November 13, 1995 SHUTDOWNHOOK_ESTABLISH 9

### NAME

shutdownhook_establish, shutdownhook_disestablish - add or remove a shutdown hook

### SYNOPSIS

```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, and timeouts and other interrupt-driven services), or even basic system integrity (because the system could be rebooting after a crash).

### 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

doshutdownhooks(9)

### BUGS

The names are clumsy, at best.