From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
To: libc-alpha@sourceware.org
Subject: [PATCH v6 10/10] manual: Add documentation for arc4random functions
Date: Wed, 18 May 2022 16:14:24 -0300 [thread overview]
Message-ID: <20220518191424.3630729-11-adhemerval.zanella@linaro.org> (raw)
In-Reply-To: <20220518191424.3630729-1-adhemerval.zanella@linaro.org>
---
manual/math.texi | 49 ++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 49 insertions(+)
diff --git a/manual/math.texi b/manual/math.texi
index 477a18b6d1..7d47aaf386 100644
--- a/manual/math.texi
+++ b/manual/math.texi
@@ -1447,6 +1447,7 @@ systems.
* ISO Random:: @code{rand} and friends.
* BSD Random:: @code{random} and friends.
* SVID Random:: @code{drand48} and friends.
+* High Quality Random:: @code{arc4random} and friends.
@end menu
@node ISO Random
@@ -1985,6 +1986,54 @@ This function is a GNU extension and should not be used in portable
programs.
@end deftypefun
+@node High Quality Random
+@subsection High Quality Random Number Functions
+
+This section describes the random number functions provided as a GNU
+extension, based on OpenBSD interfaces.
+
+@Theglibc{} uses kernel entropy obtained either through @code{getrandom}
+or by reading @file{/dev/urandom} to seed and periodically re-seed the
+internal state. A per-thread data pool is used, which allows fast output
+generation.
+
+Although these functions provide higher random quality than ISO, BSD, and
+SVID functions, these still use a Pseudo-Random generator and should not
+be used in cryptographic contexts.
+
+The internal state is cleared and reseed with kernel entropy on @code{fork}
+and @code{_Fork} (it is not cleared for either direct @code{clone} syscall
+or through glibc wrapper).
+
+The prototypes for these functions are in @file{stdlib.h}.
+@pindex stdlib.h
+
+@deftypefun int32_t arc4random (void)
+@standards{GNU, stdlib.h}
+@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acsafe{}}
+This function returns a single 32-bit value in the range of 0 to 2^32−1,
+which is twice the range of @code{rand} and @code{random}.
+@end deftypefun
+
+@deftypefun void arc4random (void *@var{buffer}, size_t @var{length})
+@standards{GNU, stdlib.h}
+@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acsafe{}}
+This function fills the region @var{buffer} of @var{length} with random data.
+@end deftypefun
+
+@deftypefun uint32_t arc4random_uniform (uint32_t @var{upper_bound})
+@standards{GNU, stdlib.h}
+@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acsafe{}}
+This function returns a single 32-bit value, uniformly distributed but
+less than the @var{upper_bound}. It avoids the @w{modulo bias} when the
+upper bound is not a power of two.
+
+The algorithm obtains the exact sampling of a discrete uniform variable
+using an optimal number of random bits for any range @var{upper_bounds},
+allowing to consume as less as possible data from the per-thread entropy
+pool.
+@end deftypefun
+
@node FP Function Optimizations
@section Is Fast Code or Small Code preferred?
@cindex Optimization
--
2.34.1
next prev parent reply other threads:[~2022-05-18 19:14 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-18 19:14 [PATCH v6 00/10] Add arc4random support Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 01/10] stdlib: Add arc4random, arc4random_buf, and arc4random_uniform (BZ #4417) Adhemerval Zanella
2022-06-02 9:44 ` Yann Droneaud
2022-06-10 17:26 ` Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 02/10] stdlib: Add arc4random tests Adhemerval Zanella
2022-06-28 11:59 ` Florian Weimer
2022-06-28 16:58 ` Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 03/10] benchtests: Add arc4random benchtest Adhemerval Zanella
2022-06-28 12:01 ` Florian Weimer
2022-06-28 17:06 ` Adhemerval Zanella
2022-06-28 17:13 ` Noah Goldstein
2022-06-28 17:14 ` Noah Goldstein
2022-06-28 17:22 ` Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 04/10] aarch64: Add optimized chacha20 Adhemerval Zanella
2022-06-28 12:03 ` Florian Weimer
2022-06-28 17:39 ` Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 05/10] x86: Add SSE2 " Adhemerval Zanella
2022-05-18 20:44 ` Noah Goldstein
2022-05-18 19:14 ` [PATCH v6 06/10] x86: Add AVX2 " Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 07/10] powerpc64: Add " Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 08/10] s390x: " Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 09/10] stdlib: Add TLS optimization to arc4random Adhemerval Zanella
2022-06-28 12:19 ` Florian Weimer
2022-06-28 19:45 ` Adhemerval Zanella
2022-05-18 19:14 ` Adhemerval Zanella [this message]
2022-06-28 12:09 ` [PATCH v6 10/10] manual: Add documentation for arc4random functions Florian Weimer
2022-06-28 19:15 ` Adhemerval Zanella
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