random(3-BSD) MISC. REFERENCE MANUAL PAGES random(3-BSD)
NAME
random, srandom, initstate, setstate - better random number
generator; routines for changing generators
SYNOPSIS
cc [ flag... ] file ... -lucb long random()
srandom(seed)
int seed;
char *initstate(seed, state, n)
unsigned seed;
char *state;
int n;
char *setstate(state)
char *state;
DESCRIPTION
random uses a non-linear additive feedback random number
generator employing a default table of size 31 long integers
to return successive pseudo-random numbers in the range from
0 to (2**31)-1. The period of this random number generator
is very large, approximately 16*((2**31)-1). random/srandom
have (almost) the same calling sequence and initialization
properties as rand/srand [see rand(3C)]. The difference is
that rand(3C) produces a much less random sequence-in fact,
the low dozen bits generated by rand go through a cyclic
pattern. All the bits generated by random are usable. For
example,
random()&01 will produce a random binary value. Unlike
srand, srandom does not return the old seed because the
amount of state information used is much more than a
single word. Two other routines are provided to deal
with restarting/changing random number generators.
Like rand(3C), however, random will, by default, pro-
duce a sequence of numbers that can be duplicated by
calling srandom with 1 as the seed. The initstate rou-
tine allows a state array, passed in as an argument, to
be initialized for future use. n specifies the size of
state in bytes. initstate uses n to decide how sophis-
ticated a random number generator it should use-the
more state, the better the random numbers will be.
Current ``optimal'' values for the amount of state
information are 8, 32, 64, 128, and 256 bytes; other
amounts will be rounded down to the nearest known
amount. Using less than 8 bytes will cause an error.
The seed for the initialization (which specifies a
starting point for the random number sequence, and pro-
vides for restarting at the same point) is also an
argument. initstate returns a pointer to the previous
state information array. Once a state has been ini-
tialized, the setstate routine provides for rapid
switching between states. setstate returns a pointer
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random(3-BSD) MISC. REFERENCE MANUAL PAGES random(3-BSD)
to the previous state array; its argument state array
is used for further random number generation until the
next call to initstate or setstate. Once a state array
has been initialized, it may be restarted at a dif-
ferent point either by calling initstate (with the
desired seed, the state array, and its size) or by cal-
ling both setstate (with the state array) and srandom
(with the desired seed). The advantage of calling both
setstate and srandom is that the size of the state
array does not have to be remembered after it is ini-
tialized. With 256 bytes of state information, the
period of the random number generator is greater than
2**69 which should be sufficient for most purposes.
EXAMPLE
/* Initialize an array and pass it in to initstate. */
static long state1[32] = {
3,
0x9a319039, 0x32d9c024, 0x9b663182, 0x5da1f342,
0x7449e56b, 0xbeb1dbb0, 0xab5c5918, 0x946554fd,
0x8c2e680f, 0xeb3d799f, 0xb11ee0b7, 0x2d436b86,
0xda672e2a, 0x1588ca88, 0xe369735d, 0x904f35f7,
0xd7158fd6, 0x6fa6f051, 0x616e6b96, 0xac94efdc,
0xde3b81e0, 0xdf0a6fb5, 0xf103bc02, 0x48f340fb,
0x36413f93, 0xc622c298, 0xf5a42ab8, 0x8a88d77b,
0xf5ad9d0e, 0x8999220b, 0x27fb47b9
};
main()
{
unsigned seed;
int n;
seed = 1;
n = 128;
initstate(seed, state1, n);
setstate(state1);
printf("%d0,random());
}
SEE ALSO
rand(3C). drand48(2), drand(3C), rand(3C), srand(3C) in the
Programmer's Reference Manual.
RETURN VALUE
If initstate is called with less than 8 bytes of state
information, or if setstate detects that the state informa-
tion has been garbled, error messages are printed on the
standard error output.
NOTES
About two-thirds the speed of rand(3C).
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