2 Linux kernel coding style
4 This is a short document describing the preferred coding style for the
5 linux kernel. Coding style is very personal, and I won't _force_ my
6 views on anybody, but this is what goes for anything that I have to be
7 able to maintain, and I'd prefer it for most other things too. Please
8 at least consider the points made here.
10 First off, I'd suggest printing out a copy of the GNU coding standards,
11 and NOT read it. Burn them, it's a great symbolic gesture.
16 Chapter 1: Indentation
18 Tabs are 8 characters, and thus indentations are also 8 characters.
19 There are heretic movements that try to make indentations 4 (or even 2!)
20 characters deep, and that is akin to trying to define the value of PI to
23 Rationale: The whole idea behind indentation is to clearly define where
24 a block of control starts and ends. Especially when you've been looking
25 at your screen for 20 straight hours, you'll find it a lot easier to see
26 how the indentation works if you have large indentations.
28 Now, some people will claim that having 8-character indentations makes
29 the code move too far to the right, and makes it hard to read on a
30 80-character terminal screen. The answer to that is that if you need
31 more than 3 levels of indentation, you're screwed anyway, and should fix
34 In short, 8-char indents make things easier to read, and have the added
35 benefit of warning you when you're nesting your functions too deep.
38 Don't put multiple statements on a single line unless you have
41 if (condition) do_this;
42 do_something_everytime;
44 Outside of comments, documentation and except in Kconfig, spaces are never
45 used for indentation, and the above example is deliberately broken.
47 Get a decent editor and don't leave whitespace at the end of lines.
50 Chapter 2: Breaking long lines and strings
52 Coding style is all about readability and maintainability using commonly
55 The limit on the length of lines is 80 columns and this is a hard limit.
57 Statements longer than 80 columns will be broken into sensible chunks.
58 Descendants are always substantially shorter than the parent and are placed
59 substantially to the right. The same applies to function headers with a long
60 argument list. Long strings are as well broken into shorter strings.
62 void fun(int a, int b, int c)
65 printk(KERN_WARNING "Warning this is a long printk with "
66 "3 parameters a: %u b: %u "
72 Chapter 3: Placing Braces
74 The other issue that always comes up in C styling is the placement of
75 braces. Unlike the indent size, there are few technical reasons to
76 choose one placement strategy over the other, but the preferred way, as
77 shown to us by the prophets Kernighan and Ritchie, is to put the opening
78 brace last on the line, and put the closing brace first, thusly:
84 However, there is one special case, namely functions: they have the
85 opening brace at the beginning of the next line, thus:
92 Heretic people all over the world have claimed that this inconsistency
93 is ... well ... inconsistent, but all right-thinking people know that
94 (a) K&R are _right_ and (b) K&R are right. Besides, functions are
95 special anyway (you can't nest them in C).
97 Note that the closing brace is empty on a line of its own, _except_ in
98 the cases where it is followed by a continuation of the same statement,
99 ie a "while" in a do-statement or an "else" in an if-statement, like
118 Also, note that this brace-placement also minimizes the number of empty
119 (or almost empty) lines, without any loss of readability. Thus, as the
120 supply of new-lines on your screen is not a renewable resource (think
121 25-line terminal screens here), you have more empty lines to put
127 C is a Spartan language, and so should your naming be. Unlike Modula-2
128 and Pascal programmers, C programmers do not use cute names like
129 ThisVariableIsATemporaryCounter. A C programmer would call that
130 variable "tmp", which is much easier to write, and not the least more
131 difficult to understand.
133 HOWEVER, while mixed-case names are frowned upon, descriptive names for
134 global variables are a must. To call a global function "foo" is a
137 GLOBAL variables (to be used only if you _really_ need them) need to
138 have descriptive names, as do global functions. If you have a function
139 that counts the number of active users, you should call that
140 "count_active_users()" or similar, you should _not_ call it "cntusr()".
142 Encoding the type of a function into the name (so-called Hungarian
143 notation) is brain damaged - the compiler knows the types anyway and can
144 check those, and it only confuses the programmer. No wonder MicroSoft
145 makes buggy programs.
147 LOCAL variable names should be short, and to the point. If you have
148 some random integer loop counter, it should probably be called "i".
149 Calling it "loop_counter" is non-productive, if there is no chance of it
150 being mis-understood. Similarly, "tmp" can be just about any type of
151 variable that is used to hold a temporary value.
153 If you are afraid to mix up your local variable names, you have another
154 problem, which is called the function-growth-hormone-imbalance syndrome.
160 Functions should be short and sweet, and do just one thing. They should
161 fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
162 as we all know), and do one thing and do that well.
164 The maximum length of a function is inversely proportional to the
165 complexity and indentation level of that function. So, if you have a
166 conceptually simple function that is just one long (but simple)
167 case-statement, where you have to do lots of small things for a lot of
168 different cases, it's OK to have a longer function.
170 However, if you have a complex function, and you suspect that a
171 less-than-gifted first-year high-school student might not even
172 understand what the function is all about, you should adhere to the
173 maximum limits all the more closely. Use helper functions with
174 descriptive names (you can ask the compiler to in-line them if you think
175 it's performance-critical, and it will probably do a better job of it
176 than you would have done).
178 Another measure of the function is the number of local variables. They
179 shouldn't exceed 5-10, or you're doing something wrong. Re-think the
180 function, and split it into smaller pieces. A human brain can
181 generally easily keep track of about 7 different things, anything more
182 and it gets confused. You know you're brilliant, but maybe you'd like
183 to understand what you did 2 weeks from now.
186 Chapter 6: Centralized exiting of functions
188 Albeit deprecated by some people, the equivalent of the goto statement is
189 used frequently by compilers in form of the unconditional jump instruction.
191 The goto statement comes in handy when a function exits from multiple
192 locations and some common work such as cleanup has to be done.
196 - unconditional statements are easier to understand and follow
198 - errors by not updating individual exit points when making
199 modifications are prevented
200 - saves the compiler work to optimize redundant code away ;)
205 char *buffer = kmalloc(SIZE);
223 Chapter 7: Commenting
225 Comments are good, but there is also a danger of over-commenting. NEVER
226 try to explain HOW your code works in a comment: it's much better to
227 write the code so that the _working_ is obvious, and it's a waste of
228 time to explain badly written code.
230 Generally, you want your comments to tell WHAT your code does, not HOW.
231 Also, try to avoid putting comments inside a function body: if the
232 function is so complex that you need to separately comment parts of it,
233 you should probably go back to chapter 5 for a while. You can make
234 small comments to note or warn about something particularly clever (or
235 ugly), but try to avoid excess. Instead, put the comments at the head
236 of the function, telling people what it does, and possibly WHY it does
239 When commenting the kernel API functions, please use the kerneldoc format.
240 See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
243 Chapter 8: You've made a mess of it
245 That's OK, we all do. You've probably been told by your long-time Unix
246 user helper that "GNU emacs" automatically formats the C sources for
247 you, and you've noticed that yes, it does do that, but the defaults it
248 uses are less than desirable (in fact, they are worse than random
249 typing - an infinite number of monkeys typing into GNU emacs would never
250 make a good program).
252 So, you can either get rid of GNU emacs, or change it to use saner
253 values. To do the latter, you can stick the following in your .emacs file:
255 (defun linux-c-mode ()
256 "C mode with adjusted defaults for use with the Linux kernel."
261 (setq indent-tabs-mode t)
262 (setq c-basic-offset 8))
264 This will define the M-x linux-c-mode command. When hacking on a
265 module, if you put the string -*- linux-c -*- somewhere on the first
266 two lines, this mode will be automatically invoked. Also, you may want
269 (setq auto-mode-alist (cons '("/usr/src/linux.*/.*\\.[ch]$" . linux-c-mode)
272 to your .emacs file if you want to have linux-c-mode switched on
273 automagically when you edit source files under /usr/src/linux.
275 But even if you fail in getting emacs to do sane formatting, not
276 everything is lost: use "indent".
278 Now, again, GNU indent has the same brain-dead settings that GNU emacs
279 has, which is why you need to give it a few command line options.
280 However, that's not too bad, because even the makers of GNU indent
281 recognize the authority of K&R (the GNU people aren't evil, they are
282 just severely misguided in this matter), so you just give indent the
283 options "-kr -i8" (stands for "K&R, 8 character indents"), or use
284 "scripts/Lindent", which indents in the latest style.
286 "indent" has a lot of options, and especially when it comes to comment
287 re-formatting you may want to take a look at the man page. But
288 remember: "indent" is not a fix for bad programming.
291 Chapter 9: Configuration-files
293 For configuration options (arch/xxx/Kconfig, and all the Kconfig files),
294 somewhat different indentation is used.
296 Help text is indented with 2 spaces.
298 if CONFIG_EXPERIMENTAL
302 Apply nitroglycerine inside the keyboard (DANGEROUS)
304 depends on CONFIG_BOOM
307 Output nice messages when you explode
310 Generally, CONFIG_EXPERIMENTAL should surround all options not considered
311 stable. All options that are known to trash data (experimental write-
312 support for file-systems, for instance) should be denoted (DANGEROUS), other
313 experimental options should be denoted (EXPERIMENTAL).
316 Chapter 10: Data structures
318 Data structures that have visibility outside the single-threaded
319 environment they are created and destroyed in should always have
320 reference counts. In the kernel, garbage collection doesn't exist (and
321 outside the kernel garbage collection is slow and inefficient), which
322 means that you absolutely _have_ to reference count all your uses.
324 Reference counting means that you can avoid locking, and allows multiple
325 users to have access to the data structure in parallel - and not having
326 to worry about the structure suddenly going away from under them just
327 because they slept or did something else for a while.
329 Note that locking is _not_ a replacement for reference counting.
330 Locking is used to keep data structures coherent, while reference
331 counting is a memory management technique. Usually both are needed, and
332 they are not to be confused with each other.
334 Many data structures can indeed have two levels of reference counting,
335 when there are users of different "classes". The subclass count counts
336 the number of subclass users, and decrements the global count just once
337 when the subclass count goes to zero.
339 Examples of this kind of "multi-level-reference-counting" can be found in
340 memory management ("struct mm_struct": mm_users and mm_count), and in
341 filesystem code ("struct super_block": s_count and s_active).
343 Remember: if another thread can find your data structure, and you don't
344 have a reference count on it, you almost certainly have a bug.
347 Chapter 11: Macros, Enums, Inline functions and RTL
349 Names of macros defining constants and labels in enums are capitalized.
351 #define CONSTANT 0x12345
353 Enums are preferred when defining several related constants.
355 CAPITALIZED macro names are appreciated but macros resembling functions
356 may be named in lower case.
358 Generally, inline functions are preferable to macros resembling functions.
360 Macros with multiple statements should be enclosed in a do - while block:
362 #define macrofun(a, b, c) \
368 Things to avoid when using macros:
370 1) macros that affect control flow:
378 is a _very_ bad idea. It looks like a function call but exits the "calling"
379 function; don't break the internal parsers of those who will read the code.
381 2) macros that depend on having a local variable with a magic name:
383 #define FOO(val) bar(index, val)
385 might look like a good thing, but it's confusing as hell when one reads the
386 code and it's prone to breakage from seemingly innocent changes.
388 3) macros with arguments that are used as l-values: FOO(x) = y; will
389 bite you if somebody e.g. turns FOO into an inline function.
391 4) forgetting about precedence: macros defining constants using expressions
392 must enclose the expression in parentheses. Beware of similar issues with
393 macros using parameters.
395 #define CONSTANT 0x4000
396 #define CONSTEXP (CONSTANT | 3)
398 The cpp manual deals with macros exhaustively. The gcc internals manual also
399 covers RTL which is used frequently with assembly language in the kernel.
402 Chapter 12: Printing kernel messages
404 Kernel developers like to be seen as literate. Do mind the spelling
405 of kernel messages to make a good impression. Do not use crippled
406 words like "dont" and use "do not" or "don't" instead.
408 Kernel messages do not have to be terminated with a period.
410 Printing numbers in parentheses (%d) adds no value and should be avoided.
413 Chapter 13: References
415 The C Programming Language, Second Edition
416 by Brian W. Kernighan and Dennis M. Ritchie.
417 Prentice Hall, Inc., 1988.
418 ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
419 URL: http://cm.bell-labs.com/cm/cs/cbook/
421 The Practice of Programming
422 by Brian W. Kernighan and Rob Pike.
423 Addison-Wesley, Inc., 1999.
425 URL: http://cm.bell-labs.com/cm/cs/tpop/
427 GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
428 gcc internals and indent, all available from http://www.gnu.org
430 WG14 is the international standardization working group for the programming
431 language C, URL: http://std.dkuug.dk/JTC1/SC22/WG14/
434 Last updated on 16 February 2004 by a community effort on LKML.