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RANDOM(3)                 NetBSD Programmer's Manual                 RANDOM(3)


NAME

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


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <stdlib.h>

     long
     random(void);

     void
     srandom(unsigned seed);

     char *
     initstate(unsigned seed, char *state, size_t 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() and srandom() have (almost) the same calling sequence and
     initialization properties as rand(3) and srand(3).  The difference is
     that rand(3) produces a much less random sequence -- in fact, the low
     dozen bits generated by rand(3) 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(3), 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.


SEE ALSO

     rand(3), srand(3), rnd(4), rnd(9)


HISTORY

     These functions appeared in 4.2BSD.


BUGS

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

NetBSD 1.4                       June 4, 1993                                2