DragonFly On-Line Manual Pages
ARC4RANDOM(3) DragonFly Library Functions Manual ARC4RANDOM(3)
arc4random, arc4random_buf, arc4random_uniform - random number generator
Standard C Library (libc, -lc)
arc4random_buf(void *buf, size_t nbytes);
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
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.
These functions are always successful, and no return value is reserved to
indicate an error.
The following produces a drop-in replacement for the traditional rand()
and random() functions using arc4random():
#define foo4random() (arc4random_uniform(RAND_MAX + 1))
rand(3), rand48(3), random(3)
Daniel J. Bernstein, ChaCha, a variant of Salsa20,
http://cr.yp.to/papers.html#chacha, 2008-01-28, Document ID:
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
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
DragonFly 6.5-DEVELOPMENT April 13, 2020 DragonFly 6.5-DEVELOPMENT