Linux内核编码风格

Linux内核编码风格
像其他⼤型软件⼀样，Linux制订了⼀套编码风格，对代码的格式、风格和布局做出了规定。

我写这篇的⽬的也就是希望⼤家能够从中借鉴，有利于⼤家提⾼编程效率。

像Linux内核这样⼤型软件中，涉及许许多多的开发者，故它的编码风格也很有参考价值。

1、左括号紧跟在语句的最后，与语句在相同的⼀⾏。

⽽右括号要另起⼀⾏，作为该⾏的第⼀个字符。

2、如果接下来的部分是相同语句的⼀部分，那么右括号就不单独占⼀⾏。

3、还有
4、函数采⽤以下的书写⽅式：
5、最后不需要⼀定使⽤括号的语句可以忽略它：
要尽可能地保证代码长度不超过80个字符，如果代码⾏超过80应该折到下⼀⾏。

将参数分⾏输⼊，在开头简单地加⼊两个标准tab：
名称中不允许使⽤混合的⼤⼩写字符。

局部变量如果能够清楚地表明它的⽤途，那么选取idx甚⾄是i这样的名称都是可⾏的。

⽽像theLoopIndex这样冗长反复的名字不在接受之列。

——匈⽛利命名法（在变量名称中加⼊变量的类别）危害极⼤。

函数
根据经验函数的代码长度不应该超过两屏，局部变量不应该超过⼗个。

1、⼀个函数应该功能单⼀并且实现精准。

2、将⼀个函数分解成⼀些更短⼩的函数的组合不会带来危害。

——如果你担⼼函数调⽤导致的开销，可以使⽤inline关键字。

⼀般情况下，注释的⽬的是描述你的代码要做什么和为什么要做，⽽不是具体通过什么⽅式实现的。

怎么实现应该由代码本⾝展现。

注释不应该包含谁写了那个函数，修改⽇期和其他那些琐碎⽽⽆实际意义的内容。

这些信息应该集中在⽂件最开头地⽅。

内核中⼀条注释看起来如下：
重要信息常常以“XXX：”开头，⽽bug通常以“FIXME"开头，就像：
总结
希望这篇博客对⼤家有所帮助！
更详尽的内容，请看"Linux 内核代码规范原⽂"
Linus 内部代码规范原⽂
1 Linux kernel coding style
2
3 This is a short document describing the preferred coding style for the
4 linux kernel. Coding style is very personal, and I won't _force_ my
5 views on anybody, but this is what goes for anything that I have to be
6 able to maintain, and I'd prefer it for most other things too. Please
7 at least consider the points made here.
8
9 First off, I'd suggest printing out a copy of the GNU coding standards,
10 and NOT read it. Burn them, it's a great symbolic gesture.
11
12 Anyway, here goes:
13
14
15 Chapter 1: Indentation
16
17 Tabs are 8 characters, and thus indentations are also 8 characters.
18 There are heretic movements that try to make indentations 4 (or even 2!)
19 characters deep, and that is akin to trying to define the value of PI to
20 be 3.
21
22 Rationale: The whole idea behind indentation is to clearly define where
23 a block of control starts and ends. Especially when you've been looking
24 at your screen for20 straight hours, you'll find it a lot easier to see
25 how the indentation works if you have large indentations.
26
27 Now, some people will claim that having 8-character indentations makes
28 the code move too far to the right, and makes it hard to read on a
2980-character terminal screen. The answer to that is that if you need
30 more than 3 levels of indentation, you're screwed anyway, and should fix
31 your program.
32
33 In short, 8-char indents make things easier to read, and have the added
34 benefit of warning you when you're nesting your functions too deep.
35 Heed that warning.
36
37 The preferred way to ease multiple indentation levels in a switch statement is
38 to align the "switch" and its subordinate "case" labels in the same column
39 instead of "double-indenting" the "case" labels. E.g.:
40
41switch (suffix) {
42case'G':
43case'g':
44 mem <<= 30;
45break;
46case'M':
47case'm':
48 mem <<= 20;
49break;
51case'k':
52 mem <<= 10;
53/* fall through */
54default:
55break;
56 }
57
58
59 Don't put multiple statements on a single line unless you have
60 something to hide:
61
62if (condition) do_this;
63 do_something_everytime;
64
65 Don't put multiple assignments on a single line either. Kernel coding style
66is super simple. Avoid tricky expressions.
67
68 Outside of comments, documentation and except in Kconfig, spaces are never
69 used for indentation, and the above example is deliberately broken.
70
71 Get a decent editor and don't leave whitespace at the end of lines.
72
73
74 Chapter 2: Breaking long lines and strings
75
76 Coding style is all about readability and maintainability using commonly
77 available tools.
78
79 The limit on the length of lines is80 columns and this is a strongly
80 preferred limit.
81
82 Statements longer than 80 columns will be broken into sensible chunks, unless
83 exceeding 80 columns significantly increases readability and does not hide
84 information. Descendants are always substantially shorter than the parent and
85 are placed substantially to the right. The same applies to function headers
86 with a long argument list. However, never break user-visible strings such as
87 printk messages, because that breaks the ability to grep for them.
88
89
90 Chapter 3: Placing Braces and Spaces
91
92 The other issue that always comes up in C styling is the placement of
93 braces. Unlike the indent size, there are few technical reasons to
94 choose one placement strategy over the other, but the preferred way, as
95 shown to us by the prophets Kernighan and Ritchie, is to put the opening
96 brace last on the line, and put the closing brace first, thusly:
97
98if (x is true) {
99 we do y
100 }
101
102 This applies to all non-function statement blocks (if, switch, for,
103while, do). E.g.:
104
105switch (action) {
106case KOBJ_ADD:
107return"add";
108case KOBJ_REMOVE:
109return"remove";
110case KOBJ_CHANGE:
111return"change";
112default:
113return NULL;
114 }
115
116 However, there is one special case, namely functions: they have the
117 opening brace at the beginning of the next line, thus:
118
119int function(int x)
120 {
121 body of function
122 }
123
124 Heretic people all over the world have claimed that this inconsistency
125is ... well ... inconsistent, but all right-thinking people know that
126 (a) K&R are _right_ and (b) K&R are right. Besides, functions are
127 special anyway (you can't nest them in C).
128
129 Note that the closing brace is empty on a line of its own, _except_ in
130 the cases where it is followed by a continuation of the same statement,
131 ie a "while"in a do-statement or an "else"in an if-statement, like
132this:
133
135 body of do-loop
136 } while (condition);
137
138 and
139
140if (x == y) {
141 ..
142 } else if (x > y) {
143 ...
144 } else {
145 ....
146 }
147
148 Rationale: K&R.
149
150 Also, note that this brace-placement also minimizes the number of empty
151 (or almost empty) lines, without any loss of readability. Thus, as the
152 supply of new-lines on your screen is not a renewable resource (think
15325-line terminal screens here), you have more empty lines to put
154 comments on.
155
156 Do not unnecessarily use braces where a single statement will do.
157
158if (condition)
159 action();
160
161 and
162
163if (condition)
164 do_this();
165else
166 do_that();
167
168 This does not apply if only one branch of a conditional statement is a single 169 statement; in the latter case use braces in both branches:
170
171if (condition) {
172 do_this();
173 do_that();
174 } else {
175 otherwise();
176 }
177
178 3.1: Spaces
179
180 Linux kernel style for use of spaces depends (mostly) on
181 function-versus-keyword usage. Use a space after (most) keywords. The 182 notable exceptions are sizeof, typeof, alignof, and __attribute__, which look 183 somewhat like functions (and are usually used with parentheses in Linux,
184 although they are not required in the language, as in: "sizeof info" after 185"struct fileinfo info;"is declared).
186
187 So use a space after these keywords:
188if, switch, case, for, do, while
189 but not with sizeof, typeof, alignof, or __attribute__. E.g.,
190 s = sizeof(struct file);
191
192 Do not add spaces around (inside) parenthesized expressions. This example is 193 *bad*:
194
195 s = sizeof( struct file );
196
197 When declaring pointer data or a function that returns a pointer type, the
198 preferred use of '*'is adjacent to the data name or function name and not
199 adjacent to the type name. Examples:
200
201char *linux_banner;
202 unsigned long long memparse(char *ptr, char **retptr);
203char *match_strdup(substring_t *s);
204
205 Use one space around (on each side of) most binary and ternary operators, 206 such as any of these:
207
208 = + - < > * / % | & ^ <= >= == != ? :
209
210 but no space after unary operators:
211 & * + - ~ ! sizeof typeof alignof __attribute__ defined
212
213 no space before the postfix increment & decrement unary operators:
214 ++ --
215
216 no space after the prefix increment & decrement unary operators:
217 ++ --
219 and no space around the '.' and "->" structure member operators.
220
221 Do not leave trailing whitespace at the ends of lines. Some editors with 222"smart" indentation will insert whitespace at the beginning of new lines as 223 appropriate, so you can start typing the next line of code right away.
224 However, some such editors do not remove the whitespace if you end up not 225 putting a line of code there, such as if you leave a blank line. As a result, 226 you end up with lines containing trailing whitespace.
227
228 Git will warn you about patches that introduce trailing whitespace, and can 229 optionally strip the trailing whitespace for you; however, if applying a series 230 of patches, this may make later patches in the series fail by changing their 231 context lines.
232
233
234 Chapter 4: Naming
235
236 C is a Spartan language, and so should your naming be. Unlike Modula-2 237 and Pascal programmers, C programmers do not use cute names like
238 ThisVariableIsATemporaryCounter. A C programmer would call that
239 variable "tmp", which is much easier to write, and not the least more
240 difficult to understand.
241
242 HOWEVER, while mixed-case names are frowned upon, descriptive names for 243global variables are a must. To call a global function "foo"is a
244 shooting offense.
245
246 GLOBAL variables (to be used only if you _really_ need them) need to
247 have descriptive names, as do global functions. If you have a function
248 that counts the number of active users, you should call that
249"count_active_users()" or similar, you should _not_ call it "cntusr()".
250
251 Encoding the type of a function into the name (so-called Hungarian
252 notation) is brain damaged - the compiler knows the types anyway and can 253 check those, and it only confuses the programmer. No wonder MicroSoft 254 makes buggy programs.
255
256 LOCAL variable names should be short, and to the point. If you have
257 some random integer loop counter, it should probably be called "i".
258 Calling it "loop_counter"is non-productive, if there is no chance of it
259 being mis-understood. Similarly, "tmp" can be just about any type of
260 variable that is used to hold a temporary value.
261
262 If you are afraid to mix up your local variable names, you have another
263 problem, which is called the function-growth-hormone-imbalance syndrome. 264 See chapter 6 (Functions).
265
266
267 Chapter 5: Typedefs
268
269 Please don't use things like "vps_t".
270
271 It's a _mistake_ to use typedef for structures and pointers. When you see a 272
273 vps_t a;
274
275in the source, what does it mean?
276
277 In contrast, if it says
278
279struct virtual_container *a;
280
281 you can actually tell what "a"is.
282
283 Lots of people think that typedefs "help readability". Not so. They are
284 useful only for:
285
286 (a) totally opaque objects (where the typedef is actively used to _hide_
287 what the object is).
288
289 Example: "pte_t" etc. opaque objects that you can only access using
290 the proper accessor functions.
291
292 NOTE! Opaqueness and "accessor functions" are not good in themselves. 293 The reason we have them for things like pte_t etc. is that there
294 really is absolutely _zero_ portably accessible information there.
295
296 (b) Clear integer types, where the abstraction _helps_ avoid confusion
297 whether it is"int" or "long".
298
299 u8/u16/u32 are perfectly fine typedefs, although they fit into
300 category (d) better than here.
301
303"unsigned long", then there's no reason to do
304
305 typedef unsigned long myflags_t;
306
307 but if there is a clear reason for why it under certain circumstances
308 might be an "unsigned int" and under other configurations might be 309"unsigned long", then by all means go ahead and use a typedef.
310
311 (c) when you use sparse to literally create a _new_ type for
312 type-checking.
313
314 (d) New types which are identical to standard C99 types, in certain
315 exceptional circumstances.
316
317 Although it would only take a short amount of time for the eyes and
318 brain to become accustomed to the standard types like 'uint32_t',
319 some people object to their use anyway.
320
321 Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
322 signed equivalents which are identical to standard types are
323 permitted -- although they are not mandatory in new code of your
324 own.
325
326 When editing existing code which already uses one or the other set
327 of types, you should conform to the existing choices in that code.
328
329 (e) Types safe for use in userspace.
330
331 In certain structures which are visible to userspace, we cannot
332 require C99 types and cannot use the 'u32' form above. Thus, we
333 use __u32 and similar types in all structures which are shared
334 with userspace.
335
336 Maybe there are other cases too, but the rule should basically be to NEVER 337 EVER use a typedef unless you can clearly match one of those rules.
338
339 In general, a pointer, or a struct that has elements that can reasonably
340 be directly accessed should _never_ be a typedef.
341
342
343 Chapter 6: Functions
344
345 Functions should be short and sweet, and do just one thing. They should
346 fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
347as we all know), and do one thing and do that well.
348
349 The maximum length of a function is inversely proportional to the
350 complexity and indentation level of that function. So, if you have a
351 conceptually simple function that is just one long (but simple)
352case-statement, where you have to do lots of small things for a lot of
353 different cases, it's OK to have a longer function.
354
355 However, if you have a complex function, and you suspect that a
356 less-than-gifted first-year high-school student might not even
357 understand what the function is all about, you should adhere to the
358 maximum limits all the more closely. Use helper functions with
359 descriptive names (you can ask the compiler to in-line them if you think
360 it's performance-critical, and it will probably do a better job of it
361 than you would have done).
362
363 Another measure of the function is the number of local variables. They
364 shouldn't exceed 5-10, or you're doing something wrong. Re-think the
365 function, and split it into smaller pieces. A human brain can
366 generally easily keep track of about 7 different things, anything more
367 and it gets confused. You know you're brilliant, but maybe you'd like
368 to understand what you did 2 weeks from now.
369
370 In source files, separate functions with one blank line. If the function is
371 exported, the EXPORT* macro for it should follow immediately after the closing 372 function brace line. E.g.:
373
374int system_is_up(void)
375 {
376return system_state == SYSTEM_RUNNING;
377 }
378 EXPORT_SYMBOL(system_is_up);
379
380 In function prototypes, include parameter names with their data types.
381 Although this is not required by the C language, it is preferred in Linux
382 because it is a simple way to add valuable information for the reader.
383
384
385 Chapter 7: Centralized exiting of functions
387 Albeit deprecated by some people, the equivalent of the goto statement is
388 used frequently by compilers in form of the unconditional jump instruction.
389
390 The goto statement comes in handy when a function exits from multiple
391 locations and some common work such as cleanup has to be done.
392
393 The rationale is:
394
395 - unconditional statements are easier to understand and follow
396 - nesting is reduced
397 - errors by not updating individual exit points when making
398 modifications are prevented
399 - saves the compiler work to optimize redundant code away ;)
400
401int fun(int a)
402 {
403int result = 0;
404char *buffer = kmalloc(SIZE);
405
406if (buffer == NULL)
407return -ENOMEM;
408
409if (condition1) {
410while (loop1) {
411 ...
412 }
413 result = 1;
414goto out;
415 }
416 ...
417out:
418 kfree(buffer);
419return result;
420 }
421
422 Chapter 8: Commenting
423
424 Comments are good, but there is also a danger of over-commenting. NEVER 425try to explain HOW your code works in a comment: it's much better to
426 write the code so that the _working_ is obvious, and it's a waste of
427 time to explain badly written code.
428
429 Generally, you want your comments to tell WHAT your code does, not HOW. 430 Also, try to avoid putting comments inside a function body: if the
431 function is so complex that you need to separately comment parts of it,
432 you should probably go back to chapter 6for a while. You can make
433 small comments to note or warn about something particularly clever (or
434 ugly), but try to avoid excess. Instead, put the comments at the head
435 of the function, telling people what it does, and possibly WHY it does
436 it.
437
438 When commenting the kernel API functions, please use the kernel-doc format. 439 See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc 440for details.
441
442 Linux style for comments is the C89 "/* ... */" style.
443 Don't use C99-style "// ..." comments.
444
445 The preferred style for long (multi-line) comments is:
446
447/*
448 * This is the preferred style for multi-line
449 * comments in the Linux kernel source code.
450 * Please use it consistently.
451 *
452 * Description: A column of asterisks on the left side,
453 * with beginning and ending almost-blank lines.
454*/
455
456 For files in net/ and drivers/net/ the preferred style for long (multi-line)
457 comments is a little different.
458
459/* The preferred comment style for files in net/ and drivers/net
460 * looks like this.
461 *
462 * It is nearly the same as the generally preferred comment style,
463 * but there is no initial almost-blank line.
464*/
465
466 It's also important to comment data, whether they are basic types or derived 467 types. To this end, use just one data declaration per line (no commas for
468 multiple data declarations). This leaves you room for a small comment on each 469 item, explaining its use.
472 Chapter 9: You've made a mess of it
473
474 That's OK, we all do. You've probably been told by your long-time Unix 475 user helper that "GNU emacs" automatically formats the C sources for
476 you, and you've noticed that yes, it does do that, but the defaults it
477 uses are less than desirable (in fact, they are worse than random
478 typing - an infinite number of monkeys typing into GNU emacs would never 479 make a good program).
480
481 So, you can either get rid of GNU emacs, or change it to use saner
482 values. To do the latter, you can stick the following in your .emacs file: 483
484 (defun c-lineup-arglist-tabs-only (ignored)
485"Line up argument lists by tabs, not spaces"
486 (let* ((anchor (c-langelem-pos c-syntactic-element))
487 (column (c-langelem-2nd-pos c-syntactic-element))
488 (offset (- (1+ column) anchor))
489 (steps (floor offset c-basic-offset)))
490 (* (max steps 1)
491 c-basic-offset)))
492
493 (add-hook 'c-mode-common-hook
494 (lambda ()
495 ;; Add kernel style
496 (c-add-style
497"linux-tabs-only"
498'("linux" (c-offsets-alist
499 (arglist-cont-nonempty
500 c-lineup-gcc-asm-reg
501 c-lineup-arglist-tabs-only))))))
502
503 (add-hook 'c-mode-hook
504 (lambda ()
505 (let ((filename (buffer-file-name)))
506 ;; Enable kernel mode for the appropriate files
507 (when (and filename
508 (string-match (expand-file-name "~/src/linux-trees")
509 filename))
510 (setq indent-tabs-mode t)
511 (c-set-style "linux-tabs-only")))))
512
513 This will make emacs go better with the kernel coding style for C
514 files below ~/src/linux-trees.
515
516 But even if you fail in getting emacs to do sane formatting, not
517 everything is lost: use "indent".
518
519 Now, again, GNU indent has the same brain-dead settings that GNU emacs 520 has, which is why you need to give it a few command line options.
521 However, that's not too bad, because even the makers of GNU indent
522 recognize the authority of K&R (the GNU people aren't evil, they are
523 just severely misguided in this matter), so you just give indent the
524 options "-kr -i8" (stands for"K&R, 8 character indents"), or use 525"scripts/Lindent", which indents in the latest style.
526
527"indent" has a lot of options, and especially when it comes to comment 528 re-formatting you may want to take a look at the man page. But
529 remember: "indent"is not a fix for bad programming.
530
531
532 Chapter 10: Kconfig configuration files
533
534 For all of the Kconfig* configuration files throughout the source tree,
535 the indentation is somewhat different. Lines under a "config" definition
536 are indented with one tab, while help text is indented an additional two
537 spaces. Example:
538
539 config AUDIT
540bool"Auditing support"
541 depends on NET
542 help
543 Enable auditing infrastructure that can be used with another
544 kernel subsystem, such as SELinux (which requires this for
545 logging of avc messages output). Does not do system-call
546 auditing without CONFIG_AUDITSYSCALL.
547
548 Features that might still be considered unstable should be defined as
549 dependent on "EXPERIMENTAL":
550
551 config SLUB
552 depends on EXPERIMENTAL && !ARCH_USES_SLAB_PAGE_STRUCT 553bool"SLUB (Unqueued Allocator)"
556while seriously dangerous features (such as write support for certain
557 filesystems) should advertise this prominently in their prompt string:
558
559 config ADFS_FS_RW
560bool"ADFS write support (DANGEROUS)"
561 depends on ADFS_FS
562 ...
563
564 For full documentation on the configuration files, see the file
565 Documentation/kbuild/kconfig-language.txt.
566
567
568 Chapter 11: Data structures
569
570 Data structures that have visibility outside the single-threaded
571 environment they are created and destroyed in should always have
572 reference counts. In the kernel, garbage collection doesn't exist (and
573 outside the kernel garbage collection is slow and inefficient), which
574 means that you absolutely _have_ to reference count all your uses.
575
576 Reference counting means that you can avoid locking, and allows multiple 577 users to have access to the data structure in parallel - and not having
578 to worry about the structure suddenly going away from under them just
579 because they slept or did something else for a while.
580
581 Note that locking is _not_ a replacement for reference counting.
582 Locking is used to keep data structures coherent, while reference
583 counting is a memory management technique. Usually both are needed, and 584 they are not to be confused with each other.
585
586 Many data structures can indeed have two levels of reference counting,
587 when there are users of different "classes". The subclass count counts
588 the number of subclass users, and decrements the global count just once 589 when the subclass count goes to zero.
590
591 Examples of this kind of "multi-level-reference-counting" can be found in
592 memory management ("struct mm_struct": mm_users and mm_count), and in 593 filesystem code ("struct super_block": s_count and s_active).
594
595 Remember: if another thread can find your data structure, and you don't
596 have a reference count on it, you almost certainly have a bug.
597
598
599 Chapter 12: Macros, Enums and RTL
600
601 Names of macros defining constants and labels in enums are capitalized. 602
603#define CONSTANT 0x12345
604
605 Enums are preferred when defining several related constants.
606
607 CAPITALIZED macro names are appreciated but macros resembling functions 608 may be named in lower case.
609
610 Generally, inline functions are preferable to macros resembling functions. 611
612 Macros with multiple statements should be enclosed in a do - while block: 613
614#define macrofun(a, b, c) \
615do { \
616if (a == 5) \
617 do_this(b, c); \
618 } while (0)
619
620 Things to avoid when using macros:
621
6221) macros that affect control flow:
623
624#define FOO(x) \
625do { \
626if (blah(x) < 0) \
627return -EBUGGERED; \
628 } while(0)
629
630is a _very_ bad idea. It looks like a function call but exits the "calling"
631 function; don't break the internal parsers of those who will read the code.
632
6332) macros that depend on having a local variable with a magic name:
634
635#define FOO(val) bar(index, val)
636
637 might look like a good thing, but it's confusing as hell when one reads the
6403) macros with arguments that are used as l-values: FOO(x) = y; will
641 bite you if somebody e.g. turns FOO into an inline function.
642
6434) forgetting about precedence: macros defining constants using expressions
644 must enclose the expression in parentheses. Beware of similar issues with
645 macros using parameters.
646
647#define CONSTANT 0x4000
648#define CONSTEXP (CONSTANT | 3)
649
650 The cpp manual deals with macros exhaustively. The gcc internals manual also 651 covers RTL which is used frequently with assembly language in the kernel.
652
653
654 Chapter 13: Printing kernel messages
655
656 Kernel developers like to be seen as literate. Do mind the spelling
657 of kernel messages to make a good impression. Do not use crippled
658 words like "dont"; use "do not" or "don't" instead. Make the messages
659 concise, clear, and unambiguous.
660
661 Kernel messages do not have to be terminated with a period.
662
663 Printing numbers in parentheses (%d) adds no value and should be avoided.
664
665 There are a number of driver model diagnostic macros in <linux/device.h>
666 which you should use to make sure messages are matched to the right device 667 and driver, and are tagged with the right level: dev_err(), dev_warn(),
668 dev_info(), and so forth. For messages that aren't associated with a
669 particular device, <linux/printk.h> defines pr_debug() and pr_info().
670
671 Coming up with good debugging messages can be quite a challenge; and once 672 you have them, they can be a huge help for remote troubleshooting. Such
673 messages should be compiled out when the DEBUG symbol is not defined (that 674is, by default they are not included). When you use dev_dbg() or pr_debug(),
675 that's automatic. Many subsystems have Kconfig options to turn on -DDEBUG. 676 A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the 677 ones already enabled by DEBUG.
678
679
680 Chapter 14: Allocating memory
681
682 The kernel provides the following general purpose memory allocators:
683 kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
684 vzalloc(). Please refer to the API documentation for further information
685 about them.
686
687 The preferred form for passing a size of a struct is the following:
688
689 p = kmalloc(sizeof(*p), ...);
690
691 The alternative form where struct name is spelled out hurts readability and
692 introduces an opportunity for a bug when the pointer variable type is changed
693 but the corresponding sizeof that is passed to a memory allocator is not.
694
695 Casting the return value which is a void pointer is redundant. The conversion
696from void pointer to any other pointer type is guaranteed by the C programming 697 language.
698
699 The preferred form for allocating an array is the following:
700
701 p = kmalloc_array(n, sizeof(...), ...);
702
703 The preferred form for allocating a zeroed array is the following:
704
705 p = kcalloc(n, sizeof(...), ...);
706
707 Both forms check for overflow on the allocation size n * sizeof(...),
708 and return NULL if that occurred.
709
710
711 Chapter 15: The inline disease
712
713 There appears to be a common misperception that gcc has a magic "make me 714 faster" speedup option called "inline". While the use of inlines can be
715 appropriate (for example as a means of replacing macros, see Chapter 12), it
716 very often is not. Abundant use of the inline keyword leads to a much bigger
717 kernel, which in turn slows the system as a whole down, due to a bigger
718 icache footprint for the CPU and simply because there is less memory
719 available for the pagecache. Just think about it; a pagecache miss causes a
720 disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
721 that can go into these 5 milliseconds.。