1.\" Copyright (c) 1983, 1991 The Regents of the University of California. 2.\" All rights reserved. 3.\" 4.\" %sccs.include.redist.man% 5.\" 6.\" @(#)random.3 6.5 (Berkeley) 04/19/91 7.\" 8.Dd 9.Dt RANDOM 3 10.Os BSD 4.2 11.Sh NAME 12.Nm random , 13.Nm srandom , 14.Nm initstate , 15.Nm setstate 16.Nd better random number generator; routines for changing generators 17.Sh SYNOPSIS 18.Fd #include <stdlib> 19.Ft long 20.Fn random void 21.Ft void 22.Fn srandom "unsigned seed" 23.Ft char * 24.Fn initstate "unsigned seed" "char *state" "int n" 25.Ft char * 26.Fn setstate "char *state" 27.Sh DESCRIPTION 28The 29.Fn random 30function 31uses a non-linear additive feedback random number generator employing a 32default table of size 31 long integers to return successive pseudo-random 33numbers in the range from 0 to 34.if t 2\u\s731\s10\d\(mi1. 35.if n (2**31)\(mi1. 36The period of this random number generator is very large, approximately 37.if t 16\(mu(2\u\s731\s10\d\(mi1). 38.if n 16*((2**31)\(mi1). 39.Pp 40The 41.Fn random Ns / Fn srandom 42have (almost) the same calling sequence and initialization properties as 43.Xr rand 3 Ns / Xr srand 3 . 44The difference is that 45.Xr rand 46produces a much less random sequence \(em in fact, the low dozen bits 47generated by rand go through a cyclic pattern. All the bits generated by 48.Fn random 49are usable. For example, 50.Sq Li random()&01 51will produce a random binary 52value. 53.Pp 54Unlike 55.Xr srand , 56.Fn srandom 57does not return the old seed; the reason for this is that the amount of 58state information used is much more than a single word. (Two other 59routines are provided to deal with restarting/changing random 60number generators). Like 61.Xr rand 3 , 62however, 63.Fn random 64will by default produce a sequence of numbers that can be duplicated 65by calling 66.Fn srandom 67with 68.Ql 1 69as the seed. 70.Pp 71The 72.Fn initstate 73routine allows a state array, passed in as an argument, to be initialized 74for future use. The size of the state array (in bytes) is used by 75.Fn initstate 76to decide how sophisticated a random number generator it should use \(em the 77more state, the better the random numbers will be. 78(Current "optimal" values for the amount of state information are 798, 32, 64, 128, and 256 bytes; other amounts will be rounded down to 80the nearest known amount. Using less than 8 bytes will cause an error.) 81The seed for the initialization (which specifies a starting point for 82the random number sequence, and provides for restarting at the same 83point) is also an argument. 84The 85.Fn initstate 86function 87returns a pointer to the previous state information array. 88.Pp 89Once a state has been initialized, the 90.Fn setstate 91routine provides for rapid switching between states. 92The 93.Fn setstate 94function 95returns a pointer to the previous state array; its 96argument state array is used for further random number generation 97until the next call to 98.Fn initstate 99or 100.Fn setstate . 101.Pp 102Once a state array has been initialized, it may be restarted at a 103different point either by calling 104.Fn initstate 105(with the desired seed, the state array, and its size) or by calling 106both 107.Fn setstate 108(with the state array) and 109.Fn srandom 110(with the desired seed). 111The advantage of calling both 112.Fn setstate 113and 114.Fn srandom 115is that the size of the state array does not have to be remembered after 116it is initialized. 117.Pp 118With 256 bytes of state information, the period of the random number 119generator is greater than 120.if t 2\u\s769\s10\d, 121.if n 2**69 122which should be sufficient for most purposes. 123.Sh AUTHOR 124Earl T. Cohen 125.Sh DIAGNOSTICS 126If 127.Fn initstate 128is called with less than 8 bytes of state information, or if 129.Fn setstate 130detects that the state information has been garbled, error 131messages are printed on the standard error output. 132.Sh SEE ALSO 133.Xr rand 3 134.Sh HISTORY 135These 136functions appeared in 137.Bx 4.2 . 138.Sh BUGS 139About 2/3 the speed of 140.Xr rand 3 . 141