Linux kernel coding style

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Linux Kernel Coding Style

Linus Torvalds

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.

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Linux kernel coding style

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, i.e. 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 {

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Linux kernel coding style

        ....

}

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.

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Linux kernel coding style

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 80×24, 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 

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Linux kernel coding style

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)

  (setq c-indent-level 8)

  (setq c-brace-imaginary-offset 0)

  (setq c-brace-offset -8)

  (setq c-argdecl-indent 8)

  (setq c-label-offset -8)

  (setq c-continued-statement-offset 8)

  (setq indent-tabs-mode nil)

  (setq tab-width 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.

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Linux kernel coding style

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.

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