如何在不扩展CI_Controller的情况下访问CodeIgniter函数

I'm trying to write an external library which has functions commonly used among various classes.

Currently I'm trying to write a log message function. The problem is I need access to the session library and a model. How do I access them without extending from CI_Controller? Any workaround?

Here's my code:

Common_functions library:

public function _send_message($message, $log_to_db=TRUE)
{
     $this->session->set_userdata("message", $message);
     if($log_to_db) $this->User_log_model->log_mesage($message);
}

Usage in other classes example:

public function new_user()
{
    $this->_set_validation_rules();

    if($this->form_validation->run())
    {
        if($user_id = $this->User_model->insert($this->_prepare_new_user_array()))
        {
            $this->common_functions->_send_message("New User created successfully. | user_id: " . $user_id);
        }
        else {
            $this->common_functions->_send_message("Unable to create new User record.");
        }
    }
}

Managed to solve it. Just moved my log message functions to the User_log_model instead.