


      RANDOM(3C)                  (bsd 4.2)                  RANDOM(3C)



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

      SYNTAX
           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 28319-1.  The period of this random number generator is
           very large, approximately 16*(28319-1).

           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
           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,



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      RANDOM(3C)                  (bsd 4.2)                  RANDOM(3C)



           and provides for restarting at the same point) is also an
           argument.  Initstate returns a pointer to the previous state
           information array.

           Once a state has been initialized, the setstate routine
           provides 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
           random number generator is greater than 28699, which should be
           sufficient for most purposes.

      AUTHOR
           University of California, Berkeley

      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)

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



















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