1.\" $OpenBSD: arc4random.3,v 1.35 2014/11/25 16:45:24 millert Exp $ 2.\" 3.\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de> 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by Niels Provos. 17.\" 4. The name of the author may not be used to endorse or promote products 18.\" derived from this software without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 21.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 22.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 23.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 24.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 25.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 26.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 27.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 28.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 29.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 30.\" 31.\" Manual page, using -mandoc macros 32.\" 33.Dd "Dec 31, 2014" 34.Dt ARC4RANDOM 3C 35.Os 36.Sh NAME 37.Nm arc4random , 38.Nm arc4random_buf , 39.Nm arc4random_uniform 40.Nd random number generator 41.Sh SYNOPSIS 42.In stdlib.h 43.Ft uint32_t 44.Fn arc4random "void" 45.Ft void 46.Fn arc4random_buf "void *buf" "size_t nbytes" 47.Ft uint32_t 48.Fn arc4random_uniform "uint32_t upper_bound" 49.Sh DESCRIPTION 50This family of functions provides higher quality data than those 51described in 52.Xr rand 3C , 53.Xr random 3C , 54and 55.Xr rand48 3C . 56.Pp 57Use of these functions is encouraged for almost all random number 58consumption because the other interfaces are deficient in either 59quality, portability, standardization, or availability. 60These functions can be called in almost all coding environments, 61including 62.Xr pthreads 5 63and 64.Xr chroot 2 . 65.Pp 66High quality 32-bit pseudo-random numbers are generated very quickly. 67On each call, a cryptographic pseudo-random number generator is used 68to generate a new result. 69One data pool is used for all consumers in a process, so that consumption 70under program flow can act as additional stirring. 71The subsystem is re-seeded from the kernel random number subsystem using 72.Xr getentropy 2 73on a regular basis, and also upon 74.Xr fork 2 . 75.Pp 76The 77.Fn arc4random 78function returns a single 32-bit value. 79.Pp 80.Fn arc4random_buf 81fills the region 82.Fa buf 83of length 84.Fa nbytes 85with random data. 86.Pp 87.Fn arc4random_uniform 88will return a single 32-bit value, uniformly distributed but less than 89.Fa upper_bound . 90This is recommended over constructions like 91.Dq Li arc4random() % upper_bound 92as it avoids "modulo bias" when the upper bound is not a power of two. 93In the worst case, this function may consume multiple iterations 94to ensure uniformity; see the source code to understand the problem 95and solution. 96.Sh RETURN VALUES 97These functions are always successful, and no return value is 98reserved to indicate an error. 99.Sh INTERFACE STABILITY 100.Sy Committed 101.Sh MT-LEVEL 102.Sy MT-Safe 103.Sh SEE ALSO 104.Xr rand 3C , 105.Xr rand48 3C , 106.Xr random 3C 107