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random, srandom, srandomdev, initstate, setstate - better random number

 

NAME

      random, srandom, srandomdev, 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 long seed);
 
      void
      srandomdev(void);
 
      char *
      initstate(unsigned long seed, char *state, long 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() functions have (almost) the same calling
      sequence and initialization properties as the rand(3) and srand(3) func‐
      tions.  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.
 
      Like rand(3), random() will by default produce a sequence of numbers that
      can be duplicated by calling srandom() with ‘1’ as the seed.
 
      The srandomdev() routine initializes a state array using the random(4)
      random number device which returns good random numbers, suitable for
      cryptographic use.  Note that this particular seeding procedure can gen‐
      erate states which are impossible to reproduce by calling srandom() with
      any value, since the succeeding terms in the state buffer are no longer
      derived from the LC algorithm applied to a fixed 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
      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 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 pur‐
      poses.
 

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.
      arc4random(3), rand(3), srand(3), random(4)
 

HISTORY

      These functions appeared in 4.2BSD.
 

AUTHORS

      Earl T. Cohen
 

BUGS

      About 2/3 the speed of rand(3).
 
      The historical implementation used to have a very weak seeding; the ran‐
      dom sequence did not vary much with the seed.  The current implementation
      employs a better pseudo-random number generator for the initial state
      calculation.
 
      Applications requiring cryptographic quality randomness should use
      arc4random(3).
 

Sections

Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.