1RANDOM(3) Library Functions Manual RANDOM(3)
2
6 random, srandom, initstate, setstate - better random number generator;
7 routines for changing generators
8
10 long random()
11 srandom(seed)
13 int seed;
14 char *initstate(seed, state, n)
16 unsigned seed;
17 char *state;
18 int n;
19 char *setstate(state)
21 char *state;
22
24 Random uses a non-linear additive feedback random number generator
25 employing a default table of size 31 long integers to return successive
26 pseudo-random numbers in the range from 0 to (2**31)−1. The period of
27 this random number generator is very large, approximately
28 16*((2**31)−1).
29 Random/srandom have (almost) the same calling sequence and initializa‐
31 tion properties as rand/srand. The difference is that rand(3) produces
32 a much less random sequence — in fact, the low dozen bits generated by
33 rand go through a cyclic pattern. All the bits generated by random are
34 usable. For example, ``random()&01'' will produce a random binary
35 value.
36 Unlike srand, srandom does not return the old seed; the reason for this
38 is that the amount of state information used is much more than a single
39 word. (Two other routines are provided to deal with restarting/chang‐
40 ing random number generators). Like rand(3), however, random will by
41 default produce a sequence of numbers that can be duplicated by calling
42 srandom with 1 as the seed.
43 The initstate routine allows a state array, passed in as an argument,
45 to be initialized for future use. The size of the state array (in
46 bytes) is used by initstate to decide how sophisticated a random number
47 generator it should use -- the more state, the better the random num‐
48 bers will be. (Current "optimal" values for the amount of state infor‐
49 mation are 8, 32, 64, 128, and 256 bytes; other amounts will be rounded
50 down to the nearest known amount. Using less than 8 bytes will cause
51 an error). The seed for the initialization (which specifies a starting
52 point for the random number sequence, and provides for restarting at
53 the same point) is also an argument. Initstate returns a pointer to
54 the previous state information array.
55 Once a state has been initialized, the setstate routine provides for
57 rapid switching between states. Setstate returns a pointer to the pre‐
58 vious state array; its argument state array is used for further random
59 number generation until the next call to initstate or setstate.
60 Once a state array has been initialized, it may be restarted at a dif‐
62 ferent point either by calling initstate (with the desired seed, the
63 state array, and its size) or by calling both setstate (with the state
64 array) and srandom (with the desired seed). The advantage of calling
65 both setstate and srandom is that the size of the state array does not
66 have to be remembered after it is initialized.
67 With 256 bytes of state information, the period of the random number
69 generator is greater than 2**69 which should be sufficient for most
70 purposes.
71
73 Earl T. Cohen
74
76 If initstate is called with less than 8 bytes of state information, or
77 if setstate detects that the state information has been garbled, error
78 messages are printed on the standard error output.
79
81 rand(3)
82
84 About 2/3 the speed of rand(3C).
854.2 Berkeley Distribution September 29, 1985 RANDOM(3)