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Linux 2.6 - man page for snmp_generic (linux section 3erl)

snmp_generic(3erl)		     Erlang Module Definition		       snmp_generic(3erl)

       snmp_generic - Generic Functions for Implementing SNMP Objects in a Database

       The module snmp_generic contains generic functions for implementing tables (and variables)
       using the SNMP built-in database or Mnesia. These default functions are used if no instru-
       mentation  function is provided for a managed object in a MIB. Sometimes, it might be nec-
       essary to customize the behaviour of the default functions. For example,  in  some  situa-
       tions  a  trap  should  be sent if a row is deleted or modified, or some hardware is to be
       informed, when information is changed.

       The overall structure is shown in the following figure:

		|   SNMP Agent	|
		+- - - - - - - -+
		|      MIB	|
		Association file       (associates a MIB object with
			|		snmp_generic:table_funct
			|		snmp_generic:variable_func)
       |	   snmp_generic 	      |  Support for get-next,
       |				      |  RowStatus operations
       |    snmpa_local_db    |    Mnesia     |  Database
       |     dets     |  ets  |
       | (persistent) |       |

       Each function takes the argument NameDb , which is a tuple {Name, Db} , to identify  which
       database  the  functions  should  use.  Name is the symbolic name of the managed object as
       defined in the MIB, and Db is either volatile , persistent , or mnesia . If it is mnesia ,
       all variables are stored in the Mnesia table snmp_variables which must be a table with two
       attributes (not a Mnesia SNMP table). The SNMP tables are stored in Mnesia tables with the
       same  names  as	the SNMP tables. All functions assume that a Mnesia table exists with the
       correct name and attributes. It is the programmer's responsibility to ensure this. Specif-
       ically, if variables are stored in Mnesia, the table snmp_variables must be created by the
       programmer.  The  record  definition   for   this   table   is	defined   in   the   file
       snmp/include/snmp_types.hrl .

       If  an instrumentation function in the association file for a variable myVar does not have
       a name when compiling an MIB, the compiler generates an entry.

       {myVar, {snmp_generic, variable_func, [{myVar, Db]}}.

       And for a table:

       {myTable, {snmp_generic, table_func, [{myTable, Db]}}.

       In the functions defined below, the following types are used:

       name_db() = {name(), db()}
       name() = atom()
       db() = volatile | persistent | mnesia
       row_index() = [int()]
       columns() = [column()] | [{column(), value()}]
       column() = int()
       value() = term()

	 row_index() :
	   Denotes the last part of the OID which specifies the index of the  row  in  the  table
	   (see RFC1212, 4.1.6 for more information about INDEX).

	 columns() :
	   Is  a list of column numbers in the case of a get operation, and a list of column num-
	   bers and values in the case of a set operation.

       get_status_col(Name, Cols)
       get_status_col(NameDb, Cols) -> {ok, StatusVal} | false

	      Types  Name = name()
		     NameDb = name_db()
		     Cols = columns()
		     StatusVal = term()

	      Gets the value of the status column from Cols .

	      This function can be used in instrumentation functions for is_set_ok , undo or  set
	      to check if the status column of a table is modified.


	      Types  Name = name()

	      Gets the index types of Name

	      This  function can be used in instrumentation functions to retrieve the index types
	      part of the table info.

       table_func(Op1, NameDb)
       table_func(Op2, RowIndex, Cols, NameDb) -> Ret

	      Types  Op1 = new | delete
		     Op2 = get | next | is_set_ok | set | undo
		     NameDb = name_db()
		     RowIndex = row_index()
		     Cols = columns()
		     Ret = term()

	      This is the default instrumentation function for tables.

		* The new function creates the table if it does not exist, but only if the  data-
		  base is the SNMP internal db.

		* The delete function does not delete the table from the database since unloading
		  an MIB does not necessarily mean that the table should be destroyed.

		* The is_set_ok function checks that a row which is to	be  modified  or  deleted
		  exists, and that a row which is to be created does not exist.

		* The undo function does nothing.

		* The set function checks if it has enough information to make the row change its
		  status from notReady to notInService (when a row has been been set  to  create-
		  AndWait  ).  If a row is set to createAndWait , columns without a value are set
		  to noinit . If Mnesia is used, the set functionality is handled within a trans-

	      If  it  is possible for a manager to create or delete rows in the table, there must
	      be a RowStatus column for is_set_ok , set and undo to work properly.

	      The function returns according to the specification of an instrumentation function.

       table_get_elements(NameDb, RowIndex, Cols) -> Values

	      Types  NameDb = name_db()
		     RowIndex = row_index()
		     Cols = columns()
		     Values = [value() | noinit]

	      Returns a list with values for all columns in Cols . If a column is undefined,  its
	      value is noinit .

       table_next(NameDb, RestOid) -> RowIndex | endOfTable

	      Types  NameDb = name_db()
		     RestOid = [int()]
		     RowIndex = row_index()

	      Finds the indices of the next row in the table. RestOid does not have to specify an
	      existing row.

       table_row_exists(NameDb, RowIndex) -> bool()

	      Types  NameDb = name_db()
		     RowIndex = row_index()

	      Checks if a row in a table exists.

       table_set_elements(NameDb, RowIndex, Cols) -> bool()

	      Types  NameDb = name_db()
		     RowIndex = row_index()
		     Cols = columns()

	      Sets the elements in Cols to the row specified by RowIndex .  No	checks	are  per-
	      formed on the new values.

	      If  the Mnesia database is used, this function calls mnesia:write to store the val-
	      ues. This means that this function must be called from within a transaction (  mne-
	      sia:transaction/1 or mnesia:dirty/1 ).

       variable_func(Op1, NameDb)
       variable_func(Op2, Val, NameDb) -> Ret

	      Types  Op1 = new | delete | get
		     Op2 = is_set_ok | set | undo
		     NameDb = name_db()
		     Val = value()
		     Ret = term()

	      This is the default instrumentation function for variables.

	      The  new	function  creates  a new variable in the database with a default value as
	      defined in the MIB, or a zero value (depending on the type).

	      The delete function does not delete the variable from the database.

	      The function returns according to the specification of an instrumentation function.

       variable_get(NameDb) -> {value, Value} | undefined

	      Types  NameDb = name_db()
		     Value = value()

	      Gets the value of a variable.

       variable_set(NameDb, NewVal) -> true | false

	      Types  NameDb = name_db()
		     NewVal = value()

	      Sets a new value to a variable. The variable is created if it does  not  exist.  No
	      checks are made on the type of the new value.

	      Returns false if the NameDb argument is incorrectly specified, otherwise true .

       The  following  example	shows an implementation of a table which is stored in Mnesia, but
       with some checks performed at set-request operations.

       myTable_func(new, NameDb) ->   % pass unchanged
	 snmp_generic:table_func(new, NameDb).

       myTable_func(delete, NameDb) ->	 % pass unchanged
	 snmp_generic:table_func(delete, NameDb).

       %% change row
       myTable_func(is_set_ok, RowIndex, Cols, NameDb) ->
	 case snmp_generic:table_func(is_set_ok, RowIndex,
				      Cols, NameDb) of
	   {noError, 0} ->
	     myApplication:is_set_ok(RowIndex, Cols);
	   Err ->

       myTable_func(set, RowIndex, Cols, NameDb) ->
	 case snmp_generic:table_func(set, RowIndex, Cols,
	   {noError, 0} ->
	     % Now the row is updated, tell the application
	     myApplication:update(RowIndex, Cols);
	   Err ->

       myTable_func(Op, RowIndex, Cols, NameDb) ->   % pass unchanged
	 snmp_generic:table_func(Op, RowIndex, Cols, NameDb).

       The .funcs file would look like:

       {myTable, {myModule, myTable_func, [{myTable, mnesia}]}}.

Ericsson AB				    snmp 4.19			       snmp_generic(3erl)

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