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Mauro Carvalho Chehab609d99a2016-09-19 08:07:56 -03001.. _codingstyle:
2
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03003Linux kernel coding style
4=========================
Linus Torvalds1da177e2005-04-16 15:20:36 -07005
6This is a short document describing the preferred coding style for the
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -03007linux kernel. Coding style is very personal, and I won't **force** my
Linus Torvalds1da177e2005-04-16 15:20:36 -07008views on anybody, but this is what goes for anything that I have to be
9able to maintain, and I'd prefer it for most other things too. Please
10at least consider the points made here.
11
12First off, I'd suggest printing out a copy of the GNU coding standards,
13and NOT read it. Burn them, it's a great symbolic gesture.
14
15Anyway, here goes:
16
17
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300181) Indentation
19--------------
Linus Torvalds1da177e2005-04-16 15:20:36 -070020
21Tabs are 8 characters, and thus indentations are also 8 characters.
22There are heretic movements that try to make indentations 4 (or even 2!)
23characters deep, and that is akin to trying to define the value of PI to
24be 3.
25
26Rationale: The whole idea behind indentation is to clearly define where
27a block of control starts and ends. Especially when you've been looking
28at your screen for 20 straight hours, you'll find it a lot easier to see
29how the indentation works if you have large indentations.
30
31Now, some people will claim that having 8-character indentations makes
32the code move too far to the right, and makes it hard to read on a
3380-character terminal screen. The answer to that is that if you need
34more than 3 levels of indentation, you're screwed anyway, and should fix
35your program.
36
37In short, 8-char indents make things easier to read, and have the added
38benefit of warning you when you're nesting your functions too deep.
39Heed that warning.
40
Randy Dunlapb3fc9942006-12-10 02:18:56 -080041The preferred way to ease multiple indentation levels in a switch statement is
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -030042to align the ``switch`` and its subordinate ``case`` labels in the same column
43instead of ``double-indenting`` the ``case`` labels. E.g.:
Randy Dunlapb3fc9942006-12-10 02:18:56 -080044
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -030045.. code-block:: c
46
Randy Dunlapb3fc9942006-12-10 02:18:56 -080047 switch (suffix) {
48 case 'G':
49 case 'g':
50 mem <<= 30;
51 break;
52 case 'M':
53 case 'm':
54 mem <<= 20;
55 break;
56 case 'K':
57 case 'k':
58 mem <<= 10;
Joe Perchesb9918bd2019-10-05 09:46:43 -070059 fallthrough;
Randy Dunlapb3fc9942006-12-10 02:18:56 -080060 default:
61 break;
62 }
63
Linus Torvalds1da177e2005-04-16 15:20:36 -070064Don't put multiple statements on a single line unless you have
65something to hide:
66
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -030067.. code-block:: c
68
Linus Torvalds1da177e2005-04-16 15:20:36 -070069 if (condition) do_this;
70 do_something_everytime;
71
Joe Perches26606ce2020-08-24 21:55:58 -070072Don't use commas to avoid using braces:
73
74.. code-block:: c
75
76 if (condition)
77 do_this(), do_that();
78
79Always uses braces for multiple statements:
80
81.. code-block:: c
82
83 if (condition) {
84 do_this();
85 do_that();
86 }
87
Randy Dunlapb3fc9942006-12-10 02:18:56 -080088Don't put multiple assignments on a single line either. Kernel coding style
89is super simple. Avoid tricky expressions.
90
Joe Perches26606ce2020-08-24 21:55:58 -070091
Linus Torvalds1da177e2005-04-16 15:20:36 -070092Outside of comments, documentation and except in Kconfig, spaces are never
93used for indentation, and the above example is deliberately broken.
94
95Get a decent editor and don't leave whitespace at the end of lines.
96
97
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300982) Breaking long lines and strings
99----------------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700100
101Coding style is all about readability and maintainability using commonly
102available tools.
103
Joe Perchesbdc48fa2020-05-29 16:12:21 -0700104The preferred limit on the length of a single line is 80 columns.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700105
Joe Perchesbdc48fa2020-05-29 16:12:21 -0700106Statements longer than 80 columns should be broken into sensible chunks,
107unless exceeding 80 columns significantly increases readability and does
108not hide information.
109
Geert Uytterhoeven77d22a42020-06-01 12:00:49 +0200110Descendants are always substantially shorter than the parent and
Joe Perchesbdc48fa2020-05-29 16:12:21 -0700111are placed substantially to the right. A very commonly used style
112is to align descendants to a function open parenthesis.
113
114These same rules are applied to function headers with a long argument list.
115
116However, never break user-visible strings such as printk messages because
117that breaks the ability to grep for them.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700118
Linus Torvalds1da177e2005-04-16 15:20:36 -0700119
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001203) Placing Braces and Spaces
121----------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700122
123The other issue that always comes up in C styling is the placement of
124braces. Unlike the indent size, there are few technical reasons to
125choose one placement strategy over the other, but the preferred way, as
126shown to us by the prophets Kernighan and Ritchie, is to put the opening
127brace last on the line, and put the closing brace first, thusly:
128
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300129.. code-block:: c
130
Linus Torvalds1da177e2005-04-16 15:20:36 -0700131 if (x is true) {
132 we do y
133 }
134
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800135This applies to all non-function statement blocks (if, switch, for,
136while, do). E.g.:
137
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300138.. code-block:: c
139
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800140 switch (action) {
141 case KOBJ_ADD:
142 return "add";
143 case KOBJ_REMOVE:
144 return "remove";
145 case KOBJ_CHANGE:
146 return "change";
147 default:
148 return NULL;
149 }
150
Linus Torvalds1da177e2005-04-16 15:20:36 -0700151However, there is one special case, namely functions: they have the
152opening brace at the beginning of the next line, thus:
153
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300154.. code-block:: c
155
Linus Torvalds1da177e2005-04-16 15:20:36 -0700156 int function(int x)
157 {
158 body of function
159 }
160
161Heretic people all over the world have claimed that this inconsistency
162is ... well ... inconsistent, but all right-thinking people know that
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300163(a) K&R are **right** and (b) K&R are right. Besides, functions are
Linus Torvalds1da177e2005-04-16 15:20:36 -0700164special anyway (you can't nest them in C).
165
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300166Note that the closing brace is empty on a line of its own, **except** in
Linus Torvalds1da177e2005-04-16 15:20:36 -0700167the cases where it is followed by a continuation of the same statement,
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300168ie a ``while`` in a do-statement or an ``else`` in an if-statement, like
Linus Torvalds1da177e2005-04-16 15:20:36 -0700169this:
170
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300171.. code-block:: c
172
Linus Torvalds1da177e2005-04-16 15:20:36 -0700173 do {
174 body of do-loop
175 } while (condition);
176
177and
178
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300179.. code-block:: c
180
Linus Torvalds1da177e2005-04-16 15:20:36 -0700181 if (x == y) {
182 ..
183 } else if (x > y) {
184 ...
185 } else {
186 ....
187 }
188
189Rationale: K&R.
190
191Also, note that this brace-placement also minimizes the number of empty
192(or almost empty) lines, without any loss of readability. Thus, as the
193supply of new-lines on your screen is not a renewable resource (think
19425-line terminal screens here), you have more empty lines to put
195comments on.
196
Oliver Neukume659ba42007-05-08 00:30:34 -0700197Do not unnecessarily use braces where a single statement will do.
198
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300199.. code-block:: c
200
Pavel Kretov09677e02015-02-16 20:26:18 +0300201 if (condition)
202 action();
Oliver Neukume659ba42007-05-08 00:30:34 -0700203
Harry Wei38829dc2011-03-22 16:35:01 -0700204and
205
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300206.. code-block:: none
207
Pavel Kretov09677e02015-02-16 20:26:18 +0300208 if (condition)
209 do_this();
210 else
211 do_that();
Harry Wei38829dc2011-03-22 16:35:01 -0700212
Antonio Ospiteb218ab02011-11-04 11:22:19 -0700213This does not apply if only one branch of a conditional statement is a single
214statement; in the latter case use braces in both branches:
Oliver Neukume659ba42007-05-08 00:30:34 -0700215
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300216.. code-block:: c
217
Pavel Kretov09677e02015-02-16 20:26:18 +0300218 if (condition) {
219 do_this();
220 do_that();
221 } else {
222 otherwise();
223 }
Oliver Neukume659ba42007-05-08 00:30:34 -0700224
Gary R Hook1dbba2c2018-03-14 17:21:38 -0500225Also, use braces when a loop contains more than a single simple statement:
226
227.. code-block:: c
228
229 while (condition) {
230 if (test)
231 do_something();
232 }
233
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03002343.1) Spaces
235***********
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800236
237Linux kernel style for use of spaces depends (mostly) on
238function-versus-keyword usage. Use a space after (most) keywords. The
239notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
240somewhat like functions (and are usually used with parentheses in Linux,
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300241although they are not required in the language, as in: ``sizeof info`` after
242``struct fileinfo info;`` is declared).
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800243
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300244So use a space after these keywords::
Pavel Kretov09677e02015-02-16 20:26:18 +0300245
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800246 if, switch, case, for, do, while
Pavel Kretov09677e02015-02-16 20:26:18 +0300247
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800248but not with sizeof, typeof, alignof, or __attribute__. E.g.,
Pavel Kretov09677e02015-02-16 20:26:18 +0300249
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300250.. code-block:: c
251
252
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800253 s = sizeof(struct file);
254
255Do not add spaces around (inside) parenthesized expressions. This example is
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300256**bad**:
257
258.. code-block:: c
259
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800260
261 s = sizeof( struct file );
262
263When declaring pointer data or a function that returns a pointer type, the
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300264preferred use of ``*`` is adjacent to the data name or function name and not
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800265adjacent to the type name. Examples:
266
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300267.. code-block:: c
268
269
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800270 char *linux_banner;
271 unsigned long long memparse(char *ptr, char **retptr);
272 char *match_strdup(substring_t *s);
273
274Use one space around (on each side of) most binary and ternary operators,
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300275such as any of these::
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800276
277 = + - < > * / % | & ^ <= >= == != ? :
278
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300279but no space after unary operators::
Pavel Kretov09677e02015-02-16 20:26:18 +0300280
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800281 & * + - ~ ! sizeof typeof alignof __attribute__ defined
282
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300283no space before the postfix increment & decrement unary operators::
Pavel Kretov09677e02015-02-16 20:26:18 +0300284
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800285 ++ --
286
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300287no space after the prefix increment & decrement unary operators::
Pavel Kretov09677e02015-02-16 20:26:18 +0300288
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800289 ++ --
290
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300291and no space around the ``.`` and ``->`` structure member operators.
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800292
Josh Tripletta923fd62007-07-15 23:41:37 -0700293Do not leave trailing whitespace at the ends of lines. Some editors with
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300294``smart`` indentation will insert whitespace at the beginning of new lines as
Josh Tripletta923fd62007-07-15 23:41:37 -0700295appropriate, so you can start typing the next line of code right away.
296However, some such editors do not remove the whitespace if you end up not
297putting a line of code there, such as if you leave a blank line. As a result,
298you end up with lines containing trailing whitespace.
299
300Git will warn you about patches that introduce trailing whitespace, and can
301optionally strip the trailing whitespace for you; however, if applying a series
302of patches, this may make later patches in the series fail by changing their
303context lines.
304
Linus Torvalds1da177e2005-04-16 15:20:36 -0700305
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03003064) Naming
307---------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700308
Tony Fischettifb0e0ff2020-02-16 19:08:26 -0500309C is a Spartan language, and your naming conventions should follow suit.
310Unlike Modula-2 and Pascal programmers, C programmers do not use cute
311names like ThisVariableIsATemporaryCounter. A C programmer would call that
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300312variable ``tmp``, which is much easier to write, and not the least more
Linus Torvalds1da177e2005-04-16 15:20:36 -0700313difficult to understand.
314
315HOWEVER, while mixed-case names are frowned upon, descriptive names for
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300316global variables are a must. To call a global function ``foo`` is a
Linus Torvalds1da177e2005-04-16 15:20:36 -0700317shooting offense.
318
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300319GLOBAL variables (to be used only if you **really** need them) need to
Linus Torvalds1da177e2005-04-16 15:20:36 -0700320have descriptive names, as do global functions. If you have a function
321that counts the number of active users, you should call that
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300322``count_active_users()`` or similar, you should **not** call it ``cntusr()``.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700323
324Encoding the type of a function into the name (so-called Hungarian
Tony Fischettifb0e0ff2020-02-16 19:08:26 -0500325notation) is asinine - the compiler knows the types anyway and can check
Yorick de Widb7592e52021-02-08 16:04:48 +0100326those, and it only confuses the programmer.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700327
328LOCAL variable names should be short, and to the point. If you have
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300329some random integer loop counter, it should probably be called ``i``.
330Calling it ``loop_counter`` is non-productive, if there is no chance of it
331being mis-understood. Similarly, ``tmp`` can be just about any type of
Linus Torvalds1da177e2005-04-16 15:20:36 -0700332variable that is used to hold a temporary value.
333
334If you are afraid to mix up your local variable names, you have another
335problem, which is called the function-growth-hormone-imbalance syndrome.
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800336See chapter 6 (Functions).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700337
Dan Williamsa5f526e2020-07-03 23:54:35 -0700338For symbol names and documentation, avoid introducing new usage of
339'master / slave' (or 'slave' independent of 'master') and 'blacklist /
340whitelist'.
341
342Recommended replacements for 'master / slave' are:
343 '{primary,main} / {secondary,replica,subordinate}'
344 '{initiator,requester} / {target,responder}'
345 '{controller,host} / {device,worker,proxy}'
346 'leader / follower'
347 'director / performer'
348
349Recommended replacements for 'blacklist/whitelist' are:
350 'denylist / allowlist'
351 'blocklist / passlist'
352
353Exceptions for introducing new usage is to maintain a userspace ABI/API,
354or when updating code for an existing (as of 2020) hardware or protocol
355specification that mandates those terms. For new specifications
356translate specification usage of the terminology to the kernel coding
357standard where possible.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700358
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03003595) Typedefs
360-----------
Randy Dunlap226a6b82006-06-23 02:05:58 -0700361
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300362Please don't use things like ``vps_t``.
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300363It's a **mistake** to use typedef for structures and pointers. When you see a
Randy Dunlap226a6b82006-06-23 02:05:58 -0700364
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300365.. code-block:: c
366
367
Randy Dunlap226a6b82006-06-23 02:05:58 -0700368 vps_t a;
369
370in the source, what does it mean?
Randy Dunlap226a6b82006-06-23 02:05:58 -0700371In contrast, if it says
372
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300373.. code-block:: c
374
Randy Dunlap226a6b82006-06-23 02:05:58 -0700375 struct virtual_container *a;
376
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300377you can actually tell what ``a`` is.
Randy Dunlap226a6b82006-06-23 02:05:58 -0700378
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300379Lots of people think that typedefs ``help readability``. Not so. They are
Randy Dunlap226a6b82006-06-23 02:05:58 -0700380useful only for:
381
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300382 (a) totally opaque objects (where the typedef is actively used to **hide**
Randy Dunlap226a6b82006-06-23 02:05:58 -0700383 what the object is).
384
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300385 Example: ``pte_t`` etc. opaque objects that you can only access using
Randy Dunlap226a6b82006-06-23 02:05:58 -0700386 the proper accessor functions.
387
Mauro Carvalho Chehab3772ec42016-09-19 08:07:47 -0300388 .. note::
389
390 Opaqueness and ``accessor functions`` are not good in themselves.
391 The reason we have them for things like pte_t etc. is that there
392 really is absolutely **zero** portably accessible information there.
Randy Dunlap226a6b82006-06-23 02:05:58 -0700393
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300394 (b) Clear integer types, where the abstraction **helps** avoid confusion
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300395 whether it is ``int`` or ``long``.
Randy Dunlap226a6b82006-06-23 02:05:58 -0700396
397 u8/u16/u32 are perfectly fine typedefs, although they fit into
398 category (d) better than here.
399
Mauro Carvalho Chehab3772ec42016-09-19 08:07:47 -0300400 .. note::
401
402 Again - there needs to be a **reason** for this. If something is
403 ``unsigned long``, then there's no reason to do
Randy Dunlap226a6b82006-06-23 02:05:58 -0700404
405 typedef unsigned long myflags_t;
406
407 but if there is a clear reason for why it under certain circumstances
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300408 might be an ``unsigned int`` and under other configurations might be
409 ``unsigned long``, then by all means go ahead and use a typedef.
Randy Dunlap226a6b82006-06-23 02:05:58 -0700410
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300411 (c) when you use sparse to literally create a **new** type for
Randy Dunlap226a6b82006-06-23 02:05:58 -0700412 type-checking.
413
414 (d) New types which are identical to standard C99 types, in certain
415 exceptional circumstances.
416
417 Although it would only take a short amount of time for the eyes and
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300418 brain to become accustomed to the standard types like ``uint32_t``,
Randy Dunlap226a6b82006-06-23 02:05:58 -0700419 some people object to their use anyway.
420
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300421 Therefore, the Linux-specific ``u8/u16/u32/u64`` types and their
Randy Dunlap226a6b82006-06-23 02:05:58 -0700422 signed equivalents which are identical to standard types are
423 permitted -- although they are not mandatory in new code of your
424 own.
425
426 When editing existing code which already uses one or the other set
427 of types, you should conform to the existing choices in that code.
428
429 (e) Types safe for use in userspace.
430
431 In certain structures which are visible to userspace, we cannot
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300432 require C99 types and cannot use the ``u32`` form above. Thus, we
Randy Dunlap226a6b82006-06-23 02:05:58 -0700433 use __u32 and similar types in all structures which are shared
434 with userspace.
435
436Maybe there are other cases too, but the rule should basically be to NEVER
437EVER use a typedef unless you can clearly match one of those rules.
438
439In general, a pointer, or a struct that has elements that can reasonably
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300440be directly accessed should **never** be a typedef.
Randy Dunlap226a6b82006-06-23 02:05:58 -0700441
442
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03004436) Functions
444------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700445
446Functions should be short and sweet, and do just one thing. They should
447fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
448as we all know), and do one thing and do that well.
449
450The maximum length of a function is inversely proportional to the
451complexity and indentation level of that function. So, if you have a
452conceptually simple function that is just one long (but simple)
453case-statement, where you have to do lots of small things for a lot of
454different cases, it's OK to have a longer function.
455
456However, if you have a complex function, and you suspect that a
457less-than-gifted first-year high-school student might not even
458understand what the function is all about, you should adhere to the
459maximum limits all the more closely. Use helper functions with
460descriptive names (you can ask the compiler to in-line them if you think
461it's performance-critical, and it will probably do a better job of it
462than you would have done).
463
464Another measure of the function is the number of local variables. They
465shouldn't exceed 5-10, or you're doing something wrong. Re-think the
466function, and split it into smaller pieces. A human brain can
467generally easily keep track of about 7 different things, anything more
468and it gets confused. You know you're brilliant, but maybe you'd like
469to understand what you did 2 weeks from now.
470
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800471In source files, separate functions with one blank line. If the function is
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300472exported, the **EXPORT** macro for it should follow immediately after the
473closing function brace line. E.g.:
474
475.. code-block:: c
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800476
Pavel Kretov09677e02015-02-16 20:26:18 +0300477 int system_is_up(void)
478 {
479 return system_state == SYSTEM_RUNNING;
480 }
481 EXPORT_SYMBOL(system_is_up);
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800482
Kees Cookd5b421f2021-10-05 08:26:11 -07004836.1) Function prototypes
484************************
485
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800486In function prototypes, include parameter names with their data types.
487Although this is not required by the C language, it is preferred in Linux
488because it is a simple way to add valuable information for the reader.
489
Kees Cookd5b421f2021-10-05 08:26:11 -0700490Do not use the ``extern`` keyword with function declarations as this makes
Alexey Dobriyan3fe5dbf2019-01-03 15:26:16 -0800491lines longer and isn't strictly necessary.
492
Kees Cookd5b421f2021-10-05 08:26:11 -0700493When writing function prototypes, please keep the `order of elements regular
494<https://lore.kernel.org/mm-commits/CAHk-=wiOCLRny5aifWNhr621kYrJwhfURsa0vFPeUEm8mF0ufg@mail.gmail.com/>`_.
495For example, using this function declaration example::
496
497 __init void * __must_check action(enum magic value, size_t size, u8 count,
498 char *fmt, ...) __printf(4, 5) __malloc;
499
500The preferred order of elements for a function prototype is:
501
502- storage class (below, ``static __always_inline``, noting that ``__always_inline``
503 is technically an attribute but is treated like ``inline``)
504- storage class attributes (here, ``__init`` -- i.e. section declarations, but also
505 things like ``__cold``)
506- return type (here, ``void *``)
507- return type attributes (here, ``__must_check``)
508- function name (here, ``action``)
509- function parameters (here, ``(enum magic value, size_t size, u8 count, char *fmt, ...)``,
510 noting that parameter names should always be included)
511- function parameter attributes (here, ``__printf(4, 5)``)
512- function behavior attributes (here, ``__malloc``)
513
514Note that for a function **definition** (i.e. the actual function body),
515the compiler does not allow function parameter attributes after the
516function parameters. In these cases, they should go after the storage
517class attributes (e.g. note the changed position of ``__printf(4, 5)``
518below, compared to the **declaration** example above)::
519
520 static __always_inline __init __printf(4, 5) void * __must_check action(enum magic value,
521 size_t size, u8 count, char *fmt, ...) __malloc
522 {
523 ...
524 }
Linus Torvalds1da177e2005-04-16 15:20:36 -0700525
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03005267) Centralized exiting of functions
527-----------------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700528
529Albeit deprecated by some people, the equivalent of the goto statement is
530used frequently by compilers in form of the unconditional jump instruction.
531
532The goto statement comes in handy when a function exits from multiple
Dan Carpenterb57a0502013-07-03 15:08:08 -0700533locations and some common work such as cleanup has to be done. If there is no
534cleanup needed then just return directly.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700535
Dan Carpenterea040362014-12-02 11:59:50 +0300536Choose label names which say what the goto does or why the goto exists. An
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300537example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``.
538Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to
Jean Delvare865a1ca2016-07-25 14:29:06 +0200539renumber them if you ever add or remove exit paths, and they make correctness
540difficult to verify anyway.
541
Dan Carpenterea040362014-12-02 11:59:50 +0300542The rationale for using gotos is:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700543
544- unconditional statements are easier to understand and follow
545- nesting is reduced
546- errors by not updating individual exit points when making
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300547 modifications are prevented
Linus Torvalds1da177e2005-04-16 15:20:36 -0700548- saves the compiler work to optimize redundant code away ;)
549
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300550.. code-block:: c
551
Pavel Kretov09677e02015-02-16 20:26:18 +0300552 int fun(int a)
553 {
554 int result = 0;
555 char *buffer;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700556
Pavel Kretov09677e02015-02-16 20:26:18 +0300557 buffer = kmalloc(SIZE, GFP_KERNEL);
558 if (!buffer)
559 return -ENOMEM;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700560
Pavel Kretov09677e02015-02-16 20:26:18 +0300561 if (condition1) {
562 while (loop1) {
563 ...
564 }
565 result = 1;
Masahiro Yamadabeab6cb2016-11-03 01:57:34 +0900566 goto out_free_buffer;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700567 }
Pavel Kretov09677e02015-02-16 20:26:18 +0300568 ...
Jonathan Corbet79c70c32016-09-21 15:46:18 -0600569 out_free_buffer:
Pavel Kretov09677e02015-02-16 20:26:18 +0300570 kfree(buffer);
571 return result;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700572 }
Linus Torvalds1da177e2005-04-16 15:20:36 -0700573
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300574A common type of bug to be aware of is ``one err bugs`` which look like this:
Dan Carpenterea040362014-12-02 11:59:50 +0300575
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300576.. code-block:: c
577
Jonathan Corbet79c70c32016-09-21 15:46:18 -0600578 err:
Pavel Kretov09677e02015-02-16 20:26:18 +0300579 kfree(foo->bar);
580 kfree(foo);
581 return ret;
Dan Carpenterea040362014-12-02 11:59:50 +0300582
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300583The bug in this code is that on some exit paths ``foo`` is NULL. Normally the
584fix for this is to split it up into two error labels ``err_free_bar:`` and
585``err_free_foo:``:
Jean Delvare865a1ca2016-07-25 14:29:06 +0200586
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300587.. code-block:: c
588
Jean Delvare865a1ca2016-07-25 14:29:06 +0200589 err_free_bar:
590 kfree(foo->bar);
591 err_free_foo:
592 kfree(foo);
593 return ret;
594
595Ideally you should simulate errors to test all exit paths.
Dan Carpenterea040362014-12-02 11:59:50 +0300596
597
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03005988) Commenting
599-------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700600
601Comments are good, but there is also a danger of over-commenting. NEVER
602try to explain HOW your code works in a comment: it's much better to
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300603write the code so that the **working** is obvious, and it's a waste of
Linus Torvalds1da177e2005-04-16 15:20:36 -0700604time to explain badly written code.
605
606Generally, you want your comments to tell WHAT your code does, not HOW.
607Also, try to avoid putting comments inside a function body: if the
608function is so complex that you need to separately comment parts of it,
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800609you should probably go back to chapter 6 for a while. You can make
Linus Torvalds1da177e2005-04-16 15:20:36 -0700610small comments to note or warn about something particularly clever (or
611ugly), but try to avoid excess. Instead, put the comments at the head
612of the function, telling people what it does, and possibly WHY it does
613it.
614
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800615When commenting the kernel API functions, please use the kernel-doc format.
Mauro Carvalho Chehab1dc4bbf2016-11-17 08:32:33 -0200616See the files at :ref:`Documentation/doc-guide/ <doc_guide>` and
617``scripts/kernel-doc`` for details.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700618
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800619The preferred style for long (multi-line) comments is:
620
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300621.. code-block:: c
622
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800623 /*
624 * This is the preferred style for multi-line
625 * comments in the Linux kernel source code.
626 * Please use it consistently.
627 *
628 * Description: A column of asterisks on the left side,
629 * with beginning and ending almost-blank lines.
630 */
631
Joe Perchesc4ff1b52012-10-04 17:13:36 -0700632For files in net/ and drivers/net/ the preferred style for long (multi-line)
633comments is a little different.
634
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300635.. code-block:: c
636
Joe Perchesc4ff1b52012-10-04 17:13:36 -0700637 /* The preferred comment style for files in net/ and drivers/net
638 * looks like this.
639 *
640 * It is nearly the same as the generally preferred comment style,
641 * but there is no initial almost-blank line.
642 */
643
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800644It's also important to comment data, whether they are basic types or derived
645types. To this end, use just one data declaration per line (no commas for
646multiple data declarations). This leaves you room for a small comment on each
647item, explaining its use.
648
649
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03006509) You've made a mess of it
651---------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700652
653That's OK, we all do. You've probably been told by your long-time Unix
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300654user helper that ``GNU emacs`` automatically formats the C sources for
Linus Torvalds1da177e2005-04-16 15:20:36 -0700655you, and you've noticed that yes, it does do that, but the defaults it
656uses are less than desirable (in fact, they are worse than random
657typing - an infinite number of monkeys typing into GNU emacs would never
658make a good program).
659
660So, you can either get rid of GNU emacs, or change it to use saner
661values. To do the latter, you can stick the following in your .emacs file:
662
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300663.. code-block:: none
Linus Torvalds1da177e2005-04-16 15:20:36 -0700664
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300665 (defun c-lineup-arglist-tabs-only (ignored)
666 "Line up argument lists by tabs, not spaces"
667 (let* ((anchor (c-langelem-pos c-syntactic-element))
668 (column (c-langelem-2nd-pos c-syntactic-element))
669 (offset (- (1+ column) anchor))
670 (steps (floor offset c-basic-offset)))
671 (* (max steps 1)
672 c-basic-offset)))
Teemu Likonen0acbc6c2009-01-29 16:28:16 -0800673
Bart Van Assche2fec7b32019-01-07 10:20:19 -0800674 (dir-locals-set-class-variables
675 'linux-kernel
676 '((c-mode . (
677 (c-basic-offset . 8)
678 (c-label-minimum-indentation . 0)
679 (c-offsets-alist . (
680 (arglist-close . c-lineup-arglist-tabs-only)
681 (arglist-cont-nonempty .
682 (c-lineup-gcc-asm-reg c-lineup-arglist-tabs-only))
683 (arglist-intro . +)
684 (brace-list-intro . +)
685 (c . c-lineup-C-comments)
686 (case-label . 0)
687 (comment-intro . c-lineup-comment)
688 (cpp-define-intro . +)
689 (cpp-macro . -1000)
690 (cpp-macro-cont . +)
691 (defun-block-intro . +)
692 (else-clause . 0)
693 (func-decl-cont . +)
694 (inclass . +)
695 (inher-cont . c-lineup-multi-inher)
696 (knr-argdecl-intro . 0)
697 (label . -1000)
698 (statement . 0)
699 (statement-block-intro . +)
700 (statement-case-intro . +)
701 (statement-cont . +)
702 (substatement . +)
703 ))
704 (indent-tabs-mode . t)
705 (show-trailing-whitespace . t)
706 ))))
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300707
Bart Van Assche2fec7b32019-01-07 10:20:19 -0800708 (dir-locals-set-directory-class
709 (expand-file-name "~/src/linux-trees")
710 'linux-kernel)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700711
Johannes Weinera7f371e2008-07-25 01:45:51 -0700712This will make emacs go better with the kernel coding style for C
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300713files below ``~/src/linux-trees``.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700714
715But even if you fail in getting emacs to do sane formatting, not
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300716everything is lost: use ``indent``.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700717
718Now, again, GNU indent has the same brain-dead settings that GNU emacs
719has, which is why you need to give it a few command line options.
720However, that's not too bad, because even the makers of GNU indent
721recognize the authority of K&R (the GNU people aren't evil, they are
722just severely misguided in this matter), so you just give indent the
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300723options ``-kr -i8`` (stands for ``K&R, 8 character indents``), or use
724``scripts/Lindent``, which indents in the latest style.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700725
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300726``indent`` has a lot of options, and especially when it comes to comment
Linus Torvalds1da177e2005-04-16 15:20:36 -0700727re-formatting you may want to take a look at the man page. But
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300728remember: ``indent`` is not a fix for bad programming.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700729
Miguel Ojedad4ef8d32018-04-10 16:32:40 -0700730Note that you can also use the ``clang-format`` tool to help you with
731these rules, to quickly re-format parts of your code automatically,
732and to review full files in order to spot coding style mistakes,
733typos and possible improvements. It is also handy for sorting ``#includes``,
734for aligning variables/macros, for reflowing text and other similar tasks.
735See the file :ref:`Documentation/process/clang-format.rst <clangformat>`
736for more details.
737
Linus Torvalds1da177e2005-04-16 15:20:36 -0700738
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -030073910) Kconfig configuration files
740-------------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700741
Robert P. J. Day6754bb42007-05-23 13:57:42 -0700742For all of the Kconfig* configuration files throughout the source tree,
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300743the indentation is somewhat different. Lines under a ``config`` definition
Robert P. J. Day6754bb42007-05-23 13:57:42 -0700744are indented with one tab, while help text is indented an additional two
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300745spaces. Example::
Linus Torvalds1da177e2005-04-16 15:20:36 -0700746
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300747 config AUDIT
Robert P. J. Day6754bb42007-05-23 13:57:42 -0700748 bool "Auditing support"
749 depends on NET
Linus Torvalds1da177e2005-04-16 15:20:36 -0700750 help
Robert P. J. Day6754bb42007-05-23 13:57:42 -0700751 Enable auditing infrastructure that can be used with another
752 kernel subsystem, such as SELinux (which requires this for
753 logging of avc messages output). Does not do system-call
754 auditing without CONFIG_AUDITSYSCALL.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700755
Kees Cook0335cb42012-10-02 11:16:15 -0700756Seriously dangerous features (such as write support for certain
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300757filesystems) should advertise this prominently in their prompt string::
Robert P. J. Day6754bb42007-05-23 13:57:42 -0700758
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300759 config ADFS_FS_RW
Robert P. J. Day6754bb42007-05-23 13:57:42 -0700760 bool "ADFS write support (DANGEROUS)"
761 depends on ADFS_FS
762 ...
763
764For full documentation on the configuration files, see the file
Mauro Carvalho Chehabcd238ef2019-06-12 14:52:48 -0300765Documentation/kbuild/kconfig-language.rst.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700766
767
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -030076811) Data structures
769-------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700770
771Data structures that have visibility outside the single-threaded
772environment they are created and destroyed in should always have
773reference counts. In the kernel, garbage collection doesn't exist (and
774outside the kernel garbage collection is slow and inefficient), which
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300775means that you absolutely **have** to reference count all your uses.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700776
777Reference counting means that you can avoid locking, and allows multiple
778users to have access to the data structure in parallel - and not having
779to worry about the structure suddenly going away from under them just
780because they slept or did something else for a while.
781
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300782Note that locking is **not** a replacement for reference counting.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700783Locking is used to keep data structures coherent, while reference
784counting is a memory management technique. Usually both are needed, and
785they are not to be confused with each other.
786
787Many data structures can indeed have two levels of reference counting,
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300788when there are users of different ``classes``. The subclass count counts
Linus Torvalds1da177e2005-04-16 15:20:36 -0700789the number of subclass users, and decrements the global count just once
790when the subclass count goes to zero.
791
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300792Examples of this kind of ``multi-level-reference-counting`` can be found in
793memory management (``struct mm_struct``: mm_users and mm_count), and in
794filesystem code (``struct super_block``: s_count and s_active).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700795
796Remember: if another thread can find your data structure, and you don't
797have a reference count on it, you almost certainly have a bug.
798
799
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -030080012) Macros, Enums and RTL
801-------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700802
803Names of macros defining constants and labels in enums are capitalized.
804
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300805.. code-block:: c
806
Pavel Kretov09677e02015-02-16 20:26:18 +0300807 #define CONSTANT 0x12345
Linus Torvalds1da177e2005-04-16 15:20:36 -0700808
809Enums are preferred when defining several related constants.
810
811CAPITALIZED macro names are appreciated but macros resembling functions
812may be named in lower case.
813
814Generally, inline functions are preferable to macros resembling functions.
815
816Macros with multiple statements should be enclosed in a do - while block:
817
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300818.. code-block:: c
819
820 #define macrofun(a, b, c) \
Pavel Kretov09677e02015-02-16 20:26:18 +0300821 do { \
822 if (a == 5) \
823 do_this(b, c); \
824 } while (0)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700825
826Things to avoid when using macros:
827
8281) macros that affect control flow:
829
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300830.. code-block:: c
831
Pavel Kretov09677e02015-02-16 20:26:18 +0300832 #define FOO(x) \
833 do { \
834 if (blah(x) < 0) \
835 return -EBUGGERED; \
Thomas Gardner32fd52d2016-01-25 15:54:39 +1000836 } while (0)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700837
Mauro Carvalho Chehab5d628b42016-09-19 08:07:46 -0300838is a **very** bad idea. It looks like a function call but exits the ``calling``
Linus Torvalds1da177e2005-04-16 15:20:36 -0700839function; don't break the internal parsers of those who will read the code.
840
8412) macros that depend on having a local variable with a magic name:
842
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300843.. code-block:: c
844
Pavel Kretov09677e02015-02-16 20:26:18 +0300845 #define FOO(val) bar(index, val)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700846
847might look like a good thing, but it's confusing as hell when one reads the
848code and it's prone to breakage from seemingly innocent changes.
849
8503) macros with arguments that are used as l-values: FOO(x) = y; will
851bite you if somebody e.g. turns FOO into an inline function.
852
8534) forgetting about precedence: macros defining constants using expressions
854must enclose the expression in parentheses. Beware of similar issues with
855macros using parameters.
856
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300857.. code-block:: c
858
Pavel Kretov09677e02015-02-16 20:26:18 +0300859 #define CONSTANT 0x4000
860 #define CONSTEXP (CONSTANT | 3)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700861
Bartosz Golaszewskif2027542015-04-16 12:43:31 -07008625) namespace collisions when defining local variables in macros resembling
863functions:
864
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300865.. code-block:: c
866
867 #define FOO(x) \
868 ({ \
869 typeof(x) ret; \
870 ret = calc_ret(x); \
871 (ret); \
872 })
Bartosz Golaszewskif2027542015-04-16 12:43:31 -0700873
874ret is a common name for a local variable - __foo_ret is less likely
875to collide with an existing variable.
876
Linus Torvalds1da177e2005-04-16 15:20:36 -0700877The cpp manual deals with macros exhaustively. The gcc internals manual also
878covers RTL which is used frequently with assembly language in the kernel.
879
880
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -030088113) Printing kernel messages
882----------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700883
884Kernel developers like to be seen as literate. Do mind the spelling
Tony Fischettifb0e0ff2020-02-16 19:08:26 -0500885of kernel messages to make a good impression. Do not use incorrect
886contractions like ``dont``; use ``do not`` or ``don't`` instead. Make the
887messages concise, clear, and unambiguous.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700888
889Kernel messages do not have to be terminated with a period.
890
891Printing numbers in parentheses (%d) adds no value and should be avoided.
892
Trevor Woernerc04639a2021-04-23 14:40:10 -0400893There are a number of driver model diagnostic macros in <linux/dev_printk.h>
David Brownell6b094482007-07-13 16:32:09 -0700894which you should use to make sure messages are matched to the right device
895and driver, and are tagged with the right level: dev_err(), dev_warn(),
896dev_info(), and so forth. For messages that aren't associated with a
Dan Streetman6e099f52014-06-04 16:11:44 -0700897particular device, <linux/printk.h> defines pr_notice(), pr_info(),
898pr_warn(), pr_err(), etc.
David Brownell6b094482007-07-13 16:32:09 -0700899
900Coming up with good debugging messages can be quite a challenge; and once
Dan Streetman6e099f52014-06-04 16:11:44 -0700901you have them, they can be a huge help for remote troubleshooting. However
902debug message printing is handled differently than printing other non-debug
903messages. While the other pr_XXX() functions print unconditionally,
904pr_debug() does not; it is compiled out by default, unless either DEBUG is
905defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also,
906and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
907the ones already enabled by DEBUG.
908
909Many subsystems have Kconfig debug options to turn on -DDEBUG in the
910corresponding Makefile; in other cases specific files #define DEBUG. And
911when a debug message should be unconditionally printed, such as if it is
Raymond L. Rivera7c18fd72014-07-24 02:39:44 -0700912already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
Dan Streetman6e099f52014-06-04 16:11:44 -0700913used.
David Brownell6b094482007-07-13 16:32:09 -0700914
Linus Torvalds1da177e2005-04-16 15:20:36 -0700915
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -030091614) Allocating memory
917---------------------
Pekka J Enbergaf4e5a22005-09-16 19:28:11 -0700918
919The kernel provides the following general purpose memory allocators:
Xi Wang158372942012-05-31 16:26:04 -0700920kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
921vzalloc(). Please refer to the API documentation for further information
Joe Perchesbba757d2019-03-30 10:25:03 -0700922about them. :ref:`Documentation/core-api/memory-allocation.rst
923<memory_allocation>`
Pekka J Enbergaf4e5a22005-09-16 19:28:11 -0700924
925The preferred form for passing a size of a struct is the following:
926
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300927.. code-block:: c
928
Pekka J Enbergaf4e5a22005-09-16 19:28:11 -0700929 p = kmalloc(sizeof(*p), ...);
930
931The alternative form where struct name is spelled out hurts readability and
932introduces an opportunity for a bug when the pointer variable type is changed
933but the corresponding sizeof that is passed to a memory allocator is not.
934
935Casting the return value which is a void pointer is redundant. The conversion
936from void pointer to any other pointer type is guaranteed by the C programming
937language.
938
Xi Wang158372942012-05-31 16:26:04 -0700939The preferred form for allocating an array is the following:
940
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300941.. code-block:: c
942
Xi Wang158372942012-05-31 16:26:04 -0700943 p = kmalloc_array(n, sizeof(...), ...);
944
945The preferred form for allocating a zeroed array is the following:
946
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300947.. code-block:: c
948
Xi Wang158372942012-05-31 16:26:04 -0700949 p = kcalloc(n, sizeof(...), ...);
950
951Both forms check for overflow on the allocation size n * sizeof(...),
952and return NULL if that occurred.
953
Joe Perchesbba757d2019-03-30 10:25:03 -0700954These generic allocation functions all emit a stack dump on failure when used
955without __GFP_NOWARN so there is no use in emitting an additional failure
956message when NULL is returned.
Pekka J Enbergaf4e5a22005-09-16 19:28:11 -0700957
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -030095815) The inline disease
959----------------------
Arjan van de Vena771f2b2006-01-08 01:05:04 -0800960
961There appears to be a common misperception that gcc has a magic "make me
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300962faster" speedup option called ``inline``. While the use of inlines can be
Jesper Juhl53ab97a2007-05-08 00:31:06 -0700963appropriate (for example as a means of replacing macros, see Chapter 12), it
Arjan van de Vena771f2b2006-01-08 01:05:04 -0800964very often is not. Abundant use of the inline keyword leads to a much bigger
965kernel, which in turn slows the system as a whole down, due to a bigger
966icache footprint for the CPU and simply because there is less memory
967available for the pagecache. Just think about it; a pagecache miss causes a
Martin Olsson19af5cd2009-04-23 11:37:37 +0200968disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
969that can go into these 5 milliseconds.
Arjan van de Vena771f2b2006-01-08 01:05:04 -0800970
971A reasonable rule of thumb is to not put inline at functions that have more
972than 3 lines of code in them. An exception to this rule are the cases where
973a parameter is known to be a compiletime constant, and as a result of this
974constantness you *know* the compiler will be able to optimize most of your
975function away at compile time. For a good example of this later case, see
976the kmalloc() inline function.
977
978Often people argue that adding inline to functions that are static and used
979only once is always a win since there is no space tradeoff. While this is
980technically correct, gcc is capable of inlining these automatically without
981help, and the maintenance issue of removing the inline when a second user
982appears outweighs the potential value of the hint that tells gcc to do
983something it would have done anyway.
984
985
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -030098616) Function return values and names
987------------------------------------
Alan Sternc16a02d62006-09-29 02:01:21 -0700988
989Functions can return values of many different kinds, and one of the
990most common is a value indicating whether the function succeeded or
991failed. Such a value can be represented as an error-code integer
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -0300992(-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure,
Alan Sternc16a02d62006-09-29 02:01:21 -0700993non-zero = success).
994
995Mixing up these two sorts of representations is a fertile source of
996difficult-to-find bugs. If the C language included a strong distinction
997between integers and booleans then the compiler would find these mistakes
998for us... but it doesn't. To help prevent such bugs, always follow this
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -0300999convention::
Alan Sternc16a02d62006-09-29 02:01:21 -07001000
1001 If the name of a function is an action or an imperative command,
1002 the function should return an error-code integer. If the name
1003 is a predicate, the function should return a "succeeded" boolean.
1004
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -03001005For example, ``add work`` is a command, and the add_work() function returns 0
1006for success or -EBUSY for failure. In the same way, ``PCI device present`` is
Alan Sternc16a02d62006-09-29 02:01:21 -07001007a predicate, and the pci_dev_present() function returns 1 if it succeeds in
1008finding a matching device or 0 if it doesn't.
1009
1010All EXPORTed functions must respect this convention, and so should all
1011public functions. Private (static) functions need not, but it is
1012recommended that they do.
1013
1014Functions whose return value is the actual result of a computation, rather
1015than an indication of whether the computation succeeded, are not subject to
1016this rule. Generally they indicate failure by returning some out-of-range
1017result. Typical examples would be functions that return pointers; they use
1018NULL or the ERR_PTR mechanism to report failure.
1019
1020
Jason Gunthorpe79676562019-01-18 15:50:47 -0700102117) Using bool
1022--------------
1023
1024The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
1025only evaluate to 0 or 1, and implicit or explicit conversion to bool
1026automatically converts the value to true or false. When using bool types the
1027!! construction is not needed, which eliminates a class of bugs.
1028
1029When working with bool values the true and false definitions should be used
1030instead of 1 and 0.
1031
1032bool function return types and stack variables are always fine to use whenever
1033appropriate. Use of bool is encouraged to improve readability and is often a
1034better option than 'int' for storing boolean values.
1035
1036Do not use bool if cache line layout or size of the value matters, as its size
1037and alignment varies based on the compiled architecture. Structures that are
1038optimized for alignment and size should not use bool.
1039
1040If a structure has many true/false values, consider consolidating them into a
1041bitfield with 1 bit members, or using an appropriate fixed width type, such as
1042u8.
1043
1044Similarly for function arguments, many true/false values can be consolidated
1045into a single bitwise 'flags' argument and 'flags' can often be a more
1046readable alternative if the call-sites have naked true/false constants.
1047
1048Otherwise limited use of bool in structures and arguments can improve
1049readability.
1050
105118) Don't re-invent the kernel macros
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001052-------------------------------------
Robert P. J. Day58637ec902006-12-22 01:09:11 -08001053
1054The header file include/linux/kernel.h contains a number of macros that
1055you should use, rather than explicitly coding some variant of them yourself.
1056For example, if you need to calculate the length of an array, take advantage
1057of the macro
1058
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001059.. code-block:: c
1060
Pavel Kretov09677e02015-02-16 20:26:18 +03001061 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
Robert P. J. Day58637ec902006-12-22 01:09:11 -08001062
1063Similarly, if you need to calculate the size of some structure member, use
1064
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001065.. code-block:: c
1066
Pankaj Bharadiyac5936422019-12-09 10:31:43 -08001067 #define sizeof_field(t, f) (sizeof(((t*)0)->f))
Robert P. J. Day58637ec902006-12-22 01:09:11 -08001068
1069There are also min() and max() macros that do strict type checking if you
1070need them. Feel free to peruse that header file to see what else is already
1071defined that you shouldn't reproduce in your code.
1072
1073
Jason Gunthorpe79676562019-01-18 15:50:47 -0700107419) Editor modelines and other cruft
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001075------------------------------------
Josh Triplett4e7bd662007-07-15 23:41:37 -07001076
1077Some editors can interpret configuration information embedded in source files,
1078indicated with special markers. For example, emacs interprets lines marked
1079like this:
1080
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001081.. code-block:: c
1082
Pavel Kretov09677e02015-02-16 20:26:18 +03001083 -*- mode: c -*-
Josh Triplett4e7bd662007-07-15 23:41:37 -07001084
1085Or like this:
1086
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001087.. code-block:: c
1088
Pavel Kretov09677e02015-02-16 20:26:18 +03001089 /*
1090 Local Variables:
1091 compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
1092 End:
1093 */
Josh Triplett4e7bd662007-07-15 23:41:37 -07001094
1095Vim interprets markers that look like this:
1096
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001097.. code-block:: c
1098
Pavel Kretov09677e02015-02-16 20:26:18 +03001099 /* vim:set sw=8 noet */
Josh Triplett4e7bd662007-07-15 23:41:37 -07001100
1101Do not include any of these in source files. People have their own personal
1102editor configurations, and your source files should not override them. This
1103includes markers for indentation and mode configuration. People may use their
1104own custom mode, or may have some other magic method for making indentation
1105work correctly.
1106
1107
Jason Gunthorpe79676562019-01-18 15:50:47 -0700110820) Inline assembly
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001109-------------------
Josh Triplett9a7c48b2012-03-30 13:37:10 -07001110
1111In architecture-specific code, you may need to use inline assembly to interface
1112with CPU or platform functionality. Don't hesitate to do so when necessary.
1113However, don't use inline assembly gratuitously when C can do the job. You can
1114and should poke hardware from C when possible.
1115
1116Consider writing simple helper functions that wrap common bits of inline
1117assembly, rather than repeatedly writing them with slight variations. Remember
1118that inline assembly can use C parameters.
1119
1120Large, non-trivial assembly functions should go in .S files, with corresponding
1121C prototypes defined in C header files. The C prototypes for assembly
Mauro Carvalho Chehabb1a34592016-09-19 08:07:45 -03001122functions should use ``asmlinkage``.
Josh Triplett9a7c48b2012-03-30 13:37:10 -07001123
1124You may need to mark your asm statement as volatile, to prevent GCC from
1125removing it if GCC doesn't notice any side effects. You don't always need to
1126do so, though, and doing so unnecessarily can limit optimization.
1127
1128When writing a single inline assembly statement containing multiple
1129instructions, put each instruction on a separate line in a separate quoted
Ayan Shafqat68f04b52017-05-18 14:32:57 -04001130string, and end each string except the last with ``\n\t`` to properly indent
1131the next instruction in the assembly output:
Josh Triplett9a7c48b2012-03-30 13:37:10 -07001132
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001133.. code-block:: c
1134
Josh Triplett9a7c48b2012-03-30 13:37:10 -07001135 asm ("magic %reg1, #42\n\t"
1136 "more_magic %reg2, %reg3"
1137 : /* outputs */ : /* inputs */ : /* clobbers */);
1138
1139
Jason Gunthorpe79676562019-01-18 15:50:47 -0700114021) Conditional Compilation
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001141---------------------------
Josh Triplett21228a12014-10-29 11:15:17 -07001142
1143Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
1144files; doing so makes code harder to read and logic harder to follow. Instead,
1145use such conditionals in a header file defining functions for use in those .c
1146files, providing no-op stub versions in the #else case, and then call those
1147functions unconditionally from .c files. The compiler will avoid generating
1148any code for the stub calls, producing identical results, but the logic will
1149remain easy to follow.
1150
1151Prefer to compile out entire functions, rather than portions of functions or
1152portions of expressions. Rather than putting an ifdef in an expression, factor
1153out part or all of the expression into a separate helper function and apply the
1154conditional to that function.
1155
1156If you have a function or variable which may potentially go unused in a
1157particular configuration, and the compiler would warn about its definition
1158going unused, mark the definition as __maybe_unused rather than wrapping it in
1159a preprocessor conditional. (However, if a function or variable *always* goes
1160unused, delete it.)
1161
1162Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
1163symbol into a C boolean expression, and use it in a normal C conditional:
1164
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001165.. code-block:: c
1166
Josh Triplett21228a12014-10-29 11:15:17 -07001167 if (IS_ENABLED(CONFIG_SOMETHING)) {
1168 ...
1169 }
1170
1171The compiler will constant-fold the conditional away, and include or exclude
1172the block of code just as with an #ifdef, so this will not add any runtime
1173overhead. However, this approach still allows the C compiler to see the code
1174inside the block, and check it for correctness (syntax, types, symbol
1175references, etc). Thus, you still have to use an #ifdef if the code inside the
1176block references symbols that will not exist if the condition is not met.
1177
1178At the end of any non-trivial #if or #ifdef block (more than a few lines),
1179place a comment after the #endif on the same line, noting the conditional
1180expression used. For instance:
1181
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001182.. code-block:: c
1183
Pavel Kretov09677e02015-02-16 20:26:18 +03001184 #ifdef CONFIG_SOMETHING
1185 ...
1186 #endif /* CONFIG_SOMETHING */
Josh Triplett21228a12014-10-29 11:15:17 -07001187
Arjan van de Vena771f2b2006-01-08 01:05:04 -08001188
Mauro Carvalho Chehabd8dbbbc2016-09-19 08:07:44 -03001189Appendix I) References
1190----------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -07001191
1192The C Programming Language, Second Edition
1193by Brian W. Kernighan and Dennis M. Ritchie.
1194Prentice Hall, Inc., 1988.
1195ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
Linus Torvalds1da177e2005-04-16 15:20:36 -07001196
1197The Practice of Programming
1198by Brian W. Kernighan and Rob Pike.
1199Addison-Wesley, Inc., 1999.
1200ISBN 0-201-61586-X.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001201
1202GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
Alexander A. Klimove7b43112020-06-21 15:36:30 +02001203gcc internals and indent, all available from https://www.gnu.org/manual/
Linus Torvalds1da177e2005-04-16 15:20:36 -07001204
1205WG14 is the international standardization working group for the programming
Xose Vazquez Perez5b0ed2c2006-01-08 01:02:49 -08001206language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
1207
Federico Vagaf77af632018-11-21 01:35:19 +01001208Kernel :ref:`process/coding-style.rst <codingstyle>`, by greg@kroah.com at OLS 2002:
Xose Vazquez Perez5b0ed2c2006-01-08 01:02:49 -08001209http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/