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EVENTHANDLER(9)		 BSD Kernel Developer's Manual	       EVENTHANDLER(9)

NAME
     EVENTHANDLER — kernel event handling functions

SYNOPSIS
     #include <sys/eventhandler.h>

     EVENTHANDLER_DECLARE(name, type);

     EVENTHANDLER_INVOKE(name, ...);

     eventhandler_tag
     EVENTHANDLER_REGISTER(name, func, arg, priority);

     EVENTHANDLER_DEREGISTER(name, tag);

     eventhandler_tag
     eventhandler_register(struct eventhandler_list *list, const char *name,
	 void *func, void *arg, int priority);

     void
     eventhandler_deregister(struct eventhandler_list *list,
	 eventhandler_tag tag);

     struct eventhandler_list *
     eventhandler_find_list(const char *name);

     void
     eventhandler_prune_list(struct eventhandler_list *list);

DESCRIPTION
     The EVENTHANDLER mechanism provides a way for kernel subsystems to regis‐
     ter interest in kernel events and have their callback functions invoked
     when these events occur.

     The normal way to use this subsystem is via the macro interface.  The
     macros that can be used for working with event handlers and callback
     function lists are:

     EVENTHANDLER_DECLARE()
	     This macro declares an event handler named by argument name with
	     callback functions of type type.

     EVENTHANDLER_REGISTER()
	     This macro registers a callback function func with event handler
	     name.  When invoked, function func will be invoked with argument
	     arg as its first parameter along with any additional parameters
	     passed in via macro EVENTHANDLER_INVOKE() (see below).  Callback
	     functions are invoked in order of priority.  The relative prior‐
	     ity of each callback among other callbacks associated with an
	     event is given by argument priority, which is an integer ranging
	     from EVENTHANDLER_PRI_FIRST (highest priority), to
	     EVENTHANDLER_PRI_LAST (lowest priority).  The symbol
	     EVENTHANDLER_PRI_ANY may be used if the handler does not have a
	     specific priority associated with it.  If registration is suc‐
	     cessful, EVENTHANDLER_REGISTER() returns a cookie of type
	     eventhandler_tag.

     EVENTHANDLER_DEREGISTER()
	     This macro removes a previously registered callback associated
	     with tag tag from the event handler named by argument name.

     EVENTHANDLER_INVOKE()
	     This macro is used to invoke all the callbacks associated with
	     event handler name.  This macro is a variadic one.	 Additional
	     arguments to the macro after the name parameter are passed as the
	     second and subsequent arguments to each registered callback func‐
	     tion.

     The macros are implemented using the following functions:

     eventhandler_register()
	     The eventhandler_register() function is used to register a call‐
	     back with a given event.  The arguments expected by this function
	     are:

	     list      A pointer to an existing event handler list, or NULL.
		       If list is NULL, the event handler list corresponding
		       to argument name is used.

	     name      The name of the event handler list.

	     func      A pointer to a callback function.  Argument arg is
		       passed to the callback function func as its first argu‐
		       ment when it is invoked.

	     priority  The relative priority of this callback among all the
		       callbacks registered for this event.  Valid values are
		       those in the range EVENTHANDLER_PRI_FIRST to
		       EVENTHANDLER_PRI_LAST.

	     The eventhandler_register() function returns a tag that can later
	     be used with eventhandler_deregister() to remove the particular
	     callback function.

     eventhandler_deregister()
	     The eventhandler_deregister() function removes the callback asso‐
	     ciated with tag tag from the event handler list pointed to by
	     list.  This function is safe to call from inside an event handler
	     callback.

     eventhandler_find_list()
	     The eventhandler_find_list() function returns a pointer to event
	     handler list structure corresponding to event name.

     eventhandler_prune_list()
	     The eventhandler_prune_list() function removes all deregistered
	     callbacks from the event list list.

   Kernel Event Handlers
     The following event handlers are present in the kernel:

     acpi_sleep_event
	     Callbacks invoked when the system is being sent to sleep.

     acpi_wakeup_event
	     Callbacks invoked when the system is being woken up.

     dev_clone
	     Callbacks invoked when a new entry is created under /dev.

     ifaddr_event
	     Callbacks invoked when an address is set up on a network inter‐
	     face.

     if_clone_event
	     Callbacks invoked when an interface is cloned.

     ifnet_arrival_event
	     Callbacks invoked when a new network interface appears.

     ifnet_departure_event
	     Callbacks invoked when a network interface is taken down.

     power_profile_change
	     Callbacks invoked when the power profile of the system changes.

     process_exec
	     Callbacks invoked when a process performs an exec() operation.

     process_exit
	     Callbacks invoked when a process exits.

     process_fork
	     Callbacks invoked when a process forks a child.

     shutdown_pre_sync
	     Callbacks invoked at shutdown time, before file systems are syn‐
	     chronized.

     shutdown_post_sync
	     Callbacks invoked at shutdown time, after all file systems are
	     synchronized.

     shutdown_final
	     Callbacks invoked just before halting the system.

     vm_lowmem
	     Callbacks invoked when virtual memory is low.

     watchdog_list
	     Callbacks invoked when the system watchdog timer is reinitial‐
	     ized.

RETURN VALUES
     The macro EVENTHANDLER_REGISTER() and function eventhandler_register()
     return a cookie of type eventhandler_tag, which may be used in a subse‐
     quent call to EVENTHANDLER_DEREGISTER() or eventhandler_deregister().

     The eventhandler_find_list() function returns a pointer to an event han‐
     dler list corresponding to parameter name, or NULL if no such list was
     found.

HISTORY
     The EVENTHANDLER facility first appeared in FreeBSD 4.0.

AUTHORS
     This manual page was written by Joseph Koshy ⟨jkoshy@FreeBSD.org⟩.

BSD				January 7, 2005				   BSD
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