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author | Xin LI <delphij@FreeBSD.org> | 2018-08-18 06:20:45 +0000 |
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committer | Xin LI <delphij@FreeBSD.org> | 2018-08-18 06:20:45 +0000 |
commit | 718863d0768e27d285d425046237a961bd4c414f (patch) | |
tree | 638446d4d71d2f7f5b9e1396d4cf04d67b331695 /lib/libc/gen/arc4random_uniform.c | |
parent | e7e0844422276f568fb21dc385e72213dc0fca77 (diff) | |
download | src-718863d0768e27d285d425046237a961bd4c414f.tar.gz src-718863d0768e27d285d425046237a961bd4c414f.zip |
Split arc4random_uniform into it's own file and sync with OpenBSD.
PR: 182610
Obtained from: OpenBSD
MFC after: 2 weeks
Notes
Notes:
svn path=/head/; revision=337997
Diffstat (limited to 'lib/libc/gen/arc4random_uniform.c')
-rw-r--r-- | lib/libc/gen/arc4random_uniform.c | 58 |
1 files changed, 58 insertions, 0 deletions
diff --git a/lib/libc/gen/arc4random_uniform.c b/lib/libc/gen/arc4random_uniform.c new file mode 100644 index 000000000000..0136c66e3289 --- /dev/null +++ b/lib/libc/gen/arc4random_uniform.c @@ -0,0 +1,58 @@ +/* $OpenBSD: arc4random_uniform.c,v 1.2 2015/09/13 08:31:47 guenther Exp $ */ + +/* + * Copyright (c) 2008, Damien Miller <djm@openbsd.org> + * + * Permission to use, copy, modify, and distribute this software for any + * purpose with or without fee is hereby granted, provided that the above + * copyright notice and this permission notice appear in all copies. + * + * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES + * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF + * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR + * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES + * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN + * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF + * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. + * + * $FreeBSD$ + */ + +#include <sys/types.h> +#include <stdlib.h> + +/* + * Calculate a uniformly distributed random number less than upper_bound + * avoiding "modulo bias". + * + * Uniformity is achieved by generating new random numbers until the one + * returned is outside the range [0, 2**32 % upper_bound). This + * guarantees the selected random number will be inside + * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound) + * after reduction modulo upper_bound. + */ +uint32_t +arc4random_uniform(uint32_t upper_bound) +{ + uint32_t r, min; + + if (upper_bound < 2) + return 0; + + /* 2**32 % x == (2**32 - x) % x */ + min = -upper_bound % upper_bound; + + /* + * This could theoretically loop forever but each retry has + * p > 0.5 (worst case, usually far better) of selecting a + * number inside the range we need, so it should rarely need + * to re-roll. + */ + for (;;) { + r = arc4random(); + if (r >= min) + break; + } + + return r % upper_bound; +} |