pre_systemCalled very early during system execution. Only the benchmark and hooks class have been loaded at this point. No routing or other processes have happened.
pre_controllerCalled immediately prior to any of your controllers being called. All base classes, routing, and security checks have been done.
post_controller_constructorCalled immediately after your controller is instantiated, but prior to any method calls happening.
post_controllerCalled immediately after your controller is fully executed.
display_overrideOverrides the _display() method, used to send the finalized page to the web browser at the end of system execution. This permits you to use your own display methodology. Note that you will need to reference the CI super-object with $this->CI =& get_instance() and then the finalized data will be available by calling $this->CI->output->get_output().
cache_overrideEnables you to call your own method instead of the _display_cache() method in the Output Library. This permits you to use your own cache display mechanism.
post_systemCalled after the final rendered page is sent to the browser, at the end of system execution after the finalized data is sent to the browser.