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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 fallthrough;
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 use commas to avoid using braces:
73
74.. code-block:: c
75
76 if (condition)
77 do_this(), do_that();
78
79Always uses braces for multiple statements:
80
81.. code-block:: c
82
83 if (condition) {
84 do_this();
85 do_that();
86 }
87
88Don't put multiple assignments on a single line either. Kernel coding style
89is super simple. Avoid tricky expressions.
90
91
92Outside of comments, documentation and except in Kconfig, spaces are never
93used for indentation, and the above example is deliberately broken.
94
95Get a decent editor and don't leave whitespace at the end of lines.
96
97
982) Breaking long lines and strings
99----------------------------------
100
101Coding style is all about readability and maintainability using commonly
102available tools.
103
104The preferred limit on the length of a single line is 80 columns.
105
106Statements longer than 80 columns should be broken into sensible chunks,
107unless exceeding 80 columns significantly increases readability and does
108not hide information.
109
110Descendants are always substantially shorter than the parent and
111are placed substantially to the right. A very commonly used style
112is to align descendants to a function open parenthesis.
113
114These same rules are applied to function headers with a long argument list.
115
116However, never break user-visible strings such as printk messages because
117that breaks the ability to grep for them.
118
119
1203) Placing Braces and Spaces
121----------------------------
122
123The other issue that always comes up in C styling is the placement of
124braces. Unlike the indent size, there are few technical reasons to
125choose one placement strategy over the other, but the preferred way, as
126shown to us by the prophets Kernighan and Ritchie, is to put the opening
127brace last on the line, and put the closing brace first, thusly:
128
129.. code-block:: c
130
131 if (x is true) {
132 we do y
133 }
134
135This applies to all non-function statement blocks (if, switch, for,
136while, do). E.g.:
137
138.. code-block:: c
139
140 switch (action) {
141 case KOBJ_ADD:
142 return "add";
143 case KOBJ_REMOVE:
144 return "remove";
145 case KOBJ_CHANGE:
146 return "change";
147 default:
148 return NULL;
149 }
150
151However, there is one special case, namely functions: they have the
152opening brace at the beginning of the next line, thus:
153
154.. code-block:: c
155
156 int function(int x)
157 {
158 body of function
159 }
160
161Heretic people all over the world have claimed that this inconsistency
162is ... well ... inconsistent, but all right-thinking people know that
163(a) K&R are **right** and (b) K&R are right. Besides, functions are
164special anyway (you can't nest them in C).
165
166Note that the closing brace is empty on a line of its own, **except** in
167the cases where it is followed by a continuation of the same statement,
168ie a ``while`` in a do-statement or an ``else`` in an if-statement, like
169this:
170
171.. code-block:: c
172
173 do {
174 body of do-loop
175 } while (condition);
176
177and
178
179.. code-block:: c
180
181 if (x == y) {
182 ..
183 } else if (x > y) {
184 ...
185 } else {
186 ....
187 }
188
189Rationale: K&R.
190
191Also, note that this brace-placement also minimizes the number of empty
192(or almost empty) lines, without any loss of readability. Thus, as the
193supply of new-lines on your screen is not a renewable resource (think
19425-line terminal screens here), you have more empty lines to put
195comments on.
196
197Do not unnecessarily use braces where a single statement will do.
198
199.. code-block:: c
200
201 if (condition)
202 action();
203
204and
205
206.. code-block:: none
207
208 if (condition)
209 do_this();
210 else
211 do_that();
212
213This does not apply if only one branch of a conditional statement is a single
214statement; in the latter case use braces in both branches:
215
216.. code-block:: c
217
218 if (condition) {
219 do_this();
220 do_that();
221 } else {
222 otherwise();
223 }
224
225Also, use braces when a loop contains more than a single simple statement:
226
227.. code-block:: c
228
229 while (condition) {
230 if (test)
231 do_something();
232 }
233
2343.1) Spaces
235***********
236
237Linux kernel style for use of spaces depends (mostly) on
238function-versus-keyword usage. Use a space after (most) keywords. The
239notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
240somewhat like functions (and are usually used with parentheses in Linux,
241although they are not required in the language, as in: ``sizeof info`` after
242``struct fileinfo info;`` is declared).
243
244So use a space after these keywords::
245
246 if, switch, case, for, do, while
247
248but not with sizeof, typeof, alignof, or __attribute__. E.g.,
249
250.. code-block:: c
251
252
253 s = sizeof(struct file);
254
255Do not add spaces around (inside) parenthesized expressions. This example is
256**bad**:
257
258.. code-block:: c
259
260
261 s = sizeof( struct file );
262
263When declaring pointer data or a function that returns a pointer type, the
264preferred use of ``*`` is adjacent to the data name or function name and not
265adjacent to the type name. Examples:
266
267.. code-block:: c
268
269
270 char *linux_banner;
271 unsigned long long memparse(char *ptr, char **retptr);
272 char *match_strdup(substring_t *s);
273
274Use one space around (on each side of) most binary and ternary operators,
275such as any of these::
276
277 = + - < > * / % | & ^ <= >= == != ? :
278
279but no space after unary operators::
280
281 & * + - ~ ! sizeof typeof alignof __attribute__ defined
282
283no space before the postfix increment & decrement unary operators::
284
285 ++ --
286
287no space after the prefix increment & decrement unary operators::
288
289 ++ --
290
291and no space around the ``.`` and ``->`` structure member operators.
292
293Do not leave trailing whitespace at the ends of lines. Some editors with
294``smart`` indentation will insert whitespace at the beginning of new lines as
295appropriate, so you can start typing the next line of code right away.
296However, some such editors do not remove the whitespace if you end up not
297putting a line of code there, such as if you leave a blank line. As a result,
298you end up with lines containing trailing whitespace.
299
300Git will warn you about patches that introduce trailing whitespace, and can
301optionally strip the trailing whitespace for you; however, if applying a series
302of patches, this may make later patches in the series fail by changing their
303context lines.
304
305
3064) Naming
307---------
308
309C is a Spartan language, and your naming conventions should follow suit.
310Unlike Modula-2 and Pascal programmers, C programmers do not use cute
311names like ThisVariableIsATemporaryCounter. A C programmer would call that
312variable ``tmp``, which is much easier to write, and not the least more
313difficult to understand.
314
315HOWEVER, while mixed-case names are frowned upon, descriptive names for
316global variables are a must. To call a global function ``foo`` is a
317shooting offense.
318
319GLOBAL variables (to be used only if you **really** need them) need to
320have descriptive names, as do global functions. If you have a function
321that counts the number of active users, you should call that
322``count_active_users()`` or similar, you should **not** call it ``cntusr()``.
323
324Encoding the type of a function into the name (so-called Hungarian
325notation) is asinine - the compiler knows the types anyway and can check
326those, and it only confuses the programmer.
327
328LOCAL variable names should be short, and to the point. If you have
329some random integer loop counter, it should probably be called ``i``.
330Calling it ``loop_counter`` is non-productive, if there is no chance of it
331being mis-understood. Similarly, ``tmp`` can be just about any type of
332variable that is used to hold a temporary value.
333
334If you are afraid to mix up your local variable names, you have another
335problem, which is called the function-growth-hormone-imbalance syndrome.
336See chapter 6 (Functions).
337
338For symbol names and documentation, avoid introducing new usage of
339'master / slave' (or 'slave' independent of 'master') and 'blacklist /
340whitelist'.
341
342Recommended replacements for 'master / slave' are:
343 '{primary,main} / {secondary,replica,subordinate}'
344 '{initiator,requester} / {target,responder}'
345 '{controller,host} / {device,worker,proxy}'
346 'leader / follower'
347 'director / performer'
348
349Recommended replacements for 'blacklist/whitelist' are:
350 'denylist / allowlist'
351 'blocklist / passlist'
352
353Exceptions for introducing new usage is to maintain a userspace ABI/API,
354or when updating code for an existing (as of 2020) hardware or protocol
355specification that mandates those terms. For new specifications
356translate specification usage of the terminology to the kernel coding
357standard where possible.
358
3595) Typedefs
360-----------
361
362Please don't use things like ``vps_t``.
363It's a **mistake** to use typedef for structures and pointers. When you see a
364
365.. code-block:: c
366
367
368 vps_t a;
369
370in the source, what does it mean?
371In contrast, if it says
372
373.. code-block:: c
374
375 struct virtual_container *a;
376
377you can actually tell what ``a`` is.
378
379Lots of people think that typedefs ``help readability``. Not so. They are
380useful only for:
381
382 (a) totally opaque objects (where the typedef is actively used to **hide**
383 what the object is).
384
385 Example: ``pte_t`` etc. opaque objects that you can only access using
386 the proper accessor functions.
387
388 .. note::
389
390 Opaqueness and ``accessor functions`` are not good in themselves.
391 The reason we have them for things like pte_t etc. is that there
392 really is absolutely **zero** portably accessible information there.
393
394 (b) Clear integer types, where the abstraction **helps** avoid confusion
395 whether it is ``int`` or ``long``.
396
397 u8/u16/u32 are perfectly fine typedefs, although they fit into
398 category (d) better than here.
399
400 .. note::
401
402 Again - there needs to be a **reason** for this. If something is
403 ``unsigned long``, then there's no reason to do
404
405 typedef unsigned long myflags_t;
406
407 but if there is a clear reason for why it under certain circumstances
408 might be an ``unsigned int`` and under other configurations might be
409 ``unsigned long``, then by all means go ahead and use a typedef.
410
411 (c) when you use sparse to literally create a **new** type for
412 type-checking.
413
414 (d) New types which are identical to standard C99 types, in certain
415 exceptional circumstances.
416
417 Although it would only take a short amount of time for the eyes and
418 brain to become accustomed to the standard types like ``uint32_t``,
419 some people object to their use anyway.
420
421 Therefore, the Linux-specific ``u8/u16/u32/u64`` types and their
422 signed equivalents which are identical to standard types are
423 permitted -- although they are not mandatory in new code of your
424 own.
425
426 When editing existing code which already uses one or the other set
427 of types, you should conform to the existing choices in that code.
428
429 (e) Types safe for use in userspace.
430
431 In certain structures which are visible to userspace, we cannot
432 require C99 types and cannot use the ``u32`` form above. Thus, we
433 use __u32 and similar types in all structures which are shared
434 with userspace.
435
436Maybe there are other cases too, but the rule should basically be to NEVER
437EVER use a typedef unless you can clearly match one of those rules.
438
439In general, a pointer, or a struct that has elements that can reasonably
440be directly accessed should **never** be a typedef.
441
442
4436) Functions
444------------
445
446Functions should be short and sweet, and do just one thing. They should
447fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
448as we all know), and do one thing and do that well.
449
450The maximum length of a function is inversely proportional to the
451complexity and indentation level of that function. So, if you have a
452conceptually simple function that is just one long (but simple)
453case-statement, where you have to do lots of small things for a lot of
454different cases, it's OK to have a longer function.
455
456However, if you have a complex function, and you suspect that a
457less-than-gifted first-year high-school student might not even
458understand what the function is all about, you should adhere to the
459maximum limits all the more closely. Use helper functions with
460descriptive names (you can ask the compiler to in-line them if you think
461it's performance-critical, and it will probably do a better job of it
462than you would have done).
463
464Another measure of the function is the number of local variables. They
465shouldn't exceed 5-10, or you're doing something wrong. Re-think the
466function, and split it into smaller pieces. A human brain can
467generally easily keep track of about 7 different things, anything more
468and it gets confused. You know you're brilliant, but maybe you'd like
469to understand what you did 2 weeks from now.
470
471In source files, separate functions with one blank line. If the function is
472exported, the **EXPORT** macro for it should follow immediately after the
473closing function brace line. E.g.:
474
475.. code-block:: c
476
477 int system_is_up(void)
478 {
479 return system_state == SYSTEM_RUNNING;
480 }
481 EXPORT_SYMBOL(system_is_up);
482
483In function prototypes, include parameter names with their data types.
484Although this is not required by the C language, it is preferred in Linux
485because it is a simple way to add valuable information for the reader.
486
487Do not use the ``extern`` keyword with function prototypes as this makes
488lines longer and isn't strictly necessary.
489
490
4917) Centralized exiting of functions
492-----------------------------------
493
494Albeit deprecated by some people, the equivalent of the goto statement is
495used frequently by compilers in form of the unconditional jump instruction.
496
497The goto statement comes in handy when a function exits from multiple
498locations and some common work such as cleanup has to be done. If there is no
499cleanup needed then just return directly.
500
501Choose label names which say what the goto does or why the goto exists. An
502example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``.
503Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to
504renumber them if you ever add or remove exit paths, and they make correctness
505difficult to verify anyway.
506
507The rationale for using gotos is:
508
509- unconditional statements are easier to understand and follow
510- nesting is reduced
511- errors by not updating individual exit points when making
512 modifications are prevented
513- saves the compiler work to optimize redundant code away ;)
514
515.. code-block:: c
516
517 int fun(int a)
518 {
519 int result = 0;
520 char *buffer;
521
522 buffer = kmalloc(SIZE, GFP_KERNEL);
523 if (!buffer)
524 return -ENOMEM;
525
526 if (condition1) {
527 while (loop1) {
528 ...
529 }
530 result = 1;
531 goto out_free_buffer;
532 }
533 ...
534 out_free_buffer:
535 kfree(buffer);
536 return result;
537 }
538
539A common type of bug to be aware of is ``one err bugs`` which look like this:
540
541.. code-block:: c
542
543 err:
544 kfree(foo->bar);
545 kfree(foo);
546 return ret;
547
548The bug in this code is that on some exit paths ``foo`` is NULL. Normally the
549fix for this is to split it up into two error labels ``err_free_bar:`` and
550``err_free_foo:``:
551
552.. code-block:: c
553
554 err_free_bar:
555 kfree(foo->bar);
556 err_free_foo:
557 kfree(foo);
558 return ret;
559
560Ideally you should simulate errors to test all exit paths.
561
562
5638) Commenting
564-------------
565
566Comments are good, but there is also a danger of over-commenting. NEVER
567try to explain HOW your code works in a comment: it's much better to
568write the code so that the **working** is obvious, and it's a waste of
569time to explain badly written code.
570
571Generally, you want your comments to tell WHAT your code does, not HOW.
572Also, try to avoid putting comments inside a function body: if the
573function is so complex that you need to separately comment parts of it,
574you should probably go back to chapter 6 for a while. You can make
575small comments to note or warn about something particularly clever (or
576ugly), but try to avoid excess. Instead, put the comments at the head
577of the function, telling people what it does, and possibly WHY it does
578it.
579
580When commenting the kernel API functions, please use the kernel-doc format.
581See the files at :ref:`Documentation/doc-guide/ <doc_guide>` and
582``scripts/kernel-doc`` for details.
583
584The preferred style for long (multi-line) comments is:
585
586.. code-block:: c
587
588 /*
589 * This is the preferred style for multi-line
590 * comments in the Linux kernel source code.
591 * Please use it consistently.
592 *
593 * Description: A column of asterisks on the left side,
594 * with beginning and ending almost-blank lines.
595 */
596
597For files in net/ and drivers/net/ the preferred style for long (multi-line)
598comments is a little different.
599
600.. code-block:: c
601
602 /* The preferred comment style for files in net/ and drivers/net
603 * looks like this.
604 *
605 * It is nearly the same as the generally preferred comment style,
606 * but there is no initial almost-blank line.
607 */
608
609It's also important to comment data, whether they are basic types or derived
610types. To this end, use just one data declaration per line (no commas for
611multiple data declarations). This leaves you room for a small comment on each
612item, explaining its use.
613
614
6159) You've made a mess of it
616---------------------------
617
618That's OK, we all do. You've probably been told by your long-time Unix
619user helper that ``GNU emacs`` automatically formats the C sources for
620you, and you've noticed that yes, it does do that, but the defaults it
621uses are less than desirable (in fact, they are worse than random
622typing - an infinite number of monkeys typing into GNU emacs would never
623make a good program).
624
625So, you can either get rid of GNU emacs, or change it to use saner
626values. To do the latter, you can stick the following in your .emacs file:
627
628.. code-block:: none
629
630 (defun c-lineup-arglist-tabs-only (ignored)
631 "Line up argument lists by tabs, not spaces"
632 (let* ((anchor (c-langelem-pos c-syntactic-element))
633 (column (c-langelem-2nd-pos c-syntactic-element))
634 (offset (- (1+ column) anchor))
635 (steps (floor offset c-basic-offset)))
636 (* (max steps 1)
637 c-basic-offset)))
638
639 (dir-locals-set-class-variables
640 'linux-kernel
641 '((c-mode . (
642 (c-basic-offset . 8)
643 (c-label-minimum-indentation . 0)
644 (c-offsets-alist . (
645 (arglist-close . c-lineup-arglist-tabs-only)
646 (arglist-cont-nonempty .
647 (c-lineup-gcc-asm-reg c-lineup-arglist-tabs-only))
648 (arglist-intro . +)
649 (brace-list-intro . +)
650 (c . c-lineup-C-comments)
651 (case-label . 0)
652 (comment-intro . c-lineup-comment)
653 (cpp-define-intro . +)
654 (cpp-macro . -1000)
655 (cpp-macro-cont . +)
656 (defun-block-intro . +)
657 (else-clause . 0)
658 (func-decl-cont . +)
659 (inclass . +)
660 (inher-cont . c-lineup-multi-inher)
661 (knr-argdecl-intro . 0)
662 (label . -1000)
663 (statement . 0)
664 (statement-block-intro . +)
665 (statement-case-intro . +)
666 (statement-cont . +)
667 (substatement . +)
668 ))
669 (indent-tabs-mode . t)
670 (show-trailing-whitespace . t)
671 ))))
672
673 (dir-locals-set-directory-class
674 (expand-file-name "~/src/linux-trees")
675 'linux-kernel)
676
677This will make emacs go better with the kernel coding style for C
678files below ``~/src/linux-trees``.
679
680But even if you fail in getting emacs to do sane formatting, not
681everything is lost: use ``indent``.
682
683Now, again, GNU indent has the same brain-dead settings that GNU emacs
684has, which is why you need to give it a few command line options.
685However, that's not too bad, because even the makers of GNU indent
686recognize the authority of K&R (the GNU people aren't evil, they are
687just severely misguided in this matter), so you just give indent the
688options ``-kr -i8`` (stands for ``K&R, 8 character indents``), or use
689``scripts/Lindent``, which indents in the latest style.
690
691``indent`` has a lot of options, and especially when it comes to comment
692re-formatting you may want to take a look at the man page. But
693remember: ``indent`` is not a fix for bad programming.
694
695Note that you can also use the ``clang-format`` tool to help you with
696these rules, to quickly re-format parts of your code automatically,
697and to review full files in order to spot coding style mistakes,
698typos and possible improvements. It is also handy for sorting ``#includes``,
699for aligning variables/macros, for reflowing text and other similar tasks.
700See the file :ref:`Documentation/process/clang-format.rst <clangformat>`
701for more details.
702
703
70410) Kconfig configuration files
705-------------------------------
706
707For all of the Kconfig* configuration files throughout the source tree,
708the indentation is somewhat different. Lines under a ``config`` definition
709are indented with one tab, while help text is indented an additional two
710spaces. Example::
711
712 config AUDIT
713 bool "Auditing support"
714 depends on NET
715 help
716 Enable auditing infrastructure that can be used with another
717 kernel subsystem, such as SELinux (which requires this for
718 logging of avc messages output). Does not do system-call
719 auditing without CONFIG_AUDITSYSCALL.
720
721Seriously dangerous features (such as write support for certain
722filesystems) should advertise this prominently in their prompt string::
723
724 config ADFS_FS_RW
725 bool "ADFS write support (DANGEROUS)"
726 depends on ADFS_FS
727 ...
728
729For full documentation on the configuration files, see the file
730Documentation/kbuild/kconfig-language.rst.
731
732
73311) Data structures
734-------------------
735
736Data structures that have visibility outside the single-threaded
737environment they are created and destroyed in should always have
738reference counts. In the kernel, garbage collection doesn't exist (and
739outside the kernel garbage collection is slow and inefficient), which
740means that you absolutely **have** to reference count all your uses.
741
742Reference counting means that you can avoid locking, and allows multiple
743users to have access to the data structure in parallel - and not having
744to worry about the structure suddenly going away from under them just
745because they slept or did something else for a while.
746
747Note that locking is **not** a replacement for reference counting.
748Locking is used to keep data structures coherent, while reference
749counting is a memory management technique. Usually both are needed, and
750they are not to be confused with each other.
751
752Many data structures can indeed have two levels of reference counting,
753when there are users of different ``classes``. The subclass count counts
754the number of subclass users, and decrements the global count just once
755when the subclass count goes to zero.
756
757Examples of this kind of ``multi-level-reference-counting`` can be found in
758memory management (``struct mm_struct``: mm_users and mm_count), and in
759filesystem code (``struct super_block``: s_count and s_active).
760
761Remember: if another thread can find your data structure, and you don't
762have a reference count on it, you almost certainly have a bug.
763
764
76512) Macros, Enums and RTL
766-------------------------
767
768Names of macros defining constants and labels in enums are capitalized.
769
770.. code-block:: c
771
772 #define CONSTANT 0x12345
773
774Enums are preferred when defining several related constants.
775
776CAPITALIZED macro names are appreciated but macros resembling functions
777may be named in lower case.
778
779Generally, inline functions are preferable to macros resembling functions.
780
781Macros with multiple statements should be enclosed in a do - while block:
782
783.. code-block:: c
784
785 #define macrofun(a, b, c) \
786 do { \
787 if (a == 5) \
788 do_this(b, c); \
789 } while (0)
790
791Things to avoid when using macros:
792
7931) macros that affect control flow:
794
795.. code-block:: c
796
797 #define FOO(x) \
798 do { \
799 if (blah(x) < 0) \
800 return -EBUGGERED; \
801 } while (0)
802
803is a **very** bad idea. It looks like a function call but exits the ``calling``
804function; don't break the internal parsers of those who will read the code.
805
8062) macros that depend on having a local variable with a magic name:
807
808.. code-block:: c
809
810 #define FOO(val) bar(index, val)
811
812might look like a good thing, but it's confusing as hell when one reads the
813code and it's prone to breakage from seemingly innocent changes.
814
8153) macros with arguments that are used as l-values: FOO(x) = y; will
816bite you if somebody e.g. turns FOO into an inline function.
817
8184) forgetting about precedence: macros defining constants using expressions
819must enclose the expression in parentheses. Beware of similar issues with
820macros using parameters.
821
822.. code-block:: c
823
824 #define CONSTANT 0x4000
825 #define CONSTEXP (CONSTANT | 3)
826
8275) namespace collisions when defining local variables in macros resembling
828functions:
829
830.. code-block:: c
831
832 #define FOO(x) \
833 ({ \
834 typeof(x) ret; \
835 ret = calc_ret(x); \
836 (ret); \
837 })
838
839ret is a common name for a local variable - __foo_ret is less likely
840to collide with an existing variable.
841
842The cpp manual deals with macros exhaustively. The gcc internals manual also
843covers RTL which is used frequently with assembly language in the kernel.
844
845
84613) Printing kernel messages
847----------------------------
848
849Kernel developers like to be seen as literate. Do mind the spelling
850of kernel messages to make a good impression. Do not use incorrect
851contractions like ``dont``; use ``do not`` or ``don't`` instead. Make the
852messages concise, clear, and unambiguous.
853
854Kernel messages do not have to be terminated with a period.
855
856Printing numbers in parentheses (%d) adds no value and should be avoided.
857
858There are a number of driver model diagnostic macros in <linux/device.h>
859which you should use to make sure messages are matched to the right device
860and driver, and are tagged with the right level: dev_err(), dev_warn(),
861dev_info(), and so forth. For messages that aren't associated with a
862particular device, <linux/printk.h> defines pr_notice(), pr_info(),
863pr_warn(), pr_err(), etc.
864
865Coming up with good debugging messages can be quite a challenge; and once
866you have them, they can be a huge help for remote troubleshooting. However
867debug message printing is handled differently than printing other non-debug
868messages. While the other pr_XXX() functions print unconditionally,
869pr_debug() does not; it is compiled out by default, unless either DEBUG is
870defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also,
871and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
872the ones already enabled by DEBUG.
873
874Many subsystems have Kconfig debug options to turn on -DDEBUG in the
875corresponding Makefile; in other cases specific files #define DEBUG. And
876when a debug message should be unconditionally printed, such as if it is
877already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
878used.
879
880
88114) Allocating memory
882---------------------
883
884The kernel provides the following general purpose memory allocators:
885kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
886vzalloc(). Please refer to the API documentation for further information
887about them. :ref:`Documentation/core-api/memory-allocation.rst
888<memory_allocation>`
889
890The preferred form for passing a size of a struct is the following:
891
892.. code-block:: c
893
894 p = kmalloc(sizeof(*p), ...);
895
896The alternative form where struct name is spelled out hurts readability and
897introduces an opportunity for a bug when the pointer variable type is changed
898but the corresponding sizeof that is passed to a memory allocator is not.
899
900Casting the return value which is a void pointer is redundant. The conversion
901from void pointer to any other pointer type is guaranteed by the C programming
902language.
903
904The preferred form for allocating an array is the following:
905
906.. code-block:: c
907
908 p = kmalloc_array(n, sizeof(...), ...);
909
910The preferred form for allocating a zeroed array is the following:
911
912.. code-block:: c
913
914 p = kcalloc(n, sizeof(...), ...);
915
916Both forms check for overflow on the allocation size n * sizeof(...),
917and return NULL if that occurred.
918
919These generic allocation functions all emit a stack dump on failure when used
920without __GFP_NOWARN so there is no use in emitting an additional failure
921message when NULL is returned.
922
92315) The inline disease
924----------------------
925
926There appears to be a common misperception that gcc has a magic "make me
927faster" speedup option called ``inline``. While the use of inlines can be
928appropriate (for example as a means of replacing macros, see Chapter 12), it
929very often is not. Abundant use of the inline keyword leads to a much bigger
930kernel, which in turn slows the system as a whole down, due to a bigger
931icache footprint for the CPU and simply because there is less memory
932available for the pagecache. Just think about it; a pagecache miss causes a
933disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
934that can go into these 5 milliseconds.
935
936A reasonable rule of thumb is to not put inline at functions that have more
937than 3 lines of code in them. An exception to this rule are the cases where
938a parameter is known to be a compiletime constant, and as a result of this
939constantness you *know* the compiler will be able to optimize most of your
940function away at compile time. For a good example of this later case, see
941the kmalloc() inline function.
942
943Often people argue that adding inline to functions that are static and used
944only once is always a win since there is no space tradeoff. While this is
945technically correct, gcc is capable of inlining these automatically without
946help, and the maintenance issue of removing the inline when a second user
947appears outweighs the potential value of the hint that tells gcc to do
948something it would have done anyway.
949
950
95116) Function return values and names
952------------------------------------
953
954Functions can return values of many different kinds, and one of the
955most common is a value indicating whether the function succeeded or
956failed. Such a value can be represented as an error-code integer
957(-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure,
958non-zero = success).
959
960Mixing up these two sorts of representations is a fertile source of
961difficult-to-find bugs. If the C language included a strong distinction
962between integers and booleans then the compiler would find these mistakes
963for us... but it doesn't. To help prevent such bugs, always follow this
964convention::
965
966 If the name of a function is an action or an imperative command,
967 the function should return an error-code integer. If the name
968 is a predicate, the function should return a "succeeded" boolean.
969
970For example, ``add work`` is a command, and the add_work() function returns 0
971for success or -EBUSY for failure. In the same way, ``PCI device present`` is
972a predicate, and the pci_dev_present() function returns 1 if it succeeds in
973finding a matching device or 0 if it doesn't.
974
975All EXPORTed functions must respect this convention, and so should all
976public functions. Private (static) functions need not, but it is
977recommended that they do.
978
979Functions whose return value is the actual result of a computation, rather
980than an indication of whether the computation succeeded, are not subject to
981this rule. Generally they indicate failure by returning some out-of-range
982result. Typical examples would be functions that return pointers; they use
983NULL or the ERR_PTR mechanism to report failure.
984
985
98617) Using bool
987--------------
988
989The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
990only evaluate to 0 or 1, and implicit or explicit conversion to bool
991automatically converts the value to true or false. When using bool types the
992!! construction is not needed, which eliminates a class of bugs.
993
994When working with bool values the true and false definitions should be used
995instead of 1 and 0.
996
997bool function return types and stack variables are always fine to use whenever
998appropriate. Use of bool is encouraged to improve readability and is often a
999better option than 'int' for storing boolean values.
1000
1001Do not use bool if cache line layout or size of the value matters, as its size
1002and alignment varies based on the compiled architecture. Structures that are
1003optimized for alignment and size should not use bool.
1004
1005If a structure has many true/false values, consider consolidating them into a
1006bitfield with 1 bit members, or using an appropriate fixed width type, such as
1007u8.
1008
1009Similarly for function arguments, many true/false values can be consolidated
1010into a single bitwise 'flags' argument and 'flags' can often be a more
1011readable alternative if the call-sites have naked true/false constants.
1012
1013Otherwise limited use of bool in structures and arguments can improve
1014readability.
1015
101618) Don't re-invent the kernel macros
1017-------------------------------------
1018
1019The header file include/linux/kernel.h contains a number of macros that
1020you should use, rather than explicitly coding some variant of them yourself.
1021For example, if you need to calculate the length of an array, take advantage
1022of the macro
1023
1024.. code-block:: c
1025
1026 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
1027
1028Similarly, if you need to calculate the size of some structure member, use
1029
1030.. code-block:: c
1031
1032 #define sizeof_field(t, f) (sizeof(((t*)0)->f))
1033
1034There are also min() and max() macros that do strict type checking if you
1035need them. Feel free to peruse that header file to see what else is already
1036defined that you shouldn't reproduce in your code.
1037
1038
103919) Editor modelines and other cruft
1040------------------------------------
1041
1042Some editors can interpret configuration information embedded in source files,
1043indicated with special markers. For example, emacs interprets lines marked
1044like this:
1045
1046.. code-block:: c
1047
1048 -*- mode: c -*-
1049
1050Or like this:
1051
1052.. code-block:: c
1053
1054 /*
1055 Local Variables:
1056 compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
1057 End:
1058 */
1059
1060Vim interprets markers that look like this:
1061
1062.. code-block:: c
1063
1064 /* vim:set sw=8 noet */
1065
1066Do not include any of these in source files. People have their own personal
1067editor configurations, and your source files should not override them. This
1068includes markers for indentation and mode configuration. People may use their
1069own custom mode, or may have some other magic method for making indentation
1070work correctly.
1071
1072
107320) Inline assembly
1074-------------------
1075
1076In architecture-specific code, you may need to use inline assembly to interface
1077with CPU or platform functionality. Don't hesitate to do so when necessary.
1078However, don't use inline assembly gratuitously when C can do the job. You can
1079and should poke hardware from C when possible.
1080
1081Consider writing simple helper functions that wrap common bits of inline
1082assembly, rather than repeatedly writing them with slight variations. Remember
1083that inline assembly can use C parameters.
1084
1085Large, non-trivial assembly functions should go in .S files, with corresponding
1086C prototypes defined in C header files. The C prototypes for assembly
1087functions should use ``asmlinkage``.
1088
1089You may need to mark your asm statement as volatile, to prevent GCC from
1090removing it if GCC doesn't notice any side effects. You don't always need to
1091do so, though, and doing so unnecessarily can limit optimization.
1092
1093When writing a single inline assembly statement containing multiple
1094instructions, put each instruction on a separate line in a separate quoted
1095string, and end each string except the last with ``\n\t`` to properly indent
1096the next instruction in the assembly output:
1097
1098.. code-block:: c
1099
1100 asm ("magic %reg1, #42\n\t"
1101 "more_magic %reg2, %reg3"
1102 : /* outputs */ : /* inputs */ : /* clobbers */);
1103
1104
110521) Conditional Compilation
1106---------------------------
1107
1108Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
1109files; doing so makes code harder to read and logic harder to follow. Instead,
1110use such conditionals in a header file defining functions for use in those .c
1111files, providing no-op stub versions in the #else case, and then call those
1112functions unconditionally from .c files. The compiler will avoid generating
1113any code for the stub calls, producing identical results, but the logic will
1114remain easy to follow.
1115
1116Prefer to compile out entire functions, rather than portions of functions or
1117portions of expressions. Rather than putting an ifdef in an expression, factor
1118out part or all of the expression into a separate helper function and apply the
1119conditional to that function.
1120
1121If you have a function or variable which may potentially go unused in a
1122particular configuration, and the compiler would warn about its definition
1123going unused, mark the definition as __maybe_unused rather than wrapping it in
1124a preprocessor conditional. (However, if a function or variable *always* goes
1125unused, delete it.)
1126
1127Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
1128symbol into a C boolean expression, and use it in a normal C conditional:
1129
1130.. code-block:: c
1131
1132 if (IS_ENABLED(CONFIG_SOMETHING)) {
1133 ...
1134 }
1135
1136The compiler will constant-fold the conditional away, and include or exclude
1137the block of code just as with an #ifdef, so this will not add any runtime
1138overhead. However, this approach still allows the C compiler to see the code
1139inside the block, and check it for correctness (syntax, types, symbol
1140references, etc). Thus, you still have to use an #ifdef if the code inside the
1141block references symbols that will not exist if the condition is not met.
1142
1143At the end of any non-trivial #if or #ifdef block (more than a few lines),
1144place a comment after the #endif on the same line, noting the conditional
1145expression used. For instance:
1146
1147.. code-block:: c
1148
1149 #ifdef CONFIG_SOMETHING
1150 ...
1151 #endif /* CONFIG_SOMETHING */
1152
1153
1154Appendix I) References
1155----------------------
1156
1157The C Programming Language, Second Edition
1158by Brian W. Kernighan and Dennis M. Ritchie.
1159Prentice Hall, Inc., 1988.
1160ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
1161
1162The Practice of Programming
1163by Brian W. Kernighan and Rob Pike.
1164Addison-Wesley, Inc., 1999.
1165ISBN 0-201-61586-X.
1166
1167GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
1168gcc internals and indent, all available from https://www.gnu.org/manual/
1169
1170WG14 is the international standardization working group for the programming
1171language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
1172
1173Kernel :ref:`process/coding-style.rst <codingstyle>`, by greg@kroah.com at OLS 2002:
1174http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/