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Simple Network Management Protocol (SNMP)
User's Guide
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# 11 Definition of Instrumentation Functions

The section Definition of Instrumentation Functions describes the user defined functions, which the agent calls at different times.

### 11.1 Variable Instrumentation

For scalar variables, a function f(Operation, ...) must be defined.

The Operation can be new, delete, get, is_set_ok, set, or undo.

In case of an error, all instrumentation functions may return either an SNMPv1 or an SNMPv2 error code. If it returns an SNMPv2 code, it is converted into an SNMPv1 code before it is sent to a SNMPv1 manager. It is recommended to use the SNMPv2 error codes for all instrumentation functions, as these provide more details. See Appendix A for a description of error code conversions.

#### f(new [, ExtraArgs])

The function f(new [, ExtraArgs]) is called for each variable in the MIB when the MIB is loaded into the agent. This makes it possible to perform necessary initialization.

This function is optional. The return value is discarded.

#### f(delete [, ExtraArgs])

The function f(delete [, ExtraArgs]) is called for each object in an MIB when the MIB is unloaded from the agent. This makes it possible to perform necessary clean-up.

This function is optional. The return value is discarded.

#### f(get [, ExtraArgs])

The function f(get [, ExtraArgs]) is called when a get-request or a get-next request refers to the variable.

This function is mandatory.

- {value, Value}. The Value must be of correct type, length and within ranges, otherwise genErr is returned in the response PDU. If the object is an enumerated integer, the symbolic enum value may be used as an atom. If the object is of type BITS, the return value shall be an integer or a list of bits that are set.
- {noValue, noSuchName}(SNMPv1)
- {noValue, noSuchObject | noSuchInstance} (SNMPv2)
- genErr. Used if an error occurred. Note, this should be an internal processing error, e.g. a caused by a programing fault somewhere. If the variable does not exist, use {noValue, noSuchName} or {noValue, noSuchInstance}.

#### f(is_set_ok, NewValue [, ExtraArgs])

The function f(is_set_ok, NewValue [, ExtraArgs]) is called in phase one of the set-request processing so that the new value can be checked for inconsistencies.

NewValue is guaranteed to be of the correct type, length and within ranges, as specified in the MIB. If the object is an enumerated integer or of type BITS, the integer value is used.

This function is optional.

If this function is called, it will be called again, either with undo or with set as first argument.

- noError
- badValue | noSuchName | genErr(SNMPv1)
- noAccess | noCreation | inconsistentValue | resourceUnavailable | inconsistentName | genErr(SNMPv2)

#### f(undo, NewValue [, ExtraArgs])

If an error occurred, this function is called after the is_set_ok function is called. If set is called for this object, undo is not called.

NewValue is guaranteed to be of the correct type, length and within ranges, as specified in the MIB. If the object is an enumerated integer or of type BITS, the integer value is used.

This function is optional.

- noError
- genErr(SNMPv1)
- undoFailed | genErr(SNMPv2)

#### f(set, NewValue [, ExtraArgs])

This function is called to perform the set in phase two of the set-request processing. It is only called if the corresponding is_set_ok function is present and returns noError.

NewValue is guaranteed to be of the correct type, length and within ranges, as specified in the MIB. If the object is an enumerated integer or of type BITS, the integer value is used.

This function is mandatory.

- noError
- genErr(SNMPv1)
- commitFailed | undoFailed | genErr(SNMPv2)

### 11.2 Table Instrumentation

For tables, a f(Operation, ...) function should be defined (the function shown is exemplified with f).

The Operation can be new, delete, get, next, is_set_ok, undo or set.

In case of an error, all instrumentation functions may return either an SNMPv1 or an SNMPv2 error code. If it returns an SNMPv2 code, it is converted into an SNMPv1 code before it is sent to a SNMPv1 manager. It is recommended to use the SNMPv2 error codes for all instrumentation functions, as these provide more details. See Appendix A for a description of error code conversions.

#### f(new [, ExtraArgs])

The function f(new [, ExtraArgs]) is called for each object in an MIB when the MIB is loaded into the agent. This makes it possible to perform the necessary initialization.

This function is optional. The return value is discarded.

#### f(delete [, ExtraArgs])

The function f(delete [, ExtraArgs]) is called for each object in an MIB when the MIB is unloaded from the agent. This makes it possible to perform any necessary clean-up.

This function is optional. The return value is discarded.

#### f(get, RowIndex, Cols [, ExtraArgs])

The function f(get, RowIndex, Cols [, ExtraArgs]) is called when a get-request refers to a table.

This function is mandatory.

- RowIndex is a list of integers which define the key values for the row. The RowIndex is the list representation (list of integers) which follow the Cols integer in the OBJECT IDENTIFIER.
- Cols is a list of integers which represent the column numbers. The Cols are sorted by increasing value and are guaranteed to be valid column numbers.

- A list with as many elements as the Cols list, where each element is the value of the corresponding column. Each element can be:

  - {value, Value}. The Value must be of correct type, length and within ranges, otherwise genErr is returned in the response PDU. If the object is an enumerated integer, the symbolic enum value may be used (as an atom). If the object is of type BITS, the return value shall be an integer or a list of bits that are set.
  - {noValue, noSuchName}(SNMPv1)
  - {noValue, noSuchObject | noSuchInstance}(SNMPv2)
- {noValue, Error}. If the row does not exist, because all columns have {noValue, Error}), the single tuple {noValue, Error} can be returned. This is a shorthand for a list with all elements {noValue, Error}.
- genErr. Used if an error occurred. Note that this should be an internal processing error, e.g. a caused by a programing fault somewhere. If some column does not exist, use {noValue, noSuchName} or {noValue, noSuchInstance}.

#### f(get_next, RowIndex, Cols [, ExtraArgs])

The function f(get_next, RowIndex, Cols [, ExtraArgs]) is called when a get-next- or a get-bulk-request refers to the table.

The RowIndex argument may refer to an existing row or a non-existing row, or it may be unspecified. The Cols list may refer to inaccessible columns or non-existing columns. For each column in the Cols list, the corresponding next instance is determined, and the last part of its OBJECT IDENTIFIER and its value is returned.

This function is mandatory.

- RowIndex is a list of integers (possibly empty) that defines the key values for a row. The RowIndex is the list representation (list of integers), which follow the Cols integer in the OBJECT IDENTIFIER.
- Cols is a list of integers, greater than or equal to zero, which represents the column numbers.

- A list with as many elements as the Cols list Each element can be:

  - {NextOid, NextValue}, where NextOid is the lexicographic next OBJECT IDENTIFIER for the corresponding column. This should be specified as the OBJECT IDENTIFER part following the table entry. This means that the first integer is the column number and the rest is a specification of the keys. NextValue is the value of this element.
  - endOfTable if there are no accessible elements after this one.
- {genErr, Column} where Column denotes the column that caused the error. Column must be one of the columns in the Cols list. Note that this should be an internal processing error, e.g. a caused by a programing fault somewhere. If some column does not exist, you must return the next accessible element (or endOfTable).

#### f(is_set_ok, RowIndex, Cols [, ExtraArgs])

The function f(is_set_ok, RowIndex, Cols [, ExtraArgs]) is called in phase one of the set-request processing so that new values can be checked for inconsistencies.

If the function is called, it will be called again with undo, or with set as first argument.

This function is optional.

- RowIndex is a list of integers which define the key values for the row. The RowIndex is the list representation (list of integers) which follow the Cols integer in the OBJECT IDENTIFIER.
- Cols is a list of {Column, NewValue}, where Column is an integer, and NewValue is guaranteed to be of the correct type, length and within ranges, as specified in the MIB. If the object is an enumerated integer or of type BITS, the integer value is used. The list is sorted by Column (increasing) and each Column is guaranteed to be a valid column number.

- {noError, 0}
- {Error, Column}, where Error is the same as for is_set_ok for variables, and Column denotes the faulty column. Column must be one of the columns in the Cols list.

#### f(undo, RowIndex, Cols [, ExtraArgs])

If an error occurs, The function f(undo, RowIndex, Cols [, ExtraArgs]) is called after the is_set_ok function. If set is called for this object, undo is not called.

This function is optional.

- RowIndex is a list of integers which define the key values for the row. The RowIndex is the list representation (list of integers) which follow the Cols integer in the OBJECT IDENTIFIER.
- Cols is a list of {Column, NewValue}, where Column is an integer, and NewValue is guaranteed to be of the correct type, length and within ranges, as specified in the MIB. If the object is an enumerated integer or of type BITS, the integer value is used. The list is sorted by Column (increasing) and each Column is guaranteed to be a valid column number.

- {noError, 0}
- {Error, Column} where Error is the same as for undo for variables, and Column denotes the faulty column. Column must be one of the columns in the Cols list.

#### f(set, RowIndex, Cols [, ExtraArgs])

The function f(set, RowIndex, Cols [, ExtraArgs]) is called to perform the set in phase two of the set-request processing. It is only called if the corresponding is_set_ok function did not exist, or returned {noError, 0}.

This function is mandatory.

- RowIndex is a list of integers that define the key values for the row. The RowIndex is the list representation (list of integers) which follow the Cols integer in the OBJECT IDENTIFIER.
- Cols is a list of {Column, NewValue}, where Column is an integer, and NewValue is guaranteed to be of the correct type, length and within ranges, as specified in the MIB. If the object is an enumerated integer or of type BITS, the integer value is used. The list is sorted by Column (increasing) and each Column is guaranteed to be a valid column number.

- {noError, 0}
- {Error, Column} where Error is the same as set for variables, and Column denotes the faulty column. Column must be one of the columns in the Cols list.