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 1.21 2013/06/05 03:39:23 tedu Exp $ 29.\" 30.Dd $Mdocdate: June 5 2013 $ 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.In 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 "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 93random numbers obtained from the kernel, 94suitable for cryptographic use. 95Note that this particular seeding procedure can generate 96states which are impossible to reproduce by calling 97.Fn srandom 98with any value, since the succeeding terms in the 99state buffer are no longer derived from the LC algorithm applied to 100a fixed seed. 101.Pp 102The 103.Fn initstate 104routine allows a state array, passed in as an argument, to be initialized 105for future use. 106The size of the state array (in bytes) is used by 107.Fn initstate 108to decide how sophisticated a random number generator it should use \(em the 109more state, the better the random numbers will be. 110(Current "optimal" values for the amount of state information are 1118, 32, 64, 128, and 256 bytes; other amounts will be rounded down to 112the nearest known amount. 113Using less than 8 bytes will cause an error.) 114The seed for the initialization (which specifies a starting point for 115the random number sequence, and provides for restarting at the same 116point) is also an argument. 117The 118.Fn initstate 119function returns a pointer to the previous state information array. 120.Pp 121Once a state has been initialized, the 122.Fn setstate 123routine provides for rapid switching between states. 124The 125.Fn setstate 126function returns a pointer to the previous state array; its 127argument state array is used for further random number generation 128until the next call to 129.Fn initstate 130or 131.Fn setstate . 132.Pp 133Once a state array has been initialized, it may be restarted at a 134different point either by calling 135.Fn initstate 136(with the desired seed, the state array, and its size) or by calling 137both 138.Fn setstate 139(with the state array) and 140.Fn srandom 141(with the desired seed). 142The advantage of calling both 143.Fn setstate 144and 145.Fn srandom 146is that the size of the state array does not have to be remembered after 147it is initialized. 148.Pp 149With 256 bytes of state information, the period of the random number 150generator is greater than 2**69 151which should be sufficient for most purposes. 152.Sh DIAGNOSTICS 153If 154.Fn initstate 155is called with less than 8 bytes of state information, or if 156.Fn setstate 157detects that the state information has been garbled, error 158messages are printed on the standard error output. 159.Sh SEE ALSO 160.Xr arc4random 3 , 161.Xr drand48 3 , 162.Xr rand 3 , 163.Xr random 4 164.Sh STANDARDS 165The 166.Fn random , 167.Fn srandom , 168.Fn initstate , 169and 170.Fn setstate 171functions conform to 172.St -xpg4.2 . 173.Pp 174The 175.Fn srandomdev 176function is an extension. 177.Sh HISTORY 178These 179functions appeared in 180.Bx 4.2 . 181.Sh AUTHORS 182.An Earl T. Cohen 183.Sh BUGS 184About 2/3 the speed of 185.Xr rand 3 . 186