ARC4RANDOM(3) Library Functions Manual ARC4RANDOM(3)

arc4random, arc4random_buf, arc4random_uniformrandom number generator

#include <stdlib.h>

uint32_t
arc4random(void);

void
arc4random_buf(void *buf, size_t nbytes);

uint32_t
arc4random_uniform(uint32_t upper_bound);

These functions use a cryptographic pseudo-random number generator to generate high quality random bytes very quickly. 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 on a regular basis, and also upon fork(2).

This family of functions provides higher quality random data than those described in rand(3), random(3), and rand48(3). They can be called in almost all environments, including chroot(2), and their use is encouraged over all other standard library functions for random numbers.

() returns a single 32-bit value.

() fills the region buf of length nbytes with random data.

() 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 require multiple iterations to ensure uniformity.

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

rand(3), rand48(3), random(3), random(4)

The original version of this random number generator used the RC4 (also known as ARC4) algorithm. In OS X 10.12 it was replaced with the NIST-approved AES cipher, and it may be replaced again in the future as cryptographic techniques advance. A good mnemonic is “A Replacement Call for Random”.

July 30, 2015 macOS 14.4