SNMP Subagent Programmer's Guide

Programming Tutorials
SNMP Subagent Programmer’s Guide—119728 2-9
Hello World Program
MIB objects included in traps must have been defined earlier in the MIB definition.
Note that helloPrintFreq is defined before the trap, helloFreqChange, in the Hello World
MIB.
Mapping Objects to Variables
The EXTEND OBJECT-TYPE statements at the end of the MIB definition tell the run-
time library where in the subagent source code the value for MIB objects is maintained.
These statements are examples of MIB constructs known as “extensions.” Extensions
are embedded in your MIB definition to map MIB objects to structures in your subagent
that contain the values of the MIB objects.
For example, the value of helloOutputText is maintained in a global C variable named
helloText:
EXTEND OBJECT-TYPE helloOutputText
WITH C_VARIABLE helloText
IN C
You need to declare variables for MIB object values globally unless you have provided
access functions in your subagent. Refer to Section 4, “Writing Subagent Logic,” for
complete information on variable declarations and access functions.
As Section 3, “Writing and Compiling MIBs,” describes, you can segregate extensions
from MIB object definitions, as in the Hello World MIB, or you can embed extensions
within a MIB object definition. In the latter case, the definition for helloOutputText
would look like this:
helloOutputText OBJECT-TYPE
SYNTAX DisplayString (SIZE (0..255))
ACCESS read-write
C_VARIABLE helloText
STATUS mandatory
DESCRIPTION
"The textual message that is displayed to stdout
by the Hello World program."
::= { hello 1 }
Isolating the extensions is the recommended approach, because this practice makes
installing your MIB on manager stations easier.
Installing the MIB
Before using a manager to interact with an executing subagent, you compile its MIB
definition on any manager station to be used to interact with the subagent. Different
managers have different requirements for MIB installation. You have to modify your
MIB definition slightly to accommodate a manager’s requirements.