# 2.11BSD#431 - /usr/man/cat3/random.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

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