RANDOM(3) MachTen Programmer’s Manual RANDOM(3)

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
The random() function 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).

The random()/ srandom() have (almost) the same calling sequence and ini-
tialization properties as rand(3)/ srand(3). The difference is that
rand 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 restart-
ing/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 gener-
ator 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. The initstate() function returns a pointer to the pre-
vious state information array.

Once a state has been initialized, the setstate() routine provides for
rapid switching between states. The setstate() function returns a point-
er to the previous state array; its argument state array is used for fur-
ther random number generation until the next call to initstate() or
setstate().

Once a state array has been initialized, it may be restarted at a differ-
ent 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 gen-
erator is greater than 2**69 which should be sufficient for most purpos-
es.

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.