2.11BSD#431 - /usr/man/cat3/srandom.0

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


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

SYNOPSIS
     long random()

     srandom(seed)
     int seed;

     char *initstate(seed, state, n)
     unsigned seed;
     char *state;
     int n;

     char *setstate(state)
     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


Printed 11/26/99       September 29, 1985			1


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


     previous 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)

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


Printed 11/26/99       September 29, 1985			2