.\" $OpenBSD: arc4random.3,v 1.34 2014/07/19 16:11:16 naddy Exp $ .\" .\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de> .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by Niels Provos. .\" 4. The name of the author may not be used to endorse or promote products .\" derived from this software without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .\" Manual page, using -mandoc macros .\" .Dd July 30, 2015 .Dt ARC4RANDOM 3 .Os .Sh NAME .Nm arc4random , .Nm arc4random_buf , .Nm arc4random_uniform .Nd random number generator .Sh SYNOPSIS .In stdlib.h .Ft uint32_t .Fn arc4random "void" .Ft void .Fn arc4random_buf "void *buf" "size_t nbytes" .Ft uint32_t .Fn arc4random_uniform "uint32_t upper_bound" .Sh DESCRIPTION .Pp 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 kernel random number subsystem on a regular basis, and also upon .Xr fork 2 . .Pp This family of functions provides higher quality random data than those described in .Xr rand 3 , .Xr random 3 , and .Xr rand48 3 . They can be called in almost all environments, including .Xr chroot 2 , and their use is encouraged over all other standard library functions for random numbers. .Pp .Fn arc4random returns a single 32-bit value. .Pp .Fn arc4random_buf fills the region .Fa buf of length .Fa nbytes with random data. .Pp .Fn arc4random_uniform will return a single 32-bit value, uniformly distributed but less than .Fa upper_bound . This is recommended over constructions like .Dq Li 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. .Sh RETURN VALUES These functions are always successful, and no return value is reserved to indicate an error. .Sh SEE ALSO .Xr rand 3 , .Xr rand48 3 , .Xr random 3 , .Xr random 4 .Sh 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 cryptographic techniques advance. A good mnemonic is .Dq A Replacement Call for Random .