---
name: "arc4random_stir"
section: "3"
platform: "linux"
package: "libbsd-dev"
version: "0.8.3-1"
release: "Debian 9 (Stretch)"
platforms: ["linux"]
url: "https://new.carta.tech/arc4random"
---

# arc4random\_stir(3)

arc4 random number generator

Part of [arc4random(3)](/arc4random/3.md).

## NAME

**arc4random**, **arc4random\_buf**, **arc4random\_uniform**, **arc4random\_stir**, **arc4random\_addrandom** – arc4 random number generator

## LIBRARY

library “libbsd”

## SYNOPSIS

**#include \<bsd/stdlib.h\>**

*uint32\_t*\

**arc4random**(*void*);

*void*\

**arc4random\_buf**(*void \*buf*, *size\_t nbytes*);

*uint32\_t*\

**arc4random\_uniform**(*uint32\_t upper\_bound*);

*void*\

**arc4random\_stir**(*void*);

*void*\

**arc4random\_addrandom**(*unsigned char \*dat*, *int datlen*);

## DESCRIPTION

This family of functions provides higher quality data than those described in [rand(3)](/rand/3.md), [random(3)](/random/3.md), and [rand48(3)](/rand48/3.md).

Use of these functions is encouraged for almost all random number consumption because the other interfaces are deficient in either quality, portability, standardization, or availability. These functions can be called in almost all coding environments, including pthreads(3) and [chroot(2)](/chroot/2.md).

High quality 32-bit pseudo-random numbers are generated very quickly. On each call, a cryptographic pseudo-random number generator is used to generate a new result. One data pool is used for all consumers in a process, so that consumption under program flow can act as additional stirring. The subsystem is re-seeded from the kernel random number subsystem using [getentropy(2)](/getentropy/2.md) on a regular basis, and also upon [fork(2)](/fork/2.md).

The **arc4random**() function returns a single 32-bit value.

The **arc4random\_buf**() function fills the region *buf* of length *nbytes* with random data.

**arc4random\_uniform**() will return a single 32-bit value, uniformly distributed but less than *upper\_bound*. This is recommended over constructions like “arc4random() % upper\_bound” as it avoids "modulo bias" when the upper bound is not a power of two. In the worst case, this function may consume multiple iterations to ensure uniformity; see the source code to understand the problem and solution.

The **arc4random\_stir**() function reads data from [getentropy(2)](/getentropy/2.md) and uses it to re-seed the subsystem via **arc4random\_addrandom**().

There is no need to call **arc4random\_stir**() before using **arc4random**() functions family, since they automatically initialize themselves.

## RETURN VALUES

These functions are always successful, and no return value is reserved to indicate an error.

## SEE ALSO

[rand(3)](/rand/3.md), [rand48(3)](/rand48/3.md), [random(3)](/random/3.md)

## HISTORY

These functions first appeared in OpenBSD 2.1.

The original version of this random number generator used the RC4 (also known as ARC4) algorithm. In OpenBSD 5.5 it was replaced with the ChaCha20 cipher, and it may be replaced again in the future as cryptographic techniques advance. A good mnemonic is “A Replacement Call for Random”.