CodeIgniter log files are typically stored in the 'application/logs/' directory. This is the default location where the framework stores log files, and developers can access them for debugging and analysis purposes.

In the context of CodeIgniter, when facing database connectivity issues, the first place to check is the Hostname settings in the database configuration file. This setting specifies the location of your database server.

In CodeIgniter, JSON is the typical format used to send responses in a RESTful API. JSON (JavaScript Object Notation) is a lightweight data interchange format that is easy for humans to read and write, and easy for machines to parse and generate. It is well-suited for representing structured data in the context of a RESTful API.

The rollback() function in CodeIgniter is used to reverse all database actions since the last commit. It's helpful when you want to discard changes made during a transaction.

Unit testing CodeIgniter applications with heavy database interactions can be challenging due to the need to manage database connections and maintain a clean state between tests. This is crucial to avoid interference between test cases and ensure reliable and accurate results.

The Active Record Class in CodeIgniter handles SQL injection prevention by using parameterized queries. This approach ensures that user inputs are treated as data rather than executable code, making it more difficult for malicious SQL injection attacks to occur. It helps prevent the injection of unauthorized SQL code into database queries, enhancing the security of the application.

To extend the functionalities of an existing CodeIgniter library, you should create a new library that extends the original library class. This way, you can add or override methods to customize the behavior without modifying the core library file. Extending the library class allows you to reuse existing functionality while introducing your modifications.

Cross-Site Request Forgery (CSRF) attacks target the integrity of a web application by forcing the victim to perform unwanted actions without their consent, often leading to actions like changing passwords or making financial transactions.

The log_message() function in CodeIgniter is used to log messages to the system log file. It's a handy way to record information or errors for later analysis, especially in production environments where direct debugging may not be possible.

In a multi-developer environment, managing configurations for third-party libraries is crucial for consistent functionality in CodeIgniter applications. This involves maintaining consistent settings and configurations across different development environments, ensuring that the libraries work uniformly across the team.