Creating a Subagent Using the eSNMP API
3.3 Creating a MIB Source File
2. Array of OBJECT Structures Section
The second section of the subtree_TBL.C file is an array of OBJECT structures.
Each MIB variable within the subtree has one OBJECT. The chess example
produces the following:
static OBJECT objects[] = {
{I_chessProductID
,{12, &elems[ 11]}, ESNMP_TYPE_ObjectId
,chess_get, NULL},
. . .
An OBJECT structure represents a MIB variable and has the following fields:
•
object_index
— The constant I_mib-variable from the subtree_TBL.H file,
which identifies this variable (in the chess example,
I_chessProductID
.)
•
oid
— The variable’s OID (points to a part of
elems[ ]
).
This variable is of type OID, which is a structure containing two elements:
the number of elements in the OID and a pointer to the correct starting place
in the array of elements (
elems[ ]
in the chess example).
In the chess example,
oid
is designated by
{12, &elemens[ 11]}
. This
indicates that:
The OID has 12 integers separated by dots in the ASCII text
representation (
"1.3.6.1.4.1.36.2.15.2.99.2"
)
The integer with index 11 in the array
elems[ ]
is the first element.
•
type
— The variable’s eSNMP data type.
•
getfunc
— The address of the method routine to call for
Get
requests (null if
no routine exists).
•
setfunc
— The address of the method routine to call for
Set
requests (null if
no routine exists).
The master agent does not access object tables or MIB variables directly. It only
maintains a registry of subtrees. When a request for a particular MIB variable
arrives, it is processed as shown in the following steps (where the MIB variable is
mib_var
and the subtree is
subtree_1
):
1.
The master agent finds
subtree_1
as the authoritative region for the
mib_var
in the register of subtrees. The authoritative region is determined as the
registered MIB subtree that has the longest prefix and the highest priority.
2.
The master agent sends a message to the subagent that registered
subtree_1
.
3.
The subagent consults its list of registered subtrees and locates
subtree_1
.
It searches the object table of
subtree_1
and locates the following:
•
mib_var
(for
Get
and
Set
routines)
•
The first object lexicographically after
mib_var
(for
Next
or
Bulk
routines)
4.
The appropriate method routine is called. If the method routine completes
successfully, the data is returned to the master agent. If the method routine
fails when doing a
Get
or
Set
, an error is returned. If the method routine
fails when doing a
GetNext
, the code keeps trying subsequent objects in the
object table of
subtree_1
until either a method routine returns successfully or
the table is exhausted. In either case, a response is returned.
5.
If the master agent detects that
subtree_1
could not return data on a
Next
routine, it recursively tries the subtree lexicographically after
subtree_1
until
a subagent returns a value or the registry of subtrees is exhausted.
3–10 Creating a Subagent Using the eSNMP API