RANDOM(3) C LIBRARY FUNCTIONS RANDOM(3)
NAME
random, srandom, initstate, setstate - better random number
generator; routines for changing generators
SYNOPSIS
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. The difference is
that rand(3V) 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; the
reason for this is that 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 gen-
erators). Like rand(3V), however, random() will by default
produce a sequence of numbers that can be duplicated by cal-
ling srandom() with 1 as the seed.
The initstate() routine allows a state array, passed in as
an argument, to be initialized for future use. The size of
the state array (in bytes) is used by initstate() to decide
how sophisticated 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 informa-
tion 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 ini-
tialization (which specifies a starting point for the random
number sequence, and provides 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 initialized, the setstate() routine
provides for rapid switching between states. setstate()
returns a pointer 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 different point either by calling initstate() (with the
desired seed, the state array, and its size) or by calling
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 initialized.
With 256 bytes of state information, the period of the ran-
dom number generator is greater than 2**69 which should be
sufficient for most purposes.
SEE ALSO
rand(3V)
EXAMPLES
/* Initialize and 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, (char *) state1, n);
setstate(state1);
printf("%d0,random());
}
DIAGNOSTICS
If initstate() is called with less than 8 bytes of state
information, or if setstate() detects that the state infor-
mation has been garbled, error messages are printed on the
standard error output.
WARNINGS
initstate() casts state to (long *), so state must be long-
aligned. If it is not long-aligned, on some architectures
the program will dump core.
BUGS
random() is only 2/3 as fast as rand(3V).
