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.Dd October 7, 2022 .Dt EVENTHANDLER 9 .Os .Sh NAME .Nm EVENTHANDLER .Nd kernel event handling functions .Sh SYNOPSIS n sys/eventhandler.h .Fn EVENTHANDLER_DECLARE name type .Fn EVENTHANDLER_DEFINE name func arg priority .Fn EVENTHANDLER_INVOKE name ... .Ft eventhandler_tag .Fn EVENTHANDLER_REGISTER name func arg priority .Fn EVENTHANDLER_DEREGISTER name tag .Fn EVENTHANDLER_DEREGISTER_NOWAIT name tag .Fn EVENTHANDLER_LIST_DECLARE name .Fn EVENTHANDLER_LIST_DEFINE name .Fn EVENTHANDLER_DIRECT_INVOKE name .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 void .Fo eventhandler_deregister_nowait .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.
p 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.
p 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 .Fn EVENTHANDLER_LIST_DEFINE so that the event handlers can be invoked directly using .Fn EVENTHANDLER_DIRECT_INVOKE (see below). This saves the invoker from having to do a locked traversal of a global list of event handler lists. l -tag -width indent t Fn EVENTHANDLER_DECLARE This macro declares an event handler named by argument .Fa name with callback functions of type .Fa type . t Fn EVENTHANDLER_DEFINE This macro uses .Xr SYSINIT 9 to register 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). t 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). If registration is successful, .Fn EVENTHANDLER_REGISTER returns a cookie of type .Vt eventhandler_tag . t 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 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. t Fn EVENTHANDLER_DEREGISTER_NOWAIT This macro removes a previously registered callback associated with tag .Fa tag from the event handler named by argument .Fa 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. t 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. t Fn EVENTHANDLER_LIST_DEFINE This macro defines a reference to an event handler list named by argument .Fa name . It uses .Xr SYSINIT 9 to initialize the reference and the eventhandler list. t Fn EVENTHANDLER_LIST_DECLARE This macro declares an event handler list named by argument .Fa name . This is only needed for users of .Fn EVENTHANDLER_DIRECT_INVOKE which are not in the same compilation unit of that list's definition. t Fn EVENTHANDLER_DIRECT_INVOKE This macro invokes the event handlers registered for the list named by argument .Fa name . This macro can only be used if the list was defined with .Fn EVENTHANDLER_LIST_DEFINE . The macro is variadic with the same semantics as .Fn EVENTHANDLER_INVOKE . .El
p The macros are implemented using the following functions: l -tag -width indent t 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: l -tag -width ".Fa priority" t 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. t Fa name The name of the event handler list. t 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. t 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
p The .Fn eventhandler_register function returns a .Fa tag that can later be used with .Fn eventhandler_deregister to remove the particular callback function. t 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 . If .Fa tag is .Va 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. t Fn eventhandler_deregister_nowait 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. t Fn eventhandler_find_list The .Fn eventhandler_find_list function returns a pointer to event handler list structure corresponding to event .Fa name . t Fn eventhandler_prune_list The .Fn eventhandler_prune_list function removes all deregistered callbacks from the event list .Fa list . .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 .
p 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 Mt jkoshy@FreeBSD.org and .An Matt Joras Aq Mt mjoras@FreeBSD.org .