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Linus Torvalds1da177e2005-04-16 15:20:36 -07001
2 Linux kernel coding style
3
4This is a short document describing the preferred coding style for the
5linux kernel. Coding style is very personal, and I won't _force_ my
6views on anybody, but this is what goes for anything that I have to be
7able to maintain, and I'd prefer it for most other things too. Please
8at least consider the points made here.
9
10First off, I'd suggest printing out a copy of the GNU coding standards,
11and NOT read it. Burn them, it's a great symbolic gesture.
12
13Anyway, here goes:
14
15
16 Chapter 1: Indentation
17
18Tabs are 8 characters, and thus indentations are also 8 characters.
19There are heretic movements that try to make indentations 4 (or even 2!)
20characters deep, and that is akin to trying to define the value of PI to
21be 3.
22
23Rationale: The whole idea behind indentation is to clearly define where
24a block of control starts and ends. Especially when you've been looking
25at your screen for 20 straight hours, you'll find it a lot easier to see
26how the indentation works if you have large indentations.
27
28Now, some people will claim that having 8-character indentations makes
29the code move too far to the right, and makes it hard to read on a
3080-character terminal screen. The answer to that is that if you need
31more than 3 levels of indentation, you're screwed anyway, and should fix
32your program.
33
34In short, 8-char indents make things easier to read, and have the added
35benefit of warning you when you're nesting your functions too deep.
36Heed that warning.
37
Randy Dunlapb3fc9942006-12-10 02:18:56 -080038The preferred way to ease multiple indentation levels in a switch statement is
39to align the "switch" and its subordinate "case" labels in the same column
40instead of "double-indenting" the "case" labels. E.g.:
41
42 switch (suffix) {
43 case 'G':
44 case 'g':
45 mem <<= 30;
46 break;
47 case 'M':
48 case 'm':
49 mem <<= 20;
50 break;
51 case 'K':
52 case 'k':
53 mem <<= 10;
54 /* fall through */
55 default:
56 break;
57 }
58
59
Linus Torvalds1da177e2005-04-16 15:20:36 -070060Don't put multiple statements on a single line unless you have
61something to hide:
62
63 if (condition) do_this;
64 do_something_everytime;
65
Randy Dunlapb3fc9942006-12-10 02:18:56 -080066Don't put multiple assignments on a single line either. Kernel coding style
67is super simple. Avoid tricky expressions.
68
Linus Torvalds1da177e2005-04-16 15:20:36 -070069Outside of comments, documentation and except in Kconfig, spaces are never
70used for indentation, and the above example is deliberately broken.
71
72Get a decent editor and don't leave whitespace at the end of lines.
73
74
75 Chapter 2: Breaking long lines and strings
76
77Coding style is all about readability and maintainability using commonly
78available tools.
79
Alan Coxdff49822007-10-16 23:27:33 -070080The limit on the length of lines is 80 columns and this is a strongly
81preferred limit.
Linus Torvalds1da177e2005-04-16 15:20:36 -070082
83Statements longer than 80 columns will be broken into sensible chunks.
84Descendants are always substantially shorter than the parent and are placed
85substantially to the right. The same applies to function headers with a long
Alan Coxdff49822007-10-16 23:27:33 -070086argument list. Long strings are as well broken into shorter strings. The
87only exception to this is where exceeding 80 columns significantly increases
88readability and does not hide information.
Linus Torvalds1da177e2005-04-16 15:20:36 -070089
90void fun(int a, int b, int c)
91{
92 if (condition)
93 printk(KERN_WARNING "Warning this is a long printk with "
94 "3 parameters a: %u b: %u "
95 "c: %u \n", a, b, c);
96 else
97 next_statement;
98}
99
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800100 Chapter 3: Placing Braces and Spaces
Linus Torvalds1da177e2005-04-16 15:20:36 -0700101
102The other issue that always comes up in C styling is the placement of
103braces. Unlike the indent size, there are few technical reasons to
104choose one placement strategy over the other, but the preferred way, as
105shown to us by the prophets Kernighan and Ritchie, is to put the opening
106brace last on the line, and put the closing brace first, thusly:
107
108 if (x is true) {
109 we do y
110 }
111
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800112This applies to all non-function statement blocks (if, switch, for,
113while, do). E.g.:
114
115 switch (action) {
116 case KOBJ_ADD:
117 return "add";
118 case KOBJ_REMOVE:
119 return "remove";
120 case KOBJ_CHANGE:
121 return "change";
122 default:
123 return NULL;
124 }
125
Linus Torvalds1da177e2005-04-16 15:20:36 -0700126However, there is one special case, namely functions: they have the
127opening brace at the beginning of the next line, thus:
128
129 int function(int x)
130 {
131 body of function
132 }
133
134Heretic people all over the world have claimed that this inconsistency
135is ... well ... inconsistent, but all right-thinking people know that
136(a) K&R are _right_ and (b) K&R are right. Besides, functions are
137special anyway (you can't nest them in C).
138
139Note that the closing brace is empty on a line of its own, _except_ in
140the cases where it is followed by a continuation of the same statement,
141ie a "while" in a do-statement or an "else" in an if-statement, like
142this:
143
144 do {
145 body of do-loop
146 } while (condition);
147
148and
149
150 if (x == y) {
151 ..
152 } else if (x > y) {
153 ...
154 } else {
155 ....
156 }
157
158Rationale: K&R.
159
160Also, note that this brace-placement also minimizes the number of empty
161(or almost empty) lines, without any loss of readability. Thus, as the
162supply of new-lines on your screen is not a renewable resource (think
16325-line terminal screens here), you have more empty lines to put
164comments on.
165
Oliver Neukume659ba42007-05-08 00:30:34 -0700166Do not unnecessarily use braces where a single statement will do.
167
168if (condition)
169 action();
170
171This does not apply if one branch of a conditional statement is a single
172statement. Use braces in both branches.
173
174if (condition) {
175 do_this();
176 do_that();
177} else {
178 otherwise();
179}
180
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800181 3.1: Spaces
182
183Linux kernel style for use of spaces depends (mostly) on
184function-versus-keyword usage. Use a space after (most) keywords. The
185notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
186somewhat like functions (and are usually used with parentheses in Linux,
187although they are not required in the language, as in: "sizeof info" after
188"struct fileinfo info;" is declared).
189
190So use a space after these keywords:
191 if, switch, case, for, do, while
192but not with sizeof, typeof, alignof, or __attribute__. E.g.,
193 s = sizeof(struct file);
194
195Do not add spaces around (inside) parenthesized expressions. This example is
196*bad*:
197
198 s = sizeof( struct file );
199
200When declaring pointer data or a function that returns a pointer type, the
201preferred use of '*' is adjacent to the data name or function name and not
202adjacent to the type name. Examples:
203
204 char *linux_banner;
205 unsigned long long memparse(char *ptr, char **retptr);
206 char *match_strdup(substring_t *s);
207
208Use one space around (on each side of) most binary and ternary operators,
209such as any of these:
210
211 = + - < > * / % | & ^ <= >= == != ? :
212
213but no space after unary operators:
214 & * + - ~ ! sizeof typeof alignof __attribute__ defined
215
216no space before the postfix increment & decrement unary operators:
217 ++ --
218
219no space after the prefix increment & decrement unary operators:
220 ++ --
221
222and no space around the '.' and "->" structure member operators.
223
Josh Tripletta923fd62007-07-15 23:41:37 -0700224Do not leave trailing whitespace at the ends of lines. Some editors with
225"smart" indentation will insert whitespace at the beginning of new lines as
226appropriate, so you can start typing the next line of code right away.
227However, some such editors do not remove the whitespace if you end up not
228putting a line of code there, such as if you leave a blank line. As a result,
229you end up with lines containing trailing whitespace.
230
231Git will warn you about patches that introduce trailing whitespace, and can
232optionally strip the trailing whitespace for you; however, if applying a series
233of patches, this may make later patches in the series fail by changing their
234context lines.
235
Linus Torvalds1da177e2005-04-16 15:20:36 -0700236
237 Chapter 4: Naming
238
239C is a Spartan language, and so should your naming be. Unlike Modula-2
240and Pascal programmers, C programmers do not use cute names like
241ThisVariableIsATemporaryCounter. A C programmer would call that
242variable "tmp", which is much easier to write, and not the least more
243difficult to understand.
244
245HOWEVER, while mixed-case names are frowned upon, descriptive names for
246global variables are a must. To call a global function "foo" is a
247shooting offense.
248
249GLOBAL variables (to be used only if you _really_ need them) need to
250have descriptive names, as do global functions. If you have a function
251that counts the number of active users, you should call that
252"count_active_users()" or similar, you should _not_ call it "cntusr()".
253
254Encoding the type of a function into the name (so-called Hungarian
255notation) is brain damaged - the compiler knows the types anyway and can
256check those, and it only confuses the programmer. No wonder MicroSoft
257makes buggy programs.
258
259LOCAL variable names should be short, and to the point. If you have
260some random integer loop counter, it should probably be called "i".
261Calling it "loop_counter" is non-productive, if there is no chance of it
262being mis-understood. Similarly, "tmp" can be just about any type of
263variable that is used to hold a temporary value.
264
265If you are afraid to mix up your local variable names, you have another
266problem, which is called the function-growth-hormone-imbalance syndrome.
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800267See chapter 6 (Functions).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700268
269
Randy Dunlap226a6b82006-06-23 02:05:58 -0700270 Chapter 5: Typedefs
271
272Please don't use things like "vps_t".
273
274It's a _mistake_ to use typedef for structures and pointers. When you see a
275
276 vps_t a;
277
278in the source, what does it mean?
279
280In contrast, if it says
281
282 struct virtual_container *a;
283
284you can actually tell what "a" is.
285
286Lots of people think that typedefs "help readability". Not so. They are
287useful only for:
288
289 (a) totally opaque objects (where the typedef is actively used to _hide_
290 what the object is).
291
292 Example: "pte_t" etc. opaque objects that you can only access using
293 the proper accessor functions.
294
295 NOTE! Opaqueness and "accessor functions" are not good in themselves.
296 The reason we have them for things like pte_t etc. is that there
297 really is absolutely _zero_ portably accessible information there.
298
299 (b) Clear integer types, where the abstraction _helps_ avoid confusion
300 whether it is "int" or "long".
301
302 u8/u16/u32 are perfectly fine typedefs, although they fit into
303 category (d) better than here.
304
305 NOTE! Again - there needs to be a _reason_ for this. If something is
306 "unsigned long", then there's no reason to do
307
308 typedef unsigned long myflags_t;
309
310 but if there is a clear reason for why it under certain circumstances
311 might be an "unsigned int" and under other configurations might be
312 "unsigned long", then by all means go ahead and use a typedef.
313
314 (c) when you use sparse to literally create a _new_ type for
315 type-checking.
316
317 (d) New types which are identical to standard C99 types, in certain
318 exceptional circumstances.
319
320 Although it would only take a short amount of time for the eyes and
321 brain to become accustomed to the standard types like 'uint32_t',
322 some people object to their use anyway.
323
324 Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
325 signed equivalents which are identical to standard types are
326 permitted -- although they are not mandatory in new code of your
327 own.
328
329 When editing existing code which already uses one or the other set
330 of types, you should conform to the existing choices in that code.
331
332 (e) Types safe for use in userspace.
333
334 In certain structures which are visible to userspace, we cannot
335 require C99 types and cannot use the 'u32' form above. Thus, we
336 use __u32 and similar types in all structures which are shared
337 with userspace.
338
339Maybe there are other cases too, but the rule should basically be to NEVER
340EVER use a typedef unless you can clearly match one of those rules.
341
342In general, a pointer, or a struct that has elements that can reasonably
343be directly accessed should _never_ be a typedef.
344
345
346 Chapter 6: Functions
Linus Torvalds1da177e2005-04-16 15:20:36 -0700347
348Functions should be short and sweet, and do just one thing. They should
349fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
350as we all know), and do one thing and do that well.
351
352The maximum length of a function is inversely proportional to the
353complexity and indentation level of that function. So, if you have a
354conceptually simple function that is just one long (but simple)
355case-statement, where you have to do lots of small things for a lot of
356different cases, it's OK to have a longer function.
357
358However, if you have a complex function, and you suspect that a
359less-than-gifted first-year high-school student might not even
360understand what the function is all about, you should adhere to the
361maximum limits all the more closely. Use helper functions with
362descriptive names (you can ask the compiler to in-line them if you think
363it's performance-critical, and it will probably do a better job of it
364than you would have done).
365
366Another measure of the function is the number of local variables. They
367shouldn't exceed 5-10, or you're doing something wrong. Re-think the
368function, and split it into smaller pieces. A human brain can
369generally easily keep track of about 7 different things, anything more
370and it gets confused. You know you're brilliant, but maybe you'd like
371to understand what you did 2 weeks from now.
372
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800373In source files, separate functions with one blank line. If the function is
374exported, the EXPORT* macro for it should follow immediately after the closing
375function brace line. E.g.:
376
377int system_is_up(void)
378{
379 return system_state == SYSTEM_RUNNING;
380}
381EXPORT_SYMBOL(system_is_up);
382
383In function prototypes, include parameter names with their data types.
384Although this is not required by the C language, it is preferred in Linux
385because it is a simple way to add valuable information for the reader.
386
Linus Torvalds1da177e2005-04-16 15:20:36 -0700387
Randy Dunlap226a6b82006-06-23 02:05:58 -0700388 Chapter 7: Centralized exiting of functions
Linus Torvalds1da177e2005-04-16 15:20:36 -0700389
390Albeit deprecated by some people, the equivalent of the goto statement is
391used frequently by compilers in form of the unconditional jump instruction.
392
393The goto statement comes in handy when a function exits from multiple
394locations and some common work such as cleanup has to be done.
395
396The rationale is:
397
398- unconditional statements are easier to understand and follow
399- nesting is reduced
400- errors by not updating individual exit points when making
401 modifications are prevented
402- saves the compiler work to optimize redundant code away ;)
403
Jesper Juhldc3d28d2006-01-09 20:53:51 -0800404int fun(int a)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700405{
406 int result = 0;
407 char *buffer = kmalloc(SIZE);
408
409 if (buffer == NULL)
410 return -ENOMEM;
411
412 if (condition1) {
413 while (loop1) {
414 ...
415 }
416 result = 1;
417 goto out;
418 }
419 ...
420out:
421 kfree(buffer);
422 return result;
423}
424
Randy Dunlap226a6b82006-06-23 02:05:58 -0700425 Chapter 8: Commenting
Linus Torvalds1da177e2005-04-16 15:20:36 -0700426
427Comments are good, but there is also a danger of over-commenting. NEVER
428try to explain HOW your code works in a comment: it's much better to
429write the code so that the _working_ is obvious, and it's a waste of
430time to explain badly written code.
431
432Generally, you want your comments to tell WHAT your code does, not HOW.
433Also, try to avoid putting comments inside a function body: if the
434function is so complex that you need to separately comment parts of it,
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800435you should probably go back to chapter 6 for a while. You can make
Linus Torvalds1da177e2005-04-16 15:20:36 -0700436small comments to note or warn about something particularly clever (or
437ugly), but try to avoid excess. Instead, put the comments at the head
438of the function, telling people what it does, and possibly WHY it does
439it.
440
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800441When commenting the kernel API functions, please use the kernel-doc format.
Pekka J Enberge776eba2005-09-10 00:26:44 -0700442See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
443for details.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700444
Randy Dunlapb3fc9942006-12-10 02:18:56 -0800445Linux style for comments is the C89 "/* ... */" style.
446Don't use C99-style "// ..." comments.
447
448The preferred style for long (multi-line) comments is:
449
450 /*
451 * This is the preferred style for multi-line
452 * comments in the Linux kernel source code.
453 * Please use it consistently.
454 *
455 * Description: A column of asterisks on the left side,
456 * with beginning and ending almost-blank lines.
457 */
458
459It's also important to comment data, whether they are basic types or derived
460types. To this end, use just one data declaration per line (no commas for
461multiple data declarations). This leaves you room for a small comment on each
462item, explaining its use.
463
464
Randy Dunlap226a6b82006-06-23 02:05:58 -0700465 Chapter 9: You've made a mess of it
Linus Torvalds1da177e2005-04-16 15:20:36 -0700466
467That's OK, we all do. You've probably been told by your long-time Unix
468user helper that "GNU emacs" automatically formats the C sources for
469you, and you've noticed that yes, it does do that, but the defaults it
470uses are less than desirable (in fact, they are worse than random
471typing - an infinite number of monkeys typing into GNU emacs would never
472make a good program).
473
474So, you can either get rid of GNU emacs, or change it to use saner
475values. To do the latter, you can stick the following in your .emacs file:
476
Johannes Weinera7f371e2008-07-25 01:45:51 -0700477(defun c-lineup-arglist-tabs-only (ignored)
478 "Line up argument lists by tabs, not spaces"
479 (let* ((anchor (c-langelem-pos c-syntactic-element))
480 (column (c-langelem-2nd-pos c-syntactic-element))
481 (offset (- (1+ column) anchor))
482 (steps (floor offset c-basic-offset)))
483 (* (max steps 1)
484 c-basic-offset)))
Linus Torvalds1da177e2005-04-16 15:20:36 -0700485
Johannes Weinera7f371e2008-07-25 01:45:51 -0700486(add-hook 'c-mode-hook
487 (lambda ()
488 (let ((filename (buffer-file-name)))
489 ;; Enable kernel mode for the appropriate files
490 (when (and filename
491 (string-match "~/src/linux-trees" filename))
492 (setq indent-tabs-mode t)
493 (c-set-style "linux")
494 (c-set-offset 'arglist-cont-nonempty
495 '(c-lineup-gcc-asm-reg
496 c-lineup-arglist-tabs-only))))))
Linus Torvalds1da177e2005-04-16 15:20:36 -0700497
Johannes Weinera7f371e2008-07-25 01:45:51 -0700498This will make emacs go better with the kernel coding style for C
499files below ~/src/linux-trees.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700500
501But even if you fail in getting emacs to do sane formatting, not
502everything is lost: use "indent".
503
504Now, again, GNU indent has the same brain-dead settings that GNU emacs
505has, which is why you need to give it a few command line options.
506However, that's not too bad, because even the makers of GNU indent
507recognize the authority of K&R (the GNU people aren't evil, they are
508just severely misguided in this matter), so you just give indent the
509options "-kr -i8" (stands for "K&R, 8 character indents"), or use
510"scripts/Lindent", which indents in the latest style.
511
512"indent" has a lot of options, and especially when it comes to comment
513re-formatting you may want to take a look at the man page. But
514remember: "indent" is not a fix for bad programming.
515
516
Robert P. J. Day6754bb42007-05-23 13:57:42 -0700517 Chapter 10: Kconfig configuration files
Linus Torvalds1da177e2005-04-16 15:20:36 -0700518
Robert P. J. Day6754bb42007-05-23 13:57:42 -0700519For all of the Kconfig* configuration files throughout the source tree,
520the indentation is somewhat different. Lines under a "config" definition
521are indented with one tab, while help text is indented an additional two
522spaces. Example:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700523
Robert P. J. Day6754bb42007-05-23 13:57:42 -0700524config AUDIT
525 bool "Auditing support"
526 depends on NET
Linus Torvalds1da177e2005-04-16 15:20:36 -0700527 help
Robert P. J. Day6754bb42007-05-23 13:57:42 -0700528 Enable auditing infrastructure that can be used with another
529 kernel subsystem, such as SELinux (which requires this for
530 logging of avc messages output). Does not do system-call
531 auditing without CONFIG_AUDITSYSCALL.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700532
Robert P. J. Day6754bb42007-05-23 13:57:42 -0700533Features that might still be considered unstable should be defined as
534dependent on "EXPERIMENTAL":
535
536config SLUB
537 depends on EXPERIMENTAL && !ARCH_USES_SLAB_PAGE_STRUCT
538 bool "SLUB (Unqueued Allocator)"
539 ...
540
541while seriously dangerous features (such as write support for certain
542filesystems) should advertise this prominently in their prompt string:
543
544config ADFS_FS_RW
545 bool "ADFS write support (DANGEROUS)"
546 depends on ADFS_FS
547 ...
548
549For full documentation on the configuration files, see the file
550Documentation/kbuild/kconfig-language.txt.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700551
552
Randy Dunlap226a6b82006-06-23 02:05:58 -0700553 Chapter 11: Data structures
Linus Torvalds1da177e2005-04-16 15:20:36 -0700554
555Data structures that have visibility outside the single-threaded
556environment they are created and destroyed in should always have
557reference counts. In the kernel, garbage collection doesn't exist (and
558outside the kernel garbage collection is slow and inefficient), which
559means that you absolutely _have_ to reference count all your uses.
560
561Reference counting means that you can avoid locking, and allows multiple
562users to have access to the data structure in parallel - and not having
563to worry about the structure suddenly going away from under them just
564because they slept or did something else for a while.
565
566Note that locking is _not_ a replacement for reference counting.
567Locking is used to keep data structures coherent, while reference
568counting is a memory management technique. Usually both are needed, and
569they are not to be confused with each other.
570
571Many data structures can indeed have two levels of reference counting,
572when there are users of different "classes". The subclass count counts
573the number of subclass users, and decrements the global count just once
574when the subclass count goes to zero.
575
576Examples of this kind of "multi-level-reference-counting" can be found in
577memory management ("struct mm_struct": mm_users and mm_count), and in
578filesystem code ("struct super_block": s_count and s_active).
579
580Remember: if another thread can find your data structure, and you don't
581have a reference count on it, you almost certainly have a bug.
582
583
Randy Dunlap226a6b82006-06-23 02:05:58 -0700584 Chapter 12: Macros, Enums and RTL
Linus Torvalds1da177e2005-04-16 15:20:36 -0700585
586Names of macros defining constants and labels in enums are capitalized.
587
588#define CONSTANT 0x12345
589
590Enums are preferred when defining several related constants.
591
592CAPITALIZED macro names are appreciated but macros resembling functions
593may be named in lower case.
594
595Generally, inline functions are preferable to macros resembling functions.
596
597Macros with multiple statements should be enclosed in a do - while block:
598
599#define macrofun(a, b, c) \
600 do { \
601 if (a == 5) \
602 do_this(b, c); \
603 } while (0)
604
605Things to avoid when using macros:
606
6071) macros that affect control flow:
608
609#define FOO(x) \
610 do { \
611 if (blah(x) < 0) \
612 return -EBUGGERED; \
613 } while(0)
614
615is a _very_ bad idea. It looks like a function call but exits the "calling"
616function; don't break the internal parsers of those who will read the code.
617
6182) macros that depend on having a local variable with a magic name:
619
620#define FOO(val) bar(index, val)
621
622might look like a good thing, but it's confusing as hell when one reads the
623code and it's prone to breakage from seemingly innocent changes.
624
6253) macros with arguments that are used as l-values: FOO(x) = y; will
626bite you if somebody e.g. turns FOO into an inline function.
627
6284) forgetting about precedence: macros defining constants using expressions
629must enclose the expression in parentheses. Beware of similar issues with
630macros using parameters.
631
632#define CONSTANT 0x4000
633#define CONSTEXP (CONSTANT | 3)
634
635The cpp manual deals with macros exhaustively. The gcc internals manual also
636covers RTL which is used frequently with assembly language in the kernel.
637
638
Randy Dunlap226a6b82006-06-23 02:05:58 -0700639 Chapter 13: Printing kernel messages
Linus Torvalds1da177e2005-04-16 15:20:36 -0700640
641Kernel developers like to be seen as literate. Do mind the spelling
642of kernel messages to make a good impression. Do not use crippled
David Brownell6b094482007-07-13 16:32:09 -0700643words like "dont"; use "do not" or "don't" instead. Make the messages
644concise, clear, and unambiguous.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700645
646Kernel messages do not have to be terminated with a period.
647
648Printing numbers in parentheses (%d) adds no value and should be avoided.
649
David Brownell6b094482007-07-13 16:32:09 -0700650There are a number of driver model diagnostic macros in <linux/device.h>
651which you should use to make sure messages are matched to the right device
652and driver, and are tagged with the right level: dev_err(), dev_warn(),
653dev_info(), and so forth. For messages that aren't associated with a
654particular device, <linux/kernel.h> defines pr_debug() and pr_info().
655
656Coming up with good debugging messages can be quite a challenge; and once
657you have them, they can be a huge help for remote troubleshooting. Such
658messages should be compiled out when the DEBUG symbol is not defined (that
659is, by default they are not included). When you use dev_dbg() or pr_debug(),
660that's automatic. Many subsystems have Kconfig options to turn on -DDEBUG.
661A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the
662ones already enabled by DEBUG.
663
Linus Torvalds1da177e2005-04-16 15:20:36 -0700664
Randy Dunlap226a6b82006-06-23 02:05:58 -0700665 Chapter 14: Allocating memory
Pekka J Enbergaf4e5a22005-09-16 19:28:11 -0700666
667The kernel provides the following general purpose memory allocators:
668kmalloc(), kzalloc(), kcalloc(), and vmalloc(). Please refer to the API
669documentation for further information about them.
670
671The preferred form for passing a size of a struct is the following:
672
673 p = kmalloc(sizeof(*p), ...);
674
675The alternative form where struct name is spelled out hurts readability and
676introduces an opportunity for a bug when the pointer variable type is changed
677but the corresponding sizeof that is passed to a memory allocator is not.
678
679Casting the return value which is a void pointer is redundant. The conversion
680from void pointer to any other pointer type is guaranteed by the C programming
681language.
682
683
Randy Dunlap226a6b82006-06-23 02:05:58 -0700684 Chapter 15: The inline disease
Arjan van de Vena771f2b2006-01-08 01:05:04 -0800685
686There appears to be a common misperception that gcc has a magic "make me
687faster" speedup option called "inline". While the use of inlines can be
Jesper Juhl53ab97a2007-05-08 00:31:06 -0700688appropriate (for example as a means of replacing macros, see Chapter 12), it
Arjan van de Vena771f2b2006-01-08 01:05:04 -0800689very often is not. Abundant use of the inline keyword leads to a much bigger
690kernel, which in turn slows the system as a whole down, due to a bigger
691icache footprint for the CPU and simply because there is less memory
692available for the pagecache. Just think about it; a pagecache miss causes a
693disk seek, which easily takes 5 miliseconds. There are a LOT of cpu cycles
694that can go into these 5 miliseconds.
695
696A reasonable rule of thumb is to not put inline at functions that have more
697than 3 lines of code in them. An exception to this rule are the cases where
698a parameter is known to be a compiletime constant, and as a result of this
699constantness you *know* the compiler will be able to optimize most of your
700function away at compile time. For a good example of this later case, see
701the kmalloc() inline function.
702
703Often people argue that adding inline to functions that are static and used
704only once is always a win since there is no space tradeoff. While this is
705technically correct, gcc is capable of inlining these automatically without
706help, and the maintenance issue of removing the inline when a second user
707appears outweighs the potential value of the hint that tells gcc to do
708something it would have done anyway.
709
710
Alan Sternc16a02d2006-09-29 02:01:21 -0700711 Chapter 16: Function return values and names
712
713Functions can return values of many different kinds, and one of the
714most common is a value indicating whether the function succeeded or
715failed. Such a value can be represented as an error-code integer
716(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
717non-zero = success).
718
719Mixing up these two sorts of representations is a fertile source of
720difficult-to-find bugs. If the C language included a strong distinction
721between integers and booleans then the compiler would find these mistakes
722for us... but it doesn't. To help prevent such bugs, always follow this
723convention:
724
725 If the name of a function is an action or an imperative command,
726 the function should return an error-code integer. If the name
727 is a predicate, the function should return a "succeeded" boolean.
728
729For example, "add work" is a command, and the add_work() function returns 0
730for success or -EBUSY for failure. In the same way, "PCI device present" is
731a predicate, and the pci_dev_present() function returns 1 if it succeeds in
732finding a matching device or 0 if it doesn't.
733
734All EXPORTed functions must respect this convention, and so should all
735public functions. Private (static) functions need not, but it is
736recommended that they do.
737
738Functions whose return value is the actual result of a computation, rather
739than an indication of whether the computation succeeded, are not subject to
740this rule. Generally they indicate failure by returning some out-of-range
741result. Typical examples would be functions that return pointers; they use
742NULL or the ERR_PTR mechanism to report failure.
743
744
Robert P. J. Day58637ec2006-12-22 01:09:11 -0800745 Chapter 17: Don't re-invent the kernel macros
746
747The header file include/linux/kernel.h contains a number of macros that
748you should use, rather than explicitly coding some variant of them yourself.
749For example, if you need to calculate the length of an array, take advantage
750of the macro
751
752 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
753
754Similarly, if you need to calculate the size of some structure member, use
755
756 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
757
758There are also min() and max() macros that do strict type checking if you
759need them. Feel free to peruse that header file to see what else is already
760defined that you shouldn't reproduce in your code.
761
762
Josh Triplett4e7bd662007-07-15 23:41:37 -0700763 Chapter 18: Editor modelines and other cruft
764
765Some editors can interpret configuration information embedded in source files,
766indicated with special markers. For example, emacs interprets lines marked
767like this:
768
769-*- mode: c -*-
770
771Or like this:
772
773/*
774Local Variables:
775compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
776End:
777*/
778
779Vim interprets markers that look like this:
780
781/* vim:set sw=8 noet */
782
783Do not include any of these in source files. People have their own personal
784editor configurations, and your source files should not override them. This
785includes markers for indentation and mode configuration. People may use their
786own custom mode, or may have some other magic method for making indentation
787work correctly.
788
789
Arjan van de Vena771f2b2006-01-08 01:05:04 -0800790
Randy Dunlap226a6b82006-06-23 02:05:58 -0700791 Appendix I: References
Linus Torvalds1da177e2005-04-16 15:20:36 -0700792
793The C Programming Language, Second Edition
794by Brian W. Kernighan and Dennis M. Ritchie.
795Prentice Hall, Inc., 1988.
796ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
797URL: http://cm.bell-labs.com/cm/cs/cbook/
798
799The Practice of Programming
800by Brian W. Kernighan and Rob Pike.
801Addison-Wesley, Inc., 1999.
802ISBN 0-201-61586-X.
803URL: http://cm.bell-labs.com/cm/cs/tpop/
804
805GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
Xose Vazquez Perez5b0ed2c2006-01-08 01:02:49 -0800806gcc internals and indent, all available from http://www.gnu.org/manual/
Linus Torvalds1da177e2005-04-16 15:20:36 -0700807
808WG14 is the international standardization working group for the programming
Xose Vazquez Perez5b0ed2c2006-01-08 01:02:49 -0800809language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
810
811Kernel CodingStyle, by greg@kroah.com at OLS 2002:
812http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
Linus Torvalds1da177e2005-04-16 15:20:36 -0700813
814--
David Brownell6b094482007-07-13 16:32:09 -0700815Last updated on 2007-July-13.
816