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Linux/Documentation/CodingStyle

  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 Choose label names which say what the goto does or why the goto exists.  An
396 example of a good name could be "out_buffer:" if the goto frees "buffer".  Avoid
397 using GW-BASIC names like "err1:" and "err2:".  Also don't name them after the
398 goto location like "err_kmalloc_failed:"
399 
400 The rationale for using gotos is:
401 
402 - unconditional statements are easier to understand and follow
403 - nesting is reduced
404 - errors by not updating individual exit points when making
405     modifications are prevented
406 - saves the compiler work to optimize redundant code away ;)
407 
408 int fun(int a)
409 {
410         int result = 0;
411         char *buffer;
412 
413         buffer = kmalloc(SIZE, GFP_KERNEL);
414         if (!buffer)
415                 return -ENOMEM;
416 
417         if (condition1) {
418                 while (loop1) {
419                         ...
420                 }
421                 result = 1;
422                 goto out_buffer;
423         }
424         ...
425 out_buffer:
426         kfree(buffer);
427         return result;
428 }
429 
430 A common type of bug to be aware of it "one err bugs" which look like this:
431 
432 err:
433         kfree(foo->bar);
434         kfree(foo);
435         return ret;
436 
437 The bug in this code is that on some exit paths "foo" is NULL.  Normally the
438 fix for this is to split it up into two error labels "err_bar:" and "err_foo:".
439 
440 
441                 Chapter 8: Commenting
442 
443 Comments are good, but there is also a danger of over-commenting.  NEVER
444 try to explain HOW your code works in a comment: it's much better to
445 write the code so that the _working_ is obvious, and it's a waste of
446 time to explain badly written code.
447 
448 Generally, you want your comments to tell WHAT your code does, not HOW.
449 Also, try to avoid putting comments inside a function body: if the
450 function is so complex that you need to separately comment parts of it,
451 you should probably go back to chapter 6 for a while.  You can make
452 small comments to note or warn about something particularly clever (or
453 ugly), but try to avoid excess.  Instead, put the comments at the head
454 of the function, telling people what it does, and possibly WHY it does
455 it.
456 
457 When commenting the kernel API functions, please use the kernel-doc format.
458 See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
459 for details.
460 
461 Linux style for comments is the C89 "/* ... */" style.
462 Don't use C99-style "// ..." comments.
463 
464 The preferred style for long (multi-line) comments is:
465 
466         /*
467          * This is the preferred style for multi-line
468          * comments in the Linux kernel source code.
469          * Please use it consistently.
470          *
471          * Description:  A column of asterisks on the left side,
472          * with beginning and ending almost-blank lines.
473          */
474 
475 For files in net/ and drivers/net/ the preferred style for long (multi-line)
476 comments is a little different.
477 
478         /* The preferred comment style for files in net/ and drivers/net
479          * looks like this.
480          *
481          * It is nearly the same as the generally preferred comment style,
482          * but there is no initial almost-blank line.
483          */
484 
485 It's also important to comment data, whether they are basic types or derived
486 types.  To this end, use just one data declaration per line (no commas for
487 multiple data declarations).  This leaves you room for a small comment on each
488 item, explaining its use.
489 
490 
491                 Chapter 9: You've made a mess of it
492 
493 That's OK, we all do.  You've probably been told by your long-time Unix
494 user helper that "GNU emacs" automatically formats the C sources for
495 you, and you've noticed that yes, it does do that, but the defaults it
496 uses are less than desirable (in fact, they are worse than random
497 typing - an infinite number of monkeys typing into GNU emacs would never
498 make a good program).
499 
500 So, you can either get rid of GNU emacs, or change it to use saner
501 values.  To do the latter, you can stick the following in your .emacs file:
502 
503 (defun c-lineup-arglist-tabs-only (ignored)
504   "Line up argument lists by tabs, not spaces"
505   (let* ((anchor (c-langelem-pos c-syntactic-element))
506          (column (c-langelem-2nd-pos c-syntactic-element))
507          (offset (- (1+ column) anchor))
508          (steps (floor offset c-basic-offset)))
509     (* (max steps 1)
510        c-basic-offset)))
511 
512 (add-hook 'c-mode-common-hook
513           (lambda ()
514             ;; Add kernel style
515             (c-add-style
516              "linux-tabs-only"
517              '("linux" (c-offsets-alist
518                         (arglist-cont-nonempty
519                          c-lineup-gcc-asm-reg
520                          c-lineup-arglist-tabs-only))))))
521 
522 (add-hook 'c-mode-hook
523           (lambda ()
524             (let ((filename (buffer-file-name)))
525               ;; Enable kernel mode for the appropriate files
526               (when (and filename
527                          (string-match (expand-file-name "~/src/linux-trees")
528                                        filename))
529                 (setq indent-tabs-mode t)
530                 (c-set-style "linux-tabs-only")))))
531 
532 This will make emacs go better with the kernel coding style for C
533 files below ~/src/linux-trees.
534 
535 But even if you fail in getting emacs to do sane formatting, not
536 everything is lost: use "indent".
537 
538 Now, again, GNU indent has the same brain-dead settings that GNU emacs
539 has, which is why you need to give it a few command line options.
540 However, that's not too bad, because even the makers of GNU indent
541 recognize the authority of K&R (the GNU people aren't evil, they are
542 just severely misguided in this matter), so you just give indent the
543 options "-kr -i8" (stands for "K&R, 8 character indents"), or use
544 "scripts/Lindent", which indents in the latest style.
545 
546 "indent" has a lot of options, and especially when it comes to comment
547 re-formatting you may want to take a look at the man page.  But
548 remember: "indent" is not a fix for bad programming.
549 
550 
551                 Chapter 10: Kconfig configuration files
552 
553 For all of the Kconfig* configuration files throughout the source tree,
554 the indentation is somewhat different.  Lines under a "config" definition
555 are indented with one tab, while help text is indented an additional two
556 spaces.  Example:
557 
558 config AUDIT
559         bool "Auditing support"
560         depends on NET
561         help
562           Enable auditing infrastructure that can be used with another
563           kernel subsystem, such as SELinux (which requires this for
564           logging of avc messages output).  Does not do system-call
565           auditing without CONFIG_AUDITSYSCALL.
566 
567 Seriously dangerous features (such as write support for certain
568 filesystems) should advertise this prominently in their prompt string:
569 
570 config ADFS_FS_RW
571         bool "ADFS write support (DANGEROUS)"
572         depends on ADFS_FS
573         ...
574 
575 For full documentation on the configuration files, see the file
576 Documentation/kbuild/kconfig-language.txt.
577 
578 
579                 Chapter 11: Data structures
580 
581 Data structures that have visibility outside the single-threaded
582 environment they are created and destroyed in should always have
583 reference counts.  In the kernel, garbage collection doesn't exist (and
584 outside the kernel garbage collection is slow and inefficient), which
585 means that you absolutely _have_ to reference count all your uses.
586 
587 Reference counting means that you can avoid locking, and allows multiple
588 users to have access to the data structure in parallel - and not having
589 to worry about the structure suddenly going away from under them just
590 because they slept or did something else for a while.
591 
592 Note that locking is _not_ a replacement for reference counting.
593 Locking is used to keep data structures coherent, while reference
594 counting is a memory management technique.  Usually both are needed, and
595 they are not to be confused with each other.
596 
597 Many data structures can indeed have two levels of reference counting,
598 when there are users of different "classes".  The subclass count counts
599 the number of subclass users, and decrements the global count just once
600 when the subclass count goes to zero.
601 
602 Examples of this kind of "multi-level-reference-counting" can be found in
603 memory management ("struct mm_struct": mm_users and mm_count), and in
604 filesystem code ("struct super_block": s_count and s_active).
605 
606 Remember: if another thread can find your data structure, and you don't
607 have a reference count on it, you almost certainly have a bug.
608 
609 
610                 Chapter 12: Macros, Enums and RTL
611 
612 Names of macros defining constants and labels in enums are capitalized.
613 
614 #define CONSTANT 0x12345
615 
616 Enums are preferred when defining several related constants.
617 
618 CAPITALIZED macro names are appreciated but macros resembling functions
619 may be named in lower case.
620 
621 Generally, inline functions are preferable to macros resembling functions.
622 
623 Macros with multiple statements should be enclosed in a do - while block:
624 
625 #define macrofun(a, b, c)                       \
626         do {                                    \
627                 if (a == 5)                     \
628                         do_this(b, c);          \
629         } while (0)
630 
631 Things to avoid when using macros:
632 
633 1) macros that affect control flow:
634 
635 #define FOO(x)                                  \
636         do {                                    \
637                 if (blah(x) < 0)                \
638                         return -EBUGGERED;      \
639         } while(0)
640 
641 is a _very_ bad idea.  It looks like a function call but exits the "calling"
642 function; don't break the internal parsers of those who will read the code.
643 
644 2) macros that depend on having a local variable with a magic name:
645 
646 #define FOO(val) bar(index, val)
647 
648 might look like a good thing, but it's confusing as hell when one reads the
649 code and it's prone to breakage from seemingly innocent changes.
650 
651 3) macros with arguments that are used as l-values: FOO(x) = y; will
652 bite you if somebody e.g. turns FOO into an inline function.
653 
654 4) forgetting about precedence: macros defining constants using expressions
655 must enclose the expression in parentheses. Beware of similar issues with
656 macros using parameters.
657 
658 #define CONSTANT 0x4000
659 #define CONSTEXP (CONSTANT | 3)
660 
661 The cpp manual deals with macros exhaustively. The gcc internals manual also
662 covers RTL which is used frequently with assembly language in the kernel.
663 
664 
665                 Chapter 13: Printing kernel messages
666 
667 Kernel developers like to be seen as literate. Do mind the spelling
668 of kernel messages to make a good impression. Do not use crippled
669 words like "dont"; use "do not" or "don't" instead.  Make the messages
670 concise, clear, and unambiguous.
671 
672 Kernel messages do not have to be terminated with a period.
673 
674 Printing numbers in parentheses (%d) adds no value and should be avoided.
675 
676 There are a number of driver model diagnostic macros in <linux/device.h>
677 which you should use to make sure messages are matched to the right device
678 and driver, and are tagged with the right level:  dev_err(), dev_warn(),
679 dev_info(), and so forth.  For messages that aren't associated with a
680 particular device, <linux/printk.h> defines pr_notice(), pr_info(),
681 pr_warn(), pr_err(), etc.
682 
683 Coming up with good debugging messages can be quite a challenge; and once
684 you have them, they can be a huge help for remote troubleshooting.  However
685 debug message printing is handled differently than printing other non-debug
686 messages.  While the other pr_XXX() functions print unconditionally,
687 pr_debug() does not; it is compiled out by default, unless either DEBUG is
688 defined or CONFIG_DYNAMIC_DEBUG is set.  That is true for dev_dbg() also,
689 and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
690 the ones already enabled by DEBUG.
691 
692 Many subsystems have Kconfig debug options to turn on -DDEBUG in the
693 corresponding Makefile; in other cases specific files #define DEBUG.  And
694 when a debug message should be unconditionally printed, such as if it is
695 already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
696 used.
697 
698 
699                 Chapter 14: Allocating memory
700 
701 The kernel provides the following general purpose memory allocators:
702 kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
703 vzalloc().  Please refer to the API documentation for further information
704 about them.
705 
706 The preferred form for passing a size of a struct is the following:
707 
708         p = kmalloc(sizeof(*p), ...);
709 
710 The alternative form where struct name is spelled out hurts readability and
711 introduces an opportunity for a bug when the pointer variable type is changed
712 but the corresponding sizeof that is passed to a memory allocator is not.
713 
714 Casting the return value which is a void pointer is redundant. The conversion
715 from void pointer to any other pointer type is guaranteed by the C programming
716 language.
717 
718 The preferred form for allocating an array is the following:
719 
720         p = kmalloc_array(n, sizeof(...), ...);
721 
722 The preferred form for allocating a zeroed array is the following:
723 
724         p = kcalloc(n, sizeof(...), ...);
725 
726 Both forms check for overflow on the allocation size n * sizeof(...),
727 and return NULL if that occurred.
728 
729 
730                 Chapter 15: The inline disease
731 
732 There appears to be a common misperception that gcc has a magic "make me
733 faster" speedup option called "inline". While the use of inlines can be
734 appropriate (for example as a means of replacing macros, see Chapter 12), it
735 very often is not. Abundant use of the inline keyword leads to a much bigger
736 kernel, which in turn slows the system as a whole down, due to a bigger
737 icache footprint for the CPU and simply because there is less memory
738 available for the pagecache. Just think about it; a pagecache miss causes a
739 disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
740 that can go into these 5 milliseconds.
741 
742 A reasonable rule of thumb is to not put inline at functions that have more
743 than 3 lines of code in them. An exception to this rule are the cases where
744 a parameter is known to be a compiletime constant, and as a result of this
745 constantness you *know* the compiler will be able to optimize most of your
746 function away at compile time. For a good example of this later case, see
747 the kmalloc() inline function.
748 
749 Often people argue that adding inline to functions that are static and used
750 only once is always a win since there is no space tradeoff. While this is
751 technically correct, gcc is capable of inlining these automatically without
752 help, and the maintenance issue of removing the inline when a second user
753 appears outweighs the potential value of the hint that tells gcc to do
754 something it would have done anyway.
755 
756 
757                 Chapter 16: Function return values and names
758 
759 Functions can return values of many different kinds, and one of the
760 most common is a value indicating whether the function succeeded or
761 failed.  Such a value can be represented as an error-code integer
762 (-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
763 non-zero = success).
764 
765 Mixing up these two sorts of representations is a fertile source of
766 difficult-to-find bugs.  If the C language included a strong distinction
767 between integers and booleans then the compiler would find these mistakes
768 for us... but it doesn't.  To help prevent such bugs, always follow this
769 convention:
770 
771         If the name of a function is an action or an imperative command,
772         the function should return an error-code integer.  If the name
773         is a predicate, the function should return a "succeeded" boolean.
774 
775 For example, "add work" is a command, and the add_work() function returns 0
776 for success or -EBUSY for failure.  In the same way, "PCI device present" is
777 a predicate, and the pci_dev_present() function returns 1 if it succeeds in
778 finding a matching device or 0 if it doesn't.
779 
780 All EXPORTed functions must respect this convention, and so should all
781 public functions.  Private (static) functions need not, but it is
782 recommended that they do.
783 
784 Functions whose return value is the actual result of a computation, rather
785 than an indication of whether the computation succeeded, are not subject to
786 this rule.  Generally they indicate failure by returning some out-of-range
787 result.  Typical examples would be functions that return pointers; they use
788 NULL or the ERR_PTR mechanism to report failure.
789 
790 
791                 Chapter 17:  Don't re-invent the kernel macros
792 
793 The header file include/linux/kernel.h contains a number of macros that
794 you should use, rather than explicitly coding some variant of them yourself.
795 For example, if you need to calculate the length of an array, take advantage
796 of the macro
797 
798   #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
799 
800 Similarly, if you need to calculate the size of some structure member, use
801 
802   #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
803 
804 There are also min() and max() macros that do strict type checking if you
805 need them.  Feel free to peruse that header file to see what else is already
806 defined that you shouldn't reproduce in your code.
807 
808 
809                 Chapter 18:  Editor modelines and other cruft
810 
811 Some editors can interpret configuration information embedded in source files,
812 indicated with special markers.  For example, emacs interprets lines marked
813 like this:
814 
815 -*- mode: c -*-
816 
817 Or like this:
818 
819 /*
820 Local Variables:
821 compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
822 End:
823 */
824 
825 Vim interprets markers that look like this:
826 
827 /* vim:set sw=8 noet */
828 
829 Do not include any of these in source files.  People have their own personal
830 editor configurations, and your source files should not override them.  This
831 includes markers for indentation and mode configuration.  People may use their
832 own custom mode, or may have some other magic method for making indentation
833 work correctly.
834 
835 
836                 Chapter 19:  Inline assembly
837 
838 In architecture-specific code, you may need to use inline assembly to interface
839 with CPU or platform functionality.  Don't hesitate to do so when necessary.
840 However, don't use inline assembly gratuitously when C can do the job.  You can
841 and should poke hardware from C when possible.
842 
843 Consider writing simple helper functions that wrap common bits of inline
844 assembly, rather than repeatedly writing them with slight variations.  Remember
845 that inline assembly can use C parameters.
846 
847 Large, non-trivial assembly functions should go in .S files, with corresponding
848 C prototypes defined in C header files.  The C prototypes for assembly
849 functions should use "asmlinkage".
850 
851 You may need to mark your asm statement as volatile, to prevent GCC from
852 removing it if GCC doesn't notice any side effects.  You don't always need to
853 do so, though, and doing so unnecessarily can limit optimization.
854 
855 When writing a single inline assembly statement containing multiple
856 instructions, put each instruction on a separate line in a separate quoted
857 string, and end each string except the last with \n\t to properly indent the
858 next instruction in the assembly output:
859 
860         asm ("magic %reg1, #42\n\t"
861              "more_magic %reg2, %reg3"
862              : /* outputs */ : /* inputs */ : /* clobbers */);
863 
864 
865                 Chapter 20: Conditional Compilation
866 
867 Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
868 files; doing so makes code harder to read and logic harder to follow.  Instead,
869 use such conditionals in a header file defining functions for use in those .c
870 files, providing no-op stub versions in the #else case, and then call those
871 functions unconditionally from .c files.  The compiler will avoid generating
872 any code for the stub calls, producing identical results, but the logic will
873 remain easy to follow.
874 
875 Prefer to compile out entire functions, rather than portions of functions or
876 portions of expressions.  Rather than putting an ifdef in an expression, factor
877 out part or all of the expression into a separate helper function and apply the
878 conditional to that function.
879 
880 If you have a function or variable which may potentially go unused in a
881 particular configuration, and the compiler would warn about its definition
882 going unused, mark the definition as __maybe_unused rather than wrapping it in
883 a preprocessor conditional.  (However, if a function or variable *always* goes
884 unused, delete it.)
885 
886 Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
887 symbol into a C boolean expression, and use it in a normal C conditional:
888 
889         if (IS_ENABLED(CONFIG_SOMETHING)) {
890                 ...
891         }
892 
893 The compiler will constant-fold the conditional away, and include or exclude
894 the block of code just as with an #ifdef, so this will not add any runtime
895 overhead.  However, this approach still allows the C compiler to see the code
896 inside the block, and check it for correctness (syntax, types, symbol
897 references, etc).  Thus, you still have to use an #ifdef if the code inside the
898 block references symbols that will not exist if the condition is not met.
899 
900 At the end of any non-trivial #if or #ifdef block (more than a few lines),
901 place a comment after the #endif on the same line, noting the conditional
902 expression used.  For instance:
903 
904 #ifdef CONFIG_SOMETHING
905 ...
906 #endif /* CONFIG_SOMETHING */
907 
908 
909                 Appendix I: References
910 
911 The C Programming Language, Second Edition
912 by Brian W. Kernighan and Dennis M. Ritchie.
913 Prentice Hall, Inc., 1988.
914 ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
915 URL: http://cm.bell-labs.com/cm/cs/cbook/
916 
917 The Practice of Programming
918 by Brian W. Kernighan and Rob Pike.
919 Addison-Wesley, Inc., 1999.
920 ISBN 0-201-61586-X.
921 URL: http://cm.bell-labs.com/cm/cs/tpop/
922 
923 GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
924 gcc internals and indent, all available from http://www.gnu.org/manual/
925 
926 WG14 is the international standardization working group for the programming
927 language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
928 
929 Kernel CodingStyle, by greg@kroah.com at OLS 2002:
930 http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
931 

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