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前阵子做驱动的时候看得文章，Torvalds Linus所作，值得借鉴一下( 当然，现在也没有能力作出什么反驳:) )。中文版本是Google而来，我也不知道是哪位仁兄翻译的，若有冒犯，还请指出。另外，英文版的内容MS要新一些。 关键字：Linux, kernel, style
这篇简短的文章描述了Linux内核首选的编码风格。编码风格是很个人化的东西，我不会把自己的观点强加给任何人。但是，Linux内核的代码毕竟是我必须有能力维护的，因此我宁愿它的编码风格是我喜欢的。请至少考虑一下这一点。 首先，我建议打印一份《GNU编码标准》，不要阅读它。烧掉它，它不过是象征性的姿态。 然后，请看： 第 1 章： 缩进 Tabs(制表符)是8个字符的大小，因此缩进也应该是8个字符的大小。有些叛逆主张试图把缩进变成4个(甚至是2个！)字符的长度，这就好象试图把PI(案，圆周率)定义成3是一样的。 依据：缩进背后的思想是:清楚地定义一个控制块从哪里开始，到哪里结束。尤其是在你连续不断的盯了20个小时的屏幕后，如果你有大尺寸的缩进。你将更容易发现缩进的好处。 现在，有些人说8个字符大小的缩进导致代码太偏右了，并且在一个80字符宽的终端屏幕上看着很不舒服。对这个问题的回答是：如果你有超过3个级别的缩进，你就有点犯糊涂了，应当修改你的程序。 简而言之，8个字符的缩进使程序更易读，而且当你把功能隐藏的太深时，多层次的缩进还会对此很直观的给出警告。要留心这种警告信息。 第 2 章： 放置花括号
C程序中另一个要主意的就是花括号的放置。与缩进尺寸不同的是，关于如何放置花括号没有技术上的理由。但是，首选的方法是象先知Brain Kernighan和Dennis Ritchie展现的那样：把左括号放在行尾，右括号放在行首。也就是： if (x is true) { we do y } 然而，还有另外一种情况，就是函数：函数应当把左右括号都放在行首。也就是： int function(int x) { body of function } 叛逆的人们所在皆有。他们说，这样会导致…嗯，不一致性(案，指函数的花括号使用与其他情况不统一)。但是所有正确思考的人都知道：(1) K&R是正确的；(2) K&R还是正确的。而且，函数与别任何东西都不一样(在C语言中你没法隐藏它)。 注意，右括号所在的行不应当有其它东西，除非跟随着一个条件判断。也就是do-while语句中的“while”和if-else语句中的“else”。象这样： do { body of do-loop } while (condition); 和： if (x == y) { .. } else if (x > y) { ... } else { .... } 依据： K&R。 而且，注意这种花括号的放置减少了空行的数目，并没损害可读性。因此，当屏幕上不可以有很多空行时(试想25行的终端屏幕)，你就有更多的空行来安插注释。 第 3 章： 命名 C是一门朴素的语言，你使用的命名也应该这样。与Modula-2和Pascal程序员不同，C程序员不使用诸如 “ThisVariableIsATemporaryCounter”这样“聪明”的名字。C程序员应该叫它“tmp”，这写起来更简单，也不会更难懂。 然而，当面对复杂情况时就有些棘手，给全局变量取一个描述性的名字是必要的。把一个全局函数叫做“foo”是一种目光短浅的行为。 全局变量(只当你确实需要时才用)应该有描述性的名字，全局函数也一样。如果你有一个统计当前用户个数的函数，应当把它命名为“count_active_user()”或者简单点些的类似名称，不应该命名为“cntusr()”。 把函数类型写进函数名(即所谓的“匈牙利命名法”)简直就是大脑有问题──编译器总是知道函数的类型并且能加以检查，这种命名法只会弄糊涂程序员自己。怪不得微软总是制造充满bug的程序。 局部变量的名字应该尽量短，而且说到点子上。如果你有个普通的整型循环计数变量，应当命名为“i”。命名为“loop_counter”并不能带来任何成效，如果它不被误解的话(案，这里的言外之意是说，如果被误解就更惨了)。与此类似，“tmp”可以作为一个用来存储任何类型临时值的变量的名字。 如果你害怕弄混淆局部变量(s)的名字，你就面临着另一个问题，也叫作“函数增长荷尔蒙失调综合症”。请参考下一章。 第 4 章： 函数 函数应当短而精美，而且只做一件事。它们应当占满1或2个屏幕(就象我们知道的那样，ISO/ANSI的屏幕大小是80X24)，只做一件事并且把它做好。 一个函数的最大长度与它的复杂度和缩进级别成反比。所以，如果如果你有一个概念上简单(案，“简单”是simple而不是easy)的函数，它恰恰包含着一个很长的case语句，这样你不得不为不同的情况准备不同的处理，那么这样的长函数是没问题的。 然而，如果你有一个复杂的函数，你猜想一个并非天才的高一学生可能看不懂得这个函数，你就应当努力把它减缩得更接近前面提到的最大函数长度限制。可以使用一些辅助函数，给它们取描述性的名字(如果你认为这些辅助函数的调用是性能关键的，可以让编译器把它们内联进来，这比在单个函数内完成所有的事情通常要好些)。 对函数还存在另一个测量标准：局部变量的数目。这不该超过5到10个，否则你可能会弄错。应当重新考虑这个函数，把它分解成小片。人类的大脑一般能同时记住7个不同的东西，超过这个数目就会犯糊涂。或许你认为自己很聪明，那么请你理解一下从现在开始的2周时间你都做什么了。 第 5 章：注释 注释是有用的，但过量的注释则是有害的。不要试图在注释中解释你的代码是如何工作的：把代码是如何工作的视为一件显然的事情会更好些，而且，给糟糕的代码作注释则是在浪费时间。 通常，你愿意自己的注释说出代码是做什么的，而不是如何做。还有，尽量避免在函数体内作注释：如果函数很复杂，你很可能需要分开来注释，回头到第4章去看看吧。你可以给一段代码──漂亮的或丑陋的──作注释以引起注意或警告，但是不要过量。取而代之，应当把注释放在函数首部，告诉人们该函数作什么，而不是为什么这样做。 第 6 章：你把事情弄乱了 好吧，我们来看看。很可能有长期使用UNIX的人告诉过你，“GNU emacs”能自动为你格式化C程序源代码，你注意到这是真的，它确实能做到，但是缺省情况下它的用处远远小于期望值──键入无数的monkeys到GNU emacs中绝不可能造出好的程序。 因此，你可以或者删除GNU emacs，或者对它进行理智的配置。对于后者，可以把下面的行粘贴到你的.emacs文件中： (defun linux-c-mode () "C mode with adjusted defaults for use with the Linux kernel." (interactive) (c-mode) (c-set-style "K&R") (setq c-basic-offset 8)) 这将会定义一个把C代码弄成linux风格的命令。当hacking一个模块时，如果你把“-*- linux-c -*-”放到了最初的两行，这个模块将被自动调用。而且，如果你打算每当在/usr/src/linux下编辑源文件时就自动调用它，也许你会把下面的命令： (setq auto-mode-alist (cons '("/usr/src/linux.*/.*\\.[ch]$" . linux-c-mode) auto-mode-alist)) 添加进你的.emacs文件。 但是，即使你没能让emacs正确做到格式化，也并非将就此一无所有：还有“indent”程序呢。 嗯，再提醒一下，GNU indent跟GNU emacs有同样的毛病，这就需要你给它一些命令行选项。然而，这不是很糟糕的事，因为即使是GNU indent也承认K&R的权威性(GNU的人不是魔鬼，他们只是在这里太过严格了，以致于误导人)，所以你可以只需给indent这样的选项： “-kr -i8”(表示“K&R风格，8个字符的缩进”)。 “indent”程序有很多选项，特别是当为重排过的程序作注释的时候，你需要看一下它的手册。记住：“indent”可不是修正糟糕程序的万能钥匙。 第 7 章： 配置文件(configuration-files) 对配置选项来说(arch/xxx/config.in和所有的Config.in文件)，使用不同的缩进风格。 若代码中的缩进级别为3，配置选项就应该为2,这样可以暗示出依赖关系。后者只是用于bool/tristate(即二态/三态)的选项。对其它情况用常识就行了。举例来说： if [ "$CONFIG_EXPERIMENTAL" = "y" ]; then tristate 'Apply nitroglycerine inside the keyboard (DANGEROUS)' CONFIG_BOOM if [ "$CONFIG_BOOM" != "n" ]; then bool ' Output nice messages when you explode' CONFIG_CHEER fi fi 通常CONFIG_EXPERIMENTAL应当在所有不稳定的选项的周围出现。所有已知会破坏数据的选项(如文件系统的实验性的写支持功能)应当被标记为(DANGEROUS)，其他实验性的选项应当被标记为(EXPERIMENTAL)。 第 8 章： 数据结构 假如数据结构在其被创建/销毁的线程环境(案：这里说的线程是一个执行实体，可能是进程、内核线程或其它)之外还具有可见性，那么他们都该有引用计数。在内核中没有垃圾收集机制(而且内核之外的垃圾收集也是缓慢而低效的)，这意味着你绝对应该为每一次使用进行引用计数。 引用计数意味着你可以避开锁，还能允许多个线程并行访问该数据结构──而且不用担心仅仅因为访问数据结构的线程睡眠了一会儿或者干别的去了，它们就会消失。 注意，锁不是引用计数的替代品。锁是用来保持数据结构的一致性的，而引用计数是一种内存管理技术。通常二者都需要，而且不会彼此混淆。 确实有许多数据结构可以有两个级别的引用计数，当使用者具有不同的“等级”(classes)时就是这样。子等级(subclass)记录了处于该等级的使用者个数，而且当它减到零的时候就把总体计数(global count)减一。 这种“多级引用计数”(multi-reference-counting)的一个实例可以在内存管理子系统("struct mm_struct":mm_users和mm_count)中找到，也可以在文件系统的代码中("struct super_block":s_count和s_active)找到。 记住：如果另一个线程能找到你的数据结构，而你有没对它做引用计数，那几乎可以肯定：这是一个bug。 英文的，MS要新一些
Linux kernel coding style Torvalds Linus.
This is a short document describing the preferred coding style for the
(Torvalds Linus)linux kernel. Coding style is very personal, and I won't _force_ my views on anybody, but this is what goes for anything that I have to be able to maintain, and I'd prefer it for most other things too. Please at least consider the points made here. First off, I'd suggest printing out a copy of the GNU coding standards, and NOT read it. Burn them, it's a great symbolic gesture. Anyway, here goes: Chapter 1: Indentation Tabs are 8 characters, and thus indentations are also 8 characters. There are heretic movements that try to make indentations 4 (or even 2!) characters deep, and that is akin to trying to define the value of PI to be 3. Rationale: The whole idea behind indentation is to clearly define where a block of control starts and ends. Especially when you've been looking at your screen for 20 straight hours, you'll find it a lot easier to see how the indentation works if you have large indentations. Now, some people will claim that having 8-character indentations makes the code move too far to the right, and makes it hard to read on a 80-character terminal screen. The answer to that is that if you need more than 3 levels of indentation, you're screwed anyway, and should fix your program. In short, 8-char indents make things easier to read, and have the added benefit of warning you when you're nesting your functions too deep. Heed that warning. Don't put multiple statements on a single line unless you have something to hide: if (condition) do_this; do_something_everytime; Outside of comments, documentation and except in Kconfig, spaces are never used for indentation, and the above example is deliberately broken. Get a decent editor and don't leave whitespace at the end of lines. Chapter 2: Breaking long lines and strings Coding style is all about readability and maintainability using commonly available tools. The limit on the length of lines is 80 columns and this is a hard limit. Statements longer than 80 columns will be broken into sensible chunks. Descendants are always substantially shorter than the parent and are placed substantially to the right. The same applies to function headers with a long argument list. Long strings are as well broken into shorter strings. void fun(int a, int b, int c) { if (condition) printk(KERN_WARNING "Warning this is a long printk with " "3 parameters a: %u b: %u " "c: %u \n", a, b, c); else next_statement; } Chapter 3: Placing Braces The other issue that always comes up in C styling is the placement of braces. Unlike the indent size, there are few technical reasons to choose one placement strategy over the other, but the preferred way, as shown to us by the prophets Kernighan and Ritchie, is to put the opening brace last on the line, and put the closing brace first, thusly: if (x is true) { we do y } However, there is one special case, namely functions: they have the opening brace at the beginning of the next line, thus: int function(int x) { body of function } Heretic people all over the world have claimed that this inconsistency is ... well ... inconsistent, but all right-thinking people know that (a) K&R are _right_ and (b) K&R are right. Besides, functions are special anyway (you can't nest them in C). Note that the closing brace is empty on a line of its own, _except_ in the cases where it is followed by a continuation of the same statement, ie a "while" in a do-statement or an "else" in an if-statement, like this: do { body of do-loop } while (condition); and if (x == y) { .. } else if (x > y) { ... } else { .... } Rationale: K&R. Also, note that this brace-placement also minimizes the number of empty (or almost empty) lines, without any loss of readability. Thus, as the supply of new-lines on your screen is not a renewable resource (think 25-line terminal screens here), you have more empty lines to put comments on. Chapter 4: Naming C is a Spartan language, and so should your naming be. Unlike Modula-2 and Pascal programmers, C programmers do not use cute names like ThisVariableIsATemporaryCounter. A C programmer would call that variable "tmp", which is much easier to write, and not the least more difficult to understand. HOWEVER, while mixed-case names are frowned upon, descriptive names for global variables are a must. To call a global function "foo" is a shooting offense. GLOBAL variables (to be used only if you _really_ need them) need to have descriptive names, as do global functions. If you have a function that counts the number of active users, you should call that "count_active_users()" or similar, you should _not_ call it "cntusr()". Encoding the type of a function into the name (so-called Hungarian notation) is brain damaged - the compiler knows the types anyway and can check those, and it only confuses the programmer. No wonder MicroSoft makes buggy programs. LOCAL variable names should be short, and to the point. If you have some random integer loop counter, it should probably be called "i". Calling it "loop_counter" is non-productive, if there is no chance of it being mis-understood. Similarly, "tmp" can be just about any type of variable that is used to hold a temporary value. If you are afraid to mix up your local variable names, you have another problem, which is called the function-growth-hormone-imbalance syndrome. See next chapter. Chapter 5: Functions Functions should be short and sweet, and do just one thing. They should fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24, as we all know), and do one thing and do that well. The maximum length of a function is inversely proportional to the complexity and indentation level of that function. So, if you have a conceptually simple function that is just one long (but simple) case-statement, where you have to do lots of small things for a lot of different cases, it's OK to have a longer function. However, if you have a complex function, and you suspect that a less-than-gifted first-year high-school student might not even understand what the function is all about, you should adhere to the maximum limits all the more closely. Use helper functions with descriptive names (you can ask the compiler to in-line them if you think it's performance-critical, and it will probably do a better job of it than you would have done). Another measure of the function is the number of local variables. They shouldn't exceed 5-10, or you're doing something wrong. Re-think the function, and split it into smaller pieces. A human brain can generally easily keep track of about 7 different things, anything more and it gets confused. You know you're brilliant, but maybe you'd like to understand what you did 2 weeks from now. Chapter 6: Centralized exiting of functions Albeit deprecated by some people, the equivalent of the goto statement is used frequently by compilers in form of the unconditional jump instruction. The goto statement comes in handy when a function exits from multiple locations and some common work such as cleanup has to be done. The rationale is: - unconditional statements are easier to understand and follow - nesting is reduced - errors by not updating individual exit points when making modifications are prevented - saves the compiler work to optimize redundant code away ;) int fun(int ) { int result = 0; char *buffer = kmalloc(SIZE); if (buffer == NULL) return -ENOMEM; if (condition1) { while (loop1) { ... } result = 1; goto out; } ... out: kfree(buffer); return result; } Chapter 7: Commenting Comments are good, but there is also a danger of over-commenting. NEVER try to explain HOW your code works in a comment: it's much better to write the code so that the _working_ is obvious, and it's a waste of time to explain badly written code. Generally, you want your comments to tell WHAT your code does, not HOW. Also, try to avoid putting comments inside a function body: if the function is so complex that you need to separately comment parts of it, you should probably go back to chapter 5 for a while. You can make small comments to note or warn about something particularly clever (or ugly), but try to avoid excess. Instead, put the comments at the head of the function, telling people what it does, and possibly WHY it does it. When commenting the kernel API functions, please use the kerneldoc format. See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc for details. Chapter 8: You've made a mess of it That's OK, we all do. You've probably been told by your long-time Unix user helper that "GNU emacs" automatically formats the C sources for you, and you've noticed that yes, it does do that, but the defaults it uses are less than desirable (in fact, they are worse than random typing - an infinite number of monkeys typing into GNU emacs would never make a good program). So, you can either get rid of GNU emacs, or change it to use saner values. To do the latter, you can stick the following in your .emacs file: (defun linux-c-mode () "C mode with adjusted defaults for use with the Linux kernel." (interactive) (c-mode) (c-set-style "K&R") (setq tab-width 8) (setq indent-tabs-mode t) (setq c-basic-offset 8)) This will define the M-x linux-c-mode command. When hacking on a module, if you put the string -*- linux-c -*- somewhere on the first two lines, this mode will be automatically invoked. Also, you may want to add (setq auto-mode-alist (cons '("/usr/src/linux.*/.*\\.[ch]$" . linux-c-mode) auto-mode-alist)) to your .emacs file if you want to have linux-c-mode switched on automagically when you edit source files under /usr/src/linux. But even if you fail in getting emacs to do sane formatting, not everything is lost: use "indent". Now, again, GNU indent has the same brain-dead settings that GNU emacs has, which is why you need to give it a few command line options. However, that's not too bad, because even the makers of GNU indent recognize the authority of K&R (the GNU people aren't evil, they are just severely misguided in this matter), so you just give indent the options "-kr -i8" (stands for "K&R, 8 character indents"), or use "scripts/Lindent", which indents in the latest style. "indent" has a lot of options, and especially when it comes to comment re-formatting you may want to take a look at the man page. But remember: "indent" is not a fix for bad programming. Chapter 9: Configuration-files For configuration options (arch/xxx/Kconfig, and all the Kconfig files), somewhat different indentation is used. Help text is indented with 2 spaces. if CONFIG_EXPERIMENTAL tristate CONFIG_BOOM default n help Apply nitroglycerine inside the keyboard (DANGEROUS) bool CONFIG_CHEER depends on CONFIG_BOOM default y help Output nice messages when you explode endif Generally, CONFIG_EXPERIMENTAL should surround all options not considered stable. All options that are known to trash data (experimental write- support for file-systems, for instance) should be denoted (DANGEROUS), other experimental options should be denoted (EXPERIMENTAL). Chapter 10: Data structures Data structures that have visibility outside the single-threaded environment they are created and destroyed in should always have reference counts. In the kernel, garbage collection doesn't exist (and outside the kernel garbage collection is slow and inefficient), which means that you absolutely _have_ to reference count all your uses. Reference counting means that you can avoid locking, and allows multiple users to have access to the data structure in parallel - and not having to worry about the structure suddenly going away from under them just because they slept or did something else for a while. Note that locking is _not_ a replacement for reference counting. Locking is used to keep data structures coherent, while reference counting is a memory management technique. Usually both are needed, and they are not to be confused with each other. Many data structures can indeed have two levels of reference counting, when there are users of different "classes". The subclass count counts the number of subclass users, and decrements the global count just once when the subclass count goes to zero. Examples of this kind of "multi-level-reference-counting" can be found in memory management ("struct mm_struct": mm_users and mm_count), and in filesystem code ("struct super_block": s_count and s_active). Remember: if another thread can find your data structure, and you don't have a reference count on it, you almost certainly have a bug. Chapter 11: Macros, Enums, Inline functions and RTL Names of macros defining constants and labels in enums are capitalized. #define CONSTANT 0x12345 Enums are preferred when defining several related constants. CAPITALIZED macro names are appreciated but macros resembling functions may be named in lower case. Generally, inline functions are preferable to macros resembling functions. Macros with multiple statements should be enclosed in a do - while block: #define macrofun(a, b, c) \ do { \ if (a == 5) \ do_this(b, c); \ } while (0) Things to avoid when using macros: 1) macros that affect control flow: #define FOO(x) \ do { \ if (blah(x) < 0) \ return -EBUGGERED; \ } while(0) is a _very_ bad idea. It looks like a function call but exits the "calling" function; don't break the internal parsers of those who will read the code. 2) macros that depend on having a local variable with a magic name: #define FOO(val) bar(index, val) might look like a good thing, but it's confusing as hell when one reads the code and it's prone to breakage from seemingly innocent changes. 3) macros with arguments that are used as l-values: FOO(x) = y; will bite you if somebody e.g. turns FOO into an inline function. 4) forgetting about precedence: macros defining constants using expressions must enclose the expression in parentheses. Beware of similar issues with macros using parameters. #define CONSTANT 0x4000 #define CONSTEXP (CONSTANT | 3) The cpp manual deals with macros exhaustively. The gcc internals manual also covers RTL which is used frequently with assembly language in the kernel. Chapter 12: Printing kernel messages Kernel developers like to be seen as literate. Do mind the spelling of kernel messages to make a good impression. Do not use crippled words like "dont" and use "do not" or "don't" instead. Kernel messages do not have to be terminated with a period. Printing numbers in parentheses (%d) adds no value and should be avoided. Chapter 13: Allocating memory The kernel provides the following general purpose memory allocators: kmalloc(), kzalloc(), kcalloc(), and vmalloc(). Please refer to the API documentation for further information about them. The preferred form for passing a size of a struct is the following: p = kmalloc(sizeof(*p), ...); The alternative form where struct name is spelled out hurts readability and introduces an opportunity for a bug when the pointer variable type is changed but the corresponding sizeof that is passed to a memory allocator is not. Casting the return value which is a void pointer is redundant. The conversion from void pointer to any other pointer type is guaranteed by the C programming language. Chapter 14: References The C Programming Language, Second Edition by Brian W. Kernighan and Dennis M. Ritchie. Prentice Hall, Inc., 1988. ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback). URL: http://cm.bell-labs.com/cm/cs/cbook/ The Practice of Programming by Brian W. Kernighan and Rob Pike. Addison-Wesley, Inc., 1999. ISBN 0-201-61586-X. URL: http://cm.bell-labs.com/cm/cs/tpop/ GNU manuals - where in compliance with K&R and this text - for cpp, gcc, gcc internals and indent, all available from http://www.gnu.org WG14 is the international standardization working group for the programming language C, URL: http://std.dkuug.dk/JTC1/SC22/WG14/ -- Last updated on 16 February 2004 by a community effort on LKML. |