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linux/Documentation/process/clang-format.rst
Miguel Ojeda d4ef8d3ff0 clang-format: add configuration file
clang-format is a tool to format C/C++/...  code according to a set of
rules and heuristics.  Like most tools, it is not perfect nor covers
every single case, but it is good enough to be helpful.

In particular, it is useful for quickly re-formatting blocks of code
automatically, for reviewing full files in order to spot coding style
mistakes, typos and possible improvements.  It is also handy for sorting
``#includes``, for aligning variables and macros, for reflowing text and
other similar tasks.  It also serves as a teaching tool/guide for
newcomers.

The tool itself has been already included in the repositories of popular
Linux distributions for a long time.  The rules in this file are
intended for clang-format >= 4, which is easily available in most
distributions.

This commit adds the configuration file that contains the rules that the
tool uses to know how to format the code according to the kernel coding
style.  This gives us several advantages:

  * clang-format works out of the box with reasonable defaults;
    avoiding that everyone has to re-do the configuration.

  * Everyone agrees (eventually) on what is the most useful default
    configuration for most of the kernel.

  * If it becomes commonplace among kernel developers, clang-format
    may feel compelled to support us better. They already recognize
    the Linux kernel and its style in their documentation and in one
    of the style sub-options.

Some of clang-format's features relevant for the kernel are:

  * Uses clang's tooling support behind the scenes to parse and rewrite
    the code. It is not based on ad-hoc regexps.

  * Supports reasonably well the Linux kernel coding style.

  * Fast enough to be used at the press of a key.

  * There are already integrations (either built-in or third-party)
    for many common editors used by kernel developers (e.g. vim,
    emacs, Sublime, Atom...) that allow you to format an entire file
    or, more usefully, just your selection.

  * Able to parse unified diffs -- you can, for instance, reformat
    only the lines changed by a git commit.

  * Able to reflow text comments as well.

  * Widely supported and used by hundreds of developers in highly
    complex projects and organizations (e.g. the LLVM project itself,
    Chromium, WebKit, Google, Mozilla...). Therefore, it will be
    supported for a long time.

See more information about the tool at:

    https://clang.llvm.org/docs/ClangFormat.html
    https://clang.llvm.org/docs/ClangFormatStyleOptions.html

Link: http://lkml.kernel.org/r/20180318171632.qfkemw3mwbcukth6@gmail.com
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2018-04-11 10:28:35 -07:00

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.. _clangformat:
clang-format
============
``clang-format`` is a tool to format C/C++/... code according to
a set of rules and heuristics. Like most tools, it is not perfect
nor covers every single case, but it is good enough to be helpful.
``clang-format`` can be used for several purposes:
- Quickly reformat a block of code to the kernel style. Specially useful
when moving code around and aligning/sorting. See clangformatreformat_.
- Spot style mistakes, typos and possible improvements in files
you maintain, patches you review, diffs, etc. See clangformatreview_.
- Help you follow the coding style rules, specially useful for those
new to kernel development or working at the same time in several
projects with different coding styles.
Its configuration file is ``.clang-format`` in the root of the kernel tree.
The rules contained there try to approximate the most common kernel
coding style. They also try to follow :ref:`Documentation/process/coding-style.rst <codingstyle>`
as much as possible. Since not all the kernel follows the same style,
it is possible that you may want to tweak the defaults for a particular
subsystem or folder. To do so, you can override the defaults by writing
another ``.clang-format`` file in a subfolder.
The tool itself has already been included in the repositories of popular
Linux distributions for a long time. Search for ``clang-format`` in
your repositories. Otherwise, you can either download pre-built
LLVM/clang binaries or build the source code from:
http://releases.llvm.org/download.html
See more information about the tool at:
https://clang.llvm.org/docs/ClangFormat.html
https://clang.llvm.org/docs/ClangFormatStyleOptions.html
.. _clangformatreview:
Review files and patches for coding style
-----------------------------------------
By running the tool in its inline mode, you can review full subsystems,
folders or individual files for code style mistakes, typos or improvements.
To do so, you can run something like::
# Make sure your working directory is clean!
clang-format -i kernel/*.[ch]
And then take a look at the git diff.
Counting the lines of such a diff is also useful for improving/tweaking
the style options in the configuration file; as well as testing new
``clang-format`` features/versions.
``clang-format`` also supports reading unified diffs, so you can review
patches and git diffs easily. See the documentation at:
https://clang.llvm.org/docs/ClangFormat.html#script-for-patch-reformatting
To avoid ``clang-format`` formatting some portion of a file, you can do::
int formatted_code;
// clang-format off
void unformatted_code ;
// clang-format on
void formatted_code_again;
While it might be tempting to use this to keep a file always in sync with
``clang-format``, specially if you are writing new files or if you are
a maintainer, please note that people might be running different
``clang-format`` versions or not have it available at all. Therefore,
you should probably refrain yourself from using this in kernel sources;
at least until we see if ``clang-format`` becomes commonplace.
.. _clangformatreformat:
Reformatting blocks of code
---------------------------
By using an integration with your text editor, you can reformat arbitrary
blocks (selections) of code with a single keystroke. This is specially
useful when moving code around, for complex code that is deeply intended,
for multi-line macros (and aligning their backslashes), etc.
Remember that you can always tweak the changes afterwards in those cases
where the tool did not do an optimal job. But as a first approximation,
it can be very useful.
There are integrations for many popular text editors. For some of them,
like vim, emacs, BBEdit and Visual Studio you can find support built-in.
For instructions, read the appropiate section at:
https://clang.llvm.org/docs/ClangFormat.html
For Atom, Eclipse, Sublime Text, Visual Studio Code, XCode and other
editors and IDEs you should be able to find ready-to-use plugins.
For this use case, consider using a secondary ``.clang-format``
so that you can tweak a few options. See clangformatextra_.
.. _clangformatmissing:
Missing support
---------------
``clang-format`` is missing support for some things that are common
in kernel code. They are easy to remember, so if you use the tool
regularly, you will quickly learn to avoid/ignore those.
In particular, some very common ones you will notice are:
- Aligned blocks of one-line ``#defines``, e.g.::
#define TRACING_MAP_BITS_DEFAULT 11
#define TRACING_MAP_BITS_MAX 17
#define TRACING_MAP_BITS_MIN 7
vs.::
#define TRACING_MAP_BITS_DEFAULT 11
#define TRACING_MAP_BITS_MAX 17
#define TRACING_MAP_BITS_MIN 7
- Aligned designated initializers, e.g.::
static const struct file_operations uprobe_events_ops = {
.owner = THIS_MODULE,
.open = probes_open,
.read = seq_read,
.llseek = seq_lseek,
.release = seq_release,
.write = probes_write,
};
vs.::
static const struct file_operations uprobe_events_ops = {
.owner = THIS_MODULE,
.open = probes_open,
.read = seq_read,
.llseek = seq_lseek,
.release = seq_release,
.write = probes_write,
};
.. _clangformatextra:
Extra features/options
----------------------
Some features/style options are not enabled by default in the configuration
file in order to minimize the differences between the output and the current
code. In other words, to make the difference as small as possible,
which makes reviewing full-file style, as well diffs and patches as easy
as possible.
In other cases (e.g. particular subsystems/folders/files), the kernel style
might be different and enabling some of these options may approximate
better the style there.
For instance:
- Aligning assignments (``AlignConsecutiveAssignments``).
- Aligning declarations (``AlignConsecutiveDeclarations``).
- Reflowing text in comments (``ReflowComments``).
- Sorting ``#includes`` (``SortIncludes``).
They are typically useful for block re-formatting, rather than full-file.
You might want to create another ``.clang-format`` file and use that one
from your editor/IDE instead.