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

NAME
     arc4random, arc4random_buf, arc4random_uniform -- random 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 ker-
     nel 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 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 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 cryp-
     tographic techniques advance.  A good mnemonic is ``A Replacement Call
     for Random''.

BSD                              July 30, 2015                             BSD