.\"	$OpenBSD: knote.9,v 1.10 2023/02/10 14:34:16 visa Exp $
.\"	$NetBSD: knote.9,v 1.9 2003/04/16 13:35:29 wiz Exp $
.\"
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.\" This documentation is derived from text contributed by
.\" Luke Mewburn.
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.Dd $Mdocdate: February 10 2023 $
.Dt KNOTE 9
.Os
.Sh NAME
.Nm knote ,
.Nm knote_locked
.Nd raise kernel event
.Sh SYNOPSIS
.In sys/param.h
.In sys/event.h
.Ft void
.Fn knote "struct klist *list" "long hint"
.Ft void
.Fn knote_locked "struct klist *list" "long hint"
.Sh DESCRIPTION
The
.Fn knote
and
.Fn knote_locked
functions provide a hook into the kqueue kernel event notification
mechanism to allow sections of the kernel to raise a kernel event
in the form of a
.Sq knote ,
which is a
.Fa struct knote
as defined in
.In sys/event.h .
.Pp
.Fn knote
takes a singly linked
.Fa list
of knotes, along with a
.Fa hint
(which is passed to the appropriate filter routine).
.Fn knote
then locks and walks the
.Fa list
making calls to the filter routine for each knote.
As each knote contains a reference to the data structure that it is
attached to, the filter may choose to examine the data structure in
deciding whether an event should be reported.
The
.Fa hint
is used to pass in additional information, which may not be present in
the data structure that the filter examines.
.Pp
If the filter decides that the event should be returned, it returns a
non-zero value and
.Fn knote
links the knote onto the tail end of the active list in the
corresponding kqueue for the application to retrieve.
If the knote is already on the active list, no action is taken, but the
call to the filter occurs in order to provide an opportunity for the
filter to record the activity.
.Pp
.Fn knote_locked
is like
.Fn knote
but assumes that the
.Fa list
is already locked.
.Pp
.Fn knote
and
.Fn knote_locked
must not be called from interrupt contexts running at an interrupt
priority level higher than
.Fn splsched .
.\" .Sh ERRORS
.Sh SEE ALSO
.Xr kqueue 2
.\" .Xr kfilter_register 9
.Sh HISTORY
The
.Fn knote
functions first appeared in
.Fx 4.1 ,
and then in
.Ox 2.9 .
.Sh AUTHORS
The
.Fn kqueue
system was written by
.An Jonathan Lemon Aq Mt jlemon@FreeBSD.org .