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2 | Linux kernel coding style | |
3 | ||
4 | This is a short document describing the preferred coding style for the | |
5 | linux kernel. Coding style is very personal, and I won't _force_ my | |
6 | views on anybody, but this is what goes for anything that I have to be | |
7 | able to maintain, and I'd prefer it for most other things too. Please | |
8 | at least consider the points made here. | |
9 | ||
10 | First off, I'd suggest printing out a copy of the GNU coding standards, | |
11 | and NOT read it. Burn them, it's a great symbolic gesture. | |
12 | ||
13 | Anyway, here goes: | |
14 | ||
15 | ||
16 | Chapter 1: Indentation | |
17 | ||
18 | Tabs are 8 characters, and thus indentations are also 8 characters. | |
19 | There are heretic movements that try to make indentations 4 (or even 2!) | |
20 | characters deep, and that is akin to trying to define the value of PI to | |
21 | be 3. | |
22 | ||
23 | Rationale: The whole idea behind indentation is to clearly define where | |
24 | a block of control starts and ends. Especially when you've been looking | |
25 | at your screen for 20 straight hours, you'll find it a lot easier to see | |
26 | how the indentation works if you have large indentations. | |
27 | ||
28 | Now, some people will claim that having 8-character indentations makes | |
29 | the code move too far to the right, and makes it hard to read on a | |
30 | 80-character terminal screen. The answer to that is that if you need | |
31 | more than 3 levels of indentation, you're screwed anyway, and should fix | |
32 | your program. | |
33 | ||
34 | In short, 8-char indents make things easier to read, and have the added | |
35 | benefit of warning you when you're nesting your functions too deep. | |
36 | Heed that warning. | |
37 | ||
38 | The preferred way to ease multiple indentation levels in a switch statement is | |
39 | to align the "switch" and its subordinate "case" labels in the same column | |
40 | instead 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 | ||
60 | Don't put multiple statements on a single line unless you have | |
61 | something to hide: | |
62 | ||
63 | if (condition) do_this; | |
64 | do_something_everytime; | |
65 | ||
66 | Don't put multiple assignments on a single line either. Kernel coding style | |
67 | is super simple. Avoid tricky expressions. | |
68 | ||
69 | Outside of comments, documentation and except in Kconfig, spaces are never | |
70 | used for indentation, and the above example is deliberately broken. | |
71 | ||
72 | Get a decent editor and don't leave whitespace at the end of lines. | |
73 | ||
74 | ||
75 | Chapter 2: Breaking long lines and strings | |
76 | ||
77 | Coding style is all about readability and maintainability using commonly | |
78 | available tools. | |
79 | ||
80 | The limit on the length of lines is 80 columns and this is a strongly | |
81 | preferred limit. | |
82 | ||
83 | Statements longer than 80 columns will be broken into sensible chunks, unless | |
84 | exceeding 80 columns significantly increases readability and does not hide | |
85 | information. Descendants are always substantially shorter than the parent and | |
86 | are placed substantially to the right. The same applies to function headers | |
87 | with a long argument list. However, never break user-visible strings such as | |
88 | printk messages, because that breaks the ability to grep for them. | |
89 | ||
90 | ||
91 | Chapter 3: Placing Braces and Spaces | |
92 | ||
93 | The other issue that always comes up in C styling is the placement of | |
94 | braces. Unlike the indent size, there are few technical reasons to | |
95 | choose one placement strategy over the other, but the preferred way, as | |
96 | shown to us by the prophets Kernighan and Ritchie, is to put the opening | |
97 | brace last on the line, and put the closing brace first, thusly: | |
98 | ||
99 | if (x is true) { | |
100 | we do y | |
101 | } | |
102 | ||
103 | This applies to all non-function statement blocks (if, switch, for, | |
104 | while, 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 | ||
117 | However, there is one special case, namely functions: they have the | |
118 | opening brace at the beginning of the next line, thus: | |
119 | ||
120 | int function(int x) | |
121 | { | |
122 | body of function | |
123 | } | |
124 | ||
125 | Heretic people all over the world have claimed that this inconsistency | |
126 | is ... well ... inconsistent, but all right-thinking people know that | |
127 | (a) K&R are _right_ and (b) K&R are right. Besides, functions are | |
128 | special anyway (you can't nest them in C). | |
129 | ||
130 | Note that the closing brace is empty on a line of its own, _except_ in | |
131 | the cases where it is followed by a continuation of the same statement, | |
132 | ie a "while" in a do-statement or an "else" in an if-statement, like | |
133 | this: | |
134 | ||
135 | do { | |
136 | body of do-loop | |
137 | } while (condition); | |
138 | ||
139 | and | |
140 | ||
141 | if (x == y) { | |
142 | .. | |
143 | } else if (x > y) { | |
144 | ... | |
145 | } else { | |
146 | .... | |
147 | } | |
148 | ||
149 | Rationale: K&R. | |
150 | ||
151 | Also, note that this brace-placement also minimizes the number of empty | |
152 | (or almost empty) lines, without any loss of readability. Thus, as the | |
153 | supply of new-lines on your screen is not a renewable resource (think | |
154 | 25-line terminal screens here), you have more empty lines to put | |
155 | comments on. | |
156 | ||
157 | Do not unnecessarily use braces where a single statement will do. | |
158 | ||
159 | if (condition) | |
160 | action(); | |
161 | ||
162 | and | |
163 | ||
164 | if (condition) | |
165 | do_this(); | |
166 | else | |
167 | do_that(); | |
168 | ||
169 | This does not apply if only one branch of a conditional statement is a single | |
170 | statement; in the latter case use braces in both branches: | |
171 | ||
172 | if (condition) { | |
173 | do_this(); | |
174 | do_that(); | |
175 | } else { | |
176 | otherwise(); | |
177 | } | |
178 | ||
179 | 3.1: Spaces | |
180 | ||
181 | Linux kernel style for use of spaces depends (mostly) on | |
182 | function-versus-keyword usage. Use a space after (most) keywords. The | |
183 | notable exceptions are sizeof, typeof, alignof, and __attribute__, which look | |
184 | somewhat like functions (and are usually used with parentheses in Linux, | |
185 | although they are not required in the language, as in: "sizeof info" after | |
186 | "struct fileinfo info;" is declared). | |
187 | ||
188 | So use a space after these keywords: | |
189 | if, switch, case, for, do, while | |
190 | but not with sizeof, typeof, alignof, or __attribute__. E.g., | |
191 | s = sizeof(struct file); | |
192 | ||
193 | Do not add spaces around (inside) parenthesized expressions. This example is | |
194 | *bad*: | |
195 | ||
196 | s = sizeof( struct file ); | |
197 | ||
198 | When declaring pointer data or a function that returns a pointer type, the | |
199 | preferred use of '*' is adjacent to the data name or function name and not | |
200 | adjacent 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 | ||
206 | Use one space around (on each side of) most binary and ternary operators, | |
207 | such as any of these: | |
208 | ||
209 | = + - < > * / % | & ^ <= >= == != ? : | |
210 | ||
211 | but no space after unary operators: | |
212 | & * + - ~ ! sizeof typeof alignof __attribute__ defined | |
213 | ||
214 | no space before the postfix increment & decrement unary operators: | |
215 | ++ -- | |
216 | ||
217 | no space after the prefix increment & decrement unary operators: | |
218 | ++ -- | |
219 | ||
220 | and no space around the '.' and "->" structure member operators. | |
221 | ||
222 | Do 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 | |
224 | appropriate, so you can start typing the next line of code right away. | |
225 | However, some such editors do not remove the whitespace if you end up not | |
226 | putting a line of code there, such as if you leave a blank line. As a result, | |
227 | you end up with lines containing trailing whitespace. | |
228 | ||
229 | Git will warn you about patches that introduce trailing whitespace, and can | |
230 | optionally strip the trailing whitespace for you; however, if applying a series | |
231 | of patches, this may make later patches in the series fail by changing their | |
232 | context lines. | |
233 | ||
234 | ||
235 | Chapter 4: Naming | |
236 | ||
237 | C is a Spartan language, and so should your naming be. Unlike Modula-2 | |
238 | and Pascal programmers, C programmers do not use cute names like | |
239 | ThisVariableIsATemporaryCounter. A C programmer would call that | |
240 | variable "tmp", which is much easier to write, and not the least more | |
241 | difficult to understand. | |
242 | ||
243 | HOWEVER, while mixed-case names are frowned upon, descriptive names for | |
244 | global variables are a must. To call a global function "foo" is a | |
245 | shooting offense. | |
246 | ||
247 | GLOBAL variables (to be used only if you _really_ need them) need to | |
248 | have descriptive names, as do global functions. If you have a function | |
249 | that counts the number of active users, you should call that | |
250 | "count_active_users()" or similar, you should _not_ call it "cntusr()". | |
251 | ||
252 | Encoding the type of a function into the name (so-called Hungarian | |
253 | notation) is brain damaged - the compiler knows the types anyway and can | |
254 | check those, and it only confuses the programmer. No wonder MicroSoft | |
255 | makes buggy programs. | |
256 | ||
257 | LOCAL variable names should be short, and to the point. If you have | |
258 | some random integer loop counter, it should probably be called "i". | |
259 | Calling it "loop_counter" is non-productive, if there is no chance of it | |
260 | being mis-understood. Similarly, "tmp" can be just about any type of | |
261 | variable that is used to hold a temporary value. | |
262 | ||
263 | If you are afraid to mix up your local variable names, you have another | |
264 | problem, which is called the function-growth-hormone-imbalance syndrome. | |
265 | See chapter 6 (Functions). | |
266 | ||
267 | ||
268 | Chapter 5: Typedefs | |
269 | ||
270 | Please don't use things like "vps_t". | |
271 | ||
272 | It's a _mistake_ to use typedef for structures and pointers. When you see a | |
273 | ||
274 | vps_t a; | |
275 | ||
276 | in the source, what does it mean? | |
277 | ||
278 | In contrast, if it says | |
279 | ||
280 | struct virtual_container *a; | |
281 | ||
282 | you can actually tell what "a" is. | |
283 | ||
284 | Lots of people think that typedefs "help readability". Not so. They are | |
285 | useful 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 | ||
337 | Maybe there are other cases too, but the rule should basically be to NEVER | |
338 | EVER use a typedef unless you can clearly match one of those rules. | |
339 | ||
340 | In general, a pointer, or a struct that has elements that can reasonably | |
341 | be directly accessed should _never_ be a typedef. | |
342 | ||
343 | ||
344 | Chapter 6: Functions | |
345 | ||
346 | Functions should be short and sweet, and do just one thing. They should | |
347 | fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24, | |
348 | as we all know), and do one thing and do that well. | |
349 | ||
350 | The maximum length of a function is inversely proportional to the | |
351 | complexity and indentation level of that function. So, if you have a | |
352 | conceptually simple function that is just one long (but simple) | |
353 | case-statement, where you have to do lots of small things for a lot of | |
354 | different cases, it's OK to have a longer function. | |
355 | ||
356 | However, if you have a complex function, and you suspect that a | |
357 | less-than-gifted first-year high-school student might not even | |
358 | understand what the function is all about, you should adhere to the | |
359 | maximum limits all the more closely. Use helper functions with | |
360 | descriptive names (you can ask the compiler to in-line them if you think | |
361 | it's performance-critical, and it will probably do a better job of it | |
362 | than you would have done). | |
363 | ||
364 | Another measure of the function is the number of local variables. They | |
365 | shouldn't exceed 5-10, or you're doing something wrong. Re-think the | |
366 | function, and split it into smaller pieces. A human brain can | |
367 | generally easily keep track of about 7 different things, anything more | |
368 | and it gets confused. You know you're brilliant, but maybe you'd like | |
369 | to understand what you did 2 weeks from now. | |
370 | ||
371 | In source files, separate functions with one blank line. If the function is | |
372 | exported, the EXPORT* macro for it should follow immediately after the closing | |
373 | function brace line. E.g.: | |
374 | ||
375 | int system_is_up(void) | |
376 | { | |
377 | return system_state == SYSTEM_RUNNING; | |
378 | } | |
379 | EXPORT_SYMBOL(system_is_up); | |
380 | ||
381 | In function prototypes, include parameter names with their data types. | |
382 | Although this is not required by the C language, it is preferred in Linux | |
383 | because it is a simple way to add valuable information for the reader. | |
384 | ||
385 | ||
386 | Chapter 7: Centralized exiting of functions | |
387 | ||
388 | Albeit deprecated by some people, the equivalent of the goto statement is | |
389 | used frequently by compilers in form of the unconditional jump instruction. | |
390 | ||
391 | The goto statement comes in handy when a function exits from multiple | |
392 | locations and some common work such as cleanup has to be done. | |
393 | ||
394 | The rationale is: | |
395 | ||
396 | - unconditional statements are easier to understand and follow | |
397 | - nesting is reduced | |
398 | - errors by not updating individual exit points when making | |
399 | modifications are prevented | |
400 | - saves the compiler work to optimize redundant code away ;) | |
401 | ||
402 | int fun(int a) | |
403 | { | |
404 | int result = 0; | |
405 | char *buffer = kmalloc(SIZE); | |
406 | ||
407 | if (buffer == NULL) | |
408 | return -ENOMEM; | |
409 | ||
410 | if (condition1) { | |
411 | while (loop1) { | |
412 | ... | |
413 | } | |
414 | result = 1; | |
415 | goto out; | |
416 | } | |
417 | ... | |
418 | out: | |
419 | kfree(buffer); | |
420 | return result; | |
421 | } | |
422 | ||
423 | Chapter 8: Commenting | |
424 | ||
425 | Comments are good, but there is also a danger of over-commenting. NEVER | |
426 | try to explain HOW your code works in a comment: it's much better to | |
427 | write the code so that the _working_ is obvious, and it's a waste of | |
428 | time to explain badly written code. | |
429 | ||
430 | Generally, you want your comments to tell WHAT your code does, not HOW. | |
431 | Also, try to avoid putting comments inside a function body: if the | |
432 | function is so complex that you need to separately comment parts of it, | |
433 | you should probably go back to chapter 6 for a while. You can make | |
434 | small comments to note or warn about something particularly clever (or | |
435 | ugly), but try to avoid excess. Instead, put the comments at the head | |
436 | of the function, telling people what it does, and possibly WHY it does | |
437 | it. | |
438 | ||
439 | When commenting the kernel API functions, please use the kernel-doc format. | |
440 | See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc | |
441 | for details. | |
442 | ||
443 | Linux style for comments is the C89 "/* ... */" style. | |
444 | Don't use C99-style "// ..." comments. | |
445 | ||
446 | The preferred style for long (multi-line) comments is: | |
447 | ||
448 | /* | |
449 | * This is the preferred style for multi-line | |
450 | * comments in the Linux kernel source code. | |
451 | * Please use it consistently. | |
452 | * | |
453 | * Description: A column of asterisks on the left side, | |
454 | * with beginning and ending almost-blank lines. | |
455 | */ | |
456 | ||
457 | It's also important to comment data, whether they are basic types or derived | |
458 | types. To this end, use just one data declaration per line (no commas for | |
459 | multiple data declarations). This leaves you room for a small comment on each | |
460 | item, explaining its use. | |
461 | ||
462 | ||
463 | Chapter 9: You've made a mess of it | |
464 | ||
465 | That's OK, we all do. You've probably been told by your long-time Unix | |
466 | user helper that "GNU emacs" automatically formats the C sources for | |
467 | you, and you've noticed that yes, it does do that, but the defaults it | |
468 | uses are less than desirable (in fact, they are worse than random | |
469 | typing - an infinite number of monkeys typing into GNU emacs would never | |
470 | make a good program). | |
471 | ||
472 | So, you can either get rid of GNU emacs, or change it to use saner | |
473 | values. To do the latter, you can stick the following in your .emacs file: | |
474 | ||
475 | (defun c-lineup-arglist-tabs-only (ignored) | |
476 | "Line up argument lists by tabs, not spaces" | |
477 | (let* ((anchor (c-langelem-pos c-syntactic-element)) | |
478 | (column (c-langelem-2nd-pos c-syntactic-element)) | |
479 | (offset (- (1+ column) anchor)) | |
480 | (steps (floor offset c-basic-offset))) | |
481 | (* (max steps 1) | |
482 | c-basic-offset))) | |
483 | ||
484 | (add-hook 'c-mode-common-hook | |
485 | (lambda () | |
486 | ;; Add kernel style | |
487 | (c-add-style | |
488 | "linux-tabs-only" | |
489 | '("linux" (c-offsets-alist | |
490 | (arglist-cont-nonempty | |
491 | c-lineup-gcc-asm-reg | |
492 | c-lineup-arglist-tabs-only)))))) | |
493 | ||
494 | (add-hook 'c-mode-hook | |
495 | (lambda () | |
496 | (let ((filename (buffer-file-name))) | |
497 | ;; Enable kernel mode for the appropriate files | |
498 | (when (and filename | |
499 | (string-match (expand-file-name "~/src/linux-trees") | |
500 | filename)) | |
501 | (setq indent-tabs-mode t) | |
502 | (c-set-style "linux-tabs-only"))))) | |
503 | ||
504 | This will make emacs go better with the kernel coding style for C | |
505 | files below ~/src/linux-trees. | |
506 | ||
507 | But even if you fail in getting emacs to do sane formatting, not | |
508 | everything is lost: use "indent". | |
509 | ||
510 | Now, again, GNU indent has the same brain-dead settings that GNU emacs | |
511 | has, which is why you need to give it a few command line options. | |
512 | However, that's not too bad, because even the makers of GNU indent | |
513 | recognize the authority of K&R (the GNU people aren't evil, they are | |
514 | just severely misguided in this matter), so you just give indent the | |
515 | options "-kr -i8" (stands for "K&R, 8 character indents"), or use | |
516 | "scripts/Lindent", which indents in the latest style. | |
517 | ||
518 | "indent" has a lot of options, and especially when it comes to comment | |
519 | re-formatting you may want to take a look at the man page. But | |
520 | remember: "indent" is not a fix for bad programming. | |
521 | ||
522 | ||
523 | Chapter 10: Kconfig configuration files | |
524 | ||
525 | For all of the Kconfig* configuration files throughout the source tree, | |
526 | the indentation is somewhat different. Lines under a "config" definition | |
527 | are indented with one tab, while help text is indented an additional two | |
528 | spaces. Example: | |
529 | ||
530 | config AUDIT | |
531 | bool "Auditing support" | |
532 | depends on NET | |
533 | help | |
534 | Enable auditing infrastructure that can be used with another | |
535 | kernel subsystem, such as SELinux (which requires this for | |
536 | logging of avc messages output). Does not do system-call | |
537 | auditing without CONFIG_AUDITSYSCALL. | |
538 | ||
539 | Features that might still be considered unstable should be defined as | |
540 | dependent on "EXPERIMENTAL": | |
541 | ||
542 | config SLUB | |
543 | depends on EXPERIMENTAL && !ARCH_USES_SLAB_PAGE_STRUCT | |
544 | bool "SLUB (Unqueued Allocator)" | |
545 | ... | |
546 | ||
547 | while seriously dangerous features (such as write support for certain | |
548 | filesystems) should advertise this prominently in their prompt string: | |
549 | ||
550 | config ADFS_FS_RW | |
551 | bool "ADFS write support (DANGEROUS)" | |
552 | depends on ADFS_FS | |
553 | ... | |
554 | ||
555 | For full documentation on the configuration files, see the file | |
556 | Documentation/kbuild/kconfig-language.txt. | |
557 | ||
558 | ||
559 | Chapter 11: Data structures | |
560 | ||
561 | Data structures that have visibility outside the single-threaded | |
562 | environment they are created and destroyed in should always have | |
563 | reference counts. In the kernel, garbage collection doesn't exist (and | |
564 | outside the kernel garbage collection is slow and inefficient), which | |
565 | means that you absolutely _have_ to reference count all your uses. | |
566 | ||
567 | Reference counting means that you can avoid locking, and allows multiple | |
568 | users to have access to the data structure in parallel - and not having | |
569 | to worry about the structure suddenly going away from under them just | |
570 | because they slept or did something else for a while. | |
571 | ||
572 | Note that locking is _not_ a replacement for reference counting. | |
573 | Locking is used to keep data structures coherent, while reference | |
574 | counting is a memory management technique. Usually both are needed, and | |
575 | they are not to be confused with each other. | |
576 | ||
577 | Many data structures can indeed have two levels of reference counting, | |
578 | when there are users of different "classes". The subclass count counts | |
579 | the number of subclass users, and decrements the global count just once | |
580 | when the subclass count goes to zero. | |
581 | ||
582 | Examples of this kind of "multi-level-reference-counting" can be found in | |
583 | memory management ("struct mm_struct": mm_users and mm_count), and in | |
584 | filesystem code ("struct super_block": s_count and s_active). | |
585 | ||
586 | Remember: if another thread can find your data structure, and you don't | |
587 | have a reference count on it, you almost certainly have a bug. | |
588 | ||
589 | ||
590 | Chapter 12: Macros, Enums and RTL | |
591 | ||
592 | Names of macros defining constants and labels in enums are capitalized. | |
593 | ||
594 | #define CONSTANT 0x12345 | |
595 | ||
596 | Enums are preferred when defining several related constants. | |
597 | ||
598 | CAPITALIZED macro names are appreciated but macros resembling functions | |
599 | may be named in lower case. | |
600 | ||
601 | Generally, inline functions are preferable to macros resembling functions. | |
602 | ||
603 | Macros with multiple statements should be enclosed in a do - while block: | |
604 | ||
605 | #define macrofun(a, b, c) \ | |
606 | do { \ | |
607 | if (a == 5) \ | |
608 | do_this(b, c); \ | |
609 | } while (0) | |
610 | ||
611 | Things to avoid when using macros: | |
612 | ||
613 | 1) macros that affect control flow: | |
614 | ||
615 | #define FOO(x) \ | |
616 | do { \ | |
617 | if (blah(x) < 0) \ | |
618 | return -EBUGGERED; \ | |
619 | } while(0) | |
620 | ||
621 | is a _very_ bad idea. It looks like a function call but exits the "calling" | |
622 | function; don't break the internal parsers of those who will read the code. | |
623 | ||
624 | 2) macros that depend on having a local variable with a magic name: | |
625 | ||
626 | #define FOO(val) bar(index, val) | |
627 | ||
628 | might look like a good thing, but it's confusing as hell when one reads the | |
629 | code and it's prone to breakage from seemingly innocent changes. | |
630 | ||
631 | 3) macros with arguments that are used as l-values: FOO(x) = y; will | |
632 | bite you if somebody e.g. turns FOO into an inline function. | |
633 | ||
634 | 4) forgetting about precedence: macros defining constants using expressions | |
635 | must enclose the expression in parentheses. Beware of similar issues with | |
636 | macros using parameters. | |
637 | ||
638 | #define CONSTANT 0x4000 | |
639 | #define CONSTEXP (CONSTANT | 3) | |
640 | ||
641 | The cpp manual deals with macros exhaustively. The gcc internals manual also | |
642 | covers RTL which is used frequently with assembly language in the kernel. | |
643 | ||
644 | ||
645 | Chapter 13: Printing kernel messages | |
646 | ||
647 | Kernel developers like to be seen as literate. Do mind the spelling | |
648 | of kernel messages to make a good impression. Do not use crippled | |
649 | words like "dont"; use "do not" or "don't" instead. Make the messages | |
650 | concise, clear, and unambiguous. | |
651 | ||
652 | Kernel messages do not have to be terminated with a period. | |
653 | ||
654 | Printing numbers in parentheses (%d) adds no value and should be avoided. | |
655 | ||
656 | There are a number of driver model diagnostic macros in <linux/device.h> | |
657 | which you should use to make sure messages are matched to the right device | |
658 | and driver, and are tagged with the right level: dev_err(), dev_warn(), | |
659 | dev_info(), and so forth. For messages that aren't associated with a | |
660 | particular device, <linux/printk.h> defines pr_debug() and pr_info(). | |
661 | ||
662 | Coming up with good debugging messages can be quite a challenge; and once | |
663 | you have them, they can be a huge help for remote troubleshooting. Such | |
664 | messages should be compiled out when the DEBUG symbol is not defined (that | |
665 | is, by default they are not included). When you use dev_dbg() or pr_debug(), | |
666 | that's automatic. Many subsystems have Kconfig options to turn on -DDEBUG. | |
667 | A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the | |
668 | ones already enabled by DEBUG. | |
669 | ||
670 | ||
671 | Chapter 14: Allocating memory | |
672 | ||
673 | The kernel provides the following general purpose memory allocators: | |
674 | kmalloc(), kzalloc(), kcalloc(), vmalloc(), and vzalloc(). Please refer to | |
675 | the API documentation for further information about them. | |
676 | ||
677 | The preferred form for passing a size of a struct is the following: | |
678 | ||
679 | p = kmalloc(sizeof(*p), ...); | |
680 | ||
681 | The alternative form where struct name is spelled out hurts readability and | |
682 | introduces an opportunity for a bug when the pointer variable type is changed | |
683 | but the corresponding sizeof that is passed to a memory allocator is not. | |
684 | ||
685 | Casting the return value which is a void pointer is redundant. The conversion | |
686 | from void pointer to any other pointer type is guaranteed by the C programming | |
687 | language. | |
688 | ||
689 | ||
690 | Chapter 15: The inline disease | |
691 | ||
692 | There appears to be a common misperception that gcc has a magic "make me | |
693 | faster" speedup option called "inline". While the use of inlines can be | |
694 | appropriate (for example as a means of replacing macros, see Chapter 12), it | |
695 | very often is not. Abundant use of the inline keyword leads to a much bigger | |
696 | kernel, which in turn slows the system as a whole down, due to a bigger | |
697 | icache footprint for the CPU and simply because there is less memory | |
698 | available for the pagecache. Just think about it; a pagecache miss causes a | |
699 | disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles | |
700 | that can go into these 5 milliseconds. | |
701 | ||
702 | A reasonable rule of thumb is to not put inline at functions that have more | |
703 | than 3 lines of code in them. An exception to this rule are the cases where | |
704 | a parameter is known to be a compiletime constant, and as a result of this | |
705 | constantness you *know* the compiler will be able to optimize most of your | |
706 | function away at compile time. For a good example of this later case, see | |
707 | the kmalloc() inline function. | |
708 | ||
709 | Often people argue that adding inline to functions that are static and used | |
710 | only once is always a win since there is no space tradeoff. While this is | |
711 | technically correct, gcc is capable of inlining these automatically without | |
712 | help, and the maintenance issue of removing the inline when a second user | |
713 | appears outweighs the potential value of the hint that tells gcc to do | |
714 | something it would have done anyway. | |
715 | ||
716 | ||
717 | Chapter 16: Function return values and names | |
718 | ||
719 | Functions can return values of many different kinds, and one of the | |
720 | most common is a value indicating whether the function succeeded or | |
721 | failed. Such a value can be represented as an error-code integer | |
722 | (-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure, | |
723 | non-zero = success). | |
724 | ||
725 | Mixing up these two sorts of representations is a fertile source of | |
726 | difficult-to-find bugs. If the C language included a strong distinction | |
727 | between integers and booleans then the compiler would find these mistakes | |
728 | for us... but it doesn't. To help prevent such bugs, always follow this | |
729 | convention: | |
730 | ||
731 | If the name of a function is an action or an imperative command, | |
732 | the function should return an error-code integer. If the name | |
733 | is a predicate, the function should return a "succeeded" boolean. | |
734 | ||
735 | For example, "add work" is a command, and the add_work() function returns 0 | |
736 | for success or -EBUSY for failure. In the same way, "PCI device present" is | |
737 | a predicate, and the pci_dev_present() function returns 1 if it succeeds in | |
738 | finding a matching device or 0 if it doesn't. | |
739 | ||
740 | All EXPORTed functions must respect this convention, and so should all | |
741 | public functions. Private (static) functions need not, but it is | |
742 | recommended that they do. | |
743 | ||
744 | Functions whose return value is the actual result of a computation, rather | |
745 | than an indication of whether the computation succeeded, are not subject to | |
746 | this rule. Generally they indicate failure by returning some out-of-range | |
747 | result. Typical examples would be functions that return pointers; they use | |
748 | NULL or the ERR_PTR mechanism to report failure. | |
749 | ||
750 | ||
751 | Chapter 17: Don't re-invent the kernel macros | |
752 | ||
753 | The header file include/linux/kernel.h contains a number of macros that | |
754 | you should use, rather than explicitly coding some variant of them yourself. | |
755 | For example, if you need to calculate the length of an array, take advantage | |
756 | of the macro | |
757 | ||
758 | #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0])) | |
759 | ||
760 | Similarly, if you need to calculate the size of some structure member, use | |
761 | ||
762 | #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f)) | |
763 | ||
764 | There are also min() and max() macros that do strict type checking if you | |
765 | need them. Feel free to peruse that header file to see what else is already | |
766 | defined that you shouldn't reproduce in your code. | |
767 | ||
768 | ||
769 | Chapter 18: Editor modelines and other cruft | |
770 | ||
771 | Some editors can interpret configuration information embedded in source files, | |
772 | indicated with special markers. For example, emacs interprets lines marked | |
773 | like this: | |
774 | ||
775 | -*- mode: c -*- | |
776 | ||
777 | Or like this: | |
778 | ||
779 | /* | |
780 | Local Variables: | |
781 | compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c" | |
782 | End: | |
783 | */ | |
784 | ||
785 | Vim interprets markers that look like this: | |
786 | ||
787 | /* vim:set sw=8 noet */ | |
788 | ||
789 | Do not include any of these in source files. People have their own personal | |
790 | editor configurations, and your source files should not override them. This | |
791 | includes markers for indentation and mode configuration. People may use their | |
792 | own custom mode, or may have some other magic method for making indentation | |
793 | work correctly. | |
794 | ||
795 | ||
796 | Chapter 19: Inline assembly | |
797 | ||
798 | In architecture-specific code, you may need to use inline assembly to interface | |
799 | with CPU or platform functionality. Don't hesitate to do so when necessary. | |
800 | However, don't use inline assembly gratuitously when C can do the job. You can | |
801 | and should poke hardware from C when possible. | |
802 | ||
803 | Consider writing simple helper functions that wrap common bits of inline | |
804 | assembly, rather than repeatedly writing them with slight variations. Remember | |
805 | that inline assembly can use C parameters. | |
806 | ||
807 | Large, non-trivial assembly functions should go in .S files, with corresponding | |
808 | C prototypes defined in C header files. The C prototypes for assembly | |
809 | functions should use "asmlinkage". | |
810 | ||
811 | You may need to mark your asm statement as volatile, to prevent GCC from | |
812 | removing it if GCC doesn't notice any side effects. You don't always need to | |
813 | do so, though, and doing so unnecessarily can limit optimization. | |
814 | ||
815 | When writing a single inline assembly statement containing multiple | |
816 | instructions, put each instruction on a separate line in a separate quoted | |
817 | string, and end each string except the last with \n\t to properly indent the | |
818 | next instruction in the assembly output: | |
819 | ||
820 | asm ("magic %reg1, #42\n\t" | |
821 | "more_magic %reg2, %reg3" | |
822 | : /* outputs */ : /* inputs */ : /* clobbers */); | |
823 | ||
824 | ||
825 | ||
826 | Appendix I: References | |
827 | ||
828 | The C Programming Language, Second Edition | |
829 | by Brian W. Kernighan and Dennis M. Ritchie. | |
830 | Prentice Hall, Inc., 1988. | |
831 | ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback). | |
832 | URL: http://cm.bell-labs.com/cm/cs/cbook/ | |
833 | ||
834 | The Practice of Programming | |
835 | by Brian W. Kernighan and Rob Pike. | |
836 | Addison-Wesley, Inc., 1999. | |
837 | ISBN 0-201-61586-X. | |
838 | URL: http://cm.bell-labs.com/cm/cs/tpop/ | |
839 | ||
840 | GNU manuals - where in compliance with K&R and this text - for cpp, gcc, | |
841 | gcc internals and indent, all available from http://www.gnu.org/manual/ | |
842 | ||
843 | WG14 is the international standardization working group for the programming | |
844 | language C, URL: http://www.open-std.org/JTC1/SC22/WG14/ | |
845 | ||
846 | Kernel CodingStyle, by greg@kroah.com at OLS 2002: | |
847 | http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/ | |
848 |