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. The preferred way to ease multiple indentation levels in a switch statement isto align the "switch" and its subordinate "case" labels in the same columninstead of "double-indenting" the "case" labels.  E.g.: 	switch (suffix) {	case 'G':	case 'g':		mem <<= 30;		break;	case 'M':	case 'm':		mem <<= 20;		break;	case 'K':	case 'k':		mem <<= 10;		/* fall through */	default:		break;	} Don't put multiple statements on a single line unless you havesomething to hide: 	if (condition) do_this;	  do_something_everytime; Don't put multiple assignments on a single line either.  Kernel coding styleis super simple.  Avoid tricky expressions. 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 stronglypreferred limit. Statements longer than 80 columns will be broken into sensible chunks, unlessexceeding 80 columns significantly increases readability and does not hideinformation. Descendants are always substantially shorter than the parent andare placed substantially to the right. The same applies to function headerswith a long argument list. However, never break user-visible strings such asprintk messages, because that breaks the ability to grep for them.  		Chapter 3: Placing Braces and Spaces 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	} This applies to all non-function statement blocks (if, switch, for,while, do).  E.g.: 	switch (action) {	case KOBJ_ADD:		return "add";	case KOBJ_REMOVE:		return "remove";	case KOBJ_CHANGE:		return "change";	default:		return NULL;	} 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. Do not unnecessarily use braces where a single statement will do. 	if (condition)		action(); and 	if (condition)		do_this();	else		do_that(); This does not apply if only one branch of a conditional statement is a singlestatement; in the latter case use braces in both branches: 	if (condition) {		do_this();		do_that();	} else {		otherwise();	} 		3.1:  Spaces Linux kernel style for use of spaces depends (mostly) onfunction-versus-keyword usage.  Use a space after (most) keywords.  Thenotable exceptions are sizeof, typeof, alignof, and __attribute__, which looksomewhat like functions (and are usually used with parentheses in Linux,although they are not required in the language, as in: "sizeof info" after"struct fileinfo info;" is declared). So use a space after these keywords: 	if, switch, case, for, do, while but not with sizeof, typeof, alignof, or __attribute__.  E.g., 	s = sizeof(struct file); Do not add spaces around (inside) parenthesized expressions.  This example is*bad*: 	s = sizeof( struct file ); When declaring pointer data or a function that returns a pointer type, thepreferred use of '*' is adjacent to the data name or function name and notadjacent to the type name.  Examples: 	char *linux_banner;	unsigned long long memparse(char *ptr, char **retptr);	char *match_strdup(substring_t *s); Use one space around (on each side of) most binary and ternary operators,such as any of these: 	=  +  -  <  >  *  /  %  |  &  ^  <=  >=  ==  !=  ?  : but no space after unary operators: 	&  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined no space before the postfix increment & decrement unary operators: 	++  -- no space after the prefix increment & decrement unary operators: 	++  -- and no space around the '.' and "->" structure member operators. Do not leave trailing whitespace at the ends of lines.  Some editors with"smart" indentation will insert whitespace at the beginning of new lines asappropriate, so you can start typing the next line of code right away.However, some such editors do not remove the whitespace if you end up notputting a line of code there, such as if you leave a blank line.  As a result,you end up with lines containing trailing whitespace. Git will warn you about patches that introduce trailing whitespace, and canoptionally strip the trailing whitespace for you; however, if applying a seriesof patches, this may make later patches in the series fail by changing theircontext lines.  		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 chapter 6 (Functions).  		Chapter 5: Typedefs Please don't use things like "vps_t".It's a _mistake_ to use typedef for structures and pointers. When you see a 	vps_t a; in the source, what does it mean?In contrast, if it says 	struct virtual_container *a; you can actually tell what "a" is. Lots of people think that typedefs "help readability". Not so. They areuseful only for:  (a) totally opaque objects (where the typedef is actively used to _hide_     what the object is).      Example: "pte_t" etc. opaque objects that you can only access using     the proper accessor functions.      NOTE! Opaqueness and "accessor functions" are not good in themselves.     The reason we have them for things like pte_t etc. is that there     really is absolutely _zero_ portably accessible information there.  (b) Clear integer types, where the abstraction _helps_ avoid confusion     whether it is "int" or "long".      u8/u16/u32 are perfectly fine typedefs, although they fit into     category (d) better than here.      NOTE! Again - there needs to be a _reason_ for this. If something is     "unsigned long", then there's no reason to do 	typedef unsigned long myflags_t;      but if there is a clear reason for why it under certain circumstances     might be an "unsigned int" and under other configurations might be     "unsigned long", then by all means go ahead and use a typedef.  (c) when you use sparse to literally create a _new_ type for     type-checking.  (d) New types which are identical to standard C99 types, in certain     exceptional circumstances.      Although it would only take a short amount of time for the eyes and     brain to become accustomed to the standard types like 'uint32_t',     some people object to their use anyway.      Therefore, the Linux-specific 'u8/u16/u32/u64' types and their     signed equivalents which are identical to standard types are     permitted -- although they are not mandatory in new code of your     own.      When editing existing code which already uses one or the other set     of types, you should conform to the existing choices in that code.  (e) Types safe for use in userspace.      In certain structures which are visible to userspace, we cannot     require C99 types and cannot use the 'u32' form above. Thus, we     use __u32 and similar types in all structures which are shared     with userspace. Maybe there are other cases too, but the rule should basically be to NEVEREVER use a typedef unless you can clearly match one of those rules. In general, a pointer, or a struct that has elements that can reasonablybe directly accessed should _never_ be a typedef.  		Chapter 6: 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. In source files, separate functions with one blank line.  If the function isexported, the EXPORT* macro for it should follow immediately after the closingfunction brace line.  E.g.: 	int system_is_up(void)	{		return system_state == SYSTEM_RUNNING;	}	EXPORT_SYMBOL(system_is_up); In function prototypes, include parameter names with their data types.Although this is not required by the C language, it is preferred in Linuxbecause it is a simple way to add valuable information for the reader.  		Chapter 7: 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.  If there is nocleanup needed then just return directly. Choose label names which say what the goto does or why the goto exists.  Anexample of a good name could be "out_buffer:" if the goto frees "buffer".  Avoidusing GW-BASIC names like "err1:" and "err2:".  Also don't name them after thegoto location like "err_kmalloc_failed:" The rationale for using gotos 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 a)	{		int result = 0;		char *buffer; 		buffer = kmalloc(SIZE, GFP_KERNEL);		if (!buffer)			return -ENOMEM; 		if (condition1) {			while (loop1) {				...			}			result = 1;			goto out_buffer;		}		...	out_buffer:		kfree(buffer);		return result;	} A common type of bug to be aware of is "one err bugs" which look like this: 	err:		kfree(foo->bar);		kfree(foo);		return ret; The bug in this code is that on some exit paths "foo" is NULL.  Normally thefix for this is to split it up into two error labels "err_bar:" and "err_foo:".  		Chapter 8: 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 6 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. When commenting the kernel API functions, please use the kernel-doc format.See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-docfor details. Linux style for comments is the C89 "/* ... */" style.Don't use C99-style "// ..." comments. The preferred style for long (multi-line) comments is: 	/*	 * This is the preferred style for multi-line	 * comments in the Linux kernel source code.	 * Please use it consistently.	 *	 * Description:  A column of asterisks on the left side,	 * with beginning and ending almost-blank lines.	 */ For files in net/ and drivers/net/ the preferred style for long (multi-line)comments is a little different. 	/* The preferred comment style for files in net/ and drivers/net	 * looks like this.	 *	 * It is nearly the same as the generally preferred comment style,	 * but there is no initial almost-blank line.	 */ It's also important to comment data, whether they are basic types or derivedtypes.  To this end, use just one data declaration per line (no commas formultiple data declarations).  This leaves you room for a small comment on eachitem, explaining its use.  		Chapter 9: 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 never

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