EVENTHANDLER(9) FreeBSD Kernel Developer's Manual EVENTHANDLER(9)
NAME
EVENTHANDLER - kernel event handling functions
SYNOPSIS
#include <sys/eventhandler.h>
EVENTHANDLER_DECLARE(name, type);
EVENTHANDLER_DEFINE(name, func, arg, priority);
EVENTHANDLER_INVOKE(name, ...);
eventhandler_tag
EVENTHANDLER_REGISTER(name, func, arg, priority);
EVENTHANDLER_DEREGISTER(name, tag);
EVENTHANDLER_DEREGISTER_NOWAIT(name, tag);
EVENTHANDLER_LIST_DECLARE(name);
EVENTHANDLER_LIST_DEFINE(name);
EVENTHANDLER_DIRECT_INVOKE(name);
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);
void
eventhandler_deregister_nowait(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
register interest in kernel events and have their callback functions
invoked when these events occur.
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 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.
The normal way to use this subsystem is via the macro interface. For
events that are high frequency it is suggested that you additionally use
EVENTHANDLER_LIST_DEFINE() so that the event handlers can be invoked
directly using EVENTHANDLER_DIRECT_INVOKE() (see below). This saves the
invoker from having to do a locked traversal of a global list of event
handler lists.
EVENTHANDLER_DECLARE()
This macro declares an event handler named by argument name with
callback functions of type type.
EVENTHANDLER_DEFINE()
This macro uses SYSINIT(9) to register 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).
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).
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. It
waits until no threads are running handlers for this event before
returning, making it safe to unload a module immediately upon
return from this function.
EVENTHANDLER_DEREGISTER_NOWAIT()
This macro removes a previously registered callback associated
with tag tag from the event handler named by argument name. Upon
return, one or more threads could still be running the removed
function(s), but no new calls will be made. To remove a handler
function from within that function, use this version of
deregister, to avoid a deadlock.
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
function.
EVENTHANDLER_LIST_DEFINE()
This macro defines a reference to an event handler list named by
argument name. It uses SYSINIT(9) to initialize the reference
and the eventhandler list.
EVENTHANDLER_LIST_DECLARE()
This macro declares an event handler list named by argument name.
This is only needed for users of EVENTHANDLER_DIRECT_INVOKE()
which are not in the same compilation unit of that list's
definition.
EVENTHANDLER_DIRECT_INVOKE()
This macro invokes the event handlers registered for the list
named by argument name. This macro can only be used if the list
was defined with EVENTHANDLER_LIST_DEFINE(). The macro is
variadic with the same semantics as EVENTHANDLER_INVOKE().
The macros are implemented using the following functions:
eventhandler_register()
The eventhandler_register() function is used to register a
callback 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
argument 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
associated with tag tag from the event handler list pointed to by
list. If tag is NULL, all callback functions for the event are
removed. This function will not return until all threads have
exited from the removed handler callback function(s). This
function is not safe to call from inside an event handler
callback.
eventhandler_deregister_nowait()
The eventhandler_deregister() function removes the callback
associated 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.
RETURN VALUES
The macro EVENTHANDLER_REGISTER() and function eventhandler_register()
return a cookie of type eventhandler_tag, which may be used in a
subsequent call to EVENTHANDLER_DEREGISTER() or
eventhandler_deregister().
The eventhandler_find_list() function returns a pointer to an event
handler 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> and
Matt Joras <mjoras@FreeBSD.org>.
FreeBSD 15.1-STABLE-HBSD January 31, 2025 EVENTHANDLER(9)