.\"	$NetBSD: kfilter_register.9,v 1.11 2020/10/31 16:03:01 christos Exp $
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.Dd October 31, 2020
.Dt KFILTER_REGISTER 9
.Os
.Sh NAME
.Nm kfilter_register ,
.Nm kfilter_unregister
.Nd add or remove kernel event filters
.Sh SYNOPSIS
.In sys/event.h
.Ft int
.Fn kfilter_register "const char *name" "struct filterops *filtops" "int *retfilter"
.Ft int
.Fn kfilter_unregister "const char *name"
.Sh DESCRIPTION
The
.Fn kfilter_register
function adds a new kernel event filter (kfilter) to the system,
for use by callers of
.Xr kqueue 2
and
.Xr kevent 2 .
.Fa name
is the name of the new filter (which must not already exist), and
.Fa filtops
is a pointer to a
.Va filterops
structure which describes the filter operations.
Both
.Fa name
and
.Fa filtops
will be copied to an internal data structure, and a new filter number
will be allocated.
If
.Fa retfilter
is not
.Dv NULL ,
then the new filter number will be returned in the address pointed at by
.Fa retfilter .
.Pp
The
.Fn kfilter_unregister
function removes a kfilter named
.Fa name
that was previously registered with
.Fn kfilter_register .
If a filter with the same
.Fa name
is later reregistered with
.Fn kfilter_register ,
it will get a different filter number
(i.e., filter numbers are not recycled).
It is not possible to unregister the system filters
(i.e., those that start with
.Dq EVFILT_
and are documented in
.Xr kqueue 2 ) .
.Pp
The
.Va filterops
structure is defined as follows:
.Bd -literal -offset indent
struct filterops {
	int	f_isfd;		/* true if ident == filedescriptor */
	int	(*f_attach)(struct knote *kn);
				/* called when knote is ADDed */
	void	(*f_detach)(struct knote *kn);
				/* called when knote is DELETEd */
	int	(*f_event)(struct knote *kn, long hint);
				/* called when event is triggered */
	void	(*f_touch)(struct knote *kn, struct kevent *kev, long hint);
				/* called during registration and event 
				 * processing to provide custom handling 
				 * of event fflags and data
				 */
};
.Ed
.Pp
If the filter operation is for a file descriptor,
.Va f_isfd
should be non-zero, otherwise it should be zero.
This controls where the
.Xr kqueue 2
system stores the knotes for an object.
.Sh RETURN VALUES
.Fn kfilter_register
returns 0 on success,
.Er EINVAL
if there's an invalid argument, or
.Er EEXIST
if the filter already exists,
.Pp
.Fn kfilter_unregister
returns 0 on success,
.Er EINVAL
if there's an invalid argument, or
.Er ENOENT
if the filter doesn't exist.
.Sh SEE ALSO
.Xr kqueue 2 ,
.Xr free 9 ,
.Xr knote 9 ,
.Xr malloc 9
.Sh HISTORY
The
.Fn kfilter_register
and
.Fn kfilter_unregister
functions first appeared in
.Nx 2.0 .
.Sh AUTHORS
The
.Fn kfilter_register
and
.Fn kfilter_unregister
functions were implemented by
.An Luke Mewburn
.Aq lukem@NetBSD.org .