tree: 03bd81cb143599873aaedd4dcfe0dbd3988a0036 [path history] [tgz]
  1. .gitignore
  2. .travis.yml
  4. Cargo.toml
  9. appveyor.yml
  10. benches/
  11. rand-derive/
  12. rand_core/
  13. src/
  14. utils/


A Rust library for random number generators and other randomness functionality.

Build Status Build status



Add this to your Cargo.toml:

rand = "0.4"

and this to your crate root:

extern crate rand;


Version 0.5 is in development and contains many breaking changes; expected release date is approx. end of March 2018 if all goes well.

Version 0.4 was released in December 2017. It contains almost no breaking changes since the 0.3 series.

For more details, see the changelog.

Rust version requirements

The 0.5 release of Rand will require Rustc version 1.22 or greater. Rand 0.4 and 0.3 (since approx. June 2017) require Rustc version 1.15 or greater. Subsets of the Rand code may work with older Rust versions, but this is not supported.

Travis CI always has a build with a pinned version of Rustc matching the oldest supported Rust release. The current policy is that this can be updated in any Rand release if required, but the change must be noted in the changelog.


There is built-in support for a random number generator (RNG) associated with each thread stored in thread-local storage. This RNG can be accessed via thread_rng, or used implicitly via random. This RNG is normally randomly seeded from an operating-system source of randomness, e.g. /dev/urandom on Unix systems, and will automatically reseed itself from this source after generating 32 KiB of random data.

let tuple = rand::random::<(f64, char)>();
println!("{:?}", tuple)
use rand::Rng;

let mut rng = rand::thread_rng();
if rng.gen() { // random bool
    println!("i32: {}, u32: {}", rng.gen::<i32>(), rng.gen::<u32>())

It is also possible to use other RNG types, which have a similar interface. The following uses the “ChaCha” algorithm instead of the default.

use rand::{Rng, ChaChaRng};

let mut rng = rand::ChaChaRng::new_unseeded();
println!("i32: {}, u32: {}", rng.gen::<i32>(), rng.gen::<u32>())


By default, Rand is built with all stable features available. The following optional features are available:

  • alloc can be used instead of std to provide Vec and Box
  • i128_support enables support for generating u128 and i128 values
  • log enables some logging via the log crate
  • nightly enables all unstable features (i128_support)
  • serde-1 enables serialisation for some types, via Serde version 1
  • std enabled by default; by setting “default-features = false” no_std mode is activated; this removes features depending on std functionality:
    • OsRng is entirely unavailable
    • JitterRng code is still present, but a nanosecond timer must be provided via JitterRng::new_with_timer
    • Since no external entropy is available, it is not possible to create generators with fresh seeds (user must provide entropy)
    • thread_rng, weak_rng and random are all disabled
    • exponential, normal and gamma type distributions are unavailable since exp and log functions are not provided in core
    • any code requiring Vec or Box


Unfortunately, cargo test does not test everything. The following tests are recommended:

# Basic tests for Rand and sub-crates
cargo test --all

# Test no_std support
cargo test --tests --no-default-features
# Test no_std+alloc support
cargo test --tests --no-default-features --features alloc

# Test log and serde support
cargo test --features serde-1,log

# Test 128-bit support (requires nightly)
cargo test --all --features nightly

# Benchmarks (requires nightly)
cargo bench
# or just to test the benchmark code:
cargo test --benches


Rand is distributed under the terms of both the MIT license and the Apache License (Version 2.0).