arc4random(3bsd) LOCAL arc4random(3bsd)
NAME
arc4random, arc4random_buf, arc4random_uniform, arc4random_stir,
arc4random_addrandom — random number generator
LIBRARY
Utility functions from BSD systems (libbsd, -lbsd)
SYNOPSIS
#include <stdlib.h>
(See libbsd(7) for include usage.)
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 de-
scribed in rand(3), random(3), and rand48(3).
Use of these functions is encouraged for almost all random number con-
sumption 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 gen-
erate 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 sub-
system using getentropy(2) on a regular basis, and also upon fork(2).
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 dis-
tributed 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 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) 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), rand48(3), random(3)
HISTORY
These functions first appeared in OpenBSD 2.1, FreeBSD 3.0, NetBSD 1.6,
and DragonFly 1.0.
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 crypto-
graphic techniques advance. A good mnemonic is “A Replacement Call for
Random”.
BSD September 29, 2019 BSD
Generated by dwww version 1.15 on Wed Jun 26 04:31:59 CEST 2024.