coding-style.rst

   1.. _codingstyle:
   2
   3Linux kernel coding style
   4=========================
   5
   6This is a short document describing the preferred coding style for the
   7linux kernel.  Coding style is very personal, and I won't **force** my
   8views on anybody, but this is what goes for anything that I have to be
   9able to maintain, and I'd prefer it for most other things too.  Please
  10at least consider the points made here.
  11
  12First off, I'd suggest printing out a copy of the GNU coding standards,
  13and NOT read it.  Burn them, it's a great symbolic gesture.
  14
  15Anyway, here goes:
  16
  17
  181) Indentation
  19--------------
  20
  21Tabs are 8 characters, and thus indentations are also 8 characters.
  22There are heretic movements that try to make indentations 4 (or even 2!)
  23characters deep, and that is akin to trying to define the value of PI to
  24be 3.
  25
  26Rationale: The whole idea behind indentation is to clearly define where
  27a block of control starts and ends.  Especially when you've been looking
  28at your screen for 20 straight hours, you'll find it a lot easier to see
  29how the indentation works if you have large indentations.
  30
  31Now, some people will claim that having 8-character indentations makes
  32the code move too far to the right, and makes it hard to read on a
  3380-character terminal screen.  The answer to that is that if you need
  34more than 3 levels of indentation, you're screwed anyway, and should fix
  35your program.
  36
  37In short, 8-char indents make things easier to read, and have the added
  38benefit of warning you when you're nesting your functions too deep.
  39Heed that warning.
  40
  41The preferred way to ease multiple indentation levels in a switch statement is
  42to align the ``switch`` and its subordinate ``case`` labels in the same column
  43instead of ``double-indenting`` the ``case`` labels.  E.g.:
  44
  45.. code-block:: c
  46
  47	switch (suffix) {
  48	case 'G':
  49	case 'g':
  50		mem <<= 30;
  51		break;
  52	case 'M':
  53	case 'm':
  54		mem <<= 20;
  55		break;
  56	case 'K':
  57	case 'k':
  58		mem <<= 10;
  59		fallthrough;
  60	default:
  61		break;
  62	}
  63
  64Don't put multiple statements on a single line unless you have
  65something to hide:
  66
  67.. code-block:: c
  68
  69	if (condition) do_this;
  70	  do_something_everytime;
  71
  72Don't use commas to avoid using braces:
  73
  74.. code-block:: c
  75
  76	if (condition)
  77		do_this(), do_that();
  78
  79Always use braces for multiple statements:
  80
  81.. code-block:: c
  82
  83	if (condition) {
  84		do_this();
  85		do_that();
  86	}
  87
  88Don't put multiple assignments on a single line either.  Kernel coding style
  89is super simple.  Avoid tricky expressions.
  90
  91
  92Outside of comments, documentation and except in Kconfig, spaces are never
  93used for indentation, and the above example is deliberately broken.
  94
  95Get a decent editor and don't leave whitespace at the end of lines.
  96
  97
  982) Breaking long lines and strings
  99----------------------------------
 100
 101Coding style is all about readability and maintainability using commonly
 102available tools.
 103
 104The preferred limit on the length of a single line is 80 columns.
 105
 106Statements longer than 80 columns should be broken into sensible chunks,
 107unless exceeding 80 columns significantly increases readability and does
 108not hide information.
 109
 110Descendants are always substantially shorter than the parent and
 111are placed substantially to the right.  A very commonly used style
 112is to align descendants to a function open parenthesis.
 113
 114These same rules are applied to function headers with a long argument list.
 115
 116However, never break user-visible strings such as printk messages because
 117that breaks the ability to grep for them.
 118
 119
 1203) Placing Braces and Spaces
 121----------------------------
 122
 123The other issue that always comes up in C styling is the placement of
 124braces.  Unlike the indent size, there are few technical reasons to
 125choose one placement strategy over the other, but the preferred way, as
 126shown to us by the prophets Kernighan and Ritchie, is to put the opening
 127brace last on the line, and put the closing brace first, thusly:
 128
 129.. code-block:: c
 130
 131	if (x is true) {
 132		we do y
 133	}
 134
 135This applies to all non-function statement blocks (if, switch, for,
 136while, do).  E.g.:
 137
 138.. code-block:: c
 139
 140	switch (action) {
 141	case KOBJ_ADD:
 142		return "add";
 143	case KOBJ_REMOVE:
 144		return "remove";
 145	case KOBJ_CHANGE:
 146		return "change";
 147	default:
 148		return NULL;
 149	}
 150
 151However, there is one special case, namely functions: they have the
 152opening brace at the beginning of the next line, thus:
 153
 154.. code-block:: c
 155
 156	int function(int x)
 157	{
 158		body of function
 159	}
 160
 161Heretic people all over the world have claimed that this inconsistency
 162is ...  well ...  inconsistent, but all right-thinking people know that
 163(a) K&R are **right** and (b) K&R are right.  Besides, functions are
 164special anyway (you can't nest them in C).
 165
 166Note that the closing brace is empty on a line of its own, **except** in
 167the cases where it is followed by a continuation of the same statement,
 168ie a ``while`` in a do-statement or an ``else`` in an if-statement, like
 169this:
 170
 171.. code-block:: c
 172
 173	do {
 174		body of do-loop
 175	} while (condition);
 176
 177and
 178
 179.. code-block:: c
 180
 181	if (x == y) {
 182		..
 183	} else if (x > y) {
 184		...
 185	} else {
 186		....
 187	}
 188
 189Rationale: K&R.
 190
 191Also, note that this brace-placement also minimizes the number of empty
 192(or almost empty) lines, without any loss of readability.  Thus, as the
 193supply of new-lines on your screen is not a renewable resource (think
 19425-line terminal screens here), you have more empty lines to put
 195comments on.
 196
 197Do not unnecessarily use braces where a single statement will do.
 198
 199.. code-block:: c
 200
 201	if (condition)
 202		action();
 203
 204and
 205
 206.. code-block:: c
 207
 208	if (condition)
 209		do_this();
 210	else
 211		do_that();
 212
 213This does not apply if only one branch of a conditional statement is a single
 214statement; in the latter case use braces in both branches:
 215
 216.. code-block:: c
 217
 218	if (condition) {
 219		do_this();
 220		do_that();
 221	} else {
 222		otherwise();
 223	}
 224
 225Also, use braces when a loop contains more than a single simple statement:
 226
 227.. code-block:: c
 228
 229	while (condition) {
 230		if (test)
 231			do_something();
 232	}
 233
 2343.1) Spaces
 235***********
 236
 237Linux kernel style for use of spaces depends (mostly) on
 238function-versus-keyword usage.  Use a space after (most) keywords.  The
 239notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
 240somewhat like functions (and are usually used with parentheses in Linux,
 241although they are not required in the language, as in: ``sizeof info`` after
 242``struct fileinfo info;`` is declared).
 243
 244So use a space after these keywords::
 245
 246	if, switch, case, for, do, while
 247
 248but not with sizeof, typeof, alignof, or __attribute__.  E.g.,
 249
 250.. code-block:: c
 251
 252
 253	s = sizeof(struct file);
 254
 255Do not add spaces around (inside) parenthesized expressions.  This example is
 256**bad**:
 257
 258.. code-block:: c
 259
 260
 261	s = sizeof( struct file );
 262
 263When declaring pointer data or a function that returns a pointer type, the
 264preferred use of ``*`` is adjacent to the data name or function name and not
 265adjacent to the type name.  Examples:
 266
 267.. code-block:: c
 268
 269
 270	char *linux_banner;
 271	unsigned long long memparse(char *ptr, char **retptr);
 272	char *match_strdup(substring_t *s);
 273
 274Use one space around (on each side of) most binary and ternary operators,
 275such as any of these::
 276
 277	=  +  -  <  >  *  /  %  |  &  ^  <=  >=  ==  !=  ?  :
 278
 279but no space after unary operators::
 280
 281	&  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined
 282
 283no space before the postfix increment & decrement unary operators::
 284
 285	++  --
 286
 287no space after the prefix increment & decrement unary operators::
 288
 289	++  --
 290
 291and no space around the ``.`` and ``->`` structure member operators.
 292
 293Do not leave trailing whitespace at the ends of lines.  Some editors with
 294``smart`` indentation will insert whitespace at the beginning of new lines as
 295appropriate, so you can start typing the next line of code right away.
 296However, some such editors do not remove the whitespace if you end up not
 297putting a line of code there, such as if you leave a blank line.  As a result,
 298you end up with lines containing trailing whitespace.
 299
 300Git will warn you about patches that introduce trailing whitespace, and can
 301optionally strip the trailing whitespace for you; however, if applying a series
 302of patches, this may make later patches in the series fail by changing their
 303context lines.
 304
 305
 3064) Naming
 307---------
 308
 309C is a Spartan language, and your naming conventions should follow suit.
 310Unlike Modula-2 and Pascal programmers, C programmers do not use cute
 311names like ThisVariableIsATemporaryCounter. A C programmer would call that
 312variable ``tmp``, which is much easier to write, and not the least more
 313difficult to understand.
 314
 315HOWEVER, while mixed-case names are frowned upon, descriptive names for
 316global variables are a must.  To call a global function ``foo`` is a
 317shooting offense.
 318
 319GLOBAL variables (to be used only if you **really** need them) need to
 320have descriptive names, as do global functions.  If you have a function
 321that counts the number of active users, you should call that
 322``count_active_users()`` or similar, you should **not** call it ``cntusr()``.
 323
 324Encoding the type of a function into the name (so-called Hungarian
 325notation) is asinine - the compiler knows the types anyway and can check
 326those, and it only confuses the programmer.
 327
 328LOCAL variable names should be short, and to the point.  If you have
 329some random integer loop counter, it should probably be called ``i``.
 330Calling it ``loop_counter`` is non-productive, if there is no chance of it
 331being mis-understood.  Similarly, ``tmp`` can be just about any type of
 332variable that is used to hold a temporary value.
 333
 334If you are afraid to mix up your local variable names, you have another
 335problem, which is called the function-growth-hormone-imbalance syndrome.
 336See chapter 6 (Functions).
 337
 338For symbol names and documentation, avoid introducing new usage of
 339'master / slave' (or 'slave' independent of 'master') and 'blacklist /
 340whitelist'.
 341
 342Recommended replacements for 'master / slave' are:
 343    '{primary,main} / {secondary,replica,subordinate}'
 344    '{initiator,requester} / {target,responder}'
 345    '{controller,host} / {device,worker,proxy}'
 346    'leader / follower'
 347    'director / performer'
 348
 349Recommended replacements for 'blacklist/whitelist' are:
 350    'denylist / allowlist'
 351    'blocklist / passlist'
 352
 353Exceptions for introducing new usage is to maintain a userspace ABI/API,
 354or when updating code for an existing (as of 2020) hardware or protocol
 355specification that mandates those terms. For new specifications
 356translate specification usage of the terminology to the kernel coding
 357standard where possible.
 358
 3595) Typedefs
 360-----------
 361
 362Please don't use things like ``vps_t``.
 363It's a **mistake** to use typedef for structures and pointers. When you see a
 364
 365.. code-block:: c
 366
 367
 368	vps_t a;
 369
 370in the source, what does it mean?
 371In contrast, if it says
 372
 373.. code-block:: c
 374
 375	struct virtual_container *a;
 376
 377you can actually tell what ``a`` is.
 378
 379Lots of people think that typedefs ``help readability``. Not so. They are
 380useful only for:
 381
 382 (a) totally opaque objects (where the typedef is actively used to **hide**
 383     what the object is).
 384
 385     Example: ``pte_t`` etc. opaque objects that you can only access using
 386     the proper accessor functions.
 387
 388     .. note::
 389
 390       Opaqueness and ``accessor functions`` are not good in themselves.
 391       The reason we have them for things like pte_t etc. is that there
 392       really is absolutely **zero** portably accessible information there.
 393
 394 (b) Clear integer types, where the abstraction **helps** avoid confusion
 395     whether it is ``int`` or ``long``.
 396
 397     u8/u16/u32 are perfectly fine typedefs, although they fit into
 398     category (d) better than here.
 399
 400     .. note::
 401
 402       Again - there needs to be a **reason** for this. If something is
 403       ``unsigned long``, then there's no reason to do
 404
 405	typedef unsigned long myflags_t;
 406
 407     but if there is a clear reason for why it under certain circumstances
 408     might be an ``unsigned int`` and under other configurations might be
 409     ``unsigned long``, then by all means go ahead and use a typedef.
 410
 411 (c) when you use sparse to literally create a **new** type for
 412     type-checking.
 413
 414 (d) New types which are identical to standard C99 types, in certain
 415     exceptional circumstances.
 416
 417     Although it would only take a short amount of time for the eyes and
 418     brain to become accustomed to the standard types like ``uint32_t``,
 419     some people object to their use anyway.
 420
 421     Therefore, the Linux-specific ``u8/u16/u32/u64`` types and their
 422     signed equivalents which are identical to standard types are
 423     permitted -- although they are not mandatory in new code of your
 424     own.
 425
 426     When editing existing code which already uses one or the other set
 427     of types, you should conform to the existing choices in that code.
 428
 429 (e) Types safe for use in userspace.
 430
 431     In certain structures which are visible to userspace, we cannot
 432     require C99 types and cannot use the ``u32`` form above. Thus, we
 433     use __u32 and similar types in all structures which are shared
 434     with userspace.
 435
 436Maybe there are other cases too, but the rule should basically be to NEVER
 437EVER use a typedef unless you can clearly match one of those rules.
 438
 439In general, a pointer, or a struct that has elements that can reasonably
 440be directly accessed should **never** be a typedef.
 441
 442
 4436) Functions
 444------------
 445
 446Functions should be short and sweet, and do just one thing.  They should
 447fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
 448as we all know), and do one thing and do that well.
 449
 450The maximum length of a function is inversely proportional to the
 451complexity and indentation level of that function.  So, if you have a
 452conceptually simple function that is just one long (but simple)
 453case-statement, where you have to do lots of small things for a lot of
 454different cases, it's OK to have a longer function.
 455
 456However, if you have a complex function, and you suspect that a
 457less-than-gifted first-year high-school student might not even
 458understand what the function is all about, you should adhere to the
 459maximum limits all the more closely.  Use helper functions with
 460descriptive names (you can ask the compiler to in-line them if you think
 461it's performance-critical, and it will probably do a better job of it
 462than you would have done).
 463
 464Another measure of the function is the number of local variables.  They
 465shouldn't exceed 5-10, or you're doing something wrong.  Re-think the
 466function, and split it into smaller pieces.  A human brain can
 467generally easily keep track of about 7 different things, anything more
 468and it gets confused.  You know you're brilliant, but maybe you'd like
 469to understand what you did 2 weeks from now.
 470
 471In source files, separate functions with one blank line.  If the function is
 472exported, the **EXPORT** macro for it should follow immediately after the
 473closing function brace line.  E.g.:
 474
 475.. code-block:: c
 476
 477	int system_is_up(void)
 478	{
 479		return system_state == SYSTEM_RUNNING;
 480	}
 481	EXPORT_SYMBOL(system_is_up);
 482
 4836.1) Function prototypes
 484************************
 485
 486In function prototypes, include parameter names with their data types.
 487Although this is not required by the C language, it is preferred in Linux
 488because it is a simple way to add valuable information for the reader.
 489
 490Do not use the ``extern`` keyword with function declarations as this makes
 491lines longer and isn't strictly necessary.
 492
 493When writing function prototypes, please keep the `order of elements regular
 494<https://lore.kernel.org/mm-commits/CAHk-=wiOCLRny5aifWNhr621kYrJwhfURsa0vFPeUEm8mF0ufg@mail.gmail.com/>`_.
 495For example, using this function declaration example::
 496
 497 __init void * __must_check action(enum magic value, size_t size, u8 count,
 498				   char *fmt, ...) __printf(4, 5) __malloc;
 499
 500The preferred order of elements for a function prototype is:
 501
 502- storage class (below, ``static __always_inline``, noting that ``__always_inline``
 503  is technically an attribute but is treated like ``inline``)
 504- storage class attributes (here, ``__init`` -- i.e. section declarations, but also
 505  things like ``__cold``)
 506- return type (here, ``void *``)
 507- return type attributes (here, ``__must_check``)
 508- function name (here, ``action``)
 509- function parameters (here, ``(enum magic value, size_t size, u8 count, char *fmt, ...)``,
 510  noting that parameter names should always be included)
 511- function parameter attributes (here, ``__printf(4, 5)``)
 512- function behavior attributes (here, ``__malloc``)
 513
 514Note that for a function **definition** (i.e. the actual function body),
 515the compiler does not allow function parameter attributes after the
 516function parameters. In these cases, they should go after the storage
 517class attributes (e.g. note the changed position of ``__printf(4, 5)``
 518below, compared to the **declaration** example above)::
 519
 520 static __always_inline __init __printf(4, 5) void * __must_check action(enum magic value,
 521		size_t size, u8 count, char *fmt, ...) __malloc
 522 {
 523	...
 524 }
 525
 5267) Centralized exiting of functions
 527-----------------------------------
 528
 529Albeit deprecated by some people, the equivalent of the goto statement is
 530used frequently by compilers in form of the unconditional jump instruction.
 531
 532The goto statement comes in handy when a function exits from multiple
 533locations and some common work such as cleanup has to be done.  If there is no
 534cleanup needed then just return directly.
 535
 536Choose label names which say what the goto does or why the goto exists.  An
 537example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``.
 538Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to
 539renumber them if you ever add or remove exit paths, and they make correctness
 540difficult to verify anyway.
 541
 542The rationale for using gotos is:
 543
 544- unconditional statements are easier to understand and follow
 545- nesting is reduced
 546- errors by not updating individual exit points when making
 547  modifications are prevented
 548- saves the compiler work to optimize redundant code away ;)
 549
 550.. code-block:: c
 551
 552	int fun(int a)
 553	{
 554		int result = 0;
 555		char *buffer;
 556
 557		buffer = kmalloc(SIZE, GFP_KERNEL);
 558		if (!buffer)
 559			return -ENOMEM;
 560
 561		if (condition1) {
 562			while (loop1) {
 563				...
 564			}
 565			result = 1;
 566			goto out_free_buffer;
 567		}
 568		...
 569	out_free_buffer:
 570		kfree(buffer);
 571		return result;
 572	}
 573
 574A common type of bug to be aware of is ``one err bugs`` which look like this:
 575
 576.. code-block:: c
 577
 578	err:
 579		kfree(foo->bar);
 580		kfree(foo);
 581		return ret;
 582
 583The bug in this code is that on some exit paths ``foo`` is NULL.  Normally the
 584fix for this is to split it up into two error labels ``err_free_bar:`` and
 585``err_free_foo:``:
 586
 587.. code-block:: c
 588
 589	err_free_bar:
 590		kfree(foo->bar);
 591	err_free_foo:
 592		kfree(foo);
 593		return ret;
 594
 595Ideally you should simulate errors to test all exit paths.
 596
 597
 5988) Commenting
 599-------------
 600
 601Comments are good, but there is also a danger of over-commenting.  NEVER
 602try to explain HOW your code works in a comment: it's much better to
 603write the code so that the **working** is obvious, and it's a waste of
 604time to explain badly written code.
 605
 606Generally, you want your comments to tell WHAT your code does, not HOW.
 607Also, try to avoid putting comments inside a function body: if the
 608function is so complex that you need to separately comment parts of it,
 609you should probably go back to chapter 6 for a while.  You can make
 610small comments to note or warn about something particularly clever (or
 611ugly), but try to avoid excess.  Instead, put the comments at the head
 612of the function, telling people what it does, and possibly WHY it does
 613it.
 614
 615When commenting the kernel API functions, please use the kernel-doc format.
 616See the files at :ref:`Documentation/doc-guide/ <doc_guide>` and
 617``scripts/kernel-doc`` for details. Note that the danger of over-commenting
 618applies to kernel-doc comments all the same. Do not add boilerplate
 619kernel-doc which simply reiterates what's obvious from the signature
 620of the function.
 621
 622The preferred style for long (multi-line) comments is:
 623
 624.. code-block:: c
 625
 626	/*
 627	 * This is the preferred style for multi-line
 628	 * comments in the Linux kernel source code.
 629	 * Please use it consistently.
 630	 *
 631	 * Description:  A column of asterisks on the left side,
 632	 * with beginning and ending almost-blank lines.
 633	 */
 634
 635It's also important to comment data, whether they are basic types or derived
 636types.  To this end, use just one data declaration per line (no commas for
 637multiple data declarations).  This leaves you room for a small comment on each
 638item, explaining its use.
 639
 640
 6419) You've made a mess of it
 642---------------------------
 643
 644That's OK, we all do.  You've probably been told by your long-time Unix
 645user helper that ``GNU emacs`` automatically formats the C sources for
 646you, and you've noticed that yes, it does do that, but the defaults it
 647uses are less than desirable (in fact, they are worse than random
 648typing - an infinite number of monkeys typing into GNU emacs would never
 649make a good program).
 650
 651So, you can either get rid of GNU emacs, or change it to use saner
 652values.  To do the latter, you can stick the following in your .emacs file:
 653
 654.. code-block:: elisp
 655
 656  (defun c-lineup-arglist-tabs-only (ignored)
 657    "Line up argument lists by tabs, not spaces"
 658    (let* ((anchor (c-langelem-pos c-syntactic-element))
 659           (column (c-langelem-2nd-pos c-syntactic-element))
 660           (offset (- (1+ column) anchor))
 661           (steps (floor offset c-basic-offset)))
 662      (* (max steps 1)
 663         c-basic-offset)))
 664
 665  (dir-locals-set-class-variables
 666   'linux-kernel
 667   '((c-mode . (
 668          (c-basic-offset . 8)
 669          (c-label-minimum-indentation . 0)
 670          (c-offsets-alist . (
 671                  (arglist-close         . c-lineup-arglist-tabs-only)
 672                  (arglist-cont-nonempty .
 673                      (c-lineup-gcc-asm-reg c-lineup-arglist-tabs-only))
 674                  (arglist-intro         . +)
 675                  (brace-list-intro      . +)
 676                  (c                     . c-lineup-C-comments)
 677                  (case-label            . 0)
 678                  (comment-intro         . c-lineup-comment)
 679                  (cpp-define-intro      . +)
 680                  (cpp-macro             . -1000)
 681                  (cpp-macro-cont        . +)
 682                  (defun-block-intro     . +)
 683                  (else-clause           . 0)
 684                  (func-decl-cont        . +)
 685                  (inclass               . +)
 686                  (inher-cont            . c-lineup-multi-inher)
 687                  (knr-argdecl-intro     . 0)
 688                  (label                 . -1000)
 689                  (statement             . 0)
 690                  (statement-block-intro . +)
 691                  (statement-case-intro  . +)
 692                  (statement-cont        . +)
 693                  (substatement          . +)
 694                  ))
 695          (indent-tabs-mode . t)
 696          (show-trailing-whitespace . t)
 697          ))))
 698
 699  (dir-locals-set-directory-class
 700   (expand-file-name "~/src/linux-trees")
 701   'linux-kernel)
 702
 703This will make emacs go better with the kernel coding style for C
 704files below ``~/src/linux-trees``.
 705
 706But even if you fail in getting emacs to do sane formatting, not
 707everything is lost: use ``indent``.
 708
 709Now, again, GNU indent has the same brain-dead settings that GNU emacs
 710has, which is why you need to give it a few command line options.
 711However, that's not too bad, because even the makers of GNU indent
 712recognize the authority of K&R (the GNU people aren't evil, they are
 713just severely misguided in this matter), so you just give indent the
 714options ``-kr -i8`` (stands for ``K&R, 8 character indents``), or use
 715``scripts/Lindent``, which indents in the latest style.
 716
 717``indent`` has a lot of options, and especially when it comes to comment
 718re-formatting you may want to take a look at the man page.  But
 719remember: ``indent`` is not a fix for bad programming.
 720
 721Note that you can also use the ``clang-format`` tool to help you with
 722these rules, to quickly re-format parts of your code automatically,
 723and to review full files in order to spot coding style mistakes,
 724typos and possible improvements. It is also handy for sorting ``#includes``,
 725for aligning variables/macros, for reflowing text and other similar tasks.
 726See the file :ref:`Documentation/dev-tools/clang-format.rst <clangformat>`
 727for more details.
 728
 729Some basic editor settings, such as indentation and line endings, will be
 730set automatically if you are using an editor that is compatible with
 731EditorConfig. See the official EditorConfig website for more information:
 732https://editorconfig.org/
 733
 73410) Kconfig configuration files
 735-------------------------------
 736
 737For all of the Kconfig* configuration files throughout the source tree,
 738the indentation is somewhat different.  Lines under a ``config`` definition
 739are indented with one tab, while help text is indented an additional two
 740spaces.  Example::
 741
 742  config AUDIT
 743	bool "Auditing support"
 744	depends on NET
 745	help
 746	  Enable auditing infrastructure that can be used with another
 747	  kernel subsystem, such as SELinux (which requires this for
 748	  logging of avc messages output).  Does not do system-call
 749	  auditing without CONFIG_AUDITSYSCALL.
 750
 751Seriously dangerous features (such as write support for certain
 752filesystems) should advertise this prominently in their prompt string::
 753
 754  config ADFS_FS_RW
 755	bool "ADFS write support (DANGEROUS)"
 756	depends on ADFS_FS
 757	...
 758
 759For full documentation on the configuration files, see the file
 760Documentation/kbuild/kconfig-language.rst.
 761
 762
 76311) Data structures
 764-------------------
 765
 766Data structures that have visibility outside the single-threaded
 767environment they are created and destroyed in should always have
 768reference counts.  In the kernel, garbage collection doesn't exist (and
 769outside the kernel garbage collection is slow and inefficient), which
 770means that you absolutely **have** to reference count all your uses.
 771
 772Reference counting means that you can avoid locking, and allows multiple
 773users to have access to the data structure in parallel - and not having
 774to worry about the structure suddenly going away from under them just
 775because they slept or did something else for a while.
 776
 777Note that locking is **not** a replacement for reference counting.
 778Locking is used to keep data structures coherent, while reference
 779counting is a memory management technique.  Usually both are needed, and
 780they are not to be confused with each other.
 781
 782Many data structures can indeed have two levels of reference counting,
 783when there are users of different ``classes``.  The subclass count counts
 784the number of subclass users, and decrements the global count just once
 785when the subclass count goes to zero.
 786
 787Examples of this kind of ``multi-level-reference-counting`` can be found in
 788memory management (``struct mm_struct``: mm_users and mm_count), and in
 789filesystem code (``struct super_block``: s_count and s_active).
 790
 791Remember: if another thread can find your data structure, and you don't
 792have a reference count on it, you almost certainly have a bug.
 793
 794
 79512) Macros, Enums and RTL
 796-------------------------
 797
 798Names of macros defining constants and labels in enums are capitalized.
 799
 800.. code-block:: c
 801
 802	#define CONSTANT 0x12345
 803
 804Enums are preferred when defining several related constants.
 805
 806CAPITALIZED macro names are appreciated but macros resembling functions
 807may be named in lower case.
 808
 809Generally, inline functions are preferable to macros resembling functions.
 810
 811Macros with multiple statements should be enclosed in a do - while block:
 812
 813.. code-block:: c
 814
 815	#define macrofun(a, b, c)			\
 816		do {					\
 817			if (a == 5)			\
 818				do_this(b, c);		\
 819		} while (0)
 820
 821Function-like macros with unused parameters should be replaced by static
 822inline functions to avoid the issue of unused variables:
 823
 824.. code-block:: c
 825
 826	static inline void fun(struct foo *foo)
 827	{
 828	}
 829
 830Due to historical practices, many files still employ the "cast to (void)"
 831approach to evaluate parameters. However, this method is not advisable.
 832Inline functions address the issue of "expression with side effects
 833evaluated more than once", circumvent unused-variable problems, and
 834are generally better documented than macros for some reason.
 835
 836.. code-block:: c
 837
 838	/*
 839	 * Avoid doing this whenever possible and instead opt for static
 840	 * inline functions
 841	 */
 842	#define macrofun(foo) do { (void) (foo); } while (0)
 843
 844Things to avoid when using macros:
 845
 8461) macros that affect control flow:
 847
 848.. code-block:: c
 849
 850	#define FOO(x)					\
 851		do {					\
 852			if (blah(x) < 0)		\
 853				return -EBUGGERED;	\
 854		} while (0)
 855
 856is a **very** bad idea.  It looks like a function call but exits the ``calling``
 857function; don't break the internal parsers of those who will read the code.
 858
 8592) macros that depend on having a local variable with a magic name:
 860
 861.. code-block:: c
 862
 863	#define FOO(val) bar(index, val)
 864
 865might look like a good thing, but it's confusing as hell when one reads the
 866code and it's prone to breakage from seemingly innocent changes.
 867
 8683) macros with arguments that are used as l-values: FOO(x) = y; will
 869bite you if somebody e.g. turns FOO into an inline function.
 870
 8714) forgetting about precedence: macros defining constants using expressions
 872must enclose the expression in parentheses. Beware of similar issues with
 873macros using parameters.
 874
 875.. code-block:: c
 876
 877	#define CONSTANT 0x4000
 878	#define CONSTEXP (CONSTANT | 3)
 879
 8805) namespace collisions when defining local variables in macros resembling
 881functions:
 882
 883.. code-block:: c
 884
 885	#define FOO(x)				\
 886	({					\
 887		typeof(x) ret;			\
 888		ret = calc_ret(x);		\
 889		(ret);				\
 890	})
 891
 892ret is a common name for a local variable - __foo_ret is less likely
 893to collide with an existing variable.
 894
 895The cpp manual deals with macros exhaustively. The gcc internals manual also
 896covers RTL which is used frequently with assembly language in the kernel.
 897
 898
 89913) Printing kernel messages
 900----------------------------
 901
 902Kernel developers like to be seen as literate. Do mind the spelling
 903of kernel messages to make a good impression. Do not use incorrect
 904contractions like ``dont``; use ``do not`` or ``don't`` instead. Make the
 905messages concise, clear, and unambiguous.
 906
 907Kernel messages do not have to be terminated with a period.
 908
 909Printing numbers in parentheses (%d) adds no value and should be avoided.
 910
 911There are a number of driver model diagnostic macros in <linux/dev_printk.h>
 912which you should use to make sure messages are matched to the right device
 913and driver, and are tagged with the right level:  dev_err(), dev_warn(),
 914dev_info(), and so forth.  For messages that aren't associated with a
 915particular device, <linux/printk.h> defines pr_notice(), pr_info(),
 916pr_warn(), pr_err(), etc. When drivers are working properly they are quiet,
 917so prefer to use dev_dbg/pr_debug unless something is wrong.
 918
 919Coming up with good debugging messages can be quite a challenge; and once
 920you have them, they can be a huge help for remote troubleshooting.  However
 921debug message printing is handled differently than printing other non-debug
 922messages.  While the other pr_XXX() functions print unconditionally,
 923pr_debug() does not; it is compiled out by default, unless either DEBUG is
 924defined or CONFIG_DYNAMIC_DEBUG is set.  That is true for dev_dbg() also,
 925and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
 926the ones already enabled by DEBUG.
 927
 928Many subsystems have Kconfig debug options to turn on -DDEBUG in the
 929corresponding Makefile; in other cases specific files #define DEBUG.  And
 930when a debug message should be unconditionally printed, such as if it is
 931already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
 932used.
 933
 934
 93514) Allocating memory
 936---------------------
 937
 938The kernel provides the following general purpose memory allocators:
 939kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
 940vzalloc().  Please refer to the API documentation for further information
 941about them.  :ref:`Documentation/core-api/memory-allocation.rst
 942<memory_allocation>`
 943
 944The preferred form for passing a size of a struct is the following:
 945
 946.. code-block:: c
 947
 948	p = kmalloc(sizeof(*p), ...);
 949
 950The alternative form where struct name is spelled out hurts readability and
 951introduces an opportunity for a bug when the pointer variable type is changed
 952but the corresponding sizeof that is passed to a memory allocator is not.
 953
 954Casting the return value which is a void pointer is redundant. The conversion
 955from void pointer to any other pointer type is guaranteed by the C programming
 956language.
 957
 958The preferred form for allocating an array is the following:
 959
 960.. code-block:: c
 961
 962	p = kmalloc_array(n, sizeof(...), ...);
 963
 964The preferred form for allocating a zeroed array is the following:
 965
 966.. code-block:: c
 967
 968	p = kcalloc(n, sizeof(...), ...);
 969
 970Both forms check for overflow on the allocation size n * sizeof(...),
 971and return NULL if that occurred.
 972
 973These generic allocation functions all emit a stack dump on failure when used
 974without __GFP_NOWARN so there is no use in emitting an additional failure
 975message when NULL is returned.
 976
 97715) The inline disease
 978----------------------
 979
 980There appears to be a common misperception that gcc has a magic "make me
 981faster" speedup option called ``inline``. While the use of inlines can be
 982appropriate (for example as a means of replacing macros, see Chapter 12), it
 983very often is not. Abundant use of the inline keyword leads to a much bigger
 984kernel, which in turn slows the system as a whole down, due to a bigger
 985icache footprint for the CPU and simply because there is less memory
 986available for the pagecache. Just think about it; a pagecache miss causes a
 987disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
 988that can go into these 5 milliseconds.
 989
 990A reasonable rule of thumb is to not put inline at functions that have more
 991than 3 lines of code in them. An exception to this rule are the cases where
 992a parameter is known to be a compile time constant, and as a result of this
 993constantness you *know* the compiler will be able to optimize most of your
 994function away at compile time. For a good example of this later case, see
 995the kmalloc() inline function.
 996
 997Often people argue that adding inline to functions that are static and used
 998only once is always a win since there is no space tradeoff. While this is
 999technically correct, gcc is capable of inlining these automatically without
1000help, and the maintenance issue of removing the inline when a second user
1001appears outweighs the potential value of the hint that tells gcc to do
1002something it would have done anyway.
1003
1004
100516) Function return values and names
1006------------------------------------
1007
1008Functions can return values of many different kinds, and one of the
1009most common is a value indicating whether the function succeeded or
1010failed.  Such a value can be represented as an error-code integer
1011(-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure,
1012non-zero = success).
1013
1014Mixing up these two sorts of representations is a fertile source of
1015difficult-to-find bugs.  If the C language included a strong distinction
1016between integers and booleans then the compiler would find these mistakes
1017for us... but it doesn't.  To help prevent such bugs, always follow this
1018convention::
1019
1020	If the name of a function is an action or an imperative command,
1021	the function should return an error-code integer.  If the name
1022	is a predicate, the function should return a "succeeded" boolean.
1023
1024For example, ``add work`` is a command, and the add_work() function returns 0
1025for success or -EBUSY for failure.  In the same way, ``PCI device present`` is
1026a predicate, and the pci_dev_present() function returns 1 if it succeeds in
1027finding a matching device or 0 if it doesn't.
1028
1029All EXPORTed functions must respect this convention, and so should all
1030public functions.  Private (static) functions need not, but it is
1031recommended that they do.
1032
1033Functions whose return value is the actual result of a computation, rather
1034than an indication of whether the computation succeeded, are not subject to
1035this rule.  Generally they indicate failure by returning some out-of-range
1036result.  Typical examples would be functions that return pointers; they use
1037NULL or the ERR_PTR mechanism to report failure.
1038
1039
104017) Using bool
1041--------------
1042
1043The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
1044only evaluate to 0 or 1, and implicit or explicit conversion to bool
1045automatically converts the value to true or false. When using bool types the
1046!! construction is not needed, which eliminates a class of bugs.
1047
1048When working with bool values the true and false definitions should be used
1049instead of 1 and 0.
1050
1051bool function return types and stack variables are always fine to use whenever
1052appropriate. Use of bool is encouraged to improve readability and is often a
1053better option than 'int' for storing boolean values.
1054
1055Do not use bool if cache line layout or size of the value matters, as its size
1056and alignment varies based on the compiled architecture. Structures that are
1057optimized for alignment and size should not use bool.
1058
1059If a structure has many true/false values, consider consolidating them into a
1060bitfield with 1 bit members, or using an appropriate fixed width type, such as
1061u8.
1062
1063Similarly for function arguments, many true/false values can be consolidated
1064into a single bitwise 'flags' argument and 'flags' can often be a more
1065readable alternative if the call-sites have naked true/false constants.
1066
1067Otherwise limited use of bool in structures and arguments can improve
1068readability.
1069
107018) Don't re-invent the kernel macros
1071-------------------------------------
1072
1073The header file include/linux/kernel.h contains a number of macros that
1074you should use, rather than explicitly coding some variant of them yourself.
1075For example, if you need to calculate the length of an array, take advantage
1076of the macro
1077
1078.. code-block:: c
1079
1080	#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
1081
1082Similarly, if you need to calculate the size of some structure member, use
1083
1084.. code-block:: c
1085
1086	#define sizeof_field(t, f) (sizeof(((t*)0)->f))
1087
1088There are also min() and max() macros that do strict type checking if you
1089need them.  Feel free to peruse that header file to see what else is already
1090defined that you shouldn't reproduce in your code.
1091
1092
109319) Editor modelines and other cruft
1094------------------------------------
1095
1096Some editors can interpret configuration information embedded in source files,
1097indicated with special markers.  For example, emacs interprets lines marked
1098like this:
1099
1100.. code-block:: c
1101
1102	-*- mode: c -*-
1103
1104Or like this:
1105
1106.. code-block:: c
1107
1108	/*
1109	Local Variables:
1110	compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
1111	End:
1112	*/
1113
1114Vim interprets markers that look like this:
1115
1116.. code-block:: c
1117
1118	/* vim:set sw=8 noet */
1119
1120Do not include any of these in source files.  People have their own personal
1121editor configurations, and your source files should not override them.  This
1122includes markers for indentation and mode configuration.  People may use their
1123own custom mode, or may have some other magic method for making indentation
1124work correctly.
1125
1126
112720) Inline assembly
1128-------------------
1129
1130In architecture-specific code, you may need to use inline assembly to interface
1131with CPU or platform functionality.  Don't hesitate to do so when necessary.
1132However, don't use inline assembly gratuitously when C can do the job.  You can
1133and should poke hardware from C when possible.
1134
1135Consider writing simple helper functions that wrap common bits of inline
1136assembly, rather than repeatedly writing them with slight variations.  Remember
1137that inline assembly can use C parameters.
1138
1139Large, non-trivial assembly functions should go in .S files, with corresponding
1140C prototypes defined in C header files.  The C prototypes for assembly
1141functions should use ``asmlinkage``.
1142
1143You may need to mark your asm statement as volatile, to prevent GCC from
1144removing it if GCC doesn't notice any side effects.  You don't always need to
1145do so, though, and doing so unnecessarily can limit optimization.
1146
1147When writing a single inline assembly statement containing multiple
1148instructions, put each instruction on a separate line in a separate quoted
1149string, and end each string except the last with ``\n\t`` to properly indent
1150the next instruction in the assembly output:
1151
1152.. code-block:: c
1153
1154	asm ("magic %reg1, #42\n\t"
1155	     "more_magic %reg2, %reg3"
1156	     : /* outputs */ : /* inputs */ : /* clobbers */);
1157
1158
115921) Conditional Compilation
1160---------------------------
1161
1162Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
1163files; doing so makes code harder to read and logic harder to follow.  Instead,
1164use such conditionals in a header file defining functions for use in those .c
1165files, providing no-op stub versions in the #else case, and then call those
1166functions unconditionally from .c files.  The compiler will avoid generating
1167any code for the stub calls, producing identical results, but the logic will
1168remain easy to follow.
1169
1170Prefer to compile out entire functions, rather than portions of functions or
1171portions of expressions.  Rather than putting an ifdef in an expression, factor
1172out part or all of the expression into a separate helper function and apply the
1173conditional to that function.
1174
1175If you have a function or variable which may potentially go unused in a
1176particular configuration, and the compiler would warn about its definition
1177going unused, mark the definition as __maybe_unused rather than wrapping it in
1178a preprocessor conditional.  (However, if a function or variable *always* goes
1179unused, delete it.)
1180
1181Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
1182symbol into a C boolean expression, and use it in a normal C conditional:
1183
1184.. code-block:: c
1185
1186	if (IS_ENABLED(CONFIG_SOMETHING)) {
1187		...
1188	}
1189
1190The compiler will constant-fold the conditional away, and include or exclude
1191the block of code just as with an #ifdef, so this will not add any runtime
1192overhead.  However, this approach still allows the C compiler to see the code
1193inside the block, and check it for correctness (syntax, types, symbol
1194references, etc).  Thus, you still have to use an #ifdef if the code inside the
1195block references symbols that will not exist if the condition is not met.
1196
1197At the end of any non-trivial #if or #ifdef block (more than a few lines),
1198place a comment after the #endif on the same line, noting the conditional
1199expression used.  For instance:
1200
1201.. code-block:: c
1202
1203	#ifdef CONFIG_SOMETHING
1204	...
1205	#endif /* CONFIG_SOMETHING */
1206
1207
120822) Do not crash the kernel
1209---------------------------
1210
1211In general, the decision to crash the kernel belongs to the user, rather
1212than to the kernel developer.
1213
1214Avoid panic()
1215*************
1216
1217panic() should be used with care and primarily only during system boot.
1218panic() is, for example, acceptable when running out of memory during boot and
1219not being able to continue.
1220
1221Use WARN() rather than BUG()
1222****************************
1223
1224Do not add new code that uses any of the BUG() variants, such as BUG(),
1225BUG_ON(), or VM_BUG_ON(). Instead, use a WARN*() variant, preferably
1226WARN_ON_ONCE(), and possibly with recovery code. Recovery code is not
1227required if there is no reasonable way to at least partially recover.
1228
1229"I'm too lazy to do error handling" is not an excuse for using BUG(). Major
1230internal corruptions with no way of continuing may still use BUG(), but need
1231good justification.
1232
1233Use WARN_ON_ONCE() rather than WARN() or WARN_ON()
1234**************************************************
1235
1236WARN_ON_ONCE() is generally preferred over WARN() or WARN_ON(), because it
1237is common for a given warning condition, if it occurs at all, to occur
1238multiple times. This can fill up and wrap the kernel log, and can even slow
1239the system enough that the excessive logging turns into its own, additional
1240problem.
1241
1242Do not WARN lightly
1243*******************
1244
1245WARN*() is intended for unexpected, this-should-never-happen situations.
1246WARN*() macros are not to be used for anything that is expected to happen
1247during normal operation. These are not pre- or post-condition asserts, for
1248example. Again: WARN*() must not be used for a condition that is expected
1249to trigger easily, for example, by user space actions. pr_warn_once() is a
1250possible alternative, if you need to notify the user of a problem.
1251
1252Do not worry about panic_on_warn users
1253**************************************
1254
1255A few more words about panic_on_warn: Remember that ``panic_on_warn`` is an
1256available kernel option, and that many users set this option. This is why
1257there is a "Do not WARN lightly" writeup, above. However, the existence of
1258panic_on_warn users is not a valid reason to avoid the judicious use
1259WARN*(). That is because, whoever enables panic_on_warn has explicitly
1260asked the kernel to crash if a WARN*() fires, and such users must be
1261prepared to deal with the consequences of a system that is somewhat more
1262likely to crash.
1263
1264Use BUILD_BUG_ON() for compile-time assertions
1265**********************************************
1266
1267The use of BUILD_BUG_ON() is acceptable and encouraged, because it is a
1268compile-time assertion that has no effect at runtime.
1269
1270Appendix I) References
1271----------------------
1272
1273The C Programming Language, Second Edition
1274by Brian W. Kernighan and Dennis M. Ritchie.
1275Prentice Hall, Inc., 1988.
1276ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
1277
1278The Practice of Programming
1279by Brian W. Kernighan and Rob Pike.
1280Addison-Wesley, Inc., 1999.
1281ISBN 0-201-61586-X.
1282
1283GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
1284gcc internals and indent, all available from https://www.gnu.org/manual/
1285
1286WG14 is the international standardization working group for the programming
1287language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
1288
1289Kernel CodingStyle, by greg@kroah.com at OLS 2002:
1290http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/