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Man Pages
EVENTHANDLER(9) FreeBSD Kernel Developer's Manual EVENTHANDLER(9)

EVENTHANDLER
kernel event handling functions

#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);

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). If registration is successful, 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.

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.
app_coredump_start
Callbacks invoked at start of application core dump.
app_coredump_progress
Callbacks invoked during progress of application core dump.
app_coredump_finish
Callbacks invoked at finish of application core dump.
app_coredump_error
Callbacks invoked on error of application core dump.
bpf_track
Callbacks invoked when a BPF listener attaches to/detaches from network interface.
cpufreq_levels_changed
Callback invoked when cpu frequency levels have changed.
cpufreq_post_change
Callback invoked after cpu frequency has changed.
cpufreq_pre_change
Callback invoked before cpu frequency has changed.
dcons_poll
Callback invoked to poll for dcons changes.
device_attach
Callback invoked after a device has attached.
device_detach
Callbacks invoked before and after a device has detached.
dev_clone
Callbacks invoked when a new entry is created under /dev.
group_attach_event
Callback invoked when an interfance has been added to an interface group.
group_change_event
Callback invoked when an change has been made to an interface group.
group_detach_event
Callback invoked when an interfance has been removed from an interface group.
ifaddr_event
Callbacks invoked when an address is set up on a network interface.
if_clone_event
Callbacks invoked when an interface is cloned.
iflladdr_event
Callback invoked when an if link layer address event has happened.
ifnet_arrival_event
Callbacks invoked when a new network interface appears.
ifnet_departure_event
Callbacks invoked when a network interface is taken down.
ifnet_link_event
Callback invoked when an interfance link event has happened.
kld_load
Callbacks invoked after a linker file has been loaded.
kld_unload
Callbacks invoked after a linker file has been successfully unloaded.
kld_unload_try
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.
lle_event
Callback invoked when an link layer event has happened.
nmbclusters_change
Callback invoked when the number of mbuf clusters has changed.
nmbufs_change
Callback invoked when the number of mbufs has changed.
maxsockets_change
Callback invoked when the maximum number of sockets has changed.
mountroot
Callback invoked when root has been mounted.
power_profile_change
Callbacks invoked when the power profile of the system changes.
power_resume
Callback invoked when the system has resumed.
power_suspend
Callback invoked just before the system is suspended.
process_ctor
Callback invoked when a process is created.
process_dtor
Callback invoked when a process is destroyed.
process_exec
Callbacks invoked when a process performs an exec() operation.
process_exit
Callbacks invoked when a process exits.
process_fini
Callback invoked when a process memory is destroyed. This is never called.
process_fork
Callbacks invoked when a process forks a child.
process_init
Callback invoked when a process is initialized.
random_adaptor_attach
Callback invoked when a new random module has been loaded.
register_framebuffer
Callback invoked when a new frame buffer is registered.
route_redirect_event
Callback invoked when a route gets redirected to a new location.
shutdown_pre_sync
Callbacks invoked at shutdown time, before file systems are synchronized.
shutdown_post_sync
Callbacks invoked at shutdown time, after all file systems are synchronized.
shutdown_final
Callbacks invoked just before halting the system.
tcp_offload_listen_start
Callback invoked for TCP Offload to start listening for new connections.
tcp_offload_listen_stop
Callback invoked ror TCP Offload to stop listening for new connections.
thread_ctor
Callback invoked when a thread object is created.
thread_dtor
Callback invoked when a thread object is destroyed.
thread_init
Callback invoked when a thread object is initialized.
thread_fini
Callback invoked when a thread object is deinitalized.
usb_dev_configured
Callback invoked when a USB device is configured
unregister_framebuffer
Callback invoked when a frame buffer is deregistered.
vfs_mounted
Callback invoked when a file system is mounted.
vfs_unmounted
Callback invoked when a file system is unmounted.
vlan_config
Callback invoked when the vlan configuration has changed.
vlan_unconfig
Callback invoked when a vlan is destroyed.
vm_lowmem
Callbacks invoked when virtual memory is low.
watchdog_list
Callbacks invoked when the system watchdog timer is reinitialized.

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.

The EVENTHANDLER facility first appeared in FreeBSD 4.0.

This manual page was written by Joseph Koshy <jkoshy@FreeBSD.org> and
Matt Joras <mjoras@FreeBSD.org>.
September 6, 2018 FreeBSD 12.0-RELEASE

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