1.\" Copyright (c) 1983, 1991 The Regents of the University of California. 2.\" 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.\" 3. 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.\" $OpenBSD: random.3,v 1.17 2003/06/02 20:18:38 millert Exp $ 29.\" 30.Dd April 19, 1991 31.Dt RANDOM 3 32.Os 33.Sh NAME 34.Nm random , 35.Nm srandom , 36.Nm srandomdev , 37.Nm initstate , 38.Nm setstate 39.Nd better random number generator; routines for changing generators 40.Sh SYNOPSIS 41.Fd #include <stdlib.h> 42.Ft long 43.Fn random void 44.Ft void 45.Fn srandom "unsigned int seed" 46.Ft void 47.Fn srandomdev void 48.Ft char * 49.Fn initstate "unsigned int seed" "char *state" "size_t n" 50.Ft char * 51.Fn setstate "const char *state" 52.Sh DESCRIPTION 53The 54.Fn random 55function uses a non-linear additive feedback random number generator employing 56a default table of size 31 long integers to return successive pseudo-random 57numbers in the range from 0 to (2**31)\-1. 58The period of this random number generator is very large, approximately 5916*((2**31)\-1). 60.Pp 61The 62.Fn random 63and 64.Fn srandom 65functions have (almost) the same calling sequence and initialization 66properties as 67.Xr rand 3 Ns / Ns Xr srand 3 . 68The difference is that 69.Xr rand 70produces a much less random sequence \(em in fact, the low dozen bits 71generated by rand go through a cyclic pattern. 72All the bits generated by 73.Fn random 74are usable. 75For example, 76.Sq Li random()&01 77will produce a random binary 78value. 79.Pp 80Like 81.Xr rand 3 , 82.Fn random 83will by default produce a sequence of numbers that can be duplicated 84by calling 85.Fn srandom 86with 87.Ql 1 88as the seed. 89.Pp 90The 91.Fn srandomdev 92routine initializes a state array using the 93.Xr arandom 4 94random number device which returns good random numbers, 95suitable for cryptographic use. 96Note that this particular seeding procedure can generate 97states which are impossible to reproduce by calling 98.Fn srandom 99with any value, since the succeeding terms in the 100state buffer are no longer derived from the LC algorithm applied to 101a fixed seed. 102.Pp 103The 104.Fn initstate 105routine allows a state array, passed in as an argument, to be initialized 106for future use. 107The size of the state array (in bytes) is used by 108.Fn initstate 109to decide how sophisticated a random number generator it should use \(em the 110more state, the better the random numbers will be. 111(Current "optimal" values for the amount of state information are 1128, 32, 64, 128, and 256 bytes; other amounts will be rounded down to 113the nearest known amount. 114Using less than 8 bytes will cause an error.) 115The seed for the initialization (which specifies a starting point for 116the random number sequence, and provides for restarting at the same 117point) is also an argument. 118The 119.Fn initstate 120function returns a pointer to the previous state information array. 121.Pp 122Once a state has been initialized, the 123.Fn setstate 124routine provides for rapid switching between states. 125The 126.Fn setstate 127function returns a pointer to the previous state array; its 128argument state array is used for further random number generation 129until the next call to 130.Fn initstate 131or 132.Fn setstate . 133.Pp 134Once a state array has been initialized, it may be restarted at a 135different point either by calling 136.Fn initstate 137(with the desired seed, the state array, and its size) or by calling 138both 139.Fn setstate 140(with the state array) and 141.Fn srandom 142(with the desired seed). 143The advantage of calling both 144.Fn setstate 145and 146.Fn srandom 147is that the size of the state array does not have to be remembered after 148it is initialized. 149.Pp 150With 256 bytes of state information, the period of the random number 151generator is greater than 2**69 152which should be sufficient for most purposes. 153.Sh DIAGNOSTICS 154If 155.Fn initstate 156is called with less than 8 bytes of state information, or if 157.Fn setstate 158detects that the state information has been garbled, error 159messages are printed on the standard error output. 160.Sh SEE ALSO 161.Xr arc4random 3 , 162.Xr drand48 3 , 163.Xr rand 3 , 164.Xr random 4 165.Sh STANDARDS 166The 167.Fn random , 168.Fn srandom , 169.Fn initstate , 170and 171.Fn setstate 172functions conform to 173.St -xpg4.2 . 174.Pp 175The 176.Fn srandomdev 177function is an extension. 178.Sh HISTORY 179These 180functions appeared in 181.Bx 4.2 . 182.Sh AUTHORS 183.An Earl T. Cohen 184.Sh BUGS 185About 2/3 the speed of 186.Xr rand 3 . 187