Emacs and Flymake Syntax-Checking

Emacs is a great tool for software development. If you’re not using a more advanced environment than Notepad and a Terminal it may be time for you to look at options. Two popular ones are Emacs and Vim. Depending on who you ask you will get widely varying responses about which is “better”. Eclipse, Visual Studio and Xcode are all popular options as well. Now to get to the point. Compiling code takes time and having a built-in syntax check is an excellent way to reduce time spent compiling. For Emacs there is Flymake and Flycheck. As you can probably guess, I will discuss Flymake.

Flymake and Flycheck are both options for syntax checking in Emacs. I use Flymake and am very happy with it so far. It is packaged with Emacs 24 but is rather dated now. Flycheck is newer and as far as I know still maintained. Notice that so far I haven’t specified which language we will be syntax checking? This is because Flymake supports a few by default and more via some extensions. Flycheck supports 30, so if Flymake doesn’t support your language of choice you should stop reading now and learn to use Flycheck. I program primarily in C++ and also frequently in python, both of which Flymake supports.

Let’s get started with python. If you already have Flymake installed, then install flymake-easy and flymake-python-pyflakes using M-x list-packages. You will also need to install pyflakes using pip install pyflakes. In case you didn’t know, pip is essentially a module/package manager for python. Now open your Emacs init file (generally ~/.emacs) and add the following lines of code to the end.

;; Load Flymake
(require 'flymake)
;; Load Flymake-python
(require 'flymake-python-pyflakes)
;; Activate it each time python mode is turned on
(add-hook 'python-mode-hook 'flymake-python-pyflakes-load)

If Emacs is having issues with finding Flymake you will need to add the appropriate folders to the load-path. In my case I had to add the following before the above require lines.

(add-to-list 'load-path "~/.emacs.d/elpa/flymake-python-pyflakes-0.9/")
(add-to-list 'load-path "~/.emacs.d/elpa/flymake-easy-0.10/")

If you are familiar with python I recommend opening up a file and testing it out to make sure everything works correctly. If it does we can move on to syntax checking C++.

The above setup will almost work for C++ code. Before delving into setting up Makefiles for your projects I will share my Flymake settings with you. I like to have the option to go to the next and previous error. To include this add the following to your init file:

;; Control-c n    next error
(global-set-key (kbd "C-c n") 'flymake-goto-next-error)
;; Control-c p    previous error
(global-set-key (kbd "C-c p") 'flymake-goto-prev-error)

To run Flymake when a file is opened add

;; Run flymake when we open a file
(add-hook 'find-file-hook 'flymake-find-file-hook)

This can be annoying for projects where you do not have Flymake setup because you will receive a warning on load. I also like having Flymake underline the line with an error or warning. This means adding

;; Underline warnings and errors from Flymake
(custom-set-faces
 '(flymake-errline ((((class color)) (:underline "red"))))
 '(flymake-warnline ((((class color)) (:underline "yellow")))))

By default Flymake shows the error in a tooltip when you hoover your mouse over the line. To instead have the error display in the mini buffer when your cursor is on the line add:

;; Display error and warning messages in minibuffer.
(custom-set-variables
 '(help-at-pt-timer-delay 0.5)
 '(help-at-pt-display-when-idle '(flymake-overlay)))

We can now move on to the fun part: adding the syntax-check to the Makefile of the project. I’m assuming you are familiar with how Makefiles work. If not, give it a quick Google to get the basics and then we can carry on. Once you have your Makefile set up you will want to add the following:

check-syntax:
	g++ -o nul -S ${CHK_SOURCES}

You can change the compiler you want to use. On my system g++ defaults to clang. If you’re compiling with MPI you’ll want to use mpic++ or whichever MPI wrapper you’re using. You can add flags too. For example, if you want C++11 support, specify the -std=c++11 flag.

The above will work if you have a simple build system. For more complex build systems where the paths of header files are referenced from a code “home” directory it will fail. I have managed to successfully deal with this by using the following syntax check instead.

# Used for flymake in eMacs
$(CODE_HOME)=/path/to/code/home
check-syntax:
	g++ -I$(CODE_HOME) -std=c++11 -s -o nul -S $(CHK_SOURCES)

Note that $(CODE_HOME) needs to be specified giving the absolute path to where your header files are referenced from.

I hope you found this useful! Any suggestions or questions are welcome in the comments below.