NAME
arc4random,
arc4random_buf,
arc4random_uniform —
random number generator
LIBRARY
library “libc”
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
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 pthread(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(4) subsystem using getentropy(3) on a regular basis, and also upon fork(2).
The
arc4random()
function returns a single 32-bit value. 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()
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.
RETURN VALUES
These functions are always successful, and no return value is reserved to indicate an error.
EXAMPLES
The following produces a drop-in replacement for the traditional
rand() and random()
functions using arc4random():
#define foo4random()
(arc4random_uniform(RAND_MAX + 1))SEE ALSO
Daniel J. Bernstein, ChaCha, a variant of Salsa20, http://cr.yp.to/papers.html#chacha, 2008-01-28, Document ID: 4027b5256e17b9796842e6d0f68b0b5e.
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”.
The arc4random() random number generator
was first introduced in FreeBSD 2.2.6. The ChaCha20
based implementation was introduced in FreeBSD 12.0,
with obsolete stir and addrandom interfaces removed at the same time.