89 lines
2.5 KiB
C
89 lines
2.5 KiB
C
/*
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FUNCTION
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<<rand>>, <<srand>>---pseudo-random numbers
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INDEX
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rand
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INDEX
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srand
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INDEX
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rand_r
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SYNOPSIS
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#include <stdlib.h>
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int rand(void);
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void srand(unsigned int <[seed]>);
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int rand_r(unsigned int *<[seed]>);
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DESCRIPTION
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<<rand>> returns a different integer each time it is called; each
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integer is chosen by an algorithm designed to be unpredictable, so
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that you can use <<rand>> when you require a random number.
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The algorithm depends on a static variable called the ``random seed'';
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starting with a given value of the random seed always produces the
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same sequence of numbers in successive calls to <<rand>>.
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You can set the random seed using <<srand>>; it does nothing beyond
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storing its argument in the static variable used by <<rand>>. You can
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exploit this to make the pseudo-random sequence less predictable, if
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you wish, by using some other unpredictable value (often the least
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significant parts of a time-varying value) as the random seed before
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beginning a sequence of calls to <<rand>>; or, if you wish to ensure
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(for example, while debugging) that successive runs of your program
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use the same ``random'' numbers, you can use <<srand>> to set the same
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random seed at the outset.
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RETURNS
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<<rand>> returns the next pseudo-random integer in sequence; it is a
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number between <<0>> and <<RAND_MAX>> (inclusive).
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<<srand>> does not return a result.
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NOTES
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<<rand>> and <<srand>> are unsafe for multi-threaded applications.
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<<rand_r>> is thread-safe and should be used instead.
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PORTABILITY
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<<rand>> is required by ANSI, but the algorithm for pseudo-random
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number generation is not specified; therefore, even if you use
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the same random seed, you cannot expect the same sequence of results
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on two different systems.
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<<rand>> requires no supporting OS subroutines.
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*/
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#ifndef _REENT_ONLY
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#include <stdlib.h>
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#include <reent.h>
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#ifdef _REENT_THREAD_LOCAL
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_Thread_local unsigned long long _tls_rand_next = 1;
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#endif
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void
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srand (unsigned int seed)
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{
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struct _reent *reent = _REENT;
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_REENT_CHECK_RAND48(reent);
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_REENT_RAND_NEXT(reent) = seed;
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}
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int
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rand (void)
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{
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struct _reent *reent = _REENT;
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/* This multiplier was obtained from Knuth, D.E., "The Art of
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Computer Programming," Vol 2, Seminumerical Algorithms, Third
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Edition, Addison-Wesley, 1998, p. 106 (line 26) & p. 108 */
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_REENT_CHECK_RAND48(reent);
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_REENT_RAND_NEXT(reent) =
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_REENT_RAND_NEXT(reent) * __extension__ 6364136223846793005LL + 1;
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return (int)((_REENT_RAND_NEXT(reent) >> 32) & RAND_MAX);
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}
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#endif /* _REENT_ONLY */
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