1.. _clangformat:
2
3clang-format
4============
5
6``clang-format`` is a tool to format C/C++/... code according to
7a set of rules and heuristics. Like most tools, it is not perfect
8nor covers every single case, but it is good enough to be helpful.
9
10``clang-format`` can be used for several purposes:
11
12  - Quickly reformat a block of code to the kernel style. Specially useful
13    when moving code around and aligning/sorting. See clangformatreformat_.
14
15  - Spot style mistakes, typos and possible improvements in files
16    you maintain, patches you review, diffs, etc. See clangformatreview_.
17
18  - Help you follow the coding style rules, specially useful for those
19    new to kernel development or working at the same time in several
20    projects with different coding styles.
21
22Its configuration file is ``.clang-format`` in the root of the kernel tree.
23The rules contained there try to approximate the most common kernel
24coding style. They also try to follow :ref:`Documentation/process/coding-style.rst <codingstyle>`
25as much as possible. Since not all the kernel follows the same style,
26it is possible that you may want to tweak the defaults for a particular
27subsystem or folder. To do so, you can override the defaults by writing
28another ``.clang-format`` file in a subfolder.
29
30The tool itself has already been included in the repositories of popular
31Linux distributions for a long time. Search for ``clang-format`` in
32your repositories. Otherwise, you can either download pre-built
33LLVM/clang binaries or build the source code from:
34
35    https://releases.llvm.org/download.html
36
37See more information about the tool at:
38
39    https://clang.llvm.org/docs/ClangFormat.html
40
41    https://clang.llvm.org/docs/ClangFormatStyleOptions.html
42
43
44.. _clangformatreview:
45
46Review files and patches for coding style
47-----------------------------------------
48
49By running the tool in its inline mode, you can review full subsystems,
50folders or individual files for code style mistakes, typos or improvements.
51
52To do so, you can run something like::
53
54    # Make sure your working directory is clean!
55    clang-format -i kernel/*.[ch]
56
57And then take a look at the git diff.
58
59Counting the lines of such a diff is also useful for improving/tweaking
60the style options in the configuration file; as well as testing new
61``clang-format`` features/versions.
62
63``clang-format`` also supports reading unified diffs, so you can review
64patches and git diffs easily. See the documentation at:
65
66    https://clang.llvm.org/docs/ClangFormat.html#script-for-patch-reformatting
67
68To avoid ``clang-format`` formatting some portion of a file, you can do::
69
70    int formatted_code;
71    // clang-format off
72        void    unformatted_code  ;
73    // clang-format on
74    void formatted_code_again;
75
76While it might be tempting to use this to keep a file always in sync with
77``clang-format``, specially if you are writing new files or if you are
78a maintainer, please note that people might be running different
79``clang-format`` versions or not have it available at all. Therefore,
80you should probably refrain yourself from using this in kernel sources;
81at least until we see if ``clang-format`` becomes commonplace.
82
83
84.. _clangformatreformat:
85
86Reformatting blocks of code
87---------------------------
88
89By using an integration with your text editor, you can reformat arbitrary
90blocks (selections) of code with a single keystroke. This is specially
91useful when moving code around, for complex code that is deeply intended,
92for multi-line macros (and aligning their backslashes), etc.
93
94Remember that you can always tweak the changes afterwards in those cases
95where the tool did not do an optimal job. But as a first approximation,
96it can be very useful.
97
98There are integrations for many popular text editors. For some of them,
99like vim, emacs, BBEdit and Visual Studio you can find support built-in.
100For instructions, read the appropriate section at:
101
102    https://clang.llvm.org/docs/ClangFormat.html
103
104For Atom, Eclipse, Sublime Text, Visual Studio Code, XCode and other
105editors and IDEs you should be able to find ready-to-use plugins.
106
107For this use case, consider using a secondary ``.clang-format``
108so that you can tweak a few options. See clangformatextra_.
109
110
111.. _clangformatmissing:
112
113Missing support
114---------------
115
116``clang-format`` is missing support for some things that are common
117in kernel code. They are easy to remember, so if you use the tool
118regularly, you will quickly learn to avoid/ignore those.
119
120In particular, some very common ones you will notice are:
121
122  - Aligned blocks of one-line ``#defines``, e.g.::
123
124        #define TRACING_MAP_BITS_DEFAULT       11
125        #define TRACING_MAP_BITS_MAX           17
126        #define TRACING_MAP_BITS_MIN           7
127
128    vs.::
129
130        #define TRACING_MAP_BITS_DEFAULT 11
131        #define TRACING_MAP_BITS_MAX 17
132        #define TRACING_MAP_BITS_MIN 7
133
134  - Aligned designated initializers, e.g.::
135
136        static const struct file_operations uprobe_events_ops = {
137                .owner          = THIS_MODULE,
138                .open           = probes_open,
139                .read           = seq_read,
140                .llseek         = seq_lseek,
141                .release        = seq_release,
142                .write          = probes_write,
143        };
144
145    vs.::
146
147        static const struct file_operations uprobe_events_ops = {
148                .owner = THIS_MODULE,
149                .open = probes_open,
150                .read = seq_read,
151                .llseek = seq_lseek,
152                .release = seq_release,
153                .write = probes_write,
154        };
155
156
157.. _clangformatextra:
158
159Extra features/options
160----------------------
161
162Some features/style options are not enabled by default in the configuration
163file in order to minimize the differences between the output and the current
164code. In other words, to make the difference as small as possible,
165which makes reviewing full-file style, as well diffs and patches as easy
166as possible.
167
168In other cases (e.g. particular subsystems/folders/files), the kernel style
169might be different and enabling some of these options may approximate
170better the style there.
171
172For instance:
173
174  - Aligning assignments (``AlignConsecutiveAssignments``).
175
176  - Aligning declarations (``AlignConsecutiveDeclarations``).
177
178  - Reflowing text in comments (``ReflowComments``).
179
180  - Sorting ``#includes`` (``SortIncludes``).
181
182They are typically useful for block re-formatting, rather than full-file.
183You might want to create another ``.clang-format`` file and use that one
184from your editor/IDE instead.
185