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Based on kernel version 3.19. Page generated on 2015-02-13 21:16 EST.

1	
2			Linux kernel coding style
3	
4	This is a short document describing the preferred coding style for the
5	linux kernel.  Coding style is very personal, and I won't _force_ my
6	views on anybody, but this is what goes for anything that I have to be
7	able to maintain, and I'd prefer it for most other things too.  Please
8	at least consider the points made here.
9	
10	First off, I'd suggest printing out a copy of the GNU coding standards,
11	and NOT read it.  Burn them, it's a great symbolic gesture.
12	
13	Anyway, here goes:
14	
15	
16		 	Chapter 1: Indentation
17	
18	Tabs are 8 characters, and thus indentations are also 8 characters.
19	There are heretic movements that try to make indentations 4 (or even 2!)
20	characters deep, and that is akin to trying to define the value of PI to
21	be 3.
22	
23	Rationale: The whole idea behind indentation is to clearly define where
24	a block of control starts and ends.  Especially when you've been looking
25	at your screen for 20 straight hours, you'll find it a lot easier to see
26	how the indentation works if you have large indentations.
27	
28	Now, some people will claim that having 8-character indentations makes
29	the code move too far to the right, and makes it hard to read on a
30	80-character terminal screen.  The answer to that is that if you need
31	more than 3 levels of indentation, you're screwed anyway, and should fix
32	your program.
33	
34	In short, 8-char indents make things easier to read, and have the added
35	benefit of warning you when you're nesting your functions too deep.
36	Heed that warning.
37	
38	The preferred way to ease multiple indentation levels in a switch statement is
39	to align the "switch" and its subordinate "case" labels in the same column
40	instead of "double-indenting" the "case" labels.  E.g.:
41	
42		switch (suffix) {
43		case 'G':
44		case 'g':
45			mem <<= 30;
46			break;
47		case 'M':
48		case 'm':
49			mem <<= 20;
50			break;
51		case 'K':
52		case 'k':
53			mem <<= 10;
54			/* fall through */
55		default:
56			break;
57		}
58	
59	
60	Don't put multiple statements on a single line unless you have
61	something to hide:
62	
63		if (condition) do_this;
64		  do_something_everytime;
65	
66	Don't put multiple assignments on a single line either.  Kernel coding style
67	is super simple.  Avoid tricky expressions.
68	
69	Outside of comments, documentation and except in Kconfig, spaces are never
70	used for indentation, and the above example is deliberately broken.
71	
72	Get a decent editor and don't leave whitespace at the end of lines.
73	
74	
75			Chapter 2: Breaking long lines and strings
76	
77	Coding style is all about readability and maintainability using commonly
78	available tools.
79	
80	The limit on the length of lines is 80 columns and this is a strongly
81	preferred limit.
82	
83	Statements longer than 80 columns will be broken into sensible chunks, unless
84	exceeding 80 columns significantly increases readability and does not hide
85	information. Descendants are always substantially shorter than the parent and
86	are placed substantially to the right. The same applies to function headers
87	with a long argument list. However, never break user-visible strings such as
88	printk messages, because that breaks the ability to grep for them.
89	
90	
91			Chapter 3: Placing Braces and Spaces
92	
93	The other issue that always comes up in C styling is the placement of
94	braces.  Unlike the indent size, there are few technical reasons to
95	choose one placement strategy over the other, but the preferred way, as
96	shown to us by the prophets Kernighan and Ritchie, is to put the opening
97	brace last on the line, and put the closing brace first, thusly:
98	
99		if (x is true) {
100			we do y
101		}
102	
103	This applies to all non-function statement blocks (if, switch, for,
104	while, do).  E.g.:
105	
106		switch (action) {
107		case KOBJ_ADD:
108			return "add";
109		case KOBJ_REMOVE:
110			return "remove";
111		case KOBJ_CHANGE:
112			return "change";
113		default:
114			return NULL;
115		}
116	
117	However, there is one special case, namely functions: they have the
118	opening brace at the beginning of the next line, thus:
119	
120		int function(int x)
121		{
122			body of function
123		}
124	
125	Heretic people all over the world have claimed that this inconsistency
126	is ...  well ...  inconsistent, but all right-thinking people know that
127	(a) K&R are _right_ and (b) K&R are right.  Besides, functions are
128	special anyway (you can't nest them in C).
129	
130	Note that the closing brace is empty on a line of its own, _except_ in
131	the cases where it is followed by a continuation of the same statement,
132	ie a "while" in a do-statement or an "else" in an if-statement, like
133	this:
134	
135		do {
136			body of do-loop
137		} while (condition);
138	
139	and
140	
141		if (x == y) {
142			..
143		} else if (x > y) {
144			...
145		} else {
146			....
147		}
148	
149	Rationale: K&R.
150	
151	Also, note that this brace-placement also minimizes the number of empty
152	(or almost empty) lines, without any loss of readability.  Thus, as the
153	supply of new-lines on your screen is not a renewable resource (think
154	25-line terminal screens here), you have more empty lines to put
155	comments on.
156	
157	Do not unnecessarily use braces where a single statement will do.
158	
159	if (condition)
160		action();
161	
162	and
163	
164	if (condition)
165		do_this();
166	else
167		do_that();
168	
169	This does not apply if only one branch of a conditional statement is a single
170	statement; in the latter case use braces in both branches:
171	
172	if (condition) {
173		do_this();
174		do_that();
175	} else {
176		otherwise();
177	}
178	
179			3.1:  Spaces
180	
181	Linux kernel style for use of spaces depends (mostly) on
182	function-versus-keyword usage.  Use a space after (most) keywords.  The
183	notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
184	somewhat like functions (and are usually used with parentheses in Linux,
185	although they are not required in the language, as in: "sizeof info" after
186	"struct fileinfo info;" is declared).
187	
188	So use a space after these keywords:
189		if, switch, case, for, do, while
190	but not with sizeof, typeof, alignof, or __attribute__.  E.g.,
191		s = sizeof(struct file);
192	
193	Do not add spaces around (inside) parenthesized expressions.  This example is
194	*bad*:
195	
196		s = sizeof( struct file );
197	
198	When declaring pointer data or a function that returns a pointer type, the
199	preferred use of '*' is adjacent to the data name or function name and not
200	adjacent to the type name.  Examples:
201	
202		char *linux_banner;
203		unsigned long long memparse(char *ptr, char **retptr);
204		char *match_strdup(substring_t *s);
205	
206	Use one space around (on each side of) most binary and ternary operators,
207	such as any of these:
208	
209		=  +  -  <  >  *  /  %  |  &  ^  <=  >=  ==  !=  ?  :
210	
211	but no space after unary operators:
212		&  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined
213	
214	no space before the postfix increment & decrement unary operators:
215		++  --
216	
217	no space after the prefix increment & decrement unary operators:
218		++  --
219	
220	and no space around the '.' and "->" structure member operators.
221	
222	Do not leave trailing whitespace at the ends of lines.  Some editors with
223	"smart" indentation will insert whitespace at the beginning of new lines as
224	appropriate, so you can start typing the next line of code right away.
225	However, some such editors do not remove the whitespace if you end up not
226	putting a line of code there, such as if you leave a blank line.  As a result,
227	you end up with lines containing trailing whitespace.
228	
229	Git will warn you about patches that introduce trailing whitespace, and can
230	optionally strip the trailing whitespace for you; however, if applying a series
231	of patches, this may make later patches in the series fail by changing their
232	context lines.
233	
234	
235			Chapter 4: Naming
236	
237	C is a Spartan language, and so should your naming be.  Unlike Modula-2
238	and Pascal programmers, C programmers do not use cute names like
239	ThisVariableIsATemporaryCounter.  A C programmer would call that
240	variable "tmp", which is much easier to write, and not the least more
241	difficult to understand.
242	
243	HOWEVER, while mixed-case names are frowned upon, descriptive names for
244	global variables are a must.  To call a global function "foo" is a
245	shooting offense.
246	
247	GLOBAL variables (to be used only if you _really_ need them) need to
248	have descriptive names, as do global functions.  If you have a function
249	that counts the number of active users, you should call that
250	"count_active_users()" or similar, you should _not_ call it "cntusr()".
251	
252	Encoding the type of a function into the name (so-called Hungarian
253	notation) is brain damaged - the compiler knows the types anyway and can
254	check those, and it only confuses the programmer.  No wonder MicroSoft
255	makes buggy programs.
256	
257	LOCAL variable names should be short, and to the point.  If you have
258	some random integer loop counter, it should probably be called "i".
259	Calling it "loop_counter" is non-productive, if there is no chance of it
260	being mis-understood.  Similarly, "tmp" can be just about any type of
261	variable that is used to hold a temporary value.
262	
263	If you are afraid to mix up your local variable names, you have another
264	problem, which is called the function-growth-hormone-imbalance syndrome.
265	See chapter 6 (Functions).
266	
267	
268			Chapter 5: Typedefs
269	
270	Please don't use things like "vps_t".
271	
272	It's a _mistake_ to use typedef for structures and pointers. When you see a
273	
274		vps_t a;
275	
276	in the source, what does it mean?
277	
278	In contrast, if it says
279	
280		struct virtual_container *a;
281	
282	you can actually tell what "a" is.
283	
284	Lots of people think that typedefs "help readability". Not so. They are
285	useful only for:
286	
287	 (a) totally opaque objects (where the typedef is actively used to _hide_
288	     what the object is).
289	
290	     Example: "pte_t" etc. opaque objects that you can only access using
291	     the proper accessor functions.
292	
293	     NOTE! Opaqueness and "accessor functions" are not good in themselves.
294	     The reason we have them for things like pte_t etc. is that there
295	     really is absolutely _zero_ portably accessible information there.
296	
297	 (b) Clear integer types, where the abstraction _helps_ avoid confusion
298	     whether it is "int" or "long".
299	
300	     u8/u16/u32 are perfectly fine typedefs, although they fit into
301	     category (d) better than here.
302	
303	     NOTE! Again - there needs to be a _reason_ for this. If something is
304	     "unsigned long", then there's no reason to do
305	
306		typedef unsigned long myflags_t;
307	
308	     but if there is a clear reason for why it under certain circumstances
309	     might be an "unsigned int" and under other configurations might be
310	     "unsigned long", then by all means go ahead and use a typedef.
311	
312	 (c) when you use sparse to literally create a _new_ type for
313	     type-checking.
314	
315	 (d) New types which are identical to standard C99 types, in certain
316	     exceptional circumstances.
317	
318	     Although it would only take a short amount of time for the eyes and
319	     brain to become accustomed to the standard types like 'uint32_t',
320	     some people object to their use anyway.
321	
322	     Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
323	     signed equivalents which are identical to standard types are
324	     permitted -- although they are not mandatory in new code of your
325	     own.
326	
327	     When editing existing code which already uses one or the other set
328	     of types, you should conform to the existing choices in that code.
329	
330	 (e) Types safe for use in userspace.
331	
332	     In certain structures which are visible to userspace, we cannot
333	     require C99 types and cannot use the 'u32' form above. Thus, we
334	     use __u32 and similar types in all structures which are shared
335	     with userspace.
336	
337	Maybe there are other cases too, but the rule should basically be to NEVER
338	EVER use a typedef unless you can clearly match one of those rules.
339	
340	In general, a pointer, or a struct that has elements that can reasonably
341	be directly accessed should _never_ be a typedef.
342	
343	
344			Chapter 6: Functions
345	
346	Functions should be short and sweet, and do just one thing.  They should
347	fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
348	as we all know), and do one thing and do that well.
349	
350	The maximum length of a function is inversely proportional to the
351	complexity and indentation level of that function.  So, if you have a
352	conceptually simple function that is just one long (but simple)
353	case-statement, where you have to do lots of small things for a lot of
354	different cases, it's OK to have a longer function.
355	
356	However, if you have a complex function, and you suspect that a
357	less-than-gifted first-year high-school student might not even
358	understand what the function is all about, you should adhere to the
359	maximum limits all the more closely.  Use helper functions with
360	descriptive names (you can ask the compiler to in-line them if you think
361	it's performance-critical, and it will probably do a better job of it
362	than you would have done).
363	
364	Another measure of the function is the number of local variables.  They
365	shouldn't exceed 5-10, or you're doing something wrong.  Re-think the
366	function, and split it into smaller pieces.  A human brain can
367	generally easily keep track of about 7 different things, anything more
368	and it gets confused.  You know you're brilliant, but maybe you'd like
369	to understand what you did 2 weeks from now.
370	
371	In source files, separate functions with one blank line.  If the function is
372	exported, the EXPORT* macro for it should follow immediately after the closing
373	function brace line.  E.g.:
374	
375	int system_is_up(void)
376	{
377		return system_state == SYSTEM_RUNNING;
378	}
379	EXPORT_SYMBOL(system_is_up);
380	
381	In function prototypes, include parameter names with their data types.
382	Although this is not required by the C language, it is preferred in Linux
383	because it is a simple way to add valuable information for the reader.
384	
385	
386			Chapter 7: Centralized exiting of functions
387	
388	Albeit deprecated by some people, the equivalent of the goto statement is
389	used frequently by compilers in form of the unconditional jump instruction.
390	
391	The goto statement comes in handy when a function exits from multiple
392	locations and some common work such as cleanup has to be done.  If there is no
393	cleanup needed then just return directly.
394	
395	Choose label names which say what the goto does or why the goto exists.  An
396	example of a good name could be "out_buffer:" if the goto frees "buffer".  Avoid
397	using GW-BASIC names like "err1:" and "err2:".  Also don't name them after the
398	goto location like "err_kmalloc_failed:"
399	
400	The rationale for using gotos is:
401	
402	- unconditional statements are easier to understand and follow
403	- nesting is reduced
404	- errors by not updating individual exit points when making
405	    modifications are prevented
406	- saves the compiler work to optimize redundant code away ;)
407	
408	int fun(int a)
409	{
410		int result = 0;
411		char *buffer;
412	
413		buffer = kmalloc(SIZE, GFP_KERNEL);
414		if (!buffer)
415			return -ENOMEM;
416	
417		if (condition1) {
418			while (loop1) {
419				...
420			}
421			result = 1;
422			goto out_buffer;
423		}
424		...
425	out_buffer:
426		kfree(buffer);
427		return result;
428	}
429	
430	A common type of bug to be aware of it "one err bugs" which look like this:
431	
432	err:
433		kfree(foo->bar);
434		kfree(foo);
435		return ret;
436	
437	The bug in this code is that on some exit paths "foo" is NULL.  Normally the
438	fix for this is to split it up into two error labels "err_bar:" and "err_foo:".
439	
440	
441			Chapter 8: Commenting
442	
443	Comments are good, but there is also a danger of over-commenting.  NEVER
444	try to explain HOW your code works in a comment: it's much better to
445	write the code so that the _working_ is obvious, and it's a waste of
446	time to explain badly written code.
447	
448	Generally, you want your comments to tell WHAT your code does, not HOW.
449	Also, try to avoid putting comments inside a function body: if the
450	function is so complex that you need to separately comment parts of it,
451	you should probably go back to chapter 6 for a while.  You can make
452	small comments to note or warn about something particularly clever (or
453	ugly), but try to avoid excess.  Instead, put the comments at the head
454	of the function, telling people what it does, and possibly WHY it does
455	it.
456	
457	When commenting the kernel API functions, please use the kernel-doc format.
458	See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
459	for details.
460	
461	Linux style for comments is the C89 "/* ... */" style.
462	Don't use C99-style "// ..." comments.
463	
464	The preferred style for long (multi-line) comments is:
465	
466		/*
467		 * This is the preferred style for multi-line
468		 * comments in the Linux kernel source code.
469		 * Please use it consistently.
470		 *
471		 * Description:  A column of asterisks on the left side,
472		 * with beginning and ending almost-blank lines.
473		 */
474	
475	For files in net/ and drivers/net/ the preferred style for long (multi-line)
476	comments is a little different.
477	
478		/* The preferred comment style for files in net/ and drivers/net
479		 * looks like this.
480		 *
481		 * It is nearly the same as the generally preferred comment style,
482		 * but there is no initial almost-blank line.
483		 */
484	
485	It's also important to comment data, whether they are basic types or derived
486	types.  To this end, use just one data declaration per line (no commas for
487	multiple data declarations).  This leaves you room for a small comment on each
488	item, explaining its use.
489	
490	
491			Chapter 9: You've made a mess of it
492	
493	That's OK, we all do.  You've probably been told by your long-time Unix
494	user helper that "GNU emacs" automatically formats the C sources for
495	you, and you've noticed that yes, it does do that, but the defaults it
496	uses are less than desirable (in fact, they are worse than random
497	typing - an infinite number of monkeys typing into GNU emacs would never
498	make a good program).
499	
500	So, you can either get rid of GNU emacs, or change it to use saner
501	values.  To do the latter, you can stick the following in your .emacs file:
502	
503	(defun c-lineup-arglist-tabs-only (ignored)
504	  "Line up argument lists by tabs, not spaces"
505	  (let* ((anchor (c-langelem-pos c-syntactic-element))
506		 (column (c-langelem-2nd-pos c-syntactic-element))
507		 (offset (- (1+ column) anchor))
508		 (steps (floor offset c-basic-offset)))
509	    (* (max steps 1)
510	       c-basic-offset)))
511	
512	(add-hook 'c-mode-common-hook
513	          (lambda ()
514	            ;; Add kernel style
515	            (c-add-style
516	             "linux-tabs-only"
517	             '("linux" (c-offsets-alist
518	                        (arglist-cont-nonempty
519	                         c-lineup-gcc-asm-reg
520	                         c-lineup-arglist-tabs-only))))))
521	
522	(add-hook 'c-mode-hook
523	          (lambda ()
524	            (let ((filename (buffer-file-name)))
525	              ;; Enable kernel mode for the appropriate files
526	              (when (and filename
527	                         (string-match (expand-file-name "~/src/linux-trees")
528	                                       filename))
529	                (setq indent-tabs-mode t)
530	                (c-set-style "linux-tabs-only")))))
531	
532	This will make emacs go better with the kernel coding style for C
533	files below ~/src/linux-trees.
534	
535	But even if you fail in getting emacs to do sane formatting, not
536	everything is lost: use "indent".
537	
538	Now, again, GNU indent has the same brain-dead settings that GNU emacs
539	has, which is why you need to give it a few command line options.
540	However, that's not too bad, because even the makers of GNU indent
541	recognize the authority of K&R (the GNU people aren't evil, they are
542	just severely misguided in this matter), so you just give indent the
543	options "-kr -i8" (stands for "K&R, 8 character indents"), or use
544	"scripts/Lindent", which indents in the latest style.
545	
546	"indent" has a lot of options, and especially when it comes to comment
547	re-formatting you may want to take a look at the man page.  But
548	remember: "indent" is not a fix for bad programming.
549	
550	
551			Chapter 10: Kconfig configuration files
552	
553	For all of the Kconfig* configuration files throughout the source tree,
554	the indentation is somewhat different.  Lines under a "config" definition
555	are indented with one tab, while help text is indented an additional two
556	spaces.  Example:
557	
558	config AUDIT
559		bool "Auditing support"
560		depends on NET
561		help
562		  Enable auditing infrastructure that can be used with another
563		  kernel subsystem, such as SELinux (which requires this for
564		  logging of avc messages output).  Does not do system-call
565		  auditing without CONFIG_AUDITSYSCALL.
566	
567	Seriously dangerous features (such as write support for certain
568	filesystems) should advertise this prominently in their prompt string:
569	
570	config ADFS_FS_RW
571		bool "ADFS write support (DANGEROUS)"
572		depends on ADFS_FS
573		...
574	
575	For full documentation on the configuration files, see the file
576	Documentation/kbuild/kconfig-language.txt.
577	
578	
579			Chapter 11: Data structures
580	
581	Data structures that have visibility outside the single-threaded
582	environment they are created and destroyed in should always have
583	reference counts.  In the kernel, garbage collection doesn't exist (and
584	outside the kernel garbage collection is slow and inefficient), which
585	means that you absolutely _have_ to reference count all your uses.
586	
587	Reference counting means that you can avoid locking, and allows multiple
588	users to have access to the data structure in parallel - and not having
589	to worry about the structure suddenly going away from under them just
590	because they slept or did something else for a while.
591	
592	Note that locking is _not_ a replacement for reference counting.
593	Locking is used to keep data structures coherent, while reference
594	counting is a memory management technique.  Usually both are needed, and
595	they are not to be confused with each other.
596	
597	Many data structures can indeed have two levels of reference counting,
598	when there are users of different "classes".  The subclass count counts
599	the number of subclass users, and decrements the global count just once
600	when the subclass count goes to zero.
601	
602	Examples of this kind of "multi-level-reference-counting" can be found in
603	memory management ("struct mm_struct": mm_users and mm_count), and in
604	filesystem code ("struct super_block": s_count and s_active).
605	
606	Remember: if another thread can find your data structure, and you don't
607	have a reference count on it, you almost certainly have a bug.
608	
609	
610			Chapter 12: Macros, Enums and RTL
611	
612	Names of macros defining constants and labels in enums are capitalized.
613	
614	#define CONSTANT 0x12345
615	
616	Enums are preferred when defining several related constants.
617	
618	CAPITALIZED macro names are appreciated but macros resembling functions
619	may be named in lower case.
620	
621	Generally, inline functions are preferable to macros resembling functions.
622	
623	Macros with multiple statements should be enclosed in a do - while block:
624	
625	#define macrofun(a, b, c) 			\
626		do {					\
627			if (a == 5)			\
628				do_this(b, c);		\
629		} while (0)
630	
631	Things to avoid when using macros:
632	
633	1) macros that affect control flow:
634	
635	#define FOO(x)					\
636		do {					\
637			if (blah(x) < 0)		\
638				return -EBUGGERED;	\
639		} while(0)
640	
641	is a _very_ bad idea.  It looks like a function call but exits the "calling"
642	function; don't break the internal parsers of those who will read the code.
643	
644	2) macros that depend on having a local variable with a magic name:
645	
646	#define FOO(val) bar(index, val)
647	
648	might look like a good thing, but it's confusing as hell when one reads the
649	code and it's prone to breakage from seemingly innocent changes.
650	
651	3) macros with arguments that are used as l-values: FOO(x) = y; will
652	bite you if somebody e.g. turns FOO into an inline function.
653	
654	4) forgetting about precedence: macros defining constants using expressions
655	must enclose the expression in parentheses. Beware of similar issues with
656	macros using parameters.
657	
658	#define CONSTANT 0x4000
659	#define CONSTEXP (CONSTANT | 3)
660	
661	The cpp manual deals with macros exhaustively. The gcc internals manual also
662	covers RTL which is used frequently with assembly language in the kernel.
663	
664	
665			Chapter 13: Printing kernel messages
666	
667	Kernel developers like to be seen as literate. Do mind the spelling
668	of kernel messages to make a good impression. Do not use crippled
669	words like "dont"; use "do not" or "don't" instead.  Make the messages
670	concise, clear, and unambiguous.
671	
672	Kernel messages do not have to be terminated with a period.
673	
674	Printing numbers in parentheses (%d) adds no value and should be avoided.
675	
676	There are a number of driver model diagnostic macros in <linux/device.h>
677	which you should use to make sure messages are matched to the right device
678	and driver, and are tagged with the right level:  dev_err(), dev_warn(),
679	dev_info(), and so forth.  For messages that aren't associated with a
680	particular device, <linux/printk.h> defines pr_notice(), pr_info(),
681	pr_warn(), pr_err(), etc.
682	
683	Coming up with good debugging messages can be quite a challenge; and once
684	you have them, they can be a huge help for remote troubleshooting.  However
685	debug message printing is handled differently than printing other non-debug
686	messages.  While the other pr_XXX() functions print unconditionally,
687	pr_debug() does not; it is compiled out by default, unless either DEBUG is
688	defined or CONFIG_DYNAMIC_DEBUG is set.  That is true for dev_dbg() also,
689	and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
690	the ones already enabled by DEBUG.
691	
692	Many subsystems have Kconfig debug options to turn on -DDEBUG in the
693	corresponding Makefile; in other cases specific files #define DEBUG.  And
694	when a debug message should be unconditionally printed, such as if it is
695	already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
696	used.
697	
698	
699			Chapter 14: Allocating memory
700	
701	The kernel provides the following general purpose memory allocators:
702	kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
703	vzalloc().  Please refer to the API documentation for further information
704	about them.
705	
706	The preferred form for passing a size of a struct is the following:
707	
708		p = kmalloc(sizeof(*p), ...);
709	
710	The alternative form where struct name is spelled out hurts readability and
711	introduces an opportunity for a bug when the pointer variable type is changed
712	but the corresponding sizeof that is passed to a memory allocator is not.
713	
714	Casting the return value which is a void pointer is redundant. The conversion
715	from void pointer to any other pointer type is guaranteed by the C programming
716	language.
717	
718	The preferred form for allocating an array is the following:
719	
720		p = kmalloc_array(n, sizeof(...), ...);
721	
722	The preferred form for allocating a zeroed array is the following:
723	
724		p = kcalloc(n, sizeof(...), ...);
725	
726	Both forms check for overflow on the allocation size n * sizeof(...),
727	and return NULL if that occurred.
728	
729	
730			Chapter 15: The inline disease
731	
732	There appears to be a common misperception that gcc has a magic "make me
733	faster" speedup option called "inline". While the use of inlines can be
734	appropriate (for example as a means of replacing macros, see Chapter 12), it
735	very often is not. Abundant use of the inline keyword leads to a much bigger
736	kernel, which in turn slows the system as a whole down, due to a bigger
737	icache footprint for the CPU and simply because there is less memory
738	available for the pagecache. Just think about it; a pagecache miss causes a
739	disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
740	that can go into these 5 milliseconds.
741	
742	A reasonable rule of thumb is to not put inline at functions that have more
743	than 3 lines of code in them. An exception to this rule are the cases where
744	a parameter is known to be a compiletime constant, and as a result of this
745	constantness you *know* the compiler will be able to optimize most of your
746	function away at compile time. For a good example of this later case, see
747	the kmalloc() inline function.
748	
749	Often people argue that adding inline to functions that are static and used
750	only once is always a win since there is no space tradeoff. While this is
751	technically correct, gcc is capable of inlining these automatically without
752	help, and the maintenance issue of removing the inline when a second user
753	appears outweighs the potential value of the hint that tells gcc to do
754	something it would have done anyway.
755	
756	
757			Chapter 16: Function return values and names
758	
759	Functions can return values of many different kinds, and one of the
760	most common is a value indicating whether the function succeeded or
761	failed.  Such a value can be represented as an error-code integer
762	(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
763	non-zero = success).
764	
765	Mixing up these two sorts of representations is a fertile source of
766	difficult-to-find bugs.  If the C language included a strong distinction
767	between integers and booleans then the compiler would find these mistakes
768	for us... but it doesn't.  To help prevent such bugs, always follow this
769	convention:
770	
771		If the name of a function is an action or an imperative command,
772		the function should return an error-code integer.  If the name
773		is a predicate, the function should return a "succeeded" boolean.
774	
775	For example, "add work" is a command, and the add_work() function returns 0
776	for success or -EBUSY for failure.  In the same way, "PCI device present" is
777	a predicate, and the pci_dev_present() function returns 1 if it succeeds in
778	finding a matching device or 0 if it doesn't.
779	
780	All EXPORTed functions must respect this convention, and so should all
781	public functions.  Private (static) functions need not, but it is
782	recommended that they do.
783	
784	Functions whose return value is the actual result of a computation, rather
785	than an indication of whether the computation succeeded, are not subject to
786	this rule.  Generally they indicate failure by returning some out-of-range
787	result.  Typical examples would be functions that return pointers; they use
788	NULL or the ERR_PTR mechanism to report failure.
789	
790	
791			Chapter 17:  Don't re-invent the kernel macros
792	
793	The header file include/linux/kernel.h contains a number of macros that
794	you should use, rather than explicitly coding some variant of them yourself.
795	For example, if you need to calculate the length of an array, take advantage
796	of the macro
797	
798	  #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
799	
800	Similarly, if you need to calculate the size of some structure member, use
801	
802	  #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
803	
804	There are also min() and max() macros that do strict type checking if you
805	need them.  Feel free to peruse that header file to see what else is already
806	defined that you shouldn't reproduce in your code.
807	
808	
809			Chapter 18:  Editor modelines and other cruft
810	
811	Some editors can interpret configuration information embedded in source files,
812	indicated with special markers.  For example, emacs interprets lines marked
813	like this:
814	
815	-*- mode: c -*-
816	
817	Or like this:
818	
819	/*
820	Local Variables:
821	compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
822	End:
823	*/
824	
825	Vim interprets markers that look like this:
826	
827	/* vim:set sw=8 noet */
828	
829	Do not include any of these in source files.  People have their own personal
830	editor configurations, and your source files should not override them.  This
831	includes markers for indentation and mode configuration.  People may use their
832	own custom mode, or may have some other magic method for making indentation
833	work correctly.
834	
835	
836			Chapter 19:  Inline assembly
837	
838	In architecture-specific code, you may need to use inline assembly to interface
839	with CPU or platform functionality.  Don't hesitate to do so when necessary.
840	However, don't use inline assembly gratuitously when C can do the job.  You can
841	and should poke hardware from C when possible.
842	
843	Consider writing simple helper functions that wrap common bits of inline
844	assembly, rather than repeatedly writing them with slight variations.  Remember
845	that inline assembly can use C parameters.
846	
847	Large, non-trivial assembly functions should go in .S files, with corresponding
848	C prototypes defined in C header files.  The C prototypes for assembly
849	functions should use "asmlinkage".
850	
851	You may need to mark your asm statement as volatile, to prevent GCC from
852	removing it if GCC doesn't notice any side effects.  You don't always need to
853	do so, though, and doing so unnecessarily can limit optimization.
854	
855	When writing a single inline assembly statement containing multiple
856	instructions, put each instruction on a separate line in a separate quoted
857	string, and end each string except the last with \n\t to properly indent the
858	next instruction in the assembly output:
859	
860		asm ("magic %reg1, #42\n\t"
861		     "more_magic %reg2, %reg3"
862		     : /* outputs */ : /* inputs */ : /* clobbers */);
863	
864	
865			Chapter 20: Conditional Compilation
866	
867	Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
868	files; doing so makes code harder to read and logic harder to follow.  Instead,
869	use such conditionals in a header file defining functions for use in those .c
870	files, providing no-op stub versions in the #else case, and then call those
871	functions unconditionally from .c files.  The compiler will avoid generating
872	any code for the stub calls, producing identical results, but the logic will
873	remain easy to follow.
874	
875	Prefer to compile out entire functions, rather than portions of functions or
876	portions of expressions.  Rather than putting an ifdef in an expression, factor
877	out part or all of the expression into a separate helper function and apply the
878	conditional to that function.
879	
880	If you have a function or variable which may potentially go unused in a
881	particular configuration, and the compiler would warn about its definition
882	going unused, mark the definition as __maybe_unused rather than wrapping it in
883	a preprocessor conditional.  (However, if a function or variable *always* goes
884	unused, delete it.)
885	
886	Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
887	symbol into a C boolean expression, and use it in a normal C conditional:
888	
889		if (IS_ENABLED(CONFIG_SOMETHING)) {
890			...
891		}
892	
893	The compiler will constant-fold the conditional away, and include or exclude
894	the block of code just as with an #ifdef, so this will not add any runtime
895	overhead.  However, this approach still allows the C compiler to see the code
896	inside the block, and check it for correctness (syntax, types, symbol
897	references, etc).  Thus, you still have to use an #ifdef if the code inside the
898	block references symbols that will not exist if the condition is not met.
899	
900	At the end of any non-trivial #if or #ifdef block (more than a few lines),
901	place a comment after the #endif on the same line, noting the conditional
902	expression used.  For instance:
903	
904	#ifdef CONFIG_SOMETHING
905	...
906	#endif /* CONFIG_SOMETHING */
907	
908	
909			Appendix I: References
910	
911	The C Programming Language, Second Edition
912	by Brian W. Kernighan and Dennis M. Ritchie.
913	Prentice Hall, Inc., 1988.
914	ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
915	URL: http://cm.bell-labs.com/cm/cs/cbook/
916	
917	The Practice of Programming
918	by Brian W. Kernighan and Rob Pike.
919	Addison-Wesley, Inc., 1999.
920	ISBN 0-201-61586-X.
921	URL: http://cm.bell-labs.com/cm/cs/tpop/
922	
923	GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
924	gcc internals and indent, all available from http://www.gnu.org/manual/
925	
926	WG14 is the international standardization working group for the programming
927	language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
928	
929	Kernel CodingStyle, by greg@kroah.com at OLS 2002:
930	http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
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