1.\" $NetBSD: random.3,v 1.18 2003/08/07 16:43:43 agc 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 4, 1993 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 long seed" 49.Ft char * 50.Fn initstate "unsigned long 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). 65.Pp 66The 67.Fn random 68and 69.Fn srandom 70have (almost) the same calling sequence and initialization properties as 71.Xr rand 3 72and 73.Xr srand 3 . 74The difference is that 75.Xr rand 3 76produces a much less random sequence \(em in fact, the low dozen bits 77generated by 78.Xr rand 3 79go through a cyclic pattern. 80All the bits generated by 81.Fn random 82are usable. 83For example, 84.Sq Li random()\*[Am]01 85will produce a random binary value. 86.Pp 87Like 88.Xr rand 3 , 89.Fn random 90will by default produce a sequence of numbers that can be duplicated 91by calling 92.Fn srandom 93with 94.Ql 1 95as the seed. 96.Pp 97The 98.Fn initstate 99routine allows a state array, passed in as an argument, to be initialized 100for future use. 101The size of the state array (in bytes) is used by 102.Fn initstate 103to decide how sophisticated a random number generator it should use \(em the 104more state, the better the random numbers will be. 105(Current "optimal" values for the amount of state information are 1068, 32, 64, 128, and 256 bytes; other amounts will be rounded down to 107the nearest known amount. 108Using less than 8 bytes will cause an error). 109The seed for the initialization (which specifies a starting point for 110the random number sequence, and provides for restarting at the same 111point) is also an argument. 112The state array passed to 113.Fn initstate 114must be aligned to a 32-bit boundary. 115This can be achieved by using 116a suitably-sized array of ints, and casting the array to char * when 117passing it to 118.Fn initstate . 119The 120.Fn initstate 121function 122returns a pointer to the previous state information array. 123.Pp 124Once a state has been initialized, the 125.Fn setstate 126routine provides for rapid switching between states. 127The 128.Fn setstate 129function 130returns a pointer to the previous state array; its 131argument state array is used for further random number generation 132until the next call to 133.Fn initstate 134or 135.Fn setstate . 136.Pp 137Once a state array has been initialized, it may be restarted at a 138different point either by calling 139.Fn initstate 140(with the desired seed, the state array, and its size) or by calling 141both 142.Fn setstate 143(with the state array) and 144.Fn srandom 145(with the desired seed). 146The advantage of calling both 147.Fn setstate 148and 149.Fn srandom 150is that the size of the state array does not have to be remembered after 151it is initialized. 152.Pp 153With 256 bytes of state information, the period of the random number 154generator is greater than 155.if t 2\u\s769\s10\d, 156.if n 2**69 157which should be sufficient for most purposes. 158.Sh DIAGNOSTICS 159If 160.Fn initstate 161is called with less than 8 bytes of state information, or if 162.Fn setstate 163detects that the state information has been garbled, error 164messages are printed on the standard error output. 165.Sh SEE ALSO 166.Xr rand 3 , 167.Xr srand 3 , 168.Xr rnd 4 , 169.Xr rnd 9 170.Sh HISTORY 171These 172functions appeared in 173.Bx 4.2 . 174.Sh AUTHORS 175.An Earl T. Cohen 176.Sh BUGS 177About 2/3 the speed of 178.Xr rand 3 . 179