Net-SNMP Agent handler and extensibility API
[The Net-SNMP agent]

The basic theory goes something like this: In the past, with the original mib module api (which derived from the original CMU SNMP code) the underlying mib modules were passed very little information (only the truly most basic information about a request). [詳細]


モジュール
	utility_handlers
	Simplify request processing A group of handlers intended to simplify certain aspects of processing a request for a MIB object.
	leaf_handlers
	Process individual leaf objects A group of handlers to implement individual leaf objects and instances (both scalar objects, and individual objects and instances within a table).
	baby_steps
	Calls your handler in baby_steps for set processing.
	old_api
	Calls mib module code written in the old style of code.
	stash_cache
	Automatically caches data for certain handlers.
	table
	Helps you implement a table.
データ構造
struct	netsnmp_mib_handler_s
	the mib handler structure to be registered [詳細]
struct	netsnmp_handler_registration_s
	Root registration info. [詳細]
struct	netsnmp_handler_args_s
struct	netsnmp_delegated_cache_s
struct	netsnmp_mib_handler_s
	the mib handler structure to be registered [詳細]
struct	netsnmp_handler_registration_s
	Root registration info. [詳細]
マクロ定義
#define	MIB_HANDLER_AUTO_NEXT 0x00000001
#define	MIB_HANDLER_AUTO_NEXT_OVERRIDE_ONCE 0x00000002
#define	MIB_HANDLER_CUSTOM4 0x10000000
#define	MIB_HANDLER_CUSTOM3 0x20000000
#define	MIB_HANDLER_CUSTOM2 0x40000000
#define	MIB_HANDLER_CUSTOM1 0x80000000
#define	HANDLER_CAN_GETANDGETNEXT 0x01
#define	HANDLER_CAN_SET 0x02
#define	HANDLER_CAN_GETBULK 0x04
#define	HANDLER_CAN_NOT_CREATE 0x08
#define	HANDLER_CAN_BABY_STEP 0x10
#define	HANDLER_CAN_STASH 0x20
#define	HANDLER_CAN_RONLY (HANDLER_CAN_GETANDGETNEXT)
#define	HANDLER_CAN_RWRITE (HANDLER_CAN_GETANDGETNEXT \| HANDLER_CAN_SET)
#define	HANDLER_CAN_SET_ONLY (HANDLER_CAN_SET \| HANDLER_CAN_NOT_CREATE)
#define	HANDLER_CAN_DEFAULT (HANDLER_CAN_RONLY \| HANDLER_CAN_NOT_CREATE)
#define	REQUEST_IS_DELEGATED 1
#define	REQUEST_IS_NOT_DELEGATED 0
型定義
typedef netsnmp_mib_handler_s	netsnmp_mib_handler
	Typedefs the netsnmp_mib_handler_s struct into netsnmp_mib_handler
typedef netsnmp_handler_registration_s	netsnmp_handler_registration
	Typedefs the netsnmp_handler_registration_s struct into netsnmp_handler_registration
typedef int(	Netsnmp_Node_Handler )(netsnmp_mib_handler handler, netsnmp_handler_registration reginfo, netsnmp_agent_request_info reqinfo, netsnmp_request_info requests)
typedef netsnmp_handler_args_s	netsnmp_handler_args
typedef netsnmp_delegated_cache_s	netsnmp_delegated_cache
関数
netsnmp_mib_handler *	netsnmp_create_handler (const char name, Netsnmp_Node_Handler handler_access_method)
	creates a netsnmp_mib_handler structure given a name and a access method.
netsnmp_handler_registration *	netsnmp_handler_registration_create (const char name, netsnmp_mib_handler handler, oid *reg_oid, size_t reg_oid_len, int modes)
	creates a handler registration structure given a name, a access_method function, a registration location oid and the modes the handler supports.
netsnmp_handler_registration *	netsnmp_create_handler_registration (const char name, Netsnmp_Node_Handler handler_access_method, oid *reg_oid, size_t reg_oid_len, int modes)
int	netsnmp_register_handler (netsnmp_handler_registration *reginfo)
	register a handler, as defined by the netsnmp_handler_registration pointer.
int	netsnmp_unregister_handler (netsnmp_handler_registration *reginfo)
	unregister a handler, as defined by the netsnmp_handler_registration pointer.
int	netsnmp_register_handler_nocallback (netsnmp_handler_registration *reginfo)
	register a handler, as defined by the netsnmp_handler_registration pointer.
int	netsnmp_inject_handler_before (netsnmp_handler_registration reginfo, netsnmp_mib_handler handler, const char *before_what)
	inject a new handler into the calling chain of the handlers definedy by the netsnmp_handler_registration pointer.
int	netsnmp_inject_handler (netsnmp_handler_registration reginfo, netsnmp_mib_handler handler)
	inject a new handler into the calling chain of the handlers definedy by the netsnmp_handler_registration pointer.
NETSNMP_INLINE int	netsnmp_call_handler (netsnmp_mib_handler next_handler, netsnmp_handler_registration reginfo, netsnmp_agent_request_info reqinfo, netsnmp_request_info requests)
	calls a handler with with appropriate NULL checking of arguments, etc.
int	netsnmp_call_handlers (netsnmp_handler_registration reginfo, netsnmp_agent_request_info reqinfo, netsnmp_request_info *requests)
NETSNMP_INLINE int	netsnmp_call_next_handler (netsnmp_mib_handler current, netsnmp_handler_registration reginfo, netsnmp_agent_request_info reqinfo, netsnmp_request_info requests)
	calls the next handler in the chain after the current one with with appropriate NULL checking, etc.
NETSNMP_INLINE int	netsnmp_call_next_handler_one_request (netsnmp_mib_handler current, netsnmp_handler_registration reginfo, netsnmp_agent_request_info reqinfo, netsnmp_request_info requests)
	calls the next handler in the chain after the current one with with appropriate NULL checking, etc.
void	netsnmp_handler_free (netsnmp_mib_handler *handler)
	free's the resourceses associated with a given handler
netsnmp_mib_handler *	netsnmp_handler_dup (netsnmp_mib_handler *handler)
	dulpicates a handler and all subsequent handlers see also _clone_handler
void	netsnmp_handler_registration_free (netsnmp_handler_registration *reginfo)
	free the resources associated with a handler registration object
netsnmp_handler_registration *	netsnmp_handler_registration_dup (netsnmp_handler_registration *reginfo)
	duplicates the handler registration object
NETSNMP_INLINE netsnmp_delegated_cache *	netsnmp_create_delegated_cache (netsnmp_mib_handler handler, netsnmp_handler_registration reginfo, netsnmp_agent_request_info reqinfo, netsnmp_request_info requests, void *localinfo)
	creates a cache of information which can be saved for future reference.
NETSNMP_INLINE netsnmp_delegated_cache *	netsnmp_handler_check_cache (netsnmp_delegated_cache *dcache)
	check's a given cache and returns it if it is still valid (ie, the agent still considers it to be an outstanding request.
NETSNMP_INLINE void	netsnmp_free_delegated_cache (netsnmp_delegated_cache *dcache)
	frees a cache once you're finished using it
void	netsnmp_handler_mark_requests_as_delegated (netsnmp_request_info *requests, int isdelegated)
	marks a list of requests as delegated (or not if isdelegaded = 0)
NETSNMP_INLINE void	netsnmp_request_add_list_data (netsnmp_request_info request, netsnmp_data_list node)
	add data to a request that can be extracted later by submodules
NETSNMP_INLINE int	netsnmp_request_remove_list_data (netsnmp_request_info request, const char name)
	remove data from a request
NETSNMP_INLINE void *	netsnmp_request_get_list_data (netsnmp_request_info request, const char name)
	extract data from a request that was added previously by a parent module
NETSNMP_INLINE void	netsnmp_free_request_data_set (netsnmp_request_info *request)
	Free the extra data stored in a request
NETSNMP_INLINE void	netsnmp_free_request_data_sets (netsnmp_request_info *request)
	Free the extra data stored in a bunch of requests (all data in the chain)
netsnmp_mib_handler *	netsnmp_find_handler_by_name (netsnmp_handler_registration reginfo, const char name)
	Returns a handler from a chain based on the name
void *	netsnmp_find_handler_data_by_name (netsnmp_handler_registration reginfo, const char name)
	Returns a handler's void * pointer from a chain based on the name.
void	handler_free_callback (void *free)
void	netsnmp_register_handler_by_name (const char name, netsnmp_mib_handler handler)
	registers a given handler by name so that it can be found easily later.
void	netsnmp_clear_handler_list (void)
	clears the entire handler-registration list
void	netsnmp_inject_handler_into_subtree (netsnmp_subtree tp, const char name, netsnmp_mib_handler handler, const char before_what)
void	parse_injectHandler_conf (const char token, char cptr)
void	netsnmp_init_handler_conf (void)
void *	netsnmp_handler_get_parent_data (netsnmp_request_info , const char )

説明

This worked well at the time but in todays world of subagents, device instrumentation, low resource consumption, etc, it just isn't flexible enough. "handlers" are here to fix all that.

With the rewrite of the agent internals for the net-snmp 5.0 release, we introduce a modular calling scheme that allows agent modules to be written in a very flexible manner, and more importantly allows reuse of code in a decent way (and without the memory and speed overheads of OO languages like C++).

Functionally, the notion of what a handler does is the same as the older api: A handler is created and then registered with the main agent at a given OID in the OID tree and gets called any time a request is made that it should respond to. You probably should use one of the convenience helpers instead of doing anything else yourself though:

Most importantly, though, is that the handlers are built on the notion of modularity and reuse. Specifically, rather than do all the really hard work (like parsing table indexes out of an incoming oid request) in each module, the API is designed to make it easy to write "helper" handlers that merely process some aspect of the request before passing it along to the final handler that returns the real answer. Most people will want to make use of the instance, table, table_iterator, table_data, or table_dataset helpers to make their life easier. These "helpers" interpert important aspects of the request and pass them on to you.

For instance, the table helper is designed to hand you a list of extracted index values from an incoming request. THe table_iterator helper is built on top of the table helper, and is designed to help you iterate through data stored elsewhere (like in a kernel) that is not in OID lexographical order (ie, don't write your own index/oid sorting routine, use this helper instead). The beauty of the table_iterator helper, as well as the instance helper is that they take care of the complex GETNEXT processing entirely for you and hand you everything you need to merely return the data as if it was a GET request. Much less code and hair pulling. I've pulled all my hair out to help you so that only one of us has to be bald.

型定義

struct netsnmp_handler_registration_s netsnmp_handler_registration

Typedefs the netsnmp_handler_registration_s struct into netsnmp_handler_registration

struct netsnmp_mib_handler_s netsnmp_mib_handler

Typedefs the netsnmp_mib_handler_s struct into netsnmp_mib_handler

関数

int netsnmp_call_handler	(	netsnmp_mib_handler *	next_handler,
		netsnmp_handler_registration *	reginfo,
		netsnmp_agent_request_info *	reqinfo,
		netsnmp_request_info *	requests
	)

calls a handler with with appropriate NULL checking of arguments, etc.

agent_handler.c の 387 行で定義されています。

参照先 netsnmp_mib_handler_s::access_method・netsnmp_mib_handler_s::handler_name・MIB_HANDLER_AUTO_NEXT・MIB_HANDLER_AUTO_NEXT_OVERRIDE_ONCE・netsnmp_agent_request_info_s::mode・netsnmp_mib_handler_s::next・NULL・snmp_log().

参照元 netsnmp_call_handlers()・netsnmp_call_next_handler()・netsnmp_call_next_handler_one_request()・netsnmp_mode_end_call_helper()・netsnmp_multiplexer_helper_handler().

int netsnmp_call_next_handler	(	netsnmp_mib_handler *	current,
		netsnmp_handler_registration *	reginfo,
		netsnmp_agent_request_info *	reqinfo,
		netsnmp_request_info *	requests
	)

calls the next handler in the chain after the current one with with appropriate NULL checking, etc.

agent_handler.c の 517 行で定義されています。

参照先 netsnmp_call_handler()・netsnmp_mib_handler_s::next・NULL・snmp_log().

int netsnmp_call_next_handler_one_request	(	netsnmp_mib_handler *	current,
		netsnmp_handler_registration *	reginfo,
		netsnmp_agent_request_info *	reqinfo,
		netsnmp_request_info *	requests
	)

calls the next handler in the chain after the current one with with appropriate NULL checking, etc.

agent_handler.c の 539 行で定義されています。

参照先 netsnmp_call_handler()・netsnmp_mib_handler_s::next・netsnmp_request_info_s::next・NULL・snmp_log().

void netsnmp_clear_handler_list ( void )

clears the entire handler-registration list

agent_handler.c の 898 行で定義されています。

参照先 netsnmp_free_all_list_data()・NULL.

参照元 shutdown_agent().

NETSNMP_INLINE netsnmp_delegated_cache * netsnmp_create_delegated_cache	(	netsnmp_mib_handler *	handler,
		netsnmp_handler_registration *	reginfo,
		netsnmp_agent_request_info *	reqinfo,
		netsnmp_request_info *	requests,
		void *	localinfo
	)

creates a cache of information which can be saved for future reference.

Use netsnmp_handler_check_cache() later to make sure it's still valid before referencing it in the future.

例:: delayed_instance.c.

agent_handler.c の 689 行で定義されています。

参照先 netsnmp_agent_request_info_s::asp・netsnmp_delegated_cache_s::handler・netsnmp_delegated_cache_s::localinfo・netsnmp_agent_session_s::pdu・netsnmp_delegated_cache_s::reginfo・netsnmp_delegated_cache_s::reqinfo・netsnmp_delegated_cache_s::requests・SNMP_MALLOC_TYPEDEF・netsnmp_delegated_cache_s::transaction_id・snmp_pdu::transid.

netsnmp_mib_handler * netsnmp_create_handler	(	const char *	name,
		Netsnmp_Node_Handler *	handler_access_method
	)

creates a netsnmp_mib_handler structure given a name and a access method.

The returned handler should then be registered.

引数:

	name	is the handler name and is copied then assigned to netsnmp_mib_handler->handler_name
	handler_access_method	is a function pointer used as the access method for this handler registration instance for whatever required needs.

戻り値:: a pointer to a populated netsnmp_mib_handler struct to be registered

参照:: netsnmp_create_handler_registration()
netsnmp_register_handler()

agent_handler.c の 105 行で定義されています。

参照先 netsnmp_mib_handler_s::access_method・netsnmp_mib_handler_s::handler_name・NULL・SNMP_FREE・SNMP_MALLOC_TYPEDEF.

netsnmp_mib_handler * netsnmp_find_handler_by_name	(	netsnmp_handler_registration *	reginfo,
		const char *	name
	)

Returns a handler from a chain based on the name

agent_handler.c の 831 行で定義されています。

参照先 netsnmp_handler_registration_s::handler・netsnmp_mib_handler_s::handler_name・netsnmp_mib_handler_s::next・NULL.

参照元 netsnmp_find_handler_data_by_name().

void * netsnmp_find_handler_data_by_name	(	netsnmp_handler_registration *	reginfo,
		const char *	name
	)

Returns a handler's void * pointer from a chain based on the name.

This probably shouldn't be used by the general public as the void * data may change as a handler evolves. Handlers should really advertise some function for you to use instead.

agent_handler.c の 848 行で定義されています。

参照先 netsnmp_mib_handler_s::myvoid・netsnmp_find_handler_by_name()・NULL.

参照元 netsnmp_find_table_registration_info().

NETSNMP_INLINE void netsnmp_free_delegated_cache ( netsnmp_delegated_cache * dcache )

frees a cache once you're finished using it

例:: delayed_instance.c.

agent_handler.c の 727 行で定義されています。

参照先 SNMP_FREE.

NETSNMP_INLINE void netsnmp_free_request_data_set ( netsnmp_request_info * request )

Free the extra data stored in a request

agent_handler.c の 813 行で定義されています。

参照先 netsnmp_free_list_data()・netsnmp_request_info_s::parent_data.

NETSNMP_INLINE void netsnmp_free_request_data_sets ( netsnmp_request_info * request )

Free the extra data stored in a bunch of requests (all data in the chain)

agent_handler.c の 821 行で定義されています。

参照先 netsnmp_free_all_list_data()・NULL・netsnmp_request_info_s::parent_data.

参照元 free_agent_snmp_session()・get_set_cache()・netsnmp_add_varbind_to_cache()・netsnmp_delete_request_infos().

NETSNMP_INLINE netsnmp_delegated_cache * netsnmp_handler_check_cache ( netsnmp_delegated_cache * dcache )

check's a given cache and returns it if it is still valid (ie, the agent still considers it to be an outstanding request.

Returns NULL if it's no longer valid.

例:: delayed_instance.c.

agent_handler.c の 713 行で定義されています。

参照先 netsnmp_check_transaction_id()・NULL・netsnmp_delegated_cache_s::transaction_id.

netsnmp_mib_handler * netsnmp_handler_dup ( netsnmp_mib_handler * handler )

dulpicates a handler and all subsequent handlers see also _clone_handler

agent_handler.c の 584 行で定義されています。

参照先 netsnmp_mib_handler_s::myvoid・netsnmp_handler_free()・netsnmp_mib_handler_s::next・NULL・netsnmp_mib_handler_s::prev.

参照元 netsnmp_handler_registration_dup().

void netsnmp_handler_free ( netsnmp_mib_handler * handler )

free's the resourceses associated with a given handler

make sure we aren't pointing to ourselves.

XXX : segv here at shutdown if SHUTDOWN_AGENT_CLEANLY defined. About 30 functions down the stack, starting in clear_context() -> clear_subtree()

agent_handler.c の 562 行で定義されています。

参照先 netsnmp_mib_handler_s::handler_name・netsnmp_mib_handler_s::next・NULL・SNMP_FREE.

参照元 handler_free_callback()・netsnmp_baby_steps_handler_get()・netsnmp_get_scalar_group_handler()・netsnmp_get_timed_bare_stash_cache_handler()・netsnmp_handler_dup()・netsnmp_handler_registration_free().

void netsnmp_handler_mark_requests_as_delegated	(	netsnmp_request_info *	requests,
		int	isdelegated
	)

marks a list of requests as delegated (or not if isdelegaded = 0)

agent_handler.c の 741 行で定義されています。

参照先 netsnmp_request_info_s::delegated・netsnmp_request_info_s::next.

netsnmp_handler_registration * netsnmp_handler_registration_create	(	const char *	name,
		netsnmp_mib_handler *	handler,
		oid *	reg_oid,
		size_t	reg_oid_len,
		int	modes
	)

creates a handler registration structure given a name, a access_method function, a registration location oid and the modes the handler supports.

If modes == 0, then modes will automatically be set to the default value of only HANDLER_CAN_DEFAULT, which is by default read-only GET and GETNEXT requests. A hander which supports sets but not row creation should set us a mode of HANDLER_CAN_SET_ONLY.

覚え書き:: This ends up calling netsnmp_create_handler(name, handler_access_method)

引数:

	name	is the handler name and is copied then assigned to netsnmp_handler_registration->handlerName.
	handler	is a function pointer used as the access method for this handler registration instance for whatever required needs.
	reg_oid	is the registration location oid.
	reg_oid_len	is the length of reg_oid, can use the macro, OID_LENGTH
	modes	is used to configure read/write access. If modes == 0, then modes will automatically be set to the default value of only HANDLER_CAN_DEFAULT, which is by default read-only GET and GETNEXT requests. The other two mode options are read only, HANDLER_CAN_RONLY, and read/write, HANDLER_CAN_RWRITE.

HANDLER_CAN_GETANDGETNEXT
HANDLER_CAN_SET
HANDLER_CAN_GETBULK

HANDLER_CAN_RONLY (HANDLER_CAN_GETANDGETNEXT)
HANDLER_CAN_RWRITE (HANDLER_CAN_GETANDGETNEXT | HANDLER_CAN_SET)
HANDLER_CAN_DEFAULT HANDLER_CAN_RONLY

戻り値:: Returns a pointer to a netsnmp_handler_registration struct. NULL is returned only when memory could not be allocated for the netsnmp_handler_registration struct.

参照:: netsnmp_create_handler()
netsnmp_register_handler()

agent_handler.c の 163 行で定義されています。

参照先 netsnmp_handler_registration_s::handler・HANDLER_CAN_DEFAULT・netsnmp_handler_registration_s::handlerName・memdup()・netsnmp_handler_registration_s::modes・NULL・netsnmp_handler_registration_s::priority・netsnmp_handler_registration_s::rootoid・netsnmp_handler_registration_s::rootoid_len・SNMP_MALLOC_TYPEDEF.

参照元 netsnmp_create_handler_registration().

netsnmp_handler_registration * netsnmp_handler_registration_dup ( netsnmp_handler_registration * reginfo )

duplicates the handler registration object

agent_handler.c の 627 行で定義されています。

参照元 netsnmp_subtree_deepcopy().

void netsnmp_handler_registration_free ( netsnmp_handler_registration * reginfo )

free the resources associated with a handler registration object

agent_handler.c の 613 行で定義されています。

参照先 netsnmp_handler_registration_s::contextName・netsnmp_handler_registration_s::handler・netsnmp_handler_registration_s::handlerName・netsnmp_handler_free()・NULL・netsnmp_handler_registration_s::rootoid・netsnmp_handler_registration_s::rootoid_len・SNMP_FREE.

参照元 netsnmp_handler_registration_dup()・netsnmp_register_mib()・netsnmp_register_mib_table_row()・netsnmp_subtree_free()・unregister_mibs_by_session().

int netsnmp_inject_handler	(	netsnmp_handler_registration *	reginfo,
		netsnmp_mib_handler *	handler
	)

inject a new handler into the calling chain of the handlers definedy by the netsnmp_handler_registration pointer.

The new handler is injected at the top of the list and hence will be the new handler to be called first.

agent_handler.c の 379 行で定義されています。

参照先 netsnmp_inject_handler_before()・NULL.

int netsnmp_inject_handler_before	(	netsnmp_handler_registration *	reginfo,
		netsnmp_mib_handler *	handler,
		const char *	before_what
	)

inject a new handler into the calling chain of the handlers definedy by the netsnmp_handler_registration pointer.

The new handler is injected after the before_what handler, or if NULL at the top of the list and hence will be the new handler to be called first.

agent_handler.c の 322 行で定義されています。

参照先 netsnmp_handler_registration_s::handler・netsnmp_mib_handler_s::handler_name・netsnmp_mib_handler_s::next・NULL・netsnmp_mib_handler_s::prev・snmp_log().

参照元 netsnmp_inject_handler()・netsnmp_inject_handler_into_subtree().

int netsnmp_register_handler ( netsnmp_handler_registration * reginfo )

agent_handler.c の 202 行で定義されています。

参照元 netsnmp_cache_handler_register()・netsnmp_register_cache_handler()・netsnmp_register_old_api()・netsnmp_register_row_merge()・netsnmp_register_serialize()・netsnmp_register_table()・netsnmp_sparse_table_register().

void netsnmp_register_handler_by_name	(	const char *	name,
		netsnmp_mib_handler *	handler
	)

registers a given handler by name so that it can be found easily later.

agent_handler.c の 886 行で定義されています。

参照先 handler_free_callback()・netsnmp_add_list_data()・netsnmp_create_data_list().

参照元 netsnmp_baby_steps_handler_init()・netsnmp_init_bulk_to_next_helper()・netsnmp_init_debug_helper()・netsnmp_init_read_only_helper()・netsnmp_init_row_merge()・netsnmp_init_serialize()・netsnmp_init_stash_cache_helper().

int netsnmp_register_handler_nocallback ( netsnmp_handler_registration * reginfo )

agent_handler.c の 270 行で定義されています。

参照元 netsnmp_register_mib_table_row().

NETSNMP_INLINE void netsnmp_request_add_list_data	(	netsnmp_request_info *	request,
		netsnmp_data_list *	node
	)

add data to a request that can be extracted later by submodules

引数:

	request	the netsnmp request info structure
	node	this is the data to be added to the linked list request->parent_data

戻り値:: void

例:: delayed_instance.c.

agent_handler.c の 761 行で定義されています。

参照先 netsnmp_add_list_data()・netsnmp_request_info_s::parent_data.

参照元 _netsnmp_tdata_helper_handler()・netsnmp_container_table_row_insert()・netsnmp_insert_iterator_context()・netsnmp_insert_table_row()・netsnmp_instance_int_handler()・netsnmp_instance_long_handler()・netsnmp_instance_num_file_handler()・netsnmp_instance_ulong_handler()・netsnmp_table_data_helper_handler()・netsnmp_table_data_set_helper_handler()・netsnmp_table_iterator_helper_handler()・netsnmp_watcher_helper_handler()・table_helper_handler().

NETSNMP_INLINE void * netsnmp_request_get_list_data	(	netsnmp_request_info *	request,
		const char *	name
	)

extract data from a request that was added previously by a parent module

引数:

	request	the netsnmp request info function
	name	used to compare against the request->parent_data->name value, if a match is found request->parent_data->data is returned

戻り値:: a void pointer(request->parent_data->data), otherwise NULL is returned if request is NULL or request->parent_data is NULL or request->parent_data object is not found.