Handling Errors and Exceptions in Proc*C

When working with embedded SQL in C using Proc*C, it is essential to handle errors and exceptions effectively to ensure the robustness of your applications. Errors can occur during database operations, and exceptional situations might arise that require special handling. In this tutorial, we will explore the techniques for handling errors and exceptions in Proc*C, along with example commands and code snippets to illustrate the process.

1. SQL Communication Area (SQLCA)

The SQL Communication Area (SQLCA) is a critical structure in Proc*C that plays a vital role in error handling. It contains status information about the most recent SQL statement execution. By checking the SQLCA after each SQL operation, you can identify errors and exceptions that occurred during database interactions.

2. Exception Handling with WHENEVER

Proc*C provides the WHENEVER statement to handle exceptions automatically. The WHENEVER statement directs the program flow to specific labels when certain conditions are met. The most commonly used conditions are SQLERROR and NOT FOUND.

      /* EXEC SQL WHENEVER SQLERROR GOTO error_handling; */
      /* EXEC SQL WHENEVER NOT FOUND GOTO not_found_handling; */
  /* EXEC SQL SELECT employee_name INTO :employee_name
             FROM employees
             WHERE employee_id = :emp_id; */
  
  printf("Employee Name: %s\n", employee_name);
  /* EXEC SQL COMMIT; */
  return 0;

error_handling:
  /* Handle SQL errors here */
  printf("SQL Error occurred!\n");
  /* EXEC SQL ROLLBACK; */
  return -1;

not_found_handling:
  /* Handle NOT FOUND condition here */
  printf("Employee not found!\n");
  /* EXEC SQL ROLLBACK; */
  return -1;

In the example above, the program flow will jump to the error_handling label if any SQL error occurs during the SELECT statement execution. It will jump to the not_found_handling label if the SELECT statement doesn't find any matching records.

3. Manual Error Handling

Besides using WHENEVER, you can also manually handle errors by checking the values of SQLCODE and SQLERRM, which are part of the SQLCA structure. SQLCODE contains the return code of the last SQL operation, and SQLERRM contains the corresponding error message.

      /* EXEC SQL SELECT COUNT(*) INTO :emp_count FROM employees; */
  if (SQLCODE != 0) {
      /* Handle error */
      printf("Error: %s\n", SQLERRM.SQLERRM_TEXT);
      /* EXEC SQL ROLLBACK; */
      return -1;
  }
  
  printf("Total Employees: %d\n", emp_count);
  /* EXEC SQL COMMIT; */
  return 0;

4. Common Mistakes with Handling Errors and Exceptions in Proc*C

• Not checking SQLCA after each SQL operation.
• Improper use of the WHENEVER statement, leading to unexpected program flow.
• Failure to handle specific exception conditions, such as NOT FOUND.

5. Frequently Asked Questions (FAQs)

• Q: Can I use the WHENEVER statement with custom exception conditions?
  A: No, the WHENEVER statement in Proc*C only supports SQLERROR and NOT FOUND conditions.
• Q: Can I use WHENEVER with dynamic SQL statements?
  A: Yes, you can use WHENEVER with both static and dynamic SQL statements.
• Q: What happens if I don't handle SQL exceptions?
  A: If SQL exceptions are not handled, the program may continue execution, leading to unexpected results or application crashes.
• Q: Can I use a single WHENEVER statement to handle multiple conditions?
  A: No, you need separate WHENEVER statements for each condition you want to handle.
• Q: How can I debug errors in my Proc*C program?
  A: You can enable debugging options during compilation and use logging mechanisms to trace the program flow and identify errors.

6. Summary

Handling errors and exceptions in Proc*C is crucial for building reliable and robust database-centric C applications. By utilizing the SQL Communication Area (SQLCA) and employing the WHENEVER statement or manual error handling techniques, you can gracefully manage errors and exceptions in your Proc*C programs. Avoid common mistakes, and refer to the FAQs for any queries related to error handling. With these techniques, you can ensure the smooth execution of your Proc*C applications and enhance their overall stability and reliability.