---
name: "eventhandler_deregister"
section: "9"
platform: "linux"
package: "freebsd-manpages"
version: "6.0-5"
release: "Debian 4.0 (Etch)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/eventhandler_deregister"
---

# eventhandler\_deregister(9)

kernel event handling functions

## NAME

**EVENTHANDLER** – kernel event handling functions

## SYNOPSIS

**#include \<sys/eventhandler.h\>**

**EVENTHANDLER\_DECLARE**(*name*, *type*);

**EVENTHANDLER\_INVOKE**(*name*, *...*);

*eventhandler\_tag*\

**EVENTHANDLER\_REGISTER**(*name*, *func*, *arg*, *priority*);

**EVENTHANDLER\_DEREGISTER**(*name*, *tag*);

*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*);

*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.

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:

- **EVENTHANDLER\_DECLARE**()

  This macro declares an event handler named by argument *name* with callback functions of type *type*.

- **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). 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. 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*.

- **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.

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 a 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*. 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*.

### Kernel Event Handlers

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.

- *dev\_clone*

  Callbacks invoked when a new entry is created under */dev*.

- *ifaddr\_event*

  Callbacks invoked when an address is set up on a network interface.

- *if\_clone\_event*

  Callbacks invoked when an interface is cloned.

- *ifnet\_arrival\_event*

  Callbacks invoked when a new network interface appears.

- *ifnet\_departure\_event*

  Callbacks invoked when a network interface is taken down.

- *power\_profile\_change*

  Callbacks invoked when the power profile of the system changes.

- *process\_exec*

  Callbacks invoked when a process performs an **exec**() operation.

- *process\_exit*

  Callbacks invoked when a process exits.

- *process\_fork*

  Callbacks invoked when a process forks a child.

- *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.

- *vm\_lowmem*

  Callbacks invoked when virtual memory is low.

- *watchdog\_list*

  Callbacks invoked when the system watchdog timer is reinitialized.

## 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⟩.