NAME

       arc4random,  arc4random_buf,  arc4random_uniform,  arc4random_stir,  arc4random_addrandom  —  arc4 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 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 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 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 number subsystem 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 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.

       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
       cryptographic techniques advance.  A good mnemonic is “A Replacement Call for Random”.

Debian                                            July 19, 2014                                 ARC4RANDOM(3bsd)