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Based on kernel version 2.6.33. Page generated on 2010-02-24 15:35 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.
84	Descendants are always substantially shorter than the parent and are placed
85	substantially to the right. The same applies to function headers with a long
86	argument list. Long strings are as well broken into shorter strings. The
87	only exception to this is where exceeding 80 columns significantly increases
88	readability and does not hide information.
89	
90	void fun(int a, int b, int c)
91	{
92		if (condition)
93			printk(KERN_WARNING "Warning this is a long printk with "
94							"3 parameters a: %u b: %u "
95							"c: %u \n", a, b, c);
96		else
97			next_statement;
98	}
99	
100			Chapter 3: Placing Braces and Spaces
101	
102	The other issue that always comes up in C styling is the placement of
103	braces.  Unlike the indent size, there are few technical reasons to
104	choose one placement strategy over the other, but the preferred way, as
105	shown to us by the prophets Kernighan and Ritchie, is to put the opening
106	brace last on the line, and put the closing brace first, thusly:
107	
108		if (x is true) {
109			we do y
110		}
111	
112	This applies to all non-function statement blocks (if, switch, for,
113	while, do).  E.g.:
114	
115		switch (action) {
116		case KOBJ_ADD:
117			return "add";
118		case KOBJ_REMOVE:
119			return "remove";
120		case KOBJ_CHANGE:
121			return "change";
122		default:
123			return NULL;
124		}
125	
126	However, there is one special case, namely functions: they have the
127	opening brace at the beginning of the next line, thus:
128	
129		int function(int x)
130		{
131			body of function
132		}
133	
134	Heretic people all over the world have claimed that this inconsistency
135	is ...  well ...  inconsistent, but all right-thinking people know that
136	(a) K&R are _right_ and (b) K&R are right.  Besides, functions are
137	special anyway (you can't nest them in C).
138	
139	Note that the closing brace is empty on a line of its own, _except_ in
140	the cases where it is followed by a continuation of the same statement,
141	ie a "while" in a do-statement or an "else" in an if-statement, like
142	this:
143	
144		do {
145			body of do-loop
146		} while (condition);
147	
148	and
149	
150		if (x == y) {
151			..
152		} else if (x > y) {
153			...
154		} else {
155			....
156		}
157	
158	Rationale: K&R.
159	
160	Also, note that this brace-placement also minimizes the number of empty
161	(or almost empty) lines, without any loss of readability.  Thus, as the
162	supply of new-lines on your screen is not a renewable resource (think
163	25-line terminal screens here), you have more empty lines to put
164	comments on.
165	
166	Do not unnecessarily use braces where a single statement will do.
167	
168	if (condition)
169		action();
170	
171	This does not apply if one branch of a conditional statement is a single
172	statement. Use braces in both branches.
173	
174	if (condition) {
175		do_this();
176		do_that();
177	} else {
178		otherwise();
179	}
180	
181			3.1:  Spaces
182	
183	Linux kernel style for use of spaces depends (mostly) on
184	function-versus-keyword usage.  Use a space after (most) keywords.  The
185	notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
186	somewhat like functions (and are usually used with parentheses in Linux,
187	although they are not required in the language, as in: "sizeof info" after
188	"struct fileinfo info;" is declared).
189	
190	So use a space after these keywords:
191		if, switch, case, for, do, while
192	but not with sizeof, typeof, alignof, or __attribute__.  E.g.,
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		&  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined
215	
216	no space before the postfix increment & decrement unary operators:
217		++  --
218	
219	no space after the prefix increment & decrement unary operators:
220		++  --
221	
222	and no space around the '.' and "->" structure member operators.
223	
224	Do not leave trailing whitespace at the ends of lines.  Some editors with
225	"smart" indentation will insert whitespace at the beginning of new lines as
226	appropriate, so you can start typing the next line of code right away.
227	However, some such editors do not remove the whitespace if you end up not
228	putting a line of code there, such as if you leave a blank line.  As a result,
229	you end up with lines containing trailing whitespace.
230	
231	Git will warn you about patches that introduce trailing whitespace, and can
232	optionally strip the trailing whitespace for you; however, if applying a series
233	of patches, this may make later patches in the series fail by changing their
234	context lines.
235	
236	
237			Chapter 4: Naming
238	
239	C is a Spartan language, and so should your naming be.  Unlike Modula-2
240	and Pascal programmers, C programmers do not use cute names like
241	ThisVariableIsATemporaryCounter.  A C programmer would call that
242	variable "tmp", which is much easier to write, and not the least more
243	difficult to understand.
244	
245	HOWEVER, while mixed-case names are frowned upon, descriptive names for
246	global variables are a must.  To call a global function "foo" is a
247	shooting offense.
248	
249	GLOBAL variables (to be used only if you _really_ need them) need to
250	have descriptive names, as do global functions.  If you have a function
251	that counts the number of active users, you should call that
252	"count_active_users()" or similar, you should _not_ call it "cntusr()".
253	
254	Encoding the type of a function into the name (so-called Hungarian
255	notation) is brain damaged - the compiler knows the types anyway and can
256	check those, and it only confuses the programmer.  No wonder MicroSoft
257	makes buggy programs.
258	
259	LOCAL variable names should be short, and to the point.  If you have
260	some random integer loop counter, it should probably be called "i".
261	Calling it "loop_counter" is non-productive, if there is no chance of it
262	being mis-understood.  Similarly, "tmp" can be just about any type of
263	variable that is used to hold a temporary value.
264	
265	If you are afraid to mix up your local variable names, you have another
266	problem, which is called the function-growth-hormone-imbalance syndrome.
267	See chapter 6 (Functions).
268	
269	
270			Chapter 5: Typedefs
271	
272	Please don't use things like "vps_t".
273	
274	It's a _mistake_ to use typedef for structures and pointers. When you see a
275	
276		vps_t a;
277	
278	in the source, what does it mean?
279	
280	In contrast, if it says
281	
282		struct virtual_container *a;
283	
284	you can actually tell what "a" is.
285	
286	Lots of people think that typedefs "help readability". Not so. They are
287	useful only for:
288	
289	 (a) totally opaque objects (where the typedef is actively used to _hide_
290	     what the object is).
291	
292	     Example: "pte_t" etc. opaque objects that you can only access using
293	     the proper accessor functions.
294	
295	     NOTE! Opaqueness and "accessor functions" are not good in themselves.
296	     The reason we have them for things like pte_t etc. is that there
297	     really is absolutely _zero_ portably accessible information there.
298	
299	 (b) Clear integer types, where the abstraction _helps_ avoid confusion
300	     whether it is "int" or "long".
301	
302	     u8/u16/u32 are perfectly fine typedefs, although they fit into
303	     category (d) better than here.
304	
305	     NOTE! Again - there needs to be a _reason_ for this. If something is
306	     "unsigned long", then there's no reason to do
307	
308		typedef unsigned long myflags_t;
309	
310	     but if there is a clear reason for why it under certain circumstances
311	     might be an "unsigned int" and under other configurations might be
312	     "unsigned long", then by all means go ahead and use a typedef.
313	
314	 (c) when you use sparse to literally create a _new_ type for
315	     type-checking.
316	
317	 (d) New types which are identical to standard C99 types, in certain
318	     exceptional circumstances.
319	
320	     Although it would only take a short amount of time for the eyes and
321	     brain to become accustomed to the standard types like 'uint32_t',
322	     some people object to their use anyway.
323	
324	     Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
325	     signed equivalents which are identical to standard types are
326	     permitted -- although they are not mandatory in new code of your
327	     own.
328	
329	     When editing existing code which already uses one or the other set
330	     of types, you should conform to the existing choices in that code.
331	
332	 (e) Types safe for use in userspace.
333	
334	     In certain structures which are visible to userspace, we cannot
335	     require C99 types and cannot use the 'u32' form above. Thus, we
336	     use __u32 and similar types in all structures which are shared
337	     with userspace.
338	
339	Maybe there are other cases too, but the rule should basically be to NEVER
340	EVER use a typedef unless you can clearly match one of those rules.
341	
342	In general, a pointer, or a struct that has elements that can reasonably
343	be directly accessed should _never_ be a typedef.
344	
345	
346			Chapter 6: Functions
347	
348	Functions should be short and sweet, and do just one thing.  They should
349	fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
350	as we all know), and do one thing and do that well.
351	
352	The maximum length of a function is inversely proportional to the
353	complexity and indentation level of that function.  So, if you have a
354	conceptually simple function that is just one long (but simple)
355	case-statement, where you have to do lots of small things for a lot of
356	different cases, it's OK to have a longer function.
357	
358	However, if you have a complex function, and you suspect that a
359	less-than-gifted first-year high-school student might not even
360	understand what the function is all about, you should adhere to the
361	maximum limits all the more closely.  Use helper functions with
362	descriptive names (you can ask the compiler to in-line them if you think
363	it's performance-critical, and it will probably do a better job of it
364	than you would have done).
365	
366	Another measure of the function is the number of local variables.  They
367	shouldn't exceed 5-10, or you're doing something wrong.  Re-think the
368	function, and split it into smaller pieces.  A human brain can
369	generally easily keep track of about 7 different things, anything more
370	and it gets confused.  You know you're brilliant, but maybe you'd like
371	to understand what you did 2 weeks from now.
372	
373	In source files, separate functions with one blank line.  If the function is
374	exported, the EXPORT* macro for it should follow immediately after the closing
375	function brace line.  E.g.:
376	
377	int system_is_up(void)
378	{
379		return system_state == SYSTEM_RUNNING;
380	}
381	EXPORT_SYMBOL(system_is_up);
382	
383	In function prototypes, include parameter names with their data types.
384	Although this is not required by the C language, it is preferred in Linux
385	because it is a simple way to add valuable information for the reader.
386	
387	
388			Chapter 7: Centralized exiting of functions
389	
390	Albeit deprecated by some people, the equivalent of the goto statement is
391	used frequently by compilers in form of the unconditional jump instruction.
392	
393	The goto statement comes in handy when a function exits from multiple
394	locations and some common work such as cleanup has to be done.
395	
396	The rationale is:
397	
398	- unconditional statements are easier to understand and follow
399	- nesting is reduced
400	- errors by not updating individual exit points when making
401	    modifications are prevented
402	- saves the compiler work to optimize redundant code away ;)
403	
404	int fun(int a)
405	{
406		int result = 0;
407		char *buffer = kmalloc(SIZE);
408	
409		if (buffer == NULL)
410			return -ENOMEM;
411	
412		if (condition1) {
413			while (loop1) {
414				...
415			}
416			result = 1;
417			goto out;
418		}
419		...
420	out:
421		kfree(buffer);
422		return result;
423	}
424	
425			Chapter 8: Commenting
426	
427	Comments are good, but there is also a danger of over-commenting.  NEVER
428	try to explain HOW your code works in a comment: it's much better to
429	write the code so that the _working_ is obvious, and it's a waste of
430	time to explain badly written code.
431	
432	Generally, you want your comments to tell WHAT your code does, not HOW.
433	Also, try to avoid putting comments inside a function body: if the
434	function is so complex that you need to separately comment parts of it,
435	you should probably go back to chapter 6 for a while.  You can make
436	small comments to note or warn about something particularly clever (or
437	ugly), but try to avoid excess.  Instead, put the comments at the head
438	of the function, telling people what it does, and possibly WHY it does
439	it.
440	
441	When commenting the kernel API functions, please use the kernel-doc format.
442	See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
443	for details.
444	
445	Linux style for comments is the C89 "/* ... */" style.
446	Don't use C99-style "// ..." comments.
447	
448	The preferred style for long (multi-line) comments is:
449	
450		/*
451		 * This is the preferred style for multi-line
452		 * comments in the Linux kernel source code.
453		 * Please use it consistently.
454		 *
455		 * Description:  A column of asterisks on the left side,
456		 * with beginning and ending almost-blank lines.
457		 */
458	
459	It's also important to comment data, whether they are basic types or derived
460	types.  To this end, use just one data declaration per line (no commas for
461	multiple data declarations).  This leaves you room for a small comment on each
462	item, explaining its use.
463	
464	
465			Chapter 9: You've made a mess of it
466	
467	That's OK, we all do.  You've probably been told by your long-time Unix
468	user helper that "GNU emacs" automatically formats the C sources for
469	you, and you've noticed that yes, it does do that, but the defaults it
470	uses are less than desirable (in fact, they are worse than random
471	typing - an infinite number of monkeys typing into GNU emacs would never
472	make a good program).
473	
474	So, you can either get rid of GNU emacs, or change it to use saner
475	values.  To do the latter, you can stick the following in your .emacs file:
476	
477	(defun c-lineup-arglist-tabs-only (ignored)
478	  "Line up argument lists by tabs, not spaces"
479	  (let* ((anchor (c-langelem-pos c-syntactic-element))
480		 (column (c-langelem-2nd-pos c-syntactic-element))
481		 (offset (- (1+ column) anchor))
482		 (steps (floor offset c-basic-offset)))
483	    (* (max steps 1)
484	       c-basic-offset)))
485	
486	(add-hook 'c-mode-common-hook
487	          (lambda ()
488	            ;; Add kernel style
489	            (c-add-style
490	             "linux-tabs-only"
491	             '("linux" (c-offsets-alist
492	                        (arglist-cont-nonempty
493	                         c-lineup-gcc-asm-reg
494	                         c-lineup-arglist-tabs-only))))))
495	
496	(add-hook 'c-mode-hook
497	          (lambda ()
498	            (let ((filename (buffer-file-name)))
499	              ;; Enable kernel mode for the appropriate files
500	              (when (and filename
501	                         (string-match (expand-file-name "~/src/linux-trees")
502	                                       filename))
503	                (setq indent-tabs-mode t)
504	                (c-set-style "linux-tabs-only")))))
505	
506	This will make emacs go better with the kernel coding style for C
507	files below ~/src/linux-trees.
508	
509	But even if you fail in getting emacs to do sane formatting, not
510	everything is lost: use "indent".
511	
512	Now, again, GNU indent has the same brain-dead settings that GNU emacs
513	has, which is why you need to give it a few command line options.
514	However, that's not too bad, because even the makers of GNU indent
515	recognize the authority of K&R (the GNU people aren't evil, they are
516	just severely misguided in this matter), so you just give indent the
517	options "-kr -i8" (stands for "K&R, 8 character indents"), or use
518	"scripts/Lindent", which indents in the latest style.
519	
520	"indent" has a lot of options, and especially when it comes to comment
521	re-formatting you may want to take a look at the man page.  But
522	remember: "indent" is not a fix for bad programming.
523	
524	
525			Chapter 10: Kconfig configuration files
526	
527	For all of the Kconfig* configuration files throughout the source tree,
528	the indentation is somewhat different.  Lines under a "config" definition
529	are indented with one tab, while help text is indented an additional two
530	spaces.  Example:
531	
532	config AUDIT
533		bool "Auditing support"
534		depends on NET
535		help
536		  Enable auditing infrastructure that can be used with another
537		  kernel subsystem, such as SELinux (which requires this for
538		  logging of avc messages output).  Does not do system-call
539		  auditing without CONFIG_AUDITSYSCALL.
540	
541	Features that might still be considered unstable should be defined as
542	dependent on "EXPERIMENTAL":
543	
544	config SLUB
545		depends on EXPERIMENTAL && !ARCH_USES_SLAB_PAGE_STRUCT
546		bool "SLUB (Unqueued Allocator)"
547		...
548	
549	while 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/kernel.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(), kcalloc(), and vmalloc().  Please refer to the API
677	documentation for further information about them.
678	
679	The preferred form for passing a size of a struct is the following:
680	
681		p = kmalloc(sizeof(*p), ...);
682	
683	The alternative form where struct name is spelled out hurts readability and
684	introduces an opportunity for a bug when the pointer variable type is changed
685	but the corresponding sizeof that is passed to a memory allocator is not.
686	
687	Casting the return value which is a void pointer is redundant. The conversion
688	from void pointer to any other pointer type is guaranteed by the C programming
689	language.
690	
691	
692			Chapter 15: The inline disease
693	
694	There appears to be a common misperception that gcc has a magic "make me
695	faster" speedup option called "inline". While the use of inlines can be
696	appropriate (for example as a means of replacing macros, see Chapter 12), it
697	very often is not. Abundant use of the inline keyword leads to a much bigger
698	kernel, which in turn slows the system as a whole down, due to a bigger
699	icache footprint for the CPU and simply because there is less memory
700	available for the pagecache. Just think about it; a pagecache miss causes a
701	disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
702	that can go into these 5 milliseconds.
703	
704	A reasonable rule of thumb is to not put inline at functions that have more
705	than 3 lines of code in them. An exception to this rule are the cases where
706	a parameter is known to be a compiletime constant, and as a result of this
707	constantness you *know* the compiler will be able to optimize most of your
708	function away at compile time. For a good example of this later case, see
709	the kmalloc() inline function.
710	
711	Often people argue that adding inline to functions that are static and used
712	only once is always a win since there is no space tradeoff. While this is
713	technically correct, gcc is capable of inlining these automatically without
714	help, and the maintenance issue of removing the inline when a second user
715	appears outweighs the potential value of the hint that tells gcc to do
716	something it would have done anyway.
717	
718	
719			Chapter 16: Function return values and names
720	
721	Functions can return values of many different kinds, and one of the
722	most common is a value indicating whether the function succeeded or
723	failed.  Such a value can be represented as an error-code integer
724	(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
725	non-zero = success).
726	
727	Mixing up these two sorts of representations is a fertile source of
728	difficult-to-find bugs.  If the C language included a strong distinction
729	between integers and booleans then the compiler would find these mistakes
730	for us... but it doesn't.  To help prevent such bugs, always follow this
731	convention:
732	
733		If the name of a function is an action or an imperative command,
734		the function should return an error-code integer.  If the name
735		is a predicate, the function should return a "succeeded" boolean.
736	
737	For example, "add work" is a command, and the add_work() function returns 0
738	for success or -EBUSY for failure.  In the same way, "PCI device present" is
739	a predicate, and the pci_dev_present() function returns 1 if it succeeds in
740	finding a matching device or 0 if it doesn't.
741	
742	All EXPORTed functions must respect this convention, and so should all
743	public functions.  Private (static) functions need not, but it is
744	recommended that they do.
745	
746	Functions whose return value is the actual result of a computation, rather
747	than an indication of whether the computation succeeded, are not subject to
748	this rule.  Generally they indicate failure by returning some out-of-range
749	result.  Typical examples would be functions that return pointers; they use
750	NULL or the ERR_PTR mechanism to report failure.
751	
752	
753			Chapter 17:  Don't re-invent the kernel macros
754	
755	The header file include/linux/kernel.h contains a number of macros that
756	you should use, rather than explicitly coding some variant of them yourself.
757	For example, if you need to calculate the length of an array, take advantage
758	of the macro
759	
760	  #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
761	
762	Similarly, if you need to calculate the size of some structure member, use
763	
764	  #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
765	
766	There are also min() and max() macros that do strict type checking if you
767	need them.  Feel free to peruse that header file to see what else is already
768	defined that you shouldn't reproduce in your code.
769	
770	
771			Chapter 18:  Editor modelines and other cruft
772	
773	Some editors can interpret configuration information embedded in source files,
774	indicated with special markers.  For example, emacs interprets lines marked
775	like this:
776	
777	-*- mode: c -*-
778	
779	Or like this:
780	
781	/*
782	Local Variables:
783	compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
784	End:
785	*/
786	
787	Vim interprets markers that look like this:
788	
789	/* vim:set sw=8 noet */
790	
791	Do not include any of these in source files.  People have their own personal
792	editor configurations, and your source files should not override them.  This
793	includes markers for indentation and mode configuration.  People may use their
794	own custom mode, or may have some other magic method for making indentation
795	work correctly.
796	
797	
798	
799			Appendix I: References
800	
801	The C Programming Language, Second Edition
802	by Brian W. Kernighan and Dennis M. Ritchie.
803	Prentice Hall, Inc., 1988.
804	ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
805	URL: http://cm.bell-labs.com/cm/cs/cbook/
806	
807	The Practice of Programming
808	by Brian W. Kernighan and Rob Pike.
809	Addison-Wesley, Inc., 1999.
810	ISBN 0-201-61586-X.
811	URL: http://cm.bell-labs.com/cm/cs/tpop/
812	
813	GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
814	gcc internals and indent, all available from http://www.gnu.org/manual/
815	
816	WG14 is the international standardization working group for the programming
817	language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
818	
819	Kernel CodingStyle, by greg[AT]kroah[DOT]com at OLS 2002:
820	http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
821	
822	--
823	Last updated on 2007-July-13.
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