How to programmatically create/update a TAGS file with emacs?

Question

Is there any plugin for emacs to automatically update the TAGS file in my C project (for example on buffer save or access) or create a new one if there is no TAGS file present?

Answer 1

This might get you close (untested):

(defvar my-auto-update-tags-alist
  (list '("/some/path/to/TAGS" "command_to_build_tags")
        '("/another/path/to/TAGS" "another_build_command")))

(defun my-auto-update-tags ()
  "Automatically update TAGS files"
  (tags-table-check-computed-list)
  (let ((filename (buffer-file-name))
        build-cmd)
    (mapc (lambda (tag-file)
            (set-buffer tag-file)
            (when (member filename (tags-table-files))
              (setq build-cmd (cdr (assoc tag-file my-auto-update-tags-alist)))
              (when build-cmd
                (call-process build-cmd nil 0))))
          tags-table-computed-list)))

(add-hook 'after-save-hook 'my-auto-update-tags)

It will only work (did I mention it's untested?) on files that are in TAGS files already. If you add a new file you'd have to regenerate the TAGS file the first time yourself. The call-process part should work asynchronously, so it might be a few moments until the TAGS file is actually rebuilt (if this even works ;)