.\"	$OpenBSD: powerhook_establish.9,v 1.2 2000/10/12 18:06:02 aaron Exp $
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.\" Copyright (c) 2000 Aaron Campbell
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.Dd July 1, 2000
.Dt POWERHOOK_ESTABLISH 9
.Os
.Sh NAME
.Nm powerhook_establish ,
.Nm powerhook_disestablish
.Nd add or remove a power hook
.Sh SYNOPSIS
.Ft void *
.Fn powerhook_establish "void (*fn)(void *)" "void *arg"
.Ft void
.Fn powerhook_disestablish "void *cookie"
.Sh DESCRIPTION
The
.Fn powerhook_establish
function adds
.Fa fn
to the list of hooks invoked by
.Xr dopowerhooks 9
during any change in
.Xr apm 4
power state (i.e.,
during a
.Dq suspend ,
.Dq standby ,
or
.Dq resume ) .
When invoked, the hook function
.Fa fn
will be passed
.Fa arg
as its only argument.
.Pp
The
.Fn powerhook_disestablish
function removes the hook described by the opaque pointer
.Fa cookie
from the list of power hooks.
If
.Fa cookie
is invalid, the result of
.Fn powerhook_disestablish
is undefined.
.Sh RETURN VALUES
If successful,
.Fn powerhook_establish
returns an opaque pointer describing the newly established
power hook.
Otherwise, it returns
.Dv NULL .
.Sh EXAMPLES
It may be appropriate to use a power hook to disestablish interrupts and
disable PCMCIA functions on PC Cards that are present when the system is put
into suspend mode.
The reverse should happen on resume.
.Pp
It may be appropriate to use a power hook to halt audio output in a sound
driver at suspend-time.
On resume, it may not be possible to recover audio output, but at least the
device can be put into a known, working state.
.Sh SEE ALSO
.Xr apm 4 ,
.Xr dopowerhooks 9