Tips : Linux kernel coding rules
Friday, 01 December 2017
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Écrit par
Grégory Soutadé

Tux, the Linux mascot

For some projects I have to work inside Linux kernel code and, as a developer, I have my own preferences for coding rules (not stable for variable/function naming in facts...) . Especially, I prefer 4 spaces for indentation, curly brackets at a newline. But, the kernel is full of narcissists dictatorscoders and a strict set of coding rules has been determined some years ago. It can be found in Documentation/CodingStyle. Linux is a big project with thousands of people working on it, so I agree that it requires some code normalization for all contribution. Even if you don't plan to verse your patches into upstream, it's good to follow these rules. Here is some tips to comply with it.

First, as an emacs user I have my own rules in my ~/.emacs configuration file. But, when I work on Linux kernel, I want "linux" rules to be applied. A tip from emacswiki allows to automatically switch when a file with "linux" in its path name is found :

(defun maybe-linux-style ()
  (when (and buffer-file-name
         (string-match "linux" buffer-file-name))
    (c-set-style "Linux")
    (c-set-indentation-style "linux")
    (indent-tabs-mode t)
    ))
(add-hook 'c-mode-hook 'maybe-linux-style)
(add-hook 'before-save-hook 'delete-trailing-whitespace)

Another tip I use is a modified pre-commit hook that will checks my modifications before validate the commit. Edit you .git/hooks/pre-commit with the following lines :

#!/bin/sh
#
# An example hook script to verify what is about to be committed.
# Called by "git commit" with no arguments.  The hook should
# exit with non-zero status after issuing an appropriate message if
# it wants to stop the commit.
#
# To enable this hook, rename this file to "pre-commit".

temp_file=`mktemp`
files=`git diff --name-only`
to_diff=""
for file in ${files} ; do
    # Filter .c and .h files
    echo ${file} | grep ".h$" > /dev/null 2>&1
    if [ $? -ne 0 ] ; then
    echo ${file} | grep ".c$" > /dev/null 2>&1
    if [ $? -ne 0 ] ; then
        continue
    fi
    fi
    # Add not deleted file
    if [ -f ${file} ] ; then
    to_diff="${to_diff} ${file}"
    fi
done
git diff --no-color -u --summary ${to_diff} > ${temp_file}
./scripts/checkpatch.pl --no-signoff --min-conf-desc-length=0 --no-summary --mailback ${temp_file}
ret=$?
rm -f ${temp_file}
exit $ret

Don't forget chmod +x .git/hooks/pre-commit

Unfortunately, this hook can't be stored into the central repository and have to be copied each time you clone it (or install a server hook). If you want to bypass it (for some reasons), just commit with "--no-verify" option.

Finally, when you already have a custom codebase ready and commited, you can use some scripts provided by the kernel team to check for coding rules. The first is scripts/Lindent that will indent your file following the kernel coding rules via indent util (needs to be installed). The second is scripts/cleanfile which remove unnecessary whitespaces. The third is scripts/checkpatch.pl -f that will checks the whole file (and not just a patch).

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