Linux kernel coding style 1
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 sym-
bolic 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 indenta-
tion works if you have large indentations.
Now, some people will claim that having 8-character indenta-
tions 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 indenta-
tion, 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 sensi-
ble 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 0, 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 num-
ber 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 under-
stand.
HOWEVER, while mixed-case names are frowned upon, descrip-
tive 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 proba-
bly be called "i". Calling it "loop_counter" is non-produc-
tive, 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-crit-
ical, 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 vari-
ables. They shouldn't exceed 5-10, or you're doing some-
thing 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 mak-
ing 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-com-
menting. NEVER try to explain HOW your code works in a com-
ment: it's much better to write the code so that the _work-
ing_ 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 com-
mand 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 mis-
guided 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 pro-
gramming.
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 cer-
tainly 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 resem-
bling functions may be named in lower case.
Generally, inline functions are preferable to macros resem-
bling 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 parenthe-
ses. 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/
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