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.\" Copyright (c) 1994 Christopher G. Demetriou
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.Dd November 13, 1995
.Dt SHUTDOWNHOOK_ESTABLISH 9
.Os
.Sh NAME
.Nm shutdownhook_establish ,
.Nm shutdownhook_disestablish
.Nd add or remove a shutdown hook
.Sh SYNOPSIS
.Ft void *
.Fn shutdownhook_establish "void (*fn)(void *)" "void *arg"
.Ft void
.Fn shutdownhook_disestablish "void *cookie"
.Sh DESCRIPTION
The
.Fn shutdownhook_establish
function adds
.Fa fn
to the list of hooks invoked by
.Xr doshutdownhooks 9
at shutdown.
When invoked, the hook function
.Fa fn
will be passed
.Fa arg
as its only argument.
.Pp
The
.Fn shutdownhook_disestablish
function removes the hook described by the opaque pointer
.Fa cookie
from the list of hooks to be invoked at shutdown.
If
.Fa cookie
is invalid, the result of
.Fn shutdownhook_disestablish
is undefined.
.Pp
Shutdown hooks should be used to perform one-time activities
that must happen immediately before the kernel exits.
Because of the environment in which they are run, shutdown hooks cannot
rely on many system services (including file systems, and timeouts
and other interrupt-driven services), or even basic system
integrity (because the system could be rebooting after a crash).
.Sh RETURN VALUES
If successful,
.Fn shutdownhook_establish
returns an opaque pointer describing the newly-established
shutdown hook.
Otherwise, it returns NULL.
.Sh EXAMPLES
It may be appropriate to use a shutdown hook to
disable a device that does direct memory access, so that
the device will not try to access memory while the system
is rebooting.
.Pp
It may be appropriate to use a shutdown hook to
inform watchdog timer hardware that the operating system
is no longer running.
.Sh SEE ALSO
.Xr doshutdownhooks 9
.Sh BUGS
The names are clumsy, at best.