--- /dev/null
+
+ 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.
+
+
+ Chapter 2: 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 3: 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 4: 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
+that 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 5: 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 4 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 6: 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 - a 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 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]$" . linux-c-mode)
+ auto-mode-alist))
+
+to your .emacs file if you want to have linux-c-mode switched on
+automagically when you edit source files under /usr/src/linux.
+
+But even if you fail in getting emacs to do sane formatting, not
+everything is lost: use "indent".
+
+Now, again, GNU indent has the same brain dead settings that GNU emacs
+has, which is why you need to give it a few command line options.
+However, that's not too bad, because even the makers of GNU indent
+recognize the authority of K&R (the GNU people aren't evil, they are
+just severely misguided in this matter), so you just give indent the
+options "-kr -i8" (stands for "K&R, 8 character indents").
+
+"indent" has a lot of options, and especially when it comes to comment
+re-formatting you may want to take a look at the manual page. But
+remember: "indent" is not a fix for bad programming.
+
+
+ Chapter 7: Configuration-files
+
+For configuration options (arch/xxx/config.in, and all the Config.in files),
+somewhat different indentation is used.
+
+An indention level of 3 is used in the code, while the text in the config-
+options should have an indention-level of 2 to indicate dependencies. The
+latter only applies to bool/tristate options. For other options, just use
+common sense. An example:
+
+if [ "$CONFIG_EXPERIMENTAL" = "y" ]; then
+ tristate 'Apply nitroglycerine inside the keyboard (DANGEROUS)' CONFIG_BOOM
+ if [ "$CONFIG_BOOM" != "n" ]; then
+ bool ' Output nice messages when you explode' CONFIG_CHEER
+ fi
+fi
+
+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).
git://git.jankratochvil.net / gnokii.git / blobdiff