Linux Debugging has some Emacs-specific debugging tips.

Syntax-error Highlighting

Ninja users get in-line highlighting of syntax errors using flymake.el on each buffer-save:

(load-file "src/tools/emacs/flymake-chromium.el")

ycmd (YouCompleteMe) + flycheck

emacs-ycmd in combination with flycheck provides:

  • advanced code completion
  • syntax checking
  • navigation to declarations and definitions (using ycmd-goto) based on on-the-fly processing using clang. A quick demo video showing code completion and flycheck highlighting a missing semicolon syntax error:

video preview


  • Your build system is set up for building with clang or wrapper+clang


  1. Clone, update external git repositories and build.sh ycmd from https://github.com/ycm-core/YouCompleteMe into a directory, e.g. ~/dev/ycmd
    • The ycmd packaging changed, the server is in a different location. To build it, run:
     cd ~/dev/ycmd
     git submodule update --init --recursive
     python3 install.py --clang-completer
  2. Test ycmd by running ~/dev/ycmd$ python third_party/ycmd/__main__.py --options_file /tmp/test.json. The options file can be created with e.g. echo '{"hmac_secret": "1234567812345678"}' > /tmp/test.json. The server should start normally.
  3. Install the following packages to emacs, for example from melpa:
    • ycmd
    • company-ycmd
    • flycheck-ycmd
  4. More info on configuring emacs-ycmd
    1. Assuming your checkout of Chromium is in ~/dev/blink, i.e. this is the directory in which you find the srcfolder, create a symbolic link as follows:

      cd ~/dev/blink
      ln -s src/tools/vim/chromium.ycm_extra_conf.py .ycm_extra_conf.py
    2. Add something like the following to your init.el

;; ycmd

;;; Googlers can replace a lot of this with (require 'google-ycmd).

(require 'ycmd)
(require 'company-ycmd)
(require 'flycheck-ycmd)


;; Show completions after 0.15 seconds
(setq company-idle-delay 0.15)

;; Activate for editing C++ files
(add-hook 'c++-mode-hook 'ycmd-mode)
(add-hook 'c++-mode-hook 'company-mode)
(add-hook 'c++-mode-hook 'flycheck-mode)

;; Replace the directory information with where you downloaded ycmd to
(set-variable 'ycmd-server-command (list "python3" (substitute-in-file-name "$HOME/dev/ycmd/third_party/ycmd/ycmd/__main__.py")))

;; Edit according to where you have your Chromium/Blink checkout
(add-to-list 'ycmd-extra-conf-whitelist (substitute-in-file-name "$HOME/dev/blink/.ycm_extra_conf.py"))

;; Show flycheck errors in idle-mode as well
(setq ycmd-parse-conditions '(save new-line mode-enabled idle-change))

;; Makes emacs-ycmd less verbose
(setq url-show-status nil)


  • If no completions show up or emacs reports errors, you can check the *ycmd-server* buffer for errors. See the next bullet point for how to handle “OS Error: No such file or directory”
  • Launching emacs from an OS menu might result in a different environment so that ycmd does not find ninja. In that case, you can use a package like exec-path from shell and add the following to your init.el:
(require 'exec-path-from-shell)
(when (memq window-system '(mac ns x))
  • ycmd depends on the installed version of python. For googlers running gLinux, a system update may have changed the python version. In this case, you need to recompile ycmd first.


There's a builtin function called ff-get-other-file which will get the “other file” based on file extension. I have this bound to C-o in c-mode ((local-set-key "\C-o" 'ff-get-other-file)). While “other file” is per-mode defined, in c-like languages it means jumping between the header and the source file. So I switch back and forth between the header and the source with C-o. If we had separate include/ and src/ directories, this would be a pain to setup, but this might just work out of the box for you. See the documentation for the variable cc-other-file-alist for more information.

One drawback of ff-get-other-file is that it will always switch to a matching buffer, even if the other file is in a different directory, so if you have A.cc,A.h,A.cc(2) then ff-get-other-file will switch to A.h from A.cc(2) rather than load A.h(2) from the appropriate directory. If you prefer something (C specific) that always finds, try this:

(defun cc-other-file()
  "Toggles source/header file"
  (let ((buf (current-buffer))
        (name (file-name-sans-extension (buffer-file-name)))
	 (cadr (assoc (concat "\\."
			      (file-name-extension (buffer-file-name))
    (dolist (e other-extens)
      (if (let ((f (concat name e)))
	    (and (file-exists-p f) (find-file f)))

Note: if you know an easy way to change the ff-get-other-file behavior, please replace this hack with that solution! - stevenjb@chromium.org

Use Google's C++ style!

We have an emacs module, google-c-style.el, which adds c-mode formatting. Then add to your .emacs:

(load "/<path/to/chromium>/src/third_party/clang-format/script/clang-format.el")
(add-hook 'c-mode-common-hook
    (function (lambda () (local-set-key (kbd "TAB") 'clang-format-region))))

Now, you can use the


key to format the current line (even a long line) or region.

BUILD.gn and .gni files

BUILD.gn and gni files are supported by the official gn-mode which can be enabled with:

(load-file "src/tools/emacs/gn.el")

Mojom IDL (.mojom files)

Mojom files are supported by mojom-mode, which can be enabled with:

(load-file "/<path/to/chromium>/src/tools/emacs/mojom-mode.el")

Alternatively, you can require the feature. (See emacs documentation on Named Features.)

(add-to-list 'load-path "/<path/to/chromium>/src/tools/emacs/")
(require 'mojom-mode)

Highlight long lines

One nice way to highlight long lines and other style issues:

(require 'whitespace)
(setq whitespace-style '(face indentation trailing empty lines-tail))
(setq whitespace-line-column nil)
(set-face-attribute 'whitespace-line nil
                    :background "purple"
                    :foreground "white"
                    :weight 'bold)
(global-whitespace-mode 1)

Note: You might need to grab the latest version of whitespace.el.

deep nesting

A couple of helpers that show a summary of where you are; the first by tracing the indentation hierarchy upwards, the second by only showing #ifs and #elses that are relevant to the current line:

(defun ami-summarize-indentation-at-point ()
  "Echo a summary of how one gets from the left-most column to
  POINT in terms of indentation changes."
    (let ((cur-indent most-positive-fixnum)
          (trace '()))
      (while (not (bobp))
        (let ((current-line (buffer-substring (line-beginning-position)
          (when (and (not (string-match "^\\s-*$" current-line))
                     (< (current-indentation) cur-indent))
            (setq cur-indent (current-indentation))
            (setq trace (cons current-line trace))
            (if (or (string-match "^\\s-*}" current-line)
                    (string-match "^\\s-*else " current-line)
                    (string-match "^\\s-*elif " current-line))
                (setq cur-indent (1+ cur-indent)))))
        (forward-line -1))
      (message "%s" (mapconcat 'identity trace "\n")))))

(require 'cl)
  (defun ami-summarize-preprocessor-branches-at-point ()
    "Summarize the C preprocessor branches needed to get to point."
    (flet ((current-line-text ()
              (buffer-substring (line-beginning-position) (line-end-position))))
        (let ((eol (or (end-of-line) (point)))
              deactivate-mark directives-stack)
          (goto-char (point-min))
          (while (re-search-forward "^#\\(if\\|else\\|endif\\)" eol t)
            (if (or (string-prefix-p "#if" (match-string 0))
                    (string-prefix-p "#else" (match-string 0)))
                (push (current-line-text) directives-stack)
            (if (string-prefix-p "#endif" (match-string 0))
                 (while (string-prefix-p "#else" (pop directives-stack)) t))))
          (message "%s" (mapconcat 'identity (reverse directives-stack) "\n"))))))


erg wrote a suite of tools that do common operations from the root of your repository, called Find Things Fast. It contains ido completion over git ls-files (or the svn find equivalent) and grepsource that only git greps files with extensions we care about (or the equivalent the find | xargs grep statement in non-git repos.)

vc-mode and find-file performance

When you first open a file under git control, vc mode kicks in and does a high level stat of your git repo. For huge repos, especially WebKit and Chromium, this makes opening a file take literally seconds. This snippet disables VC git for chrome directories:

; Turn off VC git for chrome
(when (locate-library "vc")
(defadvice vc-registered (around nochrome-vc-registered (file))
(message (format "nochrome-vc-registered %s" file))
(if (string-match ".*chrome/src.*" file)
(message (format "Skipping VC mode for %s" % file))
(setq ad-return-value nil)
(ad-activate 'vc-registered)

git tools

We're collecting Chrome-specific tools under tools/emacs. See the files there for details.

  • trybot.el: import Windows trybot output into a compilation-mode buffer.


See ErcIrc.

Compilation with M-x compile

It's usually useful to first find the top directory with locate-dominating-file, and run M-x compile

(defun chromium-compile ()
  "Run compile for chromium."
  (let ((chrome-dir (locate-dominating-file "." "chrome/BUILD.gn")))
    (when chrome-dir
      (let* ((default-directory chrome-dir))
        (compile "ninja -C out/Debug chrome chrome_sandbox ")))))