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28 .\" @(#)random.3 8.1 (Berkeley) 6/4/93
40 .Nd better random number generator; routines for changing generators
48 .Fn srandom "unsigned int seed"
52 .Fn initstate "unsigned int seed" "char *state" "size_t n"
54 .Fn setstate "char *state"
57 The functions described in this manual page are not secure.
58 Applications which require unpredictable random numbers should use
66 uses a non-linear additive feedback random number generator employing a
67 default table of size 31 long integers to return successive pseudo-random
68 numbers in the range from 0 to
69 .if t 2\u\s731\s10\d\(mi1.
71 The period of this random number generator is very large, approximately
72 .if t 16\(mu(2\u\s731\s10\d\(mi1).
73 .if n 16*((2**31)\(mi1).
79 functions have (almost) the same calling sequence and initialization properties as the
84 The difference is that
86 produces a much less random sequence \(em in fact, the low dozen bits
87 generated by rand go through a cyclic pattern.
88 All the bits generated by
93 will produce a random binary
99 will by default produce a sequence of numbers that can be duplicated
108 routine initializes a state array using
109 pseudo-random numbers obtained from the kernel.
110 Note that this particular seeding
111 procedure can generate states which are impossible to reproduce by
114 with any value, since the succeeding terms in the
115 state buffer are no longer derived from the LC algorithm applied to
120 routine allows a state array, passed in as an argument, to be initialized
122 The size of the state array (in bytes) is used by
124 to decide how sophisticated a random number generator it should use \(em the
125 more state, the better the random numbers will be.
126 (Current "optimal" values for the amount of state information are
127 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
128 the nearest known amount.
129 Using less than 8 bytes will cause an error.)
130 The seed for the initialization (which specifies a starting point for
131 the random number sequence, and provides for restarting at the same
132 point) is also an argument.
136 returns a pointer to the previous state information array.
138 Once a state has been initialized, the
140 routine provides for rapid switching between states.
144 returns a pointer to the previous state array; its
145 argument state array is used for further random number generation
146 until the next call to
151 Once a state array has been initialized, it may be restarted at a
152 different point either by calling
154 (with the desired seed, the state array, and its size) or by calling
157 (with the state array) and
159 (with the desired seed).
160 The advantage of calling both
164 is that the size of the state array does not have to be remembered after
167 With 256 bytes of state information, the period of the random number
168 generator is greater than
169 .if t 2\u\s769\s10\d,
171 which should be sufficient for most purposes.
175 is called with less than 8 bytes of state information, or if
177 detects that the state information has been garbled,
186 functions appeared in
191 About 2/3 the speed of
194 The historical implementation used to have a very weak seeding; the
195 random sequence did not vary much with the seed.
196 The current implementation employs a better pseudo-random number
197 generator for the initial state calculation.