Loading...
Note: File does not exist in v3.15.
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
203Also, use braces when a loop contains more than a single simple statement:
204
205.. code-block:: c
206
207 while (condition) {
208 if (test)
209 do_something();
210 }
211
2123.1) Spaces
213***********
214
215Linux kernel style for use of spaces depends (mostly) on
216function-versus-keyword usage. Use a space after (most) keywords. The
217notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
218somewhat like functions (and are usually used with parentheses in Linux,
219although they are not required in the language, as in: ``sizeof info`` after
220``struct fileinfo info;`` is declared).
221
222So use a space after these keywords::
223
224 if, switch, case, for, do, while
225
226but not with sizeof, typeof, alignof, or __attribute__. E.g.,
227
228.. code-block:: c
229
230
231 s = sizeof(struct file);
232
233Do not add spaces around (inside) parenthesized expressions. This example is
234**bad**:
235
236.. code-block:: c
237
238
239 s = sizeof( struct file );
240
241When declaring pointer data or a function that returns a pointer type, the
242preferred use of ``*`` is adjacent to the data name or function name and not
243adjacent to the type name. Examples:
244
245.. code-block:: c
246
247
248 char *linux_banner;
249 unsigned long long memparse(char *ptr, char **retptr);
250 char *match_strdup(substring_t *s);
251
252Use one space around (on each side of) most binary and ternary operators,
253such as any of these::
254
255 = + - < > * / % | & ^ <= >= == != ? :
256
257but no space after unary operators::
258
259 & * + - ~ ! sizeof typeof alignof __attribute__ defined
260
261no space before the postfix increment & decrement unary operators::
262
263 ++ --
264
265no space after the prefix increment & decrement unary operators::
266
267 ++ --
268
269and no space around the ``.`` and ``->`` structure member operators.
270
271Do not leave trailing whitespace at the ends of lines. Some editors with
272``smart`` indentation will insert whitespace at the beginning of new lines as
273appropriate, so you can start typing the next line of code right away.
274However, some such editors do not remove the whitespace if you end up not
275putting a line of code there, such as if you leave a blank line. As a result,
276you end up with lines containing trailing whitespace.
277
278Git will warn you about patches that introduce trailing whitespace, and can
279optionally strip the trailing whitespace for you; however, if applying a series
280of patches, this may make later patches in the series fail by changing their
281context lines.
282
283
2844) Naming
285---------
286
287C is a Spartan language, and so should your naming be. Unlike Modula-2
288and Pascal programmers, C programmers do not use cute names like
289ThisVariableIsATemporaryCounter. A C programmer would call that
290variable ``tmp``, which is much easier to write, and not the least more
291difficult to understand.
292
293HOWEVER, while mixed-case names are frowned upon, descriptive names for
294global variables are a must. To call a global function ``foo`` is a
295shooting offense.
296
297GLOBAL variables (to be used only if you **really** need them) need to
298have descriptive names, as do global functions. If you have a function
299that counts the number of active users, you should call that
300``count_active_users()`` or similar, you should **not** call it ``cntusr()``.
301
302Encoding the type of a function into the name (so-called Hungarian
303notation) is brain damaged - the compiler knows the types anyway and can
304check those, and it only confuses the programmer. No wonder MicroSoft
305makes buggy programs.
306
307LOCAL variable names should be short, and to the point. If you have
308some random integer loop counter, it should probably be called ``i``.
309Calling it ``loop_counter`` is non-productive, if there is no chance of it
310being mis-understood. Similarly, ``tmp`` can be just about any type of
311variable that is used to hold a temporary value.
312
313If you are afraid to mix up your local variable names, you have another
314problem, which is called the function-growth-hormone-imbalance syndrome.
315See chapter 6 (Functions).
316
317
3185) Typedefs
319-----------
320
321Please don't use things like ``vps_t``.
322It's a **mistake** to use typedef for structures and pointers. When you see a
323
324.. code-block:: c
325
326
327 vps_t a;
328
329in the source, what does it mean?
330In contrast, if it says
331
332.. code-block:: c
333
334 struct virtual_container *a;
335
336you can actually tell what ``a`` is.
337
338Lots of people think that typedefs ``help readability``. Not so. They are
339useful only for:
340
341 (a) totally opaque objects (where the typedef is actively used to **hide**
342 what the object is).
343
344 Example: ``pte_t`` etc. opaque objects that you can only access using
345 the proper accessor functions.
346
347 .. note::
348
349 Opaqueness and ``accessor functions`` are not good in themselves.
350 The reason we have them for things like pte_t etc. is that there
351 really is absolutely **zero** portably accessible information there.
352
353 (b) Clear integer types, where the abstraction **helps** avoid confusion
354 whether it is ``int`` or ``long``.
355
356 u8/u16/u32 are perfectly fine typedefs, although they fit into
357 category (d) better than here.
358
359 .. note::
360
361 Again - there needs to be a **reason** for this. If something is
362 ``unsigned long``, then there's no reason to do
363
364 typedef unsigned long myflags_t;
365
366 but if there is a clear reason for why it under certain circumstances
367 might be an ``unsigned int`` and under other configurations might be
368 ``unsigned long``, then by all means go ahead and use a typedef.
369
370 (c) when you use sparse to literally create a **new** type for
371 type-checking.
372
373 (d) New types which are identical to standard C99 types, in certain
374 exceptional circumstances.
375
376 Although it would only take a short amount of time for the eyes and
377 brain to become accustomed to the standard types like ``uint32_t``,
378 some people object to their use anyway.
379
380 Therefore, the Linux-specific ``u8/u16/u32/u64`` types and their
381 signed equivalents which are identical to standard types are
382 permitted -- although they are not mandatory in new code of your
383 own.
384
385 When editing existing code which already uses one or the other set
386 of types, you should conform to the existing choices in that code.
387
388 (e) Types safe for use in userspace.
389
390 In certain structures which are visible to userspace, we cannot
391 require C99 types and cannot use the ``u32`` form above. Thus, we
392 use __u32 and similar types in all structures which are shared
393 with userspace.
394
395Maybe there are other cases too, but the rule should basically be to NEVER
396EVER use a typedef unless you can clearly match one of those rules.
397
398In general, a pointer, or a struct that has elements that can reasonably
399be directly accessed should **never** be a typedef.
400
401
4026) Functions
403------------
404
405Functions should be short and sweet, and do just one thing. They should
406fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
407as we all know), and do one thing and do that well.
408
409The maximum length of a function is inversely proportional to the
410complexity and indentation level of that function. So, if you have a
411conceptually simple function that is just one long (but simple)
412case-statement, where you have to do lots of small things for a lot of
413different cases, it's OK to have a longer function.
414
415However, if you have a complex function, and you suspect that a
416less-than-gifted first-year high-school student might not even
417understand what the function is all about, you should adhere to the
418maximum limits all the more closely. Use helper functions with
419descriptive names (you can ask the compiler to in-line them if you think
420it's performance-critical, and it will probably do a better job of it
421than you would have done).
422
423Another measure of the function is the number of local variables. They
424shouldn't exceed 5-10, or you're doing something wrong. Re-think the
425function, and split it into smaller pieces. A human brain can
426generally easily keep track of about 7 different things, anything more
427and it gets confused. You know you're brilliant, but maybe you'd like
428to understand what you did 2 weeks from now.
429
430In source files, separate functions with one blank line. If the function is
431exported, the **EXPORT** macro for it should follow immediately after the
432closing function brace line. E.g.:
433
434.. code-block:: c
435
436 int system_is_up(void)
437 {
438 return system_state == SYSTEM_RUNNING;
439 }
440 EXPORT_SYMBOL(system_is_up);
441
442In function prototypes, include parameter names with their data types.
443Although this is not required by the C language, it is preferred in Linux
444because it is a simple way to add valuable information for the reader.
445
446
4477) Centralized exiting of functions
448-----------------------------------
449
450Albeit deprecated by some people, the equivalent of the goto statement is
451used frequently by compilers in form of the unconditional jump instruction.
452
453The goto statement comes in handy when a function exits from multiple
454locations and some common work such as cleanup has to be done. If there is no
455cleanup needed then just return directly.
456
457Choose label names which say what the goto does or why the goto exists. An
458example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``.
459Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to
460renumber them if you ever add or remove exit paths, and they make correctness
461difficult to verify anyway.
462
463The rationale for using gotos is:
464
465- unconditional statements are easier to understand and follow
466- nesting is reduced
467- errors by not updating individual exit points when making
468 modifications are prevented
469- saves the compiler work to optimize redundant code away ;)
470
471.. code-block:: c
472
473 int fun(int a)
474 {
475 int result = 0;
476 char *buffer;
477
478 buffer = kmalloc(SIZE, GFP_KERNEL);
479 if (!buffer)
480 return -ENOMEM;
481
482 if (condition1) {
483 while (loop1) {
484 ...
485 }
486 result = 1;
487 goto out_free_buffer;
488 }
489 ...
490 out_free_buffer:
491 kfree(buffer);
492 return result;
493 }
494
495A common type of bug to be aware of is ``one err bugs`` which look like this:
496
497.. code-block:: c
498
499 err:
500 kfree(foo->bar);
501 kfree(foo);
502 return ret;
503
504The bug in this code is that on some exit paths ``foo`` is NULL. Normally the
505fix for this is to split it up into two error labels ``err_free_bar:`` and
506``err_free_foo:``:
507
508.. code-block:: c
509
510 err_free_bar:
511 kfree(foo->bar);
512 err_free_foo:
513 kfree(foo);
514 return ret;
515
516Ideally you should simulate errors to test all exit paths.
517
518
5198) Commenting
520-------------
521
522Comments are good, but there is also a danger of over-commenting. NEVER
523try to explain HOW your code works in a comment: it's much better to
524write the code so that the **working** is obvious, and it's a waste of
525time to explain badly written code.
526
527Generally, you want your comments to tell WHAT your code does, not HOW.
528Also, try to avoid putting comments inside a function body: if the
529function is so complex that you need to separately comment parts of it,
530you should probably go back to chapter 6 for a while. You can make
531small comments to note or warn about something particularly clever (or
532ugly), but try to avoid excess. Instead, put the comments at the head
533of the function, telling people what it does, and possibly WHY it does
534it.
535
536When commenting the kernel API functions, please use the kernel-doc format.
537See the files at :ref:`Documentation/doc-guide/ <doc_guide>` and
538``scripts/kernel-doc`` for details.
539
540The preferred style for long (multi-line) comments is:
541
542.. code-block:: c
543
544 /*
545 * This is the preferred style for multi-line
546 * comments in the Linux kernel source code.
547 * Please use it consistently.
548 *
549 * Description: A column of asterisks on the left side,
550 * with beginning and ending almost-blank lines.
551 */
552
553For files in net/ and drivers/net/ the preferred style for long (multi-line)
554comments is a little different.
555
556.. code-block:: c
557
558 /* The preferred comment style for files in net/ and drivers/net
559 * looks like this.
560 *
561 * It is nearly the same as the generally preferred comment style,
562 * but there is no initial almost-blank line.
563 */
564
565It's also important to comment data, whether they are basic types or derived
566types. To this end, use just one data declaration per line (no commas for
567multiple data declarations). This leaves you room for a small comment on each
568item, explaining its use.
569
570
5719) You've made a mess of it
572---------------------------
573
574That's OK, we all do. You've probably been told by your long-time Unix
575user helper that ``GNU emacs`` automatically formats the C sources for
576you, and you've noticed that yes, it does do that, but the defaults it
577uses are less than desirable (in fact, they are worse than random
578typing - an infinite number of monkeys typing into GNU emacs would never
579make a good program).
580
581So, you can either get rid of GNU emacs, or change it to use saner
582values. To do the latter, you can stick the following in your .emacs file:
583
584.. code-block:: none
585
586 (defun c-lineup-arglist-tabs-only (ignored)
587 "Line up argument lists by tabs, not spaces"
588 (let* ((anchor (c-langelem-pos c-syntactic-element))
589 (column (c-langelem-2nd-pos c-syntactic-element))
590 (offset (- (1+ column) anchor))
591 (steps (floor offset c-basic-offset)))
592 (* (max steps 1)
593 c-basic-offset)))
594
595 (add-hook 'c-mode-common-hook
596 (lambda ()
597 ;; Add kernel style
598 (c-add-style
599 "linux-tabs-only"
600 '("linux" (c-offsets-alist
601 (arglist-cont-nonempty
602 c-lineup-gcc-asm-reg
603 c-lineup-arglist-tabs-only))))))
604
605 (add-hook 'c-mode-hook
606 (lambda ()
607 (let ((filename (buffer-file-name)))
608 ;; Enable kernel mode for the appropriate files
609 (when (and filename
610 (string-match (expand-file-name "~/src/linux-trees")
611 filename))
612 (setq indent-tabs-mode t)
613 (setq show-trailing-whitespace t)
614 (c-set-style "linux-tabs-only")))))
615
616This will make emacs go better with the kernel coding style for C
617files below ``~/src/linux-trees``.
618
619But even if you fail in getting emacs to do sane formatting, not
620everything is lost: use ``indent``.
621
622Now, again, GNU indent has the same brain-dead settings that GNU emacs
623has, which is why you need to give it a few command line options.
624However, that's not too bad, because even the makers of GNU indent
625recognize the authority of K&R (the GNU people aren't evil, they are
626just severely misguided in this matter), so you just give indent the
627options ``-kr -i8`` (stands for ``K&R, 8 character indents``), or use
628``scripts/Lindent``, which indents in the latest style.
629
630``indent`` has a lot of options, and especially when it comes to comment
631re-formatting you may want to take a look at the man page. But
632remember: ``indent`` is not a fix for bad programming.
633
634Note that you can also use the ``clang-format`` tool to help you with
635these rules, to quickly re-format parts of your code automatically,
636and to review full files in order to spot coding style mistakes,
637typos and possible improvements. It is also handy for sorting ``#includes``,
638for aligning variables/macros, for reflowing text and other similar tasks.
639See the file :ref:`Documentation/process/clang-format.rst <clangformat>`
640for more details.
641
642
64310) Kconfig configuration files
644-------------------------------
645
646For all of the Kconfig* configuration files throughout the source tree,
647the indentation is somewhat different. Lines under a ``config`` definition
648are indented with one tab, while help text is indented an additional two
649spaces. Example::
650
651 config AUDIT
652 bool "Auditing support"
653 depends on NET
654 help
655 Enable auditing infrastructure that can be used with another
656 kernel subsystem, such as SELinux (which requires this for
657 logging of avc messages output). Does not do system-call
658 auditing without CONFIG_AUDITSYSCALL.
659
660Seriously dangerous features (such as write support for certain
661filesystems) should advertise this prominently in their prompt string::
662
663 config ADFS_FS_RW
664 bool "ADFS write support (DANGEROUS)"
665 depends on ADFS_FS
666 ...
667
668For full documentation on the configuration files, see the file
669Documentation/kbuild/kconfig-language.txt.
670
671
67211) Data structures
673-------------------
674
675Data structures that have visibility outside the single-threaded
676environment they are created and destroyed in should always have
677reference counts. In the kernel, garbage collection doesn't exist (and
678outside the kernel garbage collection is slow and inefficient), which
679means that you absolutely **have** to reference count all your uses.
680
681Reference counting means that you can avoid locking, and allows multiple
682users to have access to the data structure in parallel - and not having
683to worry about the structure suddenly going away from under them just
684because they slept or did something else for a while.
685
686Note that locking is **not** a replacement for reference counting.
687Locking is used to keep data structures coherent, while reference
688counting is a memory management technique. Usually both are needed, and
689they are not to be confused with each other.
690
691Many data structures can indeed have two levels of reference counting,
692when there are users of different ``classes``. The subclass count counts
693the number of subclass users, and decrements the global count just once
694when the subclass count goes to zero.
695
696Examples of this kind of ``multi-level-reference-counting`` can be found in
697memory management (``struct mm_struct``: mm_users and mm_count), and in
698filesystem code (``struct super_block``: s_count and s_active).
699
700Remember: if another thread can find your data structure, and you don't
701have a reference count on it, you almost certainly have a bug.
702
703
70412) Macros, Enums and RTL
705-------------------------
706
707Names of macros defining constants and labels in enums are capitalized.
708
709.. code-block:: c
710
711 #define CONSTANT 0x12345
712
713Enums are preferred when defining several related constants.
714
715CAPITALIZED macro names are appreciated but macros resembling functions
716may be named in lower case.
717
718Generally, inline functions are preferable to macros resembling functions.
719
720Macros with multiple statements should be enclosed in a do - while block:
721
722.. code-block:: c
723
724 #define macrofun(a, b, c) \
725 do { \
726 if (a == 5) \
727 do_this(b, c); \
728 } while (0)
729
730Things to avoid when using macros:
731
7321) macros that affect control flow:
733
734.. code-block:: c
735
736 #define FOO(x) \
737 do { \
738 if (blah(x) < 0) \
739 return -EBUGGERED; \
740 } while (0)
741
742is a **very** bad idea. It looks like a function call but exits the ``calling``
743function; don't break the internal parsers of those who will read the code.
744
7452) macros that depend on having a local variable with a magic name:
746
747.. code-block:: c
748
749 #define FOO(val) bar(index, val)
750
751might look like a good thing, but it's confusing as hell when one reads the
752code and it's prone to breakage from seemingly innocent changes.
753
7543) macros with arguments that are used as l-values: FOO(x) = y; will
755bite you if somebody e.g. turns FOO into an inline function.
756
7574) forgetting about precedence: macros defining constants using expressions
758must enclose the expression in parentheses. Beware of similar issues with
759macros using parameters.
760
761.. code-block:: c
762
763 #define CONSTANT 0x4000
764 #define CONSTEXP (CONSTANT | 3)
765
7665) namespace collisions when defining local variables in macros resembling
767functions:
768
769.. code-block:: c
770
771 #define FOO(x) \
772 ({ \
773 typeof(x) ret; \
774 ret = calc_ret(x); \
775 (ret); \
776 })
777
778ret is a common name for a local variable - __foo_ret is less likely
779to collide with an existing variable.
780
781The cpp manual deals with macros exhaustively. The gcc internals manual also
782covers RTL which is used frequently with assembly language in the kernel.
783
784
78513) Printing kernel messages
786----------------------------
787
788Kernel developers like to be seen as literate. Do mind the spelling
789of kernel messages to make a good impression. Do not use crippled
790words like ``dont``; use ``do not`` or ``don't`` instead. Make the messages
791concise, clear, and unambiguous.
792
793Kernel messages do not have to be terminated with a period.
794
795Printing numbers in parentheses (%d) adds no value and should be avoided.
796
797There are a number of driver model diagnostic macros in <linux/device.h>
798which you should use to make sure messages are matched to the right device
799and driver, and are tagged with the right level: dev_err(), dev_warn(),
800dev_info(), and so forth. For messages that aren't associated with a
801particular device, <linux/printk.h> defines pr_notice(), pr_info(),
802pr_warn(), pr_err(), etc.
803
804Coming up with good debugging messages can be quite a challenge; and once
805you have them, they can be a huge help for remote troubleshooting. However
806debug message printing is handled differently than printing other non-debug
807messages. While the other pr_XXX() functions print unconditionally,
808pr_debug() does not; it is compiled out by default, unless either DEBUG is
809defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also,
810and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
811the ones already enabled by DEBUG.
812
813Many subsystems have Kconfig debug options to turn on -DDEBUG in the
814corresponding Makefile; in other cases specific files #define DEBUG. And
815when a debug message should be unconditionally printed, such as if it is
816already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
817used.
818
819
82014) Allocating memory
821---------------------
822
823The kernel provides the following general purpose memory allocators:
824kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
825vzalloc(). Please refer to the API documentation for further information
826about them.
827
828The preferred form for passing a size of a struct is the following:
829
830.. code-block:: c
831
832 p = kmalloc(sizeof(*p), ...);
833
834The alternative form where struct name is spelled out hurts readability and
835introduces an opportunity for a bug when the pointer variable type is changed
836but the corresponding sizeof that is passed to a memory allocator is not.
837
838Casting the return value which is a void pointer is redundant. The conversion
839from void pointer to any other pointer type is guaranteed by the C programming
840language.
841
842The preferred form for allocating an array is the following:
843
844.. code-block:: c
845
846 p = kmalloc_array(n, sizeof(...), ...);
847
848The preferred form for allocating a zeroed array is the following:
849
850.. code-block:: c
851
852 p = kcalloc(n, sizeof(...), ...);
853
854Both forms check for overflow on the allocation size n * sizeof(...),
855and return NULL if that occurred.
856
857
85815) The inline disease
859----------------------
860
861There appears to be a common misperception that gcc has a magic "make me
862faster" speedup option called ``inline``. While the use of inlines can be
863appropriate (for example as a means of replacing macros, see Chapter 12), it
864very often is not. Abundant use of the inline keyword leads to a much bigger
865kernel, which in turn slows the system as a whole down, due to a bigger
866icache footprint for the CPU and simply because there is less memory
867available for the pagecache. Just think about it; a pagecache miss causes a
868disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
869that can go into these 5 milliseconds.
870
871A reasonable rule of thumb is to not put inline at functions that have more
872than 3 lines of code in them. An exception to this rule are the cases where
873a parameter is known to be a compiletime constant, and as a result of this
874constantness you *know* the compiler will be able to optimize most of your
875function away at compile time. For a good example of this later case, see
876the kmalloc() inline function.
877
878Often people argue that adding inline to functions that are static and used
879only once is always a win since there is no space tradeoff. While this is
880technically correct, gcc is capable of inlining these automatically without
881help, and the maintenance issue of removing the inline when a second user
882appears outweighs the potential value of the hint that tells gcc to do
883something it would have done anyway.
884
885
88616) Function return values and names
887------------------------------------
888
889Functions can return values of many different kinds, and one of the
890most common is a value indicating whether the function succeeded or
891failed. Such a value can be represented as an error-code integer
892(-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure,
893non-zero = success).
894
895Mixing up these two sorts of representations is a fertile source of
896difficult-to-find bugs. If the C language included a strong distinction
897between integers and booleans then the compiler would find these mistakes
898for us... but it doesn't. To help prevent such bugs, always follow this
899convention::
900
901 If the name of a function is an action or an imperative command,
902 the function should return an error-code integer. If the name
903 is a predicate, the function should return a "succeeded" boolean.
904
905For example, ``add work`` is a command, and the add_work() function returns 0
906for success or -EBUSY for failure. In the same way, ``PCI device present`` is
907a predicate, and the pci_dev_present() function returns 1 if it succeeds in
908finding a matching device or 0 if it doesn't.
909
910All EXPORTed functions must respect this convention, and so should all
911public functions. Private (static) functions need not, but it is
912recommended that they do.
913
914Functions whose return value is the actual result of a computation, rather
915than an indication of whether the computation succeeded, are not subject to
916this rule. Generally they indicate failure by returning some out-of-range
917result. Typical examples would be functions that return pointers; they use
918NULL or the ERR_PTR mechanism to report failure.
919
920
92117) Don't re-invent the kernel macros
922-------------------------------------
923
924The header file include/linux/kernel.h contains a number of macros that
925you should use, rather than explicitly coding some variant of them yourself.
926For example, if you need to calculate the length of an array, take advantage
927of the macro
928
929.. code-block:: c
930
931 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
932
933Similarly, if you need to calculate the size of some structure member, use
934
935.. code-block:: c
936
937 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
938
939There are also min() and max() macros that do strict type checking if you
940need them. Feel free to peruse that header file to see what else is already
941defined that you shouldn't reproduce in your code.
942
943
94418) Editor modelines and other cruft
945------------------------------------
946
947Some editors can interpret configuration information embedded in source files,
948indicated with special markers. For example, emacs interprets lines marked
949like this:
950
951.. code-block:: c
952
953 -*- mode: c -*-
954
955Or like this:
956
957.. code-block:: c
958
959 /*
960 Local Variables:
961 compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
962 End:
963 */
964
965Vim interprets markers that look like this:
966
967.. code-block:: c
968
969 /* vim:set sw=8 noet */
970
971Do not include any of these in source files. People have their own personal
972editor configurations, and your source files should not override them. This
973includes markers for indentation and mode configuration. People may use their
974own custom mode, or may have some other magic method for making indentation
975work correctly.
976
977
97819) Inline assembly
979-------------------
980
981In architecture-specific code, you may need to use inline assembly to interface
982with CPU or platform functionality. Don't hesitate to do so when necessary.
983However, don't use inline assembly gratuitously when C can do the job. You can
984and should poke hardware from C when possible.
985
986Consider writing simple helper functions that wrap common bits of inline
987assembly, rather than repeatedly writing them with slight variations. Remember
988that inline assembly can use C parameters.
989
990Large, non-trivial assembly functions should go in .S files, with corresponding
991C prototypes defined in C header files. The C prototypes for assembly
992functions should use ``asmlinkage``.
993
994You may need to mark your asm statement as volatile, to prevent GCC from
995removing it if GCC doesn't notice any side effects. You don't always need to
996do so, though, and doing so unnecessarily can limit optimization.
997
998When writing a single inline assembly statement containing multiple
999instructions, put each instruction on a separate line in a separate quoted
1000string, and end each string except the last with ``\n\t`` to properly indent
1001the next instruction in the assembly output:
1002
1003.. code-block:: c
1004
1005 asm ("magic %reg1, #42\n\t"
1006 "more_magic %reg2, %reg3"
1007 : /* outputs */ : /* inputs */ : /* clobbers */);
1008
1009
101020) Conditional Compilation
1011---------------------------
1012
1013Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
1014files; doing so makes code harder to read and logic harder to follow. Instead,
1015use such conditionals in a header file defining functions for use in those .c
1016files, providing no-op stub versions in the #else case, and then call those
1017functions unconditionally from .c files. The compiler will avoid generating
1018any code for the stub calls, producing identical results, but the logic will
1019remain easy to follow.
1020
1021Prefer to compile out entire functions, rather than portions of functions or
1022portions of expressions. Rather than putting an ifdef in an expression, factor
1023out part or all of the expression into a separate helper function and apply the
1024conditional to that function.
1025
1026If you have a function or variable which may potentially go unused in a
1027particular configuration, and the compiler would warn about its definition
1028going unused, mark the definition as __maybe_unused rather than wrapping it in
1029a preprocessor conditional. (However, if a function or variable *always* goes
1030unused, delete it.)
1031
1032Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
1033symbol into a C boolean expression, and use it in a normal C conditional:
1034
1035.. code-block:: c
1036
1037 if (IS_ENABLED(CONFIG_SOMETHING)) {
1038 ...
1039 }
1040
1041The compiler will constant-fold the conditional away, and include or exclude
1042the block of code just as with an #ifdef, so this will not add any runtime
1043overhead. However, this approach still allows the C compiler to see the code
1044inside the block, and check it for correctness (syntax, types, symbol
1045references, etc). Thus, you still have to use an #ifdef if the code inside the
1046block references symbols that will not exist if the condition is not met.
1047
1048At the end of any non-trivial #if or #ifdef block (more than a few lines),
1049place a comment after the #endif on the same line, noting the conditional
1050expression used. For instance:
1051
1052.. code-block:: c
1053
1054 #ifdef CONFIG_SOMETHING
1055 ...
1056 #endif /* CONFIG_SOMETHING */
1057
1058
1059Appendix I) References
1060----------------------
1061
1062The C Programming Language, Second Edition
1063by Brian W. Kernighan and Dennis M. Ritchie.
1064Prentice Hall, Inc., 1988.
1065ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
1066
1067The Practice of Programming
1068by Brian W. Kernighan and Rob Pike.
1069Addison-Wesley, Inc., 1999.
1070ISBN 0-201-61586-X.
1071
1072GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
1073gcc internals and indent, all available from http://www.gnu.org/manual/
1074
1075WG14 is the international standardization working group for the programming
1076language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
1077
1078Kernel process/coding-style.rst, by greg@kroah.com at OLS 2002:
1079http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/