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  2                 Linux kernel coding style
  4 This is a short document describing the preferred coding style for the
  5 linux kernel.  Coding style is very personal, and I won't _force_ my
  6 views on anybody, but this is what goes for anything that I have to be
  7 able to maintain, and I'd prefer it for most other things too.  Please
  8 at least consider the points made here.
 10 First off, I'd suggest printing out a copy of the GNU coding standards,
 11 and NOT read it.  Burn them, it's a great symbolic gesture.
 13 Anyway, here goes:
 16                 Chapter 1: Indentation
 18 Tabs are 8 characters, and thus indentations are also 8 characters.
 19 There are heretic movements that try to make indentations 4 (or even 2!)
 20 characters deep, and that is akin to trying to define the value of PI to
 21 be 3.
 23 Rationale: The whole idea behind indentation is to clearly define where
 24 a block of control starts and ends.  Especially when you've been looking
 25 at your screen for 20 straight hours, you'll find it a lot easier to see
 26 how the indentation works if you have large indentations.
 28 Now, some people will claim that having 8-character indentations makes
 29 the code move too far to the right, and makes it hard to read on a
 30 80-character terminal screen.  The answer to that is that if you need
 31 more than 3 levels of indentation, you're screwed anyway, and should fix
 32 your program.
 34 In short, 8-char indents make things easier to read, and have the added
 35 benefit of warning you when you're nesting your functions too deep.
 36 Heed that warning.
 38 The preferred way to ease multiple indentation levels in a switch statement is
 39 to align the "switch" and its subordinate "case" labels in the same column
 40 instead of "double-indenting" the "case" labels.  E.g.:
 42         switch (suffix) {
 43         case 'G':
 44         case 'g':
 45                 mem <<= 30;
 46                 break;
 47         case 'M':
 48         case 'm':
 49                 mem <<= 20;
 50                 break;
 51         case 'K':
 52         case 'k':
 53                 mem <<= 10;
 54                 /* fall through */
 55         default:
 56                 break;
 57         }
 59 Don't put multiple statements on a single line unless you have
 60 something to hide:
 62         if (condition) do_this;
 63           do_something_everytime;
 65 Don't put multiple assignments on a single line either.  Kernel coding style
 66 is super simple.  Avoid tricky expressions.
 68 Outside of comments, documentation and except in Kconfig, spaces are never
 69 used for indentation, and the above example is deliberately broken.
 71 Get a decent editor and don't leave whitespace at the end of lines.
 74                 Chapter 2: Breaking long lines and strings
 76 Coding style is all about readability and maintainability using commonly
 77 available tools.
 79 The limit on the length of lines is 80 columns and this is a strongly
 80 preferred limit.
 82 Statements longer than 80 columns will be broken into sensible chunks, unless
 83 exceeding 80 columns significantly increases readability and does not hide
 84 information. Descendants are always substantially shorter than the parent and
 85 are placed substantially to the right. The same applies to function headers
 86 with a long argument list. However, never break user-visible strings such as
 87 printk messages, because that breaks the ability to grep for them.
 90                 Chapter 3: Placing Braces and Spaces
 92 The other issue that always comes up in C styling is the placement of
 93 braces.  Unlike the indent size, there are few technical reasons to
 94 choose one placement strategy over the other, but the preferred way, as
 95 shown to us by the prophets Kernighan and Ritchie, is to put the opening
 96 brace last on the line, and put the closing brace first, thusly:
 98         if (x is true) {
 99                 we do y
100         }
102 This applies to all non-function statement blocks (if, switch, for,
103 while, do).  E.g.:
105         switch (action) {
106         case KOBJ_ADD:
107                 return "add";
108         case KOBJ_REMOVE:
109                 return "remove";
110         case KOBJ_CHANGE:
111                 return "change";
112         default:
113                 return NULL;
114         }
116 However, there is one special case, namely functions: they have the
117 opening brace at the beginning of the next line, thus:
119         int function(int x)
120         {
121                 body of function
122         }
124 Heretic people all over the world have claimed that this inconsistency
125 is ...  well ...  inconsistent, but all right-thinking people know that
126 (a) K&R are _right_ and (b) K&R are right.  Besides, functions are
127 special anyway (you can't nest them in C).
129 Note that the closing brace is empty on a line of its own, _except_ in
130 the cases where it is followed by a continuation of the same statement,
131 ie a "while" in a do-statement or an "else" in an if-statement, like
132 this:
134         do {
135                 body of do-loop
136         } while (condition);
138 and
140         if (x == y) {
141                 ..
142         } else if (x > y) {
143                 ...
144         } else {
145                 ....
146         }
148 Rationale: K&R.
150 Also, note that this brace-placement also minimizes the number of empty
151 (or almost empty) lines, without any loss of readability.  Thus, as the
152 supply of new-lines on your screen is not a renewable resource (think
153 25-line terminal screens here), you have more empty lines to put
154 comments on.
156 Do not unnecessarily use braces where a single statement will do.
158         if (condition)
159                 action();
161 and
163         if (condition)
164                 do_this();
165         else
166                 do_that();
168 This does not apply if only one branch of a conditional statement is a single
169 statement; in the latter case use braces in both branches:
171         if (condition) {
172                 do_this();
173                 do_that();
174         } else {
175                 otherwise();
176         }
178                 3.1:  Spaces
180 Linux kernel style for use of spaces depends (mostly) on
181 function-versus-keyword usage.  Use a space after (most) keywords.  The
182 notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
183 somewhat like functions (and are usually used with parentheses in Linux,
184 although they are not required in the language, as in: "sizeof info" after
185 "struct fileinfo info;" is declared).
187 So use a space after these keywords:
189         if, switch, case, for, do, while
191 but not with sizeof, typeof, alignof, or __attribute__.  E.g.,
193         s = sizeof(struct file);
195 Do not add spaces around (inside) parenthesized expressions.  This example is
196 *bad*:
198         s = sizeof( struct file );
200 When declaring pointer data or a function that returns a pointer type, the
201 preferred use of '*' is adjacent to the data name or function name and not
202 adjacent to the type name.  Examples:
204         char *linux_banner;
205         unsigned long long memparse(char *ptr, char **retptr);
206         char *match_strdup(substring_t *s);
208 Use one space around (on each side of) most binary and ternary operators,
209 such as any of these:
211         =  +  -  <  >  *  /  %  |  &  ^  <=  >=  ==  !=  ?  :
213 but no space after unary operators:
215         &  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined
217 no space before the postfix increment & decrement unary operators:
219         ++  --
221 no space after the prefix increment & decrement unary operators:
223         ++  --
225 and no space around the '.' and "->" structure member operators.
227 Do not leave trailing whitespace at the ends of lines.  Some editors with
228 "smart" indentation will insert whitespace at the beginning of new lines as
229 appropriate, so you can start typing the next line of code right away.
230 However, some such editors do not remove the whitespace if you end up not
231 putting a line of code there, such as if you leave a blank line.  As a result,
232 you end up with lines containing trailing whitespace.
234 Git will warn you about patches that introduce trailing whitespace, and can
235 optionally strip the trailing whitespace for you; however, if applying a series
236 of patches, this may make later patches in the series fail by changing their
237 context lines.
240                 Chapter 4: Naming
242 C is a Spartan language, and so should your naming be.  Unlike Modula-2
243 and Pascal programmers, C programmers do not use cute names like
244 ThisVariableIsATemporaryCounter.  A C programmer would call that
245 variable "tmp", which is much easier to write, and not the least more
246 difficult to understand.
248 HOWEVER, while mixed-case names are frowned upon, descriptive names for
249 global variables are a must.  To call a global function "foo" is a
250 shooting offense.
252 GLOBAL variables (to be used only if you _really_ need them) need to
253 have descriptive names, as do global functions.  If you have a function
254 that counts the number of active users, you should call that
255 "count_active_users()" or similar, you should _not_ call it "cntusr()".
257 Encoding the type of a function into the name (so-called Hungarian
258 notation) is brain damaged - the compiler knows the types anyway and can
259 check those, and it only confuses the programmer.  No wonder MicroSoft
260 makes buggy programs.
262 LOCAL variable names should be short, and to the point.  If you have
263 some random integer loop counter, it should probably be called "i".
264 Calling it "loop_counter" is non-productive, if there is no chance of it
265 being mis-understood.  Similarly, "tmp" can be just about any type of
266 variable that is used to hold a temporary value.
268 If you are afraid to mix up your local variable names, you have another
269 problem, which is called the function-growth-hormone-imbalance syndrome.
270 See chapter 6 (Functions).
273                 Chapter 5: Typedefs
275 Please don't use things like "vps_t".
276 It's a _mistake_ to use typedef for structures and pointers. When you see a
278         vps_t a;
280 in the source, what does it mean?
281 In contrast, if it says
283         struct virtual_container *a;
285 you can actually tell what "a" is.
287 Lots of people think that typedefs "help readability". Not so. They are
288 useful only for:
290  (a) totally opaque objects (where the typedef is actively used to _hide_
291      what the object is).
293      Example: "pte_t" etc. opaque objects that you can only access using
294      the proper accessor functions.
296      NOTE! Opaqueness and "accessor functions" are not good in themselves.
297      The reason we have them for things like pte_t etc. is that there
298      really is absolutely _zero_ portably accessible information there.
300  (b) Clear integer types, where the abstraction _helps_ avoid confusion
301      whether it is "int" or "long".
303      u8/u16/u32 are perfectly fine typedefs, although they fit into
304      category (d) better than here.
306      NOTE! Again - there needs to be a _reason_ for this. If something is
307      "unsigned long", then there's no reason to do
309         typedef unsigned long myflags_t;
311      but if there is a clear reason for why it under certain circumstances
312      might be an "unsigned int" and under other configurations might be
313      "unsigned long", then by all means go ahead and use a typedef.
315  (c) when you use sparse to literally create a _new_ type for
316      type-checking.
318  (d) New types which are identical to standard C99 types, in certain
319      exceptional circumstances.
321      Although it would only take a short amount of time for the eyes and
322      brain to become accustomed to the standard types like 'uint32_t',
323      some people object to their use anyway.
325      Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
326      signed equivalents which are identical to standard types are
327      permitted -- although they are not mandatory in new code of your
328      own.
330      When editing existing code which already uses one or the other set
331      of types, you should conform to the existing choices in that code.
333  (e) Types safe for use in userspace.
335      In certain structures which are visible to userspace, we cannot
336      require C99 types and cannot use the 'u32' form above. Thus, we
337      use __u32 and similar types in all structures which are shared
338      with userspace.
340 Maybe there are other cases too, but the rule should basically be to NEVER
341 EVER use a typedef unless you can clearly match one of those rules.
343 In general, a pointer, or a struct that has elements that can reasonably
344 be directly accessed should _never_ be a typedef.
347                 Chapter 6: Functions
349 Functions should be short and sweet, and do just one thing.  They should
350 fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
351 as we all know), and do one thing and do that well.
353 The maximum length of a function is inversely proportional to the
354 complexity and indentation level of that function.  So, if you have a
355 conceptually simple function that is just one long (but simple)
356 case-statement, where you have to do lots of small things for a lot of
357 different cases, it's OK to have a longer function.
359 However, if you have a complex function, and you suspect that a
360 less-than-gifted first-year high-school student might not even
361 understand what the function is all about, you should adhere to the
362 maximum limits all the more closely.  Use helper functions with
363 descriptive names (you can ask the compiler to in-line them if you think
364 it's performance-critical, and it will probably do a better job of it
365 than you would have done).
367 Another measure of the function is the number of local variables.  They
368 shouldn't exceed 5-10, or you're doing something wrong.  Re-think the
369 function, and split it into smaller pieces.  A human brain can
370 generally easily keep track of about 7 different things, anything more
371 and it gets confused.  You know you're brilliant, but maybe you'd like
372 to understand what you did 2 weeks from now.
374 In source files, separate functions with one blank line.  If the function is
375 exported, the EXPORT* macro for it should follow immediately after the closing
376 function brace line.  E.g.:
378         int system_is_up(void)
379         {
380                 return system_state == SYSTEM_RUNNING;
381         }
382         EXPORT_SYMBOL(system_is_up);
384 In function prototypes, include parameter names with their data types.
385 Although this is not required by the C language, it is preferred in Linux
386 because it is a simple way to add valuable information for the reader.
389                 Chapter 7: Centralized exiting of functions
391 Albeit deprecated by some people, the equivalent of the goto statement is
392 used frequently by compilers in form of the unconditional jump instruction.
394 The goto statement comes in handy when a function exits from multiple
395 locations and some common work such as cleanup has to be done.  If there is no
396 cleanup needed then just return directly.
398 Choose label names which say what the goto does or why the goto exists.  An
399 example of a good name could be "out_buffer:" if the goto frees "buffer".  Avoid
400 using GW-BASIC names like "err1:" and "err2:".  Also don't name them after the
401 goto location like "err_kmalloc_failed:"
403 The rationale for using gotos is:
405 - unconditional statements are easier to understand and follow
406 - nesting is reduced
407 - errors by not updating individual exit points when making
408     modifications are prevented
409 - saves the compiler work to optimize redundant code away ;)
411         int fun(int a)
412         {
413                 int result = 0;
414                 char *buffer;
416                 buffer = kmalloc(SIZE, GFP_KERNEL);
417                 if (!buffer)
418                         return -ENOMEM;
420                 if (condition1) {
421                         while (loop1) {
422                                 ...
423                         }
424                         result = 1;
425                         goto out_buffer;
426                 }
427                 ...
428         out_buffer:
429                 kfree(buffer);
430                 return result;
431         }
433 A common type of bug to be aware of is "one err bugs" which look like this:
435         err:
436                 kfree(foo->bar);
437                 kfree(foo);
438                 return ret;
440 The bug in this code is that on some exit paths "foo" is NULL.  Normally the
441 fix for this is to split it up into two error labels "err_bar:" and "err_foo:".
444                 Chapter 8: Commenting
446 Comments are good, but there is also a danger of over-commenting.  NEVER
447 try to explain HOW your code works in a comment: it's much better to
448 write the code so that the _working_ is obvious, and it's a waste of
449 time to explain badly written code.
451 Generally, you want your comments to tell WHAT your code does, not HOW.
452 Also, try to avoid putting comments inside a function body: if the
453 function is so complex that you need to separately comment parts of it,
454 you should probably go back to chapter 6 for a while.  You can make
455 small comments to note or warn about something particularly clever (or
456 ugly), but try to avoid excess.  Instead, put the comments at the head
457 of the function, telling people what it does, and possibly WHY it does
458 it.
460 When commenting the kernel API functions, please use the kernel-doc format.
461 See the files Documentation/kernel-documentation.rst and scripts/kernel-doc
462 for details.
464 Linux style for comments is the C89 "/* ... */" style.
465 Don't use C99-style "// ..." comments.
467 The preferred style for long (multi-line) comments is:
469         /*
470          * This is the preferred style for multi-line
471          * comments in the Linux kernel source code.
472          * Please use it consistently.
473          *
474          * Description:  A column of asterisks on the left side,
475          * with beginning and ending almost-blank lines.
476          */
478 For files in net/ and drivers/net/ the preferred style for long (multi-line)
479 comments is a little different.
481         /* The preferred comment style for files in net/ and drivers/net
482          * looks like this.
483          *
484          * It is nearly the same as the generally preferred comment style,
485          * but there is no initial almost-blank line.
486          */
488 It's also important to comment data, whether they are basic types or derived
489 types.  To this end, use just one data declaration per line (no commas for
490 multiple data declarations).  This leaves you room for a small comment on each
491 item, explaining its use.
494                 Chapter 9: You've made a mess of it
496 That's OK, we all do.  You've probably been told by your long-time Unix
497 user helper that "GNU emacs" automatically formats the C sources for
498 you, and you've noticed that yes, it does do that, but the defaults it
499 uses are less than desirable (in fact, they are worse than random
500 typing - an infinite number of monkeys typing into GNU emacs would never
501 make a good program).
503 So, you can either get rid of GNU emacs, or change it to use saner
504 values.  To do the latter, you can stick the following in your .emacs file:
506 (defun c-lineup-arglist-tabs-only (ignored)
507   "Line up argument lists by tabs, not spaces"
508   (let* ((anchor (c-langelem-pos c-syntactic-element))
509          (column (c-langelem-2nd-pos c-syntactic-element))
510          (offset (- (1+ column) anchor))
511          (steps (floor offset c-basic-offset)))
512     (* (max steps 1)
513        c-basic-offset)))
515 (add-hook 'c-mode-common-hook
516           (lambda ()
517             ;; Add kernel style
518             (c-add-style
519              "linux-tabs-only"
520              '("linux" (c-offsets-alist
521                         (arglist-cont-nonempty
522                          c-lineup-gcc-asm-reg
523                          c-lineup-arglist-tabs-only))))))
525 (add-hook 'c-mode-hook
526           (lambda ()
527             (let ((filename (buffer-file-name)))
528               ;; Enable kernel mode for the appropriate files
529               (when (and filename
530                          (string-match (expand-file-name "~/src/linux-trees")
531                                        filename))
532                 (setq indent-tabs-mode t)
533                 (setq show-trailing-whitespace t)
534                 (c-set-style "linux-tabs-only")))))
536 This will make emacs go better with the kernel coding style for C
537 files below ~/src/linux-trees.
539 But even if you fail in getting emacs to do sane formatting, not
540 everything is lost: use "indent".
542 Now, again, GNU indent has the same brain-dead settings that GNU emacs
543 has, which is why you need to give it a few command line options.
544 However, that's not too bad, because even the makers of GNU indent
545 recognize the authority of K&R (the GNU people aren't evil, they are
546 just severely misguided in this matter), so you just give indent the
547 options "-kr -i8" (stands for "K&R, 8 character indents"), or use
548 "scripts/Lindent", which indents in the latest style.
550 "indent" has a lot of options, and especially when it comes to comment
551 re-formatting you may want to take a look at the man page.  But
552 remember: "indent" is not a fix for bad programming.
555                 Chapter 10: Kconfig configuration files
557 For all of the Kconfig* configuration files throughout the source tree,
558 the indentation is somewhat different.  Lines under a "config" definition
559 are indented with one tab, while help text is indented an additional two
560 spaces.  Example:
562 config AUDIT
563         bool "Auditing support"
564         depends on NET
565         help
566           Enable auditing infrastructure that can be used with another
567           kernel subsystem, such as SELinux (which requires this for
568           logging of avc messages output).  Does not do system-call
569           auditing without CONFIG_AUDITSYSCALL.
571 Seriously dangerous features (such as write support for certain
572 filesystems) should advertise this prominently in their prompt string:
574 config ADFS_FS_RW
575         bool "ADFS write support (DANGEROUS)"
576         depends on ADFS_FS
577         ...
579 For full documentation on the configuration files, see the file
580 Documentation/kbuild/kconfig-language.txt.
583                 Chapter 11: Data structures
585 Data structures that have visibility outside the single-threaded
586 environment they are created and destroyed in should always have
587 reference counts.  In the kernel, garbage collection doesn't exist (and
588 outside the kernel garbage collection is slow and inefficient), which
589 means that you absolutely _have_ to reference count all your uses.
591 Reference counting means that you can avoid locking, and allows multiple
592 users to have access to the data structure in parallel - and not having
593 to worry about the structure suddenly going away from under them just
594 because they slept or did something else for a while.
596 Note that locking is _not_ a replacement for reference counting.
597 Locking is used to keep data structures coherent, while reference
598 counting is a memory management technique.  Usually both are needed, and
599 they are not to be confused with each other.
601 Many data structures can indeed have two levels of reference counting,
602 when there are users of different "classes".  The subclass count counts
603 the number of subclass users, and decrements the global count just once
604 when the subclass count goes to zero.
606 Examples of this kind of "multi-level-reference-counting" can be found in
607 memory management ("struct mm_struct": mm_users and mm_count), and in
608 filesystem code ("struct super_block": s_count and s_active).
610 Remember: if another thread can find your data structure, and you don't
611 have a reference count on it, you almost certainly have a bug.
614                 Chapter 12: Macros, Enums and RTL
616 Names of macros defining constants and labels in enums are capitalized.
618         #define CONSTANT 0x12345
620 Enums are preferred when defining several related constants.
622 CAPITALIZED macro names are appreciated but macros resembling functions
623 may be named in lower case.
625 Generally, inline functions are preferable to macros resembling functions.
627 Macros with multiple statements should be enclosed in a do - while block:
629         #define macrofun(a, b, c)                       \
630                 do {                                    \
631                         if (a == 5)                     \
632                                 do_this(b, c);          \
633                 } while (0)
635 Things to avoid when using macros:
637 1) macros that affect control flow:
639         #define FOO(x)                                  \
640                 do {                                    \
641                         if (blah(x) < 0)                \
642                                 return -EBUGGERED;      \
643                 } while (0)
645 is a _very_ bad idea.  It looks like a function call but exits the "calling"
646 function; don't break the internal parsers of those who will read the code.
648 2) macros that depend on having a local variable with a magic name:
650         #define FOO(val) bar(index, val)
652 might look like a good thing, but it's confusing as hell when one reads the
653 code and it's prone to breakage from seemingly innocent changes.
655 3) macros with arguments that are used as l-values: FOO(x) = y; will
656 bite you if somebody e.g. turns FOO into an inline function.
658 4) forgetting about precedence: macros defining constants using expressions
659 must enclose the expression in parentheses. Beware of similar issues with
660 macros using parameters.
662         #define CONSTANT 0x4000
663         #define CONSTEXP (CONSTANT | 3)
665 5) namespace collisions when defining local variables in macros resembling
666 functions:
668 #define FOO(x)                          \
669 ({                                      \
670         typeof(x) ret;                  \
671         ret = calc_ret(x);              \
672         (ret);                          \
673 })
675 ret is a common name for a local variable - __foo_ret is less likely
676 to collide with an existing variable.
678 The cpp manual deals with macros exhaustively. The gcc internals manual also
679 covers RTL which is used frequently with assembly language in the kernel.
682                 Chapter 13: Printing kernel messages
684 Kernel developers like to be seen as literate. Do mind the spelling
685 of kernel messages to make a good impression. Do not use crippled
686 words like "dont"; use "do not" or "don't" instead.  Make the messages
687 concise, clear, and unambiguous.
689 Kernel messages do not have to be terminated with a period.
691 Printing numbers in parentheses (%d) adds no value and should be avoided.
693 There are a number of driver model diagnostic macros in <linux/device.h>
694 which you should use to make sure messages are matched to the right device
695 and driver, and are tagged with the right level:  dev_err(), dev_warn(),
696 dev_info(), and so forth.  For messages that aren't associated with a
697 particular device, <linux/printk.h> defines pr_notice(), pr_info(),
698 pr_warn(), pr_err(), etc.
700 Coming up with good debugging messages can be quite a challenge; and once
701 you have them, they can be a huge help for remote troubleshooting.  However
702 debug message printing is handled differently than printing other non-debug
703 messages.  While the other pr_XXX() functions print unconditionally,
704 pr_debug() does not; it is compiled out by default, unless either DEBUG is
705 defined or CONFIG_DYNAMIC_DEBUG is set.  That is true for dev_dbg() also,
706 and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
707 the ones already enabled by DEBUG.
709 Many subsystems have Kconfig debug options to turn on -DDEBUG in the
710 corresponding Makefile; in other cases specific files #define DEBUG.  And
711 when a debug message should be unconditionally printed, such as if it is
712 already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
713 used.
716                 Chapter 14: Allocating memory
718 The kernel provides the following general purpose memory allocators:
719 kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
720 vzalloc().  Please refer to the API documentation for further information
721 about them.
723 The preferred form for passing a size of a struct is the following:
725         p = kmalloc(sizeof(*p), ...);
727 The alternative form where struct name is spelled out hurts readability and
728 introduces an opportunity for a bug when the pointer variable type is changed
729 but the corresponding sizeof that is passed to a memory allocator is not.
731 Casting the return value which is a void pointer is redundant. The conversion
732 from void pointer to any other pointer type is guaranteed by the C programming
733 language.
735 The preferred form for allocating an array is the following:
737         p = kmalloc_array(n, sizeof(...), ...);
739 The preferred form for allocating a zeroed array is the following:
741         p = kcalloc(n, sizeof(...), ...);
743 Both forms check for overflow on the allocation size n * sizeof(...),
744 and return NULL if that occurred.
747                 Chapter 15: The inline disease
749 There appears to be a common misperception that gcc has a magic "make me
750 faster" speedup option called "inline". While the use of inlines can be
751 appropriate (for example as a means of replacing macros, see Chapter 12), it
752 very often is not. Abundant use of the inline keyword leads to a much bigger
753 kernel, which in turn slows the system as a whole down, due to a bigger
754 icache footprint for the CPU and simply because there is less memory
755 available for the pagecache. Just think about it; a pagecache miss causes a
756 disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
757 that can go into these 5 milliseconds.
759 A reasonable rule of thumb is to not put inline at functions that have more
760 than 3 lines of code in them. An exception to this rule are the cases where
761 a parameter is known to be a compiletime constant, and as a result of this
762 constantness you *know* the compiler will be able to optimize most of your
763 function away at compile time. For a good example of this later case, see
764 the kmalloc() inline function.
766 Often people argue that adding inline to functions that are static and used
767 only once is always a win since there is no space tradeoff. While this is
768 technically correct, gcc is capable of inlining these automatically without
769 help, and the maintenance issue of removing the inline when a second user
770 appears outweighs the potential value of the hint that tells gcc to do
771 something it would have done anyway.
774                 Chapter 16: Function return values and names
776 Functions can return values of many different kinds, and one of the
777 most common is a value indicating whether the function succeeded or
778 failed.  Such a value can be represented as an error-code integer
779 (-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
780 non-zero = success).
782 Mixing up these two sorts of representations is a fertile source of
783 difficult-to-find bugs.  If the C language included a strong distinction
784 between integers and booleans then the compiler would find these mistakes
785 for us... but it doesn't.  To help prevent such bugs, always follow this
786 convention:
788         If the name of a function is an action or an imperative command,
789         the function should return an error-code integer.  If the name
790         is a predicate, the function should return a "succeeded" boolean.
792 For example, "add work" is a command, and the add_work() function returns 0
793 for success or -EBUSY for failure.  In the same way, "PCI device present" is
794 a predicate, and the pci_dev_present() function returns 1 if it succeeds in
795 finding a matching device or 0 if it doesn't.
797 All EXPORTed functions must respect this convention, and so should all
798 public functions.  Private (static) functions need not, but it is
799 recommended that they do.
801 Functions whose return value is the actual result of a computation, rather
802 than an indication of whether the computation succeeded, are not subject to
803 this rule.  Generally they indicate failure by returning some out-of-range
804 result.  Typical examples would be functions that return pointers; they use
805 NULL or the ERR_PTR mechanism to report failure.
808                 Chapter 17:  Don't re-invent the kernel macros
810 The header file include/linux/kernel.h contains a number of macros that
811 you should use, rather than explicitly coding some variant of them yourself.
812 For example, if you need to calculate the length of an array, take advantage
813 of the macro
815         #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
817 Similarly, if you need to calculate the size of some structure member, use
819         #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
821 There are also min() and max() macros that do strict type checking if you
822 need them.  Feel free to peruse that header file to see what else is already
823 defined that you shouldn't reproduce in your code.
826                 Chapter 18:  Editor modelines and other cruft
828 Some editors can interpret configuration information embedded in source files,
829 indicated with special markers.  For example, emacs interprets lines marked
830 like this:
832         -*- mode: c -*-
834 Or like this:
836         /*
837         Local Variables:
838         compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
839         End:
840         */
842 Vim interprets markers that look like this:
844         /* vim:set sw=8 noet */
846 Do not include any of these in source files.  People have their own personal
847 editor configurations, and your source files should not override them.  This
848 includes markers for indentation and mode configuration.  People may use their
849 own custom mode, or may have some other magic method for making indentation
850 work correctly.
853                 Chapter 19:  Inline assembly
855 In architecture-specific code, you may need to use inline assembly to interface
856 with CPU or platform functionality.  Don't hesitate to do so when necessary.
857 However, don't use inline assembly gratuitously when C can do the job.  You can
858 and should poke hardware from C when possible.
860 Consider writing simple helper functions that wrap common bits of inline
861 assembly, rather than repeatedly writing them with slight variations.  Remember
862 that inline assembly can use C parameters.
864 Large, non-trivial assembly functions should go in .S files, with corresponding
865 C prototypes defined in C header files.  The C prototypes for assembly
866 functions should use "asmlinkage".
868 You may need to mark your asm statement as volatile, to prevent GCC from
869 removing it if GCC doesn't notice any side effects.  You don't always need to
870 do so, though, and doing so unnecessarily can limit optimization.
872 When writing a single inline assembly statement containing multiple
873 instructions, put each instruction on a separate line in a separate quoted
874 string, and end each string except the last with \n\t to properly indent the
875 next instruction in the assembly output:
877         asm ("magic %reg1, #42\n\t"
878              "more_magic %reg2, %reg3"
879              : /* outputs */ : /* inputs */ : /* clobbers */);
882                 Chapter 20: Conditional Compilation
884 Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
885 files; doing so makes code harder to read and logic harder to follow.  Instead,
886 use such conditionals in a header file defining functions for use in those .c
887 files, providing no-op stub versions in the #else case, and then call those
888 functions unconditionally from .c files.  The compiler will avoid generating
889 any code for the stub calls, producing identical results, but the logic will
890 remain easy to follow.
892 Prefer to compile out entire functions, rather than portions of functions or
893 portions of expressions.  Rather than putting an ifdef in an expression, factor
894 out part or all of the expression into a separate helper function and apply the
895 conditional to that function.
897 If you have a function or variable which may potentially go unused in a
898 particular configuration, and the compiler would warn about its definition
899 going unused, mark the definition as __maybe_unused rather than wrapping it in
900 a preprocessor conditional.  (However, if a function or variable *always* goes
901 unused, delete it.)
903 Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
904 symbol into a C boolean expression, and use it in a normal C conditional:
907                 ...
908         }
910 The compiler will constant-fold the conditional away, and include or exclude
911 the block of code just as with an #ifdef, so this will not add any runtime
912 overhead.  However, this approach still allows the C compiler to see the code
913 inside the block, and check it for correctness (syntax, types, symbol
914 references, etc).  Thus, you still have to use an #ifdef if the code inside the
915 block references symbols that will not exist if the condition is not met.
917 At the end of any non-trivial #if or #ifdef block (more than a few lines),
918 place a comment after the #endif on the same line, noting the conditional
919 expression used.  For instance:
921         #ifdef CONFIG_SOMETHING
922         ...
923         #endif /* CONFIG_SOMETHING */
926                 Appendix I: References
928 The C Programming Language, Second Edition
929 by Brian W. Kernighan and Dennis M. Ritchie.
930 Prentice Hall, Inc., 1988.
931 ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
933 The Practice of Programming
934 by Brian W. Kernighan and Rob Pike.
935 Addison-Wesley, Inc., 1999.
936 ISBN 0-201-61586-X.
938 GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
939 gcc internals and indent, all available from
941 WG14 is the international standardization working group for the programming
942 language C, URL:
944 Kernel CodingStyle, by at OLS 2002:

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