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 /* fall through */ 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 put multiple assignments on a single line either. Kernel coding style 73is super simple. Avoid tricky expressions. 74 75Outside of comments, documentation and except in Kconfig, spaces are never 76used for indentation, and the above example is deliberately broken. 77 78Get a decent editor and don't leave whitespace at the end of lines. 79 80 812) Breaking long lines and strings 82---------------------------------- 83 84Coding style is all about readability and maintainability using commonly 85available tools. 86 87The limit on the length of lines is 80 columns and this is a strongly 88preferred limit. 89 90Statements longer than 80 columns will be broken into sensible chunks, unless 91exceeding 80 columns significantly increases readability and does not hide 92information. Descendants are always substantially shorter than the parent and 93are placed substantially to the right. The same applies to function headers 94with a long argument list. However, never break user-visible strings such as 95printk messages, because that breaks the ability to grep for them. 96 97 983) Placing Braces and Spaces 99---------------------------- 100 101The other issue that always comes up in C styling is the placement of 102braces. Unlike the indent size, there are few technical reasons to 103choose one placement strategy over the other, but the preferred way, as 104shown to us by the prophets Kernighan and Ritchie, is to put the opening 105brace last on the line, and put the closing brace first, thusly: 106 107.. code-block:: c 108 109 if (x is true) { 110 we do y 111 } 112 113This applies to all non-function statement blocks (if, switch, for, 114while, do). E.g.: 115 116.. code-block:: c 117 118 switch (action) { 119 case KOBJ_ADD: 120 return "add"; 121 case KOBJ_REMOVE: 122 return "remove"; 123 case KOBJ_CHANGE: 124 return "change"; 125 default: 126 return NULL; 127 } 128 129However, there is one special case, namely functions: they have the 130opening brace at the beginning of the next line, thus: 131 132.. code-block:: c 133 134 int function(int x) 135 { 136 body of function 137 } 138 139Heretic people all over the world have claimed that this inconsistency 140is ... well ... inconsistent, but all right-thinking people know that 141(a) K&R are **right** and (b) K&R are right. Besides, functions are 142special anyway (you can't nest them in C). 143 144Note that the closing brace is empty on a line of its own, **except** in 145the cases where it is followed by a continuation of the same statement, 146ie a ``while`` in a do-statement or an ``else`` in an if-statement, like 147this: 148 149.. code-block:: c 150 151 do { 152 body of do-loop 153 } while (condition); 154 155and 156 157.. code-block:: c 158 159 if (x == y) { 160 .. 161 } else if (x > y) { 162 ... 163 } else { 164 .... 165 } 166 167Rationale: K&R. 168 169Also, note that this brace-placement also minimizes the number of empty 170(or almost empty) lines, without any loss of readability. Thus, as the 171supply of new-lines on your screen is not a renewable resource (think 17225-line terminal screens here), you have more empty lines to put 173comments on. 174 175Do not unnecessarily use braces where a single statement will do. 176 177.. code-block:: c 178 179 if (condition) 180 action(); 181 182and 183 184.. code-block:: none 185 186 if (condition) 187 do_this(); 188 else 189 do_that(); 190 191This does not apply if only one branch of a conditional statement is a single 192statement; in the latter case use braces in both branches: 193 194.. code-block:: c 195 196 if (condition) { 197 do_this(); 198 do_that(); 199 } else { 200 otherwise(); 201 } 202 203Also, use braces when a loop contains more than a single simple statement: 204 205.. code-block:: c 206 207 while (condition) { 208 if (test) 209 do_something(); 210 } 211 2123.1) Spaces 213*********** 214 215Linux kernel style for use of spaces depends (mostly) on 216function-versus-keyword usage. Use a space after (most) keywords. The 217notable exceptions are sizeof, typeof, alignof, and __attribute__, which look 218somewhat like functions (and are usually used with parentheses in Linux, 219although they are not required in the language, as in: ``sizeof info`` after 220``struct fileinfo info;`` is declared). 221 222So use a space after these keywords:: 223 224 if, switch, case, for, do, while 225 226but not with sizeof, typeof, alignof, or __attribute__. E.g., 227 228.. code-block:: c 229 230 231 s = sizeof(struct file); 232 233Do not add spaces around (inside) parenthesized expressions. This example is 234**bad**: 235 236.. code-block:: c 237 238 239 s = sizeof( struct file ); 240 241When declaring pointer data or a function that returns a pointer type, the 242preferred use of ``*`` is adjacent to the data name or function name and not 243adjacent to the type name. Examples: 244 245.. code-block:: c 246 247 248 char *linux_banner; 249 unsigned long long memparse(char *ptr, char **retptr); 250 char *match_strdup(substring_t *s); 251 252Use one space around (on each side of) most binary and ternary operators, 253such as any of these:: 254 255 = + - < > * / % | & ^ <= >= == != ? : 256 257but no space after unary operators:: 258 259 & * + - ~ ! sizeof typeof alignof __attribute__ defined 260 261no space before the postfix increment & decrement unary operators:: 262 263 ++ -- 264 265no space after the prefix increment & decrement unary operators:: 266 267 ++ -- 268 269and no space around the ``.`` and ``->`` structure member operators. 270 271Do not leave trailing whitespace at the ends of lines. Some editors with 272``smart`` indentation will insert whitespace at the beginning of new lines as 273appropriate, so you can start typing the next line of code right away. 274However, some such editors do not remove the whitespace if you end up not 275putting a line of code there, such as if you leave a blank line. As a result, 276you end up with lines containing trailing whitespace. 277 278Git will warn you about patches that introduce trailing whitespace, and can 279optionally strip the trailing whitespace for you; however, if applying a series 280of patches, this may make later patches in the series fail by changing their 281context lines. 282 283 2844) Naming 285--------- 286 287C is a Spartan language, and so should your naming be. Unlike Modula-2 288and Pascal programmers, C programmers do not use cute names like 289ThisVariableIsATemporaryCounter. A C programmer would call that 290variable ``tmp``, which is much easier to write, and not the least more 291difficult to understand. 292 293HOWEVER, while mixed-case names are frowned upon, descriptive names for 294global variables are a must. To call a global function ``foo`` is a 295shooting offense. 296 297GLOBAL variables (to be used only if you **really** need them) need to 298have descriptive names, as do global functions. If you have a function 299that counts the number of active users, you should call that 300``count_active_users()`` or similar, you should **not** call it ``cntusr()``. 301 302Encoding the type of a function into the name (so-called Hungarian 303notation) is brain damaged - the compiler knows the types anyway and can 304check those, and it only confuses the programmer. No wonder MicroSoft 305makes buggy programs. 306 307LOCAL variable names should be short, and to the point. If you have 308some random integer loop counter, it should probably be called ``i``. 309Calling it ``loop_counter`` is non-productive, if there is no chance of it 310being mis-understood. Similarly, ``tmp`` can be just about any type of 311variable that is used to hold a temporary value. 312 313If you are afraid to mix up your local variable names, you have another 314problem, which is called the function-growth-hormone-imbalance syndrome. 315See chapter 6 (Functions). 316 317 3185) Typedefs 319----------- 320 321Please don't use things like ``vps_t``. 322It's a **mistake** to use typedef for structures and pointers. When you see a 323 324.. code-block:: c 325 326 327 vps_t a; 328 329in the source, what does it mean? 330In contrast, if it says 331 332.. code-block:: c 333 334 struct virtual_container *a; 335 336you can actually tell what ``a`` is. 337 338Lots of people think that typedefs ``help readability``. Not so. They are 339useful only for: 340 341 (a) totally opaque objects (where the typedef is actively used to **hide** 342 what the object is). 343 344 Example: ``pte_t`` etc. opaque objects that you can only access using 345 the proper accessor functions. 346 347 .. note:: 348 349 Opaqueness and ``accessor functions`` are not good in themselves. 350 The reason we have them for things like pte_t etc. is that there 351 really is absolutely **zero** portably accessible information there. 352 353 (b) Clear integer types, where the abstraction **helps** avoid confusion 354 whether it is ``int`` or ``long``. 355 356 u8/u16/u32 are perfectly fine typedefs, although they fit into 357 category (d) better than here. 358 359 .. note:: 360 361 Again - there needs to be a **reason** for this. If something is 362 ``unsigned long``, then there's no reason to do 363 364 typedef unsigned long myflags_t; 365 366 but if there is a clear reason for why it under certain circumstances 367 might be an ``unsigned int`` and under other configurations might be 368 ``unsigned long``, then by all means go ahead and use a typedef. 369 370 (c) when you use sparse to literally create a **new** type for 371 type-checking. 372 373 (d) New types which are identical to standard C99 types, in certain 374 exceptional circumstances. 375 376 Although it would only take a short amount of time for the eyes and 377 brain to become accustomed to the standard types like ``uint32_t``, 378 some people object to their use anyway. 379 380 Therefore, the Linux-specific ``u8/u16/u32/u64`` types and their 381 signed equivalents which are identical to standard types are 382 permitted -- although they are not mandatory in new code of your 383 own. 384 385 When editing existing code which already uses one or the other set 386 of types, you should conform to the existing choices in that code. 387 388 (e) Types safe for use in userspace. 389 390 In certain structures which are visible to userspace, we cannot 391 require C99 types and cannot use the ``u32`` form above. Thus, we 392 use __u32 and similar types in all structures which are shared 393 with userspace. 394 395Maybe there are other cases too, but the rule should basically be to NEVER 396EVER use a typedef unless you can clearly match one of those rules. 397 398In general, a pointer, or a struct that has elements that can reasonably 399be directly accessed should **never** be a typedef. 400 401 4026) Functions 403------------ 404 405Functions should be short and sweet, and do just one thing. They should 406fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24, 407as we all know), and do one thing and do that well. 408 409The maximum length of a function is inversely proportional to the 410complexity and indentation level of that function. So, if you have a 411conceptually simple function that is just one long (but simple) 412case-statement, where you have to do lots of small things for a lot of 413different cases, it's OK to have a longer function. 414 415However, if you have a complex function, and you suspect that a 416less-than-gifted first-year high-school student might not even 417understand what the function is all about, you should adhere to the 418maximum limits all the more closely. Use helper functions with 419descriptive names (you can ask the compiler to in-line them if you think 420it's performance-critical, and it will probably do a better job of it 421than you would have done). 422 423Another measure of the function is the number of local variables. They 424shouldn't exceed 5-10, or you're doing something wrong. Re-think the 425function, and split it into smaller pieces. A human brain can 426generally easily keep track of about 7 different things, anything more 427and it gets confused. You know you're brilliant, but maybe you'd like 428to understand what you did 2 weeks from now. 429 430In source files, separate functions with one blank line. If the function is 431exported, the **EXPORT** macro for it should follow immediately after the 432closing function brace line. E.g.: 433 434.. code-block:: c 435 436 int system_is_up(void) 437 { 438 return system_state == SYSTEM_RUNNING; 439 } 440 EXPORT_SYMBOL(system_is_up); 441 442In function prototypes, include parameter names with their data types. 443Although this is not required by the C language, it is preferred in Linux 444because it is a simple way to add valuable information for the reader. 445 446Do not use the ``extern`` keyword with function prototypes as this makes 447lines longer and isn't strictly necessary. 448 449 4507) Centralized exiting of functions 451----------------------------------- 452 453Albeit deprecated by some people, the equivalent of the goto statement is 454used frequently by compilers in form of the unconditional jump instruction. 455 456The goto statement comes in handy when a function exits from multiple 457locations and some common work such as cleanup has to be done. If there is no 458cleanup needed then just return directly. 459 460Choose label names which say what the goto does or why the goto exists. An 461example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``. 462Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to 463renumber them if you ever add or remove exit paths, and they make correctness 464difficult to verify anyway. 465 466The rationale for using gotos is: 467 468- unconditional statements are easier to understand and follow 469- nesting is reduced 470- errors by not updating individual exit points when making 471 modifications are prevented 472- saves the compiler work to optimize redundant code away ;) 473 474.. code-block:: c 475 476 int fun(int a) 477 { 478 int result = 0; 479 char *buffer; 480 481 buffer = kmalloc(SIZE, GFP_KERNEL); 482 if (!buffer) 483 return -ENOMEM; 484 485 if (condition1) { 486 while (loop1) { 487 ... 488 } 489 result = 1; 490 goto out_free_buffer; 491 } 492 ... 493 out_free_buffer: 494 kfree(buffer); 495 return result; 496 } 497 498A common type of bug to be aware of is ``one err bugs`` which look like this: 499 500.. code-block:: c 501 502 err: 503 kfree(foo->bar); 504 kfree(foo); 505 return ret; 506 507The bug in this code is that on some exit paths ``foo`` is NULL. Normally the 508fix for this is to split it up into two error labels ``err_free_bar:`` and 509``err_free_foo:``: 510 511.. code-block:: c 512 513 err_free_bar: 514 kfree(foo->bar); 515 err_free_foo: 516 kfree(foo); 517 return ret; 518 519Ideally you should simulate errors to test all exit paths. 520 521 5228) Commenting 523------------- 524 525Comments are good, but there is also a danger of over-commenting. NEVER 526try to explain HOW your code works in a comment: it's much better to 527write the code so that the **working** is obvious, and it's a waste of 528time to explain badly written code. 529 530Generally, you want your comments to tell WHAT your code does, not HOW. 531Also, try to avoid putting comments inside a function body: if the 532function is so complex that you need to separately comment parts of it, 533you should probably go back to chapter 6 for a while. You can make 534small comments to note or warn about something particularly clever (or 535ugly), but try to avoid excess. Instead, put the comments at the head 536of the function, telling people what it does, and possibly WHY it does 537it. 538 539When commenting the kernel API functions, please use the kernel-doc format. 540See the files at :ref:`Documentation/doc-guide/ <doc_guide>` and 541``scripts/kernel-doc`` for details. 542 543The preferred style for long (multi-line) comments is: 544 545.. code-block:: c 546 547 /* 548 * This is the preferred style for multi-line 549 * comments in the Linux kernel source code. 550 * Please use it consistently. 551 * 552 * Description: A column of asterisks on the left side, 553 * with beginning and ending almost-blank lines. 554 */ 555 556For files in net/ and drivers/net/ the preferred style for long (multi-line) 557comments is a little different. 558 559.. code-block:: c 560 561 /* The preferred comment style for files in net/ and drivers/net 562 * looks like this. 563 * 564 * It is nearly the same as the generally preferred comment style, 565 * but there is no initial almost-blank line. 566 */ 567 568It's also important to comment data, whether they are basic types or derived 569types. To this end, use just one data declaration per line (no commas for 570multiple data declarations). This leaves you room for a small comment on each 571item, explaining its use. 572 573 5749) You've made a mess of it 575--------------------------- 576 577That's OK, we all do. You've probably been told by your long-time Unix 578user helper that ``GNU emacs`` automatically formats the C sources for 579you, and you've noticed that yes, it does do that, but the defaults it 580uses are less than desirable (in fact, they are worse than random 581typing - an infinite number of monkeys typing into GNU emacs would never 582make a good program). 583 584So, you can either get rid of GNU emacs, or change it to use saner 585values. To do the latter, you can stick the following in your .emacs file: 586 587.. code-block:: none 588 589 (defun c-lineup-arglist-tabs-only (ignored) 590 "Line up argument lists by tabs, not spaces" 591 (let* ((anchor (c-langelem-pos c-syntactic-element)) 592 (column (c-langelem-2nd-pos c-syntactic-element)) 593 (offset (- (1+ column) anchor)) 594 (steps (floor offset c-basic-offset))) 595 (* (max steps 1) 596 c-basic-offset))) 597 598 (dir-locals-set-class-variables 599 'linux-kernel 600 '((c-mode . ( 601 (c-basic-offset . 8) 602 (c-label-minimum-indentation . 0) 603 (c-offsets-alist . ( 604 (arglist-close . c-lineup-arglist-tabs-only) 605 (arglist-cont-nonempty . 606 (c-lineup-gcc-asm-reg c-lineup-arglist-tabs-only)) 607 (arglist-intro . +) 608 (brace-list-intro . +) 609 (c . c-lineup-C-comments) 610 (case-label . 0) 611 (comment-intro . c-lineup-comment) 612 (cpp-define-intro . +) 613 (cpp-macro . -1000) 614 (cpp-macro-cont . +) 615 (defun-block-intro . +) 616 (else-clause . 0) 617 (func-decl-cont . +) 618 (inclass . +) 619 (inher-cont . c-lineup-multi-inher) 620 (knr-argdecl-intro . 0) 621 (label . -1000) 622 (statement . 0) 623 (statement-block-intro . +) 624 (statement-case-intro . +) 625 (statement-cont . +) 626 (substatement . +) 627 )) 628 (indent-tabs-mode . t) 629 (show-trailing-whitespace . t) 630 )))) 631 632 (dir-locals-set-directory-class 633 (expand-file-name "~/src/linux-trees") 634 'linux-kernel) 635 636This will make emacs go better with the kernel coding style for C 637files below ``~/src/linux-trees``. 638 639But even if you fail in getting emacs to do sane formatting, not 640everything is lost: use ``indent``. 641 642Now, again, GNU indent has the same brain-dead settings that GNU emacs 643has, which is why you need to give it a few command line options. 644However, that's not too bad, because even the makers of GNU indent 645recognize the authority of K&R (the GNU people aren't evil, they are 646just severely misguided in this matter), so you just give indent the 647options ``-kr -i8`` (stands for ``K&R, 8 character indents``), or use 648``scripts/Lindent``, which indents in the latest style. 649 650``indent`` has a lot of options, and especially when it comes to comment 651re-formatting you may want to take a look at the man page. But 652remember: ``indent`` is not a fix for bad programming. 653 654Note that you can also use the ``clang-format`` tool to help you with 655these rules, to quickly re-format parts of your code automatically, 656and to review full files in order to spot coding style mistakes, 657typos and possible improvements. It is also handy for sorting ``#includes``, 658for aligning variables/macros, for reflowing text and other similar tasks. 659See the file :ref:`Documentation/process/clang-format.rst <clangformat>` 660for more details. 661 662 66310) Kconfig configuration files 664------------------------------- 665 666For all of the Kconfig* configuration files throughout the source tree, 667the indentation is somewhat different. Lines under a ``config`` definition 668are indented with one tab, while help text is indented an additional two 669spaces. Example:: 670 671 config AUDIT 672 bool "Auditing support" 673 depends on NET 674 help 675 Enable auditing infrastructure that can be used with another 676 kernel subsystem, such as SELinux (which requires this for 677 logging of avc messages output). Does not do system-call 678 auditing without CONFIG_AUDITSYSCALL. 679 680Seriously dangerous features (such as write support for certain 681filesystems) should advertise this prominently in their prompt string:: 682 683 config ADFS_FS_RW 684 bool "ADFS write support (DANGEROUS)" 685 depends on ADFS_FS 686 ... 687 688For full documentation on the configuration files, see the file 689Documentation/kbuild/kconfig-language.rst. 690 691 69211) Data structures 693------------------- 694 695Data structures that have visibility outside the single-threaded 696environment they are created and destroyed in should always have 697reference counts. In the kernel, garbage collection doesn't exist (and 698outside the kernel garbage collection is slow and inefficient), which 699means that you absolutely **have** to reference count all your uses. 700 701Reference counting means that you can avoid locking, and allows multiple 702users to have access to the data structure in parallel - and not having 703to worry about the structure suddenly going away from under them just 704because they slept or did something else for a while. 705 706Note that locking is **not** a replacement for reference counting. 707Locking is used to keep data structures coherent, while reference 708counting is a memory management technique. Usually both are needed, and 709they are not to be confused with each other. 710 711Many data structures can indeed have two levels of reference counting, 712when there are users of different ``classes``. The subclass count counts 713the number of subclass users, and decrements the global count just once 714when the subclass count goes to zero. 715 716Examples of this kind of ``multi-level-reference-counting`` can be found in 717memory management (``struct mm_struct``: mm_users and mm_count), and in 718filesystem code (``struct super_block``: s_count and s_active). 719 720Remember: if another thread can find your data structure, and you don't 721have a reference count on it, you almost certainly have a bug. 722 723 72412) Macros, Enums and RTL 725------------------------- 726 727Names of macros defining constants and labels in enums are capitalized. 728 729.. code-block:: c 730 731 #define CONSTANT 0x12345 732 733Enums are preferred when defining several related constants. 734 735CAPITALIZED macro names are appreciated but macros resembling functions 736may be named in lower case. 737 738Generally, inline functions are preferable to macros resembling functions. 739 740Macros with multiple statements should be enclosed in a do - while block: 741 742.. code-block:: c 743 744 #define macrofun(a, b, c) \ 745 do { \ 746 if (a == 5) \ 747 do_this(b, c); \ 748 } while (0) 749 750Things to avoid when using macros: 751 7521) macros that affect control flow: 753 754.. code-block:: c 755 756 #define FOO(x) \ 757 do { \ 758 if (blah(x) < 0) \ 759 return -EBUGGERED; \ 760 } while (0) 761 762is a **very** bad idea. It looks like a function call but exits the ``calling`` 763function; don't break the internal parsers of those who will read the code. 764 7652) macros that depend on having a local variable with a magic name: 766 767.. code-block:: c 768 769 #define FOO(val) bar(index, val) 770 771might look like a good thing, but it's confusing as hell when one reads the 772code and it's prone to breakage from seemingly innocent changes. 773 7743) macros with arguments that are used as l-values: FOO(x) = y; will 775bite you if somebody e.g. turns FOO into an inline function. 776 7774) forgetting about precedence: macros defining constants using expressions 778must enclose the expression in parentheses. Beware of similar issues with 779macros using parameters. 780 781.. code-block:: c 782 783 #define CONSTANT 0x4000 784 #define CONSTEXP (CONSTANT | 3) 785 7865) namespace collisions when defining local variables in macros resembling 787functions: 788 789.. code-block:: c 790 791 #define FOO(x) \ 792 ({ \ 793 typeof(x) ret; \ 794 ret = calc_ret(x); \ 795 (ret); \ 796 }) 797 798ret is a common name for a local variable - __foo_ret is less likely 799to collide with an existing variable. 800 801The cpp manual deals with macros exhaustively. The gcc internals manual also 802covers RTL which is used frequently with assembly language in the kernel. 803 804 80513) Printing kernel messages 806---------------------------- 807 808Kernel developers like to be seen as literate. Do mind the spelling 809of kernel messages to make a good impression. Do not use crippled 810words like ``dont``; use ``do not`` or ``don't`` instead. Make the messages 811concise, clear, and unambiguous. 812 813Kernel messages do not have to be terminated with a period. 814 815Printing numbers in parentheses (%d) adds no value and should be avoided. 816 817There are a number of driver model diagnostic macros in <linux/device.h> 818which you should use to make sure messages are matched to the right device 819and driver, and are tagged with the right level: dev_err(), dev_warn(), 820dev_info(), and so forth. For messages that aren't associated with a 821particular device, <linux/printk.h> defines pr_notice(), pr_info(), 822pr_warn(), pr_err(), etc. 823 824Coming up with good debugging messages can be quite a challenge; and once 825you have them, they can be a huge help for remote troubleshooting. However 826debug message printing is handled differently than printing other non-debug 827messages. While the other pr_XXX() functions print unconditionally, 828pr_debug() does not; it is compiled out by default, unless either DEBUG is 829defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also, 830and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to 831the ones already enabled by DEBUG. 832 833Many subsystems have Kconfig debug options to turn on -DDEBUG in the 834corresponding Makefile; in other cases specific files #define DEBUG. And 835when a debug message should be unconditionally printed, such as if it is 836already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be 837used. 838 839 84014) Allocating memory 841--------------------- 842 843The kernel provides the following general purpose memory allocators: 844kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and 845vzalloc(). Please refer to the API documentation for further information 846about them. :ref:`Documentation/core-api/memory-allocation.rst 847<memory_allocation>` 848 849The preferred form for passing a size of a struct is the following: 850 851.. code-block:: c 852 853 p = kmalloc(sizeof(*p), ...); 854 855The alternative form where struct name is spelled out hurts readability and 856introduces an opportunity for a bug when the pointer variable type is changed 857but the corresponding sizeof that is passed to a memory allocator is not. 858 859Casting the return value which is a void pointer is redundant. The conversion 860from void pointer to any other pointer type is guaranteed by the C programming 861language. 862 863The preferred form for allocating an array is the following: 864 865.. code-block:: c 866 867 p = kmalloc_array(n, sizeof(...), ...); 868 869The preferred form for allocating a zeroed array is the following: 870 871.. code-block:: c 872 873 p = kcalloc(n, sizeof(...), ...); 874 875Both forms check for overflow on the allocation size n * sizeof(...), 876and return NULL if that occurred. 877 878These generic allocation functions all emit a stack dump on failure when used 879without __GFP_NOWARN so there is no use in emitting an additional failure 880message when NULL is returned. 881 88215) The inline disease 883---------------------- 884 885There appears to be a common misperception that gcc has a magic "make me 886faster" speedup option called ``inline``. While the use of inlines can be 887appropriate (for example as a means of replacing macros, see Chapter 12), it 888very often is not. Abundant use of the inline keyword leads to a much bigger 889kernel, which in turn slows the system as a whole down, due to a bigger 890icache footprint for the CPU and simply because there is less memory 891available for the pagecache. Just think about it; a pagecache miss causes a 892disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles 893that can go into these 5 milliseconds. 894 895A reasonable rule of thumb is to not put inline at functions that have more 896than 3 lines of code in them. An exception to this rule are the cases where 897a parameter is known to be a compiletime constant, and as a result of this 898constantness you *know* the compiler will be able to optimize most of your 899function away at compile time. For a good example of this later case, see 900the kmalloc() inline function. 901 902Often people argue that adding inline to functions that are static and used 903only once is always a win since there is no space tradeoff. While this is 904technically correct, gcc is capable of inlining these automatically without 905help, and the maintenance issue of removing the inline when a second user 906appears outweighs the potential value of the hint that tells gcc to do 907something it would have done anyway. 908 909 91016) Function return values and names 911------------------------------------ 912 913Functions can return values of many different kinds, and one of the 914most common is a value indicating whether the function succeeded or 915failed. Such a value can be represented as an error-code integer 916(-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure, 917non-zero = success). 918 919Mixing up these two sorts of representations is a fertile source of 920difficult-to-find bugs. If the C language included a strong distinction 921between integers and booleans then the compiler would find these mistakes 922for us... but it doesn't. To help prevent such bugs, always follow this 923convention:: 924 925 If the name of a function is an action or an imperative command, 926 the function should return an error-code integer. If the name 927 is a predicate, the function should return a "succeeded" boolean. 928 929For example, ``add work`` is a command, and the add_work() function returns 0 930for success or -EBUSY for failure. In the same way, ``PCI device present`` is 931a predicate, and the pci_dev_present() function returns 1 if it succeeds in 932finding a matching device or 0 if it doesn't. 933 934All EXPORTed functions must respect this convention, and so should all 935public functions. Private (static) functions need not, but it is 936recommended that they do. 937 938Functions whose return value is the actual result of a computation, rather 939than an indication of whether the computation succeeded, are not subject to 940this rule. Generally they indicate failure by returning some out-of-range 941result. Typical examples would be functions that return pointers; they use 942NULL or the ERR_PTR mechanism to report failure. 943 944 94517) Using bool 946-------------- 947 948The Linux kernel bool type is an alias for the C99 _Bool type. bool values can 949only evaluate to 0 or 1, and implicit or explicit conversion to bool 950automatically converts the value to true or false. When using bool types the 951!! construction is not needed, which eliminates a class of bugs. 952 953When working with bool values the true and false definitions should be used 954instead of 1 and 0. 955 956bool function return types and stack variables are always fine to use whenever 957appropriate. Use of bool is encouraged to improve readability and is often a 958better option than 'int' for storing boolean values. 959 960Do not use bool if cache line layout or size of the value matters, as its size 961and alignment varies based on the compiled architecture. Structures that are 962optimized for alignment and size should not use bool. 963 964If a structure has many true/false values, consider consolidating them into a 965bitfield with 1 bit members, or using an appropriate fixed width type, such as 966u8. 967 968Similarly for function arguments, many true/false values can be consolidated 969into a single bitwise 'flags' argument and 'flags' can often be a more 970readable alternative if the call-sites have naked true/false constants. 971 972Otherwise limited use of bool in structures and arguments can improve 973readability. 974 97518) Don't re-invent the kernel macros 976------------------------------------- 977 978The header file include/linux/kernel.h contains a number of macros that 979you should use, rather than explicitly coding some variant of them yourself. 980For example, if you need to calculate the length of an array, take advantage 981of the macro 982 983.. code-block:: c 984 985 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0])) 986 987Similarly, if you need to calculate the size of some structure member, use 988 989.. code-block:: c 990 991 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f)) 992 993There are also min() and max() macros that do strict type checking if you 994need them. Feel free to peruse that header file to see what else is already 995defined that you shouldn't reproduce in your code. 996 997 99819) Editor modelines and other cruft 999------------------------------------ 1000 1001Some editors can interpret configuration information embedded in source files, 1002indicated with special markers. For example, emacs interprets lines marked 1003like this: 1004 1005.. code-block:: c 1006 1007 -*- mode: c -*- 1008 1009Or like this: 1010 1011.. code-block:: c 1012 1013 /* 1014 Local Variables: 1015 compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c" 1016 End: 1017 */ 1018 1019Vim interprets markers that look like this: 1020 1021.. code-block:: c 1022 1023 /* vim:set sw=8 noet */ 1024 1025Do not include any of these in source files. People have their own personal 1026editor configurations, and your source files should not override them. This 1027includes markers for indentation and mode configuration. People may use their 1028own custom mode, or may have some other magic method for making indentation 1029work correctly. 1030 1031 103220) Inline assembly 1033------------------- 1034 1035In architecture-specific code, you may need to use inline assembly to interface 1036with CPU or platform functionality. Don't hesitate to do so when necessary. 1037However, don't use inline assembly gratuitously when C can do the job. You can 1038and should poke hardware from C when possible. 1039 1040Consider writing simple helper functions that wrap common bits of inline 1041assembly, rather than repeatedly writing them with slight variations. Remember 1042that inline assembly can use C parameters. 1043 1044Large, non-trivial assembly functions should go in .S files, with corresponding 1045C prototypes defined in C header files. The C prototypes for assembly 1046functions should use ``asmlinkage``. 1047 1048You may need to mark your asm statement as volatile, to prevent GCC from 1049removing it if GCC doesn't notice any side effects. You don't always need to 1050do so, though, and doing so unnecessarily can limit optimization. 1051 1052When writing a single inline assembly statement containing multiple 1053instructions, put each instruction on a separate line in a separate quoted 1054string, and end each string except the last with ``\n\t`` to properly indent 1055the next instruction in the assembly output: 1056 1057.. code-block:: c 1058 1059 asm ("magic %reg1, #42\n\t" 1060 "more_magic %reg2, %reg3" 1061 : /* outputs */ : /* inputs */ : /* clobbers */); 1062 1063 106421) Conditional Compilation 1065--------------------------- 1066 1067Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c 1068files; doing so makes code harder to read and logic harder to follow. Instead, 1069use such conditionals in a header file defining functions for use in those .c 1070files, providing no-op stub versions in the #else case, and then call those 1071functions unconditionally from .c files. The compiler will avoid generating 1072any code for the stub calls, producing identical results, but the logic will 1073remain easy to follow. 1074 1075Prefer to compile out entire functions, rather than portions of functions or 1076portions of expressions. Rather than putting an ifdef in an expression, factor 1077out part or all of the expression into a separate helper function and apply the 1078conditional to that function. 1079 1080If you have a function or variable which may potentially go unused in a 1081particular configuration, and the compiler would warn about its definition 1082going unused, mark the definition as __maybe_unused rather than wrapping it in 1083a preprocessor conditional. (However, if a function or variable *always* goes 1084unused, delete it.) 1085 1086Within code, where possible, use the IS_ENABLED macro to convert a Kconfig 1087symbol into a C boolean expression, and use it in a normal C conditional: 1088 1089.. code-block:: c 1090 1091 if (IS_ENABLED(CONFIG_SOMETHING)) { 1092 ... 1093 } 1094 1095The compiler will constant-fold the conditional away, and include or exclude 1096the block of code just as with an #ifdef, so this will not add any runtime 1097overhead. However, this approach still allows the C compiler to see the code 1098inside the block, and check it for correctness (syntax, types, symbol 1099references, etc). Thus, you still have to use an #ifdef if the code inside the 1100block references symbols that will not exist if the condition is not met. 1101 1102At the end of any non-trivial #if or #ifdef block (more than a few lines), 1103place a comment after the #endif on the same line, noting the conditional 1104expression used. For instance: 1105 1106.. code-block:: c 1107 1108 #ifdef CONFIG_SOMETHING 1109 ... 1110 #endif /* CONFIG_SOMETHING */ 1111 1112 1113Appendix I) References 1114---------------------- 1115 1116The C Programming Language, Second Edition 1117by Brian W. Kernighan and Dennis M. Ritchie. 1118Prentice Hall, Inc., 1988. 1119ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback). 1120 1121The Practice of Programming 1122by Brian W. Kernighan and Rob Pike. 1123Addison-Wesley, Inc., 1999. 1124ISBN 0-201-61586-X. 1125 1126GNU manuals - where in compliance with K&R and this text - for cpp, gcc, 1127gcc internals and indent, all available from http://www.gnu.org/manual/ 1128 1129WG14 is the international standardization working group for the programming 1130language C, URL: http://www.open-std.org/JTC1/SC22/WG14/ 1131 1132Kernel :ref:`process/coding-style.rst <codingstyle>`, by greg@kroah.com at OLS 2002: 1133http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/ 1134