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+.. highlight:: c
+
+Hooks
+=====
+
+Libfrr provides type-safe subscribable hook points where other pieces of
+code can add one or more callback functions.  "type-safe" in this case
+applies to the function pointers used for subscriptions.  The
+implementations checks (at compile-time) whether a callback to be added has
+the appropriate function signature (parameters) for the hook.
+
+Example:
+
+.. code-block:: c
+   :caption: mydaemon.h
+
+   #include "hook.h"
+   DECLARE_HOOK(some_update_event, (struct eventinfo *info), (info));
+
+.. code-block:: c
+   :caption: mydaemon.c
+
+   #include "mydaemon.h"
+   DEFINE_HOOK(some_update_event, (struct eventinfo *info), (info));
+   ...
+   hook_call(some_update_event, info);
+
+.. code-block:: c
+   :caption: mymodule.c
+
+   #include "mydaemon.h"
+   static int event_handler(struct eventinfo *info);
+   ...
+   hook_register(some_update_event, event_handler);
+
+Do not use parameter names starting with "hook", these can collide with
+names used by the hook code itself.
+
+
+Return values
+-------------
+
+Callbacks to be placed on hooks always return "int" for now;  hook_call will
+sum up the return values from each called function.  (The default is 0 if no
+callbacks are registered.)
+
+There are no pre-defined semantics for the value, in most cases it is
+ignored.  For success/failure indication, 0 should be success, and
+handlers should make sure to only return 0 or 1 (not -1 or other values).
+
+There is no built-in way to abort executing a chain after a failure of one
+of the callbacks.  If this is needed, the hook can use an extra
+``bool *aborted`` argument.
+
+
+Priorities
+----------
+
+Hooks support a "priority" value for ordering registered calls
+relative to each other.  The priority is a signed integer where lower
+values are called earlier.  There are also "Koohs", which is hooks with
+reverse priority ordering (for cleanup/deinit hooks, so you can use the
+same priority value).
+
+Recommended priority value ranges are:
+
+======================== ===================================================
+Range                    Usage
+------------------------ ---------------------------------------------------
+ -999 ...     0 ...  999 main executable / daemon, or library
+
+-1999 ... -1000          modules registering calls that should run before
+                         the daemon's bits
+
+1000 ... 1999            modules' calls that should run after daemon's
+                         (includes default value: 1000)
+======================== ===================================================
+
+Note: the default value is 1000, based on the following 2 expectations:
+
+- most hook_register() usage will be in loadable modules
+- usage of hook_register() in the daemon itself may need relative ordering
+  to itself, making an explicit value the expected case
+
+The priority value is passed as extra argument on hook_register_prio() /
+hook_register_arg_prio().  Whether a hook runs in reverse is determined
+solely by the code defining / calling the hook.  (DECLARE_KOOH is actually
+the same thing as DECLARE_HOOK, it's just there to make it obvious.)
+
+
+Definition
+----------
+
+.. c:macro:: DECLARE_HOOK(name, arglist, passlist)
+.. c:macro:: DECLARE_KOOH(name, arglist, passlist)
+
+   :param name: Name of the hook to be defined
+   :param arglist: Function definition style parameter list in braces.
+   :param passlist: List of the same parameters without their types.
+
+   Note:  the second and third macro args must be the hook function's
+   parameter list, with the same names for each parameter.  The second
+   macro arg is with types (used for defining things), the third arg is
+   just the names (used for passing along parameters).
+
+   This macro must be placed in a header file;  this header file must be
+   included to register a callback on the hook.
+
+   Examples:
+
+   .. code-block:: c
+
+      DECLARE_HOOK(foo, (), ());
+      DECLARE_HOOK(bar, (int arg), (arg));
+      DECLARE_HOOK(baz, (const void *x, in_addr_t y), (x, y));
+
+.. c:macro:: DEFINE_HOOK(name, arglist, passlist)
+
+   Implements an hook.  Each ``DECLARE_HOOK`` must have be accompanied by
+   exactly one ``DEFINE_HOOK``, which needs to be placed in a source file.
+   **The hook can only be called from this source file.**  This is intentional
+   to avoid overloading and/or misusing hooks for distinct purposes.
+
+   The compiled source file will include a global symbol with the name of the
+   hook prefixed by `_hook_`.  Trying to register a callback for a hook that
+   doesn't exist will therefore result in a linker error, or a module
+   load-time error for dynamic modules.
+
+.. c:macro:: DEFINE_KOOH(name, arglist, passlist)
+
+   Same as ``DEFINE_HOOK``, but the sense of priorities / order of callbacks
+   is reversed.  This should be used for cleanup hooks.
+
+.. c:function:: int hook_call(name, ...)
+
+   Calls the specified named hook.  Parameters to the hook are passed right
+   after the hook name, e.g.:
+
+   .. code-block:: c
+
+      hook_call(foo);
+      hook_call(bar, 0);
+      hook_call(baz, NULL, INADDR_ANY);
+
+   Returns the sum of return values from all callbacks.  The ``DEFINE_HOOK``
+   statement for the hook must be placed in the file before any ``hook_call``
+   use of the hook.
+
+
+Callback registration
+---------------------
+
+.. c:function:: void hook_register(name, int (*callback)(...))
+.. c:function:: void hook_register_prio(name, int priority, int (*callback)(...))
+.. c:function:: void hook_register_arg(name, int (*callback)(void *arg, ...), void *arg)
+.. c:function:: void hook_register_arg_prio(name, int priority, int (*callback)(void *arg, ...), void *arg)
+
+   Register a callback with an hook.  If the caller needs to pass an extra
+   argument to the callback, the _arg variant can be used and the extra
+   parameter will be passed as first argument to the callback.  There is no
+   typechecking for this argument.
+
+   The priority value is used as described above.  The variants without a
+   priority parameter use 1000 as priority value.
+
+.. c:function:: void hook_unregister(name, int (*callback)(...))
+.. c:function:: void hook_unregister_arg(name, int (*callback)(void *arg, ...), void *arg)
+
+   Removes a previously registered callback from a hook.  Note that there
+   is no _prio variant of these calls.  The priority value is only used during
+   registration.