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1	.. _codingstyle:
2
3	Linux kernel coding style
4	=========================
5
6	This is a short document describing the preferred coding style for the
7	linux kernel. Coding style is very personal, and I won't **force** my
8	views on anybody, but this is what goes for anything that I have to be
9	able to maintain, and I'd prefer it for most other things too. Please
10	at least consider the points made here.
11
12	First off, I'd suggest printing out a copy of the GNU coding standards,
13	and NOT read it. Burn them, it's a great symbolic gesture.
14
15	Anyway, here goes:
16
17
18	1) Indentation
19	--------------
20
21	Tabs are 8 characters, and thus indentations are also 8 characters.
22	There are heretic movements that try to make indentations 4 (or even 2!)
23	characters deep, and that is akin to trying to define the value of PI to
24	be 3.
25
26	Rationale: The whole idea behind indentation is to clearly define where
27	a block of control starts and ends. Especially when you've been looking
28	at your screen for 20 straight hours, you'll find it a lot easier to see
29	how the indentation works if you have large indentations.
30
31	Now, some people will claim that having 8-character indentations makes
32	the code move too far to the right, and makes it hard to read on a
33	80-character terminal screen. The answer to that is that if you need
34	more than 3 levels of indentation, you're screwed anyway, and should fix
35	your program.
36
37	In short, 8-char indents make things easier to read, and have the added
38	benefit of warning you when you're nesting your functions too deep.
39	Heed that warning.
40
41	The preferred way to ease multiple indentation levels in a switch statement is
42	to align the ``switch`` and its subordinate ``case`` labels in the same column
43	instead 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
64	Don't put multiple statements on a single line unless you have
65	something to hide:
66
67	.. code-block:: c
68
69	if (condition) do_this;
70	do_something_everytime;
71
72	Don't put multiple assignments on a single line either. Kernel coding style
73	is super simple. Avoid tricky expressions.
74
75	Outside of comments, documentation and except in Kconfig, spaces are never
76	used for indentation, and the above example is deliberately broken.
77
78	Get a decent editor and don't leave whitespace at the end of lines.
79
80
81	2) Breaking long lines and strings
82	----------------------------------
83
84	Coding style is all about readability and maintainability using commonly
85	available tools.
86
87	The limit on the length of lines is 80 columns and this is a strongly
88	preferred limit.
89
90	Statements longer than 80 columns will be broken into sensible chunks, unless
91	exceeding 80 columns significantly increases readability and does not hide
92	information. Descendants are always substantially shorter than the parent and
93	are placed substantially to the right. The same applies to function headers
94	with a long argument list. However, never break user-visible strings such as
95	printk messages, because that breaks the ability to grep for them.
96
97
98	3) Placing Braces and Spaces
99	----------------------------
100
101	The other issue that always comes up in C styling is the placement of
102	braces. Unlike the indent size, there are few technical reasons to
103	choose one placement strategy over the other, but the preferred way, as
104	shown to us by the prophets Kernighan and Ritchie, is to put the opening
105	brace 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
113	This applies to all non-function statement blocks (if, switch, for,
114	while, 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
129	However, there is one special case, namely functions: they have the
130	opening 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
139	Heretic people all over the world have claimed that this inconsistency
140	is ... well ... inconsistent, but all right-thinking people know that
141	(a) K&R are **right** and (b) K&R are right. Besides, functions are
142	special anyway (you can't nest them in C).
143
144	Note that the closing brace is empty on a line of its own, **except** in
145	the cases where it is followed by a continuation of the same statement,
146	ie a ``while`` in a do-statement or an ``else`` in an if-statement, like
147	this:
148
149	.. code-block:: c
150
151	do {
152	body of do-loop
153	} while (condition);
154
155	and
156
157	.. code-block:: c
158
159	if (x == y) {
160	..
161	} else if (x > y) {
162	...
163	} else {
164	....
165	}
166
167	Rationale: K&R.
168
169	Also, note that this brace-placement also minimizes the number of empty
170	(or almost empty) lines, without any loss of readability. Thus, as the
171	supply of new-lines on your screen is not a renewable resource (think
172	25-line terminal screens here), you have more empty lines to put
173	comments on.
174
175	Do not unnecessarily use braces where a single statement will do.
176
177	.. code-block:: c
178
179	if (condition)
180	action();
181
182	and
183
184	.. code-block:: none
185
186	if (condition)
187	do_this();
188	else
189	do_that();
190
191	This does not apply if only one branch of a conditional statement is a single
192	statement; 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
203	3.1) Spaces
204	***********
205
206	Linux kernel style for use of spaces depends (mostly) on
207	function-versus-keyword usage. Use a space after (most) keywords. The
208	notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
209	somewhat like functions (and are usually used with parentheses in Linux,
210	although they are not required in the language, as in: ``sizeof info`` after
211	``struct fileinfo info;`` is declared).
212
213	So use a space after these keywords::
214
215	if, switch, case, for, do, while
216
217	but not with sizeof, typeof, alignof, or __attribute__. E.g.,
218
219	.. code-block:: c
220
221
222	s = sizeof(struct file);
223
224	Do 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
232	When declaring pointer data or a function that returns a pointer type, the
233	preferred use of ``*`` is adjacent to the data name or function name and not
234	adjacent 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
243	Use one space around (on each side of) most binary and ternary operators,
244	such as any of these::
245
246	= + - < > * / % | & ^ <= >= == != ? :
247
248	but no space after unary operators::
249
250	& * + - ~ ! sizeof typeof alignof __attribute__ defined
251
252	no space before the postfix increment & decrement unary operators::
253
254	++ --
255
256	no space after the prefix increment & decrement unary operators::
257
258	++ --
259
260	and no space around the ``.`` and ``->`` structure member operators.
261
262	Do 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
264	appropriate, so you can start typing the next line of code right away.
265	However, some such editors do not remove the whitespace if you end up not
266	putting a line of code there, such as if you leave a blank line. As a result,
267	you end up with lines containing trailing whitespace.
268
269	Git will warn you about patches that introduce trailing whitespace, and can
270	optionally strip the trailing whitespace for you; however, if applying a series
271	of patches, this may make later patches in the series fail by changing their
272	context lines.
273
274
275	4) Naming
276	---------
277
278	C is a Spartan language, and so should your naming be. Unlike Modula-2
279	and Pascal programmers, C programmers do not use cute names like
280	ThisVariableIsATemporaryCounter. A C programmer would call that
281	variable ``tmp``, which is much easier to write, and not the least more
282	difficult to understand.
283
284	HOWEVER, while mixed-case names are frowned upon, descriptive names for
285	global variables are a must. To call a global function ``foo`` is a
286	shooting offense.
287
288	GLOBAL variables (to be used only if you **really** need them) need to
289	have descriptive names, as do global functions. If you have a function
290	that counts the number of active users, you should call that
291	``count_active_users()`` or similar, you should **not** call it ``cntusr()``.
292
293	Encoding the type of a function into the name (so-called Hungarian
294	notation) is brain damaged - the compiler knows the types anyway and can
295	check those, and it only confuses the programmer. No wonder MicroSoft
296	makes buggy programs.
297
298	LOCAL variable names should be short, and to the point. If you have
299	some random integer loop counter, it should probably be called ``i``.
300	Calling it ``loop_counter`` is non-productive, if there is no chance of it
301	being mis-understood. Similarly, ``tmp`` can be just about any type of
302	variable that is used to hold a temporary value.
303
304	If you are afraid to mix up your local variable names, you have another
305	problem, which is called the function-growth-hormone-imbalance syndrome.
306	See chapter 6 (Functions).
307
308
309	5) Typedefs
310	-----------
311
312	Please don't use things like ``vps_t``.
313	It'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
320	in the source, what does it mean?
321	In contrast, if it says
322
323	.. code-block:: c
324
325	struct virtual_container *a;
326
327	you can actually tell what ``a`` is.
328
329	Lots of people think that typedefs ``help readability``. Not so. They are
330	useful 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
386	Maybe there are other cases too, but the rule should basically be to NEVER
387	EVER use a typedef unless you can clearly match one of those rules.
388
389	In general, a pointer, or a struct that has elements that can reasonably
390	be directly accessed should **never** be a typedef.
391
392
393	6) Functions
394	------------
395
396	Functions should be short and sweet, and do just one thing. They should
397	fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
398	as we all know), and do one thing and do that well.
399
400	The maximum length of a function is inversely proportional to the
401	complexity and indentation level of that function. So, if you have a
402	conceptually simple function that is just one long (but simple)
403	case-statement, where you have to do lots of small things for a lot of
404	different cases, it's OK to have a longer function.
405
406	However, if you have a complex function, and you suspect that a
407	less-than-gifted first-year high-school student might not even
408	understand what the function is all about, you should adhere to the
409	maximum limits all the more closely. Use helper functions with
410	descriptive names (you can ask the compiler to in-line them if you think
411	it's performance-critical, and it will probably do a better job of it
412	than you would have done).
413
414	Another measure of the function is the number of local variables. They
415	shouldn't exceed 5-10, or you're doing something wrong. Re-think the
416	function, and split it into smaller pieces. A human brain can
417	generally easily keep track of about 7 different things, anything more
418	and it gets confused. You know you're brilliant, but maybe you'd like
419	to understand what you did 2 weeks from now.
420
421	In source files, separate functions with one blank line. If the function is
422	exported, the **EXPORT** macro for it should follow immediately after the
423	closing 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
433	In function prototypes, include parameter names with their data types.
434	Although this is not required by the C language, it is preferred in Linux
435	because it is a simple way to add valuable information for the reader.
436
437
438	7) Centralized exiting of functions
439	-----------------------------------
440
441	Albeit deprecated by some people, the equivalent of the goto statement is
442	used frequently by compilers in form of the unconditional jump instruction.
443
444	The goto statement comes in handy when a function exits from multiple
445	locations and some common work such as cleanup has to be done. If there is no
446	cleanup needed then just return directly.
447
448	Choose label names which say what the goto does or why the goto exists. An
449	example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``.
450	Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to
451	renumber them if you ever add or remove exit paths, and they make correctness
452	difficult to verify anyway.
453
454	The 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_buffer;
479	}
480	...
481	out_free_buffer:
482	kfree(buffer);
483	return result;
484	}
485
486	A 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
495	The bug in this code is that on some exit paths ``foo`` is NULL. Normally the
496	fix 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
507	Ideally you should simulate errors to test all exit paths.
508
509
510	8) Commenting
511	-------------
512
513	Comments are good, but there is also a danger of over-commenting. NEVER
514	try to explain HOW your code works in a comment: it's much better to
515	write the code so that the **working** is obvious, and it's a waste of
516	time to explain badly written code.
517
518	Generally, you want your comments to tell WHAT your code does, not HOW.
519	Also, try to avoid putting comments inside a function body: if the
520	function is so complex that you need to separately comment parts of it,
521	you should probably go back to chapter 6 for a while. You can make
522	small comments to note or warn about something particularly clever (or
523	ugly), but try to avoid excess. Instead, put the comments at the head
524	of the function, telling people what it does, and possibly WHY it does
525	it.
526
527	When commenting the kernel API functions, please use the kernel-doc format.
528	See the files Documentation/kernel-documentation.rst and scripts/kernel-doc
529	for details.
530
531	The 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
544	For files in net/ and drivers/net/ the preferred style for long (multi-line)
545	comments 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
556	It's also important to comment data, whether they are basic types or derived
557	types. To this end, use just one data declaration per line (no commas for
558	multiple data declarations). This leaves you room for a small comment on each
559	item, explaining its use.
560
561
562	9) You've made a mess of it
563	---------------------------
564
565	That's OK, we all do. You've probably been told by your long-time Unix
566	user helper that ``GNU emacs`` automatically formats the C sources for
567	you, and you've noticed that yes, it does do that, but the defaults it
568	uses are less than desirable (in fact, they are worse than random
569	typing - an infinite number of monkeys typing into GNU emacs would never
570	make a good program).
571
572	So, you can either get rid of GNU emacs, or change it to use saner
573	values. 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
607	This will make emacs go better with the kernel coding style for C
608	files below ``~/src/linux-trees``.
609
610	But even if you fail in getting emacs to do sane formatting, not
611	everything is lost: use ``indent``.
612
613	Now, again, GNU indent has the same brain-dead settings that GNU emacs
614	has, which is why you need to give it a few command line options.
615	However, that's not too bad, because even the makers of GNU indent
616	recognize the authority of K&R (the GNU people aren't evil, they are
617	just severely misguided in this matter), so you just give indent the
618	options ``-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
622	re-formatting you may want to take a look at the man page. But
623	remember: ``indent`` is not a fix for bad programming.
624
625
626	10) Kconfig configuration files
627	-------------------------------
628
629	For all of the Kconfig* configuration files throughout the source tree,
630	the indentation is somewhat different. Lines under a ``config`` definition
631	are indented with one tab, while help text is indented an additional two
632	spaces. 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
643	Seriously dangerous features (such as write support for certain
644	filesystems) 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
651	For full documentation on the configuration files, see the file
652	Documentation/kbuild/kconfig-language.txt.
653
654
655	11) Data structures
656	-------------------
657
658	Data structures that have visibility outside the single-threaded
659	environment they are created and destroyed in should always have
660	reference counts. In the kernel, garbage collection doesn't exist (and
661	outside the kernel garbage collection is slow and inefficient), which
662	means that you absolutely **have** to reference count all your uses.
663
664	Reference counting means that you can avoid locking, and allows multiple
665	users to have access to the data structure in parallel - and not having
666	to worry about the structure suddenly going away from under them just
667	because they slept or did something else for a while.
668
669	Note that locking is **not** a replacement for reference counting.
670	Locking is used to keep data structures coherent, while reference
671	counting is a memory management technique. Usually both are needed, and
672	they are not to be confused with each other.
673
674	Many data structures can indeed have two levels of reference counting,
675	when there are users of different ``classes``. The subclass count counts
676	the number of subclass users, and decrements the global count just once
677	when the subclass count goes to zero.
678
679	Examples of this kind of ``multi-level-reference-counting`` can be found in
680	memory management (``struct mm_struct``: mm_users and mm_count), and in
681	filesystem code (``struct super_block``: s_count and s_active).
682
683	Remember: if another thread can find your data structure, and you don't
684	have a reference count on it, you almost certainly have a bug.
685
686
687	12) Macros, Enums and RTL
688	-------------------------
689
690	Names of macros defining constants and labels in enums are capitalized.
691
692	.. code-block:: c
693
694	#define CONSTANT 0x12345
695
696	Enums are preferred when defining several related constants.
697
698	CAPITALIZED macro names are appreciated but macros resembling functions
699	may be named in lower case.
700
701	Generally, inline functions are preferable to macros resembling functions.
702
703	Macros 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
713	Things to avoid when using macros:
714
715	1) 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
725	is a **very** bad idea. It looks like a function call but exits the ``calling``
726	function; don't break the internal parsers of those who will read the code.
727
728	2) 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
734	might look like a good thing, but it's confusing as hell when one reads the
735	code and it's prone to breakage from seemingly innocent changes.
736
737	3) macros with arguments that are used as l-values: FOO(x) = y; will
738	bite you if somebody e.g. turns FOO into an inline function.
739
740	4) forgetting about precedence: macros defining constants using expressions
741	must enclose the expression in parentheses. Beware of similar issues with
742	macros using parameters.
743
744	.. code-block:: c
745
746	#define CONSTANT 0x4000
747	#define CONSTEXP (CONSTANT | 3)
748
749	5) namespace collisions when defining local variables in macros resembling
750	functions:
751
752	.. code-block:: c
753
754	#define FOO(x) \
755	({ \
756	typeof(x) ret; \
757	ret = calc_ret(x); \
758	(ret); \
759	})
760
761	ret is a common name for a local variable - __foo_ret is less likely
762	to collide with an existing variable.
763
764	The cpp manual deals with macros exhaustively. The gcc internals manual also
765	covers RTL which is used frequently with assembly language in the kernel.
766
767
768	13) Printing kernel messages
769	----------------------------
770
771	Kernel developers like to be seen as literate. Do mind the spelling
772	of kernel messages to make a good impression. Do not use crippled
773	words like ``dont``; use ``do not`` or ``don't`` instead. Make the messages
774	concise, clear, and unambiguous.
775
776	Kernel messages do not have to be terminated with a period.
777
778	Printing numbers in parentheses (%d) adds no value and should be avoided.
779
780	There are a number of driver model diagnostic macros in <linux/device.h>
781	which you should use to make sure messages are matched to the right device
782	and driver, and are tagged with the right level: dev_err(), dev_warn(),
783	dev_info(), and so forth. For messages that aren't associated with a
784	particular device, <linux/printk.h> defines pr_notice(), pr_info(),
785	pr_warn(), pr_err(), etc.
786
787	Coming up with good debugging messages can be quite a challenge; and once
788	you have them, they can be a huge help for remote troubleshooting. However
789	debug message printing is handled differently than printing other non-debug
790	messages. While the other pr_XXX() functions print unconditionally,
791	pr_debug() does not; it is compiled out by default, unless either DEBUG is
792	defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also,
793	and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
794	the ones already enabled by DEBUG.
795
796	Many subsystems have Kconfig debug options to turn on -DDEBUG in the
797	corresponding Makefile; in other cases specific files #define DEBUG. And
798	when a debug message should be unconditionally printed, such as if it is
799	already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
800	used.
801
802
803	14) Allocating memory
804	---------------------
805
806	The kernel provides the following general purpose memory allocators:
807	kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
808	vzalloc(). Please refer to the API documentation for further information
809	about them.
810
811	The preferred form for passing a size of a struct is the following:
812
813	.. code-block:: c
814
815	p = kmalloc(sizeof(*p), ...);
816
817	The alternative form where struct name is spelled out hurts readability and
818	introduces an opportunity for a bug when the pointer variable type is changed
819	but the corresponding sizeof that is passed to a memory allocator is not.
820
821	Casting the return value which is a void pointer is redundant. The conversion
822	from void pointer to any other pointer type is guaranteed by the C programming
823	language.
824
825	The preferred form for allocating an array is the following:
826
827	.. code-block:: c
828
829	p = kmalloc_array(n, sizeof(...), ...);
830
831	The preferred form for allocating a zeroed array is the following:
832
833	.. code-block:: c
834
835	p = kcalloc(n, sizeof(...), ...);
836
837	Both forms check for overflow on the allocation size n * sizeof(...),
838	and return NULL if that occurred.
839
840
841	15) The inline disease
842	----------------------
843
844	There appears to be a common misperception that gcc has a magic "make me
845	faster" speedup option called ``inline``. While the use of inlines can be
846	appropriate (for example as a means of replacing macros, see Chapter 12), it
847	very often is not. Abundant use of the inline keyword leads to a much bigger
848	kernel, which in turn slows the system as a whole down, due to a bigger
849	icache footprint for the CPU and simply because there is less memory
850	available for the pagecache. Just think about it; a pagecache miss causes a
851	disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
852	that can go into these 5 milliseconds.
853
854	A reasonable rule of thumb is to not put inline at functions that have more
855	than 3 lines of code in them. An exception to this rule are the cases where
856	a parameter is known to be a compiletime constant, and as a result of this
857	constantness you *know* the compiler will be able to optimize most of your
858	function away at compile time. For a good example of this later case, see
859	the kmalloc() inline function.
860
861	Often people argue that adding inline to functions that are static and used
862	only once is always a win since there is no space tradeoff. While this is
863	technically correct, gcc is capable of inlining these automatically without
864	help, and the maintenance issue of removing the inline when a second user
865	appears outweighs the potential value of the hint that tells gcc to do
866	something it would have done anyway.
867
868
869	16) Function return values and names
870	------------------------------------
871
872	Functions can return values of many different kinds, and one of the
873	most common is a value indicating whether the function succeeded or
874	failed. Such a value can be represented as an error-code integer
875	(-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure,
876	non-zero = success).
877
878	Mixing up these two sorts of representations is a fertile source of
879	difficult-to-find bugs. If the C language included a strong distinction
880	between integers and booleans then the compiler would find these mistakes
881	for us... but it doesn't. To help prevent such bugs, always follow this
882	convention::
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
888	For example, ``add work`` is a command, and the add_work() function returns 0
889	for success or -EBUSY for failure. In the same way, ``PCI device present`` is
890	a predicate, and the pci_dev_present() function returns 1 if it succeeds in
891	finding a matching device or 0 if it doesn't.
892
893	All EXPORTed functions must respect this convention, and so should all
894	public functions. Private (static) functions need not, but it is
895	recommended that they do.
896
897	Functions whose return value is the actual result of a computation, rather
898	than an indication of whether the computation succeeded, are not subject to
899	this rule. Generally they indicate failure by returning some out-of-range
900	result. Typical examples would be functions that return pointers; they use
901	NULL or the ERR_PTR mechanism to report failure.
902
903
904	17) Don't re-invent the kernel macros
905	-------------------------------------
906
907	The header file include/linux/kernel.h contains a number of macros that
908	you should use, rather than explicitly coding some variant of them yourself.
909	For example, if you need to calculate the length of an array, take advantage
910	of the macro
911
912	.. code-block:: c
913
914	#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
915
916	Similarly, 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
922	There are also min() and max() macros that do strict type checking if you
923	need them. Feel free to peruse that header file to see what else is already
924	defined that you shouldn't reproduce in your code.
925
926
927	18) Editor modelines and other cruft
928	------------------------------------
929
930	Some editors can interpret configuration information embedded in source files,
931	indicated with special markers. For example, emacs interprets lines marked
932	like this:
933
934	.. code-block:: c
935
936	-*- mode: c -*-
937
938	Or 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
948	Vim interprets markers that look like this:
949
950	.. code-block:: c
951
952	/* vim:set sw=8 noet */
953
954	Do not include any of these in source files. People have their own personal
955	editor configurations, and your source files should not override them. This
956	includes markers for indentation and mode configuration. People may use their
957	own custom mode, or may have some other magic method for making indentation
958	work correctly.
959
960
961	19) Inline assembly
962	-------------------
963
964	In architecture-specific code, you may need to use inline assembly to interface
965	with CPU or platform functionality. Don't hesitate to do so when necessary.
966	However, don't use inline assembly gratuitously when C can do the job. You can
967	and should poke hardware from C when possible.
968
969	Consider writing simple helper functions that wrap common bits of inline
970	assembly, rather than repeatedly writing them with slight variations. Remember
971	that inline assembly can use C parameters.
972
973	Large, non-trivial assembly functions should go in .S files, with corresponding
974	C prototypes defined in C header files. The C prototypes for assembly
975	functions should use ``asmlinkage``.
976
977	You may need to mark your asm statement as volatile, to prevent GCC from
978	removing it if GCC doesn't notice any side effects. You don't always need to
979	do so, though, and doing so unnecessarily can limit optimization.
980
981	When writing a single inline assembly statement containing multiple
982	instructions, put each instruction on a separate line in a separate quoted
983	string, and end each string except the last with \n\t to properly indent the
984	next 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
993	20) Conditional Compilation
994	---------------------------
995
996	Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
997	files; doing so makes code harder to read and logic harder to follow. Instead,
998	use such conditionals in a header file defining functions for use in those .c
999	files, providing no-op stub versions in the #else case, and then call those
1000	functions unconditionally from .c files. The compiler will avoid generating
1001	any code for the stub calls, producing identical results, but the logic will
1002	remain easy to follow.
1003
1004	Prefer to compile out entire functions, rather than portions of functions or
1005	portions of expressions. Rather than putting an ifdef in an expression, factor
1006	out part or all of the expression into a separate helper function and apply the
1007	conditional to that function.
1008
1009	If you have a function or variable which may potentially go unused in a
1010	particular configuration, and the compiler would warn about its definition
1011	going unused, mark the definition as __maybe_unused rather than wrapping it in
1012	a preprocessor conditional. (However, if a function or variable *always* goes
1013	unused, delete it.)
1014
1015	Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
1016	symbol 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
1024	The compiler will constant-fold the conditional away, and include or exclude
1025	the block of code just as with an #ifdef, so this will not add any runtime
1026	overhead. However, this approach still allows the C compiler to see the code
1027	inside the block, and check it for correctness (syntax, types, symbol
1028	references, etc). Thus, you still have to use an #ifdef if the code inside the
1029	block references symbols that will not exist if the condition is not met.
1030
1031	At the end of any non-trivial #if or #ifdef block (more than a few lines),
1032	place a comment after the #endif on the same line, noting the conditional
1033	expression used. For instance:
1034
1035	.. code-block:: c
1036
1037	#ifdef CONFIG_SOMETHING
1038	...
1039	#endif /* CONFIG_SOMETHING */
1040
1041
1042	Appendix I) References
1043	----------------------
1044
1045	The C Programming Language, Second Edition
1046	by Brian W. Kernighan and Dennis M. Ritchie.
1047	Prentice Hall, Inc., 1988.
1048	ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
1049
1050	The Practice of Programming
1051	by Brian W. Kernighan and Rob Pike.
1052	Addison-Wesley, Inc., 1999.
1053	ISBN 0-201-61586-X.
1054
1055	GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
1056	gcc internals and indent, all available from http://www.gnu.org/manual/
1057
1058	WG14 is the international standardization working group for the programming
1059	language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
1060
1061	Kernel CodingStyle, by greg@kroah.com at OLS 2002:
1062	http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
1063