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 
  ", 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.