[glibc] manual: Add documentation for arc4random functions

[glibc] manual: Add documentation for arc4random functions

[Adhemerval Zanella]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=ca4d3ea5130d66e66c5af14e958e99341bf20689

commit ca4d3ea5130d66e66c5af14e958e99341bf20689
Author: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Date:   Thu Jul 21 10:05:07 2022 -0300

    manual: Add documentation for arc4random functions

Diff:
---
 manual/math.texi | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/manual/math.texi b/manual/math.texi
index 477a18b6d1..141695cc30 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 +1986,51 @@ 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 reseeded with kernel entropy on @code{fork}
+and @code{_Fork}.  It is not cleared on either a direct @code{clone} syscall
+or when using @theglibc{} @code{syscall} function.
+
+The prototypes for these functions are in @file{stdlib.h}.
+@pindex stdlib.h
+
+@deftypefun uint32_t arc4random (void)
+@standards{BSD, stdlib.h}
+@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acsafe{}}
+This function returns a single 32-bit value in the range of @code{0} to
+@code{2^32−1} (inclusive), which is twice the range of @code{rand} and
+@code{random}.
+@end deftypefun
+
+@deftypefun void arc4random_buf (void *@var{buffer}, size_t @var{length})
+@standards{BSD, stdlib.h}
+@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acsafe{}}
+This function fills the region @var{buffer} of length @var{length} bytes
+with random data.
+@end deftypefun
+
+@deftypefun uint32_t arc4random_uniform (uint32_t @var{upper_bound})
+@standards{BSD, 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.
+@end deftypefun
+
 @node FP Function Optimizations
 @section Is Fast Code or Small Code preferred?
 @cindex Optimization