Handling an exception in the callback

To indicate that the callback function executes instead of the default exception handling, an exception callback function returns the MI_CB_EXC_HANDLED status. This return status tells the that the actions of the callback have completely handled the exception.

An exception callback function returns the MI_CB_EXC_HANDLED status to indicate that the callback has completely handled the exception. That is, the actions of the callback have provided the exception handling. When the receives the MI_CB_EXC_HANDLED return status, it does not perform its default exception handling. It assumes that the callback has executed instead of the default exception handling.

When a callback returns MI_CB_EXC_HANDLED, the does not propagate the exception up the calling sequence. Therefore, a client application that has executed an SQL expression that contains a UDR does not receive an error from the execution of the UDR (unless the callback uses a user-provided error buffer). If the SQL expression contains no other exceptions, the client application would have an SQLSTATE value of 00000 (success).

The following code fragment shows the excpt_callback() exception callback, which is written to handle the MI_Exception event. It returns MI_CB_EXC_HANDLED to indicate that no further exception handling is required.
Figure 1. A simple exception callback 
static MI_CALLBACK_STATUS MI_PROC_CALLBACK
excpt_callback(event_type, conn, event_data, user_data)
   MI_EVENT_TYPE event_type;
   MI_CONNECTION *conn;
   void *event_data;
   void *user_data;
{
   /* claim to have handled the exception */
   return MI_CB_EXC_HANDLED;
}

The excpt_callback() function in A simple exception callback returns MI_CB_EXC_HANDLED, which prevents the from taking any further exception-handling steps, such as invoking other callbacks that handle MI_Exception or aborting the current statement. This callback executes instead of the default exception handling.

For the has_exception_handling() routine (which A C UDR with exception handling defines), the takes the following steps when the mi_exec() function executes:
  1. Executes the excpt_callback() callback when mi_exec() throws an exception
  2. Returns control to the first statement in has_exception_handling() after mi_exec(). As a result, execution of the has_exception_handling() routine returns from the mi_exec() call with a return value of MI_ERROR.
Important: Because excpt_callback() returns MI_CB_EXC_HANDLED, the database server assumes that the MI_Exception event does not require any further handling. However, excpt_callback() does not actually take any exception-handling steps; it contains only a return statement to return an MI_CB_EXC_HANDLED status. In an actual module, you would probably want to add code to excpt_callback() that provides exception handling.