Documentation/CodingStyle - android_kernel_htc_msm8960 - Gitiles


 		Linux kernel coding style

 This is a short document describing the preferred coding style for the
 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: 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 are
 useful 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 NEVER
 EVER use a typedef unless you can clearly match one of those rules.

 In general, a pointer, or a struct that has elements that can reasonably
 be directly accessed should _never_ be a typedef.


 		Chapter 6: 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 7: 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 a)
 {
 	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 8: 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 9: 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 10: 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 11: 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 12: Macros, Enums 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 13: 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 14: 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 15: The inline disease

 There appears to be a common misperception that gcc has a magic "make me
 faster" speedup option called "inline". While the use of inlines can be
 appropriate (for example as a means of replacing macros, see Chapter 11), it
 very often is not. Abundant use of the inline keyword leads to a much bigger
 kernel, which in turn slows the system as a whole down, due to a bigger
 icache footprint for the CPU and simply because there is less memory
 available for the pagecache. Just think about it; a pagecache miss causes a
 disk seek, which easily takes 5 miliseconds. There are a LOT of cpu cycles
 that can go into these 5 miliseconds.

 A reasonable rule of thumb is to not put inline at functions that have more
 than 3 lines of code in them. An exception to this rule are the cases where
 a parameter is known to be a compiletime constant, and as a result of this
 constantness you *know* the compiler will be able to optimize most of your
 function away at compile time. For a good example of this later case, see
 the kmalloc() inline function.

 Often people argue that adding inline to functions that are static and used
 only once is always a win since there is no space tradeoff. While this is
 technically correct, gcc is capable of inlining these automatically without
 help, and the maintenance issue of removing the inline when a second user
 appears outweighs the potential value of the hint that tells gcc to do
 something it would have done anyway.


 		Appendix I: 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/manual/

 WG14 is the international standardization working group for the programming
 language C, URL: http://www.open-std.org/JTC1/SC22/WG14/

 Kernel CodingStyle, by greg@kroah.com at OLS 2002:
 http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/

 --
 Last updated on 30 April 2006.

	Linux kernel coding style

	This is a short document describing the preferred coding style for the
	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: 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 are
	useful 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 NEVER
	EVER use a typedef unless you can clearly match one of those rules.

	In general, a pointer, or a struct that has elements that can reasonably
	be directly accessed should _never_ be a typedef.


	Chapter 6: 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 7: 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 a)
	{
	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 8: 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 9: 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 10: 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 11: 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 12: Macros, Enums 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 13: 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 14: 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 15: The inline disease

	There appears to be a common misperception that gcc has a magic "make me
	faster" speedup option called "inline". While the use of inlines can be
	appropriate (for example as a means of replacing macros, see Chapter 11), it
	very often is not. Abundant use of the inline keyword leads to a much bigger
	kernel, which in turn slows the system as a whole down, due to a bigger
	icache footprint for the CPU and simply because there is less memory
	available for the pagecache. Just think about it; a pagecache miss causes a
	disk seek, which easily takes 5 miliseconds. There are a LOT of cpu cycles
	that can go into these 5 miliseconds.

	A reasonable rule of thumb is to not put inline at functions that have more
	than 3 lines of code in them. An exception to this rule are the cases where
	a parameter is known to be a compiletime constant, and as a result of this
	constantness you know the compiler will be able to optimize most of your
	function away at compile time. For a good example of this later case, see
	the kmalloc() inline function.

	Often people argue that adding inline to functions that are static and used
	only once is always a win since there is no space tradeoff. While this is
	technically correct, gcc is capable of inlining these automatically without
	help, and the maintenance issue of removing the inline when a second user
	appears outweighs the potential value of the hint that tells gcc to do
	something it would have done anyway.



	Appendix I: 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/manual/

	WG14 is the international standardization working group for the programming
	language C, URL: http://www.open-std.org/JTC1/SC22/WG14/

	Kernel CodingStyle, by greg@kroah.com at OLS 2002:
	http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/

	--
	Last updated on 30 April 2006.