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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 extensionsYou 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 GuideThe 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 eventsThis section describes exceptions and events, and explains how to handle them in DataBlade® API modules.
Callback functionsTo 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 functionWithin 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 informationWhen 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 descriptorThe 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

Access an error descriptor

The error descriptor is an opaque structure. You must use the DataBlade® API functions in the following table to access information within it.

Table 1. Accessor functions for an error descriptor
Error-descriptor information	Description	DataBlade API function
Error or warning message	The text of the error or warning message	mi_errmsg()
Exception or error level	An MI_Exception event has an exception level that indicates the type of exception that has occurred: a warning (MI_WARNING) or a runtime error (MI_EXCEPTION) An MI_Client_Library_Error event has an error level to indicate the type of error that has occurred. For a list of possible client LIBMI error levels, see Client LIBMI errors.	mi_error_level()
SQLSTATE value	A five-character status value that is compliant with ANSI and X/Open standards. For more information, see The SQLSTATE status value.	mi_error_sql_state()
SQLCODE value	A database server-specific status value that contains an integer value. For more information, see SQLCODE status value.	mi_error_sqlcode()

Each of the DataBlade API functions in preceding table requires that you pass in a pointer to a valid error descriptor.

Restriction: The MI_ERROR_DESC structure is an opaque structure to DataBlade API modules. Do not access its internal fields directly. The internal structure of MI_ERROR_DESC might change in future releases. Therefore, to create portable code, always use the accessor functions in Accessor functions for an error descriptor to obtain values from this structure.

For a sample callback that obtains information from an error descriptor, see the excpt_callback2() function in Associate a user-defined error structure with a callback.

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