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+
+                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: 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.
+
+
+                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: 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.