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| -rw-r--r-- | manual/math.texi | 46 |
1 files changed, 46 insertions, 0 deletions
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 |