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/* Copyright (c) 2016 Joel Sherrill <joel@rtems.org> */
/*
FUNCTION
<<random>>, <<srandom>>---pseudo-random numbers
INDEX
random
INDEX
srandom
SYNOPSIS
#define _XOPEN_SOURCE 500
#include <stdlib.h>
long int random(void);
void srandom(unsigned int <[seed]>);
DESCRIPTION
<<random>> returns a different integer each time it is called; each
integer is chosen by an algorithm designed to be unpredictable, so
that you can use <<random>> when you require a random number.
The algorithm depends on a static variable called the ``random seed'';
starting with a given value of the random seed always produces the
same sequence of numbers in successive calls to <<random>>.
You can set the random seed using <<srandom>>; it does nothing beyond
storing its argument in the static variable used by <<rand>>. You can
exploit this to make the pseudo-random sequence less predictable, if
you wish, by using some other unpredictable value (often the least
significant parts of a time-varying value) as the random seed before
beginning a sequence of calls to <<rand>>; or, if you wish to ensure
(for example, while debugging) that successive runs of your program
use the same ``random'' numbers, you can use <<srandom>> to set the same
random seed at the outset.
RETURNS
<<random>> returns the next pseudo-random integer in sequence; it is a
number between <<0>> and <<2**31 - 1>> (inclusive).
<<srandom>> does not return a result.
NOTES
<<random>> and <<srandom>> are unsafe for multi-threaded applications.
_XOPEN_SOURCE may be any value >= 500.
PORTABILITY
<<random>> is required by XSI. This implementation uses the same
algorithm as <<rand>>.
<<random>> requires no supporting OS subroutines.
*/
#define _DEFAULT_SOURCE
#include <stdlib.h>
#include <stdint.h>
__THREAD_LOCAL uint64_t _rand_next = 1;
long int
random(void)
{
/* This multiplier was obtained from Knuth, D.E., "The Art of
Computer Programming," Vol 2, Seminumerical Algorithms, Third
Edition, Addison-Wesley, 1998, p. 106 (line 26) & p. 108 */
_rand_next = _rand_next * 6364136223846793005ULL + 1;
return (long int)((_rand_next >> 32) & 0x7fffffffL);
}
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