MirOS Manual: shutdownhook_disestablish(9), shutdownhook_establish(9)

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 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 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

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