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1.. _codingstyle:
2
3Linux kernel coding style
4=========================
5
6This is a short document describing the preferred coding style for the
7linux kernel. Coding style is very personal, and I won't **force** my
8views on anybody, but this is what goes for anything that I have to be
9able to maintain, and I'd prefer it for most other things too. Please
10at least consider the points made here.
11
12First off, I'd suggest printing out a copy of the GNU coding standards,
13and NOT read it. Burn them, it's a great symbolic gesture.
14
15Anyway, here goes:
16
17
181) Indentation
19--------------
20
21Tabs are 8 characters, and thus indentations are also 8 characters.
22There are heretic movements that try to make indentations 4 (or even 2!)
23characters deep, and that is akin to trying to define the value of PI to
24be 3.
25
26Rationale: The whole idea behind indentation is to clearly define where
27a block of control starts and ends. Especially when you've been looking
28at your screen for 20 straight hours, you'll find it a lot easier to see
29how the indentation works if you have large indentations.
30
31Now, some people will claim that having 8-character indentations makes
32the code move too far to the right, and makes it hard to read on a
3380-character terminal screen. The answer to that is that if you need
34more than 3 levels of indentation, you're screwed anyway, and should fix
35your program.
36
37In short, 8-char indents make things easier to read, and have the added
38benefit of warning you when you're nesting your functions too deep.
39Heed that warning.
40
41The preferred way to ease multiple indentation levels in a switch statement is
42to align the ``switch`` and its subordinate ``case`` labels in the same column
43instead of ``double-indenting`` the ``case`` labels. E.g.:
44
45.. code-block:: c
46
47 switch (suffix) {
48 case 'G':
49 case 'g':
50 mem <<= 30;
51 break;
52 case 'M':
53 case 'm':
54 mem <<= 20;
55 break;
56 case 'K':
57 case 'k':
58 mem <<= 10;
59 /* fall through */
60 default:
61 break;
62 }
63
64Don't put multiple statements on a single line unless you have
65something to hide:
66
67.. code-block:: c
68
69 if (condition) do_this;
70 do_something_everytime;
71
72Don't put multiple assignments on a single line either. Kernel coding style
73is super simple. Avoid tricky expressions.
74
75Outside of comments, documentation and except in Kconfig, spaces are never
76used for indentation, and the above example is deliberately broken.
77
78Get a decent editor and don't leave whitespace at the end of lines.
79
80
812) Breaking long lines and strings
82----------------------------------
83
84Coding style is all about readability and maintainability using commonly
85available tools.
86
87The limit on the length of lines is 80 columns and this is a strongly
88preferred limit.
89
90Statements longer than 80 columns will be broken into sensible chunks, unless
91exceeding 80 columns significantly increases readability and does not hide
92information. Descendants are always substantially shorter than the parent and
93are placed substantially to the right. The same applies to function headers
94with a long argument list. However, never break user-visible strings such as
95printk messages, because that breaks the ability to grep for them.
96
97
983) Placing Braces and Spaces
99----------------------------
100
101The other issue that always comes up in C styling is the placement of
102braces. Unlike the indent size, there are few technical reasons to
103choose one placement strategy over the other, but the preferred way, as
104shown to us by the prophets Kernighan and Ritchie, is to put the opening
105brace last on the line, and put the closing brace first, thusly:
106
107.. code-block:: c
108
109 if (x is true) {
110 we do y
111 }
112
113This applies to all non-function statement blocks (if, switch, for,
114while, do). E.g.:
115
116.. code-block:: c
117
118 switch (action) {
119 case KOBJ_ADD:
120 return "add";
121 case KOBJ_REMOVE:
122 return "remove";
123 case KOBJ_CHANGE:
124 return "change";
125 default:
126 return NULL;
127 }
128
129However, there is one special case, namely functions: they have the
130opening brace at the beginning of the next line, thus:
131
132.. code-block:: c
133
134 int function(int x)
135 {
136 body of function
137 }
138
139Heretic people all over the world have claimed that this inconsistency
140is ... well ... inconsistent, but all right-thinking people know that
141(a) K&R are **right** and (b) K&R are right. Besides, functions are
142special anyway (you can't nest them in C).
143
144Note that the closing brace is empty on a line of its own, **except** in
145the cases where it is followed by a continuation of the same statement,
146ie a ``while`` in a do-statement or an ``else`` in an if-statement, like
147this:
148
149.. code-block:: c
150
151 do {
152 body of do-loop
153 } while (condition);
154
155and
156
157.. code-block:: c
158
159 if (x == y) {
160 ..
161 } else if (x > y) {
162 ...
163 } else {
164 ....
165 }
166
167Rationale: K&R.
168
169Also, note that this brace-placement also minimizes the number of empty
170(or almost empty) lines, without any loss of readability. Thus, as the
171supply of new-lines on your screen is not a renewable resource (think
17225-line terminal screens here), you have more empty lines to put
173comments on.
174
175Do not unnecessarily use braces where a single statement will do.
176
177.. code-block:: c
178
179 if (condition)
180 action();
181
182and
183
184.. code-block:: none
185
186 if (condition)
187 do_this();
188 else
189 do_that();
190
191This does not apply if only one branch of a conditional statement is a single
192statement; in the latter case use braces in both branches:
193
194.. code-block:: c
195
196 if (condition) {
197 do_this();
198 do_that();
199 } else {
200 otherwise();
201 }
202
2033.1) Spaces
204***********
205
206Linux kernel style for use of spaces depends (mostly) on
207function-versus-keyword usage. Use a space after (most) keywords. The
208notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
209somewhat like functions (and are usually used with parentheses in Linux,
210although they are not required in the language, as in: ``sizeof info`` after
211``struct fileinfo info;`` is declared).
212
213So use a space after these keywords::
214
215 if, switch, case, for, do, while
216
217but not with sizeof, typeof, alignof, or __attribute__. E.g.,
218
219.. code-block:: c
220
221
222 s = sizeof(struct file);
223
224Do not add spaces around (inside) parenthesized expressions. This example is
225**bad**:
226
227.. code-block:: c
228
229
230 s = sizeof( struct file );
231
232When declaring pointer data or a function that returns a pointer type, the
233preferred use of ``*`` is adjacent to the data name or function name and not
234adjacent to the type name. Examples:
235
236.. code-block:: c
237
238
239 char *linux_banner;
240 unsigned long long memparse(char *ptr, char **retptr);
241 char *match_strdup(substring_t *s);
242
243Use one space around (on each side of) most binary and ternary operators,
244such as any of these::
245
246 = + - < > * / % | & ^ <= >= == != ? :
247
248but no space after unary operators::
249
250 & * + - ~ ! sizeof typeof alignof __attribute__ defined
251
252no space before the postfix increment & decrement unary operators::
253
254 ++ --
255
256no space after the prefix increment & decrement unary operators::
257
258 ++ --
259
260and no space around the ``.`` and ``->`` structure member operators.
261
262Do not leave trailing whitespace at the ends of lines. Some editors with
263``smart`` indentation will insert whitespace at the beginning of new lines as
264appropriate, so you can start typing the next line of code right away.
265However, some such editors do not remove the whitespace if you end up not
266putting a line of code there, such as if you leave a blank line. As a result,
267you end up with lines containing trailing whitespace.
268
269Git will warn you about patches that introduce trailing whitespace, and can
270optionally strip the trailing whitespace for you; however, if applying a series
271of patches, this may make later patches in the series fail by changing their
272context lines.
273
274
2754) Naming
276---------
277
278C is a Spartan language, and so should your naming be. Unlike Modula-2
279and Pascal programmers, C programmers do not use cute names like
280ThisVariableIsATemporaryCounter. A C programmer would call that
281variable ``tmp``, which is much easier to write, and not the least more
282difficult to understand.
283
284HOWEVER, while mixed-case names are frowned upon, descriptive names for
285global variables are a must. To call a global function ``foo`` is a
286shooting offense.
287
288GLOBAL variables (to be used only if you **really** need them) need to
289have descriptive names, as do global functions. If you have a function
290that counts the number of active users, you should call that
291``count_active_users()`` or similar, you should **not** call it ``cntusr()``.
292
293Encoding the type of a function into the name (so-called Hungarian
294notation) is brain damaged - the compiler knows the types anyway and can
295check those, and it only confuses the programmer. No wonder MicroSoft
296makes buggy programs.
297
298LOCAL variable names should be short, and to the point. If you have
299some random integer loop counter, it should probably be called ``i``.
300Calling it ``loop_counter`` is non-productive, if there is no chance of it
301being mis-understood. Similarly, ``tmp`` can be just about any type of
302variable that is used to hold a temporary value.
303
304If you are afraid to mix up your local variable names, you have another
305problem, which is called the function-growth-hormone-imbalance syndrome.
306See chapter 6 (Functions).
307
308
3095) Typedefs
310-----------
311
312Please don't use things like ``vps_t``.
313It's a **mistake** to use typedef for structures and pointers. When you see a
314
315.. code-block:: c
316
317
318 vps_t a;
319
320in the source, what does it mean?
321In contrast, if it says
322
323.. code-block:: c
324
325 struct virtual_container *a;
326
327you can actually tell what ``a`` is.
328
329Lots of people think that typedefs ``help readability``. Not so. They are
330useful only for:
331
332 (a) totally opaque objects (where the typedef is actively used to **hide**
333 what the object is).
334
335 Example: ``pte_t`` etc. opaque objects that you can only access using
336 the proper accessor functions.
337
338 .. note::
339
340 Opaqueness and ``accessor functions`` are not good in themselves.
341 The reason we have them for things like pte_t etc. is that there
342 really is absolutely **zero** portably accessible information there.
343
344 (b) Clear integer types, where the abstraction **helps** avoid confusion
345 whether it is ``int`` or ``long``.
346
347 u8/u16/u32 are perfectly fine typedefs, although they fit into
348 category (d) better than here.
349
350 .. note::
351
352 Again - there needs to be a **reason** for this. If something is
353 ``unsigned long``, then there's no reason to do
354
355 typedef unsigned long myflags_t;
356
357 but if there is a clear reason for why it under certain circumstances
358 might be an ``unsigned int`` and under other configurations might be
359 ``unsigned long``, then by all means go ahead and use a typedef.
360
361 (c) when you use sparse to literally create a **new** type for
362 type-checking.
363
364 (d) New types which are identical to standard C99 types, in certain
365 exceptional circumstances.
366
367 Although it would only take a short amount of time for the eyes and
368 brain to become accustomed to the standard types like ``uint32_t``,
369 some people object to their use anyway.
370
371 Therefore, the Linux-specific ``u8/u16/u32/u64`` types and their
372 signed equivalents which are identical to standard types are
373 permitted -- although they are not mandatory in new code of your
374 own.
375
376 When editing existing code which already uses one or the other set
377 of types, you should conform to the existing choices in that code.
378
379 (e) Types safe for use in userspace.
380
381 In certain structures which are visible to userspace, we cannot
382 require C99 types and cannot use the ``u32`` form above. Thus, we
383 use __u32 and similar types in all structures which are shared
384 with userspace.
385
386Maybe there are other cases too, but the rule should basically be to NEVER
387EVER use a typedef unless you can clearly match one of those rules.
388
389In general, a pointer, or a struct that has elements that can reasonably
390be directly accessed should **never** be a typedef.
391
392
3936) Functions
394------------
395
396Functions should be short and sweet, and do just one thing. They should
397fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
398as we all know), and do one thing and do that well.
399
400The maximum length of a function is inversely proportional to the
401complexity and indentation level of that function. So, if you have a
402conceptually simple function that is just one long (but simple)
403case-statement, where you have to do lots of small things for a lot of
404different cases, it's OK to have a longer function.
405
406However, if you have a complex function, and you suspect that a
407less-than-gifted first-year high-school student might not even
408understand what the function is all about, you should adhere to the
409maximum limits all the more closely. Use helper functions with
410descriptive names (you can ask the compiler to in-line them if you think
411it's performance-critical, and it will probably do a better job of it
412than you would have done).
413
414Another measure of the function is the number of local variables. They
415shouldn't exceed 5-10, or you're doing something wrong. Re-think the
416function, and split it into smaller pieces. A human brain can
417generally easily keep track of about 7 different things, anything more
418and it gets confused. You know you're brilliant, but maybe you'd like
419to understand what you did 2 weeks from now.
420
421In source files, separate functions with one blank line. If the function is
422exported, the **EXPORT** macro for it should follow immediately after the
423closing function brace line. E.g.:
424
425.. code-block:: c
426
427 int system_is_up(void)
428 {
429 return system_state == SYSTEM_RUNNING;
430 }
431 EXPORT_SYMBOL(system_is_up);
432
433In function prototypes, include parameter names with their data types.
434Although this is not required by the C language, it is preferred in Linux
435because it is a simple way to add valuable information for the reader.
436
437
4387) Centralized exiting of functions
439-----------------------------------
440
441Albeit deprecated by some people, the equivalent of the goto statement is
442used frequently by compilers in form of the unconditional jump instruction.
443
444The goto statement comes in handy when a function exits from multiple
445locations and some common work such as cleanup has to be done. If there is no
446cleanup needed then just return directly.
447
448Choose label names which say what the goto does or why the goto exists. An
449example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``.
450Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to
451renumber them if you ever add or remove exit paths, and they make correctness
452difficult to verify anyway.
453
454The rationale for using gotos is:
455
456- unconditional statements are easier to understand and follow
457- nesting is reduced
458- errors by not updating individual exit points when making
459 modifications are prevented
460- saves the compiler work to optimize redundant code away ;)
461
462.. code-block:: c
463
464 int fun(int a)
465 {
466 int result = 0;
467 char *buffer;
468
469 buffer = kmalloc(SIZE, GFP_KERNEL);
470 if (!buffer)
471 return -ENOMEM;
472
473 if (condition1) {
474 while (loop1) {
475 ...
476 }
477 result = 1;
478 goto out_free_buffer;
479 }
480 ...
481 out_free_buffer:
482 kfree(buffer);
483 return result;
484 }
485
486A common type of bug to be aware of is ``one err bugs`` which look like this:
487
488.. code-block:: c
489
490 err:
491 kfree(foo->bar);
492 kfree(foo);
493 return ret;
494
495The bug in this code is that on some exit paths ``foo`` is NULL. Normally the
496fix for this is to split it up into two error labels ``err_free_bar:`` and
497``err_free_foo:``:
498
499.. code-block:: c
500
501 err_free_bar:
502 kfree(foo->bar);
503 err_free_foo:
504 kfree(foo);
505 return ret;
506
507Ideally you should simulate errors to test all exit paths.
508
509
5108) Commenting
511-------------
512
513Comments are good, but there is also a danger of over-commenting. NEVER
514try to explain HOW your code works in a comment: it's much better to
515write the code so that the **working** is obvious, and it's a waste of
516time to explain badly written code.
517
518Generally, you want your comments to tell WHAT your code does, not HOW.
519Also, try to avoid putting comments inside a function body: if the
520function is so complex that you need to separately comment parts of it,
521you should probably go back to chapter 6 for a while. You can make
522small comments to note or warn about something particularly clever (or
523ugly), but try to avoid excess. Instead, put the comments at the head
524of the function, telling people what it does, and possibly WHY it does
525it.
526
527When commenting the kernel API functions, please use the kernel-doc format.
528See the files at :ref:`Documentation/doc-guide/ <doc_guide>` and
529``scripts/kernel-doc`` for details.
530
531The preferred style for long (multi-line) comments is:
532
533.. code-block:: c
534
535 /*
536 * This is the preferred style for multi-line
537 * comments in the Linux kernel source code.
538 * Please use it consistently.
539 *
540 * Description: A column of asterisks on the left side,
541 * with beginning and ending almost-blank lines.
542 */
543
544For files in net/ and drivers/net/ the preferred style for long (multi-line)
545comments is a little different.
546
547.. code-block:: c
548
549 /* The preferred comment style for files in net/ and drivers/net
550 * looks like this.
551 *
552 * It is nearly the same as the generally preferred comment style,
553 * but there is no initial almost-blank line.
554 */
555
556It's also important to comment data, whether they are basic types or derived
557types. To this end, use just one data declaration per line (no commas for
558multiple data declarations). This leaves you room for a small comment on each
559item, explaining its use.
560
561
5629) You've made a mess of it
563---------------------------
564
565That's OK, we all do. You've probably been told by your long-time Unix
566user helper that ``GNU emacs`` automatically formats the C sources for
567you, and you've noticed that yes, it does do that, but the defaults it
568uses are less than desirable (in fact, they are worse than random
569typing - an infinite number of monkeys typing into GNU emacs would never
570make a good program).
571
572So, you can either get rid of GNU emacs, or change it to use saner
573values. To do the latter, you can stick the following in your .emacs file:
574
575.. code-block:: none
576
577 (defun c-lineup-arglist-tabs-only (ignored)
578 "Line up argument lists by tabs, not spaces"
579 (let* ((anchor (c-langelem-pos c-syntactic-element))
580 (column (c-langelem-2nd-pos c-syntactic-element))
581 (offset (- (1+ column) anchor))
582 (steps (floor offset c-basic-offset)))
583 (* (max steps 1)
584 c-basic-offset)))
585
586 (add-hook 'c-mode-common-hook
587 (lambda ()
588 ;; Add kernel style
589 (c-add-style
590 "linux-tabs-only"
591 '("linux" (c-offsets-alist
592 (arglist-cont-nonempty
593 c-lineup-gcc-asm-reg
594 c-lineup-arglist-tabs-only))))))
595
596 (add-hook 'c-mode-hook
597 (lambda ()
598 (let ((filename (buffer-file-name)))
599 ;; Enable kernel mode for the appropriate files
600 (when (and filename
601 (string-match (expand-file-name "~/src/linux-trees")
602 filename))
603 (setq indent-tabs-mode t)
604 (setq show-trailing-whitespace t)
605 (c-set-style "linux-tabs-only")))))
606
607This will make emacs go better with the kernel coding style for C
608files below ``~/src/linux-trees``.
609
610But even if you fail in getting emacs to do sane formatting, not
611everything is lost: use ``indent``.
612
613Now, again, GNU indent has the same brain-dead settings that GNU emacs
614has, which is why you need to give it a few command line options.
615However, that's not too bad, because even the makers of GNU indent
616recognize the authority of K&R (the GNU people aren't evil, they are
617just severely misguided in this matter), so you just give indent the
618options ``-kr -i8`` (stands for ``K&R, 8 character indents``), or use
619``scripts/Lindent``, which indents in the latest style.
620
621``indent`` has a lot of options, and especially when it comes to comment
622re-formatting you may want to take a look at the man page. But
623remember: ``indent`` is not a fix for bad programming.
624
625
62610) Kconfig configuration files
627-------------------------------
628
629For all of the Kconfig* configuration files throughout the source tree,
630the indentation is somewhat different. Lines under a ``config`` definition
631are indented with one tab, while help text is indented an additional two
632spaces. Example::
633
634 config AUDIT
635 bool "Auditing support"
636 depends on NET
637 help
638 Enable auditing infrastructure that can be used with another
639 kernel subsystem, such as SELinux (which requires this for
640 logging of avc messages output). Does not do system-call
641 auditing without CONFIG_AUDITSYSCALL.
642
643Seriously dangerous features (such as write support for certain
644filesystems) should advertise this prominently in their prompt string::
645
646 config ADFS_FS_RW
647 bool "ADFS write support (DANGEROUS)"
648 depends on ADFS_FS
649 ...
650
651For full documentation on the configuration files, see the file
652Documentation/kbuild/kconfig-language.txt.
653
654
65511) Data structures
656-------------------
657
658Data structures that have visibility outside the single-threaded
659environment they are created and destroyed in should always have
660reference counts. In the kernel, garbage collection doesn't exist (and
661outside the kernel garbage collection is slow and inefficient), which
662means that you absolutely **have** to reference count all your uses.
663
664Reference counting means that you can avoid locking, and allows multiple
665users to have access to the data structure in parallel - and not having
666to worry about the structure suddenly going away from under them just
667because they slept or did something else for a while.
668
669Note that locking is **not** a replacement for reference counting.
670Locking is used to keep data structures coherent, while reference
671counting is a memory management technique. Usually both are needed, and
672they are not to be confused with each other.
673
674Many data structures can indeed have two levels of reference counting,
675when there are users of different ``classes``. The subclass count counts
676the number of subclass users, and decrements the global count just once
677when the subclass count goes to zero.
678
679Examples of this kind of ``multi-level-reference-counting`` can be found in
680memory management (``struct mm_struct``: mm_users and mm_count), and in
681filesystem code (``struct super_block``: s_count and s_active).
682
683Remember: if another thread can find your data structure, and you don't
684have a reference count on it, you almost certainly have a bug.
685
686
68712) Macros, Enums and RTL
688-------------------------
689
690Names of macros defining constants and labels in enums are capitalized.
691
692.. code-block:: c
693
694 #define CONSTANT 0x12345
695
696Enums are preferred when defining several related constants.
697
698CAPITALIZED macro names are appreciated but macros resembling functions
699may be named in lower case.
700
701Generally, inline functions are preferable to macros resembling functions.
702
703Macros with multiple statements should be enclosed in a do - while block:
704
705.. code-block:: c
706
707 #define macrofun(a, b, c) \
708 do { \
709 if (a == 5) \
710 do_this(b, c); \
711 } while (0)
712
713Things to avoid when using macros:
714
7151) macros that affect control flow:
716
717.. code-block:: c
718
719 #define FOO(x) \
720 do { \
721 if (blah(x) < 0) \
722 return -EBUGGERED; \
723 } while (0)
724
725is a **very** bad idea. It looks like a function call but exits the ``calling``
726function; don't break the internal parsers of those who will read the code.
727
7282) macros that depend on having a local variable with a magic name:
729
730.. code-block:: c
731
732 #define FOO(val) bar(index, val)
733
734might look like a good thing, but it's confusing as hell when one reads the
735code and it's prone to breakage from seemingly innocent changes.
736
7373) macros with arguments that are used as l-values: FOO(x) = y; will
738bite you if somebody e.g. turns FOO into an inline function.
739
7404) forgetting about precedence: macros defining constants using expressions
741must enclose the expression in parentheses. Beware of similar issues with
742macros using parameters.
743
744.. code-block:: c
745
746 #define CONSTANT 0x4000
747 #define CONSTEXP (CONSTANT | 3)
748
7495) namespace collisions when defining local variables in macros resembling
750functions:
751
752.. code-block:: c
753
754 #define FOO(x) \
755 ({ \
756 typeof(x) ret; \
757 ret = calc_ret(x); \
758 (ret); \
759 })
760
761ret is a common name for a local variable - __foo_ret is less likely
762to collide with an existing variable.
763
764The cpp manual deals with macros exhaustively. The gcc internals manual also
765covers RTL which is used frequently with assembly language in the kernel.
766
767
76813) Printing kernel messages
769----------------------------
770
771Kernel developers like to be seen as literate. Do mind the spelling
772of kernel messages to make a good impression. Do not use crippled
773words like ``dont``; use ``do not`` or ``don't`` instead. Make the messages
774concise, clear, and unambiguous.
775
776Kernel messages do not have to be terminated with a period.
777
778Printing numbers in parentheses (%d) adds no value and should be avoided.
779
780There are a number of driver model diagnostic macros in <linux/device.h>
781which you should use to make sure messages are matched to the right device
782and driver, and are tagged with the right level: dev_err(), dev_warn(),
783dev_info(), and so forth. For messages that aren't associated with a
784particular device, <linux/printk.h> defines pr_notice(), pr_info(),
785pr_warn(), pr_err(), etc.
786
787Coming up with good debugging messages can be quite a challenge; and once
788you have them, they can be a huge help for remote troubleshooting. However
789debug message printing is handled differently than printing other non-debug
790messages. While the other pr_XXX() functions print unconditionally,
791pr_debug() does not; it is compiled out by default, unless either DEBUG is
792defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also,
793and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
794the ones already enabled by DEBUG.
795
796Many subsystems have Kconfig debug options to turn on -DDEBUG in the
797corresponding Makefile; in other cases specific files #define DEBUG. And
798when a debug message should be unconditionally printed, such as if it is
799already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
800used.
801
802
80314) Allocating memory
804---------------------
805
806The kernel provides the following general purpose memory allocators:
807kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
808vzalloc(). Please refer to the API documentation for further information
809about them.
810
811The preferred form for passing a size of a struct is the following:
812
813.. code-block:: c
814
815 p = kmalloc(sizeof(*p), ...);
816
817The alternative form where struct name is spelled out hurts readability and
818introduces an opportunity for a bug when the pointer variable type is changed
819but the corresponding sizeof that is passed to a memory allocator is not.
820
821Casting the return value which is a void pointer is redundant. The conversion
822from void pointer to any other pointer type is guaranteed by the C programming
823language.
824
825The preferred form for allocating an array is the following:
826
827.. code-block:: c
828
829 p = kmalloc_array(n, sizeof(...), ...);
830
831The preferred form for allocating a zeroed array is the following:
832
833.. code-block:: c
834
835 p = kcalloc(n, sizeof(...), ...);
836
837Both forms check for overflow on the allocation size n * sizeof(...),
838and return NULL if that occurred.
839
840
84115) The inline disease
842----------------------
843
844There appears to be a common misperception that gcc has a magic "make me
845faster" speedup option called ``inline``. While the use of inlines can be
846appropriate (for example as a means of replacing macros, see Chapter 12), it
847very often is not. Abundant use of the inline keyword leads to a much bigger
848kernel, which in turn slows the system as a whole down, due to a bigger
849icache footprint for the CPU and simply because there is less memory
850available for the pagecache. Just think about it; a pagecache miss causes a
851disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
852that can go into these 5 milliseconds.
853
854A reasonable rule of thumb is to not put inline at functions that have more
855than 3 lines of code in them. An exception to this rule are the cases where
856a parameter is known to be a compiletime constant, and as a result of this
857constantness you *know* the compiler will be able to optimize most of your
858function away at compile time. For a good example of this later case, see
859the kmalloc() inline function.
860
861Often people argue that adding inline to functions that are static and used
862only once is always a win since there is no space tradeoff. While this is
863technically correct, gcc is capable of inlining these automatically without
864help, and the maintenance issue of removing the inline when a second user
865appears outweighs the potential value of the hint that tells gcc to do
866something it would have done anyway.
867
868
86916) Function return values and names
870------------------------------------
871
872Functions can return values of many different kinds, and one of the
873most common is a value indicating whether the function succeeded or
874failed. Such a value can be represented as an error-code integer
875(-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure,
876non-zero = success).
877
878Mixing up these two sorts of representations is a fertile source of
879difficult-to-find bugs. If the C language included a strong distinction
880between integers and booleans then the compiler would find these mistakes
881for us... but it doesn't. To help prevent such bugs, always follow this
882convention::
883
884 If the name of a function is an action or an imperative command,
885 the function should return an error-code integer. If the name
886 is a predicate, the function should return a "succeeded" boolean.
887
888For example, ``add work`` is a command, and the add_work() function returns 0
889for success or -EBUSY for failure. In the same way, ``PCI device present`` is
890a predicate, and the pci_dev_present() function returns 1 if it succeeds in
891finding a matching device or 0 if it doesn't.
892
893All EXPORTed functions must respect this convention, and so should all
894public functions. Private (static) functions need not, but it is
895recommended that they do.
896
897Functions whose return value is the actual result of a computation, rather
898than an indication of whether the computation succeeded, are not subject to
899this rule. Generally they indicate failure by returning some out-of-range
900result. Typical examples would be functions that return pointers; they use
901NULL or the ERR_PTR mechanism to report failure.
902
903
90417) Don't re-invent the kernel macros
905-------------------------------------
906
907The header file include/linux/kernel.h contains a number of macros that
908you should use, rather than explicitly coding some variant of them yourself.
909For example, if you need to calculate the length of an array, take advantage
910of the macro
911
912.. code-block:: c
913
914 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
915
916Similarly, if you need to calculate the size of some structure member, use
917
918.. code-block:: c
919
920 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
921
922There are also min() and max() macros that do strict type checking if you
923need them. Feel free to peruse that header file to see what else is already
924defined that you shouldn't reproduce in your code.
925
926
92718) Editor modelines and other cruft
928------------------------------------
929
930Some editors can interpret configuration information embedded in source files,
931indicated with special markers. For example, emacs interprets lines marked
932like this:
933
934.. code-block:: c
935
936 -*- mode: c -*-
937
938Or like this:
939
940.. code-block:: c
941
942 /*
943 Local Variables:
944 compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
945 End:
946 */
947
948Vim interprets markers that look like this:
949
950.. code-block:: c
951
952 /* vim:set sw=8 noet */
953
954Do not include any of these in source files. People have their own personal
955editor configurations, and your source files should not override them. This
956includes markers for indentation and mode configuration. People may use their
957own custom mode, or may have some other magic method for making indentation
958work correctly.
959
960
96119) Inline assembly
962-------------------
963
964In architecture-specific code, you may need to use inline assembly to interface
965with CPU or platform functionality. Don't hesitate to do so when necessary.
966However, don't use inline assembly gratuitously when C can do the job. You can
967and should poke hardware from C when possible.
968
969Consider writing simple helper functions that wrap common bits of inline
970assembly, rather than repeatedly writing them with slight variations. Remember
971that inline assembly can use C parameters.
972
973Large, non-trivial assembly functions should go in .S files, with corresponding
974C prototypes defined in C header files. The C prototypes for assembly
975functions should use ``asmlinkage``.
976
977You may need to mark your asm statement as volatile, to prevent GCC from
978removing it if GCC doesn't notice any side effects. You don't always need to
979do so, though, and doing so unnecessarily can limit optimization.
980
981When writing a single inline assembly statement containing multiple
982instructions, put each instruction on a separate line in a separate quoted
983string, and end each string except the last with \n\t to properly indent the
984next instruction in the assembly output:
985
986.. code-block:: c
987
988 asm ("magic %reg1, #42\n\t"
989 "more_magic %reg2, %reg3"
990 : /* outputs */ : /* inputs */ : /* clobbers */);
991
992
99320) Conditional Compilation
994---------------------------
995
996Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
997files; doing so makes code harder to read and logic harder to follow. Instead,
998use such conditionals in a header file defining functions for use in those .c
999files, providing no-op stub versions in the #else case, and then call those
1000functions unconditionally from .c files. The compiler will avoid generating
1001any code for the stub calls, producing identical results, but the logic will
1002remain easy to follow.
1003
1004Prefer to compile out entire functions, rather than portions of functions or
1005portions of expressions. Rather than putting an ifdef in an expression, factor
1006out part or all of the expression into a separate helper function and apply the
1007conditional to that function.
1008
1009If you have a function or variable which may potentially go unused in a
1010particular configuration, and the compiler would warn about its definition
1011going unused, mark the definition as __maybe_unused rather than wrapping it in
1012a preprocessor conditional. (However, if a function or variable *always* goes
1013unused, delete it.)
1014
1015Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
1016symbol into a C boolean expression, and use it in a normal C conditional:
1017
1018.. code-block:: c
1019
1020 if (IS_ENABLED(CONFIG_SOMETHING)) {
1021 ...
1022 }
1023
1024The compiler will constant-fold the conditional away, and include or exclude
1025the block of code just as with an #ifdef, so this will not add any runtime
1026overhead. However, this approach still allows the C compiler to see the code
1027inside the block, and check it for correctness (syntax, types, symbol
1028references, etc). Thus, you still have to use an #ifdef if the code inside the
1029block references symbols that will not exist if the condition is not met.
1030
1031At the end of any non-trivial #if or #ifdef block (more than a few lines),
1032place a comment after the #endif on the same line, noting the conditional
1033expression used. For instance:
1034
1035.. code-block:: c
1036
1037 #ifdef CONFIG_SOMETHING
1038 ...
1039 #endif /* CONFIG_SOMETHING */
1040
1041
1042Appendix I) References
1043----------------------
1044
1045The C Programming Language, Second Edition
1046by Brian W. Kernighan and Dennis M. Ritchie.
1047Prentice Hall, Inc., 1988.
1048ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
1049
1050The Practice of Programming
1051by Brian W. Kernighan and Rob Pike.
1052Addison-Wesley, Inc., 1999.
1053ISBN 0-201-61586-X.
1054
1055GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
1056gcc internals and indent, all available from http://www.gnu.org/manual/
1057
1058WG14 is the international standardization working group for the programming
1059language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
1060
1061Kernel process/coding-style.rst, by greg@kroah.com at OLS 2002:
1062http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/