arc4random_buf(3) arc4 random number generator

Other Alias

arc4random, arc4random_uniform, arc4random_stir, arc4random_addrandom


Lb libbsd


In bsd/stdlib.h Ft uint32_t Fn arc4random void Ft void Fn arc4random_buf void *buf size_t nbytes Ft uint32_t Fn arc4random_uniform uint32_t upper_bound Ft void Fn arc4random_stir void Ft void Fn arc4random_addrandom unsigned char *dat int datlen


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

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).

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) on a regular basis, and also upon fork(2).

The Fn arc4random function returns a single 32-bit value.

The Fn arc4random_buf function fills the region Fa buf of length Fa nbytes with random data.

Fn arc4random_uniform will return a single 32-bit value, uniformly distributed but less than Fa 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 Fn arc4random_stir function reads data from getentropy(2) and uses it to re-seed the subsystem via Fn arc4random_addrandom .

There is no need to call Fn arc4random_stir before using Fn arc4random functions family, since they automatically initialize themselves.


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


These functions first appeared in Ox 2.1 .

The original version of this random number generator used the RC4 (also known as ARC4) algorithm. In Ox 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''