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.\" $FreeBSD: head/share/man/man9/random_harvest.9 285458 2015-07-13 14:25:15Z brueffer $
.\"
.Dd July 13, 2015
.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_direct
.Fa "void *entropy"
.Fa "u_int size"
.Fa "u_int bits"
.Fa "enum esource source"
.Fc
.Ft void
.Fo random_harvest_fast
.Fa "void *entropy"
.Fa "u_int size"
.Fa "u_int bits"
.Fa "enum esource source"
.Fc
.Ft void
.Fo random_harvest_queue
.Fa "void *entropy"
.Fa "u_int size"
.Fa "u_int bits"
.Fa "enum esource source"
.Fc
.Sh DESCRIPTION
The
.Fn random_harvest_*
functions are 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 pointing to the
.Dq random
data in
.Fa entropy .
The argument
.Fa size
contains the number of bytes pointed to.
The
.Fa source
is chosen from one of
the values enumerated in
.Pa sys/dev/random.h .
and is used to indicate the source of the entropy.
.Pp
The
.Fo random_harvest_direct
.Fc
variant is used
for early harvesting
before any multitasking
is enabled.
.Pp
The
.Fn random_harvest_fast
variant is used
by sources that
should not take
a performance hit
from harvesting,
as they are high-rate
sources.
Some entropy is sacrificed,
but the high rate of supply
will compensate for this.
.Pp
The
.Fn random_harvest_queue
variant is used
for general harvesting
and is the default
choice for most entropy sources
such as interrupts
or console events.
.Pp
The
.Fa bits
argument is only used
by the deprecated Yarrow algorithm.
For compatibility,
the caller should
.Em "very conservatively"
estimate the number of random bits
in the sample,
and pass this in
.Fa bits .
.Pp
Interrupt harvesting has been
in part 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.
This should be done wherever practicable.
.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 .