Development: Coding Style - Revision history

CityK: add cat.

2009-01-15T23:52:38Z

add cat.

← Older revision		Revision as of 23:52, 15 January 2009
Line 421:		Line 421:

	Last updated on 16 February 2004 by a community effort on LKML.		Last updated on 16 February 2004 by a community effort on LKML.
		+
		+	[[Category:Development]]

CityK: Documentation/CodingStyle moved to Development: Coding Style: name

2009-01-15T00:45:46Z

Documentation/CodingStyle moved to Development: Coding Style: name

← Older revision

Revision as of 00:45, 15 January 2009

CityK: Add link to online documentation

2009-01-15T00:45:23Z

Add link to online documentation

← Older revision		Revision as of 00:45, 15 January 2009
Line 1:		Line 1:
-	The following content is a copy of the <code>/usr/src/linux/Documentation/CodingStyle</code> file (modified slightly for visual presentation).	+	The following content is a copy of the <code>/usr/src/linux/Documentation/CodingStyle</code> file (modified slightly for visual presentation). A current copy can also be found online [http://lxr.linux.no/source/Documentation/CodingStyle here].

CityK: add description and format

2009-01-15T00:21:15Z

add description and format

← Older revision		Revision as of 00:21, 15 January 2009
Line 1:		Line 1:
	The following content is a copy of the <code>/usr/src/linux/Documentation/CodingStyle</code> file (modified slightly for visual presentation).		The following content is a copy of the <code>/usr/src/linux/Documentation/CodingStyle</code> file (modified slightly for visual presentation).
		+

-	= <center>Linux kernel coding style </center>=	+	<center>'''Linux kernel coding style''' </center>

	This is a short document describing the preferred coding style for the		This is a short document describing the preferred coding style for the

CityK: add description and format

2009-01-15T00:10:18Z

add description and format

← Older revision		Revision as of 00:10, 15 January 2009
Line 1:		Line 1:
		+	The following content is a copy of the <code>/usr/src/linux/Documentation/CodingStyle</code> file (modified slightly for visual presentation).

-	== Linux kernel coding style ==	+	= <center>Linux kernel coding style </center>=
-		+

	This is a short document describing the preferred coding style for the		This is a short document describing the preferred coding style for the
Line 13:		Line 13:

	Anyway, here goes:		Anyway, here goes:
-
-

	== Chapter 1: Indentation ==		== Chapter 1: Indentation ==
-

	Tabs are 8 characters, and thus indentations are also 8 characters.		Tabs are 8 characters, and thus indentations are also 8 characters.
Line 49:		Line 46:

	Get a decent editor and don't leave whitespace at the end of lines.		Get a decent editor and don't leave whitespace at the end of lines.
-
-

	== Chapter 2: Breaking long lines and strings ==		== Chapter 2: Breaking long lines and strings ==
Line 73:		Line 68:
	next_statement;		next_statement;
	}		}
-

	== Chapter 3: Placing Braces ==		== Chapter 3: Placing Braces ==
Line 128:		Line 122:

	== Chapter 4: Naming ==		== Chapter 4: Naming ==
-

	C is a Spartan language, and so should your naming be. Unlike Modula-2		C is a Spartan language, and so should your naming be. Unlike Modula-2
Line 159:		Line 152:
	problem, which is called the function-growth-hormone-imbalance syndrome.		problem, which is called the function-growth-hormone-imbalance syndrome.
	See next chapter.		See next chapter.
-
-

	== Chapter 5: Functions ==		== Chapter 5: Functions ==
-

	Functions should be short and sweet, and do just one thing. They should		Functions should be short and sweet, and do just one thing. They should
Line 189:		Line 179:
	and it gets confused. You know you're brilliant, but maybe you'd like		and it gets confused. You know you're brilliant, but maybe you'd like
	to understand what you did 2 weeks from now.		to understand what you did 2 weeks from now.
-

	== Chapter 6: Centralized exiting of functions ==		== Chapter 6: Centralized exiting of functions ==
-

	Albeit deprecated by some people, the equivalent of the goto statement is		Albeit deprecated by some people, the equivalent of the goto statement is
Line 227:		Line 215:
	return result;		return result;
	}		}
-

	== Chapter 7: Commenting ==		== Chapter 7: Commenting ==
-

	Comments are good, but there is also a danger of over-commenting. NEVER		Comments are good, but there is also a danger of over-commenting. NEVER
Line 245:		Line 231:
	of the function, telling people what it does, and possibly WHY it does		of the function, telling people what it does, and possibly WHY it does
	it.		it.
-
-

	== Chapter 8: You've made a mess of 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		That's OK, we all do. You've probably been told by your long-time Unix
Line 295:		Line 278:
	re-formatting you may want to take a look at the man page. But		re-formatting you may want to take a look at the man page. But
	remember: "indent" is not a fix for bad programming.		remember: "indent" is not a fix for bad programming.
-
-

	== Chapter 9: Configuration-files ==		== Chapter 9: Configuration-files ==
-

	For configuration options (arch/xxx/Kconfig, and all the Kconfig files),		For configuration options (arch/xxx/Kconfig, and all the Kconfig files),
Line 322:		Line 302:
	support for file-systems, for instance) should be denoted (DANGEROUS), other		support for file-systems, for instance) should be denoted (DANGEROUS), other
	experimental options should be denoted (EXPERIMENTAL).		experimental options should be denoted (EXPERIMENTAL).
-
-

	== Chapter 10: Data structures ==		== Chapter 10: Data structures ==
-

	Data structures that have visibility outside the single-threaded		Data structures that have visibility outside the single-threaded
Line 355:		Line 332:
	Remember: if another thread can find your data structure, and you don't		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.		have a reference count on it, you almost certainly have a bug.
-
-

	== Chapter 11: Macros, Enums, Inline functions and RTL ==		== Chapter 11: Macros, Enums, Inline functions and RTL ==
-

	Names of macros defining constants and labels in enums are capitalized.		Names of macros defining constants and labels in enums are capitalized.
Line 412:		Line 386:
	The cpp manual deals with macros exhaustively. The gcc internals manual also		The cpp manual deals with macros exhaustively. The gcc internals manual also
	covers RTL which is used frequently with assembly language in the kernel.		covers RTL which is used frequently with assembly language in the kernel.
-
-

	== Chapter 12: Printing kernel messages ==		== Chapter 12: Printing kernel messages ==
-

	Kernel developers like to be seen as literate. Do mind the spelling		Kernel developers like to be seen as literate. Do mind the spelling
Line 425:		Line 396:

	Printing numbers in parentheses (%d) adds no value and should be avoided.		Printing numbers in parentheses (%d) adds no value and should be avoided.
-
-

	== Chapter 13: References ==		== Chapter 13: References ==
-

	The C Programming Language, Second Edition		The C Programming Language, Second Edition
Line 449:		Line 417:
	language C, URL: http://std.dkuug.dk/JTC1/SC22/WG14/		language C, URL: http://std.dkuug.dk/JTC1/SC22/WG14/

-	--	+
		+
	Last updated on 16 February 2004 by a community effort on LKML.		Last updated on 16 February 2004 by a community effort on LKML.

CityK: 3 revision(s)

2008-11-20T02:48:39Z

3 revision(s)

← Older revision

Revision as of 02:48, 20 November 2008

Halftime: Finished updating the rest of the code blocks to wiki format.

2006-01-31T17:13:29Z

Finished updating the rest of the code blocks to wiki format.

← Older revision		Revision as of 17:13, 31 January 2006
Line 42:		Line 42:
	something to hide:		something to hide:

-	if (condition) do_this;	+	if (condition) do_this;
-	do_something_everytime;	+	do_something_everytime;

	Outside of comments, documentation and except in Kconfig, spaces are never		Outside of comments, documentation and except in Kconfig, spaces are never
Line 64:		Line 64:
	argument list. Long strings are as well broken into shorter strings.		argument list. Long strings are as well broken into shorter strings.

-	void fun(int a, int b, int c)	+	void fun(int a, int b, int c)
-	{	+	{
-	if (condition)	+	if (condition)
-	printk(KERN_WARNING "Warning this is a long printk with "	+	printk(KERN_WARNING "Warning this is a long printk with "
-	"3 parameters a: %u b: %u "	+	"3 parameters a: %u b: %u "
-	"c: %u \n", a, b, c);	+	"c: %u \n", a, b, c);
-	else	+	else
-	next_statement;	+	next_statement;
-	}	+	}


Line 204:		Line 204:
	- unconditional statements are easier to understand and follow		- unconditional statements are easier to understand and follow
	- nesting is reduced		- nesting is reduced
-	- errors by not updating individual exit points when making	+	- errors by not updating individual exit points when making modifications are prevented
-	modifications are prevented	+
	- saves the compiler work to optimize redundant code away ;)		- saves the compiler work to optimize redundant code away ;)

-	int fun(int )	+	int fun(int )
-	{	+	{
-	int result = 0;	+	int result = 0;
-	char *buffer = kmalloc(SIZE);	+	char *buffer = kmalloc(SIZE);
-		+
-	if (buffer == NULL)	+	if (buffer == NULL)
-	return -ENOMEM;	+	return -ENOMEM;
-		+
-	if (condition1) {	+	if (condition1) {
-	while (loop1) {	+	while (loop1) {
-	...	+	...
-	}	+	}
-	result = 1;	+	result = 1;
-	goto out;	+	goto out;
-	}	+	}
-	...	+	...
-	out:	+	out:
-	kfree(buffer);	+	kfree(buffer);
-	return result;	+	return result;
-	}	+	}


Line 262:		Line 261:
	values. To do the latter, you can stick the following in your .emacs file:		values. To do the latter, you can stick the following in your .emacs file:

-	(defun linux-c-mode ()	+	(defun linux-c-mode ()
-	"C mode with adjusted defaults for use with the Linux kernel."	+	"C mode with adjusted defaults for use with the Linux kernel."
-	(interactive)	+	(interactive)
-	(c-mode)	+	(c-mode)
-	(c-set-style "K&R")	+	(c-set-style "K&R")
-	(setq tab-width 8)	+	(setq tab-width 8)
-	(setq indent-tabs-mode t)	+	(setq indent-tabs-mode t)
-	(setq c-basic-offset 8))	+	(setq c-basic-offset 8))

	This will define the M-x linux-c-mode command. When hacking on a		This will define the M-x linux-c-mode command. When hacking on a
Line 276:		Line 275:
	to add		to add

-	(setq auto-mode-alist (cons '("/usr/src/linux./.\\.[ch]$" . linux-c-mode)	+	(setq auto-mode-alist (cons '("/usr/src/linux./.\\.[ch]$" . linux-c-mode)
-	auto-mode-alist))	+	auto-mode-alist))

	to your .emacs file if you want to have linux-c-mode switched on		to your .emacs file if you want to have linux-c-mode switched on
Line 307:		Line 306:
	Help text is indented with 2 spaces.		Help text is indented with 2 spaces.

-	if CONFIG_EXPERIMENTAL	+	if CONFIG_EXPERIMENTAL
-	tristate CONFIG_BOOM	+	tristate CONFIG_BOOM
-	default n	+	default n
-	help	+	help
-	Apply nitroglycerine inside the keyboard (DANGEROUS)	+	Apply nitroglycerine inside the keyboard (DANGEROUS)
-	bool CONFIG_CHEER	+	bool CONFIG_CHEER
-	depends on CONFIG_BOOM	+	depends on CONFIG_BOOM
-	default y	+	default y
-	help	+	help
-	Output nice messages when you explode	+	Output nice messages when you explode
-	endif	+	endif

	Generally, CONFIG_EXPERIMENTAL should surround all options not considered		Generally, CONFIG_EXPERIMENTAL should surround all options not considered
Line 364:		Line 363:
	Names of macros defining constants and labels in enums are capitalized.		Names of macros defining constants and labels in enums are capitalized.

-	#define CONSTANT 0x12345	+	#define CONSTANT 0x12345

	Enums are preferred when defining several related constants.		Enums are preferred when defining several related constants.
Line 375:		Line 374:
	Macros with multiple statements should be enclosed in a do - while block:		Macros with multiple statements should be enclosed in a do - while block:

-	#define macrofun(a, b, c) \	+	#define macrofun(a, b, c) \
-	do { \	+	do { \
-	if (a == 5) \	+	if (a == 5) \
-	do_this(b, c); \	+	do_this(b, c); \
-	} while (0)	+	} while (0)

	Things to avoid when using macros:		Things to avoid when using macros:
Line 385:		Line 384:
	1) macros that affect control flow:		1) macros that affect control flow:

-	#define FOO(x) \	+	#define FOO(x) \
-	do { \	+	do { \
-	if (blah(x) < 0) \	+	if (blah(x) < 0) \
-	return -EBUGGERED; \	+	return -EBUGGERED; \
-	} while(0)	+	} while(0)

	is a _very_ bad idea. It looks like a function call but exits the "calling"		is a _very_ bad idea. It looks like a function call but exits the "calling"
Line 396:		Line 395:
	2) macros that depend on having a local variable with a magic name:		2) macros that depend on having a local variable with a magic name:

-	#define FOO(val) bar(index, val)	+	#define FOO(val) bar(index, val)

	might look like a good thing, but it's confusing as hell when one reads the		might look like a good thing, but it's confusing as hell when one reads the
Line 408:		Line 407:
	macros using parameters.		macros using parameters.

-	#define CONSTANT 0x4000	+	#define CONSTANT 0x4000
-	#define CONSTEXP (CONSTANT \| 3)	+	#define CONSTEXP (CONSTANT \| 3)

	The cpp manual deals with macros exhaustively. The gcc internals manual also		The cpp manual deals with macros exhaustively. The gcc internals manual also

Halftime: /* Chapter 3: Placing Braces */ Updated the wiki formatting so code is displayed correctly.

2006-01-31T17:05:54Z

Chapter 3: Placing Braces: Updated the wiki formatting so code is displayed correctly.

← Older revision		Revision as of 17:05, 31 January 2006
Line 83:		Line 83:
	brace last on the line, and put the closing brace first, thusly:		brace last on the line, and put the closing brace first, thusly:

-	if (x is true) {	+	if (x is true) {
-	we do y	+	we do y
-	}	+	}

	However, there is one special case, namely functions: they have the		However, there is one special case, namely functions: they have the
	opening brace at the beginning of the next line, thus:		opening brace at the beginning of the next line, thus:

-	int function(int x)	+	int function(int x)
-	{	+	{
-	body of function	+	body of function
-	}	+	}

	Heretic people all over the world have claimed that this inconsistency		Heretic people all over the world have claimed that this inconsistency
Line 105:		Line 105:
	this:		this:

-	do {	+	do {
-	body of do-loop	+	body of do-loop
-	} while (condition);	+	} while (condition);

	and		and

-	if (x == y) {	+	if (x == y) {
-	..	+	..
-	} else if (x > y) {	+	} else if (x > y) {
-	...	+	...
-	} else {	+	} else {
-	....	+	....
-	}	+	}

	Rationale: K&R.		Rationale: K&R.
Line 127:		Line 127:
	comments on.		comments on.

-
-
	== Chapter 4: Naming ==		== Chapter 4: Naming ==

Mauro Carvalho Chehab at 16:08, 28 June 2005

2005-06-28T16:08:09Z

New page

== 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/

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Last updated on 16 February 2004 by a community effort on LKML.