I got these lines:
;; turn on semantic
(semantic-mode 1)
;; define a function which adds semantic as backup to auto-complete
(defun my:add-semantic-to-autocomplete()
(add-to-list 'ac-sources 'ac-sources-semantic))
(add-hook 'c-mode-common-hook 'my:add-semantic-to-autocomplete)
From this video here.But for me it does not work at all.Save me please,i lost all day fighting with it...