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.Dd August 1, 2013
.Dt EVENTHANDLER 9
.Os
.Sh NAME
.Nm EVENTHANDLER
.Nd kernel event handling functions
.Sh SYNOPSIS
.In sys/eventhandler.h
.Fn EVENTHANDLER_DECLARE name type
.Fn EVENTHANDLER_INVOKE name ...
.Ft eventhandler_tag
.Fn EVENTHANDLER_REGISTER name func arg priority
.Fn EVENTHANDLER_DEREGISTER name tag
.Ft eventhandler_tag
.Fo eventhandler_register
.Fa "struct eventhandler_list *list"
.Fa "const char *name"
.Fa "void *func"
.Fa "void *arg"
.Fa "int priority"
.Fc
.Ft void
.Fo eventhandler_deregister
.Fa "struct eventhandler_list *list"
.Fa "eventhandler_tag tag"
.Fc
.Ft "struct eventhandler_list *"
.Fn eventhandler_find_list "const char *name"
.Ft void
.Fn eventhandler_prune_list "struct eventhandler_list *list"
.Sh DESCRIPTION
The
.Nm
mechanism provides a way for kernel subsystems to register interest in
kernel events and have their callback functions invoked when these
events occur.
.Pp
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:
.Bl -tag -width indent
.It Fn EVENTHANDLER_DECLARE
This macro declares an event handler named by argument
.Fa name
with callback functions of type
.Fa type .
.It Fn EVENTHANDLER_REGISTER
This macro registers a callback function
.Fa func
with event handler
.Fa name .
When invoked, function
.Fa func
will be invoked with argument
.Fa arg
as its first parameter along with any additional parameters passed in
via macro
.Fn EVENTHANDLER_INVOKE
(see below).
Callback functions are invoked in order of priority.
The relative priority of each callback among other callbacks
associated with an event is given by argument
.Fa priority ,
which is an integer ranging from
.Dv EVENTHANDLER_PRI_FIRST
(highest priority), to
.Dv EVENTHANDLER_PRI_LAST
(lowest priority).
The symbol
.Dv EVENTHANDLER_PRI_ANY
may be used if the handler does not have a specific priority
associated with it.
If registration is successful,
.Fn EVENTHANDLER_REGISTER
returns a cookie of type
.Vt eventhandler_tag .
.It Fn EVENTHANDLER_DEREGISTER
This macro removes a previously registered callback associated with tag
.Fa tag
from the event handler named by argument
.Fa name .
.It Fn EVENTHANDLER_INVOKE
This macro is used to invoke all the callbacks associated with event
handler
.Fa name .
This macro is a variadic one.
Additional arguments to the macro after the
.Fa name
parameter are passed as the second and subsequent arguments to each
registered callback function.
.El
.Pp
The macros are implemented using the following functions:
.Bl -tag -width indent
.It Fn eventhandler_register
The
.Fn eventhandler_register
function is used to register a callback with a given event.
The arguments expected by this function are:
.Bl -tag -width ".Fa priority"
.It Fa list
A pointer to an existing event handler list, or
.Dv NULL .
If
.Fa list
is
.Dv NULL ,
the event handler list corresponding to argument
.Fa name
is used.
.It Fa name
The name of the event handler list.
.It Fa func
A pointer to a callback function.
Argument
.Fa arg
is passed to the callback function
.Fa func
as its first argument when it is invoked.
.It Fa priority
The relative priority of this callback among all the callbacks
registered for this event.
Valid values are those in the range
.Dv EVENTHANDLER_PRI_FIRST
to
.Dv EVENTHANDLER_PRI_LAST .
.El
.Pp
The
.Fn eventhandler_register
function returns a
.Fa tag
that can later be used with
.Fn eventhandler_deregister
to remove the particular callback function.
.It Fn eventhandler_deregister
The
.Fn eventhandler_deregister
function removes the callback associated with tag
.Fa tag
from the event handler list pointed to by
.Fa list .
This function is safe to call from inside an event handler
callback.
.It Fn eventhandler_find_list
The
.Fn eventhandler_find_list
function returns a pointer to event handler list structure corresponding
to event
.Fa name .
.It Fn eventhandler_prune_list
The
.Fn eventhandler_prune_list
function removes all deregistered callbacks from the event list
.Fa list .
.El
.Ss Kernel Event Handlers
The following event handlers are present in the kernel:
.Bl -tag -width indent
.It Vt acpi_sleep_event
Callbacks invoked when the system is being sent to sleep.
.It Vt acpi_wakeup_event
Callbacks invoked when the system is being woken up.
.It Vt dev_clone
Callbacks invoked when a new entry is created under
.Pa /dev .
.It Vt ifaddr_event
Callbacks invoked when an address is set up on a network interface.
.It Vt if_clone_event
Callbacks invoked when an interface is cloned.
.It Vt ifnet_arrival_event
Callbacks invoked when a new network interface appears.
.It Vt ifnet_departure_event
Callbacks invoked when a network interface is taken down.
.It Vt bpf_track
Callbacks invoked when a BPF listener attaches to/detaches from network interface.
.It Vt kld_load
Callbacks invoked after a linker file has been loaded.
.It Vt kld_unload
Callbacks invoked before a linker file is about to be unloaded.
These callbacks may be used to return an error and prevent the unload from
proceeding.
.It Vt power_profile_change
Callbacks invoked when the power profile of the system changes.
.It Vt process_exec
Callbacks invoked when a process performs an
.Fn exec
operation.
.It Vt process_exit
Callbacks invoked when a process exits.
.It Vt process_fork
Callbacks invoked when a process forks a child.
.It Vt shutdown_pre_sync
Callbacks invoked at shutdown time, before file systems are synchronized.
.It Vt shutdown_post_sync
Callbacks invoked at shutdown time, after all file systems are synchronized.
.It Vt shutdown_final
Callbacks invoked just before halting the system.
.It Vt vm_lowmem
Callbacks invoked when virtual memory is low.
.It Vt watchdog_list
Callbacks invoked when the system watchdog timer is reinitialized.
.El
.Sh RETURN VALUES
The macro
.Fn EVENTHANDLER_REGISTER
and function
.Fn eventhandler_register
return a cookie of type
.Vt eventhandler_tag ,
which may be used in a subsequent call to
.Fn EVENTHANDLER_DEREGISTER
or
.Fn eventhandler_deregister .
.Pp
The
.Fn eventhandler_find_list
function
returns a pointer to an event handler list corresponding to parameter
.Fa name ,
or
.Dv NULL
if no such list was found.
.Sh HISTORY
The
.Nm
facility first appeared in
.Fx 4.0 .
.Sh AUTHORS
This manual page was written by
.An Joseph Koshy Aq jkoshy@FreeBSD.org .