RANDOM(3)                 386BSD Programmer's Manual                 RANDOM(3)

NAME
     random, srandom, initstate, setstate - better random number generator;
     routines for changing generators

SYNOPSIS
     #include <stdlib>

     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
     initialization 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
     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.  The initstate() function returns a pointer to the
     previous state information array.

     Once a state has been initialized, the setstate() routine provides for
     rapid switching between states.  The setstate() function 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)

HISTORY
     These functions appeared in 4.2BSD.

BUGS
     About 2/3 the speed of rand(3).

4.2 Berkeley Distribution       April 19, 1991                               2