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1 | .\" Copyright (c) 1983, 1991 The Regents of the University of California. |
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32 | .\" @(#)random.3 6.5 (Berkeley) 4/19/91 | |
33 | .\" | |
34 | .Dd April 19, 1991 | |
35 | .Dt RANDOM 3 | |
36 | .Os BSD 4.2 | |
37 | .Sh NAME | |
38 | .Nm random , | |
39 | .Nm srandom , | |
40 | .Nm initstate , | |
41 | .Nm setstate | |
42 | .Nd better random number generator; routines for changing generators | |
43 | .Sh SYNOPSIS | |
c6d691fc | 44 | .Fd #include <stdlib.h> |
15637ed4 RG |
45 | .Ft long |
46 | .Fn random void | |
47 | .Ft void | |
48 | .Fn srandom "unsigned seed" | |
49 | .Ft char * | |
50 | .Fn initstate "unsigned seed" "char *state" "int n" | |
51 | .Ft char * | |
52 | .Fn setstate "char *state" | |
53 | .Sh DESCRIPTION | |
54 | The | |
55 | .Fn random | |
56 | function | |
57 | uses a non-linear additive feedback random number generator employing a | |
58 | default table of size 31 long integers to return successive pseudo-random | |
59 | numbers in the range from 0 to | |
60 | .if t 2\u\s731\s10\d\(mi1. | |
61 | .if n (2**31)\(mi1. | |
62 | The 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 | |
66 | The | |
67 | .Fn random Ns / Fn srandom | |
68 | have (almost) the same calling sequence and initialization properties as | |
69 | .Xr rand 3 Ns / Xr srand 3 . | |
70 | The difference is that | |
71 | .Xr rand | |
72 | produces a much less random sequence \(em in fact, the low dozen bits | |
73 | generated by rand go through a cyclic pattern. All the bits generated by | |
74 | .Fn random | |
75 | are usable. For example, | |
76 | .Sq Li random()&01 | |
77 | will produce a random binary | |
78 | value. | |
79 | .Pp | |
80 | Unlike | |
81 | .Xr srand , | |
82 | .Fn srandom | |
83 | does not return the old seed; the reason for this is that the amount of | |
84 | state information used is much more than a single word. (Two other | |
85 | routines are provided to deal with restarting/changing random | |
86 | number generators). Like | |
87 | .Xr rand 3 , | |
88 | however, | |
89 | .Fn random | |
90 | will by default produce a sequence of numbers that can be duplicated | |
91 | by calling | |
92 | .Fn srandom | |
93 | with | |
94 | .Ql 1 | |
95 | as the seed. | |
96 | .Pp | |
97 | The | |
98 | .Fn initstate | |
99 | routine allows a state array, passed in as an argument, to be initialized | |
100 | for future use. The size of the state array (in bytes) is used by | |
101 | .Fn initstate | |
102 | to decide how sophisticated a random number generator it should use \(em the | |
103 | more state, the better the random numbers will be. | |
104 | (Current "optimal" values for the amount of state information are | |
105 | 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to | |
106 | the nearest known amount. Using less than 8 bytes will cause an error.) | |
107 | The seed for the initialization (which specifies a starting point for | |
108 | the random number sequence, and provides for restarting at the same | |
109 | point) is also an argument. | |
110 | The | |
111 | .Fn initstate | |
112 | function | |
113 | returns a pointer to the previous state information array. | |
114 | .Pp | |
115 | Once a state has been initialized, the | |
116 | .Fn setstate | |
117 | routine provides for rapid switching between states. | |
118 | The | |
119 | .Fn setstate | |
120 | function | |
121 | returns a pointer to the previous state array; its | |
122 | argument state array is used for further random number generation | |
123 | until the next call to | |
124 | .Fn initstate | |
125 | or | |
126 | .Fn setstate . | |
127 | .Pp | |
128 | Once a state array has been initialized, it may be restarted at a | |
129 | different point either by calling | |
130 | .Fn initstate | |
131 | (with the desired seed, the state array, and its size) or by calling | |
132 | both | |
133 | .Fn setstate | |
134 | (with the state array) and | |
135 | .Fn srandom | |
136 | (with the desired seed). | |
137 | The advantage of calling both | |
138 | .Fn setstate | |
139 | and | |
140 | .Fn srandom | |
141 | is that the size of the state array does not have to be remembered after | |
142 | it is initialized. | |
143 | .Pp | |
144 | With 256 bytes of state information, the period of the random number | |
145 | generator is greater than | |
146 | .if t 2\u\s769\s10\d, | |
147 | .if n 2**69 | |
148 | which should be sufficient for most purposes. | |
149 | .Sh AUTHOR | |
150 | Earl T. Cohen | |
151 | .Sh DIAGNOSTICS | |
152 | If | |
153 | .Fn initstate | |
154 | is called with less than 8 bytes of state information, or if | |
155 | .Fn setstate | |
156 | detects that the state information has been garbled, error | |
157 | messages are printed on the standard error output. | |
158 | .Sh SEE ALSO | |
159 | .Xr rand 3 | |
160 | .Sh HISTORY | |
161 | These | |
162 | functions appeared in | |
163 | .Bx 4.2 . | |
164 | .Sh BUGS | |
165 | About 2/3 the speed of | |
166 | .Xr rand 3 . |