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Based on kernel version 3.13. Page generated on 2014-01-20 22:00 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	The rationale is:
396	
397	- unconditional statements are easier to understand and follow
398	- nesting is reduced
399	- errors by not updating individual exit points when making
400	    modifications are prevented
401	- saves the compiler work to optimize redundant code away ;)
402	
403	int fun(int a)
404	{
405		int result = 0;
406		char *buffer = kmalloc(SIZE);
407	
408		if (buffer == NULL)
409			return -ENOMEM;
410	
411		if (condition1) {
412			while (loop1) {
413				...
414			}
415			result = 1;
416			goto out;
417		}
418		...
419	out:
420		kfree(buffer);
421		return result;
422	}
423	
424			Chapter 8: Commenting
425	
426	Comments are good, but there is also a danger of over-commenting.  NEVER
427	try to explain HOW your code works in a comment: it's much better to
428	write the code so that the _working_ is obvious, and it's a waste of
429	time to explain badly written code.
430	
431	Generally, you want your comments to tell WHAT your code does, not HOW.
432	Also, try to avoid putting comments inside a function body: if the
433	function is so complex that you need to separately comment parts of it,
434	you should probably go back to chapter 6 for a while.  You can make
435	small comments to note or warn about something particularly clever (or
436	ugly), but try to avoid excess.  Instead, put the comments at the head
437	of the function, telling people what it does, and possibly WHY it does
438	it.
439	
440	When commenting the kernel API functions, please use the kernel-doc format.
441	See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
442	for details.
443	
444	Linux style for comments is the C89 "/* ... */" style.
445	Don't use C99-style "// ..." comments.
446	
447	The preferred style for long (multi-line) comments is:
448	
449		/*
450		 * This is the preferred style for multi-line
451		 * comments in the Linux kernel source code.
452		 * Please use it consistently.
453		 *
454		 * Description:  A column of asterisks on the left side,
455		 * with beginning and ending almost-blank lines.
456		 */
457	
458	For files in net/ and drivers/net/ the preferred style for long (multi-line)
459	comments is a little different.
460	
461		/* The preferred comment style for files in net/ and drivers/net
462		 * looks like this.
463		 *
464		 * It is nearly the same as the generally preferred comment style,
465		 * but there is no initial almost-blank line.
466		 */
467	
468	It's also important to comment data, whether they are basic types or derived
469	types.  To this end, use just one data declaration per line (no commas for
470	multiple data declarations).  This leaves you room for a small comment on each
471	item, explaining its use.
472	
473	
474			Chapter 9: You've made a mess of it
475	
476	That's OK, we all do.  You've probably been told by your long-time Unix
477	user helper that "GNU emacs" automatically formats the C sources for
478	you, and you've noticed that yes, it does do that, but the defaults it
479	uses are less than desirable (in fact, they are worse than random
480	typing - an infinite number of monkeys typing into GNU emacs would never
481	make a good program).
482	
483	So, you can either get rid of GNU emacs, or change it to use saner
484	values.  To do the latter, you can stick the following in your .emacs file:
485	
486	(defun c-lineup-arglist-tabs-only (ignored)
487	  "Line up argument lists by tabs, not spaces"
488	  (let* ((anchor (c-langelem-pos c-syntactic-element))
489		 (column (c-langelem-2nd-pos c-syntactic-element))
490		 (offset (- (1+ column) anchor))
491		 (steps (floor offset c-basic-offset)))
492	    (* (max steps 1)
493	       c-basic-offset)))
494	
495	(add-hook 'c-mode-common-hook
496	          (lambda ()
497	            ;; Add kernel style
498	            (c-add-style
499	             "linux-tabs-only"
500	             '("linux" (c-offsets-alist
501	                        (arglist-cont-nonempty
502	                         c-lineup-gcc-asm-reg
503	                         c-lineup-arglist-tabs-only))))))
504	
505	(add-hook 'c-mode-hook
506	          (lambda ()
507	            (let ((filename (buffer-file-name)))
508	              ;; Enable kernel mode for the appropriate files
509	              (when (and filename
510	                         (string-match (expand-file-name "~/src/linux-trees")
511	                                       filename))
512	                (setq indent-tabs-mode t)
513	                (c-set-style "linux-tabs-only")))))
514	
515	This will make emacs go better with the kernel coding style for C
516	files below ~/src/linux-trees.
517	
518	But even if you fail in getting emacs to do sane formatting, not
519	everything is lost: use "indent".
520	
521	Now, again, GNU indent has the same brain-dead settings that GNU emacs
522	has, which is why you need to give it a few command line options.
523	However, that's not too bad, because even the makers of GNU indent
524	recognize the authority of K&R (the GNU people aren't evil, they are
525	just severely misguided in this matter), so you just give indent the
526	options "-kr -i8" (stands for "K&R, 8 character indents"), or use
527	"scripts/Lindent", which indents in the latest style.
528	
529	"indent" has a lot of options, and especially when it comes to comment
530	re-formatting you may want to take a look at the man page.  But
531	remember: "indent" is not a fix for bad programming.
532	
533	
534			Chapter 10: Kconfig configuration files
535	
536	For all of the Kconfig* configuration files throughout the source tree,
537	the indentation is somewhat different.  Lines under a "config" definition
538	are indented with one tab, while help text is indented an additional two
539	spaces.  Example:
540	
541	config AUDIT
542		bool "Auditing support"
543		depends on NET
544		help
545		  Enable auditing infrastructure that can be used with another
546		  kernel subsystem, such as SELinux (which requires this for
547		  logging of avc messages output).  Does not do system-call
548		  auditing without CONFIG_AUDITSYSCALL.
549	
550	Seriously dangerous features (such as write support for certain
551	filesystems) should advertise this prominently in their prompt string:
552	
553	config ADFS_FS_RW
554		bool "ADFS write support (DANGEROUS)"
555		depends on ADFS_FS
556		...
557	
558	For full documentation on the configuration files, see the file
559	Documentation/kbuild/kconfig-language.txt.
560	
561	
562			Chapter 11: Data structures
563	
564	Data structures that have visibility outside the single-threaded
565	environment they are created and destroyed in should always have
566	reference counts.  In the kernel, garbage collection doesn't exist (and
567	outside the kernel garbage collection is slow and inefficient), which
568	means that you absolutely _have_ to reference count all your uses.
569	
570	Reference counting means that you can avoid locking, and allows multiple
571	users to have access to the data structure in parallel - and not having
572	to worry about the structure suddenly going away from under them just
573	because they slept or did something else for a while.
574	
575	Note that locking is _not_ a replacement for reference counting.
576	Locking is used to keep data structures coherent, while reference
577	counting is a memory management technique.  Usually both are needed, and
578	they are not to be confused with each other.
579	
580	Many data structures can indeed have two levels of reference counting,
581	when there are users of different "classes".  The subclass count counts
582	the number of subclass users, and decrements the global count just once
583	when the subclass count goes to zero.
584	
585	Examples of this kind of "multi-level-reference-counting" can be found in
586	memory management ("struct mm_struct": mm_users and mm_count), and in
587	filesystem code ("struct super_block": s_count and s_active).
588	
589	Remember: if another thread can find your data structure, and you don't
590	have a reference count on it, you almost certainly have a bug.
591	
592	
593			Chapter 12: Macros, Enums and RTL
594	
595	Names of macros defining constants and labels in enums are capitalized.
596	
597	#define CONSTANT 0x12345
598	
599	Enums are preferred when defining several related constants.
600	
601	CAPITALIZED macro names are appreciated but macros resembling functions
602	may be named in lower case.
603	
604	Generally, inline functions are preferable to macros resembling functions.
605	
606	Macros with multiple statements should be enclosed in a do - while block:
607	
608	#define macrofun(a, b, c) 			\
609		do {					\
610			if (a == 5)			\
611				do_this(b, c);		\
612		} while (0)
613	
614	Things to avoid when using macros:
615	
616	1) macros that affect control flow:
617	
618	#define FOO(x)					\
619		do {					\
620			if (blah(x) < 0)		\
621				return -EBUGGERED;	\
622		} while(0)
623	
624	is a _very_ bad idea.  It looks like a function call but exits the "calling"
625	function; don't break the internal parsers of those who will read the code.
626	
627	2) macros that depend on having a local variable with a magic name:
628	
629	#define FOO(val) bar(index, val)
630	
631	might look like a good thing, but it's confusing as hell when one reads the
632	code and it's prone to breakage from seemingly innocent changes.
633	
634	3) macros with arguments that are used as l-values: FOO(x) = y; will
635	bite you if somebody e.g. turns FOO into an inline function.
636	
637	4) forgetting about precedence: macros defining constants using expressions
638	must enclose the expression in parentheses. Beware of similar issues with
639	macros using parameters.
640	
641	#define CONSTANT 0x4000
642	#define CONSTEXP (CONSTANT | 3)
643	
644	The cpp manual deals with macros exhaustively. The gcc internals manual also
645	covers RTL which is used frequently with assembly language in the kernel.
646	
647	
648			Chapter 13: Printing kernel messages
649	
650	Kernel developers like to be seen as literate. Do mind the spelling
651	of kernel messages to make a good impression. Do not use crippled
652	words like "dont"; use "do not" or "don't" instead.  Make the messages
653	concise, clear, and unambiguous.
654	
655	Kernel messages do not have to be terminated with a period.
656	
657	Printing numbers in parentheses (%d) adds no value and should be avoided.
658	
659	There are a number of driver model diagnostic macros in <linux/device.h>
660	which you should use to make sure messages are matched to the right device
661	and driver, and are tagged with the right level:  dev_err(), dev_warn(),
662	dev_info(), and so forth.  For messages that aren't associated with a
663	particular device, <linux/printk.h> defines pr_debug() and pr_info().
664	
665	Coming up with good debugging messages can be quite a challenge; and once
666	you have them, they can be a huge help for remote troubleshooting.  Such
667	messages should be compiled out when the DEBUG symbol is not defined (that
668	is, by default they are not included).  When you use dev_dbg() or pr_debug(),
669	that's automatic.  Many subsystems have Kconfig options to turn on -DDEBUG.
670	A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the
671	ones already enabled by DEBUG.
672	
673	
674			Chapter 14: Allocating memory
675	
676	The kernel provides the following general purpose memory allocators:
677	kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
678	vzalloc().  Please refer to the API documentation for further information
679	about them.
680	
681	The preferred form for passing a size of a struct is the following:
682	
683		p = kmalloc(sizeof(*p), ...);
684	
685	The alternative form where struct name is spelled out hurts readability and
686	introduces an opportunity for a bug when the pointer variable type is changed
687	but the corresponding sizeof that is passed to a memory allocator is not.
688	
689	Casting the return value which is a void pointer is redundant. The conversion
690	from void pointer to any other pointer type is guaranteed by the C programming
691	language.
692	
693	The preferred form for allocating an array is the following:
694	
695		p = kmalloc_array(n, sizeof(...), ...);
696	
697	The preferred form for allocating a zeroed array is the following:
698	
699		p = kcalloc(n, sizeof(...), ...);
700	
701	Both forms check for overflow on the allocation size n * sizeof(...),
702	and return NULL if that occurred.
703	
704	
705			Chapter 15: The inline disease
706	
707	There appears to be a common misperception that gcc has a magic "make me
708	faster" speedup option called "inline". While the use of inlines can be
709	appropriate (for example as a means of replacing macros, see Chapter 12), it
710	very often is not. Abundant use of the inline keyword leads to a much bigger
711	kernel, which in turn slows the system as a whole down, due to a bigger
712	icache footprint for the CPU and simply because there is less memory
713	available for the pagecache. Just think about it; a pagecache miss causes a
714	disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
715	that can go into these 5 milliseconds.
716	
717	A reasonable rule of thumb is to not put inline at functions that have more
718	than 3 lines of code in them. An exception to this rule are the cases where
719	a parameter is known to be a compiletime constant, and as a result of this
720	constantness you *know* the compiler will be able to optimize most of your
721	function away at compile time. For a good example of this later case, see
722	the kmalloc() inline function.
723	
724	Often people argue that adding inline to functions that are static and used
725	only once is always a win since there is no space tradeoff. While this is
726	technically correct, gcc is capable of inlining these automatically without
727	help, and the maintenance issue of removing the inline when a second user
728	appears outweighs the potential value of the hint that tells gcc to do
729	something it would have done anyway.
730	
731	
732			Chapter 16: Function return values and names
733	
734	Functions can return values of many different kinds, and one of the
735	most common is a value indicating whether the function succeeded or
736	failed.  Such a value can be represented as an error-code integer
737	(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
738	non-zero = success).
739	
740	Mixing up these two sorts of representations is a fertile source of
741	difficult-to-find bugs.  If the C language included a strong distinction
742	between integers and booleans then the compiler would find these mistakes
743	for us... but it doesn't.  To help prevent such bugs, always follow this
744	convention:
745	
746		If the name of a function is an action or an imperative command,
747		the function should return an error-code integer.  If the name
748		is a predicate, the function should return a "succeeded" boolean.
749	
750	For example, "add work" is a command, and the add_work() function returns 0
751	for success or -EBUSY for failure.  In the same way, "PCI device present" is
752	a predicate, and the pci_dev_present() function returns 1 if it succeeds in
753	finding a matching device or 0 if it doesn't.
754	
755	All EXPORTed functions must respect this convention, and so should all
756	public functions.  Private (static) functions need not, but it is
757	recommended that they do.
758	
759	Functions whose return value is the actual result of a computation, rather
760	than an indication of whether the computation succeeded, are not subject to
761	this rule.  Generally they indicate failure by returning some out-of-range
762	result.  Typical examples would be functions that return pointers; they use
763	NULL or the ERR_PTR mechanism to report failure.
764	
765	
766			Chapter 17:  Don't re-invent the kernel macros
767	
768	The header file include/linux/kernel.h contains a number of macros that
769	you should use, rather than explicitly coding some variant of them yourself.
770	For example, if you need to calculate the length of an array, take advantage
771	of the macro
772	
773	  #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
774	
775	Similarly, if you need to calculate the size of some structure member, use
776	
777	  #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
778	
779	There are also min() and max() macros that do strict type checking if you
780	need them.  Feel free to peruse that header file to see what else is already
781	defined that you shouldn't reproduce in your code.
782	
783	
784			Chapter 18:  Editor modelines and other cruft
785	
786	Some editors can interpret configuration information embedded in source files,
787	indicated with special markers.  For example, emacs interprets lines marked
788	like this:
789	
790	-*- mode: c -*-
791	
792	Or like this:
793	
794	/*
795	Local Variables:
796	compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
797	End:
798	*/
799	
800	Vim interprets markers that look like this:
801	
802	/* vim:set sw=8 noet */
803	
804	Do not include any of these in source files.  People have their own personal
805	editor configurations, and your source files should not override them.  This
806	includes markers for indentation and mode configuration.  People may use their
807	own custom mode, or may have some other magic method for making indentation
808	work correctly.
809	
810	
811			Chapter 19:  Inline assembly
812	
813	In architecture-specific code, you may need to use inline assembly to interface
814	with CPU or platform functionality.  Don't hesitate to do so when necessary.
815	However, don't use inline assembly gratuitously when C can do the job.  You can
816	and should poke hardware from C when possible.
817	
818	Consider writing simple helper functions that wrap common bits of inline
819	assembly, rather than repeatedly writing them with slight variations.  Remember
820	that inline assembly can use C parameters.
821	
822	Large, non-trivial assembly functions should go in .S files, with corresponding
823	C prototypes defined in C header files.  The C prototypes for assembly
824	functions should use "asmlinkage".
825	
826	You may need to mark your asm statement as volatile, to prevent GCC from
827	removing it if GCC doesn't notice any side effects.  You don't always need to
828	do so, though, and doing so unnecessarily can limit optimization.
829	
830	When writing a single inline assembly statement containing multiple
831	instructions, put each instruction on a separate line in a separate quoted
832	string, and end each string except the last with \n\t to properly indent the
833	next instruction in the assembly output:
834	
835		asm ("magic %reg1, #42\n\t"
836		     "more_magic %reg2, %reg3"
837		     : /* outputs */ : /* inputs */ : /* clobbers */);
838	
839	
840	
841			Appendix I: References
842	
843	The C Programming Language, Second Edition
844	by Brian W. Kernighan and Dennis M. Ritchie.
845	Prentice Hall, Inc., 1988.
846	ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
847	URL: http://cm.bell-labs.com/cm/cs/cbook/
848	
849	The Practice of Programming
850	by Brian W. Kernighan and Rob Pike.
851	Addison-Wesley, Inc., 1999.
852	ISBN 0-201-61586-X.
853	URL: http://cm.bell-labs.com/cm/cs/tpop/
854	
855	GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
856	gcc internals and indent, all available from http://www.gnu.org/manual/
857	
858	WG14 is the international standardization working group for the programming
859	language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
860	
861	Kernel CodingStyle, by greg@kroah.com at OLS 2002:
862	http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
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