NAME
random, srandom, initstate, setstate - better random number
generator; routines for changing generators
SYNOPSIS
long random()
srandom(seed)
unsigned int seed;
char *initstate(seed, state, n)
unsigned int seed;
char *state;
int n;
void *setstate(state)
void *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(3) 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 generators). Like rand(3),
however, random will by default produce a sequence of
numbers that
can be duplicated by calling 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 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 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 random
number generator is greater than 2**69 which should be
sufficient
for most purposes.
AUTHOR
Earl T. Cohen
DIAGNOSTICS
If initstate is called with less than 8 bytes of state
information,
or if setstate detects that the state information has been
garbled,
error messages are printed on the standard error output.
SEE ALSO
rand(3)
BUGS
About 2/3 the speed of rand(3).