ptional (opt-in) backends, which allow users to customize the source
of randomness based on their specific needs:

| Backend name      | Target               | Target Triple            | Implementation
| ----------------- | -------------------- | ------------------------ | --------------
| `linux_getrandom` | Linux, Android       | `*‑linux‑*`              | [`getrandom`][1] system call (without `/dev/urandom` fallback). Bumps minimum supported Linux kernel version to 3.17 and Android API level to 23 (Marshmallow).
| `linux_raw`       | Linux, Android       | `*‑linux‑*`              | Same as `linux_getrandom`, but uses raw `asm!`-based syscalls instead of `libc`.
| `rdrand`          | x86, x86-64          | `x86_64-*`, `i686-*`     | [`RDRAND`] instruction
| `rndr`            | AArch64              | `aarch64-*`              | [`RNDR`] register
| `wasm_js`         | Web Browser, Node.js | `wasm32‑unknown‑unknown`, `wasm32v1-none` | [`Crypto.getRandomValues`]. Requires feature `wasm_js` ([see below](#webassembly-support)).
| `efi_rng`         | UEFI                 | `*-unknown‑uefi`         | [`EFI_RNG_PROTOCOL`] with `EFI_RNG_ALGORITHM_RAW` (requires `std` and Nigthly compiler)
| `custom`          | All targets          | `*`                      | User-provided custom implementation (see [custom backend])

Opt-in backends can be enabled using the `getrandom_backend` configuration flag.
The flag can be set either by specifying the `rustflags` field in [`.cargo/config.toml`]:
```toml
# It's recommended to set the flag on a per-target basis:
[target.wasm32-unknown-unknown]
rustflags = ['--cfg', 'getrandom_backend="wasm_js"']
```

Or by using the `RUSTFLAGS` environment variable:

```sh
RUSTFLAGS='--cfg getrandom_backend="linux_getrandom"' cargo build
```

Enabling an opt-in backend will replace the backend used by default. Doing this for
an incorrect target (e.g. using `linux_getrandom` while compiling for a Windows target)
will result in a compilation error. Be extremely careful while using opt-in backends,
as incorrect configuration may result in vulnerable applications or applications
that always panic.

Note that using an opt-in backend in a library (e.g. for tests or benchmarks)
WILL NOT have any effect on its downstream users.

[`.cargo/config.toml`]: https://doc.rust-lang.org/cargo/reference/config.html

### Raw Linux syscall support

Currently the `linux_raw` backend supports only targets with stabilized `asm!` macro,
i.e. `arm`, `aarch64`, `loongarch64`, `riscv32`, `riscv64`, `s390x`, `x86`, and `x86_64`.

Note that the raw syscall backend may be slower than backends based on `libc::getrandom`,
e.g. it does not implement vDSO optimizations and on `x86` it uses the infamously slow
`int 0x80` instruction to perform syscall.

### WebAssembly support

This crate fully supports the [WASI] and [Emscripten] targets. However,
the `wasm32-unknown-unknown` target (i.e. the target used by `wasm-pack`)
is not automatically supported since, from the target name alone, we cannot deduce
which JavaScript interface should be used (or if JavaScript is available at all).

To enable `getrandom`'s functionality on `wasm32-unknown-unknown` using the Web
Crypto methods [described above][opt-in] via [`wasm-bindgen`], do
*both* of the following:

-   Use the `wasm_js` feature flag, i.e.
    `getrandom = { version = "0.3", features = ["wasm_js"] }`.
    On its own, this only makes the backend available. (As a side effect this
    will make your `Cargo.lock` significantly larger if you are not already
    using [`wasm-bindgen`], but otherwise enabling this feature is harmless.)
-   Set `RUSTFLAGS='--cfg getrandom_backend="wasm_js"'` ([see above][opt-in]).

This backend supports both web browsers (main window and Web Workers)
and Node.js (v19 or later) environments.

WARNING: It is highly recommended to enable the `wasm_js` feature only for
binary crates and tests, i.e. avoid unconditionally enabling it in library crates.

### Custom backend

If this crate does not support your target out of the box or you have to use
a non-default entropy source, then you can provide a custom implementation.
You need to enable the custom backend as described in the
[opt-in backends][opt-in] section.

Next, you need to define an `extern` function with the following signature:

```rust
use getrandom::Error;

#[no_mangle]
unsafe extern "Rust" fn __getrandom_v03_custom(
    dest: *mut u8,
    len: usize,
) -> Result<(), Error> {
    todo!()
}
```

This function should, ideally, be defined in the root crate of your project,
e.g. in your `main.rs`. This function MUST be defined only once for your
project, i.e. upstream library crates SHOULD NOT define it outside of
tests and benchmarks. Improper configuration of this backend may result
in linking errors.

The function accepts a pointer to a buffer that should be filled with random
data and its length in bytes. Note that the buffer MAY be uninitialized.
On success, the function should return `Ok(())` and fully fill the input buffer;
otherwise, it should return an error value.

While wrapping functions which work with byte slices you should fully initialize
the buffer before passing it to the function:
```rust
use getrandom::Error;

fn my_entropy_source(buf: &mut [u8]) -> Result<(), getrandom::Error> {
    // ...
    Ok(())
}

#[no_mangle]
unsafe extern "Rust" fn __getrandom_v03_custom(
    dest: *mut u8,
    len: usize,
) -> Result<(), Error> {
    let buf = unsafe {
        // fill the buffer with zeros
        core::ptr::write_bytes(dest, 0, len);
        // create mutable byte slice
        core::slice::from_raw_parts_mut(dest, len)
    };
    my_entropy_source(buf)
}
```

If you are confident that `getrandom` is not used in your project, but
it gets pulled nevertheless by one of your dependencies, then you can
use the following custom backend, which always returns the "unsupported" error:
```rust
use getrandom::Error;

#[no_mangle]
unsafe extern "Rust" fn __getrandom_v03_custom(
    dest: *mut u8,
    len: usize,
) -> Result<(), Error> {
    Err(Error::UNSUPPORTED)
}
```

### Platform Support

This crate generally supports the same operating system and platform versions
that the Rust standard library does. Additional targets may be supported using
the opt-in custom backend.

This means that as Rust drops support for old versions of operating systems
(such as old Linux kernel versions, Android API levels, etc.) in stable releases,
`getrandom` may create new patch releases that remove support for
outdated platform versions.

### `/dev/urandom` fallback on Linux and Android

On Linux targets, the `/dev/urandom` fallback is present only if either `target_env`
is `musl`, or `target_arch` is one of the following: `aarch64`, `arm`, `powerpc`,
`powerpc64`, `s390x`, `x86`, `x86_64`. Other supported targets [require][platform-support]
kernel versions that support the `getrandom` system call, so the fallback is not needed.

On Android targets the fallback is present only for the following `target_arch`es:
`aarch64`, `arm`, `x86`, `x86_64`. Other `target_arch`es (e.g. RISC-V) require
sufficiently high API levels.

The fallback can be disabled by enabling the `linux_getrandom` opt-in backend.
Note that doing so will bump minimum supported Linux kernel version to 3.17
and Android API level to 23 (Marshmallow).

### Early boot

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.

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.

On Linux (when the `getrandom` system call is not available), reading from
`/dev/urandom` never blocks, even when the OS hasn't collected enough
entropy yet. To avoid returning low-entropy bytes, we first poll
`/dev/random` and only switch to `/dev/urandom` once this has succeeded.

On OpenBSD, this kind of entropy accounting isn't available, and on
NetBSD, blocking on it is discouraged. On these platforms, nonblocking
interfaces are used, even when reliable entropy may not be available.
On the platforms where it is used, the reliability of entropy accounting
itself isn't free from controversy. This library provides randomness
sourced according to the platform's best practices, but each platform has
its own limits on the grade of randomness it can promise in environments
with few sources of entropy.

On ESP-IDF, if `esp_fill_random` is used before enabling WiFi, BT, or the
voltage noise entropy source (SAR ADC), the Hardware RNG will only be seeded
via RC_FAST_CLK. This can occur during early boot unless
`bootloader_random_enable()` is called. For more information see the
[ESP-IDF RNG Docs][esp-idf-rng] or the
[RNG section of the ESP32 Technical Reference Manual][esp-trng-docs].

## Error handling

We always prioritize failure over returning known insecure "random" bytes.
Generally, on supported platforms, failure is highly unlikely, though not
impossible. If an error does occur, it is likely that it will occur
on every call to `getrandom`. Therefore, after the first successful call,
one can be reasonably confident that no errors will occur.

## Panic handling

We strive to eliminate all potential panics from our backend implementations.
In other words, when compiled with optimizations enabled, the generated
binary code for `getrandom` functions should not contain any panic branches.
Even if the platform misbehaves and returns an unexpected result,
our code should correctly handle it and return an error, e.g.
[`Error::UNEXPECTED`].

## Sanitizer support

If your code uses [`fill_uninit`] and you enable
[MemorySanitizer](https://doc.rust-lang.org/beta/unstable-book/compiler-flags/sanitizer.html#memorysanitizer)
(i.e. `-Zsanitizer=memory`), we will automatically handle unpoisoning
of the destination buffer filled by `fill_uninit`.

You can run sanitizer tests for your crate dependent on `getrandom` like this:
```sh
RUSTFLAGS="-Zsanitizer=memory" cargo test -Zbuild-std --target=x86_64-unknown-linux-gnu
```

## Minimum Supported Rust Version

This crate requires Rust 1.63 or later.

## License

The `getrandom` library is distributed under either of

 * [Apache License, Version 2.0][LICENSE-APACHE]
 * [MIT license][LICENSE-MIT]

at your option.

### Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
dual licensed as above, without any additional terms or conditions.

[//]: # (badges)

[GitHub Actions]: https://github.com/rust-random/getrandom/actions?query=branch:master
[Build Status]: https://github.com/rust-random/getrandom/actions/workflows/tests.yml/badge.svg?branch=master
[crates.io]: https://crates.io/crates/getrandom
[Crate]: https://img.shields.io/crates/v/getrandom
[docs.rs]: https://docs.rs/getrandom
[Documentation]: https://docs.rs/getrandom/badge.svg
[deps.rs]: https://deps.rs/repo/github/rust-random/getrandom
[Dependency Status]: https://deps.rs/repo/github/rust-random/getrandom/status.svg
[Downloads]: https://img.shields.io/crates/d/getrandom
[License]: https://img.shields.io/crates/l/getrandom

[//]: # (supported targets)

[1]: https://manned.org/getrandom.2
[2]: https://manned.org/urandom.4
[3]: https://www.unix.com/man-page/mojave/2/getentropy/
[4]: https://www.unix.com/man-page/mojave/4/urandom/
[5]: https://www.freebsd.org/cgi/man.cgi?query=getrandom&manpath=FreeBSD+12.0-stable
[7]: https://man.openbsd.org/getentropy.2
[8]: https://man.netbsd.org/sysctl.7
[9]: https://leaf.dragonflybsd.org/cgi/web-man?command=getrandom
[11]: https://docs.oracle.com/cd/E88353_01/html/E37841/getrandom-2.html
[12]: https://illumos.org/man/2/getrandom
[13]: https://github.com/emscripten-core/emscripten/pull/12240
[14]: https://www.qnx.com/developers/docs/7.1/index.html#com.qnx.doc.neutrino.utilities/topic/r/random.html
[15]: https://www.ibm.com/docs/en/aix/7.3?topic=files-random-urandom-devices
[16]: https://man.netbsd.org/getrandom.2
[17]: https://www.gnu.org/software/libc/manual/html_mono/libc.html#index-getrandom
[18]: https://github.com/rust3ds/shim-3ds/commit/b01d2568836dea2a65d05d662f8e5f805c64389d
[19]: https://github.com/vitasdk/newlib/blob/2d869fe47aaf02b8e52d04e9a2b79d5b210fd016/newlib/libc/sys/vita/getentropy.c
[20]: https://github.com/cygwin/cygwin/blob/main/winsup/cygwin/libc/getentropy.cc

[`ProcessPrng`]: https://learn.microsoft.com/en-us/windows/win32/seccng/processprng
[`RtlGenRandom`]: https://learn.microsoft.com/en-us/windows/win32/api/ntsecapi/nf-ntsecapi-rtlgenrandom
[`Crypto.getRandomValues`]: https://www.w3.org/TR/WebCryptoAPI/#Crypto-method-getRandomValues
[`RDRAND`]: https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide
[`RNDR`]: https://developer.arm.com/documentation/ddi0601/2024-06/AArch64-Registers/RNDR--Random-Number
[`CCRandomGenerateBytes`]: https://opensource.apple.com/source/CommonCrypto/CommonCrypto-60074/include/CommonRandom.h.auto.html
[`cprng_draw`]: https://fuchsia.dev/fuchsia-src/zircon/syscalls/cprng_draw
[`esp_fill_random`]: https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-reference/system/random.html#functions
[esp-idf-rng]: https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-reference/system/random.html
[esp-trng-docs]: https://www.espressif.com/sites/default/files/documentation/esp32_technical_reference_manual_en.pdf#rng
[`EFI_RNG_PROTOCOL`]: https://uefi.org/specs/UEFI/2.10/37_Secure_Technologies.html#efi-rng-protocol
[`random_get`]: https://github.com/WebAssembly/WASI/blob/snapshot-01/phases/snapshot/docs.md#-random_getbuf-pointeru8-buf_len-size---errno
[`get-random-u64`]: https://github.com/WebAssembly/WASI/blob/v0.2.1/wasip2/random/random.wit#L23-L28
[configuration flags]: #configuration-flags
[custom backend]: #custom-backend
[`wasm-bindgen`]: https://github.com/rustwasm/wasm-bindgen
[`module`]: https://rustwasm.github.io/wasm-bindgen/reference/attributes/on-js-imports/module.html
[`sys_read_entropy`]: https://github.com/hermit-os/kernel/blob/315f58ff5efc81d9bf0618af85a59963ff55f8b1/src/syscalls/entropy.rs#L47-L55
[platform-support]: https://doc.rust-lang.org/stable/rustc/platform-support.html
[WASI]: https://github.com/WebAssembly/WASI
[Emscripten]: https://emscripten.org
[opt-in]: #opt-in-backends

[//]: # (licenses)

[LICENSE-APACHE]: https://github.com/rust-random/getrandom/blob/master/LICENSE-APACHE
[LICENSE-MIT]: https://github.com/rust-random/getrandom/blob/master/LICENSE-MIT

[`Error::UNEXPECTED`]: https://docs.rs/getrandom/latest/getrandom/struct.Error.html#associatedconstant.UNEXPECTED
[`fill_uninit`]: https://docs.rs/getrandom/latest/getrandom/fn.fill_uninit.html
