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ea99f21ce6
By default, Newlib uses a huge object of type struct _reent to store thread-specific data. This object is returned by __getreent() if the __DYNAMIC_REENT__ Newlib configuration option is defined. The reentrancy structure contains for example errno and the standard input, output, and error file streams. This means that if an application only uses errno it has a dependency on the file stream support even if it does not use it. This is an issue for lower end targets and applications which need to qualify the software according to safety standards (for example ECSS-E-ST-40C, ECSS-Q-ST-80C, IEC 61508, ISO 26262, DO-178, DO-330, DO-333). If the new _REENT_THREAD_LOCAL configuration option is enabled, then struct _reent is replaced by dedicated thread-local objects for each struct _reent member. The thread-local objects are defined in translation units which use the corresponding object.
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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