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Based on kernel version 3.9. Page generated on 2013-05-02 22:55 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.
393	
394	The rationale is:
395	
396	- unconditional statements are easier to understand and follow
397	- nesting is reduced
398	- errors by not updating individual exit points when making
399	    modifications are prevented
400	- saves the compiler work to optimize redundant code away ;)
401	
402	int fun(int a)
403	{
404		int result = 0;
405		char *buffer = kmalloc(SIZE);
406	
407		if (buffer == NULL)
408			return -ENOMEM;
409	
410		if (condition1) {
411			while (loop1) {
412				...
413			}
414			result = 1;
415			goto out;
416		}
417		...
418	out:
419		kfree(buffer);
420		return result;
421	}
422	
423			Chapter 8: Commenting
424	
425	Comments are good, but there is also a danger of over-commenting.  NEVER
426	try to explain HOW your code works in a comment: it's much better to
427	write the code so that the _working_ is obvious, and it's a waste of
428	time to explain badly written code.
429	
430	Generally, you want your comments to tell WHAT your code does, not HOW.
431	Also, try to avoid putting comments inside a function body: if the
432	function is so complex that you need to separately comment parts of it,
433	you should probably go back to chapter 6 for a while.  You can make
434	small comments to note or warn about something particularly clever (or
435	ugly), but try to avoid excess.  Instead, put the comments at the head
436	of the function, telling people what it does, and possibly WHY it does
437	it.
438	
439	When commenting the kernel API functions, please use the kernel-doc format.
440	See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
441	for details.
442	
443	Linux style for comments is the C89 "/* ... */" style.
444	Don't use C99-style "// ..." comments.
445	
446	The preferred style for long (multi-line) comments is:
447	
448		/*
449		 * This is the preferred style for multi-line
450		 * comments in the Linux kernel source code.
451		 * Please use it consistently.
452		 *
453		 * Description:  A column of asterisks on the left side,
454		 * with beginning and ending almost-blank lines.
455		 */
456	
457	For files in net/ and drivers/net/ the preferred style for long (multi-line)
458	comments is a little different.
459	
460		/* The preferred comment style for files in net/ and drivers/net
461		 * looks like this.
462		 *
463		 * It is nearly the same as the generally preferred comment style,
464		 * but there is no initial almost-blank line.
465		 */
466	
467	It's also important to comment data, whether they are basic types or derived
468	types.  To this end, use just one data declaration per line (no commas for
469	multiple data declarations).  This leaves you room for a small comment on each
470	item, explaining its use.
471	
472	
473			Chapter 9: You've made a mess of it
474	
475	That's OK, we all do.  You've probably been told by your long-time Unix
476	user helper that "GNU emacs" automatically formats the C sources for
477	you, and you've noticed that yes, it does do that, but the defaults it
478	uses are less than desirable (in fact, they are worse than random
479	typing - an infinite number of monkeys typing into GNU emacs would never
480	make a good program).
481	
482	So, you can either get rid of GNU emacs, or change it to use saner
483	values.  To do the latter, you can stick the following in your .emacs file:
484	
485	(defun c-lineup-arglist-tabs-only (ignored)
486	  "Line up argument lists by tabs, not spaces"
487	  (let* ((anchor (c-langelem-pos c-syntactic-element))
488		 (column (c-langelem-2nd-pos c-syntactic-element))
489		 (offset (- (1+ column) anchor))
490		 (steps (floor offset c-basic-offset)))
491	    (* (max steps 1)
492	       c-basic-offset)))
493	
494	(add-hook 'c-mode-common-hook
495	          (lambda ()
496	            ;; Add kernel style
497	            (c-add-style
498	             "linux-tabs-only"
499	             '("linux" (c-offsets-alist
500	                        (arglist-cont-nonempty
501	                         c-lineup-gcc-asm-reg
502	                         c-lineup-arglist-tabs-only))))))
503	
504	(add-hook 'c-mode-hook
505	          (lambda ()
506	            (let ((filename (buffer-file-name)))
507	              ;; Enable kernel mode for the appropriate files
508	              (when (and filename
509	                         (string-match (expand-file-name "~/src/linux-trees")
510	                                       filename))
511	                (setq indent-tabs-mode t)
512	                (c-set-style "linux-tabs-only")))))
513	
514	This will make emacs go better with the kernel coding style for C
515	files below ~/src/linux-trees.
516	
517	But even if you fail in getting emacs to do sane formatting, not
518	everything is lost: use "indent".
519	
520	Now, again, GNU indent has the same brain-dead settings that GNU emacs
521	has, which is why you need to give it a few command line options.
522	However, that's not too bad, because even the makers of GNU indent
523	recognize the authority of K&R (the GNU people aren't evil, they are
524	just severely misguided in this matter), so you just give indent the
525	options "-kr -i8" (stands for "K&R, 8 character indents"), or use
526	"scripts/Lindent", which indents in the latest style.
527	
528	"indent" has a lot of options, and especially when it comes to comment
529	re-formatting you may want to take a look at the man page.  But
530	remember: "indent" is not a fix for bad programming.
531	
532	
533			Chapter 10: Kconfig configuration files
534	
535	For all of the Kconfig* configuration files throughout the source tree,
536	the indentation is somewhat different.  Lines under a "config" definition
537	are indented with one tab, while help text is indented an additional two
538	spaces.  Example:
539	
540	config AUDIT
541		bool "Auditing support"
542		depends on NET
543		help
544		  Enable auditing infrastructure that can be used with another
545		  kernel subsystem, such as SELinux (which requires this for
546		  logging of avc messages output).  Does not do system-call
547		  auditing without CONFIG_AUDITSYSCALL.
548	
549	Seriously dangerous features (such as write support for certain
550	filesystems) should advertise this prominently in their prompt string:
551	
552	config ADFS_FS_RW
553		bool "ADFS write support (DANGEROUS)"
554		depends on ADFS_FS
555		...
556	
557	For full documentation on the configuration files, see the file
558	Documentation/kbuild/kconfig-language.txt.
559	
560	
561			Chapter 11: Data structures
562	
563	Data structures that have visibility outside the single-threaded
564	environment they are created and destroyed in should always have
565	reference counts.  In the kernel, garbage collection doesn't exist (and
566	outside the kernel garbage collection is slow and inefficient), which
567	means that you absolutely _have_ to reference count all your uses.
568	
569	Reference counting means that you can avoid locking, and allows multiple
570	users to have access to the data structure in parallel - and not having
571	to worry about the structure suddenly going away from under them just
572	because they slept or did something else for a while.
573	
574	Note that locking is _not_ a replacement for reference counting.
575	Locking is used to keep data structures coherent, while reference
576	counting is a memory management technique.  Usually both are needed, and
577	they are not to be confused with each other.
578	
579	Many data structures can indeed have two levels of reference counting,
580	when there are users of different "classes".  The subclass count counts
581	the number of subclass users, and decrements the global count just once
582	when the subclass count goes to zero.
583	
584	Examples of this kind of "multi-level-reference-counting" can be found in
585	memory management ("struct mm_struct": mm_users and mm_count), and in
586	filesystem code ("struct super_block": s_count and s_active).
587	
588	Remember: if another thread can find your data structure, and you don't
589	have a reference count on it, you almost certainly have a bug.
590	
591	
592			Chapter 12: Macros, Enums and RTL
593	
594	Names of macros defining constants and labels in enums are capitalized.
595	
596	#define CONSTANT 0x12345
597	
598	Enums are preferred when defining several related constants.
599	
600	CAPITALIZED macro names are appreciated but macros resembling functions
601	may be named in lower case.
602	
603	Generally, inline functions are preferable to macros resembling functions.
604	
605	Macros with multiple statements should be enclosed in a do - while block:
606	
607	#define macrofun(a, b, c) 			\
608		do {					\
609			if (a == 5)			\
610				do_this(b, c);		\
611		} while (0)
612	
613	Things to avoid when using macros:
614	
615	1) macros that affect control flow:
616	
617	#define FOO(x)					\
618		do {					\
619			if (blah(x) < 0)		\
620				return -EBUGGERED;	\
621		} while(0)
622	
623	is a _very_ bad idea.  It looks like a function call but exits the "calling"
624	function; don't break the internal parsers of those who will read the code.
625	
626	2) macros that depend on having a local variable with a magic name:
627	
628	#define FOO(val) bar(index, val)
629	
630	might look like a good thing, but it's confusing as hell when one reads the
631	code and it's prone to breakage from seemingly innocent changes.
632	
633	3) macros with arguments that are used as l-values: FOO(x) = y; will
634	bite you if somebody e.g. turns FOO into an inline function.
635	
636	4) forgetting about precedence: macros defining constants using expressions
637	must enclose the expression in parentheses. Beware of similar issues with
638	macros using parameters.
639	
640	#define CONSTANT 0x4000
641	#define CONSTEXP (CONSTANT | 3)
642	
643	The cpp manual deals with macros exhaustively. The gcc internals manual also
644	covers RTL which is used frequently with assembly language in the kernel.
645	
646	
647			Chapter 13: Printing kernel messages
648	
649	Kernel developers like to be seen as literate. Do mind the spelling
650	of kernel messages to make a good impression. Do not use crippled
651	words like "dont"; use "do not" or "don't" instead.  Make the messages
652	concise, clear, and unambiguous.
653	
654	Kernel messages do not have to be terminated with a period.
655	
656	Printing numbers in parentheses (%d) adds no value and should be avoided.
657	
658	There are a number of driver model diagnostic macros in <linux/device.h>
659	which you should use to make sure messages are matched to the right device
660	and driver, and are tagged with the right level:  dev_err(), dev_warn(),
661	dev_info(), and so forth.  For messages that aren't associated with a
662	particular device, <linux/printk.h> defines pr_debug() and pr_info().
663	
664	Coming up with good debugging messages can be quite a challenge; and once
665	you have them, they can be a huge help for remote troubleshooting.  Such
666	messages should be compiled out when the DEBUG symbol is not defined (that
667	is, by default they are not included).  When you use dev_dbg() or pr_debug(),
668	that's automatic.  Many subsystems have Kconfig options to turn on -DDEBUG.
669	A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the
670	ones already enabled by DEBUG.
671	
672	
673			Chapter 14: Allocating memory
674	
675	The kernel provides the following general purpose memory allocators:
676	kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
677	vzalloc().  Please refer to the API documentation for further information
678	about them.
679	
680	The preferred form for passing a size of a struct is the following:
681	
682		p = kmalloc(sizeof(*p), ...);
683	
684	The alternative form where struct name is spelled out hurts readability and
685	introduces an opportunity for a bug when the pointer variable type is changed
686	but the corresponding sizeof that is passed to a memory allocator is not.
687	
688	Casting the return value which is a void pointer is redundant. The conversion
689	from void pointer to any other pointer type is guaranteed by the C programming
690	language.
691	
692	The preferred form for allocating an array is the following:
693	
694		p = kmalloc_array(n, sizeof(...), ...);
695	
696	The preferred form for allocating a zeroed array is the following:
697	
698		p = kcalloc(n, sizeof(...), ...);
699	
700	Both forms check for overflow on the allocation size n * sizeof(...),
701	and return NULL if that occurred.
702	
703	
704			Chapter 15: The inline disease
705	
706	There appears to be a common misperception that gcc has a magic "make me
707	faster" speedup option called "inline". While the use of inlines can be
708	appropriate (for example as a means of replacing macros, see Chapter 12), it
709	very often is not. Abundant use of the inline keyword leads to a much bigger
710	kernel, which in turn slows the system as a whole down, due to a bigger
711	icache footprint for the CPU and simply because there is less memory
712	available for the pagecache. Just think about it; a pagecache miss causes a
713	disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
714	that can go into these 5 milliseconds.
715	
716	A reasonable rule of thumb is to not put inline at functions that have more
717	than 3 lines of code in them. An exception to this rule are the cases where
718	a parameter is known to be a compiletime constant, and as a result of this
719	constantness you *know* the compiler will be able to optimize most of your
720	function away at compile time. For a good example of this later case, see
721	the kmalloc() inline function.
722	
723	Often people argue that adding inline to functions that are static and used
724	only once is always a win since there is no space tradeoff. While this is
725	technically correct, gcc is capable of inlining these automatically without
726	help, and the maintenance issue of removing the inline when a second user
727	appears outweighs the potential value of the hint that tells gcc to do
728	something it would have done anyway.
729	
730	
731			Chapter 16: Function return values and names
732	
733	Functions can return values of many different kinds, and one of the
734	most common is a value indicating whether the function succeeded or
735	failed.  Such a value can be represented as an error-code integer
736	(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
737	non-zero = success).
738	
739	Mixing up these two sorts of representations is a fertile source of
740	difficult-to-find bugs.  If the C language included a strong distinction
741	between integers and booleans then the compiler would find these mistakes
742	for us... but it doesn't.  To help prevent such bugs, always follow this
743	convention:
744	
745		If the name of a function is an action or an imperative command,
746		the function should return an error-code integer.  If the name
747		is a predicate, the function should return a "succeeded" boolean.
748	
749	For example, "add work" is a command, and the add_work() function returns 0
750	for success or -EBUSY for failure.  In the same way, "PCI device present" is
751	a predicate, and the pci_dev_present() function returns 1 if it succeeds in
752	finding a matching device or 0 if it doesn't.
753	
754	All EXPORTed functions must respect this convention, and so should all
755	public functions.  Private (static) functions need not, but it is
756	recommended that they do.
757	
758	Functions whose return value is the actual result of a computation, rather
759	than an indication of whether the computation succeeded, are not subject to
760	this rule.  Generally they indicate failure by returning some out-of-range
761	result.  Typical examples would be functions that return pointers; they use
762	NULL or the ERR_PTR mechanism to report failure.
763	
764	
765			Chapter 17:  Don't re-invent the kernel macros
766	
767	The header file include/linux/kernel.h contains a number of macros that
768	you should use, rather than explicitly coding some variant of them yourself.
769	For example, if you need to calculate the length of an array, take advantage
770	of the macro
771	
772	  #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
773	
774	Similarly, if you need to calculate the size of some structure member, use
775	
776	  #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
777	
778	There are also min() and max() macros that do strict type checking if you
779	need them.  Feel free to peruse that header file to see what else is already
780	defined that you shouldn't reproduce in your code.
781	
782	
783			Chapter 18:  Editor modelines and other cruft
784	
785	Some editors can interpret configuration information embedded in source files,
786	indicated with special markers.  For example, emacs interprets lines marked
787	like this:
788	
789	-*- mode: c -*-
790	
791	Or like this:
792	
793	/*
794	Local Variables:
795	compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
796	End:
797	*/
798	
799	Vim interprets markers that look like this:
800	
801	/* vim:set sw=8 noet */
802	
803	Do not include any of these in source files.  People have their own personal
804	editor configurations, and your source files should not override them.  This
805	includes markers for indentation and mode configuration.  People may use their
806	own custom mode, or may have some other magic method for making indentation
807	work correctly.
808	
809	
810			Chapter 19:  Inline assembly
811	
812	In architecture-specific code, you may need to use inline assembly to interface
813	with CPU or platform functionality.  Don't hesitate to do so when necessary.
814	However, don't use inline assembly gratuitously when C can do the job.  You can
815	and should poke hardware from C when possible.
816	
817	Consider writing simple helper functions that wrap common bits of inline
818	assembly, rather than repeatedly writing them with slight variations.  Remember
819	that inline assembly can use C parameters.
820	
821	Large, non-trivial assembly functions should go in .S files, with corresponding
822	C prototypes defined in C header files.  The C prototypes for assembly
823	functions should use "asmlinkage".
824	
825	You may need to mark your asm statement as volatile, to prevent GCC from
826	removing it if GCC doesn't notice any side effects.  You don't always need to
827	do so, though, and doing so unnecessarily can limit optimization.
828	
829	When writing a single inline assembly statement containing multiple
830	instructions, put each instruction on a separate line in a separate quoted
831	string, and end each string except the last with \n\t to properly indent the
832	next instruction in the assembly output:
833	
834		asm ("magic %reg1, #42\n\t"
835		     "more_magic %reg2, %reg3"
836		     : /* outputs */ : /* inputs */ : /* clobbers */);
837	
838	
839	
840			Appendix I: References
841	
842	The C Programming Language, Second Edition
843	by Brian W. Kernighan and Dennis M. Ritchie.
844	Prentice Hall, Inc., 1988.
845	ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
846	URL: http://cm.bell-labs.com/cm/cs/cbook/
847	
848	The Practice of Programming
849	by Brian W. Kernighan and Rob Pike.
850	Addison-Wesley, Inc., 1999.
851	ISBN 0-201-61586-X.
852	URL: http://cm.bell-labs.com/cm/cs/tpop/
853	
854	GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
855	gcc internals and indent, all available from http://www.gnu.org/manual/
856	
857	WG14 is the international standardization working group for the programming
858	language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
859	
860	Kernel CodingStyle, by greg@kroah.com at OLS 2002:
861	http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
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