Extras: Remove deprecated consumer commands in bash completion
[lttng-tools.git] / doc / kernel-CodingStyle.txt
2 Linux kernel coding style
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.
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.
13 Anyway, here goes:
16 Chapter 1: Indentation
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.
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.
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.
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.
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.:
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 }
60 Don't put multiple statements on a single line unless you have
61 something to hide:
63 if (condition) do_this;
64 do_something_everytime;
66 Don't put multiple assignments on a single line either. Kernel coding style
67 is super simple. Avoid tricky expressions.
69 Outside of comments, documentation and except in Kconfig, spaces are never
70 used for indentation, and the above example is deliberately broken.
72 Get a decent editor and don't leave whitespace at the end of lines.
75 Chapter 2: Breaking long lines and strings
77 Coding style is all about readability and maintainability using commonly
78 available tools.
80 The limit on the length of lines is 80 columns and this is a strongly
81 preferred limit.
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.
91 Chapter 3: Placing Braces and Spaces
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:
99 if (x is true) {
100 we do y
101 }
103 This applies to all non-function statement blocks (if, switch, for,
104 while, do). E.g.:
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 }
117 However, there is one special case, namely functions: they have the
118 opening brace at the beginning of the next line, thus:
120 int function(int x)
121 {
122 body of function
123 }
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).
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:
135 do {
136 body of do-loop
137 } while (condition);
139 and
141 if (x == y) {
142 ..
143 } else if (x > y) {
144 ...
145 } else {
146 ....
147 }
149 Rationale: K&R.
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.
157 Do not unnecessarily use braces where a single statement will do.
159 if (condition)
160 action();
162 and
164 if (condition)
165 do_this();
166 else
167 do_that();
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:
172 if (condition) {
173 do_this();
174 do_that();
175 } else {
176 otherwise();
177 }
179 3.1: Spaces
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).
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);
193 Do not add spaces around (inside) parenthesized expressions. This example is
194 *bad*:
196 s = sizeof( struct file );
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:
202 char *linux_banner;
203 unsigned long long memparse(char *ptr, char **retptr);
204 char *match_strdup(substring_t *s);
206 Use one space around (on each side of) most binary and ternary operators,
207 such as any of these:
209 = + - < > * / % | & ^ <= >= == != ? :
211 but no space after unary operators:
212 & * + - ~ ! sizeof typeof alignof __attribute__ defined
214 no space before the postfix increment & decrement unary operators:
215 ++ --
217 no space after the prefix increment & decrement unary operators:
218 ++ --
220 and no space around the '.' and "->" structure member operators.
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.
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.
235 Chapter 4: Naming
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.
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.
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()".
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.
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.
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).
268 Chapter 5: Typedefs
270 Please don't use things like "vps_t".
272 It's a _mistake_ to use typedef for structures and pointers. When you see a
274 vps_t a;
276 in the source, what does it mean?
278 In contrast, if it says
280 struct virtual_container *a;
282 you can actually tell what "a" is.
284 Lots of people think that typedefs "help readability". Not so. They are
285 useful only for:
287 (a) totally opaque objects (where the typedef is actively used to _hide_
288 what the object is).
290 Example: "pte_t" etc. opaque objects that you can only access using
291 the proper accessor functions.
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.
297 (b) Clear integer types, where the abstraction _helps_ avoid confusion
298 whether it is "int" or "long".
300 u8/u16/u32 are perfectly fine typedefs, although they fit into
301 category (d) better than here.
303 NOTE! Again - there needs to be a _reason_ for this. If something is
304 "unsigned long", then there's no reason to do
306 typedef unsigned long myflags_t;
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.
312 (c) when you use sparse to literally create a _new_ type for
313 type-checking.
315 (d) New types which are identical to standard C99 types, in certain
316 exceptional circumstances.
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.
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.
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.
330 (e) Types safe for use in userspace.
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.
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.
340 In general, a pointer, or a struct that has elements that can reasonably
341 be directly accessed should _never_ be a typedef.
344 Chapter 6: Functions
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.
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.
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).
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.
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.:
375 int system_is_up(void)
376 {
377 return system_state == SYSTEM_RUNNING;
378 }
379 EXPORT_SYMBOL(system_is_up);
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.
386 Chapter 7: Centralized exiting of functions
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.
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.
394 The rationale is:
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 ;)
402 int fun(int a)
403 {
404 int result = 0;
405 char *buffer = kmalloc(SIZE);
407 if (buffer == NULL)
408 return -ENOMEM;
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 }
423 Chapter 8: Commenting
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.
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.
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.
443 Linux style for comments is the C89 "/* ... */" style.
444 Don't use C99-style "// ..." comments.
446 The preferred style for long (multi-line) comments is:
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 */
457 It's also important to comment data, whether they are basic types or derived
458 types. To this end, use just one data declaration per line (no commas for
459 multiple data declarations). This leaves you room for a small comment on each
460 item, explaining its use.
463 Chapter 9: You've made a mess of it
465 That's OK, we all do. You've probably been told by your long-time Unix
466 user helper that "GNU emacs" automatically formats the C sources for
467 you, and you've noticed that yes, it does do that, but the defaults it
468 uses are less than desirable (in fact, they are worse than random
469 typing - an infinite number of monkeys typing into GNU emacs would never
470 make a good program).
472 So, you can either get rid of GNU emacs, or change it to use saner
473 values. To do the latter, you can stick the following in your .emacs file:
475 (defun c-lineup-arglist-tabs-only (ignored)
476 "Line up argument lists by tabs, not spaces"
477 (let* ((anchor (c-langelem-pos c-syntactic-element))
478 (column (c-langelem-2nd-pos c-syntactic-element))
479 (offset (- (1+ column) anchor))
480 (steps (floor offset c-basic-offset)))
481 (* (max steps 1)
482 c-basic-offset)))
484 (add-hook 'c-mode-common-hook
485 (lambda ()
486 ;; Add kernel style
487 (c-add-style
488 "linux-tabs-only"
489 '("linux" (c-offsets-alist
490 (arglist-cont-nonempty
491 c-lineup-gcc-asm-reg
492 c-lineup-arglist-tabs-only))))))
494 (add-hook 'c-mode-hook
495 (lambda ()
496 (let ((filename (buffer-file-name)))
497 ;; Enable kernel mode for the appropriate files
498 (when (and filename
499 (string-match (expand-file-name "~/src/linux-trees")
500 filename))
501 (setq indent-tabs-mode t)
502 (c-set-style "linux-tabs-only")))))
504 This will make emacs go better with the kernel coding style for C
505 files below ~/src/linux-trees.
507 But even if you fail in getting emacs to do sane formatting, not
508 everything is lost: use "indent".
510 Now, again, GNU indent has the same brain-dead settings that GNU emacs
511 has, which is why you need to give it a few command line options.
512 However, that's not too bad, because even the makers of GNU indent
513 recognize the authority of K&R (the GNU people aren't evil, they are
514 just severely misguided in this matter), so you just give indent the
515 options "-kr -i8" (stands for "K&R, 8 character indents"), or use
516 "scripts/Lindent", which indents in the latest style.
518 "indent" has a lot of options, and especially when it comes to comment
519 re-formatting you may want to take a look at the man page. But
520 remember: "indent" is not a fix for bad programming.
523 Chapter 10: Kconfig configuration files
525 For all of the Kconfig* configuration files throughout the source tree,
526 the indentation is somewhat different. Lines under a "config" definition
527 are indented with one tab, while help text is indented an additional two
528 spaces. Example:
530 config AUDIT
531 bool "Auditing support"
532 depends on NET
533 help
534 Enable auditing infrastructure that can be used with another
535 kernel subsystem, such as SELinux (which requires this for
536 logging of avc messages output). Does not do system-call
537 auditing without CONFIG_AUDITSYSCALL.
539 Features that might still be considered unstable should be defined as
540 dependent on "EXPERIMENTAL":
542 config SLUB
544 bool "SLUB (Unqueued Allocator)"
545 ...
547 while seriously dangerous features (such as write support for certain
548 filesystems) should advertise this prominently in their prompt string:
550 config ADFS_FS_RW
551 bool "ADFS write support (DANGEROUS)"
552 depends on ADFS_FS
553 ...
555 For full documentation on the configuration files, see the file
556 Documentation/kbuild/kconfig-language.txt.
559 Chapter 11: Data structures
561 Data structures that have visibility outside the single-threaded
562 environment they are created and destroyed in should always have
563 reference counts. In the kernel, garbage collection doesn't exist (and
564 outside the kernel garbage collection is slow and inefficient), which
565 means that you absolutely _have_ to reference count all your uses.
567 Reference counting means that you can avoid locking, and allows multiple
568 users to have access to the data structure in parallel - and not having
569 to worry about the structure suddenly going away from under them just
570 because they slept or did something else for a while.
572 Note that locking is _not_ a replacement for reference counting.
573 Locking is used to keep data structures coherent, while reference
574 counting is a memory management technique. Usually both are needed, and
575 they are not to be confused with each other.
577 Many data structures can indeed have two levels of reference counting,
578 when there are users of different "classes". The subclass count counts
579 the number of subclass users, and decrements the global count just once
580 when the subclass count goes to zero.
582 Examples of this kind of "multi-level-reference-counting" can be found in
583 memory management ("struct mm_struct": mm_users and mm_count), and in
584 filesystem code ("struct super_block": s_count and s_active).
586 Remember: if another thread can find your data structure, and you don't
587 have a reference count on it, you almost certainly have a bug.
590 Chapter 12: Macros, Enums and RTL
592 Names of macros defining constants and labels in enums are capitalized.
594 #define CONSTANT 0x12345
596 Enums are preferred when defining several related constants.
598 CAPITALIZED macro names are appreciated but macros resembling functions
599 may be named in lower case.
601 Generally, inline functions are preferable to macros resembling functions.
603 Macros with multiple statements should be enclosed in a do - while block:
605 #define macrofun(a, b, c) \
606 do { \
607 if (a == 5) \
608 do_this(b, c); \
609 } while (0)
611 Things to avoid when using macros:
613 1) macros that affect control flow:
615 #define FOO(x) \
616 do { \
617 if (blah(x) < 0) \
618 return -EBUGGERED; \
619 } while(0)
621 is a _very_ bad idea. It looks like a function call but exits the "calling"
622 function; don't break the internal parsers of those who will read the code.
624 2) macros that depend on having a local variable with a magic name:
626 #define FOO(val) bar(index, val)
628 might look like a good thing, but it's confusing as hell when one reads the
629 code and it's prone to breakage from seemingly innocent changes.
631 3) macros with arguments that are used as l-values: FOO(x) = y; will
632 bite you if somebody e.g. turns FOO into an inline function.
634 4) forgetting about precedence: macros defining constants using expressions
635 must enclose the expression in parentheses. Beware of similar issues with
636 macros using parameters.
638 #define CONSTANT 0x4000
639 #define CONSTEXP (CONSTANT | 3)
641 The cpp manual deals with macros exhaustively. The gcc internals manual also
642 covers RTL which is used frequently with assembly language in the kernel.
645 Chapter 13: Printing kernel messages
647 Kernel developers like to be seen as literate. Do mind the spelling
648 of kernel messages to make a good impression. Do not use crippled
649 words like "dont"; use "do not" or "don't" instead. Make the messages
650 concise, clear, and unambiguous.
652 Kernel messages do not have to be terminated with a period.
654 Printing numbers in parentheses (%d) adds no value and should be avoided.
656 There are a number of driver model diagnostic macros in <linux/device.h>
657 which you should use to make sure messages are matched to the right device
658 and driver, and are tagged with the right level: dev_err(), dev_warn(),
659 dev_info(), and so forth. For messages that aren't associated with a
660 particular device, <linux/printk.h> defines pr_debug() and pr_info().
662 Coming up with good debugging messages can be quite a challenge; and once
663 you have them, they can be a huge help for remote troubleshooting. Such
664 messages should be compiled out when the DEBUG symbol is not defined (that
665 is, by default they are not included). When you use dev_dbg() or pr_debug(),
666 that's automatic. Many subsystems have Kconfig options to turn on -DDEBUG.
667 A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the
668 ones already enabled by DEBUG.
671 Chapter 14: Allocating memory
673 The kernel provides the following general purpose memory allocators:
674 kmalloc(), kzalloc(), kcalloc(), vmalloc(), and vzalloc(). Please refer to
675 the API documentation for further information about them.
677 The preferred form for passing a size of a struct is the following:
679 p = kmalloc(sizeof(*p), ...);
681 The alternative form where struct name is spelled out hurts readability and
682 introduces an opportunity for a bug when the pointer variable type is changed
683 but the corresponding sizeof that is passed to a memory allocator is not.
685 Casting the return value which is a void pointer is redundant. The conversion
686 from void pointer to any other pointer type is guaranteed by the C programming
687 language.
690 Chapter 15: The inline disease
692 There appears to be a common misperception that gcc has a magic "make me
693 faster" speedup option called "inline". While the use of inlines can be
694 appropriate (for example as a means of replacing macros, see Chapter 12), it
695 very often is not. Abundant use of the inline keyword leads to a much bigger
696 kernel, which in turn slows the system as a whole down, due to a bigger
697 icache footprint for the CPU and simply because there is less memory
698 available for the pagecache. Just think about it; a pagecache miss causes a
699 disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
700 that can go into these 5 milliseconds.
702 A reasonable rule of thumb is to not put inline at functions that have more
703 than 3 lines of code in them. An exception to this rule are the cases where
704 a parameter is known to be a compiletime constant, and as a result of this
705 constantness you *know* the compiler will be able to optimize most of your
706 function away at compile time. For a good example of this later case, see
707 the kmalloc() inline function.
709 Often people argue that adding inline to functions that are static and used
710 only once is always a win since there is no space tradeoff. While this is
711 technically correct, gcc is capable of inlining these automatically without
712 help, and the maintenance issue of removing the inline when a second user
713 appears outweighs the potential value of the hint that tells gcc to do
714 something it would have done anyway.
717 Chapter 16: Function return values and names
719 Functions can return values of many different kinds, and one of the
720 most common is a value indicating whether the function succeeded or
721 failed. Such a value can be represented as an error-code integer
722 (-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
723 non-zero = success).
725 Mixing up these two sorts of representations is a fertile source of
726 difficult-to-find bugs. If the C language included a strong distinction
727 between integers and booleans then the compiler would find these mistakes
728 for us... but it doesn't. To help prevent such bugs, always follow this
729 convention:
731 If the name of a function is an action or an imperative command,
732 the function should return an error-code integer. If the name
733 is a predicate, the function should return a "succeeded" boolean.
735 For example, "add work" is a command, and the add_work() function returns 0
736 for success or -EBUSY for failure. In the same way, "PCI device present" is
737 a predicate, and the pci_dev_present() function returns 1 if it succeeds in
738 finding a matching device or 0 if it doesn't.
740 All EXPORTed functions must respect this convention, and so should all
741 public functions. Private (static) functions need not, but it is
742 recommended that they do.
744 Functions whose return value is the actual result of a computation, rather
745 than an indication of whether the computation succeeded, are not subject to
746 this rule. Generally they indicate failure by returning some out-of-range
747 result. Typical examples would be functions that return pointers; they use
748 NULL or the ERR_PTR mechanism to report failure.
751 Chapter 17: Don't re-invent the kernel macros
753 The header file include/linux/kernel.h contains a number of macros that
754 you should use, rather than explicitly coding some variant of them yourself.
755 For example, if you need to calculate the length of an array, take advantage
756 of the macro
758 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
760 Similarly, if you need to calculate the size of some structure member, use
762 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
764 There are also min() and max() macros that do strict type checking if you
765 need them. Feel free to peruse that header file to see what else is already
766 defined that you shouldn't reproduce in your code.
769 Chapter 18: Editor modelines and other cruft
771 Some editors can interpret configuration information embedded in source files,
772 indicated with special markers. For example, emacs interprets lines marked
773 like this:
775 -*- mode: c -*-
777 Or like this:
779 /*
780 Local Variables:
781 compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
782 End:
783 */
785 Vim interprets markers that look like this:
787 /* vim:set sw=8 noet */
789 Do not include any of these in source files. People have their own personal
790 editor configurations, and your source files should not override them. This
791 includes markers for indentation and mode configuration. People may use their
792 own custom mode, or may have some other magic method for making indentation
793 work correctly.
796 Chapter 19: Inline assembly
798 In architecture-specific code, you may need to use inline assembly to interface
799 with CPU or platform functionality. Don't hesitate to do so when necessary.
800 However, don't use inline assembly gratuitously when C can do the job. You can
801 and should poke hardware from C when possible.
803 Consider writing simple helper functions that wrap common bits of inline
804 assembly, rather than repeatedly writing them with slight variations. Remember
805 that inline assembly can use C parameters.
807 Large, non-trivial assembly functions should go in .S files, with corresponding
808 C prototypes defined in C header files. The C prototypes for assembly
809 functions should use "asmlinkage".
811 You may need to mark your asm statement as volatile, to prevent GCC from
812 removing it if GCC doesn't notice any side effects. You don't always need to
813 do so, though, and doing so unnecessarily can limit optimization.
815 When writing a single inline assembly statement containing multiple
816 instructions, put each instruction on a separate line in a separate quoted
817 string, and end each string except the last with \n\t to properly indent the
818 next instruction in the assembly output:
820 asm ("magic %reg1, #42\n\t"
821 "more_magic %reg2, %reg3"
822 : /* outputs */ : /* inputs */ : /* clobbers */);
826 Appendix I: References
828 The C Programming Language, Second Edition
829 by Brian W. Kernighan and Dennis M. Ritchie.
830 Prentice Hall, Inc., 1988.
831 ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
832 URL: http://cm.bell-labs.com/cm/cs/cbook/
834 The Practice of Programming
835 by Brian W. Kernighan and Rob Pike.
836 Addison-Wesley, Inc., 1999.
837 ISBN 0-201-61586-X.
838 URL: http://cm.bell-labs.com/cm/cs/tpop/
840 GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
841 gcc internals and indent, all available from http://www.gnu.org/manual/
843 WG14 is the international standardization working group for the programming
844 language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
846 Kernel CodingStyle, by greg@kroah.com at OLS 2002:
847 http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
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