Linux kernel coding style This is a short document describing the preferred coding style for thelinux kernel.  Coding style is very personal, and I won't _force_ myviews on anybody, but this is what goes for anything that I have to beable to maintain, and I'd prefer it for most other things too.  Pleaseat 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 tobe 3. Rationale: The whole idea behind indentation is to clearly define wherea block of control starts and ends.  Especially when you've been lookingat your screen for 20 straight hours, you'll find it a lot easier to seehow the indentation works if you have large indentations. Now, some people will claim that having 8-character indentations makesthe code move too far to the right, and makes it hard to read on a80-character terminal screen.  The answer to that is that if you needmore than 3 levels of indentation, you're screwed anyway, and should fixyour program. In short, 8-char indents make things easier to read, and have the addedbenefit 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 havesomething to hide: 	if (condition) do_this;	  do_something_everytime; Outside of comments, documentation and except in Kconfig, spaces are neverused 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 commonlyavailable 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 placedsubstantially to the right. The same applies to function headers with a longargument 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 ofbraces.  Unlike the indent size, there are few technical reasons tochoose one placement strategy over the other, but the preferred way, asshown to us by the prophets Kernighan and Ritchie, is to put the openingbrace 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 theopening 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 inconsistencyis ...  well ...  inconsistent, but all right-thinking people know that(a) K&R are _right_ and (b) K&R are right.  Besides, functions arespecial anyway (you can't nest them in C). Note that the closing brace is empty on a line of its own, _except_ inthe 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, likethis: 	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 thesupply of new-lines on your screen is not a renewable resource (think25-line terminal screens here), you have more empty lines to putcomments on.  		Chapter 4: Naming C is a Spartan language, and so should your naming be.  Unlike Modula-2and Pascal programmers, C programmers do not use cute names likeThisVariableIsATemporaryCounter.  A C programmer would call thatvariable "tmp", which is much easier to write, and not the least moredifficult to understand. HOWEVER, while mixed-case names are frowned upon, descriptive names forglobal variables are a must.  To call a global function "foo" is ashooting offense. GLOBAL variables (to be used only if you _really_ need them) need tohave descriptive names, as do global functions.  If you have a functionthat 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 Hungariannotation) is brain damaged - the compiler knows the types anyway and cancheck those, and it only confuses the programmer.  No wonder MicroSoftmakes buggy programs. LOCAL variable names should be short, and to the point.  If you havesome random integer loop counter, it should probably be called "i".Calling it "loop_counter" is non-productive, if there is no chance of itbeing mis-understood.  Similarly, "tmp" can be just about any type ofvariable that is used to hold a temporary value. If you are afraid to mix up your local variable names, you have anotherproblem, 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 shouldfit 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 thecomplexity and indentation level of that function.  So, if you have aconceptually simple function that is just one long (but simple)case-statement, where you have to do lots of small things for a lot ofdifferent cases, it's OK to have a longer function. However, if you have a complex function, and you suspect that aless-than-gifted first-year high-school student might not evenunderstand what the function is all about, you should adhere to themaximum limits all the more closely.  Use helper functions withdescriptive names (you can ask the compiler to in-line them if you thinkit's performance-critical, and it will probably do a better job of itthan you would have done). Another measure of the function is the number of local variables.  Theyshouldn't exceed 5-10, or you're doing something wrong.  Re-think thefunction, and split it into smaller pieces.  A human brain cangenerally easily keep track of about 7 different things, anything moreand it gets confused.  You know you're brilliant, but maybe you'd liketo 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 isused frequently by compilers in form of the unconditional jump instruction. The goto statement comes in handy when a function exits from multiplelocations 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.  NEVERtry to explain HOW your code works in a comment: it's much better towrite the code so that the _working_ is obvious, and it's a waste oftime 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 thefunction 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 makesmall comments to note or warn about something particularly clever (orugly), but try to avoid excess.  Instead, put the comments at the headof the function, telling people what it does, and possibly WHY it doesit.  		Chapter 8: You've made a mess of it That's OK, we all do.  You've probably been told by your long-time Unixuser helper that "GNU emacs" automatically formats the C sources foryou, and you've noticed that yes, it does do that, but the defaults ituses are less than desirable (in fact, they are worse than randomtyping - an infinite number of monkeys typing into GNU emacs would nevermake a good program). So, you can either get rid of GNU emacs, or change it to use sanervalues.  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 amodule, if you put the string -*- linux-c -*- somewhere on the firsttwo lines, this mode will be automatically invoked. Also, you may wantto 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 onautomagically when you edit source files under /usr/src/linux. But even if you fail in getting emacs to do sane formatting, noteverything is lost: use "indent". Now, again, GNU indent has the same brain-dead settings that GNU emacshas, 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 indentrecognize the authority of K&R (the GNU people aren't evil, they arejust severely misguided in this matter), so you just give indent theoptions "-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 commentre-formatting you may want to take a look at the man page.  Butremember: "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 explodeendif Generally, CONFIG_EXPERIMENTAL should surround all options not consideredstable. All options that are known to trash data (experimental write-support for file-systems, for instance) should be denoted (DANGEROUS), otherexperimental options should be denoted (EXPERIMENTAL).  		Chapter 10: Data structures Data structures that have visibility outside the single-threadedenvironment they are created and destroyed in should always havereference counts.  In the kernel, garbage collection doesn't exist (andoutside the kernel garbage collection is slow and inefficient), whichmeans that you absolutely _have_ to reference count all your uses. Reference counting means that you can avoid locking, and allows multipleusers to have access to the data structure in parallel - and not havingto worry about the structure suddenly going away from under them justbecause 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 referencecounting is a memory management technique.  Usually both are needed, andthey 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 countsthe number of subclass users, and decrements the global count just oncewhen the subclass count goes to zero. Examples of this kind of "multi-level-reference-counting" can be found inmemory management ("struct mm_struct": mm_users and mm_count), and infilesystem code ("struct super_block": s_count and s_active). Remember: if another thread can find your data structure, and you don'thave 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 functionsmay 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 thecode and it's prone to breakage from seemingly innocent changes. 3) macros with arguments that are used as l-values: FOO(x) = y; willbite you if somebody e.g. turns FOO into an inline function. 4) forgetting about precedence: macros defining constants using expressionsmust enclose the expression in parentheses. Beware of similar issues withmacros using parameters. #define CONSTANT 0x4000#define CONSTEXP (CONSTANT | 3) The cpp manual deals with macros exhaustively. The gcc internals manual alsocovers 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 spellingof kernel messages to make a good impression. Do not use crippledwords 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: References The C Programming Language, Second Editionby 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 Programmingby 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 programminglanguage C, URL: http://std.dkuug.dk/JTC1/SC22/WG14/ --Last updated on 16 February 2004 by a community effort on LKML.