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1
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
38The 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
59Don't put multiple statements on a single line unless you have
60something to hide:
61
62 if (condition) do_this;
63 do_something_everytime;
64
65Don't put multiple assignments on a single line either. Kernel coding style
66is super simple. Avoid tricky expressions.
67
68Outside of comments, documentation and except in Kconfig, spaces are never
69used for indentation, and the above example is deliberately broken.
70
71Get a decent editor and don't leave whitespace at the end of lines.
72
73
74 Chapter 2: Breaking long lines and strings
75
76Coding style is all about readability and maintainability using commonly
77available tools.
78
79The limit on the length of lines is 80 columns and this is a strongly
80preferred limit.
81
82Statements longer than 80 columns will be broken into sensible chunks, unless
83exceeding 80 columns significantly increases readability and does not hide
84information. Descendants are always substantially shorter than the parent and
85are placed substantially to the right. The same applies to function headers
86with a long argument list. However, never break user-visible strings such as
87printk messages, because that breaks the ability to grep for them.
88
89
90 Chapter 3: Placing Braces and Spaces
91
92The other issue that always comes up in C styling is the placement of
93braces. Unlike the indent size, there are few technical reasons to
94choose one placement strategy over the other, but the preferred way, as
95shown to us by the prophets Kernighan and Ritchie, is to put the opening
96brace last on the line, and put the closing brace first, thusly:
97
98 if (x is true) {
99 we do y
100 }
101
102This applies to all non-function statement blocks (if, switch, for,
103while, do). E.g.:
104
105 switch (action) {
106 case KOBJ_ADD:
107 return "add";
108 case KOBJ_REMOVE:
109 return "remove";
110 case KOBJ_CHANGE:
111 return "change";
112 default:
113 return NULL;
114 }
115
116However, there is one special case, namely functions: they have the
117opening brace at the beginning of the next line, thus:
118
119 int function(int x)
120 {
121 body of function
122 }
123
124Heretic people all over the world have claimed that this inconsistency
125is ... well ... inconsistent, but all right-thinking people know that
126(a) K&R are _right_ and (b) K&R are right. Besides, functions are
127special anyway (you can't nest them in C).
128
129Note that the closing brace is empty on a line of its own, _except_ in
130the cases where it is followed by a continuation of the same statement,
131ie a "while" in a do-statement or an "else" in an if-statement, like
132this:
133
134 do {
135 body of do-loop
136 } while (condition);
137
138and
139
140 if (x == y) {
141 ..
142 } else if (x > y) {
143 ...
144 } else {
145 ....
146 }
147
148Rationale: K&R.
149
150Also, note that this brace-placement also minimizes the number of empty
151(or almost empty) lines, without any loss of readability. Thus, as the
152supply of new-lines on your screen is not a renewable resource (think
15325-line terminal screens here), you have more empty lines to put
154comments on.
155
156Do not unnecessarily use braces where a single statement will do.
157
158 if (condition)
159 action();
160
161and
162
163 if (condition)
164 do_this();
165 else
166 do_that();
167
168This does not apply if only one branch of a conditional statement is a single
169statement; in the latter case use braces in both branches:
170
171 if (condition) {
172 do_this();
173 do_that();
174 } else {
175 otherwise();
176 }
177
178 3.1: Spaces
179
180Linux kernel style for use of spaces depends (mostly) on
181function-versus-keyword usage. Use a space after (most) keywords. The
182notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
183somewhat like functions (and are usually used with parentheses in Linux,
184although they are not required in the language, as in: "sizeof info" after
185"struct fileinfo info;" is declared).
186
187So use a space after these keywords:
188
189 if, switch, case, for, do, while
190
191but not with sizeof, typeof, alignof, or __attribute__. E.g.,
192
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
215 & * + - ~ ! sizeof typeof alignof __attribute__ defined
216
217no space before the postfix increment & decrement unary operators:
218
219 ++ --
220
221no space after the prefix increment & decrement unary operators:
222
223 ++ --
224
225and no space around the '.' and "->" structure member operators.
226
227Do not leave trailing whitespace at the ends of lines. Some editors with
228"smart" indentation will insert whitespace at the beginning of new lines as
229appropriate, so you can start typing the next line of code right away.
230However, some such editors do not remove the whitespace if you end up not
231putting a line of code there, such as if you leave a blank line. As a result,
232you end up with lines containing trailing whitespace.
233
234Git will warn you about patches that introduce trailing whitespace, and can
235optionally strip the trailing whitespace for you; however, if applying a series
236of patches, this may make later patches in the series fail by changing their
237context lines.
238
239
240 Chapter 4: Naming
241
242C is a Spartan language, and so should your naming be. Unlike Modula-2
243and Pascal programmers, C programmers do not use cute names like
244ThisVariableIsATemporaryCounter. A C programmer would call that
245variable "tmp", which is much easier to write, and not the least more
246difficult to understand.
247
248HOWEVER, while mixed-case names are frowned upon, descriptive names for
249global variables are a must. To call a global function "foo" is a
250shooting offense.
251
252GLOBAL variables (to be used only if you _really_ need them) need to
253have descriptive names, as do global functions. If you have a function
254that counts the number of active users, you should call that
255"count_active_users()" or similar, you should _not_ call it "cntusr()".
256
257Encoding the type of a function into the name (so-called Hungarian
258notation) is brain damaged - the compiler knows the types anyway and can
259check those, and it only confuses the programmer. No wonder MicroSoft
260makes buggy programs.
261
262LOCAL variable names should be short, and to the point. If you have
263some random integer loop counter, it should probably be called "i".
264Calling it "loop_counter" is non-productive, if there is no chance of it
265being mis-understood. Similarly, "tmp" can be just about any type of
266variable that is used to hold a temporary value.
267
268If you are afraid to mix up your local variable names, you have another
269problem, which is called the function-growth-hormone-imbalance syndrome.
270See chapter 6 (Functions).
271
272
273 Chapter 5: Typedefs
274
275Please don't use things like "vps_t".
276It's a _mistake_ to use typedef for structures and pointers. When you see a
277
278 vps_t a;
279
280in the source, what does it mean?
281In contrast, if it says
282
283 struct virtual_container *a;
284
285you can actually tell what "a" is.
286
287Lots of people think that typedefs "help readability". Not so. They are
288useful only for:
289
290 (a) totally opaque objects (where the typedef is actively used to _hide_
291 what the object is).
292
293 Example: "pte_t" etc. opaque objects that you can only access using
294 the proper accessor functions.
295
296 NOTE! Opaqueness and "accessor functions" are not good in themselves.
297 The reason we have them for things like pte_t etc. is that there
298 really is absolutely _zero_ portably accessible information there.
299
300 (b) Clear integer types, where the abstraction _helps_ avoid confusion
301 whether it is "int" or "long".
302
303 u8/u16/u32 are perfectly fine typedefs, although they fit into
304 category (d) better than here.
305
306 NOTE! Again - there needs to be a _reason_ for this. If something is
307 "unsigned long", then there's no reason to do
308
309 typedef unsigned long myflags_t;
310
311 but if there is a clear reason for why it under certain circumstances
312 might be an "unsigned int" and under other configurations might be
313 "unsigned long", then by all means go ahead and use a typedef.
314
315 (c) when you use sparse to literally create a _new_ type for
316 type-checking.
317
318 (d) New types which are identical to standard C99 types, in certain
319 exceptional circumstances.
320
321 Although it would only take a short amount of time for the eyes and
322 brain to become accustomed to the standard types like 'uint32_t',
323 some people object to their use anyway.
324
325 Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
326 signed equivalents which are identical to standard types are
327 permitted -- although they are not mandatory in new code of your
328 own.
329
330 When editing existing code which already uses one or the other set
331 of types, you should conform to the existing choices in that code.
332
333 (e) Types safe for use in userspace.
334
335 In certain structures which are visible to userspace, we cannot
336 require C99 types and cannot use the 'u32' form above. Thus, we
337 use __u32 and similar types in all structures which are shared
338 with userspace.
339
340Maybe there are other cases too, but the rule should basically be to NEVER
341EVER use a typedef unless you can clearly match one of those rules.
342
343In general, a pointer, or a struct that has elements that can reasonably
344be directly accessed should _never_ be a typedef.
345
346
347 Chapter 6: Functions
348
349Functions should be short and sweet, and do just one thing. They should
350fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
351as we all know), and do one thing and do that well.
352
353The maximum length of a function is inversely proportional to the
354complexity and indentation level of that function. So, if you have a
355conceptually simple function that is just one long (but simple)
356case-statement, where you have to do lots of small things for a lot of
357different cases, it's OK to have a longer function.
358
359However, if you have a complex function, and you suspect that a
360less-than-gifted first-year high-school student might not even
361understand what the function is all about, you should adhere to the
362maximum limits all the more closely. Use helper functions with
363descriptive names (you can ask the compiler to in-line them if you think
364it's performance-critical, and it will probably do a better job of it
365than you would have done).
366
367Another measure of the function is the number of local variables. They
368shouldn't exceed 5-10, or you're doing something wrong. Re-think the
369function, and split it into smaller pieces. A human brain can
370generally easily keep track of about 7 different things, anything more
371and it gets confused. You know you're brilliant, but maybe you'd like
372to understand what you did 2 weeks from now.
373
374In source files, separate functions with one blank line. If the function is
375exported, the EXPORT* macro for it should follow immediately after the closing
376function brace line. E.g.:
377
378 int system_is_up(void)
379 {
380 return system_state == SYSTEM_RUNNING;
381 }
382 EXPORT_SYMBOL(system_is_up);
383
384In function prototypes, include parameter names with their data types.
385Although this is not required by the C language, it is preferred in Linux
386because it is a simple way to add valuable information for the reader.
387
388
389 Chapter 7: Centralized exiting of functions
390
391Albeit deprecated by some people, the equivalent of the goto statement is
392used frequently by compilers in form of the unconditional jump instruction.
393
394The goto statement comes in handy when a function exits from multiple
395locations and some common work such as cleanup has to be done. If there is no
396cleanup needed then just return directly.
397
398Choose label names which say what the goto does or why the goto exists. An
399example of a good name could be "out_buffer:" if the goto frees "buffer". Avoid
400using GW-BASIC names like "err1:" and "err2:". Also don't name them after the
401goto location like "err_kmalloc_failed:"
402
403The rationale for using gotos is:
404
405- unconditional statements are easier to understand and follow
406- nesting is reduced
407- errors by not updating individual exit points when making
408 modifications are prevented
409- saves the compiler work to optimize redundant code away ;)
410
411 int fun(int a)
412 {
413 int result = 0;
414 char *buffer;
415
416 buffer = kmalloc(SIZE, GFP_KERNEL);
417 if (!buffer)
418 return -ENOMEM;
419
420 if (condition1) {
421 while (loop1) {
422 ...
423 }
424 result = 1;
425 goto out_buffer;
426 }
427 ...
428 out_buffer:
429 kfree(buffer);
430 return result;
431 }
432
433A common type of bug to be aware of is "one err bugs" which look like this:
434
435 err:
436 kfree(foo->bar);
437 kfree(foo);
438 return ret;
439
440The bug in this code is that on some exit paths "foo" is NULL. Normally the
441fix for this is to split it up into two error labels "err_bar:" and "err_foo:".
442
443
444 Chapter 8: Commenting
445
446Comments are good, but there is also a danger of over-commenting. NEVER
447try to explain HOW your code works in a comment: it's much better to
448write the code so that the _working_ is obvious, and it's a waste of
449time to explain badly written code.
450
451Generally, you want your comments to tell WHAT your code does, not HOW.
452Also, try to avoid putting comments inside a function body: if the
453function is so complex that you need to separately comment parts of it,
454you should probably go back to chapter 6 for a while. You can make
455small comments to note or warn about something particularly clever (or
456ugly), but try to avoid excess. Instead, put the comments at the head
457of the function, telling people what it does, and possibly WHY it does
458it.
459
460When commenting the kernel API functions, please use the kernel-doc format.
461See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
462for details.
463
464Linux style for comments is the C89 "/* ... */" style.
465Don't use C99-style "// ..." comments.
466
467The preferred style for long (multi-line) comments is:
468
469 /*
470 * This is the preferred style for multi-line
471 * comments in the Linux kernel source code.
472 * Please use it consistently.
473 *
474 * Description: A column of asterisks on the left side,
475 * with beginning and ending almost-blank lines.
476 */
477
478For files in net/ and drivers/net/ the preferred style for long (multi-line)
479comments is a little different.
480
481 /* The preferred comment style for files in net/ and drivers/net
482 * looks like this.
483 *
484 * It is nearly the same as the generally preferred comment style,
485 * but there is no initial almost-blank line.
486 */
487
488It's also important to comment data, whether they are basic types or derived
489types. To this end, use just one data declaration per line (no commas for
490multiple data declarations). This leaves you room for a small comment on each
491item, explaining its use.
492
493
494 Chapter 9: You've made a mess of it
495
496That's OK, we all do. You've probably been told by your long-time Unix
497user helper that "GNU emacs" automatically formats the C sources for
498you, and you've noticed that yes, it does do that, but the defaults it
499uses are less than desirable (in fact, they are worse than random
500typing - an infinite number of monkeys typing into GNU emacs would never
501make a good program).
502
503So, you can either get rid of GNU emacs, or change it to use saner
504values. To do the latter, you can stick the following in your .emacs file:
505
506(defun c-lineup-arglist-tabs-only (ignored)
507 "Line up argument lists by tabs, not spaces"
508 (let* ((anchor (c-langelem-pos c-syntactic-element))
509 (column (c-langelem-2nd-pos c-syntactic-element))
510 (offset (- (1+ column) anchor))
511 (steps (floor offset c-basic-offset)))
512 (* (max steps 1)
513 c-basic-offset)))
514
515(add-hook 'c-mode-common-hook
516 (lambda ()
517 ;; Add kernel style
518 (c-add-style
519 "linux-tabs-only"
520 '("linux" (c-offsets-alist
521 (arglist-cont-nonempty
522 c-lineup-gcc-asm-reg
523 c-lineup-arglist-tabs-only))))))
524
525(add-hook 'c-mode-hook
526 (lambda ()
527 (let ((filename (buffer-file-name)))
528 ;; Enable kernel mode for the appropriate files
529 (when (and filename
530 (string-match (expand-file-name "~/src/linux-trees")
531 filename))
532 (setq indent-tabs-mode t)
533 (setq show-trailing-whitespace t)
534 (c-set-style "linux-tabs-only")))))
535
536This will make emacs go better with the kernel coding style for C
537files below ~/src/linux-trees.
538
539But even if you fail in getting emacs to do sane formatting, not
540everything is lost: use "indent".
541
542Now, again, GNU indent has the same brain-dead settings that GNU emacs
543has, which is why you need to give it a few command line options.
544However, that's not too bad, because even the makers of GNU indent
545recognize the authority of K&R (the GNU people aren't evil, they are
546just severely misguided in this matter), so you just give indent the
547options "-kr -i8" (stands for "K&R, 8 character indents"), or use
548"scripts/Lindent", which indents in the latest style.
549
550"indent" has a lot of options, and especially when it comes to comment
551re-formatting you may want to take a look at the man page. But
552remember: "indent" is not a fix for bad programming.
553
554
555 Chapter 10: Kconfig configuration files
556
557For all of the Kconfig* configuration files throughout the source tree,
558the indentation is somewhat different. Lines under a "config" definition
559are indented with one tab, while help text is indented an additional two
560spaces. Example:
561
562config AUDIT
563 bool "Auditing support"
564 depends on NET
565 help
566 Enable auditing infrastructure that can be used with another
567 kernel subsystem, such as SELinux (which requires this for
568 logging of avc messages output). Does not do system-call
569 auditing without CONFIG_AUDITSYSCALL.
570
571Seriously dangerous features (such as write support for certain
572filesystems) should advertise this prominently in their prompt string:
573
574config ADFS_FS_RW
575 bool "ADFS write support (DANGEROUS)"
576 depends on ADFS_FS
577 ...
578
579For full documentation on the configuration files, see the file
580Documentation/kbuild/kconfig-language.txt.
581
582
583 Chapter 11: Data structures
584
585Data structures that have visibility outside the single-threaded
586environment they are created and destroyed in should always have
587reference counts. In the kernel, garbage collection doesn't exist (and
588outside the kernel garbage collection is slow and inefficient), which
589means that you absolutely _have_ to reference count all your uses.
590
591Reference counting means that you can avoid locking, and allows multiple
592users to have access to the data structure in parallel - and not having
593to worry about the structure suddenly going away from under them just
594because they slept or did something else for a while.
595
596Note that locking is _not_ a replacement for reference counting.
597Locking is used to keep data structures coherent, while reference
598counting is a memory management technique. Usually both are needed, and
599they are not to be confused with each other.
600
601Many data structures can indeed have two levels of reference counting,
602when there are users of different "classes". The subclass count counts
603the number of subclass users, and decrements the global count just once
604when the subclass count goes to zero.
605
606Examples of this kind of "multi-level-reference-counting" can be found in
607memory management ("struct mm_struct": mm_users and mm_count), and in
608filesystem code ("struct super_block": s_count and s_active).
609
610Remember: if another thread can find your data structure, and you don't
611have a reference count on it, you almost certainly have a bug.
612
613
614 Chapter 12: Macros, Enums and RTL
615
616Names of macros defining constants and labels in enums are capitalized.
617
618 #define CONSTANT 0x12345
619
620Enums are preferred when defining several related constants.
621
622CAPITALIZED macro names are appreciated but macros resembling functions
623may be named in lower case.
624
625Generally, inline functions are preferable to macros resembling functions.
626
627Macros with multiple statements should be enclosed in a do - while block:
628
629 #define macrofun(a, b, c) \
630 do { \
631 if (a == 5) \
632 do_this(b, c); \
633 } while (0)
634
635Things to avoid when using macros:
636
6371) macros that affect control flow:
638
639 #define FOO(x) \
640 do { \
641 if (blah(x) < 0) \
642 return -EBUGGERED; \
643 } while (0)
644
645is a _very_ bad idea. It looks like a function call but exits the "calling"
646function; don't break the internal parsers of those who will read the code.
647
6482) macros that depend on having a local variable with a magic name:
649
650 #define FOO(val) bar(index, val)
651
652might look like a good thing, but it's confusing as hell when one reads the
653code and it's prone to breakage from seemingly innocent changes.
654
6553) macros with arguments that are used as l-values: FOO(x) = y; will
656bite you if somebody e.g. turns FOO into an inline function.
657
6584) forgetting about precedence: macros defining constants using expressions
659must enclose the expression in parentheses. Beware of similar issues with
660macros using parameters.
661
662 #define CONSTANT 0x4000
663 #define CONSTEXP (CONSTANT | 3)
664
6655) namespace collisions when defining local variables in macros resembling
666functions:
667
668#define FOO(x) \
669({ \
670 typeof(x) ret; \
671 ret = calc_ret(x); \
672 (ret); \
673})
674
675ret is a common name for a local variable - __foo_ret is less likely
676to collide with an existing variable.
677
678The cpp manual deals with macros exhaustively. The gcc internals manual also
679covers RTL which is used frequently with assembly language in the kernel.
680
681
682 Chapter 13: Printing kernel messages
683
684Kernel developers like to be seen as literate. Do mind the spelling
685of kernel messages to make a good impression. Do not use crippled
686words like "dont"; use "do not" or "don't" instead. Make the messages
687concise, clear, and unambiguous.
688
689Kernel messages do not have to be terminated with a period.
690
691Printing numbers in parentheses (%d) adds no value and should be avoided.
692
693There are a number of driver model diagnostic macros in <linux/device.h>
694which you should use to make sure messages are matched to the right device
695and driver, and are tagged with the right level: dev_err(), dev_warn(),
696dev_info(), and so forth. For messages that aren't associated with a
697particular device, <linux/printk.h> defines pr_notice(), pr_info(),
698pr_warn(), pr_err(), etc.
699
700Coming up with good debugging messages can be quite a challenge; and once
701you have them, they can be a huge help for remote troubleshooting. However
702debug message printing is handled differently than printing other non-debug
703messages. While the other pr_XXX() functions print unconditionally,
704pr_debug() does not; it is compiled out by default, unless either DEBUG is
705defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also,
706and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
707the ones already enabled by DEBUG.
708
709Many subsystems have Kconfig debug options to turn on -DDEBUG in the
710corresponding Makefile; in other cases specific files #define DEBUG. And
711when a debug message should be unconditionally printed, such as if it is
712already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
713used.
714
715
716 Chapter 14: Allocating memory
717
718The kernel provides the following general purpose memory allocators:
719kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
720vzalloc(). Please refer to the API documentation for further information
721about them.
722
723The preferred form for passing a size of a struct is the following:
724
725 p = kmalloc(sizeof(*p), ...);
726
727The alternative form where struct name is spelled out hurts readability and
728introduces an opportunity for a bug when the pointer variable type is changed
729but the corresponding sizeof that is passed to a memory allocator is not.
730
731Casting the return value which is a void pointer is redundant. The conversion
732from void pointer to any other pointer type is guaranteed by the C programming
733language.
734
735The preferred form for allocating an array is the following:
736
737 p = kmalloc_array(n, sizeof(...), ...);
738
739The preferred form for allocating a zeroed array is the following:
740
741 p = kcalloc(n, sizeof(...), ...);
742
743Both forms check for overflow on the allocation size n * sizeof(...),
744and return NULL if that occurred.
745
746
747 Chapter 15: The inline disease
748
749There appears to be a common misperception that gcc has a magic "make me
750faster" speedup option called "inline". While the use of inlines can be
751appropriate (for example as a means of replacing macros, see Chapter 12), it
752very often is not. Abundant use of the inline keyword leads to a much bigger
753kernel, which in turn slows the system as a whole down, due to a bigger
754icache footprint for the CPU and simply because there is less memory
755available for the pagecache. Just think about it; a pagecache miss causes a
756disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
757that can go into these 5 milliseconds.
758
759A reasonable rule of thumb is to not put inline at functions that have more
760than 3 lines of code in them. An exception to this rule are the cases where
761a parameter is known to be a compiletime constant, and as a result of this
762constantness you *know* the compiler will be able to optimize most of your
763function away at compile time. For a good example of this later case, see
764the kmalloc() inline function.
765
766Often people argue that adding inline to functions that are static and used
767only once is always a win since there is no space tradeoff. While this is
768technically correct, gcc is capable of inlining these automatically without
769help, and the maintenance issue of removing the inline when a second user
770appears outweighs the potential value of the hint that tells gcc to do
771something it would have done anyway.
772
773
774 Chapter 16: Function return values and names
775
776Functions can return values of many different kinds, and one of the
777most common is a value indicating whether the function succeeded or
778failed. Such a value can be represented as an error-code integer
779(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
780non-zero = success).
781
782Mixing up these two sorts of representations is a fertile source of
783difficult-to-find bugs. If the C language included a strong distinction
784between integers and booleans then the compiler would find these mistakes
785for us... but it doesn't. To help prevent such bugs, always follow this
786convention:
787
788 If the name of a function is an action or an imperative command,
789 the function should return an error-code integer. If the name
790 is a predicate, the function should return a "succeeded" boolean.
791
792For example, "add work" is a command, and the add_work() function returns 0
793for success or -EBUSY for failure. In the same way, "PCI device present" is
794a predicate, and the pci_dev_present() function returns 1 if it succeeds in
795finding a matching device or 0 if it doesn't.
796
797All EXPORTed functions must respect this convention, and so should all
798public functions. Private (static) functions need not, but it is
799recommended that they do.
800
801Functions whose return value is the actual result of a computation, rather
802than an indication of whether the computation succeeded, are not subject to
803this rule. Generally they indicate failure by returning some out-of-range
804result. Typical examples would be functions that return pointers; they use
805NULL or the ERR_PTR mechanism to report failure.
806
807
808 Chapter 17: Don't re-invent the kernel macros
809
810The header file include/linux/kernel.h contains a number of macros that
811you should use, rather than explicitly coding some variant of them yourself.
812For example, if you need to calculate the length of an array, take advantage
813of the macro
814
815 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
816
817Similarly, if you need to calculate the size of some structure member, use
818
819 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
820
821There are also min() and max() macros that do strict type checking if you
822need them. Feel free to peruse that header file to see what else is already
823defined that you shouldn't reproduce in your code.
824
825
826 Chapter 18: Editor modelines and other cruft
827
828Some editors can interpret configuration information embedded in source files,
829indicated with special markers. For example, emacs interprets lines marked
830like this:
831
832 -*- mode: c -*-
833
834Or like this:
835
836 /*
837 Local Variables:
838 compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
839 End:
840 */
841
842Vim interprets markers that look like this:
843
844 /* vim:set sw=8 noet */
845
846Do not include any of these in source files. People have their own personal
847editor configurations, and your source files should not override them. This
848includes markers for indentation and mode configuration. People may use their
849own custom mode, or may have some other magic method for making indentation
850work correctly.
851
852
853 Chapter 19: Inline assembly
854
855In architecture-specific code, you may need to use inline assembly to interface
856with CPU or platform functionality. Don't hesitate to do so when necessary.
857However, don't use inline assembly gratuitously when C can do the job. You can
858and should poke hardware from C when possible.
859
860Consider writing simple helper functions that wrap common bits of inline
861assembly, rather than repeatedly writing them with slight variations. Remember
862that inline assembly can use C parameters.
863
864Large, non-trivial assembly functions should go in .S files, with corresponding
865C prototypes defined in C header files. The C prototypes for assembly
866functions should use "asmlinkage".
867
868You may need to mark your asm statement as volatile, to prevent GCC from
869removing it if GCC doesn't notice any side effects. You don't always need to
870do so, though, and doing so unnecessarily can limit optimization.
871
872When writing a single inline assembly statement containing multiple
873instructions, put each instruction on a separate line in a separate quoted
874string, and end each string except the last with \n\t to properly indent the
875next instruction in the assembly output:
876
877 asm ("magic %reg1, #42\n\t"
878 "more_magic %reg2, %reg3"
879 : /* outputs */ : /* inputs */ : /* clobbers */);
880
881
882 Chapter 20: Conditional Compilation
883
884Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
885files; doing so makes code harder to read and logic harder to follow. Instead,
886use such conditionals in a header file defining functions for use in those .c
887files, providing no-op stub versions in the #else case, and then call those
888functions unconditionally from .c files. The compiler will avoid generating
889any code for the stub calls, producing identical results, but the logic will
890remain easy to follow.
891
892Prefer to compile out entire functions, rather than portions of functions or
893portions of expressions. Rather than putting an ifdef in an expression, factor
894out part or all of the expression into a separate helper function and apply the
895conditional to that function.
896
897If you have a function or variable which may potentially go unused in a
898particular configuration, and the compiler would warn about its definition
899going unused, mark the definition as __maybe_unused rather than wrapping it in
900a preprocessor conditional. (However, if a function or variable *always* goes
901unused, delete it.)
902
903Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
904symbol into a C boolean expression, and use it in a normal C conditional:
905
906 if (IS_ENABLED(CONFIG_SOMETHING)) {
907 ...
908 }
909
910The compiler will constant-fold the conditional away, and include or exclude
911the block of code just as with an #ifdef, so this will not add any runtime
912overhead. However, this approach still allows the C compiler to see the code
913inside the block, and check it for correctness (syntax, types, symbol
914references, etc). Thus, you still have to use an #ifdef if the code inside the
915block references symbols that will not exist if the condition is not met.
916
917At the end of any non-trivial #if or #ifdef block (more than a few lines),
918place a comment after the #endif on the same line, noting the conditional
919expression used. For instance:
920
921 #ifdef CONFIG_SOMETHING
922 ...
923 #endif /* CONFIG_SOMETHING */
924
925
926 Appendix I: References
927
928The C Programming Language, Second Edition
929by Brian W. Kernighan and Dennis M. Ritchie.
930Prentice Hall, Inc., 1988.
931ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
932
933The Practice of Programming
934by Brian W. Kernighan and Rob Pike.
935Addison-Wesley, Inc., 1999.
936ISBN 0-201-61586-X.
937
938GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
939gcc internals and indent, all available from http://www.gnu.org/manual/
940
941WG14 is the international standardization working group for the programming
942language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
943
944Kernel CodingStyle, by greg@kroah.com at OLS 2002:
945http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
946
1
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
38The 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
60Don'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
66Don't put multiple assignments on a single line either. Kernel coding style
67is super simple. Avoid tricky expressions.
68
69Outside 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
80The limit on the length of lines is 80 columns and this is a strongly
81preferred limit.
82
83Statements longer than 80 columns will be broken into sensible chunks, unless
84exceeding 80 columns significantly increases readability and does not hide
85information. Descendants are always substantially shorter than the parent and
86are placed substantially to the right. The same applies to function headers
87with a long argument list. However, never break user-visible strings such as
88printk messages, because that breaks the ability to grep for them.
89
90
91 Chapter 3: Placing Braces and Spaces
92
93The other issue that always comes up in C styling is the placement of
94braces. Unlike the indent size, there are few technical reasons to
95choose one placement strategy over the other, but the preferred way, as
96shown to us by the prophets Kernighan and Ritchie, is to put the opening
97brace last on the line, and put the closing brace first, thusly:
98
99 if (x is true) {
100 we do y
101 }
102
103This applies to all non-function statement blocks (if, switch, for,
104while, do). E.g.:
105
106 switch (action) {
107 case KOBJ_ADD:
108 return "add";
109 case KOBJ_REMOVE:
110 return "remove";
111 case KOBJ_CHANGE:
112 return "change";
113 default:
114 return NULL;
115 }
116
117However, there is one special case, namely functions: they have the
118opening brace at the beginning of the next line, thus:
119
120 int function(int x)
121 {
122 body of function
123 }
124
125Heretic people all over the world have claimed that this inconsistency
126is ... well ... inconsistent, but all right-thinking people know that
127(a) K&R are _right_ and (b) K&R are right. Besides, functions are
128special anyway (you can't nest them in C).
129
130Note that the closing brace is empty on a line of its own, _except_ in
131the cases where it is followed by a continuation of the same statement,
132ie a "while" in a do-statement or an "else" in an if-statement, like
133this:
134
135 do {
136 body of do-loop
137 } while (condition);
138
139and
140
141 if (x == y) {
142 ..
143 } else if (x > y) {
144 ...
145 } else {
146 ....
147 }
148
149Rationale: K&R.
150
151Also, note that this brace-placement also minimizes the number of empty
152(or almost empty) lines, without any loss of readability. Thus, as the
153supply of new-lines on your screen is not a renewable resource (think
15425-line terminal screens here), you have more empty lines to put
155comments on.
156
157Do not unnecessarily use braces where a single statement will do.
158
159if (condition)
160 action();
161
162and
163
164if (condition)
165 do_this();
166else
167 do_that();
168
169This does not apply if only one branch of a conditional statement is a single
170statement; in the latter case use braces in both branches:
171
172if (condition) {
173 do_this();
174 do_that();
175} else {
176 otherwise();
177}
178
179 3.1: Spaces
180
181Linux kernel style for use of spaces depends (mostly) on
182function-versus-keyword usage. Use a space after (most) keywords. The
183notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
184somewhat like functions (and are usually used with parentheses in Linux,
185although they are not required in the language, as in: "sizeof info" after
186"struct fileinfo info;" is declared).
187
188So use a space after these keywords:
189 if, switch, case, for, do, while
190but not with sizeof, typeof, alignof, or __attribute__. E.g.,
191 s = sizeof(struct file);
192
193Do not add spaces around (inside) parenthesized expressions. This example is
194*bad*:
195
196 s = sizeof( struct file );
197
198When declaring pointer data or a function that returns a pointer type, the
199preferred use of '*' is adjacent to the data name or function name and not
200adjacent to the type name. Examples:
201
202 char *linux_banner;
203 unsigned long long memparse(char *ptr, char **retptr);
204 char *match_strdup(substring_t *s);
205
206Use one space around (on each side of) most binary and ternary operators,
207such as any of these:
208
209 = + - < > * / % | & ^ <= >= == != ? :
210
211but no space after unary operators:
212 & * + - ~ ! sizeof typeof alignof __attribute__ defined
213
214no space before the postfix increment & decrement unary operators:
215 ++ --
216
217no space after the prefix increment & decrement unary operators:
218 ++ --
219
220and no space around the '.' and "->" structure member operators.
221
222Do not leave trailing whitespace at the ends of lines. Some editors with
223"smart" indentation will insert whitespace at the beginning of new lines as
224appropriate, so you can start typing the next line of code right away.
225However, some such editors do not remove the whitespace if you end up not
226putting a line of code there, such as if you leave a blank line. As a result,
227you end up with lines containing trailing whitespace.
228
229Git will warn you about patches that introduce trailing whitespace, and can
230optionally strip the trailing whitespace for you; however, if applying a series
231of patches, this may make later patches in the series fail by changing their
232context lines.
233
234
235 Chapter 4: Naming
236
237C is a Spartan language, and so should your naming be. Unlike Modula-2
238and Pascal programmers, C programmers do not use cute names like
239ThisVariableIsATemporaryCounter. A C programmer would call that
240variable "tmp", which is much easier to write, and not the least more
241difficult to understand.
242
243HOWEVER, while mixed-case names are frowned upon, descriptive names for
244global variables are a must. To call a global function "foo" is a
245shooting offense.
246
247GLOBAL variables (to be used only if you _really_ need them) need to
248have descriptive names, as do global functions. If you have a function
249that counts the number of active users, you should call that
250"count_active_users()" or similar, you should _not_ call it "cntusr()".
251
252Encoding the type of a function into the name (so-called Hungarian
253notation) is brain damaged - the compiler knows the types anyway and can
254check those, and it only confuses the programmer. No wonder MicroSoft
255makes buggy programs.
256
257LOCAL variable names should be short, and to the point. If you have
258some random integer loop counter, it should probably be called "i".
259Calling it "loop_counter" is non-productive, if there is no chance of it
260being mis-understood. Similarly, "tmp" can be just about any type of
261variable that is used to hold a temporary value.
262
263If you are afraid to mix up your local variable names, you have another
264problem, which is called the function-growth-hormone-imbalance syndrome.
265See chapter 6 (Functions).
266
267
268 Chapter 5: Typedefs
269
270Please don't use things like "vps_t".
271
272It's a _mistake_ to use typedef for structures and pointers. When you see a
273
274 vps_t a;
275
276in the source, what does it mean?
277
278In contrast, if it says
279
280 struct virtual_container *a;
281
282you can actually tell what "a" is.
283
284Lots of people think that typedefs "help readability". Not so. They are
285useful only for:
286
287 (a) totally opaque objects (where the typedef is actively used to _hide_
288 what the object is).
289
290 Example: "pte_t" etc. opaque objects that you can only access using
291 the proper accessor functions.
292
293 NOTE! Opaqueness and "accessor functions" are not good in themselves.
294 The reason we have them for things like pte_t etc. is that there
295 really is absolutely _zero_ portably accessible information there.
296
297 (b) Clear integer types, where the abstraction _helps_ avoid confusion
298 whether it is "int" or "long".
299
300 u8/u16/u32 are perfectly fine typedefs, although they fit into
301 category (d) better than here.
302
303 NOTE! Again - there needs to be a _reason_ for this. If something is
304 "unsigned long", then there's no reason to do
305
306 typedef unsigned long myflags_t;
307
308 but if there is a clear reason for why it under certain circumstances
309 might be an "unsigned int" and under other configurations might be
310 "unsigned long", then by all means go ahead and use a typedef.
311
312 (c) when you use sparse to literally create a _new_ type for
313 type-checking.
314
315 (d) New types which are identical to standard C99 types, in certain
316 exceptional circumstances.
317
318 Although it would only take a short amount of time for the eyes and
319 brain to become accustomed to the standard types like 'uint32_t',
320 some people object to their use anyway.
321
322 Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
323 signed equivalents which are identical to standard types are
324 permitted -- although they are not mandatory in new code of your
325 own.
326
327 When editing existing code which already uses one or the other set
328 of types, you should conform to the existing choices in that code.
329
330 (e) Types safe for use in userspace.
331
332 In certain structures which are visible to userspace, we cannot
333 require C99 types and cannot use the 'u32' form above. Thus, we
334 use __u32 and similar types in all structures which are shared
335 with userspace.
336
337Maybe there are other cases too, but the rule should basically be to NEVER
338EVER use a typedef unless you can clearly match one of those rules.
339
340In general, a pointer, or a struct that has elements that can reasonably
341be directly accessed should _never_ be a typedef.
342
343
344 Chapter 6: Functions
345
346Functions should be short and sweet, and do just one thing. They should
347fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
348as we all know), and do one thing and do that well.
349
350The maximum length of a function is inversely proportional to the
351complexity and indentation level of that function. So, if you have a
352conceptually simple function that is just one long (but simple)
353case-statement, where you have to do lots of small things for a lot of
354different cases, it's OK to have a longer function.
355
356However, if you have a complex function, and you suspect that a
357less-than-gifted first-year high-school student might not even
358understand what the function is all about, you should adhere to the
359maximum limits all the more closely. Use helper functions with
360descriptive names (you can ask the compiler to in-line them if you think
361it's performance-critical, and it will probably do a better job of it
362than you would have done).
363
364Another measure of the function is the number of local variables. They
365shouldn't exceed 5-10, or you're doing something wrong. Re-think the
366function, and split it into smaller pieces. A human brain can
367generally easily keep track of about 7 different things, anything more
368and it gets confused. You know you're brilliant, but maybe you'd like
369to understand what you did 2 weeks from now.
370
371In source files, separate functions with one blank line. If the function is
372exported, the EXPORT* macro for it should follow immediately after the closing
373function brace line. E.g.:
374
375int system_is_up(void)
376{
377 return system_state == SYSTEM_RUNNING;
378}
379EXPORT_SYMBOL(system_is_up);
380
381In function prototypes, include parameter names with their data types.
382Although this is not required by the C language, it is preferred in Linux
383because it is a simple way to add valuable information for the reader.
384
385
386 Chapter 7: Centralized exiting of functions
387
388Albeit deprecated by some people, the equivalent of the goto statement is
389used frequently by compilers in form of the unconditional jump instruction.
390
391The goto statement comes in handy when a function exits from multiple
392locations and some common work such as cleanup has to be done. If there is no
393cleanup needed then just return directly.
394
395The rationale is:
396
397- unconditional statements are easier to understand and follow
398- nesting is reduced
399- errors by not updating individual exit points when making
400 modifications are prevented
401- saves the compiler work to optimize redundant code away ;)
402
403int fun(int a)
404{
405 int result = 0;
406 char *buffer = kmalloc(SIZE);
407
408 if (buffer == NULL)
409 return -ENOMEM;
410
411 if (condition1) {
412 while (loop1) {
413 ...
414 }
415 result = 1;
416 goto out;
417 }
418 ...
419out:
420 kfree(buffer);
421 return result;
422}
423
424 Chapter 8: Commenting
425
426Comments are good, but there is also a danger of over-commenting. NEVER
427try to explain HOW your code works in a comment: it's much better to
428write the code so that the _working_ is obvious, and it's a waste of
429time to explain badly written code.
430
431Generally, you want your comments to tell WHAT your code does, not HOW.
432Also, try to avoid putting comments inside a function body: if the
433function is so complex that you need to separately comment parts of it,
434you should probably go back to chapter 6 for a while. You can make
435small comments to note or warn about something particularly clever (or
436ugly), but try to avoid excess. Instead, put the comments at the head
437of the function, telling people what it does, and possibly WHY it does
438it.
439
440When commenting the kernel API functions, please use the kernel-doc format.
441See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
442for details.
443
444Linux style for comments is the C89 "/* ... */" style.
445Don't use C99-style "// ..." comments.
446
447The preferred style for long (multi-line) comments is:
448
449 /*
450 * This is the preferred style for multi-line
451 * comments in the Linux kernel source code.
452 * Please use it consistently.
453 *
454 * Description: A column of asterisks on the left side,
455 * with beginning and ending almost-blank lines.
456 */
457
458For files in net/ and drivers/net/ the preferred style for long (multi-line)
459comments is a little different.
460
461 /* The preferred comment style for files in net/ and drivers/net
462 * looks like this.
463 *
464 * It is nearly the same as the generally preferred comment style,
465 * but there is no initial almost-blank line.
466 */
467
468It's also important to comment data, whether they are basic types or derived
469types. To this end, use just one data declaration per line (no commas for
470multiple data declarations). This leaves you room for a small comment on each
471item, explaining its use.
472
473
474 Chapter 9: You've made a mess of it
475
476That's OK, we all do. You've probably been told by your long-time Unix
477user helper that "GNU emacs" automatically formats the C sources for
478you, and you've noticed that yes, it does do that, but the defaults it
479uses are less than desirable (in fact, they are worse than random
480typing - an infinite number of monkeys typing into GNU emacs would never
481make a good program).
482
483So, you can either get rid of GNU emacs, or change it to use saner
484values. To do the latter, you can stick the following in your .emacs file:
485
486(defun c-lineup-arglist-tabs-only (ignored)
487 "Line up argument lists by tabs, not spaces"
488 (let* ((anchor (c-langelem-pos c-syntactic-element))
489 (column (c-langelem-2nd-pos c-syntactic-element))
490 (offset (- (1+ column) anchor))
491 (steps (floor offset c-basic-offset)))
492 (* (max steps 1)
493 c-basic-offset)))
494
495(add-hook 'c-mode-common-hook
496 (lambda ()
497 ;; Add kernel style
498 (c-add-style
499 "linux-tabs-only"
500 '("linux" (c-offsets-alist
501 (arglist-cont-nonempty
502 c-lineup-gcc-asm-reg
503 c-lineup-arglist-tabs-only))))))
504
505(add-hook 'c-mode-hook
506 (lambda ()
507 (let ((filename (buffer-file-name)))
508 ;; Enable kernel mode for the appropriate files
509 (when (and filename
510 (string-match (expand-file-name "~/src/linux-trees")
511 filename))
512 (setq indent-tabs-mode t)
513 (c-set-style "linux-tabs-only")))))
514
515This will make emacs go better with the kernel coding style for C
516files below ~/src/linux-trees.
517
518But even if you fail in getting emacs to do sane formatting, not
519everything is lost: use "indent".
520
521Now, again, GNU indent has the same brain-dead settings that GNU emacs
522has, which is why you need to give it a few command line options.
523However, that's not too bad, because even the makers of GNU indent
524recognize the authority of K&R (the GNU people aren't evil, they are
525just severely misguided in this matter), so you just give indent the
526options "-kr -i8" (stands for "K&R, 8 character indents"), or use
527"scripts/Lindent", which indents in the latest style.
528
529"indent" has a lot of options, and especially when it comes to comment
530re-formatting you may want to take a look at the man page. But
531remember: "indent" is not a fix for bad programming.
532
533
534 Chapter 10: Kconfig configuration files
535
536For all of the Kconfig* configuration files throughout the source tree,
537the indentation is somewhat different. Lines under a "config" definition
538are indented with one tab, while help text is indented an additional two
539spaces. Example:
540
541config AUDIT
542 bool "Auditing support"
543 depends on NET
544 help
545 Enable auditing infrastructure that can be used with another
546 kernel subsystem, such as SELinux (which requires this for
547 logging of avc messages output). Does not do system-call
548 auditing without CONFIG_AUDITSYSCALL.
549
550Seriously dangerous features (such as write support for certain
551filesystems) should advertise this prominently in their prompt string:
552
553config ADFS_FS_RW
554 bool "ADFS write support (DANGEROUS)"
555 depends on ADFS_FS
556 ...
557
558For full documentation on the configuration files, see the file
559Documentation/kbuild/kconfig-language.txt.
560
561
562 Chapter 11: Data structures
563
564Data structures that have visibility outside the single-threaded
565environment they are created and destroyed in should always have
566reference counts. In the kernel, garbage collection doesn't exist (and
567outside the kernel garbage collection is slow and inefficient), which
568means that you absolutely _have_ to reference count all your uses.
569
570Reference counting means that you can avoid locking, and allows multiple
571users to have access to the data structure in parallel - and not having
572to worry about the structure suddenly going away from under them just
573because they slept or did something else for a while.
574
575Note that locking is _not_ a replacement for reference counting.
576Locking is used to keep data structures coherent, while reference
577counting is a memory management technique. Usually both are needed, and
578they are not to be confused with each other.
579
580Many data structures can indeed have two levels of reference counting,
581when there are users of different "classes". The subclass count counts
582the number of subclass users, and decrements the global count just once
583when the subclass count goes to zero.
584
585Examples of this kind of "multi-level-reference-counting" can be found in
586memory management ("struct mm_struct": mm_users and mm_count), and in
587filesystem code ("struct super_block": s_count and s_active).
588
589Remember: if another thread can find your data structure, and you don't
590have a reference count on it, you almost certainly have a bug.
591
592
593 Chapter 12: Macros, Enums and RTL
594
595Names of macros defining constants and labels in enums are capitalized.
596
597#define CONSTANT 0x12345
598
599Enums are preferred when defining several related constants.
600
601CAPITALIZED macro names are appreciated but macros resembling functions
602may be named in lower case.
603
604Generally, inline functions are preferable to macros resembling functions.
605
606Macros with multiple statements should be enclosed in a do - while block:
607
608#define macrofun(a, b, c) \
609 do { \
610 if (a == 5) \
611 do_this(b, c); \
612 } while (0)
613
614Things to avoid when using macros:
615
6161) macros that affect control flow:
617
618#define FOO(x) \
619 do { \
620 if (blah(x) < 0) \
621 return -EBUGGERED; \
622 } while(0)
623
624is a _very_ bad idea. It looks like a function call but exits the "calling"
625function; don't break the internal parsers of those who will read the code.
626
6272) macros that depend on having a local variable with a magic name:
628
629#define FOO(val) bar(index, val)
630
631might look like a good thing, but it's confusing as hell when one reads the
632code and it's prone to breakage from seemingly innocent changes.
633
6343) macros with arguments that are used as l-values: FOO(x) = y; will
635bite you if somebody e.g. turns FOO into an inline function.
636
6374) forgetting about precedence: macros defining constants using expressions
638must enclose the expression in parentheses. Beware of similar issues with
639macros using parameters.
640
641#define CONSTANT 0x4000
642#define CONSTEXP (CONSTANT | 3)
643
644The cpp manual deals with macros exhaustively. The gcc internals manual also
645covers RTL which is used frequently with assembly language in the kernel.
646
647
648 Chapter 13: Printing kernel messages
649
650Kernel developers like to be seen as literate. Do mind the spelling
651of kernel messages to make a good impression. Do not use crippled
652words like "dont"; use "do not" or "don't" instead. Make the messages
653concise, clear, and unambiguous.
654
655Kernel messages do not have to be terminated with a period.
656
657Printing numbers in parentheses (%d) adds no value and should be avoided.
658
659There are a number of driver model diagnostic macros in <linux/device.h>
660which you should use to make sure messages are matched to the right device
661and driver, and are tagged with the right level: dev_err(), dev_warn(),
662dev_info(), and so forth. For messages that aren't associated with a
663particular device, <linux/printk.h> defines pr_debug() and pr_info().
664
665Coming up with good debugging messages can be quite a challenge; and once
666you have them, they can be a huge help for remote troubleshooting. Such
667messages should be compiled out when the DEBUG symbol is not defined (that
668is, by default they are not included). When you use dev_dbg() or pr_debug(),
669that's automatic. Many subsystems have Kconfig options to turn on -DDEBUG.
670A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the
671ones already enabled by DEBUG.
672
673
674 Chapter 14: Allocating memory
675
676The kernel provides the following general purpose memory allocators:
677kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
678vzalloc(). Please refer to the API documentation for further information
679about them.
680
681The preferred form for passing a size of a struct is the following:
682
683 p = kmalloc(sizeof(*p), ...);
684
685The alternative form where struct name is spelled out hurts readability and
686introduces an opportunity for a bug when the pointer variable type is changed
687but the corresponding sizeof that is passed to a memory allocator is not.
688
689Casting the return value which is a void pointer is redundant. The conversion
690from void pointer to any other pointer type is guaranteed by the C programming
691language.
692
693The preferred form for allocating an array is the following:
694
695 p = kmalloc_array(n, sizeof(...), ...);
696
697The preferred form for allocating a zeroed array is the following:
698
699 p = kcalloc(n, sizeof(...), ...);
700
701Both forms check for overflow on the allocation size n * sizeof(...),
702and return NULL if that occurred.
703
704
705 Chapter 15: The inline disease
706
707There appears to be a common misperception that gcc has a magic "make me
708faster" speedup option called "inline". While the use of inlines can be
709appropriate (for example as a means of replacing macros, see Chapter 12), it
710very often is not. Abundant use of the inline keyword leads to a much bigger
711kernel, which in turn slows the system as a whole down, due to a bigger
712icache footprint for the CPU and simply because there is less memory
713available for the pagecache. Just think about it; a pagecache miss causes a
714disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
715that can go into these 5 milliseconds.
716
717A reasonable rule of thumb is to not put inline at functions that have more
718than 3 lines of code in them. An exception to this rule are the cases where
719a parameter is known to be a compiletime constant, and as a result of this
720constantness you *know* the compiler will be able to optimize most of your
721function away at compile time. For a good example of this later case, see
722the kmalloc() inline function.
723
724Often people argue that adding inline to functions that are static and used
725only once is always a win since there is no space tradeoff. While this is
726technically correct, gcc is capable of inlining these automatically without
727help, and the maintenance issue of removing the inline when a second user
728appears outweighs the potential value of the hint that tells gcc to do
729something it would have done anyway.
730
731
732 Chapter 16: Function return values and names
733
734Functions can return values of many different kinds, and one of the
735most common is a value indicating whether the function succeeded or
736failed. Such a value can be represented as an error-code integer
737(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
738non-zero = success).
739
740Mixing up these two sorts of representations is a fertile source of
741difficult-to-find bugs. If the C language included a strong distinction
742between integers and booleans then the compiler would find these mistakes
743for us... but it doesn't. To help prevent such bugs, always follow this
744convention:
745
746 If the name of a function is an action or an imperative command,
747 the function should return an error-code integer. If the name
748 is a predicate, the function should return a "succeeded" boolean.
749
750For example, "add work" is a command, and the add_work() function returns 0
751for success or -EBUSY for failure. In the same way, "PCI device present" is
752a predicate, and the pci_dev_present() function returns 1 if it succeeds in
753finding a matching device or 0 if it doesn't.
754
755All EXPORTed functions must respect this convention, and so should all
756public functions. Private (static) functions need not, but it is
757recommended that they do.
758
759Functions whose return value is the actual result of a computation, rather
760than an indication of whether the computation succeeded, are not subject to
761this rule. Generally they indicate failure by returning some out-of-range
762result. Typical examples would be functions that return pointers; they use
763NULL or the ERR_PTR mechanism to report failure.
764
765
766 Chapter 17: Don't re-invent the kernel macros
767
768The header file include/linux/kernel.h contains a number of macros that
769you should use, rather than explicitly coding some variant of them yourself.
770For example, if you need to calculate the length of an array, take advantage
771of the macro
772
773 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
774
775Similarly, if you need to calculate the size of some structure member, use
776
777 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
778
779There are also min() and max() macros that do strict type checking if you
780need them. Feel free to peruse that header file to see what else is already
781defined that you shouldn't reproduce in your code.
782
783
784 Chapter 18: Editor modelines and other cruft
785
786Some editors can interpret configuration information embedded in source files,
787indicated with special markers. For example, emacs interprets lines marked
788like this:
789
790-*- mode: c -*-
791
792Or like this:
793
794/*
795Local Variables:
796compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
797End:
798*/
799
800Vim interprets markers that look like this:
801
802/* vim:set sw=8 noet */
803
804Do not include any of these in source files. People have their own personal
805editor configurations, and your source files should not override them. This
806includes markers for indentation and mode configuration. People may use their
807own custom mode, or may have some other magic method for making indentation
808work correctly.
809
810
811 Chapter 19: Inline assembly
812
813In architecture-specific code, you may need to use inline assembly to interface
814with CPU or platform functionality. Don't hesitate to do so when necessary.
815However, don't use inline assembly gratuitously when C can do the job. You can
816and should poke hardware from C when possible.
817
818Consider writing simple helper functions that wrap common bits of inline
819assembly, rather than repeatedly writing them with slight variations. Remember
820that inline assembly can use C parameters.
821
822Large, non-trivial assembly functions should go in .S files, with corresponding
823C prototypes defined in C header files. The C prototypes for assembly
824functions should use "asmlinkage".
825
826You may need to mark your asm statement as volatile, to prevent GCC from
827removing it if GCC doesn't notice any side effects. You don't always need to
828do so, though, and doing so unnecessarily can limit optimization.
829
830When writing a single inline assembly statement containing multiple
831instructions, put each instruction on a separate line in a separate quoted
832string, and end each string except the last with \n\t to properly indent the
833next instruction in the assembly output:
834
835 asm ("magic %reg1, #42\n\t"
836 "more_magic %reg2, %reg3"
837 : /* outputs */ : /* inputs */ : /* clobbers */);
838
839
840
841 Appendix I: References
842
843The C Programming Language, Second Edition
844by Brian W. Kernighan and Dennis M. Ritchie.
845Prentice Hall, Inc., 1988.
846ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
847URL: http://cm.bell-labs.com/cm/cs/cbook/
848
849The Practice of Programming
850by Brian W. Kernighan and Rob Pike.
851Addison-Wesley, Inc., 1999.
852ISBN 0-201-61586-X.
853URL: http://cm.bell-labs.com/cm/cs/tpop/
854
855GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
856gcc internals and indent, all available from http://www.gnu.org/manual/
857
858WG14 is the international standardization working group for the programming
859language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
860
861Kernel CodingStyle, by greg@kroah.com at OLS 2002:
862http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
863