Adding error message to global container

hi Folks

I am new to CRM , need to validate a input in DO_VALIDATE_INPUT method and if it fails need to add messsage to global container so that next standard method on_save can read it from global container and throw it to user. please suggest the best possible way for this

here is the copy of the standard on_save method where SAP reads the error message . i need to add my message in do_validate_input which is called before this on_save method