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Based on kernel version 4.1. Page generated on 2015-06-28 12:08 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	Don't put multiple statements on a single line unless you have
60	something to hide:
61	
62		if (condition) do_this;
63		  do_something_everytime;
64	
65	Don't put multiple assignments on a single line either.  Kernel coding style
66	is super simple.  Avoid tricky expressions.
67	
68	Outside of comments, documentation and except in Kconfig, spaces are never
69	used for indentation, and the above example is deliberately broken.
70	
71	Get a decent editor and don't leave whitespace at the end of lines.
72	
73	
74			Chapter 2: Breaking long lines and strings
75	
76	Coding style is all about readability and maintainability using commonly
77	available tools.
78	
79	The limit on the length of lines is 80 columns and this is a strongly
80	preferred limit.
81	
82	Statements longer than 80 columns will be broken into sensible chunks, unless
83	exceeding 80 columns significantly increases readability and does not hide
84	information. Descendants are always substantially shorter than the parent and
85	are placed substantially to the right. The same applies to function headers
86	with a long argument list. However, never break user-visible strings such as
87	printk messages, because that breaks the ability to grep for them.
88	
89	
90			Chapter 3: Placing Braces and Spaces
91	
92	The other issue that always comes up in C styling is the placement of
93	braces.  Unlike the indent size, there are few technical reasons to
94	choose one placement strategy over the other, but the preferred way, as
95	shown to us by the prophets Kernighan and Ritchie, is to put the opening
96	brace last on the line, and put the closing brace first, thusly:
97	
98		if (x is true) {
99			we do y
100		}
101	
102	This applies to all non-function statement blocks (if, switch, for,
103	while, do).  E.g.:
104	
105		switch (action) {
106		case KOBJ_ADD:
107			return "add";
108		case KOBJ_REMOVE:
109			return "remove";
110		case KOBJ_CHANGE:
111			return "change";
112		default:
113			return NULL;
114		}
115	
116	However, there is one special case, namely functions: they have the
117	opening brace at the beginning of the next line, thus:
118	
119		int function(int x)
120		{
121			body of function
122		}
123	
124	Heretic people all over the world have claimed that this inconsistency
125	is ...  well ...  inconsistent, but all right-thinking people know that
126	(a) K&R are _right_ and (b) K&R are right.  Besides, functions are
127	special anyway (you can't nest them in C).
128	
129	Note that the closing brace is empty on a line of its own, _except_ in
130	the cases where it is followed by a continuation of the same statement,
131	ie a "while" in a do-statement or an "else" in an if-statement, like
132	this:
133	
134		do {
135			body of do-loop
136		} while (condition);
137	
138	and
139	
140		if (x == y) {
141			..
142		} else if (x > y) {
143			...
144		} else {
145			....
146		}
147	
148	Rationale: K&R.
149	
150	Also, note that this brace-placement also minimizes the number of empty
151	(or almost empty) lines, without any loss of readability.  Thus, as the
152	supply of new-lines on your screen is not a renewable resource (think
153	25-line terminal screens here), you have more empty lines to put
154	comments on.
155	
156	Do not unnecessarily use braces where a single statement will do.
157	
158		if (condition)
159			action();
160	
161	and
162	
163		if (condition)
164			do_this();
165		else
166			do_that();
167	
168	This does not apply if only one branch of a conditional statement is a single
169	statement; in the latter case use braces in both branches:
170	
171		if (condition) {
172			do_this();
173			do_that();
174		} else {
175			otherwise();
176		}
177	
178			3.1:  Spaces
179	
180	Linux kernel style for use of spaces depends (mostly) on
181	function-versus-keyword usage.  Use a space after (most) keywords.  The
182	notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
183	somewhat like functions (and are usually used with parentheses in Linux,
184	although they are not required in the language, as in: "sizeof info" after
185	"struct fileinfo info;" is declared).
186	
187	So use a space after these keywords:
188	
189		if, switch, case, for, do, while
190	
191	but not with sizeof, typeof, alignof, or __attribute__.  E.g.,
192	
193		s = sizeof(struct file);
194	
195	Do not add spaces around (inside) parenthesized expressions.  This example is
196	*bad*:
197	
198		s = sizeof( struct file );
199	
200	When declaring pointer data or a function that returns a pointer type, the
201	preferred use of '*' is adjacent to the data name or function name and not
202	adjacent to the type name.  Examples:
203	
204		char *linux_banner;
205		unsigned long long memparse(char *ptr, char **retptr);
206		char *match_strdup(substring_t *s);
207	
208	Use one space around (on each side of) most binary and ternary operators,
209	such as any of these:
210	
211		=  +  -  <  >  *  /  %  |  &  ^  <=  >=  ==  !=  ?  :
212	
213	but no space after unary operators:
214	
215		&  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined
216	
217	no space before the postfix increment & decrement unary operators:
218	
219		++  --
220	
221	no space after the prefix increment & decrement unary operators:
222	
223		++  --
224	
225	and no space around the '.' and "->" structure member operators.
226	
227	Do not leave trailing whitespace at the ends of lines.  Some editors with
228	"smart" indentation will insert whitespace at the beginning of new lines as
229	appropriate, so you can start typing the next line of code right away.
230	However, some such editors do not remove the whitespace if you end up not
231	putting a line of code there, such as if you leave a blank line.  As a result,
232	you end up with lines containing trailing whitespace.
233	
234	Git will warn you about patches that introduce trailing whitespace, and can
235	optionally strip the trailing whitespace for you; however, if applying a series
236	of patches, this may make later patches in the series fail by changing their
237	context lines.
238	
239	
240			Chapter 4: Naming
241	
242	C is a Spartan language, and so should your naming be.  Unlike Modula-2
243	and Pascal programmers, C programmers do not use cute names like
244	ThisVariableIsATemporaryCounter.  A C programmer would call that
245	variable "tmp", which is much easier to write, and not the least more
246	difficult to understand.
247	
248	HOWEVER, while mixed-case names are frowned upon, descriptive names for
249	global variables are a must.  To call a global function "foo" is a
250	shooting offense.
251	
252	GLOBAL variables (to be used only if you _really_ need them) need to
253	have descriptive names, as do global functions.  If you have a function
254	that counts the number of active users, you should call that
255	"count_active_users()" or similar, you should _not_ call it "cntusr()".
256	
257	Encoding the type of a function into the name (so-called Hungarian
258	notation) is brain damaged - the compiler knows the types anyway and can
259	check those, and it only confuses the programmer.  No wonder MicroSoft
260	makes buggy programs.
261	
262	LOCAL variable names should be short, and to the point.  If you have
263	some random integer loop counter, it should probably be called "i".
264	Calling it "loop_counter" is non-productive, if there is no chance of it
265	being mis-understood.  Similarly, "tmp" can be just about any type of
266	variable that is used to hold a temporary value.
267	
268	If you are afraid to mix up your local variable names, you have another
269	problem, which is called the function-growth-hormone-imbalance syndrome.
270	See chapter 6 (Functions).
271	
272	
273			Chapter 5: Typedefs
274	
275	Please don't use things like "vps_t".
276	It's a _mistake_ to use typedef for structures and pointers. When you see a
277	
278		vps_t a;
279	
280	in the source, what does it mean?
281	In contrast, if it says
282	
283		struct virtual_container *a;
284	
285	you can actually tell what "a" is.
286	
287	Lots of people think that typedefs "help readability". Not so. They are
288	useful only for:
289	
290	 (a) totally opaque objects (where the typedef is actively used to _hide_
291	     what the object is).
292	
293	     Example: "pte_t" etc. opaque objects that you can only access using
294	     the proper accessor functions.
295	
296	     NOTE! Opaqueness and "accessor functions" are not good in themselves.
297	     The reason we have them for things like pte_t etc. is that there
298	     really is absolutely _zero_ portably accessible information there.
299	
300	 (b) Clear integer types, where the abstraction _helps_ avoid confusion
301	     whether it is "int" or "long".
302	
303	     u8/u16/u32 are perfectly fine typedefs, although they fit into
304	     category (d) better than here.
305	
306	     NOTE! Again - there needs to be a _reason_ for this. If something is
307	     "unsigned long", then there's no reason to do
308	
309		typedef unsigned long myflags_t;
310	
311	     but if there is a clear reason for why it under certain circumstances
312	     might be an "unsigned int" and under other configurations might be
313	     "unsigned long", then by all means go ahead and use a typedef.
314	
315	 (c) when you use sparse to literally create a _new_ type for
316	     type-checking.
317	
318	 (d) New types which are identical to standard C99 types, in certain
319	     exceptional circumstances.
320	
321	     Although it would only take a short amount of time for the eyes and
322	     brain to become accustomed to the standard types like 'uint32_t',
323	     some people object to their use anyway.
324	
325	     Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
326	     signed equivalents which are identical to standard types are
327	     permitted -- although they are not mandatory in new code of your
328	     own.
329	
330	     When editing existing code which already uses one or the other set
331	     of types, you should conform to the existing choices in that code.
332	
333	 (e) Types safe for use in userspace.
334	
335	     In certain structures which are visible to userspace, we cannot
336	     require C99 types and cannot use the 'u32' form above. Thus, we
337	     use __u32 and similar types in all structures which are shared
338	     with userspace.
339	
340	Maybe there are other cases too, but the rule should basically be to NEVER
341	EVER use a typedef unless you can clearly match one of those rules.
342	
343	In general, a pointer, or a struct that has elements that can reasonably
344	be directly accessed should _never_ be a typedef.
345	
346	
347			Chapter 6: Functions
348	
349	Functions should be short and sweet, and do just one thing.  They should
350	fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
351	as we all know), and do one thing and do that well.
352	
353	The maximum length of a function is inversely proportional to the
354	complexity and indentation level of that function.  So, if you have a
355	conceptually simple function that is just one long (but simple)
356	case-statement, where you have to do lots of small things for a lot of
357	different cases, it's OK to have a longer function.
358	
359	However, if you have a complex function, and you suspect that a
360	less-than-gifted first-year high-school student might not even
361	understand what the function is all about, you should adhere to the
362	maximum limits all the more closely.  Use helper functions with
363	descriptive names (you can ask the compiler to in-line them if you think
364	it's performance-critical, and it will probably do a better job of it
365	than you would have done).
366	
367	Another measure of the function is the number of local variables.  They
368	shouldn't exceed 5-10, or you're doing something wrong.  Re-think the
369	function, and split it into smaller pieces.  A human brain can
370	generally easily keep track of about 7 different things, anything more
371	and it gets confused.  You know you're brilliant, but maybe you'd like
372	to understand what you did 2 weeks from now.
373	
374	In source files, separate functions with one blank line.  If the function is
375	exported, the EXPORT* macro for it should follow immediately after the closing
376	function brace line.  E.g.:
377	
378		int system_is_up(void)
379		{
380			return system_state == SYSTEM_RUNNING;
381		}
382		EXPORT_SYMBOL(system_is_up);
383	
384	In function prototypes, include parameter names with their data types.
385	Although this is not required by the C language, it is preferred in Linux
386	because it is a simple way to add valuable information for the reader.
387	
388	
389			Chapter 7: Centralized exiting of functions
390	
391	Albeit deprecated by some people, the equivalent of the goto statement is
392	used frequently by compilers in form of the unconditional jump instruction.
393	
394	The goto statement comes in handy when a function exits from multiple
395	locations and some common work such as cleanup has to be done.  If there is no
396	cleanup needed then just return directly.
397	
398	Choose label names which say what the goto does or why the goto exists.  An
399	example of a good name could be "out_buffer:" if the goto frees "buffer".  Avoid
400	using GW-BASIC names like "err1:" and "err2:".  Also don't name them after the
401	goto location like "err_kmalloc_failed:"
402	
403	The rationale for using gotos is:
404	
405	- unconditional statements are easier to understand and follow
406	- nesting is reduced
407	- errors by not updating individual exit points when making
408	    modifications are prevented
409	- saves the compiler work to optimize redundant code away ;)
410	
411		int fun(int a)
412		{
413			int result = 0;
414			char *buffer;
415	
416			buffer = kmalloc(SIZE, GFP_KERNEL);
417			if (!buffer)
418				return -ENOMEM;
419	
420			if (condition1) {
421				while (loop1) {
422					...
423				}
424				result = 1;
425				goto out_buffer;
426			}
427			...
428		out_buffer:
429			kfree(buffer);
430			return result;
431		}
432	
433	A common type of bug to be aware of it "one err bugs" which look like this:
434	
435		err:
436			kfree(foo->bar);
437			kfree(foo);
438			return ret;
439	
440	The bug in this code is that on some exit paths "foo" is NULL.  Normally the
441	fix for this is to split it up into two error labels "err_bar:" and "err_foo:".
442	
443	
444			Chapter 8: Commenting
445	
446	Comments are good, but there is also a danger of over-commenting.  NEVER
447	try to explain HOW your code works in a comment: it's much better to
448	write the code so that the _working_ is obvious, and it's a waste of
449	time to explain badly written code.
450	
451	Generally, you want your comments to tell WHAT your code does, not HOW.
452	Also, try to avoid putting comments inside a function body: if the
453	function is so complex that you need to separately comment parts of it,
454	you should probably go back to chapter 6 for a while.  You can make
455	small comments to note or warn about something particularly clever (or
456	ugly), but try to avoid excess.  Instead, put the comments at the head
457	of the function, telling people what it does, and possibly WHY it does
458	it.
459	
460	When commenting the kernel API functions, please use the kernel-doc format.
461	See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
462	for details.
463	
464	Linux style for comments is the C89 "/* ... */" style.
465	Don't use C99-style "// ..." comments.
466	
467	The preferred style for long (multi-line) comments is:
468	
469		/*
470		 * This is the preferred style for multi-line
471		 * comments in the Linux kernel source code.
472		 * Please use it consistently.
473		 *
474		 * Description:  A column of asterisks on the left side,
475		 * with beginning and ending almost-blank lines.
476		 */
477	
478	For files in net/ and drivers/net/ the preferred style for long (multi-line)
479	comments is a little different.
480	
481		/* The preferred comment style for files in net/ and drivers/net
482		 * looks like this.
483		 *
484		 * It is nearly the same as the generally preferred comment style,
485		 * but there is no initial almost-blank line.
486		 */
487	
488	It's also important to comment data, whether they are basic types or derived
489	types.  To this end, use just one data declaration per line (no commas for
490	multiple data declarations).  This leaves you room for a small comment on each
491	item, explaining its use.
492	
493	
494			Chapter 9: You've made a mess of it
495	
496	That's OK, we all do.  You've probably been told by your long-time Unix
497	user helper that "GNU emacs" automatically formats the C sources for
498	you, and you've noticed that yes, it does do that, but the defaults it
499	uses are less than desirable (in fact, they are worse than random
500	typing - an infinite number of monkeys typing into GNU emacs would never
501	make a good program).
502	
503	So, you can either get rid of GNU emacs, or change it to use saner
504	values.  To do the latter, you can stick the following in your .emacs file:
505	
506	(defun c-lineup-arglist-tabs-only (ignored)
507	  "Line up argument lists by tabs, not spaces"
508	  (let* ((anchor (c-langelem-pos c-syntactic-element))
509	         (column (c-langelem-2nd-pos c-syntactic-element))
510	         (offset (- (1+ column) anchor))
511	         (steps (floor offset c-basic-offset)))
512	    (* (max steps 1)
513	       c-basic-offset)))
514	
515	(add-hook 'c-mode-common-hook
516	          (lambda ()
517	            ;; Add kernel style
518	            (c-add-style
519	             "linux-tabs-only"
520	             '("linux" (c-offsets-alist
521	                        (arglist-cont-nonempty
522	                         c-lineup-gcc-asm-reg
523	                         c-lineup-arglist-tabs-only))))))
524	
525	(add-hook 'c-mode-hook
526	          (lambda ()
527	            (let ((filename (buffer-file-name)))
528	              ;; Enable kernel mode for the appropriate files
529	              (when (and filename
530	                         (string-match (expand-file-name "~/src/linux-trees")
531	                                       filename))
532	                (setq indent-tabs-mode t)
533	                (setq show-trailing-whitespace t)
534	                (c-set-style "linux-tabs-only")))))
535	
536	This will make emacs go better with the kernel coding style for C
537	files below ~/src/linux-trees.
538	
539	But even if you fail in getting emacs to do sane formatting, not
540	everything is lost: use "indent".
541	
542	Now, again, GNU indent has the same brain-dead settings that GNU emacs
543	has, which is why you need to give it a few command line options.
544	However, that's not too bad, because even the makers of GNU indent
545	recognize the authority of K&R (the GNU people aren't evil, they are
546	just severely misguided in this matter), so you just give indent the
547	options "-kr -i8" (stands for "K&R, 8 character indents"), or use
548	"scripts/Lindent", which indents in the latest style.
549	
550	"indent" has a lot of options, and especially when it comes to comment
551	re-formatting you may want to take a look at the man page.  But
552	remember: "indent" is not a fix for bad programming.
553	
554	
555			Chapter 10: Kconfig configuration files
556	
557	For all of the Kconfig* configuration files throughout the source tree,
558	the indentation is somewhat different.  Lines under a "config" definition
559	are indented with one tab, while help text is indented an additional two
560	spaces.  Example:
561	
562	config AUDIT
563		bool "Auditing support"
564		depends on NET
565		help
566		  Enable auditing infrastructure that can be used with another
567		  kernel subsystem, such as SELinux (which requires this for
568		  logging of avc messages output).  Does not do system-call
569		  auditing without CONFIG_AUDITSYSCALL.
570	
571	Seriously dangerous features (such as write support for certain
572	filesystems) should advertise this prominently in their prompt string:
573	
574	config ADFS_FS_RW
575		bool "ADFS write support (DANGEROUS)"
576		depends on ADFS_FS
577		...
578	
579	For full documentation on the configuration files, see the file
580	Documentation/kbuild/kconfig-language.txt.
581	
582	
583			Chapter 11: Data structures
584	
585	Data structures that have visibility outside the single-threaded
586	environment they are created and destroyed in should always have
587	reference counts.  In the kernel, garbage collection doesn't exist (and
588	outside the kernel garbage collection is slow and inefficient), which
589	means that you absolutely _have_ to reference count all your uses.
590	
591	Reference counting means that you can avoid locking, and allows multiple
592	users to have access to the data structure in parallel - and not having
593	to worry about the structure suddenly going away from under them just
594	because they slept or did something else for a while.
595	
596	Note that locking is _not_ a replacement for reference counting.
597	Locking is used to keep data structures coherent, while reference
598	counting is a memory management technique.  Usually both are needed, and
599	they are not to be confused with each other.
600	
601	Many data structures can indeed have two levels of reference counting,
602	when there are users of different "classes".  The subclass count counts
603	the number of subclass users, and decrements the global count just once
604	when the subclass count goes to zero.
605	
606	Examples of this kind of "multi-level-reference-counting" can be found in
607	memory management ("struct mm_struct": mm_users and mm_count), and in
608	filesystem code ("struct super_block": s_count and s_active).
609	
610	Remember: if another thread can find your data structure, and you don't
611	have a reference count on it, you almost certainly have a bug.
612	
613	
614			Chapter 12: Macros, Enums and RTL
615	
616	Names of macros defining constants and labels in enums are capitalized.
617	
618		#define CONSTANT 0x12345
619	
620	Enums are preferred when defining several related constants.
621	
622	CAPITALIZED macro names are appreciated but macros resembling functions
623	may be named in lower case.
624	
625	Generally, inline functions are preferable to macros resembling functions.
626	
627	Macros with multiple statements should be enclosed in a do - while block:
628	
629		#define macrofun(a, b, c) 			\
630			do {					\
631				if (a == 5)			\
632					do_this(b, c);		\
633			} while (0)
634	
635	Things to avoid when using macros:
636	
637	1) macros that affect control flow:
638	
639		#define FOO(x)					\
640			do {					\
641				if (blah(x) < 0)		\
642					return -EBUGGERED;	\
643			} while(0)
644	
645	is a _very_ bad idea.  It looks like a function call but exits the "calling"
646	function; don't break the internal parsers of those who will read the code.
647	
648	2) macros that depend on having a local variable with a magic name:
649	
650		#define FOO(val) bar(index, val)
651	
652	might look like a good thing, but it's confusing as hell when one reads the
653	code and it's prone to breakage from seemingly innocent changes.
654	
655	3) macros with arguments that are used as l-values: FOO(x) = y; will
656	bite you if somebody e.g. turns FOO into an inline function.
657	
658	4) forgetting about precedence: macros defining constants using expressions
659	must enclose the expression in parentheses. Beware of similar issues with
660	macros using parameters.
661	
662		#define CONSTANT 0x4000
663		#define CONSTEXP (CONSTANT | 3)
664	
665	5) namespace collisions when defining local variables in macros resembling
666	functions:
667	
668	#define FOO(x)				\
669	({					\
670		typeof(x) ret;			\
671		ret = calc_ret(x);		\
672		(ret);				\
673	)}
674	
675	ret is a common name for a local variable - __foo_ret is less likely
676	to collide with an existing variable.
677	
678	The cpp manual deals with macros exhaustively. The gcc internals manual also
679	covers RTL which is used frequently with assembly language in the kernel.
680	
681	
682			Chapter 13: Printing kernel messages
683	
684	Kernel developers like to be seen as literate. Do mind the spelling
685	of kernel messages to make a good impression. Do not use crippled
686	words like "dont"; use "do not" or "don't" instead.  Make the messages
687	concise, clear, and unambiguous.
688	
689	Kernel messages do not have to be terminated with a period.
690	
691	Printing numbers in parentheses (%d) adds no value and should be avoided.
692	
693	There are a number of driver model diagnostic macros in <linux/device.h>
694	which you should use to make sure messages are matched to the right device
695	and driver, and are tagged with the right level:  dev_err(), dev_warn(),
696	dev_info(), and so forth.  For messages that aren't associated with a
697	particular device, <linux/printk.h> defines pr_notice(), pr_info(),
698	pr_warn(), pr_err(), etc.
699	
700	Coming up with good debugging messages can be quite a challenge; and once
701	you have them, they can be a huge help for remote troubleshooting.  However
702	debug message printing is handled differently than printing other non-debug
703	messages.  While the other pr_XXX() functions print unconditionally,
704	pr_debug() does not; it is compiled out by default, unless either DEBUG is
705	defined or CONFIG_DYNAMIC_DEBUG is set.  That is true for dev_dbg() also,
706	and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
707	the ones already enabled by DEBUG.
708	
709	Many subsystems have Kconfig debug options to turn on -DDEBUG in the
710	corresponding Makefile; in other cases specific files #define DEBUG.  And
711	when a debug message should be unconditionally printed, such as if it is
712	already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
713	used.
714	
715	
716			Chapter 14: Allocating memory
717	
718	The kernel provides the following general purpose memory allocators:
719	kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
720	vzalloc().  Please refer to the API documentation for further information
721	about them.
722	
723	The preferred form for passing a size of a struct is the following:
724	
725		p = kmalloc(sizeof(*p), ...);
726	
727	The alternative form where struct name is spelled out hurts readability and
728	introduces an opportunity for a bug when the pointer variable type is changed
729	but the corresponding sizeof that is passed to a memory allocator is not.
730	
731	Casting the return value which is a void pointer is redundant. The conversion
732	from void pointer to any other pointer type is guaranteed by the C programming
733	language.
734	
735	The preferred form for allocating an array is the following:
736	
737		p = kmalloc_array(n, sizeof(...), ...);
738	
739	The preferred form for allocating a zeroed array is the following:
740	
741		p = kcalloc(n, sizeof(...), ...);
742	
743	Both forms check for overflow on the allocation size n * sizeof(...),
744	and return NULL if that occurred.
745	
746	
747			Chapter 15: The inline disease
748	
749	There appears to be a common misperception that gcc has a magic "make me
750	faster" speedup option called "inline". While the use of inlines can be
751	appropriate (for example as a means of replacing macros, see Chapter 12), it
752	very often is not. Abundant use of the inline keyword leads to a much bigger
753	kernel, which in turn slows the system as a whole down, due to a bigger
754	icache footprint for the CPU and simply because there is less memory
755	available for the pagecache. Just think about it; a pagecache miss causes a
756	disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
757	that can go into these 5 milliseconds.
758	
759	A reasonable rule of thumb is to not put inline at functions that have more
760	than 3 lines of code in them. An exception to this rule are the cases where
761	a parameter is known to be a compiletime constant, and as a result of this
762	constantness you *know* the compiler will be able to optimize most of your
763	function away at compile time. For a good example of this later case, see
764	the kmalloc() inline function.
765	
766	Often people argue that adding inline to functions that are static and used
767	only once is always a win since there is no space tradeoff. While this is
768	technically correct, gcc is capable of inlining these automatically without
769	help, and the maintenance issue of removing the inline when a second user
770	appears outweighs the potential value of the hint that tells gcc to do
771	something it would have done anyway.
772	
773	
774			Chapter 16: Function return values and names
775	
776	Functions can return values of many different kinds, and one of the
777	most common is a value indicating whether the function succeeded or
778	failed.  Such a value can be represented as an error-code integer
779	(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
780	non-zero = success).
781	
782	Mixing up these two sorts of representations is a fertile source of
783	difficult-to-find bugs.  If the C language included a strong distinction
784	between integers and booleans then the compiler would find these mistakes
785	for us... but it doesn't.  To help prevent such bugs, always follow this
786	convention:
787	
788		If the name of a function is an action or an imperative command,
789		the function should return an error-code integer.  If the name
790		is a predicate, the function should return a "succeeded" boolean.
791	
792	For example, "add work" is a command, and the add_work() function returns 0
793	for success or -EBUSY for failure.  In the same way, "PCI device present" is
794	a predicate, and the pci_dev_present() function returns 1 if it succeeds in
795	finding a matching device or 0 if it doesn't.
796	
797	All EXPORTed functions must respect this convention, and so should all
798	public functions.  Private (static) functions need not, but it is
799	recommended that they do.
800	
801	Functions whose return value is the actual result of a computation, rather
802	than an indication of whether the computation succeeded, are not subject to
803	this rule.  Generally they indicate failure by returning some out-of-range
804	result.  Typical examples would be functions that return pointers; they use
805	NULL or the ERR_PTR mechanism to report failure.
806	
807	
808			Chapter 17:  Don't re-invent the kernel macros
809	
810	The header file include/linux/kernel.h contains a number of macros that
811	you should use, rather than explicitly coding some variant of them yourself.
812	For example, if you need to calculate the length of an array, take advantage
813	of the macro
814	
815		#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
816	
817	Similarly, if you need to calculate the size of some structure member, use
818	
819		#define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
820	
821	There are also min() and max() macros that do strict type checking if you
822	need them.  Feel free to peruse that header file to see what else is already
823	defined that you shouldn't reproduce in your code.
824	
825	
826			Chapter 18:  Editor modelines and other cruft
827	
828	Some editors can interpret configuration information embedded in source files,
829	indicated with special markers.  For example, emacs interprets lines marked
830	like this:
831	
832		-*- mode: c -*-
833	
834	Or like this:
835	
836		/*
837		Local Variables:
838		compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
839		End:
840		*/
841	
842	Vim interprets markers that look like this:
843	
844		/* vim:set sw=8 noet */
845	
846	Do not include any of these in source files.  People have their own personal
847	editor configurations, and your source files should not override them.  This
848	includes markers for indentation and mode configuration.  People may use their
849	own custom mode, or may have some other magic method for making indentation
850	work correctly.
851	
852	
853			Chapter 19:  Inline assembly
854	
855	In architecture-specific code, you may need to use inline assembly to interface
856	with CPU or platform functionality.  Don't hesitate to do so when necessary.
857	However, don't use inline assembly gratuitously when C can do the job.  You can
858	and should poke hardware from C when possible.
859	
860	Consider writing simple helper functions that wrap common bits of inline
861	assembly, rather than repeatedly writing them with slight variations.  Remember
862	that inline assembly can use C parameters.
863	
864	Large, non-trivial assembly functions should go in .S files, with corresponding
865	C prototypes defined in C header files.  The C prototypes for assembly
866	functions should use "asmlinkage".
867	
868	You may need to mark your asm statement as volatile, to prevent GCC from
869	removing it if GCC doesn't notice any side effects.  You don't always need to
870	do so, though, and doing so unnecessarily can limit optimization.
871	
872	When writing a single inline assembly statement containing multiple
873	instructions, put each instruction on a separate line in a separate quoted
874	string, and end each string except the last with \n\t to properly indent the
875	next instruction in the assembly output:
876	
877		asm ("magic %reg1, #42\n\t"
878		     "more_magic %reg2, %reg3"
879		     : /* outputs */ : /* inputs */ : /* clobbers */);
880	
881	
882			Chapter 20: Conditional Compilation
883	
884	Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
885	files; doing so makes code harder to read and logic harder to follow.  Instead,
886	use such conditionals in a header file defining functions for use in those .c
887	files, providing no-op stub versions in the #else case, and then call those
888	functions unconditionally from .c files.  The compiler will avoid generating
889	any code for the stub calls, producing identical results, but the logic will
890	remain easy to follow.
891	
892	Prefer to compile out entire functions, rather than portions of functions or
893	portions of expressions.  Rather than putting an ifdef in an expression, factor
894	out part or all of the expression into a separate helper function and apply the
895	conditional to that function.
896	
897	If you have a function or variable which may potentially go unused in a
898	particular configuration, and the compiler would warn about its definition
899	going unused, mark the definition as __maybe_unused rather than wrapping it in
900	a preprocessor conditional.  (However, if a function or variable *always* goes
901	unused, delete it.)
902	
903	Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
904	symbol into a C boolean expression, and use it in a normal C conditional:
905	
906		if (IS_ENABLED(CONFIG_SOMETHING)) {
907			...
908		}
909	
910	The compiler will constant-fold the conditional away, and include or exclude
911	the block of code just as with an #ifdef, so this will not add any runtime
912	overhead.  However, this approach still allows the C compiler to see the code
913	inside the block, and check it for correctness (syntax, types, symbol
914	references, etc).  Thus, you still have to use an #ifdef if the code inside the
915	block references symbols that will not exist if the condition is not met.
916	
917	At the end of any non-trivial #if or #ifdef block (more than a few lines),
918	place a comment after the #endif on the same line, noting the conditional
919	expression used.  For instance:
920	
921		#ifdef CONFIG_SOMETHING
922		...
923		#endif /* CONFIG_SOMETHING */
924	
925	
926			Appendix I: References
927	
928	The C Programming Language, Second Edition
929	by Brian W. Kernighan and Dennis M. Ritchie.
930	Prentice Hall, Inc., 1988.
931	ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
932	URL: http://cm.bell-labs.com/cm/cs/cbook/
933	
934	The Practice of Programming
935	by Brian W. Kernighan and Rob Pike.
936	Addison-Wesley, Inc., 1999.
937	ISBN 0-201-61586-X.
938	URL: http://cm.bell-labs.com/cm/cs/tpop/
939	
940	GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
941	gcc internals and indent, all available from http://www.gnu.org/manual/
942	
943	WG14 is the international standardization working group for the programming
944	language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
945	
946	Kernel CodingStyle, by greg@kroah.com at OLS 2002:
947	http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
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