ARC4RANDOM(3)          FreeBSD Library Functions Manual          ARC4RANDOM(3)

NAME
     arc4random, arc4random_buf, arc4random_uniform - 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);

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
     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:
     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.

FreeBSD 13.1-RELEASE-p6         April 13, 2020         FreeBSD 13.1-RELEASE-p6