SHUTDOWNHOOK_ESTABLISH(9)

NAME
shutdownhook_establish, shutdownhook_disestablish - add or remove a shutdown 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 invoked 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 shutdown. 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 ALSO
dohooks(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 engines 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 inconsistency, 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.