.\"	$OpenBSD: hook_establish.9,v 1.12 2015/12/12 11:26:32 mpi Exp $
.\"
.\" Copyright (c) 2001 Niklas Hallqvist.
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. The name of the author may not be used to endorse or promote products
.\"    derived from this software without specific prior written permission
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: December 12 2015 $
.Dt HOOK_ESTABLISH 9
.Os
.Sh NAME
.Nm hook_establish ,
.Nm hook_disestablish
.Nd add or remove a hook from a specified list
.Sh SYNOPSIS
.In sys/types.h
.In sys/systm.h
.Ft void *
.Fn hook_establish "struct hook_desc_head *head" "int tail" "void (*fn)(void *)" "void *arg"
.Ft void
.Fn hook_disestablish "struct hook_desc_head *head" "void *cookie"
.Sh DESCRIPTION
The
.Fn hook_establish
function adds
.Fa fn
to the list of hooks invoked by
.Xr dohooks 9 .
If
.Fa tail
is non-zero, the hook is added to the tail of the list
denoted by the TAILQ_HEAD pointer
.Fa head ,
otherwise to the front.
The
.Xr dohooks 9
function will at its invocation call each hook from the front of this
list.
When invoked, the hook function
.Fa fn
will be passed
.Fa arg
as its only argument.
.Pp
The
.Fn hook_disestablish
function removes the hook described by the opaque pointer
.Fa cookie
from the list of hooks denoted by the TAILQ_HEAD pointer
.Fa head .
If
.Fa cookie
is invalid, the result of
.Fn hook_disestablish
is undefined.
.Pp
The startup and mountroot systems use this API for their
implementation.
.Sh RETURN VALUES
If successful,
.Fn hook_establish
returns an opaque pointer describing the newly established
hook.
Otherwise, it returns
.Dv NULL .
.Sh SEE ALSO
.Xr dohooks 9 ,
.Xr dostartuphooks 9 ,
.Xr startuphook_establish 9