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>.