.\"
.\" Copyright (c) 2002 Mark R V Murray
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.\" $FreeBSD$
.\"
.Dd February 6, 2002
.Dt RANDOM_HARVEST 9
.Os
.Sh NAME
.Nm random_harvest
.Nd gather entropy from the kernel for the entropy device
.Sh SYNOPSIS
.In sys/types.h
.In sys/random.h
.Ft void
.Fo random_harvest
.Fa "void *entropy"
.Fa "u_int size"
.Fa "u_int bits"
.Fa "u_int frac"
.Fa "enum esource source"
.Fc
.Sh DESCRIPTION
The
.Fn random_harvest
function is used by device drivers
and other kernel processes to pass data
that is considered (at least partially) stochastic
to the entropy device.
.Pp
The caller should pass a pointer (to no more than 16 bytes) of
the
.Dq random
data in
.Fa entropy .
The argument
.Fa size
contains the number of bytes pointed to.
The caller should
.Em "very conservatively"
estimate the number of random bits
in the sample,
and pass this in
.Fa bits
or
.Fa frac .
If the estimated number of bits per sample is an integer, then
.Fa bits
is used, and
.Fa frac
is 0.
Otherwise,
for low-entropy samples,
.Dq fractional
entropy can be supplied in
.Fa frac .
(This is considered to be
.Fa frac /
1024 bits of entropy.)
The
.Fa source
is chosen from
.Dv RANDOM_WRITE , RANDOM_KEYBOARD , RANDOM_MOUSE , RANDOM_NET
and
.Dv RANDOM_INTERRUPT ,
and is used to indicate the source of the entropy.
.Pp
Interrupt harvesting has been simplified
for the kernel programmer.
If a device driver registers an interrupt handler
with
.Xr BUS_SETUP_INTR 9
or
.Xr bus_setup_intr 9 ,
then it is only necessary to
include the
.Dv INTR_ENTROPY
bit in the
.Fa flags
argument to have that interrupt source
be used for entropy harvesting.
.Sh SEE ALSO
.Xr random 4 ,
.Xr BUS_SETUP_INTR 9
.Sh AUTHORS
The
.Fx
.Xr random 4
entropy device and supporting documentation was written by
.An Mark R V Murray .
