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30 .\" from: @(#)random.3 8.1 (Berkeley) 6/4/93
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32 .Dd June 12, 2014
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 int seed"
49 .Ft char *
50 .Fn initstate "unsigned int seed" "char *state" "size_t 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 The maximum value
66 .Dv RANDOM_MAX
67 is defined in
68 .In stdlib.h .
69 .Pp
70 The
71 .Fn random
72 and
73 .Fn srandom
74 have (almost) the same calling sequence and initialization properties as
75 .Xr rand 3
76 and
77 .Xr srand 3 .
78 The difference is that
79 .Xr rand 3
80 produces a much less random sequence \(em in fact, the low dozen bits
81 generated by
82 .Xr rand 3
83 go through a cyclic pattern.
84 All the bits generated by
85 .Fn random
86 are usable.
87 For example,
88 .Sq Li random()\*[Am]01
89 will produce a random binary value.
90 .Pp
91 Like
92 .Xr rand 3 ,
93 .Fn random
94 will by default produce a sequence of numbers that can be duplicated
95 by calling
96 .Fn srandom
97 with
98 .Ql 1
99 as the seed.
100 .Pp
101 The
102 .Fn initstate
103 routine allows a state array, passed in as an argument, to be initialized
104 for future use.
105 The size of the state array (in bytes) is used by
106 .Fn initstate
107 to decide how sophisticated a random number generator it should use \(em the
108 more state, the better the random numbers will be.
109 (Current "optimal" values for the amount of state information are
110 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
111 the nearest known amount.
112 Using less than 8 bytes will cause an error).
113 The seed for the initialization (which specifies a starting point for
114 the random number sequence, and provides for restarting at the same
115 point) is also an argument.
116 The state array passed to
117 .Fn initstate
118 must be aligned to a 32-bit boundary.
119 This can be achieved by using
120 a suitably-sized array of ints, and casting the array to char * when
121 passing it to
122 .Fn initstate .
123 The
124 .Fn initstate
125 function
126 returns a pointer to the previous state information array.
127 .Pp
128 Once a state has been initialized, the
129 .Fn setstate
130 routine provides for rapid switching between states.
131 The
132 .Fn setstate
133 function
134 returns a pointer to the previous state array; its
135 argument state array is used for further random number generation
136 until the next call to
137 .Fn initstate
138 or
139 .Fn setstate .
140 .Pp
141 Once a state array has been initialized, it may be restarted at a
142 different point either by calling
143 .Fn initstate
144 (with the desired seed, the state array, and its size) or by calling
145 both
146 .Fn setstate
147 (with the state array) and
148 .Fn srandom
149 (with the desired seed).
150 The advantage of calling both
151 .Fn setstate
152 and
153 .Fn srandom
154 is that the size of the state array does not have to be remembered after
155 it is initialized.
156 .Pp
157 With 256 bytes of state information, the period of the random number
158 generator is greater than
159 .if t 2\u\s769\s10\d,
160 .if n 2**69
161 which should be sufficient for most purposes.
162 .Sh DIAGNOSTICS
163 If
164 .Fn initstate
165 is called with less than 8 bytes of state information, or if
166 .Fn setstate
167 detects that the state information has been garbled, error
168 messages are printed on the standard error output.
169 .Sh SEE ALSO
170 .Xr rand 3 ,
171 .Xr srand 3 ,
172 .Xr rnd 4 ,
173 .Xr rnd 9
174 .Sh STANDARDS
175 The
176 .Fn random ,
177 .Fn srandom ,
178 .Fn initstate
179 and
180 .Fn setstate
181 functions conform to
182 .St -p1003.1-2008 .
183 .Sh HISTORY
184 These
185 functions appeared in
186 .Bx 4.2 .
187 .Sh AUTHORS
188 .An Earl T. Cohen
189 .Sh BUGS
190 About 2/3 the speed of
191 .Xr rand 3 .