.. _codingstyle:

  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.:

  .. code-block:: c

  	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:

  .. code-block:: c

  	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:

  .. code-block:: c

  	if (x is true) {
  		we do y
  	}

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

  .. code-block:: c

  	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:

  .. code-block:: c

  	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:

  .. code-block:: c

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

  .. code-block:: c

  	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.

  .. code-block:: c

  	if (condition)
  		action();

  and

  .. code-block:: none

  	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:

  .. code-block:: c

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

  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.,

  .. code-block:: c

  	s = sizeof(struct file);
  
  Do not add spaces around (inside) parenthesized expressions.  This example is

  **bad**:
  
  .. code-block:: c

  
  	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:

  .. code-block:: c

  	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

  .. code-block:: c

  	vps_t a;
  
  in the source, what does it mean?

  In contrast, if it says

  .. code-block:: c

  	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.

   (c) 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.:
  
  .. code-block:: c

  	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.

  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 ;)

  .. code-block:: c

  	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_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:

  .. code-block:: c

  	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:``:

  .. code-block:: c

  	 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 Documentation/kernel-documentation.rst and scripts/kernel-doc

  for details.

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

  .. code-block:: c

  	/*
  	 * 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.

  .. code-block:: c

  	/* 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:

  .. code-block:: none
  
    (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.

  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.

  .. code-block:: c

  	#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:

  .. code-block:: c
  
  	#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:

  .. code-block:: c

  	#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:

  .. code-block:: c

  	#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.

  .. code-block:: c

  	#define CONSTANT 0x4000
  	#define CONSTEXP (CONSTANT | 3)

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

  .. code-block:: c
  
  	#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:

  .. code-block:: c

  	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:

  .. code-block:: c

  	p = kmalloc_array(n, sizeof(...), ...);
  
  The preferred form for allocating a zeroed array is the following:

  .. code-block:: c

  	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

  .. code-block:: c

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

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

  .. code-block:: c

  	#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:

  .. code-block:: c

  	-*- mode: c -*-

  
  Or like this:

  .. code-block:: c

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

  
  Vim interprets markers that look like this:

  .. code-block:: c

  	/* 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 
  \t to properly indent the
  next instruction in the assembly output:

  .. code-block:: c

  	asm ("magic %reg1, #42
  \t"
  	     "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:

  .. code-block:: c

  	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:

  .. code-block:: c

  	#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 CodingStyle, by greg@kroah.com at OLS 2002:
  http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/