SNMP Subagent Programmer's Guide

Callback Functions
4-26 119728—SNMP Subagent Programmer’s Guide
Writing Subagent Logic
MIB Extensions
You let the run-time library know about callback functions by encoding extensions in
MIB object definitions for objects on which the functions operate.
Locator Functions
Locator functions are identified in the ACCESS clause for a MIB object. The following
MIB definition, taken from the MIB used by the Indexed Trap Generator sample
program, identifies a locator function named locate_entry. This function is called when
the run-time library needs to find entries that have a particular value for cardNumber,
one of three objects constituting a table entry:
TableEntry ::=
SEQUENCE {
cardNumber INTEGER,
portNumber INTEGER,
ratio DisplayString
}
cardNumber OBJECT-TYPE
SYNTAX INTEGER (1..1000)
ACCESS read-only
C_STRUCT mytable, FIELD card
C_LOCATOR_FUNC locate_entry
STATUS mandatory
DESCRIPTION
"The card number index of the table"
::= { tblTrapTableEntry 1 }
Table 4-4. When to Use Callback Functions
Condition Functions Description
Request preprocessing Locator function The subagent needs to perform some
operation before a Get or Set operation on a
particular MIB object.
Dynamic table handling Next function
Locator function
A tabular MIB object contains entries that
are added or deleted during subagent
execution or a cache must be checked and
sometimes refreshed before a variable is
accessed. To process GetNext requests, the
run-time library calls the next function, the
locator function, then the Get access
function. To process Get and Set requests,
it calls the locator function, then the Get or
Set access function.
Unique operations Get access function
Test access function
Set access function
The access functions generated by GDMOC
are not able to handle the request processing
required. To process read-only objects, the
Get function is called. To process Set
requests, the Test function is called before
the Set function.