1.\" $NetBSD: random.3,v 1.21 2014/06/12 18:46:12 christos Exp $ 2.\" 3.\" Copyright (c) 1983, 1991, 1993 4.\" The Regents of the University of California. 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. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" from: @(#)random.3 8.1 (Berkeley) 6/4/93 31.\" 32.Dd June 12, 2014 33.Dt RANDOM 3 34.Os 35.Sh NAME 36.Nm random , 37.Nm srandom , 38.Nm initstate , 39.Nm setstate 40.Nd better random number generator; routines for changing generators 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In stdlib.h 45.Ft long 46.Fn random void 47.Ft void 48.Fn srandom "unsigned int seed" 49.Ft char * 50.Fn initstate "unsigned int seed" "char *state" "size_t n" 51.Ft char * 52.Fn setstate "char *state" 53.Sh DESCRIPTION 54The 55.Fn random 56function 57uses a non-linear additive feedback random number generator employing a 58default table of size 31 long integers to return successive pseudo-random 59numbers in the range from 0 to 60.if t 2\u\s731\s10\d\(mi1. 61.if n (2**31)\(mi1. 62The period of this random number generator is very large, approximately 63.if t 16\(mu(2\u\s731\s10\d\(mi1). 64.if n 16*((2**31)\(mi1). 65The maximum value 66.Dv RANDOM_MAX 67is defined in 68.In stdlib.h . 69.Pp 70The 71.Fn random 72and 73.Fn srandom 74have (almost) the same calling sequence and initialization properties as 75.Xr rand 3 76and 77.Xr srand 3 . 78The difference is that 79.Xr rand 3 80produces a much less random sequence \(em in fact, the low dozen bits 81generated by 82.Xr rand 3 83go through a cyclic pattern. 84All the bits generated by 85.Fn random 86are usable. 87For example, 88.Sq Li random()\*[Am]01 89will produce a random binary value. 90.Pp 91Like 92.Xr rand 3 , 93.Fn random 94will by default produce a sequence of numbers that can be duplicated 95by calling 96.Fn srandom 97with 98.Ql 1 99as the seed. 100.Pp 101The 102.Fn initstate 103routine allows a state array, passed in as an argument, to be initialized 104for future use. 105The size of the state array (in bytes) is used by 106.Fn initstate 107to decide how sophisticated a random number generator it should use \(em the 108more state, the better the random numbers will be. 109(Current "optimal" values for the amount of state information are 1108, 32, 64, 128, and 256 bytes; other amounts will be rounded down to 111the nearest known amount. 112Using less than 8 bytes will cause an error). 113The seed for the initialization (which specifies a starting point for 114the random number sequence, and provides for restarting at the same 115point) is also an argument. 116The state array passed to 117.Fn initstate 118must be aligned to a 32-bit boundary. 119This can be achieved by using 120a suitably-sized array of ints, and casting the array to char * when 121passing it to 122.Fn initstate . 123The 124.Fn initstate 125function 126returns a pointer to the previous state information array. 127.Pp 128Once a state has been initialized, the 129.Fn setstate 130routine provides for rapid switching between states. 131The 132.Fn setstate 133function 134returns a pointer to the previous state array; its 135argument state array is used for further random number generation 136until the next call to 137.Fn initstate 138or 139.Fn setstate . 140.Pp 141Once a state array has been initialized, it may be restarted at a 142different point either by calling 143.Fn initstate 144(with the desired seed, the state array, and its size) or by calling 145both 146.Fn setstate 147(with the state array) and 148.Fn srandom 149(with the desired seed). 150The advantage of calling both 151.Fn setstate 152and 153.Fn srandom 154is that the size of the state array does not have to be remembered after 155it is initialized. 156.Pp 157With 256 bytes of state information, the period of the random number 158generator is greater than 159.if t 2\u\s769\s10\d, 160.if n 2**69 161which should be sufficient for most purposes. 162.Sh DIAGNOSTICS 163If 164.Fn initstate 165is called with less than 8 bytes of state information, or if 166.Fn setstate 167detects that the state information has been garbled, error 168messages are printed on the standard error output. 169.Sh SEE ALSO 170.Xr rand 3 , 171.Xr srand 3 , 172.Xr rnd 4 , 173.Xr rnd 9 174.Sh STANDARDS 175The 176.Fn random , 177.Fn srandom , 178.Fn initstate 179and 180.Fn setstate 181functions conform to 182.St -p1003.1-2008 . 183.Sh HISTORY 184These 185functions appeared in 186.Bx 4.2 . 187.Sh AUTHORS 188.An Earl T. Cohen 189.Sh BUGS 190About 2/3 the speed of 191.Xr rand 3 . 192