SHUTDOWNHOOK_ESTABLISH(9) BSD Kernel Manual SHUTDOWNHOOK_ESTABLISH(9)NAME
shutdownhook_establish, shutdownhook_disestablish - add or remove a shut-
down hook
SYNOPSIS
#include <sys/types.h>
#include <sys/systm.h>
void *
shutdownhook_establish(void (*fn)(void *), void *arg);
void
shutdownhook_disestablish(void *cookie);
DESCRIPTION
The shutdownhook_establish() function adds fn to the list of hooks in-
voked by doshutdownhooks(9) at shutdown. When invoked, the hook function
fn will be passed arg as its only argument.
The shutdownhook_disestablish() function removes the hook described by
the opaque pointer cookie from the list of hooks to be invoked at shut-
down. If cookie is invalid, the result of shutdownhook_disestablish() is
undefined.
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, timeouts, and other interrupt-driven services)
or even basic system integrity (because the system could be rebooting
after a crash).
Shutdown hooks are, like startup hooks, implemented via the more general
dohooks(9) API.
RETURN VALUES
If successful, shutdownhook_establish() returns an opaque pointer
describing the newly established shutdown hook. Otherwise, it returns
NULL.
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.
It may be appropriate to use a shutdown hook to inform watchdog timer
hardware that the operating system is no longer running.
SEE ALSOdohooks(9), doshutdownhooks(9), dostartuphooks(9)CAVEATS
Shutdown hooks should only be used to do what's strictly necessary to do
to ensure a correct reboot. Since shutdown hooks are run even after a
panic, a panic caused by a shutdown hook will automatically cause the
shutdown hook to be run again causing an endless loop. An example of
things that need to be done in a shutdown hook could be stopping DMA en-
gines that might corrupt memory when rebooting. An example of things that
shouldn't be done in a shutdown hook is syncing the file systems. Once
again, since the system could be rebooting because of an internal incon-
sistency, writing down anything to permanent storage or trusting the
internal state of the system is a very bad idea.
BUGS
The names are clumsy, at best.
MirOS BSD #10-current November 13, 1995 1
