# arc4random(3) [mojave man page]

ARC4RANDOM(3) BSD Library Functions Manual ARC4RANDOM(3)NAME

arc4random, arc4random_buf, arc4random_uniformrandom number generator--SYNOPSIS

#include <stdlib.h> uint32_t arc4random(void); void arc4random_buf(void *buf, size_t nbytes); uint32_t arc4random_uniform(uint32_t upper_bound);DESCRIPTION

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. arc4random() returns a single 32-bit value. arc4random_buf() 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 construc- tions 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.RETURN VALUES

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

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

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''.BSD

July 30, 2015 BSD

## Check Out this Related Man Page

ARC4RANDOM(3) BSD Library Functions Manual ARC4RANDOM(3)NAME

arc4random, arc4random_buf, arc4random_uniform, arc4random_stir, arc4random_addrandomarc4 random number generator--LIBRARY

Standard C Library (libc, -lc)SYNOPSIS

#include <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

The arc4random() function uses the key stream generator employed by the arc4 cipher, which uses 8*8 8 bit S-Boxes. The S-Boxes can be in about (2**1700) states. The arc4random() function returns pseudo-random numbers in the range of 0 to (2**32)-1, and therefore has twice the range of rand(3) and random(3). arc4random_buf() function fills the region buf of length nbytes with ARC4-derived random data. arc4random_uniform() will return a uniformly distributed random number less than upper_bound. arc4random_uniform() is recommended over con- structions like ``arc4random() % upper_bound'' as it avoids "modulo bias" when the upper bound is not a power of two. The arc4random_stir() function reads data from /dev/urandom and uses it to permute the S-Boxes via arc4random_addrandom(). There is no need to call arc4random_stir() before using arc4random() functions family, since they automatically initialize themselves.EXAMPLES

The following produces a drop-in replacement for the traditional rand() and random() functions using arc4random(): #define foo4random() (arc4random() % ((unsigned)RAND_MAX + 1))SEE ALSO

rand(3), random(3), srandomdev(3)HISTORY

RC4 has been designed by RSA Data Security, Inc. It was posted anonymously to the USENET and was confirmed to be equivalent by several sources who had access to the original cipher. Since RC4 used to be a trade secret, the cipher is now referred to as ARC4.BSD

April 15, 1997 BSD