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 (setq show-trailing-whitespace t) 531 (c-set-style "linux-tabs-only"))))) 532 533This will make emacs go better with the kernel coding style for C 534files below ~/src/linux-trees. 535 536But even if you fail in getting emacs to do sane formatting, not 537everything is lost: use "indent". 538 539Now, again, GNU indent has the same brain-dead settings that GNU emacs 540has, which is why you need to give it a few command line options. 541However, that's not too bad, because even the makers of GNU indent 542recognize the authority of K&R (the GNU people aren't evil, they are 543just severely misguided in this matter), so you just give indent the 544options "-kr -i8" (stands for "K&R, 8 character indents"), or use 545"scripts/Lindent", which indents in the latest style. 546 547"indent" has a lot of options, and especially when it comes to comment 548re-formatting you may want to take a look at the man page. But 549remember: "indent" is not a fix for bad programming. 550 551 552 Chapter 10: Kconfig configuration files 553 554For all of the Kconfig* configuration files throughout the source tree, 555the indentation is somewhat different. Lines under a "config" definition 556are indented with one tab, while help text is indented an additional two 557spaces. Example: 558 559config AUDIT 560 bool "Auditing support" 561 depends on NET 562 help 563 Enable auditing infrastructure that can be used with another 564 kernel subsystem, such as SELinux (which requires this for 565 logging of avc messages output). Does not do system-call 566 auditing without CONFIG_AUDITSYSCALL. 567 568Seriously dangerous features (such as write support for certain 569filesystems) should advertise this prominently in their prompt string: 570 571config ADFS_FS_RW 572 bool "ADFS write support (DANGEROUS)" 573 depends on ADFS_FS 574 ... 575 576For full documentation on the configuration files, see the file 577Documentation/kbuild/kconfig-language.txt. 578 579 580 Chapter 11: Data structures 581 582Data structures that have visibility outside the single-threaded 583environment they are created and destroyed in should always have 584reference counts. In the kernel, garbage collection doesn't exist (and 585outside the kernel garbage collection is slow and inefficient), which 586means that you absolutely _have_ to reference count all your uses. 587 588Reference counting means that you can avoid locking, and allows multiple 589users to have access to the data structure in parallel - and not having 590to worry about the structure suddenly going away from under them just 591because they slept or did something else for a while. 592 593Note that locking is _not_ a replacement for reference counting. 594Locking is used to keep data structures coherent, while reference 595counting is a memory management technique. Usually both are needed, and 596they are not to be confused with each other. 597 598Many data structures can indeed have two levels of reference counting, 599when there are users of different "classes". The subclass count counts 600the number of subclass users, and decrements the global count just once 601when the subclass count goes to zero. 602 603Examples of this kind of "multi-level-reference-counting" can be found in 604memory management ("struct mm_struct": mm_users and mm_count), and in 605filesystem code ("struct super_block": s_count and s_active). 606 607Remember: if another thread can find your data structure, and you don't 608have a reference count on it, you almost certainly have a bug. 609 610 611 Chapter 12: Macros, Enums and RTL 612 613Names of macros defining constants and labels in enums are capitalized. 614 615#define CONSTANT 0x12345 616 617Enums are preferred when defining several related constants. 618 619CAPITALIZED macro names are appreciated but macros resembling functions 620may be named in lower case. 621 622Generally, inline functions are preferable to macros resembling functions. 623 624Macros with multiple statements should be enclosed in a do - while block: 625 626#define macrofun(a, b, c) \ 627 do { \ 628 if (a == 5) \ 629 do_this(b, c); \ 630 } while (0) 631 632Things to avoid when using macros: 633 6341) macros that affect control flow: 635 636#define FOO(x) \ 637 do { \ 638 if (blah(x) < 0) \ 639 return -EBUGGERED; \ 640 } while(0) 641 642is a _very_ bad idea. It looks like a function call but exits the "calling" 643function; don't break the internal parsers of those who will read the code. 644 6452) macros that depend on having a local variable with a magic name: 646 647#define FOO(val) bar(index, val) 648 649might look like a good thing, but it's confusing as hell when one reads the 650code and it's prone to breakage from seemingly innocent changes. 651 6523) macros with arguments that are used as l-values: FOO(x) = y; will 653bite you if somebody e.g. turns FOO into an inline function. 654 6554) forgetting about precedence: macros defining constants using expressions 656must enclose the expression in parentheses. Beware of similar issues with 657macros using parameters. 658 659#define CONSTANT 0x4000 660#define CONSTEXP (CONSTANT | 3) 661 662The cpp manual deals with macros exhaustively. The gcc internals manual also 663covers RTL which is used frequently with assembly language in the kernel. 664 665 666 Chapter 13: Printing kernel messages 667 668Kernel developers like to be seen as literate. Do mind the spelling 669of kernel messages to make a good impression. Do not use crippled 670words like "dont"; use "do not" or "don't" instead. Make the messages 671concise, clear, and unambiguous. 672 673Kernel messages do not have to be terminated with a period. 674 675Printing numbers in parentheses (%d) adds no value and should be avoided. 676 677There are a number of driver model diagnostic macros in <linux/device.h> 678which you should use to make sure messages are matched to the right device 679and driver, and are tagged with the right level: dev_err(), dev_warn(), 680dev_info(), and so forth. For messages that aren't associated with a 681particular device, <linux/printk.h> defines pr_notice(), pr_info(), 682pr_warn(), pr_err(), etc. 683 684Coming up with good debugging messages can be quite a challenge; and once 685you have them, they can be a huge help for remote troubleshooting. However 686debug message printing is handled differently than printing other non-debug 687messages. While the other pr_XXX() functions print unconditionally, 688pr_debug() does not; it is compiled out by default, unless either DEBUG is 689defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also, 690and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to 691the ones already enabled by DEBUG. 692 693Many subsystems have Kconfig debug options to turn on -DDEBUG in the 694corresponding Makefile; in other cases specific files #define DEBUG. And 695when a debug message should be unconditionally printed, such as if it is 696already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be 697used. 698 699 700 Chapter 14: Allocating memory 701 702The kernel provides the following general purpose memory allocators: 703kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and 704vzalloc(). Please refer to the API documentation for further information 705about them. 706 707The preferred form for passing a size of a struct is the following: 708 709 p = kmalloc(sizeof(*p), ...); 710 711The alternative form where struct name is spelled out hurts readability and 712introduces an opportunity for a bug when the pointer variable type is changed 713but the corresponding sizeof that is passed to a memory allocator is not. 714 715Casting the return value which is a void pointer is redundant. The conversion 716from void pointer to any other pointer type is guaranteed by the C programming 717language. 718 719The preferred form for allocating an array is the following: 720 721 p = kmalloc_array(n, sizeof(...), ...); 722 723The preferred form for allocating a zeroed array is the following: 724 725 p = kcalloc(n, sizeof(...), ...); 726 727Both forms check for overflow on the allocation size n * sizeof(...), 728and return NULL if that occurred. 729 730 731 Chapter 15: The inline disease 732 733There appears to be a common misperception that gcc has a magic "make me 734faster" speedup option called "inline". While the use of inlines can be 735appropriate (for example as a means of replacing macros, see Chapter 12), it 736very often is not. Abundant use of the inline keyword leads to a much bigger 737kernel, which in turn slows the system as a whole down, due to a bigger 738icache footprint for the CPU and simply because there is less memory 739available for the pagecache. Just think about it; a pagecache miss causes a 740disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles 741that can go into these 5 milliseconds. 742 743A reasonable rule of thumb is to not put inline at functions that have more 744than 3 lines of code in them. An exception to this rule are the cases where 745a parameter is known to be a compiletime constant, and as a result of this 746constantness you *know* the compiler will be able to optimize most of your 747function away at compile time. For a good example of this later case, see 748the kmalloc() inline function. 749 750Often people argue that adding inline to functions that are static and used 751only once is always a win since there is no space tradeoff. While this is 752technically correct, gcc is capable of inlining these automatically without 753help, and the maintenance issue of removing the inline when a second user 754appears outweighs the potential value of the hint that tells gcc to do 755something it would have done anyway. 756 757 758 Chapter 16: Function return values and names 759 760Functions can return values of many different kinds, and one of the 761most common is a value indicating whether the function succeeded or 762failed. Such a value can be represented as an error-code integer 763(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure, 764non-zero = success). 765 766Mixing up these two sorts of representations is a fertile source of 767difficult-to-find bugs. If the C language included a strong distinction 768between integers and booleans then the compiler would find these mistakes 769for us... but it doesn't. To help prevent such bugs, always follow this 770convention: 771 772 If the name of a function is an action or an imperative command, 773 the function should return an error-code integer. If the name 774 is a predicate, the function should return a "succeeded" boolean. 775 776For example, "add work" is a command, and the add_work() function returns 0 777for success or -EBUSY for failure. In the same way, "PCI device present" is 778a predicate, and the pci_dev_present() function returns 1 if it succeeds in 779finding a matching device or 0 if it doesn't. 780 781All EXPORTed functions must respect this convention, and so should all 782public functions. Private (static) functions need not, but it is 783recommended that they do. 784 785Functions whose return value is the actual result of a computation, rather 786than an indication of whether the computation succeeded, are not subject to 787this rule. Generally they indicate failure by returning some out-of-range 788result. Typical examples would be functions that return pointers; they use 789NULL or the ERR_PTR mechanism to report failure. 790 791 792 Chapter 17: Don't re-invent the kernel macros 793 794The header file include/linux/kernel.h contains a number of macros that 795you should use, rather than explicitly coding some variant of them yourself. 796For example, if you need to calculate the length of an array, take advantage 797of the macro 798 799 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0])) 800 801Similarly, if you need to calculate the size of some structure member, use 802 803 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f)) 804 805There are also min() and max() macros that do strict type checking if you 806need them. Feel free to peruse that header file to see what else is already 807defined that you shouldn't reproduce in your code. 808 809 810 Chapter 18: Editor modelines and other cruft 811 812Some editors can interpret configuration information embedded in source files, 813indicated with special markers. For example, emacs interprets lines marked 814like this: 815 816-*- mode: c -*- 817 818Or like this: 819 820/* 821Local Variables: 822compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c" 823End: 824*/ 825 826Vim interprets markers that look like this: 827 828/* vim:set sw=8 noet */ 829 830Do not include any of these in source files. People have their own personal 831editor configurations, and your source files should not override them. This 832includes markers for indentation and mode configuration. People may use their 833own custom mode, or may have some other magic method for making indentation 834work correctly. 835 836 837 Chapter 19: Inline assembly 838 839In architecture-specific code, you may need to use inline assembly to interface 840with CPU or platform functionality. Don't hesitate to do so when necessary. 841However, don't use inline assembly gratuitously when C can do the job. You can 842and should poke hardware from C when possible. 843 844Consider writing simple helper functions that wrap common bits of inline 845assembly, rather than repeatedly writing them with slight variations. Remember 846that inline assembly can use C parameters. 847 848Large, non-trivial assembly functions should go in .S files, with corresponding 849C prototypes defined in C header files. The C prototypes for assembly 850functions should use "asmlinkage". 851 852You may need to mark your asm statement as volatile, to prevent GCC from 853removing it if GCC doesn't notice any side effects. You don't always need to 854do so, though, and doing so unnecessarily can limit optimization. 855 856When writing a single inline assembly statement containing multiple 857instructions, put each instruction on a separate line in a separate quoted 858string, and end each string except the last with \n\t to properly indent the 859next instruction in the assembly output: 860 861 asm ("magic %reg1, #42\n\t" 862 "more_magic %reg2, %reg3" 863 : /* outputs */ : /* inputs */ : /* clobbers */); 864 865 866 Chapter 20: Conditional Compilation 867 868Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c 869files; doing so makes code harder to read and logic harder to follow. Instead, 870use such conditionals in a header file defining functions for use in those .c 871files, providing no-op stub versions in the #else case, and then call those 872functions unconditionally from .c files. The compiler will avoid generating 873any code for the stub calls, producing identical results, but the logic will 874remain easy to follow. 875 876Prefer to compile out entire functions, rather than portions of functions or 877portions of expressions. Rather than putting an ifdef in an expression, factor 878out part or all of the expression into a separate helper function and apply the 879conditional to that function. 880 881If you have a function or variable which may potentially go unused in a 882particular configuration, and the compiler would warn about its definition 883going unused, mark the definition as __maybe_unused rather than wrapping it in 884a preprocessor conditional. (However, if a function or variable *always* goes 885unused, delete it.) 886 887Within code, where possible, use the IS_ENABLED macro to convert a Kconfig 888symbol into a C boolean expression, and use it in a normal C conditional: 889 890 if (IS_ENABLED(CONFIG_SOMETHING)) { 891 ... 892 } 893 894The compiler will constant-fold the conditional away, and include or exclude 895the block of code just as with an #ifdef, so this will not add any runtime 896overhead. However, this approach still allows the C compiler to see the code 897inside the block, and check it for correctness (syntax, types, symbol 898references, etc). Thus, you still have to use an #ifdef if the code inside the 899block references symbols that will not exist if the condition is not met. 900 901At the end of any non-trivial #if or #ifdef block (more than a few lines), 902place a comment after the #endif on the same line, noting the conditional 903expression used. For instance: 904 905#ifdef CONFIG_SOMETHING 906... 907#endif /* CONFIG_SOMETHING */ 908 909 910 Appendix I: References 911 912The C Programming Language, Second Edition 913by Brian W. Kernighan and Dennis M. Ritchie. 914Prentice Hall, Inc., 1988. 915ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback). 916URL: http://cm.bell-labs.com/cm/cs/cbook/ 917 918The Practice of Programming 919by Brian W. Kernighan and Rob Pike. 920Addison-Wesley, Inc., 1999. 921ISBN 0-201-61586-X. 922URL: http://cm.bell-labs.com/cm/cs/tpop/ 923 924GNU manuals - where in compliance with K&R and this text - for cpp, gcc, 925gcc internals and indent, all available from http://www.gnu.org/manual/ 926 927WG14 is the international standardization working group for the programming 928language C, URL: http://www.open-std.org/JTC1/SC22/WG14/ 929 930Kernel CodingStyle, by greg@kroah.com at OLS 2002: 931http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/ 932 933