(require 'auto-complete)
(require 'auto-complete-config)
(ac-config-default)

;; Function which initializes auto-complete-c-headers and gets called for
;; c/c++ hooks
(defun my:ac-c-header-init ()
  (add-to-list 'ac-sources 'ac-source-c-headers)

;; List of directories where c/c++ headers are located. This list
;; may change from machine to machine. Type the following command
;; in a terminal to see your list:
;;
;;   >> gcc -xc++ -E -v -
;;
;; and add whatever paths come after '#include <...> search starts here:'.
;;
  (setq achead:include-directories
    (append '("/usr/include/c++/5"
              "/usr/include/x86_64-linux-gnu/c++/5"
              "/usr/include/c++/5/backward"
              "/usr/lib/gcc/x86_64-linux-gnu/5/include"
              "/usr/local/include"
              "/usr/lib/gcc/x86_64-linux-gnu/5/include-fixed"
              "/usr/include/x86_64-linux-gnu")
              achead:include-directories)))

;; Call this function from c/c++ hooks
(add-hook 'c++-mode-hook 'my:ac-c-header-init)
(add-hook 'c-mode-hook 'my:ac-c-header-init)