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Linux/Documentation/CodingStyle

  1 .. _codingstyle:
  2 
  3 Linux kernel coding style
  4 =========================
  5 
  6 This is a short document describing the preferred coding style for the
  7 linux kernel.  Coding style is very personal, and I won't **force** my
  8 views on anybody, but this is what goes for anything that I have to be
  9 able to maintain, and I'd prefer it for most other things too.  Please
 10 at least consider the points made here.
 11 
 12 First off, I'd suggest printing out a copy of the GNU coding standards,
 13 and NOT read it.  Burn them, it's a great symbolic gesture.
 14 
 15 Anyway, here goes:
 16 
 17 
 18 1) Indentation
 19 --------------
 20 
 21 Tabs are 8 characters, and thus indentations are also 8 characters.
 22 There are heretic movements that try to make indentations 4 (or even 2!)
 23 characters deep, and that is akin to trying to define the value of PI to
 24 be 3.
 25 
 26 Rationale: The whole idea behind indentation is to clearly define where
 27 a block of control starts and ends.  Especially when you've been looking
 28 at your screen for 20 straight hours, you'll find it a lot easier to see
 29 how the indentation works if you have large indentations.
 30 
 31 Now, some people will claim that having 8-character indentations makes
 32 the code move too far to the right, and makes it hard to read on a
 33 80-character terminal screen.  The answer to that is that if you need
 34 more than 3 levels of indentation, you're screwed anyway, and should fix
 35 your program.
 36 
 37 In short, 8-char indents make things easier to read, and have the added
 38 benefit of warning you when you're nesting your functions too deep.
 39 Heed that warning.
 40 
 41 The preferred way to ease multiple indentation levels in a switch statement is
 42 to align the ``switch`` and its subordinate ``case`` labels in the same column
 43 instead of ``double-indenting`` the ``case`` labels.  E.g.:
 44 
 45 .. code-block:: c
 46 
 47         switch (suffix) {
 48         case 'G':
 49         case 'g':
 50                 mem <<= 30;
 51                 break;
 52         case 'M':
 53         case 'm':
 54                 mem <<= 20;
 55                 break;
 56         case 'K':
 57         case 'k':
 58                 mem <<= 10;
 59                 /* fall through */
 60         default:
 61                 break;
 62         }
 63 
 64 Don't put multiple statements on a single line unless you have
 65 something to hide:
 66 
 67 .. code-block:: c
 68 
 69         if (condition) do_this;
 70           do_something_everytime;
 71 
 72 Don't put multiple assignments on a single line either.  Kernel coding style
 73 is super simple.  Avoid tricky expressions.
 74 
 75 Outside of comments, documentation and except in Kconfig, spaces are never
 76 used for indentation, and the above example is deliberately broken.
 77 
 78 Get a decent editor and don't leave whitespace at the end of lines.
 79 
 80 
 81 2) Breaking long lines and strings
 82 ----------------------------------
 83 
 84 Coding style is all about readability and maintainability using commonly
 85 available tools.
 86 
 87 The limit on the length of lines is 80 columns and this is a strongly
 88 preferred limit.
 89 
 90 Statements longer than 80 columns will be broken into sensible chunks, unless
 91 exceeding 80 columns significantly increases readability and does not hide
 92 information. Descendants are always substantially shorter than the parent and
 93 are placed substantially to the right. The same applies to function headers
 94 with a long argument list. However, never break user-visible strings such as
 95 printk messages, because that breaks the ability to grep for them.
 96 
 97 
 98 3) Placing Braces and Spaces
 99 ----------------------------
100 
101 The other issue that always comes up in C styling is the placement of
102 braces.  Unlike the indent size, there are few technical reasons to
103 choose one placement strategy over the other, but the preferred way, as
104 shown to us by the prophets Kernighan and Ritchie, is to put the opening
105 brace last on the line, and put the closing brace first, thusly:
106 
107 .. code-block:: c
108 
109         if (x is true) {
110                 we do y
111         }
112 
113 This applies to all non-function statement blocks (if, switch, for,
114 while, do).  E.g.:
115 
116 .. code-block:: c
117 
118         switch (action) {
119         case KOBJ_ADD:
120                 return "add";
121         case KOBJ_REMOVE:
122                 return "remove";
123         case KOBJ_CHANGE:
124                 return "change";
125         default:
126                 return NULL;
127         }
128 
129 However, there is one special case, namely functions: they have the
130 opening brace at the beginning of the next line, thus:
131 
132 .. code-block:: c
133 
134         int function(int x)
135         {
136                 body of function
137         }
138 
139 Heretic people all over the world have claimed that this inconsistency
140 is ...  well ...  inconsistent, but all right-thinking people know that
141 (a) K&R are **right** and (b) K&R are right.  Besides, functions are
142 special anyway (you can't nest them in C).
143 
144 Note that the closing brace is empty on a line of its own, **except** in
145 the cases where it is followed by a continuation of the same statement,
146 ie a ``while`` in a do-statement or an ``else`` in an if-statement, like
147 this:
148 
149 .. code-block:: c
150 
151         do {
152                 body of do-loop
153         } while (condition);
154 
155 and
156 
157 .. code-block:: c
158 
159         if (x == y) {
160                 ..
161         } else if (x > y) {
162                 ...
163         } else {
164                 ....
165         }
166 
167 Rationale: K&R.
168 
169 Also, note that this brace-placement also minimizes the number of empty
170 (or almost empty) lines, without any loss of readability.  Thus, as the
171 supply of new-lines on your screen is not a renewable resource (think
172 25-line terminal screens here), you have more empty lines to put
173 comments on.
174 
175 Do not unnecessarily use braces where a single statement will do.
176 
177 .. code-block:: c
178 
179         if (condition)
180                 action();
181 
182 and
183 
184 .. code-block:: none
185 
186         if (condition)
187                 do_this();
188         else
189                 do_that();
190 
191 This does not apply if only one branch of a conditional statement is a single
192 statement; in the latter case use braces in both branches:
193 
194 .. code-block:: c
195 
196         if (condition) {
197                 do_this();
198                 do_that();
199         } else {
200                 otherwise();
201         }
202 
203 3.1) Spaces
204 ***********
205 
206 Linux kernel style for use of spaces depends (mostly) on
207 function-versus-keyword usage.  Use a space after (most) keywords.  The
208 notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
209 somewhat like functions (and are usually used with parentheses in Linux,
210 although they are not required in the language, as in: ``sizeof info`` after
211 ``struct fileinfo info;`` is declared).
212 
213 So use a space after these keywords::
214 
215         if, switch, case, for, do, while
216 
217 but not with sizeof, typeof, alignof, or __attribute__.  E.g.,
218 
219 .. code-block:: c
220 
221 
222         s = sizeof(struct file);
223 
224 Do not add spaces around (inside) parenthesized expressions.  This example is
225 **bad**:
226 
227 .. code-block:: c
228 
229 
230         s = sizeof( struct file );
231 
232 When declaring pointer data or a function that returns a pointer type, the
233 preferred use of ``*`` is adjacent to the data name or function name and not
234 adjacent to the type name.  Examples:
235 
236 .. code-block:: c
237 
238 
239         char *linux_banner;
240         unsigned long long memparse(char *ptr, char **retptr);
241         char *match_strdup(substring_t *s);
242 
243 Use one space around (on each side of) most binary and ternary operators,
244 such as any of these::
245 
246         =  +  -  <  >  *  /  %  |  &  ^  <=  >=  ==  !=  ?  :
247 
248 but no space after unary operators::
249 
250         &  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined
251 
252 no space before the postfix increment & decrement unary operators::
253 
254         ++  --
255 
256 no space after the prefix increment & decrement unary operators::
257 
258         ++  --
259 
260 and no space around the ``.`` and ``->`` structure member operators.
261 
262 Do not leave trailing whitespace at the ends of lines.  Some editors with
263 ``smart`` indentation will insert whitespace at the beginning of new lines as
264 appropriate, so you can start typing the next line of code right away.
265 However, some such editors do not remove the whitespace if you end up not
266 putting a line of code there, such as if you leave a blank line.  As a result,
267 you end up with lines containing trailing whitespace.
268 
269 Git will warn you about patches that introduce trailing whitespace, and can
270 optionally strip the trailing whitespace for you; however, if applying a series
271 of patches, this may make later patches in the series fail by changing their
272 context lines.
273 
274 
275 4) Naming
276 ---------
277 
278 C is a Spartan language, and so should your naming be.  Unlike Modula-2
279 and Pascal programmers, C programmers do not use cute names like
280 ThisVariableIsATemporaryCounter.  A C programmer would call that
281 variable ``tmp``, which is much easier to write, and not the least more
282 difficult to understand.
283 
284 HOWEVER, while mixed-case names are frowned upon, descriptive names for
285 global variables are a must.  To call a global function ``foo`` is a
286 shooting offense.
287 
288 GLOBAL variables (to be used only if you **really** need them) need to
289 have descriptive names, as do global functions.  If you have a function
290 that counts the number of active users, you should call that
291 ``count_active_users()`` or similar, you should **not** call it ``cntusr()``.
292 
293 Encoding the type of a function into the name (so-called Hungarian
294 notation) is brain damaged - the compiler knows the types anyway and can
295 check those, and it only confuses the programmer.  No wonder MicroSoft
296 makes buggy programs.
297 
298 LOCAL variable names should be short, and to the point.  If you have
299 some random integer loop counter, it should probably be called ``i``.
300 Calling it ``loop_counter`` is non-productive, if there is no chance of it
301 being mis-understood.  Similarly, ``tmp`` can be just about any type of
302 variable that is used to hold a temporary value.
303 
304 If you are afraid to mix up your local variable names, you have another
305 problem, which is called the function-growth-hormone-imbalance syndrome.
306 See chapter 6 (Functions).
307 
308 
309 5) Typedefs
310 -----------
311 
312 Please don't use things like ``vps_t``.
313 It's a **mistake** to use typedef for structures and pointers. When you see a
314 
315 .. code-block:: c
316 
317 
318         vps_t a;
319 
320 in the source, what does it mean?
321 In contrast, if it says
322 
323 .. code-block:: c
324 
325         struct virtual_container *a;
326 
327 you can actually tell what ``a`` is.
328 
329 Lots of people think that typedefs ``help readability``. Not so. They are
330 useful only for:
331 
332  (a) totally opaque objects (where the typedef is actively used to **hide**
333      what the object is).
334 
335      Example: ``pte_t`` etc. opaque objects that you can only access using
336      the proper accessor functions.
337 
338      .. note::
339 
340        Opaqueness and ``accessor functions`` are not good in themselves.
341        The reason we have them for things like pte_t etc. is that there
342        really is absolutely **zero** portably accessible information there.
343 
344  (b) Clear integer types, where the abstraction **helps** avoid confusion
345      whether it is ``int`` or ``long``.
346 
347      u8/u16/u32 are perfectly fine typedefs, although they fit into
348      category (d) better than here.
349 
350      .. note::
351 
352        Again - there needs to be a **reason** for this. If something is
353        ``unsigned long``, then there's no reason to do
354 
355         typedef unsigned long myflags_t;
356 
357      but if there is a clear reason for why it under certain circumstances
358      might be an ``unsigned int`` and under other configurations might be
359      ``unsigned long``, then by all means go ahead and use a typedef.
360 
361  (c) when you use sparse to literally create a **new** type for
362      type-checking.
363 
364  (d) New types which are identical to standard C99 types, in certain
365      exceptional circumstances.
366 
367      Although it would only take a short amount of time for the eyes and
368      brain to become accustomed to the standard types like ``uint32_t``,
369      some people object to their use anyway.
370 
371      Therefore, the Linux-specific ``u8/u16/u32/u64`` types and their
372      signed equivalents which are identical to standard types are
373      permitted -- although they are not mandatory in new code of your
374      own.
375 
376      When editing existing code which already uses one or the other set
377      of types, you should conform to the existing choices in that code.
378 
379  (e) Types safe for use in userspace.
380 
381      In certain structures which are visible to userspace, we cannot
382      require C99 types and cannot use the ``u32`` form above. Thus, we
383      use __u32 and similar types in all structures which are shared
384      with userspace.
385 
386 Maybe there are other cases too, but the rule should basically be to NEVER
387 EVER use a typedef unless you can clearly match one of those rules.
388 
389 In general, a pointer, or a struct that has elements that can reasonably
390 be directly accessed should **never** be a typedef.
391 
392 
393 6) Functions
394 ------------
395 
396 Functions should be short and sweet, and do just one thing.  They should
397 fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
398 as we all know), and do one thing and do that well.
399 
400 The maximum length of a function is inversely proportional to the
401 complexity and indentation level of that function.  So, if you have a
402 conceptually simple function that is just one long (but simple)
403 case-statement, where you have to do lots of small things for a lot of
404 different cases, it's OK to have a longer function.
405 
406 However, if you have a complex function, and you suspect that a
407 less-than-gifted first-year high-school student might not even
408 understand what the function is all about, you should adhere to the
409 maximum limits all the more closely.  Use helper functions with
410 descriptive names (you can ask the compiler to in-line them if you think
411 it's performance-critical, and it will probably do a better job of it
412 than you would have done).
413 
414 Another measure of the function is the number of local variables.  They
415 shouldn't exceed 5-10, or you're doing something wrong.  Re-think the
416 function, and split it into smaller pieces.  A human brain can
417 generally easily keep track of about 7 different things, anything more
418 and it gets confused.  You know you're brilliant, but maybe you'd like
419 to understand what you did 2 weeks from now.
420 
421 In source files, separate functions with one blank line.  If the function is
422 exported, the **EXPORT** macro for it should follow immediately after the
423 closing function brace line.  E.g.:
424 
425 .. code-block:: c
426 
427         int system_is_up(void)
428         {
429                 return system_state == SYSTEM_RUNNING;
430         }
431         EXPORT_SYMBOL(system_is_up);
432 
433 In function prototypes, include parameter names with their data types.
434 Although this is not required by the C language, it is preferred in Linux
435 because it is a simple way to add valuable information for the reader.
436 
437 
438 7) Centralized exiting of functions
439 -----------------------------------
440 
441 Albeit deprecated by some people, the equivalent of the goto statement is
442 used frequently by compilers in form of the unconditional jump instruction.
443 
444 The goto statement comes in handy when a function exits from multiple
445 locations and some common work such as cleanup has to be done.  If there is no
446 cleanup needed then just return directly.
447 
448 Choose label names which say what the goto does or why the goto exists.  An
449 example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``.
450 Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to
451 renumber them if you ever add or remove exit paths, and they make correctness
452 difficult to verify anyway.
453 
454 The rationale for using gotos is:
455 
456 - unconditional statements are easier to understand and follow
457 - nesting is reduced
458 - errors by not updating individual exit points when making
459   modifications are prevented
460 - saves the compiler work to optimize redundant code away ;)
461 
462 .. code-block:: c
463 
464         int fun(int a)
465         {
466                 int result = 0;
467                 char *buffer;
468 
469                 buffer = kmalloc(SIZE, GFP_KERNEL);
470                 if (!buffer)
471                         return -ENOMEM;
472 
473                 if (condition1) {
474                         while (loop1) {
475                                 ...
476                         }
477                         result = 1;
478                         goto out_buffer;
479                 }
480                 ...
481         out_free_buffer:
482                 kfree(buffer);
483                 return result;
484         }
485 
486 A common type of bug to be aware of is ``one err bugs`` which look like this:
487 
488 .. code-block:: c
489 
490         err:
491                 kfree(foo->bar);
492                 kfree(foo);
493                 return ret;
494 
495 The bug in this code is that on some exit paths ``foo`` is NULL.  Normally the
496 fix for this is to split it up into two error labels ``err_free_bar:`` and
497 ``err_free_foo:``:
498 
499 .. code-block:: c
500 
501          err_free_bar:
502                 kfree(foo->bar);
503          err_free_foo:
504                 kfree(foo);
505                 return ret;
506 
507 Ideally you should simulate errors to test all exit paths.
508 
509 
510 8) Commenting
511 -------------
512 
513 Comments are good, but there is also a danger of over-commenting.  NEVER
514 try to explain HOW your code works in a comment: it's much better to
515 write the code so that the **working** is obvious, and it's a waste of
516 time to explain badly written code.
517 
518 Generally, you want your comments to tell WHAT your code does, not HOW.
519 Also, try to avoid putting comments inside a function body: if the
520 function is so complex that you need to separately comment parts of it,
521 you should probably go back to chapter 6 for a while.  You can make
522 small comments to note or warn about something particularly clever (or
523 ugly), but try to avoid excess.  Instead, put the comments at the head
524 of the function, telling people what it does, and possibly WHY it does
525 it.
526 
527 When commenting the kernel API functions, please use the kernel-doc format.
528 See the files Documentation/kernel-documentation.rst and scripts/kernel-doc
529 for details.
530 
531 The preferred style for long (multi-line) comments is:
532 
533 .. code-block:: c
534 
535         /*
536          * This is the preferred style for multi-line
537          * comments in the Linux kernel source code.
538          * Please use it consistently.
539          *
540          * Description:  A column of asterisks on the left side,
541          * with beginning and ending almost-blank lines.
542          */
543 
544 For files in net/ and drivers/net/ the preferred style for long (multi-line)
545 comments is a little different.
546 
547 .. code-block:: c
548 
549         /* The preferred comment style for files in net/ and drivers/net
550          * looks like this.
551          *
552          * It is nearly the same as the generally preferred comment style,
553          * but there is no initial almost-blank line.
554          */
555 
556 It's also important to comment data, whether they are basic types or derived
557 types.  To this end, use just one data declaration per line (no commas for
558 multiple data declarations).  This leaves you room for a small comment on each
559 item, explaining its use.
560 
561 
562 9) You've made a mess of it
563 ---------------------------
564 
565 That's OK, we all do.  You've probably been told by your long-time Unix
566 user helper that ``GNU emacs`` automatically formats the C sources for
567 you, and you've noticed that yes, it does do that, but the defaults it
568 uses are less than desirable (in fact, they are worse than random
569 typing - an infinite number of monkeys typing into GNU emacs would never
570 make a good program).
571 
572 So, you can either get rid of GNU emacs, or change it to use saner
573 values.  To do the latter, you can stick the following in your .emacs file:
574 
575 .. code-block:: none
576 
577   (defun c-lineup-arglist-tabs-only (ignored)
578     "Line up argument lists by tabs, not spaces"
579     (let* ((anchor (c-langelem-pos c-syntactic-element))
580            (column (c-langelem-2nd-pos c-syntactic-element))
581            (offset (- (1+ column) anchor))
582            (steps (floor offset c-basic-offset)))
583       (* (max steps 1)
584          c-basic-offset)))
585 
586   (add-hook 'c-mode-common-hook
587             (lambda ()
588               ;; Add kernel style
589               (c-add-style
590                "linux-tabs-only"
591                '("linux" (c-offsets-alist
592                           (arglist-cont-nonempty
593                            c-lineup-gcc-asm-reg
594                            c-lineup-arglist-tabs-only))))))
595 
596   (add-hook 'c-mode-hook
597             (lambda ()
598               (let ((filename (buffer-file-name)))
599                 ;; Enable kernel mode for the appropriate files
600                 (when (and filename
601                            (string-match (expand-file-name "~/src/linux-trees")
602                                          filename))
603                   (setq indent-tabs-mode t)
604                   (setq show-trailing-whitespace t)
605                   (c-set-style "linux-tabs-only")))))
606 
607 This will make emacs go better with the kernel coding style for C
608 files below ``~/src/linux-trees``.
609 
610 But even if you fail in getting emacs to do sane formatting, not
611 everything is lost: use ``indent``.
612 
613 Now, again, GNU indent has the same brain-dead settings that GNU emacs
614 has, which is why you need to give it a few command line options.
615 However, that's not too bad, because even the makers of GNU indent
616 recognize the authority of K&R (the GNU people aren't evil, they are
617 just severely misguided in this matter), so you just give indent the
618 options ``-kr -i8`` (stands for ``K&R, 8 character indents``), or use
619 ``scripts/Lindent``, which indents in the latest style.
620 
621 ``indent`` has a lot of options, and especially when it comes to comment
622 re-formatting you may want to take a look at the man page.  But
623 remember: ``indent`` is not a fix for bad programming.
624 
625 
626 10) Kconfig configuration files
627 -------------------------------
628 
629 For all of the Kconfig* configuration files throughout the source tree,
630 the indentation is somewhat different.  Lines under a ``config`` definition
631 are indented with one tab, while help text is indented an additional two
632 spaces.  Example::
633 
634   config AUDIT
635         bool "Auditing support"
636         depends on NET
637         help
638           Enable auditing infrastructure that can be used with another
639           kernel subsystem, such as SELinux (which requires this for
640           logging of avc messages output).  Does not do system-call
641           auditing without CONFIG_AUDITSYSCALL.
642 
643 Seriously dangerous features (such as write support for certain
644 filesystems) should advertise this prominently in their prompt string::
645 
646   config ADFS_FS_RW
647         bool "ADFS write support (DANGEROUS)"
648         depends on ADFS_FS
649         ...
650 
651 For full documentation on the configuration files, see the file
652 Documentation/kbuild/kconfig-language.txt.
653 
654 
655 11) Data structures
656 -------------------
657 
658 Data structures that have visibility outside the single-threaded
659 environment they are created and destroyed in should always have
660 reference counts.  In the kernel, garbage collection doesn't exist (and
661 outside the kernel garbage collection is slow and inefficient), which
662 means that you absolutely **have** to reference count all your uses.
663 
664 Reference counting means that you can avoid locking, and allows multiple
665 users to have access to the data structure in parallel - and not having
666 to worry about the structure suddenly going away from under them just
667 because they slept or did something else for a while.
668 
669 Note that locking is **not** a replacement for reference counting.
670 Locking is used to keep data structures coherent, while reference
671 counting is a memory management technique.  Usually both are needed, and
672 they are not to be confused with each other.
673 
674 Many data structures can indeed have two levels of reference counting,
675 when there are users of different ``classes``.  The subclass count counts
676 the number of subclass users, and decrements the global count just once
677 when the subclass count goes to zero.
678 
679 Examples of this kind of ``multi-level-reference-counting`` can be found in
680 memory management (``struct mm_struct``: mm_users and mm_count), and in
681 filesystem code (``struct super_block``: s_count and s_active).
682 
683 Remember: if another thread can find your data structure, and you don't
684 have a reference count on it, you almost certainly have a bug.
685 
686 
687 12) Macros, Enums and RTL
688 -------------------------
689 
690 Names of macros defining constants and labels in enums are capitalized.
691 
692 .. code-block:: c
693 
694         #define CONSTANT 0x12345
695 
696 Enums are preferred when defining several related constants.
697 
698 CAPITALIZED macro names are appreciated but macros resembling functions
699 may be named in lower case.
700 
701 Generally, inline functions are preferable to macros resembling functions.
702 
703 Macros with multiple statements should be enclosed in a do - while block:
704 
705 .. code-block:: c
706 
707         #define macrofun(a, b, c)                       \
708                 do {                                    \
709                         if (a == 5)                     \
710                                 do_this(b, c);          \
711                 } while (0)
712 
713 Things to avoid when using macros:
714 
715 1) macros that affect control flow:
716 
717 .. code-block:: c
718 
719         #define FOO(x)                                  \
720                 do {                                    \
721                         if (blah(x) < 0)                \
722                                 return -EBUGGERED;      \
723                 } while (0)
724 
725 is a **very** bad idea.  It looks like a function call but exits the ``calling``
726 function; don't break the internal parsers of those who will read the code.
727 
728 2) macros that depend on having a local variable with a magic name:
729 
730 .. code-block:: c
731 
732         #define FOO(val) bar(index, val)
733 
734 might look like a good thing, but it's confusing as hell when one reads the
735 code and it's prone to breakage from seemingly innocent changes.
736 
737 3) macros with arguments that are used as l-values: FOO(x) = y; will
738 bite you if somebody e.g. turns FOO into an inline function.
739 
740 4) forgetting about precedence: macros defining constants using expressions
741 must enclose the expression in parentheses. Beware of similar issues with
742 macros using parameters.
743 
744 .. code-block:: c
745 
746         #define CONSTANT 0x4000
747         #define CONSTEXP (CONSTANT | 3)
748 
749 5) namespace collisions when defining local variables in macros resembling
750 functions:
751 
752 .. code-block:: c
753 
754         #define FOO(x)                          \
755         ({                                      \
756                 typeof(x) ret;                  \
757                 ret = calc_ret(x);              \
758                 (ret);                          \
759         })
760 
761 ret is a common name for a local variable - __foo_ret is less likely
762 to collide with an existing variable.
763 
764 The cpp manual deals with macros exhaustively. The gcc internals manual also
765 covers RTL which is used frequently with assembly language in the kernel.
766 
767 
768 13) Printing kernel messages
769 ----------------------------
770 
771 Kernel developers like to be seen as literate. Do mind the spelling
772 of kernel messages to make a good impression. Do not use crippled
773 words like ``dont``; use ``do not`` or ``don't`` instead.  Make the messages
774 concise, clear, and unambiguous.
775 
776 Kernel messages do not have to be terminated with a period.
777 
778 Printing numbers in parentheses (%d) adds no value and should be avoided.
779 
780 There are a number of driver model diagnostic macros in <linux/device.h>
781 which you should use to make sure messages are matched to the right device
782 and driver, and are tagged with the right level:  dev_err(), dev_warn(),
783 dev_info(), and so forth.  For messages that aren't associated with a
784 particular device, <linux/printk.h> defines pr_notice(), pr_info(),
785 pr_warn(), pr_err(), etc.
786 
787 Coming up with good debugging messages can be quite a challenge; and once
788 you have them, they can be a huge help for remote troubleshooting.  However
789 debug message printing is handled differently than printing other non-debug
790 messages.  While the other pr_XXX() functions print unconditionally,
791 pr_debug() does not; it is compiled out by default, unless either DEBUG is
792 defined or CONFIG_DYNAMIC_DEBUG is set.  That is true for dev_dbg() also,
793 and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
794 the ones already enabled by DEBUG.
795 
796 Many subsystems have Kconfig debug options to turn on -DDEBUG in the
797 corresponding Makefile; in other cases specific files #define DEBUG.  And
798 when a debug message should be unconditionally printed, such as if it is
799 already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
800 used.
801 
802 
803 14) Allocating memory
804 ---------------------
805 
806 The kernel provides the following general purpose memory allocators:
807 kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
808 vzalloc().  Please refer to the API documentation for further information
809 about them.
810 
811 The preferred form for passing a size of a struct is the following:
812 
813 .. code-block:: c
814 
815         p = kmalloc(sizeof(*p), ...);
816 
817 The alternative form where struct name is spelled out hurts readability and
818 introduces an opportunity for a bug when the pointer variable type is changed
819 but the corresponding sizeof that is passed to a memory allocator is not.
820 
821 Casting the return value which is a void pointer is redundant. The conversion
822 from void pointer to any other pointer type is guaranteed by the C programming
823 language.
824 
825 The preferred form for allocating an array is the following:
826 
827 .. code-block:: c
828 
829         p = kmalloc_array(n, sizeof(...), ...);
830 
831 The preferred form for allocating a zeroed array is the following:
832 
833 .. code-block:: c
834 
835         p = kcalloc(n, sizeof(...), ...);
836 
837 Both forms check for overflow on the allocation size n * sizeof(...),
838 and return NULL if that occurred.
839 
840 
841 15) The inline disease
842 ----------------------
843 
844 There appears to be a common misperception that gcc has a magic "make me
845 faster" speedup option called ``inline``. While the use of inlines can be
846 appropriate (for example as a means of replacing macros, see Chapter 12), it
847 very often is not. Abundant use of the inline keyword leads to a much bigger
848 kernel, which in turn slows the system as a whole down, due to a bigger
849 icache footprint for the CPU and simply because there is less memory
850 available for the pagecache. Just think about it; a pagecache miss causes a
851 disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
852 that can go into these 5 milliseconds.
853 
854 A reasonable rule of thumb is to not put inline at functions that have more
855 than 3 lines of code in them. An exception to this rule are the cases where
856 a parameter is known to be a compiletime constant, and as a result of this
857 constantness you *know* the compiler will be able to optimize most of your
858 function away at compile time. For a good example of this later case, see
859 the kmalloc() inline function.
860 
861 Often people argue that adding inline to functions that are static and used
862 only once is always a win since there is no space tradeoff. While this is
863 technically correct, gcc is capable of inlining these automatically without
864 help, and the maintenance issue of removing the inline when a second user
865 appears outweighs the potential value of the hint that tells gcc to do
866 something it would have done anyway.
867 
868 
869 16) Function return values and names
870 ------------------------------------
871 
872 Functions can return values of many different kinds, and one of the
873 most common is a value indicating whether the function succeeded or
874 failed.  Such a value can be represented as an error-code integer
875 (-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure,
876 non-zero = success).
877 
878 Mixing up these two sorts of representations is a fertile source of
879 difficult-to-find bugs.  If the C language included a strong distinction
880 between integers and booleans then the compiler would find these mistakes
881 for us... but it doesn't.  To help prevent such bugs, always follow this
882 convention::
883 
884         If the name of a function is an action or an imperative command,
885         the function should return an error-code integer.  If the name
886         is a predicate, the function should return a "succeeded" boolean.
887 
888 For example, ``add work`` is a command, and the add_work() function returns 0
889 for success or -EBUSY for failure.  In the same way, ``PCI device present`` is
890 a predicate, and the pci_dev_present() function returns 1 if it succeeds in
891 finding a matching device or 0 if it doesn't.
892 
893 All EXPORTed functions must respect this convention, and so should all
894 public functions.  Private (static) functions need not, but it is
895 recommended that they do.
896 
897 Functions whose return value is the actual result of a computation, rather
898 than an indication of whether the computation succeeded, are not subject to
899 this rule.  Generally they indicate failure by returning some out-of-range
900 result.  Typical examples would be functions that return pointers; they use
901 NULL or the ERR_PTR mechanism to report failure.
902 
903 
904 17) Don't re-invent the kernel macros
905 -------------------------------------
906 
907 The header file include/linux/kernel.h contains a number of macros that
908 you should use, rather than explicitly coding some variant of them yourself.
909 For example, if you need to calculate the length of an array, take advantage
910 of the macro
911 
912 .. code-block:: c
913 
914         #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
915 
916 Similarly, if you need to calculate the size of some structure member, use
917 
918 .. code-block:: c
919 
920         #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
921 
922 There are also min() and max() macros that do strict type checking if you
923 need them.  Feel free to peruse that header file to see what else is already
924 defined that you shouldn't reproduce in your code.
925 
926 
927 18) Editor modelines and other cruft
928 ------------------------------------
929 
930 Some editors can interpret configuration information embedded in source files,
931 indicated with special markers.  For example, emacs interprets lines marked
932 like this:
933 
934 .. code-block:: c
935 
936         -*- mode: c -*-
937 
938 Or like this:
939 
940 .. code-block:: c
941 
942         /*
943         Local Variables:
944         compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
945         End:
946         */
947 
948 Vim interprets markers that look like this:
949 
950 .. code-block:: c
951 
952         /* vim:set sw=8 noet */
953 
954 Do not include any of these in source files.  People have their own personal
955 editor configurations, and your source files should not override them.  This
956 includes markers for indentation and mode configuration.  People may use their
957 own custom mode, or may have some other magic method for making indentation
958 work correctly.
959 
960 
961 19) Inline assembly
962 -------------------
963 
964 In architecture-specific code, you may need to use inline assembly to interface
965 with CPU or platform functionality.  Don't hesitate to do so when necessary.
966 However, don't use inline assembly gratuitously when C can do the job.  You can
967 and should poke hardware from C when possible.
968 
969 Consider writing simple helper functions that wrap common bits of inline
970 assembly, rather than repeatedly writing them with slight variations.  Remember
971 that inline assembly can use C parameters.
972 
973 Large, non-trivial assembly functions should go in .S files, with corresponding
974 C prototypes defined in C header files.  The C prototypes for assembly
975 functions should use ``asmlinkage``.
976 
977 You may need to mark your asm statement as volatile, to prevent GCC from
978 removing it if GCC doesn't notice any side effects.  You don't always need to
979 do so, though, and doing so unnecessarily can limit optimization.
980 
981 When writing a single inline assembly statement containing multiple
982 instructions, put each instruction on a separate line in a separate quoted
983 string, and end each string except the last with \n\t to properly indent the
984 next instruction in the assembly output:
985 
986 .. code-block:: c
987 
988         asm ("magic %reg1, #42\n\t"
989              "more_magic %reg2, %reg3"
990              : /* outputs */ : /* inputs */ : /* clobbers */);
991 
992 
993 20) Conditional Compilation
994 ---------------------------
995 
996 Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
997 files; doing so makes code harder to read and logic harder to follow.  Instead,
998 use such conditionals in a header file defining functions for use in those .c
999 files, providing no-op stub versions in the #else case, and then call those
1000 functions unconditionally from .c files.  The compiler will avoid generating
1001 any code for the stub calls, producing identical results, but the logic will
1002 remain easy to follow.
1003 
1004 Prefer to compile out entire functions, rather than portions of functions or
1005 portions of expressions.  Rather than putting an ifdef in an expression, factor
1006 out part or all of the expression into a separate helper function and apply the
1007 conditional to that function.
1008 
1009 If you have a function or variable which may potentially go unused in a
1010 particular configuration, and the compiler would warn about its definition
1011 going unused, mark the definition as __maybe_unused rather than wrapping it in
1012 a preprocessor conditional.  (However, if a function or variable *always* goes
1013 unused, delete it.)
1014 
1015 Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
1016 symbol into a C boolean expression, and use it in a normal C conditional:
1017 
1018 .. code-block:: c
1019 
1020         if (IS_ENABLED(CONFIG_SOMETHING)) {
1021                 ...
1022         }
1023 
1024 The compiler will constant-fold the conditional away, and include or exclude
1025 the block of code just as with an #ifdef, so this will not add any runtime
1026 overhead.  However, this approach still allows the C compiler to see the code
1027 inside the block, and check it for correctness (syntax, types, symbol
1028 references, etc).  Thus, you still have to use an #ifdef if the code inside the
1029 block references symbols that will not exist if the condition is not met.
1030 
1031 At the end of any non-trivial #if or #ifdef block (more than a few lines),
1032 place a comment after the #endif on the same line, noting the conditional
1033 expression used.  For instance:
1034 
1035 .. code-block:: c
1036 
1037         #ifdef CONFIG_SOMETHING
1038         ...
1039         #endif /* CONFIG_SOMETHING */
1040 
1041 
1042 Appendix I) References
1043 ----------------------
1044 
1045 The C Programming Language, Second Edition
1046 by Brian W. Kernighan and Dennis M. Ritchie.
1047 Prentice Hall, Inc., 1988.
1048 ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
1049 
1050 The Practice of Programming
1051 by Brian W. Kernighan and Rob Pike.
1052 Addison-Wesley, Inc., 1999.
1053 ISBN 0-201-61586-X.
1054 
1055 GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
1056 gcc internals and indent, all available from http://www.gnu.org/manual/
1057 
1058 WG14 is the international standardization working group for the programming
1059 language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
1060 
1061 Kernel CodingStyle, by greg@kroah.com at OLS 2002:
1062 http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/

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