Linux内核编码风格 像其他⼤型软件⼀样，Linux制订了⼀套编码风格，对代码的格式、风格和布局做出了规定。
我写这篇的⽬的也就是希望⼤家能够从中借鉴，有利于⼤家提⾼编程效率。
像Linux内核这样⼤型软件中，涉及许许多多的开发者，故它的编码风格也很有参考价值。
1、左括号紧跟在语句的最后，与语句在相同的⼀⾏。
⽽右括号要另起⼀⾏，作为该⾏的第⼀个字符。
2、如果接下来的部分是相同语句的⼀部分，那么右括号就不单独占⼀⾏。
3、还有 4、函数采⽤以下的书写⽅式： 5、最后不需要⼀定使⽤括号的语句可以忽略它： 要尽可能地保证代码长度不超过80个字符，如果代码⾏超过80应该折到下⼀⾏。
将参数分⾏输⼊，在开头简单地加⼊两个标准tab： 名称中不允许使⽤混合的⼤⼩写字符。
局部变量如果能够清楚地表明它的⽤途，那么选取idx甚⾄是i这样的名称都是可⾏的。
⽽像theLoopIndex这样冗长反复的名字不在接受之列。
——匈⽛利命名法（在变量名称中加⼊变量的类别）危害极⼤。
函数 根据经验函数的代码长度不应该超过两屏，局部变量不应该超过⼗个。
1、⼀个函数应该功能单⼀并且实现精准。
2、将⼀个函数分解成⼀些更短⼩的函数的组合不会带来危害。
——如果你担⼼函数调⽤导致的开销，可以使⽤inline关键字。
⼀般情况下，注释的⽬的是描述你的代码要做什么和为什么要做，⽽不是具体通过什么⽅式实现的。
怎么实现应该由代码本⾝展现。
注释不应该包含谁写了那个函数，修改⽇期和其他那些琐碎⽽⽆实际意义的内容。
这些信息应该集中在⽂件最开头地⽅。
内核中⼀条注释看起来如下： 重要信息常常以“XXX：”开头，⽽bug通常以“FIXME"开头，就像：总结 希望这篇博客对⼤家有所帮助！更详尽的内容，请看"Linux 内核代码规范原⽂"Linus 内部代码规范原⽂1 Linux kernel coding style23 This is a short document describing the preferred coding style for the4 linux kernel. Coding style is very personal, and I won't _force_ my5 views on anybody, but this is what goes for anything that I have to be6 able to maintain, and I'd prefer it for most other things too. Please7 at least consider the points made here.89 First off, I'd suggest printing out a copy of the GNU coding standards,10 and NOT read it. Burn them, it's a great symbolic gesture.1112 Anyway, here goes:131415 Chapter 1: Indentation1617 Tabs are 8 characters, and thus indentations are also 8 characters.18 There are heretic movements that try to make indentations 4 (or even 2!)19 characters deep, and that is akin to trying to define the value of PI to20 be 3.2122 Rationale: The whole idea behind indentation is to clearly define where23 a block of control starts and ends. Especially when you've been looking24 at your screen for20 straight hours, you'll find it a lot easier to see25 how the indentation works if you have large indentations.2627 Now, some people will claim that having 8-character indentations makes28 the code move too far to the right, and makes it hard to read on a2980-character terminal screen. The answer to that is that if you need30 more than 3 levels of indentation, you're screwed anyway, and should fix31 your program.3233 In short, 8-char indents make things easier to read, and have the added34 benefit of warning you when you're nesting your functions too deep.35 Heed that warning.3637 The preferred way to ease multiple indentation levels in a switch statement is38 to align the "switch" and its subordinate "case" labels in the same column39 instead of "double-indenting" the "case" labels. E.g.:4041switch (suffix) {42case'G':43case'g':44 mem <<= 30;45break;46case'M':47case'm':48 mem <<= 20;49break;51case'k':52 mem <<= 10;53/* fall through */54default:55break;56 }575859 Don't put multiple statements on a single line unless you have60 something to hide:6162if (condition) do_this;63 do_something_everytime;6465 Don't put multiple assignments on a single line either. Kernel coding style66is super simple. Avoid tricky expressions.6768 Outside of comments, documentation and except in Kconfig, spaces are never69 used for indentation, and the above example is deliberately broken.7071 Get a decent editor and don't leave whitespace at the end of lines.727374 Chapter 2: Breaking long lines and strings7576 Coding style is all about readability and maintainability using commonly77 available tools.7879 The limit on the length of lines is80 columns and this is a strongly80 preferred limit.8182 Statements longer than 80 columns will be broken into sensible chunks, unless83 exceeding 80 columns significantly increases readability and does not hide84 information. Descendants are always substantially shorter than the parent and85 are placed substantially to the right. The same applies to function headers86 with a long argument list. However, never break user-visible strings such as87 printk messages, because that breaks the ability to grep for them.888990 Chapter 3: Placing Braces and Spaces9192 The other issue that always comes up in C styling is the placement of93 braces. Unlike the indent size, there are few technical reasons to94 choose one placement strategy over the other, but the preferred way, as95 shown to us by the prophets Kernighan and Ritchie, is to put the opening96 brace last on the line, and put the closing brace first, thusly:9798if (x is true) {99 we do y100 }101102 This applies to all non-function statement blocks (if, switch, for,103while, do). E.g.:104105switch (action) {106case KOBJ_ADD:107return"add";108case KOBJ_REMOVE:109return"remove";110case KOBJ_CHANGE:111return"change";112default:113return NULL;114 }115116 However, there is one special case, namely functions: they have the117 opening brace at the beginning of the next line, thus:118119int function(int x)120 {121 body of function122 }123124 Heretic people all over the world have claimed that this inconsistency125is ... well ... inconsistent, but all right-thinking people know that126 (a) K&R are _right_ and (b) K&R are right. Besides, functions are127 special anyway (you can't nest them in C).128129 Note that the closing brace is empty on a line of its own, _except_ in130 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, like132this:133135 body of do-loop136 } while (condition);137138 and139140if (x == y) {141 ..142 } else if (x > y) {143 ...144 } else {145 ....146 }147148 Rationale: K&R.149150 Also, note that this brace-placement also minimizes the number of empty151 (or almost empty) lines, without any loss of readability. Thus, as the152 supply of new-lines on your screen is not a renewable resource (think15325-line terminal screens here), you have more empty lines to put154 comments on.155156 Do not unnecessarily use braces where a single statement will do.157158if (condition)159 action();160161 and162163if (condition)164 do_this();165else166 do_that();167168 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:170171if (condition) {172 do_this();173 do_that();174 } else {175 otherwise();176 }177178 3.1: Spaces179180 Linux kernel style for use of spaces depends (mostly) on181 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).186187 So use a space after these keywords:188if, switch, case, for, do, while189 but not with sizeof, typeof, alignof, or __attribute__. E.g.,190 s = sizeof(struct file);191192 Do not add spaces around (inside) parenthesized expressions. This example is 193 *bad*:194195 s = sizeof( struct file );196197 When declaring pointer data or a function that returns a pointer type, the198 preferred use of '*'is adjacent to the data name or function name and not199 adjacent to the type name. Examples:200201char *linux_banner;202 unsigned long long memparse(char *ptr, char **retptr);203char *match_strdup(substring_t *s);204205 Use one space around (on each side of) most binary and ternary operators, 206 such as any of these:207208 = + - < > * / % | & ^ <= >= == != ? :209210 but no space after unary operators:211 & * + - ~ ! sizeof typeof alignof __attribute__ defined212213 no space before the postfix increment & decrement unary operators:214 ++ --215216 no space after the prefix increment & decrement unary operators:217 ++ --219 and no space around the '.' and "->" structure member operators.220221 Do not leave trailing whitespace at the ends of lines. Some editors with 222"smart" indentation will insert whitespace at the beginning of new lines as 223 appropriate, so you can start typing the next line of code right away.224 However, some such editors do not remove the whitespace if you end up not 225 putting a line of code there, such as if you leave a blank line. As a result, 226 you end up with lines containing trailing whitespace.227228 Git will warn you about patches that introduce trailing whitespace, and can 229 optionally strip the trailing whitespace for you; however, if applying a series 230 of patches, this may make later patches in the series fail by changing their 231 context lines.232233234 Chapter 4: Naming235236 C is a Spartan language, and so should your naming be. Unlike Modula-2 237 and Pascal programmers, C programmers do not use cute names like238 ThisVariableIsATemporaryCounter. A C programmer would call that239 variable "tmp", which is much easier to write, and not the least more240 difficult to understand.241242 HOWEVER, while mixed-case names are frowned upon, descriptive names for 243global variables are a must. To call a global function "foo"is a244 shooting offense.245246 GLOBAL variables (to be used only if you _really_ need them) need to247 have descriptive names, as do global functions. If you have a function248 that counts the number of active users, you should call that249"count_active_users()" or similar, you should _not_ call it "cntusr()".250251 Encoding the type of a function into the name (so-called Hungarian252 notation) is brain damaged - the compiler knows the types anyway and can 253 check those, and it only confuses the programmer. No wonder MicroSoft 254 makes buggy programs.255256 LOCAL variable names should be short, and to the point. If you have257 some random integer loop counter, it should probably be called "i".258 Calling it "loop_counter"is non-productive, if there is no chance of it259 being mis-understood. Similarly, "tmp" can be just about any type of260 variable that is used to hold a temporary value.261262 If you are afraid to mix up your local variable names, you have another263 problem, which is called the function-growth-hormone-imbalance syndrome. 264 See chapter 6 (Functions).265266267 Chapter 5: Typedefs268269 Please don't use things like "vps_t".270271 It's a _mistake_ to use typedef for structures and pointers. When you see a 272273 vps_t a;274275in the source, what does it mean?276277 In contrast, if it says278279struct virtual_container *a;280281 you can actually tell what "a"is.282283 Lots of people think that typedefs "help readability". Not so. They are284 useful only for:285286 (a) totally opaque objects (where the typedef is actively used to _hide_287 what the object is).288289 Example: "pte_t" etc. opaque objects that you can only access using290 the proper accessor functions.291292 NOTE! Opaqueness and "accessor functions" are not good in themselves. 293 The reason we have them for things like pte_t etc. is that there294 really is absolutely _zero_ portably accessible information there.295296 (b) Clear integer types, where the abstraction _helps_ avoid confusion297 whether it is"int" or "long".298299 u8/u16/u32 are perfectly fine typedefs, although they fit into300 category (d) better than here.301303"unsigned long", then there's no reason to do304305 typedef unsigned long myflags_t;306307 but if there is a clear reason for why it under certain circumstances308 might be an "unsigned int" and under other configurations might be 309"unsigned long", then by all means go ahead and use a typedef.310311 (c) when you use sparse to literally create a _new_ type for312 type-checking.313314 (d) New types which are identical to standard C99 types, in certain315 exceptional circumstances.316317 Although it would only take a short amount of time for the eyes and318 brain to become accustomed to the standard types like 'uint32_t',319 some people object to their use anyway.320321 Therefore, the Linux-specific 'u8/u16/u32/u64' types and their322 signed equivalents which are identical to standard types are323 permitted -- although they are not mandatory in new code of your324 own.325326 When editing existing code which already uses one or the other set327 of types, you should conform to the existing choices in that code.328329 (e) Types safe for use in userspace.330331 In certain structures which are visible to userspace, we cannot332 require C99 types and cannot use the 'u32' form above. Thus, we333 use __u32 and similar types in all structures which are shared334 with userspace.335336 Maybe there are other cases too, but the rule should basically be to NEVER 337 EVER use a typedef unless you can clearly match one of those rules.338339 In general, a pointer, or a struct that has elements that can reasonably340 be directly accessed should _never_ be a typedef.341342343 Chapter 6: Functions344345 Functions should be short and sweet, and do just one thing. They should346 fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,347as we all know), and do one thing and do that well.348349 The maximum length of a function is inversely proportional to the350 complexity and indentation level of that function. So, if you have a351 conceptually simple function that is just one long (but simple)352case-statement, where you have to do lots of small things for a lot of353 different cases, it's OK to have a longer function.354355 However, if you have a complex function, and you suspect that a356 less-than-gifted first-year high-school student might not even357 understand what the function is all about, you should adhere to the358 maximum limits all the more closely. Use helper functions with359 descriptive names (you can ask the compiler to in-line them if you think360 it's performance-critical, and it will probably do a better job of it361 than you would have done).362363 Another measure of the function is the number of local variables. They364 shouldn't exceed 5-10, or you're doing something wrong. Re-think the365 function, and split it into smaller pieces. A human brain can366 generally easily keep track of about 7 different things, anything more367 and it gets confused. You know you're brilliant, but maybe you'd like368 to understand what you did 2 weeks from now.369370 In source files, separate functions with one blank line. If the function is371 exported, the EXPORT* macro for it should follow immediately after the closing 372 function brace line. E.g.:373374int system_is_up(void)375 {376return system_state == SYSTEM_RUNNING;377 }378 EXPORT_SYMBOL(system_is_up);379380 In function prototypes, include parameter names with their data types.381 Although this is not required by the C language, it is preferred in Linux382 because it is a simple way to add valuable information for the reader.383384385 Chapter 7: Centralized exiting of functions387 Albeit deprecated by some people, the equivalent of the goto statement is388 used frequently by compilers in form of the unconditional jump instruction.389390 The goto statement comes in handy when a function exits from multiple391 locations and some common work such as cleanup has to be done.392393 The rationale is:394395 - unconditional statements are easier to understand and follow396 - nesting is reduced397 - errors by not updating individual exit points when making398 modifications are prevented399 - saves the compiler work to optimize redundant code away ;)400401int fun(int a)402 {403int result = 0;404char *buffer = kmalloc(SIZE);405406if (buffer == NULL)407return -ENOMEM;408409if (condition1) {410while (loop1) {411 ...412 }413 result = 1;414goto out;415 }416 ...417out:418 kfree(buffer);419return result;420 }421422 Chapter 8: Commenting423424 Comments are good, but there is also a danger of over-commenting. NEVER 425try to explain HOW your code works in a comment: it's much better to426 write the code so that the _working_ is obvious, and it's a waste of427 time to explain badly written code.428429 Generally, you want your comments to tell WHAT your code does, not HOW. 430 Also, try to avoid putting comments inside a function body: if the431 function is so complex that you need to separately comment parts of it,432 you should probably go back to chapter 6for a while. You can make433 small comments to note or warn about something particularly clever (or434 ugly), but try to avoid excess. Instead, put the comments at the head435 of the function, telling people what it does, and possibly WHY it does436 it.437438 When commenting the kernel API functions, please use the kernel-doc format. 439 See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc 440for details.441442 Linux style for comments is the C89 "/* ... */" style.443 Don't use C99-style "// ..." comments.444445 The preferred style for long (multi-line) comments is:446447/*448 * This is the preferred style for multi-line449 * comments in the Linux kernel source code.450 * Please use it consistently.451 *452 * Description: A column of asterisks on the left side,453 * with beginning and ending almost-blank lines.454*/455456 For files in net/ and drivers/net/ the preferred style for long (multi-line)457 comments is a little different.458459/* The preferred comment style for files in net/ and drivers/net460 * looks like this.461 *462 * It is nearly the same as the generally preferred comment style,463 * but there is no initial almost-blank line.464*/465466 It's also important to comment data, whether they are basic types or derived 467 types. To this end, use just one data declaration per line (no commas for468 multiple data declarations). This leaves you room for a small comment on each 469 item, explaining its use.472 Chapter 9: You've made a mess of it473474 That's OK, we all do. You've probably been told by your long-time Unix 475 user helper that "GNU emacs" automatically formats the C sources for476 you, and you've noticed that yes, it does do that, but the defaults it477 uses are less than desirable (in fact, they are worse than random478 typing - an infinite number of monkeys typing into GNU emacs would never 479 make a good program).480481 So, you can either get rid of GNU emacs, or change it to use saner482 values. To do the latter, you can stick the following in your .emacs file: 483484 (defun c-lineup-arglist-tabs-only (ignored)485"Line up argument lists by tabs, not spaces"486 (let* ((anchor (c-langelem-pos c-syntactic-element))487 (column (c-langelem-2nd-pos c-syntactic-element))488 (offset (- (1+ column) anchor))489 (steps (floor offset c-basic-offset)))490 (* (max steps 1)491 c-basic-offset)))492493 (add-hook 'c-mode-common-hook494 (lambda ()495 ;; Add kernel style496 (c-add-style497"linux-tabs-only"498'("linux" (c-offsets-alist499 (arglist-cont-nonempty500 c-lineup-gcc-asm-reg501 c-lineup-arglist-tabs-only))))))502503 (add-hook 'c-mode-hook504 (lambda ()505 (let ((filename (buffer-file-name)))506 ;; Enable kernel mode for the appropriate files507 (when (and filename508 (string-match (expand-file-name "~/src/linux-trees")509 filename))510 (setq indent-tabs-mode t)511 (c-set-style "linux-tabs-only")))))512513 This will make emacs go better with the kernel coding style for C514 files below ~/src/linux-trees.515516 But even if you fail in getting emacs to do sane formatting, not517 everything is lost: use "indent".518519 Now, again, GNU indent has the same brain-dead settings that GNU emacs 520 has, which is why you need to give it a few command line options.521 However, that's not too bad, because even the makers of GNU indent522 recognize the authority of K&R (the GNU people aren't evil, they are523 just severely misguided in this matter), so you just give indent the524 options "-kr -i8" (stands for"K&R, 8 character indents"), or use 525"scripts/Lindent", which indents in the latest style.526527"indent" has a lot of options, and especially when it comes to comment 528 re-formatting you may want to take a look at the man page. But529 remember: "indent"is not a fix for bad programming.530531532 Chapter 10: Kconfig configuration files533534 For all of the Kconfig* configuration files throughout the source tree,535 the indentation is somewhat different. Lines under a "config" definition536 are indented with one tab, while help text is indented an additional two537 spaces. Example:538539 config AUDIT540bool"Auditing support"541 depends on NET542 help543 Enable auditing infrastructure that can be used with another544 kernel subsystem, such as SELinux (which requires this for545 logging of avc messages output). Does not do system-call546 auditing without CONFIG_AUDITSYSCALL.547548 Features that might still be considered unstable should be defined as549 dependent on "EXPERIMENTAL":550551 config SLUB552 depends on EXPERIMENTAL && !ARCH_USES_SLAB_PAGE_STRUCT 553bool"SLUB (Unqueued Allocator)"556while seriously dangerous features (such as write support for certain557 filesystems) should advertise this prominently in their prompt string:558559 config ADFS_FS_RW560bool"ADFS write support (DANGEROUS)"561 depends on ADFS_FS562 ...563564 For full documentation on the configuration files, see the file565 Documentation/kbuild/kconfig-language.txt.566567568 Chapter 11: Data structures569570 Data structures that have visibility outside the single-threaded571 environment they are created and destroyed in should always have572 reference counts. In the kernel, garbage collection doesn't exist (and573 outside the kernel garbage collection is slow and inefficient), which574 means that you absolutely _have_ to reference count all your uses.575576 Reference counting means that you can avoid locking, and allows multiple 577 users to have access to the data structure in parallel - and not having578 to worry about the structure suddenly going away from under them just579 because they slept or did something else for a while.580581 Note that locking is _not_ a replacement for reference counting.582 Locking is used to keep data structures coherent, while reference583 counting is a memory management technique. Usually both are needed, and 584 they are not to be confused with each other.585586 Many data structures can indeed have two levels of reference counting,587 when there are users of different "classes". The subclass count counts588 the number of subclass users, and decrements the global count just once 589 when the subclass count goes to zero.590591 Examples of this kind of "multi-level-reference-counting" can be found in592 memory management ("struct mm_struct": mm_users and mm_count), and in 593 filesystem code ("struct super_block": s_count and s_active).594595 Remember: if another thread can find your data structure, and you don't596 have a reference count on it, you almost certainly have a bug.597598599 Chapter 12: Macros, Enums and RTL600601 Names of macros defining constants and labels in enums are capitalized. 602603#define CONSTANT 0x12345604605 Enums are preferred when defining several related constants.606607 CAPITALIZED macro names are appreciated but macros resembling functions 608 may be named in lower case.609610 Generally, inline functions are preferable to macros resembling functions. 611612 Macros with multiple statements should be enclosed in a do - while block: 613614#define macrofun(a, b, c) \615do { \616if (a == 5) \617 do_this(b, c); \618 } while (0)619620 Things to avoid when using macros:6216221) macros that affect control flow:623624#define FOO(x) \625do { \626if (blah(x) < 0) \627return -EBUGGERED; \628 } while(0)629630is a _very_ bad idea. It looks like a function call but exits the "calling"631 function; don't break the internal parsers of those who will read the code.6326332) macros that depend on having a local variable with a magic name:634635#define FOO(val) bar(index, val)636637 might look like a good thing, but it's confusing as hell when one reads the6403) macros with arguments that are used as l-values: FOO(x) = y; will641 bite you if somebody e.g. turns FOO into an inline function.6426434) forgetting about precedence: macros defining constants using expressions644 must enclose the expression in parentheses. Beware of similar issues with645 macros using parameters.646647#define CONSTANT 0x4000648#define CONSTEXP (CONSTANT | 3)649650 The cpp manual deals with macros exhaustively. The gcc internals manual also 651 covers RTL which is used frequently with assembly language in the kernel.652653654 Chapter 13: Printing kernel messages655656 Kernel developers like to be seen as literate. Do mind the spelling657 of kernel messages to make a good impression. Do not use crippled658 words like "dont"; use "do not" or "don't" instead. Make the messages659 concise, clear, and unambiguous.660661 Kernel messages do not have to be terminated with a period.662663 Printing numbers in parentheses (%d) adds no value and should be avoided.664665 There are a number of driver model diagnostic macros in <linux/device.h>666 which you should use to make sure messages are matched to the right device 667 and driver, and are tagged with the right level: dev_err(), dev_warn(),668 dev_info(), and so forth. For messages that aren't associated with a669 particular device, <linux/printk.h> defines pr_debug() and pr_info().670671 Coming up with good debugging messages can be quite a challenge; and once 672 you have them, they can be a huge help for remote troubleshooting. Such673 messages should be compiled out when the DEBUG symbol is not defined (that 674is, by default they are not included). When you use dev_dbg() or pr_debug(),675 that's automatic. Many subsystems have Kconfig options to turn on -DDEBUG. 676 A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the 677 ones already enabled by DEBUG.678679680 Chapter 14: Allocating memory681682 The kernel provides the following general purpose memory allocators:683 kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and684 vzalloc(). Please refer to the API documentation for further information685 about them.686687 The preferred form for passing a size of a struct is the following:688689 p = kmalloc(sizeof(*p), ...);690691 The alternative form where struct name is spelled out hurts readability and692 introduces an opportunity for a bug when the pointer variable type is changed693 but the corresponding sizeof that is passed to a memory allocator is not.694695 Casting the return value which is a void pointer is redundant. The conversion696from void pointer to any other pointer type is guaranteed by the C programming 697 language.698699 The preferred form for allocating an array is the following:700701 p = kmalloc_array(n, sizeof(...), ...);702703 The preferred form for allocating a zeroed array is the following:704705 p = kcalloc(n, sizeof(...), ...);706707 Both forms check for overflow on the allocation size n * sizeof(...),708 and return NULL if that occurred.709710711 Chapter 15: The inline disease712713 There appears to be a common misperception that gcc has a magic "make me 714 faster" speedup option called "inline". While the use of inlines can be715 appropriate (for example as a means of replacing macros, see Chapter 12), it716 very often is not. Abundant use of the inline keyword leads to a much bigger717 kernel, which in turn slows the system as a whole down, due to a bigger718 icache footprint for the CPU and simply because there is less memory719 available for the pagecache. Just think about it; a pagecache miss causes a720 disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles721 that can go into these 5 milliseconds.。
