newlib/libc/stdlib/rand.c

   1 /*
   2 FUNCTION
   3 <<rand>>, <<srand>>---pseudo-random numbers
   4
   5 INDEX
   6         rand
   7 INDEX
   8         srand
   9 INDEX
  10         rand_r
  11
  12 SYNOPSIS
  13         #include <stdlib.h>
  14         int rand(void);
  15         void srand(unsigned int <[seed]>);
  16         int rand_r(unsigned int *<[seed]>);
  17
  18 DESCRIPTION
  19 <<rand>> 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 <<rand>> 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 <<rand>>.
  25
  26 You can set the random seed using <<srand>>; 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 <<srand>> to set the same
  34 random seed at the outset.
  35
  36 RETURNS
  37 <<rand>> returns the next pseudo-random integer in sequence; it is a
  38 number between <<0>> and <<RAND_MAX>> (inclusive).
  39
  40 <<srand>> does not return a result.
  41
  42 NOTES
  43 <<rand>> and <<srand>> are unsafe for multi-threaded applications.
  44 <<rand_r>> is thread-safe and should be used instead.
  45
  46
  47 PORTABILITY
  48 <<rand>> is required by ANSI, but the algorithm for pseudo-random
  49 number generation is not specified; therefore, even if you use
  50 the same random seed, you cannot expect the same sequence of results
  51 on two different systems.
  52
  53 <<rand>> requires no supporting OS subroutines.
  54 */
  55
  56 #ifndef _REENT_ONLY
  57
  58 #include <stdlib.h>
  59 #include <reent.h>
  60
  61 #ifdef _REENT_THREAD_LOCAL
  62 _Thread_local unsigned long long _tls_rand_next = 1;
  63 #endif
  64
  65 void
  66 srand (unsigned int seed)
  67 {
  68   struct _reent *reent = _REENT;
  69
  70   _REENT_CHECK_RAND48(reent);
  71   _REENT_RAND_NEXT(reent) = seed;
  72 }
  73
  74 int
  75 rand (void)
  76 {
  77   struct _reent *reent = _REENT;
  78
  79   /* This multiplier was obtained from Knuth, D.E., "The Art of
  80      Computer Programming," Vol 2, Seminumerical Algorithms, Third
  81      Edition, Addison-Wesley, 1998, p. 106 (line 26) & p. 108 */
  82   _REENT_CHECK_RAND48(reent);
  83   _REENT_RAND_NEXT(reent) =
  84      _REENT_RAND_NEXT(reent) * __extension__ 6364136223846793005LL + 1;
  85   return (int)((_REENT_RAND_NEXT(reent) >> 32) & RAND_MAX);
  86 }
  87
  88 #endif /* _REENT_ONLY */