1.\" Copyright (c) 1983, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 4. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)random.3 8.1 (Berkeley) 6/4/93 29.\" $FreeBSD: src/lib/libc/stdlib/random.3,v 1.22 2007/01/09 00:28:10 imp Exp $ 30.\" $DragonFly: src/lib/libc/stdlib/random.3,v 1.5 2007/06/29 19:34:41 swildner Exp $ 31.\" 32.Dd June 4, 1993 33.Dt RANDOM 3 34.Os 35.Sh NAME 36.Nm random , 37.Nm srandom , 38.Nm srandomdev , 39.Nm initstate , 40.Nm setstate 41.Nd better random number generator; routines for changing generators 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In stdlib.h 46.Ft long 47.Fn random void 48.Ft void 49.Fn srandom "unsigned long seed" 50.Ft void 51.Fn srandomdev void 52.Ft char * 53.Fn initstate "unsigned long seed" "char *state" "long n" 54.Ft char * 55.Fn setstate "char *state" 56.Sh DESCRIPTION 57The 58.Fn random 59function 60uses a non-linear additive feedback random number generator employing a 61default table of size 31 long integers to return successive pseudo-random 62numbers in the range from 0 to 63.if t 2\u\s731\s10\d\(mi1. 64.if n (2**31)\(mi1. 65The period of this random number generator is very large, approximately 66.if t 16\(mu(2\u\s731\s10\d\(mi1). 67.if n 16*((2**31)\(mi1). 68.Pp 69The 70.Fn random 71and 72.Fn srandom 73functions have (almost) the same calling sequence and initialization properties as the 74.Xr rand 3 75and 76.Xr srand 3 77functions. 78The difference is that 79.Xr rand 3 80produces a much less random sequence \(em in fact, the low dozen bits 81generated by rand go through a cyclic pattern. 82All the bits generated by 83.Fn random 84are usable. 85For example, 86.Sq Li random()&01 87will produce a random binary 88value. 89.Pp 90Like 91.Xr rand 3 , 92.Fn random 93will by default produce a sequence of numbers that can be duplicated 94by calling 95.Fn srandom 96with 97.Ql 1 98as the seed. 99.Pp 100The 101.Fn srandomdev 102routine initializes a state array using the 103.Xr random 4 104random number device which returns good random numbers, 105suitable for cryptographic use. 106Note that this particular seeding 107procedure can generate states which are impossible to reproduce by 108calling 109.Fn srandom 110with any value, since the succeeding terms in the 111state buffer are no longer derived from the LC algorithm applied to 112a fixed seed. 113.Pp 114The 115.Fn initstate 116routine allows a state array, passed in as an argument, to be initialized 117for future use. 118The size of the state array (in bytes) is used by 119.Fn initstate 120to decide how sophisticated a random number generator it should use \(em the 121more state, the better the random numbers will be. 122(Current "optimal" values for the amount of state information are 1238, 32, 64, 128, and 256 bytes; other amounts will be rounded down to 124the nearest known amount. 125Using less than 8 bytes will cause an error.) 126The seed for the initialization (which specifies a starting point for 127the random number sequence, and provides for restarting at the same 128point) is also an argument. 129The 130.Fn initstate 131function 132returns a pointer to the previous state information array. 133.Pp 134Once a state has been initialized, the 135.Fn setstate 136routine provides for rapid switching between states. 137The 138.Fn setstate 139function 140returns a pointer to the previous state array; its 141argument state array is used for further random number generation 142until the next call to 143.Fn initstate 144or 145.Fn setstate . 146.Pp 147Once a state array has been initialized, it may be restarted at a 148different point either by calling 149.Fn initstate 150(with the desired seed, the state array, and its size) or by calling 151both 152.Fn setstate 153(with the state array) and 154.Fn srandom 155(with the desired seed). 156The advantage of calling both 157.Fn setstate 158and 159.Fn srandom 160is that the size of the state array does not have to be remembered after 161it is initialized. 162.Pp 163With 256 bytes of state information, the period of the random number 164generator is greater than 165.if t 2\u\s769\s10\d, 166.if n 2**69 167which should be sufficient for most purposes. 168.Sh DIAGNOSTICS 169If 170.Fn initstate 171is called with less than 8 bytes of state information, or if 172.Fn setstate 173detects that the state information has been garbled, error 174messages are printed on the standard error output. 175.Sh SEE ALSO 176.Xr arc4random 3 , 177.Xr rand 3 , 178.Xr srand 3 , 179.Xr random 4 180.Sh HISTORY 181These 182functions appeared in 183.Bx 4.2 . 184.Sh AUTHORS 185.An Earl T. Cohen 186.Sh BUGS 187About 2/3 the speed of 188.Xr rand 3 . 189.Pp 190The historical implementation used to have a very weak seeding; the 191random sequence did not vary much with the seed. 192The current implementation employs a better pseudo-random number 193generator for the initial state calculation. 194.Pp 195Applications requiring cryptographic quality randomness should use 196.Xr arc4random 3 . 197