NAME
random, srandom, initstate, setstate - better random number
generator; routines for changing generators
SYNOPSIS
#include <stdlib.h>
long random(void)
void srandom(unsigned seed)
char *initstate(unsigned seed, char *state, int n)
char *setstate(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(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 rea-
son for this is that the amount of state information used is
much more than a single word. (Two other routines are pro-
vided to deal with restarting/changing random number genera-
tors). Like rand(3), 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 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 previ-
ous state information array.
Once a state has been initialized, the setstate routine pro-
vides 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.
AUTHOR
Earl T. Cohen
DIAGNOSTICS
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.
SEE ALSO
rand(3).
NOTES
initstate and setstate are not declared in <stdlib.h>, pro-
grammers must provide their own declarations.
BUGS
About 2/3 the speed of rand(3).