random(3)

NAME

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

SYNOPSIS

     #include <stdlib.h>

     long random(void)
     void srandom(unsigned seed)
     char *initstate(unsigned seed, char *state, int n)
     char *setstate(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 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.   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  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).

NOTES

     initstate and setstate are not declared in <stdlib.h>,  programmers  must
     provide their own declarations.

BUGS

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