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Extending Informix®
Beyond standard relational database objects, HCL Informix® can be extended to handle specialized data types, access methods, routines, and other objects. Informix includes many built-in extensions that are fully integrated in the database server. Informix also provides DataBlade® modules, which are packages of extended database objects for a particular purpose and that are installed separately from the database server. Alternatively, you can create your own user-defined objects for Informix.
Creating extensions
You can create user-defined data types, routines, access methods, and other database objects to suit your needs. You can use application programming interfaces to write user-defined routines and applications that access data in Informix® databases.
DataBlade® API Programmer's Guide
The Informix® DataBlade® API Programmer's Guide describes information about the DataBlade API, the C-language application programming interface (API) provided with HCL Informix.
Database access
Exceptions and events
This section describes exceptions and events, and explains how to handle them in DataBlade® API modules.
Callback functions
To catch or handle an event, you create a C function called a callback function. In your DataBlade® API module, you can register callback functions to handle recovery from events. The DataBlade API invokes a registered (and enabled) callback when the event associated with the callback occurs.
Write a callback function
Within the body of a callback function, you provide the code that handles a particular event or events. Only certain tasks are valid within a callback. When a callback function is invoked for an event, the DataBlade® API passes information about the event to the callback.
Event information
When a callback function is invoked for a particular event, the DataBlade® API passes an event-type structure as the third parameter of this function. This event-type structure contains information about the event that triggered the callback.
An error descriptor
The DataBlade® API stores information about exceptions and errors in an error descriptor. An error descriptor is an MI_ERROR_DESC structure. It holds information for the MI_Exception and MI_Client_Library_Error event types.
Create a copy of an error descriptor
The DataBlade® API passes the error descriptor as an argument to the callback. Therefore, the DataBlade API allocates memory for the error descriptor when it invokes the callback and deallocates this memory when the callback exits. To preserve the error information for the calling routine, you can create a user copy of the error descriptor within the callback.

Extending Informix®
Beyond standard relational database objects, HCL Informix® can be extended to handle specialized data types, access methods, routines, and other objects. Informix includes many built-in extensions that are fully integrated in the database server. Informix also provides DataBlade® modules, which are packages of extended database objects for a particular purpose and that are installed separately from the database server. Alternatively, you can create your own user-defined objects for Informix.
- Informix® extensions and DataBlade® modules
  These topics describe how to use built-in database extensions and separately installed DataBlade® modules.
- Creating extensions
  You can create user-defined data types, routines, access methods, and other database objects to suit your needs. You can use application programming interfaces to write user-defined routines and applications that access data in Informix® databases.
  - DataBlade® API Programmer's Guide
    The Informix® DataBlade® API Programmer's Guide describes information about the DataBlade API, the C-language application programming interface (API) provided with HCL Informix.
    - DataBlade® API overview
    - Data manipulation
    - Database access
      - Connection handling
        This section describes how to initialize, establish, and close connections.
      - Execute SQL statements
        One basic task of a DataBlade® API module is to send SQL statements to the database server for execution.
      - Execute user-defined routines
        This section provides information about how to call a user-defined routine (UDR).
      - Exceptions and events
        This section describes exceptions and events, and explains how to handle them in DataBlade® API modules.
        DataBlade® API event types
        For a DataBlade® API module, an event is a communication from HCL Informix® software that indicates the existence of some predefined condition, usually the occurrence of an exception (warning or error). The DataBlade API represents an event as one of the enumerated values of the MI_EVENT_TYPE data type.
        Event-handling mechanisms
        An event-handling mechanism provides a way for one DataBlade® API module to inform another module (or another part of the same module) that an event has occurred during execution of a function.
        Callback functions
        To catch or handle an event, you create a C function called a callback function. In your DataBlade® API module, you can register callback functions to handle recovery from events. The DataBlade API invokes a registered (and enabled) callback when the event associated with the callback occurs.
        Declare a callback function
        To declare a callback function, you provide the MI_CALLBACK_STATUS return type, the optional MI_PROC_CALLBACK modifier, and the parameter declarations.
        Write a callback function
        Within the body of a callback function, you provide the code that handles a particular event or events. Only certain tasks are valid within a callback. When a callback function is invoked for an event, the DataBlade® API passes information about the event to the callback.
        Restrictions on content
        Event information
        When a callback function is invoked for a particular event, the DataBlade® API passes an event-type structure as the third parameter of this function. This event-type structure contains information about the event that triggered the callback.
        An error descriptor
        The DataBlade® API stores information about exceptions and errors in an error descriptor. An error descriptor is an MI_ERROR_DESC structure. It holds information for the MI_Exception and MI_Client_Library_Error event types.
        Access an error descriptor
        Create a copy of an error descriptor
        The DataBlade® API passes the error descriptor as an argument to the callback. Therefore, the DataBlade API allocates memory for the error descriptor when it invokes the callback and deallocates this memory when the callback exits. To preserve the error information for the calling routine, you can create a user copy of the error descriptor within the callback.
        A transition descriptor
        The transition descriptor, MI_TRANSITION_DESC, stores information about a transition in the processing state of the database server. It holds information for all state-transition events.
        Database server exceptions
        When the database server or a UDR raises a database server exception, the DataBlade® API invokes any callbacks that are registered for the exception.
        State-transition events
        When the database server raises a state-transition event, the database server invokes any callbacks registered for the state transition. This section provides information about state-transition handling in DataBlade® API modules, including an explanation of state-transition events and a description of how to handle a state-transition event in a C UDR and a client LIBMI application.
        Client LIBMI errors
        The DataBlade® API throws the MI_Client_Library_Error event to indicate an error in the client LIBMI library.
      - XA-compliant external data sources
    - Create user-defined routines
    - Appendix
      This section contains additional reference information.
  - DataBlade® API Function Reference
    The Informix® DataBlade® API Function Reference describes the DataBlade API functions and the subset of Informix ESQL/C functions that the DataBlade API supports.
  - DataBlade® Developers Kit
    The DataBlade® Developers Kit (DBDK) guides contain information about the tools you can use to develop and package DataBlade modules, which extend the functionality of HCL Informix® databases.
  - J/Foundation Developer's Guide
    The J/Foundation Developer's Guide describes how to write user-defined routines (UDRs) in the Java™ programming language for Informix®.
  - R-Tree Index User's Guide
    The Informix® R-Tree Index User's Guide describes the HCL Informix R-tree secondary access method and how to access and use its components.
  - User-Defined Routines and Data Types Developer's Guide
    The Informix® User-Defined Routines and Data Types Developer's Guide describes how to define new data types and enable user-defined routines (UDRs) to extend HCL Informix.
  - Virtual-Table Interface Guide
    The Informix® Virtual-Table Interface Programmer's Guide explains how to create a primary access method with the Virtual-Table Interface (VTI) so that users have a single SQL interface to HCL Informix tables and to data that does not conform to the storage scheme of HCL Informix.
  - Virtual-Index Interface Guide
    The Informix® Virtual-Index Interface Programmer's Guide explains how to create a secondary access method with the Virtual-Index Interface (VII) to extend the built-in indexing schemes of HCL Informix typically with a DataBlade® module.

Create a copy of an error descriptor

The DataBlade® API passes the error descriptor as an argument to the callback. Therefore, the DataBlade® API allocates memory for the error descriptor when it invokes the callback and deallocates this memory when the callback exits. To preserve the error information for the calling routine, you can create a user copy of the error descriptor within the callback.

The following DataBlade® API functions facilitate an error-descriptor copy.


DataBlade® API function	Description
mi_error_desc_copy()	Allocates memory for a user copy of a specified error descriptor and returns a pointer to this user copy
mi_error_desc_is_copy()	Determines whether the specified error descriptor is a user copy
mi_error_desc_destroy()	Frees memory for a specified user copy of an error descriptor (which was allocated with mi_error_desc_copy())