linux/Documentation/CodingStyle
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   1
   2                Linux kernel coding style
   3
   4This is a short document describing the preferred coding style for the
   5linux kernel.  Coding style is very personal, and I won't _force_ my
   6views on anybody, but this is what goes for anything that I have to be
   7able to maintain, and I'd prefer it for most other things too.  Please
   8at least consider the points made here.
   9
  10First off, I'd suggest printing out a copy of the GNU coding standards,
  11and NOT read it.  Burn them, it's a great symbolic gesture.
  12
  13Anyway, here goes:
  14
  15
  16                Chapter 1: Indentation
  17
  18Tabs are 8 characters, and thus indentations are also 8 characters.
  19There are heretic movements that try to make indentations 4 (or even 2!)
  20characters deep, and that is akin to trying to define the value of PI to
  21be 3.
  22
  23Rationale: The whole idea behind indentation is to clearly define where
  24a block of control starts and ends.  Especially when you've been looking
  25at your screen for 20 straight hours, you'll find it a lot easier to see
  26how the indentation works if you have large indentations.
  27
  28Now, some people will claim that having 8-character indentations makes
  29the code move too far to the right, and makes it hard to read on a
  3080-character terminal screen.  The answer to that is that if you need
  31more than 3 levels of indentation, you're screwed anyway, and should fix
  32your program.
  33
  34In short, 8-char indents make things easier to read, and have the added
  35benefit of warning you when you're nesting your functions too deep.
  36Heed that warning.
  37
  38The preferred way to ease multiple indentation levels in a switch statement is
  39to align the "switch" and its subordinate "case" labels in the same column
  40instead of "double-indenting" the "case" labels.  E.g.:
  41
  42        switch (suffix) {
  43        case 'G':
  44        case 'g':
  45                mem <<= 30;
  46                break;
  47        case 'M':
  48        case 'm':
  49                mem <<= 20;
  50                break;
  51        case 'K':
  52        case 'k':
  53                mem <<= 10;
  54                /* fall through */
  55        default:
  56                break;
  57        }
  58
  59
  60Don't put multiple statements on a single line unless you have
  61something to hide:
  62
  63        if (condition) do_this;
  64          do_something_everytime;
  65
  66Don't put multiple assignments on a single line either.  Kernel coding style
  67is super simple.  Avoid tricky expressions.
  68
  69Outside of comments, documentation and except in Kconfig, spaces are never
  70used for indentation, and the above example is deliberately broken.
  71
  72Get a decent editor and don't leave whitespace at the end of lines.
  73
  74
  75                Chapter 2: Breaking long lines and strings
  76
  77Coding style is all about readability and maintainability using commonly
  78available tools.
  79
  80The limit on the length of lines is 80 columns and this is a strongly
  81preferred limit.
  82
  83Statements longer than 80 columns will be broken into sensible chunks, unless
  84exceeding 80 columns significantly increases readability and does not hide
  85information. Descendants are always substantially shorter than the parent and
  86are placed substantially to the right. The same applies to function headers
  87with a long argument list. However, never break user-visible strings such as
  88printk messages, because that breaks the ability to grep for them.
  89
  90
  91                Chapter 3: Placing Braces and Spaces
  92
  93The other issue that always comes up in C styling is the placement of
  94braces.  Unlike the indent size, there are few technical reasons to
  95choose one placement strategy over the other, but the preferred way, as
  96shown to us by the prophets Kernighan and Ritchie, is to put the opening
  97brace last on the line, and put the closing brace first, thusly:
  98
  99        if (x is true) {
 100                we do y
 101        }
 102
 103This applies to all non-function statement blocks (if, switch, for,
 104while, do).  E.g.:
 105
 106        switch (action) {
 107        case KOBJ_ADD:
 108                return "add";
 109        case KOBJ_REMOVE:
 110                return "remove";
 111        case KOBJ_CHANGE:
 112                return "change";
 113        default:
 114                return NULL;
 115        }
 116
 117However, there is one special case, namely functions: they have the
 118opening brace at the beginning of the next line, thus:
 119
 120        int function(int x)
 121        {
 122                body of function
 123        }
 124
 125Heretic people all over the world have claimed that this inconsistency
 126is ...  well ...  inconsistent, but all right-thinking people know that
 127(a) K&R are _right_ and (b) K&R are right.  Besides, functions are
 128special anyway (you can't nest them in C).
 129
 130Note that the closing brace is empty on a line of its own, _except_ in
 131the cases where it is followed by a continuation of the same statement,
 132ie a "while" in a do-statement or an "else" in an if-statement, like
 133this:
 134
 135        do {
 136                body of do-loop
 137        } while (condition);
 138
 139and
 140
 141        if (x == y) {
 142                ..
 143        } else if (x > y) {
 144                ...
 145        } else {
 146                ....
 147        }
 148
 149Rationale: K&R.
 150
 151Also, note that this brace-placement also minimizes the number of empty
 152(or almost empty) lines, without any loss of readability.  Thus, as the
 153supply of new-lines on your screen is not a renewable resource (think
 15425-line terminal screens here), you have more empty lines to put
 155comments on.
 156
 157Do not unnecessarily use braces where a single statement will do.
 158
 159if (condition)
 160        action();
 161
 162and
 163
 164if (condition)
 165        do_this();
 166else
 167        do_that();
 168
 169This does not apply if only one branch of a conditional statement is a single
 170statement; in the latter case use braces in both branches:
 171
 172if (condition) {
 173        do_this();
 174        do_that();
 175} else {
 176        otherwise();
 177}
 178
 179                3.1:  Spaces
 180
 181Linux kernel style for use of spaces depends (mostly) on
 182function-versus-keyword usage.  Use a space after (most) keywords.  The
 183notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
 184somewhat like functions (and are usually used with parentheses in Linux,
 185although they are not required in the language, as in: "sizeof info" after
 186"struct fileinfo info;" is declared).
 187
 188So use a space after these keywords:
 189        if, switch, case, for, do, while
 190but not with sizeof, typeof, alignof, or __attribute__.  E.g.,
 191        s = sizeof(struct file);
 192
 193Do not add spaces around (inside) parenthesized expressions.  This example is
 194*bad*:
 195
 196        s = sizeof( struct file );
 197
 198When declaring pointer data or a function that returns a pointer type, the
 199preferred use of '*' is adjacent to the data name or function name and not
 200adjacent to the type name.  Examples:
 201
 202        char *linux_banner;
 203        unsigned long long memparse(char *ptr, char **retptr);
 204        char *match_strdup(substring_t *s);
 205
 206Use one space around (on each side of) most binary and ternary operators,
 207such as any of these:
 208
 209        =  +  -  <  >  *  /  %  |  &  ^  <=  >=  ==  !=  ?  :
 210
 211but no space after unary operators:
 212        &  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined
 213
 214no space before the postfix increment & decrement unary operators:
 215        ++  --
 216
 217no space after the prefix increment & decrement unary operators:
 218        ++  --
 219
 220and no space around the '.' and "->" structure member operators.
 221
 222Do not leave trailing whitespace at the ends of lines.  Some editors with
 223"smart" indentation will insert whitespace at the beginning of new lines as
 224appropriate, so you can start typing the next line of code right away.
 225However, some such editors do not remove the whitespace if you end up not
 226putting a line of code there, such as if you leave a blank line.  As a result,
 227you end up with lines containing trailing whitespace.
 228
 229Git will warn you about patches that introduce trailing whitespace, and can
 230optionally strip the trailing whitespace for you; however, if applying a series
 231of patches, this may make later patches in the series fail by changing their
 232context lines.
 233
 234
 235                Chapter 4: Naming
 236
 237C is a Spartan language, and so should your naming be.  Unlike Modula-2
 238and Pascal programmers, C programmers do not use cute names like
 239ThisVariableIsATemporaryCounter.  A C programmer would call that
 240variable "tmp", which is much easier to write, and not the least more
 241difficult to understand.
 242
 243HOWEVER, while mixed-case names are frowned upon, descriptive names for
 244global variables are a must.  To call a global function "foo" is a
 245shooting offense.
 246
 247GLOBAL variables (to be used only if you _really_ need them) need to
 248have descriptive names, as do global functions.  If you have a function
 249that counts the number of active users, you should call that
 250"count_active_users()" or similar, you should _not_ call it "cntusr()".
 251
 252Encoding the type of a function into the name (so-called Hungarian
 253notation) is brain damaged - the compiler knows the types anyway and can
 254check those, and it only confuses the programmer.  No wonder MicroSoft
 255makes buggy programs.
 256
 257LOCAL variable names should be short, and to the point.  If you have
 258some random integer loop counter, it should probably be called "i".
 259Calling it "loop_counter" is non-productive, if there is no chance of it
 260being mis-understood.  Similarly, "tmp" can be just about any type of
 261variable that is used to hold a temporary value.
 262
 263If you are afraid to mix up your local variable names, you have another
 264problem, which is called the function-growth-hormone-imbalance syndrome.
 265See chapter 6 (Functions).
 266
 267
 268                Chapter 5: Typedefs
 269
 270Please don't use things like "vps_t".
 271
 272It's a _mistake_ to use typedef for structures and pointers. When you see a
 273
 274        vps_t a;
 275
 276in the source, what does it mean?
 277
 278In contrast, if it says
 279
 280        struct virtual_container *a;
 281
 282you can actually tell what "a" is.
 283
 284Lots of people think that typedefs "help readability". Not so. They are
 285useful only for:
 286
 287 (a) totally opaque objects (where the typedef is actively used to _hide_
 288     what the object is).
 289
 290     Example: "pte_t" etc. opaque objects that you can only access using
 291     the proper accessor functions.
 292
 293     NOTE! Opaqueness and "accessor functions" are not good in themselves.
 294     The reason we have them for things like pte_t etc. is that there
 295     really is absolutely _zero_ portably accessible information there.
 296
 297 (b) Clear integer types, where the abstraction _helps_ avoid confusion
 298     whether it is "int" or "long".
 299
 300     u8/u16/u32 are perfectly fine typedefs, although they fit into
 301     category (d) better than here.
 302
 303     NOTE! Again - there needs to be a _reason_ for this. If something is
 304     "unsigned long", then there's no reason to do
 305
 306        typedef unsigned long myflags_t;
 307
 308     but if there is a clear reason for why it under certain circumstances
 309     might be an "unsigned int" and under other configurations might be
 310     "unsigned long", then by all means go ahead and use a typedef.
 311
 312 (c) when you use sparse to literally create a _new_ type for
 313     type-checking.
 314
 315 (d) New types which are identical to standard C99 types, in certain
 316     exceptional circumstances.
 317
 318     Although it would only take a short amount of time for the eyes and
 319     brain to become accustomed to the standard types like 'uint32_t',
 320     some people object to their use anyway.
 321
 322     Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
 323     signed equivalents which are identical to standard types are
 324     permitted -- although they are not mandatory in new code of your
 325     own.
 326
 327     When editing existing code which already uses one or the other set
 328     of types, you should conform to the existing choices in that code.
 329
 330 (e) Types safe for use in userspace.
 331
 332     In certain structures which are visible to userspace, we cannot
 333     require C99 types and cannot use the 'u32' form above. Thus, we
 334     use __u32 and similar types in all structures which are shared
 335     with userspace.
 336
 337Maybe there are other cases too, but the rule should basically be to NEVER
 338EVER use a typedef unless you can clearly match one of those rules.
 339
 340In general, a pointer, or a struct that has elements that can reasonably
 341be directly accessed should _never_ be a typedef.
 342
 343
 344                Chapter 6: Functions
 345
 346Functions should be short and sweet, and do just one thing.  They should
 347fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
 348as we all know), and do one thing and do that well.
 349
 350The maximum length of a function is inversely proportional to the
 351complexity and indentation level of that function.  So, if you have a
 352conceptually simple function that is just one long (but simple)
 353case-statement, where you have to do lots of small things for a lot of
 354different cases, it's OK to have a longer function.
 355
 356However, if you have a complex function, and you suspect that a
 357less-than-gifted first-year high-school student might not even
 358understand what the function is all about, you should adhere to the
 359maximum limits all the more closely.  Use helper functions with
 360descriptive names (you can ask the compiler to in-line them if you think
 361it's performance-critical, and it will probably do a better job of it
 362than you would have done).
 363
 364Another measure of the function is the number of local variables.  They
 365shouldn't exceed 5-10, or you're doing something wrong.  Re-think the
 366function, and split it into smaller pieces.  A human brain can
 367generally easily keep track of about 7 different things, anything more
 368and it gets confused.  You know you're brilliant, but maybe you'd like
 369to understand what you did 2 weeks from now.
 370
 371In source files, separate functions with one blank line.  If the function is
 372exported, the EXPORT* macro for it should follow immediately after the closing
 373function brace line.  E.g.:
 374
 375int system_is_up(void)
 376{
 377        return system_state == SYSTEM_RUNNING;
 378}
 379EXPORT_SYMBOL(system_is_up);
 380
 381In function prototypes, include parameter names with their data types.
 382Although this is not required by the C language, it is preferred in Linux
 383because it is a simple way to add valuable information for the reader.
 384
 385
 386                Chapter 7: Centralized exiting of functions
 387
 388Albeit deprecated by some people, the equivalent of the goto statement is
 389used frequently by compilers in form of the unconditional jump instruction.
 390
 391The goto statement comes in handy when a function exits from multiple
 392locations and some common work such as cleanup has to be done.  If there is no
 393cleanup needed then just return directly.
 394
 395Choose label names which say what the goto does or why the goto exists.  An
 396example of a good name could be "out_buffer:" if the goto frees "buffer".  Avoid
 397using GW-BASIC names like "err1:" and "err2:".  Also don't name them after the
 398goto location like "err_kmalloc_failed:"
 399
 400The rationale for using gotos is:
 401
 402- unconditional statements are easier to understand and follow
 403- nesting is reduced
 404- errors by not updating individual exit points when making
 405    modifications are prevented
 406- saves the compiler work to optimize redundant code away ;)
 407
 408int fun(int a)
 409{
 410        int result = 0;
 411        char *buffer;
 412
 413        buffer = kmalloc(SIZE, GFP_KERNEL);
 414        if (!buffer)
 415                return -ENOMEM;
 416
 417        if (condition1) {
 418                while (loop1) {
 419                        ...
 420                }
 421                result = 1;
 422                goto out_buffer;
 423        }
 424        ...
 425out_buffer:
 426        kfree(buffer);
 427        return result;
 428}
 429
 430A common type of bug to be aware of it "one err bugs" which look like this:
 431
 432err:
 433        kfree(foo->bar);
 434        kfree(foo);
 435        return ret;
 436
 437The bug in this code is that on some exit paths "foo" is NULL.  Normally the
 438fix for this is to split it up into two error labels "err_bar:" and "err_foo:".
 439
 440
 441                Chapter 8: Commenting
 442
 443Comments are good, but there is also a danger of over-commenting.  NEVER
 444try to explain HOW your code works in a comment: it's much better to
 445write the code so that the _working_ is obvious, and it's a waste of
 446time to explain badly written code.
 447
 448Generally, you want your comments to tell WHAT your code does, not HOW.
 449Also, try to avoid putting comments inside a function body: if the
 450function is so complex that you need to separately comment parts of it,
 451you should probably go back to chapter 6 for a while.  You can make
 452small comments to note or warn about something particularly clever (or
 453ugly), but try to avoid excess.  Instead, put the comments at the head
 454of the function, telling people what it does, and possibly WHY it does
 455it.
 456
 457When commenting the kernel API functions, please use the kernel-doc format.
 458See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
 459for details.
 460
 461Linux style for comments is the C89 "/* ... */" style.
 462Don't use C99-style "// ..." comments.
 463
 464The preferred style for long (multi-line) comments is:
 465
 466        /*
 467         * This is the preferred style for multi-line
 468         * comments in the Linux kernel source code.
 469         * Please use it consistently.
 470         *
 471         * Description:  A column of asterisks on the left side,
 472         * with beginning and ending almost-blank lines.
 473         */
 474
 475For files in net/ and drivers/net/ the preferred style for long (multi-line)
 476comments is a little different.
 477
 478        /* The preferred comment style for files in net/ and drivers/net
 479         * looks like this.
 480         *
 481         * It is nearly the same as the generally preferred comment style,
 482         * but there is no initial almost-blank line.
 483         */
 484
 485It's also important to comment data, whether they are basic types or derived
 486types.  To this end, use just one data declaration per line (no commas for
 487multiple data declarations).  This leaves you room for a small comment on each
 488item, explaining its use.
 489
 490
 491                Chapter 9: You've made a mess of it
 492
 493That's OK, we all do.  You've probably been told by your long-time Unix
 494user helper that "GNU emacs" automatically formats the C sources for
 495you, and you've noticed that yes, it does do that, but the defaults it
 496uses are less than desirable (in fact, they are worse than random
 497typing - an infinite number of monkeys typing into GNU emacs would never
 498make a good program).
 499
 500So, you can either get rid of GNU emacs, or change it to use saner
 501values.  To do the latter, you can stick the following in your .emacs file:
 502
 503(defun c-lineup-arglist-tabs-only (ignored)
 504  "Line up argument lists by tabs, not spaces"
 505  (let* ((anchor (c-langelem-pos c-syntactic-element))
 506         (column (c-langelem-2nd-pos c-syntactic-element))
 507         (offset (- (1+ column) anchor))
 508         (steps (floor offset c-basic-offset)))
 509    (* (max steps 1)
 510       c-basic-offset)))
 511
 512(add-hook 'c-mode-common-hook
 513          (lambda ()
 514            ;; Add kernel style
 515            (c-add-style
 516             "linux-tabs-only"
 517             '("linux" (c-offsets-alist
 518                        (arglist-cont-nonempty
 519                         c-lineup-gcc-asm-reg
 520                         c-lineup-arglist-tabs-only))))))
 521
 522(add-hook 'c-mode-hook
 523          (lambda ()
 524            (let ((filename (buffer-file-name)))
 525              ;; Enable kernel mode for the appropriate files
 526              (when (and filename
 527                         (string-match (expand-file-name "~/src/linux-trees")
 528                                       filename))
 529                (setq indent-tabs-mode t)
 530                (c-set-style "linux-tabs-only")))))
 531
 532This will make emacs go better with the kernel coding style for C
 533files below ~/src/linux-trees.
 534
 535But even if you fail in getting emacs to do sane formatting, not
 536everything is lost: use "indent".
 537
 538Now, again, GNU indent has the same brain-dead settings that GNU emacs
 539has, which is why you need to give it a few command line options.
 540However, that's not too bad, because even the makers of GNU indent
 541recognize the authority of K&R (the GNU people aren't evil, they are
 542just severely misguided in this matter), so you just give indent the
 543options "-kr -i8" (stands for "K&R, 8 character indents"), or use
 544"scripts/Lindent", which indents in the latest style.
 545
 546"indent" has a lot of options, and especially when it comes to comment
 547re-formatting you may want to take a look at the man page.  But
 548remember: "indent" is not a fix for bad programming.
 549
 550
 551                Chapter 10: Kconfig configuration files
 552
 553For all of the Kconfig* configuration files throughout the source tree,
 554the indentation is somewhat different.  Lines under a "config" definition
 555are indented with one tab, while help text is indented an additional two
 556spaces.  Example:
 557
 558config AUDIT
 559        bool "Auditing support"
 560        depends on NET
 561        help
 562          Enable auditing infrastructure that can be used with another
 563          kernel subsystem, such as SELinux (which requires this for
 564          logging of avc messages output).  Does not do system-call
 565          auditing without CONFIG_AUDITSYSCALL.
 566
 567Seriously dangerous features (such as write support for certain
 568filesystems) should advertise this prominently in their prompt string:
 569
 570config ADFS_FS_RW
 571        bool "ADFS write support (DANGEROUS)"
 572        depends on ADFS_FS
 573        ...
 574
 575For full documentation on the configuration files, see the file
 576Documentation/kbuild/kconfig-language.txt.
 577
 578
 579                Chapter 11: Data structures
 580
 581Data structures that have visibility outside the single-threaded
 582environment they are created and destroyed in should always have
 583reference counts.  In the kernel, garbage collection doesn't exist (and
 584outside the kernel garbage collection is slow and inefficient), which
 585means that you absolutely _have_ to reference count all your uses.
 586
 587Reference counting means that you can avoid locking, and allows multiple
 588users to have access to the data structure in parallel - and not having
 589to worry about the structure suddenly going away from under them just
 590because they slept or did something else for a while.
 591
 592Note that locking is _not_ a replacement for reference counting.
 593Locking is used to keep data structures coherent, while reference
 594counting is a memory management technique.  Usually both are needed, and
 595they are not to be confused with each other.
 596
 597Many data structures can indeed have two levels of reference counting,
 598when there are users of different "classes".  The subclass count counts
 599the number of subclass users, and decrements the global count just once
 600when the subclass count goes to zero.
 601
 602Examples of this kind of "multi-level-reference-counting" can be found in
 603memory management ("struct mm_struct": mm_users and mm_count), and in
 604filesystem code ("struct super_block": s_count and s_active).
 605
 606Remember: if another thread can find your data structure, and you don't
 607have a reference count on it, you almost certainly have a bug.
 608
 609
 610                Chapter 12: Macros, Enums and RTL
 611
 612Names of macros defining constants and labels in enums are capitalized.
 613
 614#define CONSTANT 0x12345
 615
 616Enums are preferred when defining several related constants.
 617
 618CAPITALIZED macro names are appreciated but macros resembling functions
 619may be named in lower case.
 620
 621Generally, inline functions are preferable to macros resembling functions.
 622
 623Macros with multiple statements should be enclosed in a do - while block:
 624
 625#define macrofun(a, b, c)                       \
 626        do {                                    \
 627                if (a == 5)                     \
 628                        do_this(b, c);          \
 629        } while (0)
 630
 631Things to avoid when using macros:
 632
 6331) macros that affect control flow:
 634
 635#define FOO(x)                                  \
 636        do {                                    \
 637                if (blah(x) < 0)                \
 638                        return -EBUGGERED;      \
 639        } while(0)
 640
 641is a _very_ bad idea.  It looks like a function call but exits the "calling"
 642function; don't break the internal parsers of those who will read the code.
 643
 6442) macros that depend on having a local variable with a magic name:
 645
 646#define FOO(val) bar(index, val)
 647
 648might look like a good thing, but it's confusing as hell when one reads the
 649code and it's prone to breakage from seemingly innocent changes.
 650
 6513) macros with arguments that are used as l-values: FOO(x) = y; will
 652bite you if somebody e.g. turns FOO into an inline function.
 653
 6544) forgetting about precedence: macros defining constants using expressions
 655must enclose the expression in parentheses. Beware of similar issues with
 656macros using parameters.
 657
 658#define CONSTANT 0x4000
 659#define CONSTEXP (CONSTANT | 3)
 660
 661The cpp manual deals with macros exhaustively. The gcc internals manual also
 662covers RTL which is used frequently with assembly language in the kernel.
 663
 664
 665                Chapter 13: Printing kernel messages
 666
 667Kernel developers like to be seen as literate. Do mind the spelling
 668of kernel messages to make a good impression. Do not use crippled
 669words like "dont"; use "do not" or "don't" instead.  Make the messages
 670concise, clear, and unambiguous.
 671
 672Kernel messages do not have to be terminated with a period.
 673
 674Printing numbers in parentheses (%d) adds no value and should be avoided.
 675
 676There are a number of driver model diagnostic macros in <linux/device.h>
 677which you should use to make sure messages are matched to the right device
 678and driver, and are tagged with the right level:  dev_err(), dev_warn(),
 679dev_info(), and so forth.  For messages that aren't associated with a
 680particular device, <linux/printk.h> defines pr_notice(), pr_info(),
 681pr_warn(), pr_err(), etc.
 682
 683Coming up with good debugging messages can be quite a challenge; and once
 684you have them, they can be a huge help for remote troubleshooting.  However
 685debug message printing is handled differently than printing other non-debug
 686messages.  While the other pr_XXX() functions print unconditionally,
 687pr_debug() does not; it is compiled out by default, unless either DEBUG is
 688defined or CONFIG_DYNAMIC_DEBUG is set.  That is true for dev_dbg() also,
 689and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
 690the ones already enabled by DEBUG.
 691
 692Many subsystems have Kconfig debug options to turn on -DDEBUG in the
 693corresponding Makefile; in other cases specific files #define DEBUG.  And
 694when a debug message should be unconditionally printed, such as if it is
 695already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
 696used.
 697
 698
 699                Chapter 14: Allocating memory
 700
 701The kernel provides the following general purpose memory allocators:
 702kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
 703vzalloc().  Please refer to the API documentation for further information
 704about them.
 705
 706The preferred form for passing a size of a struct is the following:
 707
 708        p = kmalloc(sizeof(*p), ...);
 709
 710The alternative form where struct name is spelled out hurts readability and
 711introduces an opportunity for a bug when the pointer variable type is changed
 712but the corresponding sizeof that is passed to a memory allocator is not.
 713
 714Casting the return value which is a void pointer is redundant. The conversion
 715from void pointer to any other pointer type is guaranteed by the C programming
 716language.
 717
 718The preferred form for allocating an array is the following:
 719
 720        p = kmalloc_array(n, sizeof(...), ...);
 721
 722The preferred form for allocating a zeroed array is the following:
 723
 724        p = kcalloc(n, sizeof(...), ...);
 725
 726Both forms check for overflow on the allocation size n * sizeof(...),
 727and return NULL if that occurred.
 728
 729
 730                Chapter 15: The inline disease
 731
 732There appears to be a common misperception that gcc has a magic "make me
 733faster" speedup option called "inline". While the use of inlines can be
 734appropriate (for example as a means of replacing macros, see Chapter 12), it
 735very often is not. Abundant use of the inline keyword leads to a much bigger
 736kernel, which in turn slows the system as a whole down, due to a bigger
 737icache footprint for the CPU and simply because there is less memory
 738available for the pagecache. Just think about it; a pagecache miss causes a
 739disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
 740that can go into these 5 milliseconds.
 741
 742A reasonable rule of thumb is to not put inline at functions that have more
 743than 3 lines of code in them. An exception to this rule are the cases where
 744a parameter is known to be a compiletime constant, and as a result of this
 745constantness you *know* the compiler will be able to optimize most of your
 746function away at compile time. For a good example of this later case, see
 747the kmalloc() inline function.
 748
 749Often people argue that adding inline to functions that are static and used
 750only once is always a win since there is no space tradeoff. While this is
 751technically correct, gcc is capable of inlining these automatically without
 752help, and the maintenance issue of removing the inline when a second user
 753appears outweighs the potential value of the hint that tells gcc to do
 754something it would have done anyway.
 755
 756
 757                Chapter 16: Function return values and names
 758
 759Functions can return values of many different kinds, and one of the
 760most common is a value indicating whether the function succeeded or
 761failed.  Such a value can be represented as an error-code integer
 762(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
 763non-zero = success).
 764
 765Mixing up these two sorts of representations is a fertile source of
 766difficult-to-find bugs.  If the C language included a strong distinction
 767between integers and booleans then the compiler would find these mistakes
 768for us... but it doesn't.  To help prevent such bugs, always follow this
 769convention:
 770
 771        If the name of a function is an action or an imperative command,
 772        the function should return an error-code integer.  If the name
 773        is a predicate, the function should return a "succeeded" boolean.
 774
 775For example, "add work" is a command, and the add_work() function returns 0
 776for success or -EBUSY for failure.  In the same way, "PCI device present" is
 777a predicate, and the pci_dev_present() function returns 1 if it succeeds in
 778finding a matching device or 0 if it doesn't.
 779
 780All EXPORTed functions must respect this convention, and so should all
 781public functions.  Private (static) functions need not, but it is
 782recommended that they do.
 783
 784Functions whose return value is the actual result of a computation, rather
 785than an indication of whether the computation succeeded, are not subject to
 786this rule.  Generally they indicate failure by returning some out-of-range
 787result.  Typical examples would be functions that return pointers; they use
 788NULL or the ERR_PTR mechanism to report failure.
 789
 790
 791                Chapter 17:  Don't re-invent the kernel macros
 792
 793The header file include/linux/kernel.h contains a number of macros that
 794you should use, rather than explicitly coding some variant of them yourself.
 795For example, if you need to calculate the length of an array, take advantage
 796of the macro
 797
 798  #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
 799
 800Similarly, if you need to calculate the size of some structure member, use
 801
 802  #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
 803
 804There are also min() and max() macros that do strict type checking if you
 805need them.  Feel free to peruse that header file to see what else is already
 806defined that you shouldn't reproduce in your code.
 807
 808
 809                Chapter 18:  Editor modelines and other cruft
 810
 811Some editors can interpret configuration information embedded in source files,
 812indicated with special markers.  For example, emacs interprets lines marked
 813like this:
 814
 815-*- mode: c -*-
 816
 817Or like this:
 818
 819/*
 820Local Variables:
 821compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
 822End:
 823*/
 824
 825Vim interprets markers that look like this:
 826
 827/* vim:set sw=8 noet */
 828
 829Do not include any of these in source files.  People have their own personal
 830editor configurations, and your source files should not override them.  This
 831includes markers for indentation and mode configuration.  People may use their
 832own custom mode, or may have some other magic method for making indentation
 833work correctly.
 834
 835
 836                Chapter 19:  Inline assembly
 837
 838In architecture-specific code, you may need to use inline assembly to interface
 839with CPU or platform functionality.  Don't hesitate to do so when necessary.
 840However, don't use inline assembly gratuitously when C can do the job.  You can
 841and should poke hardware from C when possible.
 842
 843Consider writing simple helper functions that wrap common bits of inline
 844assembly, rather than repeatedly writing them with slight variations.  Remember
 845that inline assembly can use C parameters.
 846
 847Large, non-trivial assembly functions should go in .S files, with corresponding
 848C prototypes defined in C header files.  The C prototypes for assembly
 849functions should use "asmlinkage".
 850
 851You may need to mark your asm statement as volatile, to prevent GCC from
 852removing it if GCC doesn't notice any side effects.  You don't always need to
 853do so, though, and doing so unnecessarily can limit optimization.
 854
 855When writing a single inline assembly statement containing multiple
 856instructions, put each instruction on a separate line in a separate quoted
 857string, and end each string except the last with \n\t to properly indent the
 858next instruction in the assembly output:
 859
 860        asm ("magic %reg1, #42\n\t"
 861             "more_magic %reg2, %reg3"
 862             : /* outputs */ : /* inputs */ : /* clobbers */);
 863
 864
 865                Chapter 20: Conditional Compilation
 866
 867Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
 868files; doing so makes code harder to read and logic harder to follow.  Instead,
 869use such conditionals in a header file defining functions for use in those .c
 870files, providing no-op stub versions in the #else case, and then call those
 871functions unconditionally from .c files.  The compiler will avoid generating
 872any code for the stub calls, producing identical results, but the logic will
 873remain easy to follow.
 874
 875Prefer to compile out entire functions, rather than portions of functions or
 876portions of expressions.  Rather than putting an ifdef in an expression, factor
 877out part or all of the expression into a separate helper function and apply the
 878conditional to that function.
 879
 880If you have a function or variable which may potentially go unused in a
 881particular configuration, and the compiler would warn about its definition
 882going unused, mark the definition as __maybe_unused rather than wrapping it in
 883a preprocessor conditional.  (However, if a function or variable *always* goes
 884unused, delete it.)
 885
 886Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
 887symbol into a C boolean expression, and use it in a normal C conditional:
 888
 889        if (IS_ENABLED(CONFIG_SOMETHING)) {
 890                ...
 891        }
 892
 893The compiler will constant-fold the conditional away, and include or exclude
 894the block of code just as with an #ifdef, so this will not add any runtime
 895overhead.  However, this approach still allows the C compiler to see the code
 896inside the block, and check it for correctness (syntax, types, symbol
 897references, etc).  Thus, you still have to use an #ifdef if the code inside the
 898block references symbols that will not exist if the condition is not met.
 899
 900At the end of any non-trivial #if or #ifdef block (more than a few lines),
 901place a comment after the #endif on the same line, noting the conditional
 902expression used.  For instance:
 903
 904#ifdef CONFIG_SOMETHING
 905...
 906#endif /* CONFIG_SOMETHING */
 907
 908
 909                Appendix I: References
 910
 911The C Programming Language, Second Edition
 912by Brian W. Kernighan and Dennis M. Ritchie.
 913Prentice Hall, Inc., 1988.
 914ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
 915URL: http://cm.bell-labs.com/cm/cs/cbook/
 916
 917The Practice of Programming
 918by Brian W. Kernighan and Rob Pike.
 919Addison-Wesley, Inc., 1999.
 920ISBN 0-201-61586-X.
 921URL: http://cm.bell-labs.com/cm/cs/tpop/
 922
 923GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
 924gcc internals and indent, all available from http://www.gnu.org/manual/
 925
 926WG14 is the international standardization working group for the programming
 927language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
 928
 929Kernel CodingStyle, by greg@kroah.com at OLS 2002:
 930http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
 931
 932
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