Linux kernel coding style

转自https://www.kernel.org/doc/html/latest/process/coding-style.html#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:

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.

The preferred way to ease multiple indentation levels in a switch
statement is to align the switch and its subordinate case labels in
the same column instead of double-indenting the case labels. E.g.:

switch (suffix) {
case 'G':
case 'g':
mem <<= 30;
break;
case 'M':
case 'm':
mem <<= 20;
break;
case 'K':
case 'k':
mem <<= 10;
/* fall through */
default:
break;
}

Don’t put multiple statements on a single line unless you have something
to hide:

if (condition) do_this;
do_something_everytime;

Don’t put multiple assignments on a single line either. Kernel coding
style is super simple. Avoid tricky expressions.

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.

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 strongly
preferred limit.

Statements longer than 80 columns will be broken into sensible chunks,
unless exceeding 80 columns significantly increases readability and does
not hide information. 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. However, never break
user-visible strings such as printk messages, because that breaks the
ability to grep for them.

3. Placing Braces and Spaces

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
}

This applies to all non-function statement blocks (if, switch, for,
while, do). E.g.:

switch (action) {
case KOBJ_ADD:
return "add";
case KOBJ_REMOVE:
return "remove";
case KOBJ_CHANGE:
return "change";
default:
return NULL;
}

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.

Do not unnecessarily use braces where a single statement will do.

if (condition)
action();

and

if (condition)
do_this();
else
do_that();

This does not apply if only one branch of a conditional statement is a
single statement; in the latter case use braces in both branches:

if (condition) {
do_this();
do_that();
} else {
otherwise();
}

Also, use braces when a loop contains more than a single simple
statement:

while (condition) {
if (test)
do_something();
}

3.1) Spaces

Linux kernel style for use of spaces depends (mostly) on
function-versus-keyword usage. Use a space after (most) keywords. The
notable exceptions are sizeof, typeof, alignof, and __attribute__,
which look somewhat like functions (and are usually used with
parentheses in Linux, although they are not required in the language, as
in: sizeof info after struct fileinfo info; is declared).

So use a space after these keywords:

if, switch, case, for, do, while

but not with sizeof, typeof, alignof, or __attribute__. E.g.,

s = sizeof(struct file);

Do not add spaces around (inside) parenthesized expressions. This
example is bad:

s = sizeof( struct file );

When declaring pointer data or a function that returns a pointer type,
the preferred use of * is adjacent to the data name or function name
and not adjacent to the type name. Examples:

char *linux_banner;
unsigned long long memparse(char *ptr, char **retptr);
char *match_strdup(substring_t *s);

Use one space around (on each side of) most binary and ternary
operators, such as any of these:

=
+
-
<
>
*
/
%
|
&
^
<=
>=
==
!=
?
:

but no space after unary operators:

&
*
+
-
~
!
sizeof
typeof
alignof
__attribute__
defined

no space before the postfix increment & decrement unary operators:

++
--

no space after the prefix increment & decrement unary operators:

++
--

and no space around the . and -> structure member operators.

Do not leave trailing whitespace at the ends of lines. Some editors with
smart indentation will insert whitespace at the beginning of new lines
as appropriate, so you can start typing the next line of code right
away. However, some such editors do not remove the whitespace if you end
up not putting a line of code there, such as if you leave a blank line.
As a result, you end up with lines containing trailing whitespace.

Git will warn you about patches that introduce trailing whitespace, and
can optionally strip the trailing whitespace for you; however, if
applying a series of patches, this may make later patches in the series
fail by changing their context lines.

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 chapter 6 (Functions).

5. Typedefs

Please don’t use things like vps_t. It’s a mistake to use typedef
for structures and pointers. When you see a

vps_t a;

in the source, what does it mean? In contrast, if it says

struct virtual_container *a;

you can actually tell what a is.

Lots of people think that typedefs help readability. Not so. They are
useful only for:

(a) totally opaque objects (where the typedef is actively used to
hide what the object is).

Example: `pte_t` etc. opaque objects that you can only access
using the proper accessor functions.
> **note**
>
> Opaqueness and `accessor functions` are not good in themselves.
> The reason we have them for things like pte_t etc. is that
> there really is absolutely **zero** portably accessible
> information there.

(b) Clear integer types, where the abstraction helps avoid
confusion whether it is int or long.

u8/u16/u32 are perfectly fine typedefs, although they fit into
category (d) better than here.
> **note**
>
> Again - there needs to be a **reason** for this. If something is
> `unsigned long`, then there's no reason to do

typedef unsigned long myflags_t;

but if there is a clear reason for why it under certain
circumstances might be an unsigned int and under other
configurations might be unsigned long, then by all means go
ahead and use a typedef.

© when you use sparse to literally create a new type for
type-checking.
(d) New types which are identical to standard C99 types, in certain
exceptional circumstances.

Although it would only take a short amount of time for the eyes
and brain to become accustomed to the standard types like
`uint32_t`, some people object to their use anyway.
Therefore, the Linux-specific `u8/u16/u32/u64` types and their
signed equivalents which are identical to standard types are
permitted -- although they are not mandatory in new code of your
own.
When editing existing code which already uses one or the other set
of types, you should conform to the existing choices in that code.

(e) Types safe for use in userspace.

In certain structures which are visible to userspace, we cannot
require C99 types and cannot use the `u32` form above. Thus, we
use __u32 and similar types in all structures which are shared
with userspace.

Maybe there are other cases too, but the rule should basically be to
NEVER EVER use a typedef unless you can clearly match one of those
rules.

In general, a pointer, or a struct that has elements that can reasonably
be directly accessed should never be a typedef.

6. 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.

In source files, separate functions with one blank line. If the function
is exported, the EXPORT macro for it should follow immediately after
the closing function brace line. E.g.:

int system_is_up(void)
{
return system_state == SYSTEM_RUNNING;
}
EXPORT_SYMBOL(system_is_up);

In function prototypes, include parameter names with their data types.
Although this is not required by the C language, it is preferred in
Linux because it is a simple way to add valuable information for the
reader.

Do not use the extern’ keyword with function prototypes as this makes
lines longer and isn’t strictly necessary.

7. 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. If there is no
cleanup needed then just return directly.

Choose label names which say what the goto does or why the goto exists. An
example of a good name could be out_free_buffer:` if the goto frees
buffer. Avoid using GW-BASIC names like err1: and err2:, as you
would have to renumber them if you ever add or remove exit paths, and
they make correctness difficult to verify anyway.

The rationale for using gotos 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 a)
{
int result = 0;
char *buffer;
buffer = kmalloc(SIZE, GFP_KERNEL);
if (!buffer)
return -ENOMEM;
if (condition1) {
while (loop1) {
...
}
result = 1;
goto out_free_buffer;
}
...
out_free_buffer:
kfree(buffer);
return result;
}

A common type of bug to be aware of is one err bugs which look like
this:

err:
kfree(foo->bar);
kfree(foo);
return ret;

The bug in this code is that on some exit paths foo is NULL. Normally
the fix for this is to split it up into two error labels err_free_bar:
and err_free_foo::

err_free_bar:
kfree(foo->bar);
err_free_foo:
kfree(foo);
return ret;

Ideally you should simulate errors to test all exit paths.

8. 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 6 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.

When commenting the kernel API functions, please use the kernel-doc
format. See the files at Documentation/doc-guide/ <doc_guide> and
scripts/kernel-doc for details.

The preferred style for long (multi-line) comments is:

/*
* This is the preferred style for multi-line
* comments in the Linux kernel source code.
* Please use it consistently.
*
* Description:
A column of asterisks on the left side,
* with beginning and ending almost-blank lines.
*/

For files in net/ and drivers/net/ the preferred style for long
(multi-line) comments is a little different.

/* The preferred comment style for files in net/ and drivers/net
* looks like this.
*
* It is nearly the same as the generally preferred comment style,
* but there is no initial almost-blank line.
*/

It’s also important to comment data, whether they are basic types or
derived types. To this end, use just one data declaration per line (no
commas for multiple data declarations). This leaves you room for a small
comment on each item, explaining its use.

9. 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 c-lineup-arglist-tabs-only (ignored)
"Line up argument lists by tabs, not spaces"
(let* ((anchor (c-langelem-pos c-syntactic-element))
(column (c-langelem-2nd-pos c-syntactic-element))
(offset (- (1+ column) anchor))
(steps (floor offset c-basic-offset)))
(* (max steps 1)
c-basic-offset)))
(add-hook 'c-mode-common-hook
(lambda ()
;; Add kernel style
(c-add-style
"linux-tabs-only"
'("linux" (c-offsets-alist
(arglist-cont-nonempty
c-lineup-gcc-asm-reg
c-lineup-arglist-tabs-only))))))
(add-hook 'c-mode-hook
(lambda ()
(let ((filename (buffer-file-name)))
;; Enable kernel mode for the appropriate files
(when (and filename
(string-match (expand-file-name "~/src/linux-trees")
filename))
(setq indent-tabs-mode t)
(setq show-trailing-whitespace t)
(c-set-style "linux-tabs-only")))))

This will make emacs go better with the kernel coding style for C files
below ~/src/linux-trees.

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.

Note that you can also use the clang-format tool to help you with
these rules, to quickly re-format parts of your code automatically, and
to review full files in order to spot coding style mistakes, typos and
possible improvements. It is also handy for sorting #includes, for
aligning variables/macros, for reflowing text and other similar tasks.
See the file Documentation/process/clang-format.rst <clangformat> for
more details.

10. Kconfig configuration files

For all of the Kconfig* configuration files throughout the source tree,
the indentation is somewhat different. Lines under a config definition
are indented with one tab, while help text is indented an additional two
spaces. Example:

config AUDIT
bool "Auditing support"
depends on NET
help
Enable auditing infrastructure that can be used with another
kernel subsystem, such as SELinux (which requires this for
logging of avc messages output).
Does not do system-call
auditing without CONFIG_AUDITSYSCALL.

Seriously dangerous features (such as write support for certain
filesystems) should advertise this prominently in their prompt string:

config ADFS_FS_RW
bool "ADFS write support (DANGEROUS)"
depends on ADFS_FS
...

For full documentation on the configuration files, see the file
Documentation/kbuild/kconfig-language.txt.

11. 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.

12. Macros, Enums 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)

​5) namespace collisions when defining local variables in macros
resembling functions:

#define FOO(x)

({

typeof(x) ret;

ret = calc_ret(x);

(ret);

})

ret is a common name for a local variable - __foo_ret is less likely
to collide with an existing variable.

The cpp manual deals with macros exhaustively. The gcc internals manual
also covers RTL which is used frequently with assembly language in the
kernel.

13. 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; use do not or don't instead. Make the messages concise,
clear, and unambiguous.

Kernel messages do not have to be terminated with a period.

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

There are a number of driver model diagnostic macros in
<linux/device.h> which you should use to make sure messages are
matched to the right device and driver, and are tagged with the right
level: dev_err(), dev_warn(), dev_info(), and so forth. For messages
that aren’t associated with a particular device, <linux/printk.h>
defines pr_notice(), pr_info(), pr_warn(), pr_err(), etc.

Coming up with good debugging messages can be quite a challenge; and
once you have them, they can be a huge help for remote troubleshooting.
However debug message printing is handled differently than printing
other non-debug messages. While the other pr_XXX() functions print
unconditionally, pr_debug() does not; it is compiled out by default,
unless either DEBUG is defined or CONFIG_DYNAMIC_DEBUG is set. That is
true for dev_dbg() also, and a related convention uses VERBOSE_DEBUG
to add dev_vdbg() messages to the ones already enabled by DEBUG.

Many subsystems have Kconfig debug options to turn on -DDEBUG in the
corresponding Makefile; in other cases specific files #define DEBUG.
And when a debug message should be unconditionally printed, such as if
it is already inside a debug-related #ifdef section, printk(KERN_DEBUG
…) can be used.

14. Allocating memory

The kernel provides the following general purpose memory allocators:
kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
vzalloc(). Please refer to the API documentation for further information
about them.

The preferred form for passing a size of a struct is the following:

p = kmalloc(sizeof(*p), ...);

The alternative form where struct name is spelled out hurts readability
and introduces an opportunity for a bug when the pointer variable type
is changed but the corresponding sizeof that is passed to a memory
allocator is not.

Casting the return value which is a void pointer is redundant. The
conversion from void pointer to any other pointer type is guaranteed by
the C programming language.

The preferred form for allocating an array is the following:

p = kmalloc_array(n, sizeof(...), ...);

The preferred form for allocating a zeroed array is the following:

p = kcalloc(n, sizeof(...), ...);

Both forms check for overflow on the allocation size n * sizeof(…),
and return NULL if that occurred.

15. The inline disease

There appears to be a common misperception that gcc has a magic “make me
faster” speedup option called inline. While the use of inlines can be
appropriate (for example as a means of replacing macros, see Chapter
12), it very often is not. Abundant use of the inline keyword leads to a
much bigger kernel, which in turn slows the system as a whole down, due
to a bigger icache footprint for the CPU and simply because there is
less memory available for the pagecache. Just think about it; a
pagecache miss causes a disk seek, which easily takes 5 milliseconds.
There are a LOT of cpu cycles that can go into these 5 milliseconds.

A reasonable rule of thumb is to not put inline at functions that have
more than 3 lines of code in them. An exception to this rule are the
cases where a parameter is known to be a compiletime constant, and as a
result of this constantness you know the compiler will be able to
optimize most of your function away at compile time. For a good example
of this later case, see the kmalloc() inline function.

Often people argue that adding inline to functions that are static and
used only once is always a win since there is no space tradeoff. While
this is technically correct, gcc is capable of inlining these
automatically without help, and the maintenance issue of removing the
inline when a second user appears outweighs the potential value of the
hint that tells gcc to do something it would have done anyway.

16. Function return values and names

Functions can return values of many different kinds, and one of the most
common is a value indicating whether the function succeeded or failed.
Such a value can be represented as an error-code integer (-Exxx =
failure, 0 = success) or a succeeded boolean (0 = failure, non-zero =
success).

Mixing up these two sorts of representations is a fertile source of
difficult-to-find bugs. If the C language included a strong distinction
between integers and booleans then the compiler would find these
mistakes for us… but it doesn’t. To help prevent such bugs, always
follow this convention:

If the name of a function is an action or an imperative command,
the function should return an error-code integer.
If the name
is a predicate, the function should return a "succeeded" boolean.

For example, add work is a command, and the add_work() function
returns 0 for success or -EBUSY for failure. In the same way,
PCI device present is a predicate, and the pci_dev_present()
function returns 1 if it succeeds in finding a matching device or 0 if
it doesn’t.

All EXPORTed functions must respect this convention, and so should all
public functions. Private (static) functions need not, but it is
recommended that they do.

Functions whose return value is the actual result of a computation,
rather than an indication of whether the computation succeeded, are not
subject to this rule. Generally they indicate failure by returning some
out-of-range result. Typical examples would be functions that return
pointers; they use NULL or the ERR_PTR mechanism to report failure.

17. Don’t re-invent the kernel macros

The header file include/linux/kernel.h contains a number of macros that
you should use, rather than explicitly coding some variant of them
yourself. For example, if you need to calculate the length of an array,
take advantage of the macro

#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))

Similarly, if you need to calculate the size of some structure member,
use

#define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))

There are also min() and max() macros that do strict type checking if
you need them. Feel free to peruse that header file to see what else is
already defined that you shouldn’t reproduce in your code.

18. Editor modelines and other cruft

Some editors can interpret configuration information embedded in source
files, indicated with special markers. For example, emacs interprets
lines marked like this:

-*- mode: c -*-

Or like this:

/*
Local Variables:
compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
End:
*/

Vim interprets markers that look like this:

/* vim:set sw=8 noet */

Do not include any of these in source files. People have their own
personal editor configurations, and your source files should not
override them. This includes markers for indentation and mode
configuration. People may use their own custom mode, or may have some
other magic method for making indentation work correctly.

19. Inline assembly

In architecture-specific code, you may need to use inline assembly to
interface with CPU or platform functionality. Don’t hesitate to do so
when necessary. However, don’t use inline assembly gratuitously when C
can do the job. You can and should poke hardware from C when possible.

Consider writing simple helper functions that wrap common bits of inline
assembly, rather than repeatedly writing them with slight variations.
Remember that inline assembly can use C parameters.

Large, non-trivial assembly functions should go in .S files, with
corresponding C prototypes defined in C header files. The C prototypes
for assembly functions should use asmlinkage.

You may need to mark your asm statement as volatile, to prevent GCC from
removing it if GCC doesn’t notice any side effects. You don’t always
need to do so, though, and doing so unnecessarily can limit
optimization.

When writing a single inline assembly statement containing multiple
instructions, put each instruction on a separate line in a separate
quoted string, and end each string except the last with nt to
properly indent the next instruction in the assembly output:

asm ("magic %reg1, #42nt"
"more_magic %reg2, %reg3"
: /* outputs */ : /* inputs */ : /* clobbers */);

20. Conditional Compilation

Wherever possible, don’t use preprocessor conditionals (#if, #ifdef)
in .c files; doing so makes code harder to read and logic harder to
follow. Instead, use such conditionals in a header file defining
functions for use in those .c files, providing no-op stub versions in
the #else case, and then call those functions unconditionally from .c
files. The compiler will avoid generating any code for the stub calls,
producing identical results, but the logic will remain easy to follow.

Prefer to compile out entire functions, rather than portions of
functions or portions of expressions. Rather than putting an ifdef in an
expression, factor out part or all of the expression into a separate
helper function and apply the conditional to that function.

If you have a function or variable which may potentially go unused in a
particular configuration, and the compiler would warn about its
definition going unused, mark the definition as __maybe_unused rather
than wrapping it in a preprocessor conditional. (However, if a function
or variable always goes unused, delete it.)

Within code, where possible, use the IS_ENABLED macro to convert a
Kconfig symbol into a C boolean expression, and use it in a normal C
conditional:

if (IS_ENABLED(CONFIG_SOMETHING)) {
...
}

The compiler will constant-fold the conditional away, and include or
exclude the block of code just as with an #ifdef, so this will not add
any runtime overhead. However, this approach still allows the C compiler
to see the code inside the block, and check it for correctness (syntax,
types, symbol references, etc). Thus, you still have to use an #ifdef
if the code inside the block references symbols that will not exist if
the condition is not met.

At the end of any non-trivial #if or #ifdef block (more than a few
lines), place a comment after the #endif on the same line, noting the
conditional expression used. For instance:

#ifdef CONFIG_SOMETHING
...
#endif /* CONFIG_SOMETHING */

Appendix I) 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).

The Practice of Programming by Brian W. Kernighan and Rob Pike.
Addison-Wesley, Inc., 1999. ISBN 0-201-61586-X.

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/manual/

WG14 is the international standardization working group for the
programming language C, URL: http://www.open-std.org/JTC1/SC22/WG14/

Kernel process/coding-style.rst <codingstyle>, by greg@kroah.com at
OLS 2002:
http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/

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