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

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