newlib/libc/stdlib/random.c

   1 /*
   2 FUNCTION
   3 <<random>>, <<srandom>>---pseudo-random numbers
   4
   5 INDEX
   6         random
   7 INDEX
   8         srandom
   9
  10 SYNOPSIS
  11         #define _XOPEN_SOURCE 500
  12         #include <stdlib.h>
  13         long int random(void);
  14         void srandom(unsigned int <[seed]>);
  15
  16
  17
  18 DESCRIPTION
  19 <<random>> returns a different integer each time it is called; each
  20 integer is chosen by an algorithm designed to be unpredictable, so
  21 that you can use <<random>> when you require a random number.
  22 The algorithm depends on a static variable called the ``random seed'';
  23 starting with a given value of the random seed always produces the
  24 same sequence of numbers in successive calls to <<random>>.
  25
  26 You can set the random seed using <<srandom>>; it does nothing beyond
  27 storing its argument in the static variable used by <<rand>>.  You can
  28 exploit this to make the pseudo-random sequence less predictable, if
  29 you wish, by using some other unpredictable value (often the least
  30 significant parts of a time-varying value) as the random seed before
  31 beginning a sequence of calls to <<rand>>; or, if you wish to ensure
  32 (for example, while debugging) that successive runs of your program
  33 use the same ``random'' numbers, you can use <<srandom>> to set the same
  34 random seed at the outset.
  35
  36 RETURNS
  37 <<random>> returns the next pseudo-random integer in sequence; it is a
  38 number between <<0>> and <<RAND_MAX>> (inclusive).
  39
  40 <<srandom>> does not return a result.
  41
  42 NOTES
  43 <<random>> and <<srandom>> are unsafe for multi-threaded applications.
  44
  45 _XOPEN_SOURCE may be any value >= 500.
  46
  47 PORTABILITY
  48 <<random>> is required by XSI. This implementation uses the same
  49 algorithm as <<rand>>.
  50
  51 <<random>> requires no supporting OS subroutines.
  52 */
  53
  54 #ifndef _REENT_ONLY
  55
  56 #include <stdlib.h>
  57 #include <reent.h>
  58
  59 void
  60 srandom (unsigned int seed)
  61 {
  62   struct _reent *reent = _REENT;
  63
  64   _REENT_CHECK_RAND48(reent);
  65   _REENT_RAND_NEXT(reent) = seed;
  66 }
  67
  68 long int
  69 random (void)
  70 {
  71   struct _reent *reent = _REENT;
  72
  73   /* This multiplier was obtained from Knuth, D.E., "The Art of
  74      Computer Programming," Vol 2, Seminumerical Algorithms, Third
  75      Edition, Addison-Wesley, 1998, p. 106 (line 26) & p. 108 */
  76   _REENT_CHECK_RAND48(reent);
  77   _REENT_RAND_NEXT(reent) =
  78      _REENT_RAND_NEXT(reent) * __extension__ 6364136223846793005LL + 1;
  79   return (long int)((_REENT_RAND_NEXT(reent) >> 32) & RAND_MAX);
  80 }
  81
  82 #endif /* _REENT_ONLY */