dest: &mut [u8])

Fill dest with random data.

RNGs must implement at least one method from this trait directly. In the case this method is not implemented directly, it can be implemented via [next_u*][impls::fill_bytes_via_next] or via try_fill_bytes; if this generator can fail the implementation must choose how best to handle errors here (e.g. panic with a descriptive message or log a warning and retry a few times).

This method should guarantee that dest is entirely filled with new data, and may panic if this is impossible (e.g. reading past the end of a file that is being used as the source of randomness).

fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error>

Fill dest entirely with random data.

This is the only method which allows an RNG to report errors while generating random data thus making this the primary method implemented by external (true) RNGs (e.g. OsRng) which can fail. It may be used directly to generate keys and to seed (infallible) PRNGs.

Other than error handling, this method is identical to fill_bytes; thus this may be implemented using Ok(self.fill_bytes(dest)) or fill_bytes may be implemented with self.try_fill_bytes(dest).unwrap() or more specific error handling.

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Trait Implementations

impl Read for dyn RngCore + 'static[src]

fn read_vectored(&mut self, bufs: &mut [IoSliceMut]) -> Result<usize, Error>1.36.0[src]

Like read, except that it reads into a slice of buffers. Read more

unsafe fn initializer(&self) -> Initializer[src]

🔬 This is a nightly-only experimental API. (read_initializer)

Determines if this Reader can work with buffers of uninitialized memory. Read more

fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize, Error>1.0.0[src]

Read all bytes until EOF in this source, placing them into buf. Read more

fn read_to_string(&mut self, buf: &mut String) -> Result<usize, Error>1.0.0[src]

Read all bytes until EOF in this source, appending them to buf. Read more

fn read_exact(&mut self, buf: &mut [u8]) -> Result<(), Error>1.6.0[src]

Read the exact number of bytes required to fill buf. Read more

fn by_ref(&mut self) -> &mut Self1.0.0[src]

Creates a "by reference" adaptor for this instance of Read. Read more

fn bytes(self) -> Bytes<Self>1.0.0[src]

Transforms this Read instance to an [Iterator] over its bytes. Read more

fn chain<R>(self, next: R) -> Chain<Self, R> where
    R: Read

Creates an adaptor which will chain this stream with another. Read more

fn take(self, limit: u64) -> Take<Self>1.0.0[src]

Creates an adaptor which will read at most limit bytes from it. Read more

Implementations on Foreign Types

impl<R> RngCore for BlockRng64<R> where
    R: BlockRngCore<Item = u64>,
    <R as BlockRngCore>::Results: AsRef<[u64]>,
    <R as BlockRngCore>::Results: AsMut<[u64]>, 

impl<'a, R> RngCore for &'a mut R where
    R: RngCore + ?Sized

impl<R> RngCore for BlockRng<R> where
    R: BlockRngCore<Item = u32>,
    <R as BlockRngCore>::Results: AsRef<[u32]>,
    <R as BlockRngCore>::Results: AsMut<[u32]>, 

impl<R> RngCore for Box<R> where
    R: RngCore + ?Sized

impl RngCore for ChaCha20Rng[src]

impl RngCore for ChaCha12Rng[src]

impl RngCore for ChaCha8Rng[src]

impl RngCore for Lcg64Xsh32[src]

impl RngCore for Mcg128Xsl64[src]

impl RngCore for Lcg128Xsl64[src]

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impl RngCore for StepRng[src]

impl RngCore for EntropyRng[src]

impl RngCore for OsRng[src]

impl RngCore for SmallRng[src]

impl RngCore for StdRng[src]

impl RngCore for ThreadRng[src]

impl<R, Rsdr: RngCore> RngCore for ReseedingRng<R, Rsdr> where
    R: BlockRngCore<Item = u32> + SeedableRng,
    <R as BlockRngCore>::Results: AsRef<[u32]> + AsMut<[u32]>, 

impl<R: Read> RngCore for ReadRng<R>[src]

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