.\"	$OpenBSD: hook_establish.9,v 1.8 2004/11/28 22:10:02 jmc Exp $
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.\" Copyright (c) 2001 Niklas Hallqvist.
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.Dd July 27, 2001
.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
.Fd #include <sys/types.h>
.Fd #include <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-, mountroot, and shutdownhooks 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 doshutdownhooks 9 ,
.Xr dostartuphooks 9 ,
.Xr mountroothook_establish 9 ,
.Xr shutdownhook_establish 9 ,
.Xr startuphook_establish 9