How to use ctags in Emacs effectively

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Exuberant Ctags is a code navigation tool. It supports many language and could be integrated into Emacs well.

Please read EmacsWiki for basic usage.

I will talk about how I manage my ctags.

Basically ctags will produce a index file with file name TAGS. The full path of TAGS will be stored in a global list "tags-table-list".

An example of tags-table-list:

(setq tags-table-list '("~/wxWidgets-master/TAGS" "~/projs/Loris/src/desktop/TAGS"))

Every time we you "M-x find-tag", the TAGS file in above list will be read from the scratch to locate the definition of the symbol under cursor.

Here is my strategy to manage TAGS automatically:

  • I hard coded full path of TAGS in .emacs because I usually don't change project path.
  • In major mode hook like c++-mode-hook or js2-mode-hook I will check the directory path of current file. If it contains certain string, I suppose the file belong to certain project.
  • Then I will create TAGS for that project if needed
  • Every time when I save the file, I may update TAGS according to the value of tags-table-list.

Here is the code:

(defun my-project-name-contains-substring (REGEX)
  (let ((dir (if (buffer-file-name)
                 (file-name-directory (buffer-file-name))
    (string-match-p REGEX dir)))

(defun my-create-tags-if-needed (SRC-DIR &optional FORCE)
  "return the full path of tags file"
  (let ((dir (file-name-as-directory (file-truename SRC-DIR)) )
    (setq file (concat dir "TAGS"))
    (when (or FORCE (not (file-exists-p file)))
      (message "Creating TAGS in %s ..." dir)
       (format "ctags -f %s -e -R %s" file dir))

(defvar my-tags-updated-time nil)

(defun my-update-tags ()
  "check the tags in tags-table-list and re-create it"
  (dolist (tag tags-table-list)
    (my-create-tags-if-needed (file-name-directory tag) t)

(defun my-auto-update-tags-when-save ()
   ((not my-tags-updated-time)
    (setq my-tags-updated-time (current-time)))
   ((< (- (float-time (current-time)) (float-time my-tags-updated-time)) 300)
    ;; < 300 seconds
    ;; do nothing
    (setq my-tags-updated-time (current-time))
    (message "updated tags after %d seconds." (- (float-time (current-time))  (float-time my-tags-updated-time)))

(defun my-setup-develop-environment ()
    (when (my-project-name-contains-substring "Loris")
       ((my-project-name-contains-substring "src/desktop")
        ;; C++ project don't need html tags
        (setq tags-table-list (list
                                (concat (file-name-as-directory (getenv "WXWIN")) "include"))
                               (my-create-tags-if-needed "~/projs/Loris/loris/src/desktop")))
       ((my-project-name-contains-substring "src/html")
        ;; html project donot need C++ tags
        (setq tags-table-list (list (my-create-tags-if-needed "~/projs/Loris/loris/src/html")))

(add-hook 'after-save-hook 'my-auto-update-tags-when-save)
(add-hook 'js2-mode-hook 'my-setup-develop-environment)
(add-hook 'web-mode-hook 'my-setup-develop-environment)
(add-hook 'c++-mode-hook 'my-setup-develop-environment)
(add-hook 'c-mode-hook 'my-setup-develop-environment)

UPDATE: There is some discussion at Google Plus about using ctags. Kaushal Modi recommended three emacs plugins:

  1. ctags-update
  2. etags-table
  3. etags-select

I tried these three plugins. ctags-update and etags-table duplicate my above elisp code. I prefer my own code because it's simpler and totally controllable. For example, the fact that I need only care about only one global variable tags-table-list makes my code shorter.

But I do like etags-select, it provide better UI for finding tag and I will use it from now on.

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