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.
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
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
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.
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.:
59 Don't put multiple statements on a single line unless you have
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
79 The limit on the length of lines is 80 columns and this is a strongly
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:
102 This applies to all non-function statement blocks (if, switch, for,
116 However, there is one special case, namely functions: they have the
117 opening brace at the beginning of the next line, thus:
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
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
156 Do not unnecessarily use braces where a single statement will do.
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:
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
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:
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:
221 no space after the prefix increment & decrement unary operators:
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
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
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).
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
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
290 (a) totally opaque objects (where the typedef is actively used to _hide_
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
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
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
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.
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)
380 return system_state == SYSTEM_RUNNING;
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
407 - errors by not updating individual exit points when making
408 modifications are prevented
409 - saves the compiler work to optimize redundant code away ;)
416 buffer = kmalloc(SIZE, GFP_KERNEL);
433 A common type of bug to be aware of it "one err bugs" which look like this:
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
460 When commenting the kernel API functions, please use the kernel-doc format.
461 See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
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:
470 * This is the preferred style for multi-line
471 * comments in the Linux kernel source code.
472 * Please use it consistently.
474 * Description: A column of asterisks on the left side,
475 * with beginning and ending almost-blank lines.
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
484 * It is nearly the same as the generally preferred comment style,
485 * but there is no initial almost-blank line.
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)))
515 (add-hook 'c-mode-common-hook
520 '("linux" (c-offsets-alist
521 (arglist-cont-nonempty
523 c-lineup-arglist-tabs-only))))))
525 (add-hook 'c-mode-hook
527 (let ((filename (buffer-file-name)))
528 ;; Enable kernel mode for the appropriate files
530 (string-match (expand-file-name "~/src/linux-trees")
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
563 bool "Auditing support"
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:
575 bool "ADFS write support (DANGEROUS)"
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) \
635 Things to avoid when using macros:
637 1) macros that affect control flow:
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 The cpp manual deals with macros exhaustively. The gcc internals manual also
666 covers RTL which is used frequently with assembly language in the kernel.
669 Chapter 13: Printing kernel messages
671 Kernel developers like to be seen as literate. Do mind the spelling
672 of kernel messages to make a good impression. Do not use crippled
673 words like "dont"; use "do not" or "don't" instead. Make the messages
674 concise, clear, and unambiguous.
676 Kernel messages do not have to be terminated with a period.
678 Printing numbers in parentheses (%d) adds no value and should be avoided.
680 There are a number of driver model diagnostic macros in <linux/device.h>
681 which you should use to make sure messages are matched to the right device
682 and driver, and are tagged with the right level: dev_err(), dev_warn(),
683 dev_info(), and so forth. For messages that aren't associated with a
684 particular device, <linux/printk.h> defines pr_notice(), pr_info(),
685 pr_warn(), pr_err(), etc.
687 Coming up with good debugging messages can be quite a challenge; and once
688 you have them, they can be a huge help for remote troubleshooting. However
689 debug message printing is handled differently than printing other non-debug
690 messages. While the other pr_XXX() functions print unconditionally,
691 pr_debug() does not; it is compiled out by default, unless either DEBUG is
692 defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also,
693 and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
694 the ones already enabled by DEBUG.
696 Many subsystems have Kconfig debug options to turn on -DDEBUG in the
697 corresponding Makefile; in other cases specific files #define DEBUG. And
698 when a debug message should be unconditionally printed, such as if it is
699 already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
703 Chapter 14: Allocating memory
705 The kernel provides the following general purpose memory allocators:
706 kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
707 vzalloc(). Please refer to the API documentation for further information
710 The preferred form for passing a size of a struct is the following:
712 p = kmalloc(sizeof(*p), ...);
714 The alternative form where struct name is spelled out hurts readability and
715 introduces an opportunity for a bug when the pointer variable type is changed
716 but the corresponding sizeof that is passed to a memory allocator is not.
718 Casting the return value which is a void pointer is redundant. The conversion
719 from void pointer to any other pointer type is guaranteed by the C programming
722 The preferred form for allocating an array is the following:
724 p = kmalloc_array(n, sizeof(...), ...);
726 The preferred form for allocating a zeroed array is the following:
728 p = kcalloc(n, sizeof(...), ...);
730 Both forms check for overflow on the allocation size n * sizeof(...),
731 and return NULL if that occurred.
734 Chapter 15: The inline disease
736 There appears to be a common misperception that gcc has a magic "make me
737 faster" speedup option called "inline". While the use of inlines can be
738 appropriate (for example as a means of replacing macros, see Chapter 12), it
739 very often is not. Abundant use of the inline keyword leads to a much bigger
740 kernel, which in turn slows the system as a whole down, due to a bigger
741 icache footprint for the CPU and simply because there is less memory
742 available for the pagecache. Just think about it; a pagecache miss causes a
743 disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
744 that can go into these 5 milliseconds.
746 A reasonable rule of thumb is to not put inline at functions that have more
747 than 3 lines of code in them. An exception to this rule are the cases where
748 a parameter is known to be a compiletime constant, and as a result of this
749 constantness you *know* the compiler will be able to optimize most of your
750 function away at compile time. For a good example of this later case, see
751 the kmalloc() inline function.
753 Often people argue that adding inline to functions that are static and used
754 only once is always a win since there is no space tradeoff. While this is
755 technically correct, gcc is capable of inlining these automatically without
756 help, and the maintenance issue of removing the inline when a second user
757 appears outweighs the potential value of the hint that tells gcc to do
758 something it would have done anyway.
761 Chapter 16: Function return values and names
763 Functions can return values of many different kinds, and one of the
764 most common is a value indicating whether the function succeeded or
765 failed. Such a value can be represented as an error-code integer
766 (-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
769 Mixing up these two sorts of representations is a fertile source of
770 difficult-to-find bugs. If the C language included a strong distinction
771 between integers and booleans then the compiler would find these mistakes
772 for us... but it doesn't. To help prevent such bugs, always follow this
775 If the name of a function is an action or an imperative command,
776 the function should return an error-code integer. If the name
777 is a predicate, the function should return a "succeeded" boolean.
779 For example, "add work" is a command, and the add_work() function returns 0
780 for success or -EBUSY for failure. In the same way, "PCI device present" is
781 a predicate, and the pci_dev_present() function returns 1 if it succeeds in
782 finding a matching device or 0 if it doesn't.
784 All EXPORTed functions must respect this convention, and so should all
785 public functions. Private (static) functions need not, but it is
786 recommended that they do.
788 Functions whose return value is the actual result of a computation, rather
789 than an indication of whether the computation succeeded, are not subject to
790 this rule. Generally they indicate failure by returning some out-of-range
791 result. Typical examples would be functions that return pointers; they use
792 NULL or the ERR_PTR mechanism to report failure.
795 Chapter 17: Don't re-invent the kernel macros
797 The header file include/linux/kernel.h contains a number of macros that
798 you should use, rather than explicitly coding some variant of them yourself.
799 For example, if you need to calculate the length of an array, take advantage
802 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
804 Similarly, if you need to calculate the size of some structure member, use
806 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
808 There are also min() and max() macros that do strict type checking if you
809 need them. Feel free to peruse that header file to see what else is already
810 defined that you shouldn't reproduce in your code.
813 Chapter 18: Editor modelines and other cruft
815 Some editors can interpret configuration information embedded in source files,
816 indicated with special markers. For example, emacs interprets lines marked
825 compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
829 Vim interprets markers that look like this:
831 /* vim:set sw=8 noet */
833 Do not include any of these in source files. People have their own personal
834 editor configurations, and your source files should not override them. This
835 includes markers for indentation and mode configuration. People may use their
836 own custom mode, or may have some other magic method for making indentation
840 Chapter 19: Inline assembly
842 In architecture-specific code, you may need to use inline assembly to interface
843 with CPU or platform functionality. Don't hesitate to do so when necessary.
844 However, don't use inline assembly gratuitously when C can do the job. You can
845 and should poke hardware from C when possible.
847 Consider writing simple helper functions that wrap common bits of inline
848 assembly, rather than repeatedly writing them with slight variations. Remember
849 that inline assembly can use C parameters.
851 Large, non-trivial assembly functions should go in .S files, with corresponding
852 C prototypes defined in C header files. The C prototypes for assembly
853 functions should use "asmlinkage".
855 You may need to mark your asm statement as volatile, to prevent GCC from
856 removing it if GCC doesn't notice any side effects. You don't always need to
857 do so, though, and doing so unnecessarily can limit optimization.
859 When writing a single inline assembly statement containing multiple
860 instructions, put each instruction on a separate line in a separate quoted
861 string, and end each string except the last with \n\t to properly indent the
862 next instruction in the assembly output:
864 asm ("magic %reg1, #42\n\t"
865 "more_magic %reg2, %reg3"
866 : /* outputs */ : /* inputs */ : /* clobbers */);
869 Chapter 20: Conditional Compilation
871 Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
872 files; doing so makes code harder to read and logic harder to follow. Instead,
873 use such conditionals in a header file defining functions for use in those .c
874 files, providing no-op stub versions in the #else case, and then call those
875 functions unconditionally from .c files. The compiler will avoid generating
876 any code for the stub calls, producing identical results, but the logic will
877 remain easy to follow.
879 Prefer to compile out entire functions, rather than portions of functions or
880 portions of expressions. Rather than putting an ifdef in an expression, factor
881 out part or all of the expression into a separate helper function and apply the
882 conditional to that function.
884 If you have a function or variable which may potentially go unused in a
885 particular configuration, and the compiler would warn about its definition
886 going unused, mark the definition as __maybe_unused rather than wrapping it in
887 a preprocessor conditional. (However, if a function or variable *always* goes
890 Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
891 symbol into a C boolean expression, and use it in a normal C conditional:
893 if (IS_ENABLED(CONFIG_SOMETHING)) {
897 The compiler will constant-fold the conditional away, and include or exclude
898 the block of code just as with an #ifdef, so this will not add any runtime
899 overhead. However, this approach still allows the C compiler to see the code
900 inside the block, and check it for correctness (syntax, types, symbol
901 references, etc). Thus, you still have to use an #ifdef if the code inside the
902 block references symbols that will not exist if the condition is not met.
904 At the end of any non-trivial #if or #ifdef block (more than a few lines),
905 place a comment after the #endif on the same line, noting the conditional
906 expression used. For instance:
908 #ifdef CONFIG_SOMETHING
910 #endif /* CONFIG_SOMETHING */
913 Appendix I: References
915 The C Programming Language, Second Edition
916 by Brian W. Kernighan and Dennis M. Ritchie.
917 Prentice Hall, Inc., 1988.
918 ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
919 URL: http://cm.bell-labs.com/cm/cs/cbook/
921 The Practice of Programming
922 by Brian W. Kernighan and Rob Pike.
923 Addison-Wesley, Inc., 1999.
925 URL: http://cm.bell-labs.com/cm/cs/tpop/
927 GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
928 gcc internals and indent, all available from http://www.gnu.org/manual/
930 WG14 is the international standardization working group for the programming
931 language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
933 Kernel CodingStyle, by greg@kroah.com at OLS 2002:
934 http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/