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.Dd $Mdocdate: April 28 2018 $
.Dt ARC4RANDOM 9
.Os
.Sh NAME
.Nm arc4random ,
.Nm arc4random_buf ,
.Nm arc4random_uniform ,
.Nm enqueue_randomness
.Nd kernel random subsystem
.Sh SYNOPSIS
.In sys/systm.h
.Ft u_int32_t
.Fn arc4random "void"
.Ft void
.Fn arc4random_buf "void *buf" "size_t nbytes"
.Ft u_int32_t
.Fn arc4random_uniform "u_int32_t upper_bound"
.In dev/rndvar.h
.Ft void
.Fn enqueue_randomness "int"
.Sh DESCRIPTION
The
.Fn add_random
functions below mix input into the system entropy pool, which is then used
to create a key for the ChaCha stream cipher used in the
.Fn arc4random
series of functions.
.Pp
.Fn enqueue_randomness
causes the supplied data argument to be added to the entropy pool,
increasing the entropy counter by 32 bits.
.Pp
.Fn arc4random
and
.Fn arc4random_buf
provide random numbers and are intended to be called in any
circumstance where random numbers are required.
.Pp
.Fn arc4random_uniform
will return a uniformly distributed random number less than
.Fa upper_bound ,
avoiding "modulo bias" when the upper bound is not a power of two.
In the worst case, this function may consume multiple iterations
to ensure uniformity; see the source code to understand the problem
and solution.
.Pp
.Fn arc4random ,
.Fn arc4random_buf ,
and
.Fn arc4random_uniform
may be called in nearly any kernel context.
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr pchb 4 ,
.Xr random 4