1 .. _codingstyle: 1 .. _codingstyle:
2 2
3 Linux kernel coding style 3 Linux kernel coding style
4 ========================= 4 =========================
5 5
6 This is a short document describing the prefer 6 This is a short document describing the preferred coding style for the
7 linux kernel. Coding style is very personal, 7 linux kernel. Coding style is very personal, and I won't **force** my
8 views on anybody, but this is what goes for an 8 views on anybody, but this is what goes for anything that I have to be
9 able to maintain, and I'd prefer it for most o 9 able to maintain, and I'd prefer it for most other things too. Please
10 at least consider the points made here. 10 at least consider the points made here.
11 11
12 First off, I'd suggest printing out a copy of 12 First off, I'd suggest printing out a copy of the GNU coding standards,
13 and NOT read it. Burn them, it's a great symb 13 and NOT read it. Burn them, it's a great symbolic gesture.
14 14
15 Anyway, here goes: 15 Anyway, here goes:
16 16
17 17
18 1) Indentation 18 1) Indentation
19 -------------- 19 --------------
20 20
21 Tabs are 8 characters, and thus indentations a 21 Tabs are 8 characters, and thus indentations are also 8 characters.
22 There are heretic movements that try to make i 22 There are heretic movements that try to make indentations 4 (or even 2!)
23 characters deep, and that is akin to trying to 23 characters deep, and that is akin to trying to define the value of PI to
24 be 3. 24 be 3.
25 25
26 Rationale: The whole idea behind indentation i 26 Rationale: The whole idea behind indentation is to clearly define where
27 a block of control starts and ends. Especiall 27 a block of control starts and ends. Especially when you've been looking
28 at your screen for 20 straight hours, you'll f 28 at your screen for 20 straight hours, you'll find it a lot easier to see
29 how the indentation works if you have large in 29 how the indentation works if you have large indentations.
30 30
31 Now, some people will claim that having 8-char 31 Now, some people will claim that having 8-character indentations makes
32 the code move too far to the right, and makes 32 the code move too far to the right, and makes it hard to read on a
33 80-character terminal screen. The answer to t 33 80-character terminal screen. The answer to that is that if you need
34 more than 3 levels of indentation, you're scre 34 more than 3 levels of indentation, you're screwed anyway, and should fix
35 your program. 35 your program.
36 36
37 In short, 8-char indents make things easier to 37 In short, 8-char indents make things easier to read, and have the added
38 benefit of warning you when you're nesting you 38 benefit of warning you when you're nesting your functions too deep.
39 Heed that warning. 39 Heed that warning.
40 40
41 The preferred way to ease multiple indentation 41 The preferred way to ease multiple indentation levels in a switch statement is
42 to align the ``switch`` and its subordinate `` 42 to align the ``switch`` and its subordinate ``case`` labels in the same column
43 instead of ``double-indenting`` the ``case`` l 43 instead of ``double-indenting`` the ``case`` labels. E.g.:
44 44
45 .. code-block:: c 45 .. code-block:: c
46 46
47 switch (suffix) { 47 switch (suffix) {
48 case 'G': 48 case 'G':
49 case 'g': 49 case 'g':
50 mem <<= 30; 50 mem <<= 30;
51 break; 51 break;
52 case 'M': 52 case 'M':
53 case 'm': 53 case 'm':
54 mem <<= 20; 54 mem <<= 20;
55 break; 55 break;
56 case 'K': 56 case 'K':
57 case 'k': 57 case 'k':
58 mem <<= 10; 58 mem <<= 10;
59 fallthrough; 59 fallthrough;
60 default: 60 default:
61 break; 61 break;
62 } 62 }
63 63
64 Don't put multiple statements on a single line 64 Don't put multiple statements on a single line unless you have
65 something to hide: 65 something to hide:
66 66
67 .. code-block:: c 67 .. code-block:: c
68 68
69 if (condition) do_this; 69 if (condition) do_this;
70 do_something_everytime; 70 do_something_everytime;
71 71
72 Don't use commas to avoid using braces: 72 Don't use commas to avoid using braces:
73 73
74 .. code-block:: c 74 .. code-block:: c
75 75
76 if (condition) 76 if (condition)
77 do_this(), do_that(); 77 do_this(), do_that();
78 78
79 Always uses braces for multiple statements: 79 Always uses braces for multiple statements:
80 80
81 .. code-block:: c 81 .. code-block:: c
82 82
83 if (condition) { 83 if (condition) {
84 do_this(); 84 do_this();
85 do_that(); 85 do_that();
86 } 86 }
87 87
88 Don't put multiple assignments on a single lin 88 Don't put multiple assignments on a single line either. Kernel coding style
89 is super simple. Avoid tricky expressions. 89 is super simple. Avoid tricky expressions.
90 90
91 91
92 Outside of comments, documentation and except 92 Outside of comments, documentation and except in Kconfig, spaces are never
93 used for indentation, and the above example is 93 used for indentation, and the above example is deliberately broken.
94 94
95 Get a decent editor and don't leave whitespace 95 Get a decent editor and don't leave whitespace at the end of lines.
96 96
97 97
98 2) Breaking long lines and strings 98 2) Breaking long lines and strings
99 ---------------------------------- 99 ----------------------------------
100 100
101 Coding style is all about readability and main 101 Coding style is all about readability and maintainability using commonly
102 available tools. 102 available tools.
103 103
104 The preferred limit on the length of a single 104 The preferred limit on the length of a single line is 80 columns.
105 105
106 Statements longer than 80 columns should be br 106 Statements longer than 80 columns should be broken into sensible chunks,
107 unless exceeding 80 columns significantly incr 107 unless exceeding 80 columns significantly increases readability and does
108 not hide information. 108 not hide information.
109 109
110 Descendants are always substantially shorter t 110 Descendants are always substantially shorter than the parent and
111 are placed substantially to the right. A very 111 are placed substantially to the right. A very commonly used style
112 is to align descendants to a function open par 112 is to align descendants to a function open parenthesis.
113 113
114 These same rules are applied to function heade 114 These same rules are applied to function headers with a long argument list.
115 115
116 However, never break user-visible strings such 116 However, never break user-visible strings such as printk messages because
117 that breaks the ability to grep for them. 117 that breaks the ability to grep for them.
118 118
119 119
120 3) Placing Braces and Spaces 120 3) Placing Braces and Spaces
121 ---------------------------- 121 ----------------------------
122 122
123 The other issue that always comes up in C styl 123 The other issue that always comes up in C styling is the placement of
124 braces. Unlike the indent size, there are few 124 braces. Unlike the indent size, there are few technical reasons to
125 choose one placement strategy over the other, 125 choose one placement strategy over the other, but the preferred way, as
126 shown to us by the prophets Kernighan and Ritc 126 shown to us by the prophets Kernighan and Ritchie, is to put the opening
127 brace last on the line, and put the closing br 127 brace last on the line, and put the closing brace first, thusly:
128 128
129 .. code-block:: c 129 .. code-block:: c
130 130
131 if (x is true) { 131 if (x is true) {
132 we do y 132 we do y
133 } 133 }
134 134
135 This applies to all non-function statement blo 135 This applies to all non-function statement blocks (if, switch, for,
136 while, do). E.g.: 136 while, do). E.g.:
137 137
138 .. code-block:: c 138 .. code-block:: c
139 139
140 switch (action) { 140 switch (action) {
141 case KOBJ_ADD: 141 case KOBJ_ADD:
142 return "add"; 142 return "add";
143 case KOBJ_REMOVE: 143 case KOBJ_REMOVE:
144 return "remove"; 144 return "remove";
145 case KOBJ_CHANGE: 145 case KOBJ_CHANGE:
146 return "change"; 146 return "change";
147 default: 147 default:
148 return NULL; 148 return NULL;
149 } 149 }
150 150
151 However, there is one special case, namely fun 151 However, there is one special case, namely functions: they have the
152 opening brace at the beginning of the next lin 152 opening brace at the beginning of the next line, thus:
153 153
154 .. code-block:: c 154 .. code-block:: c
155 155
156 int function(int x) 156 int function(int x)
157 { 157 {
158 body of function 158 body of function
159 } 159 }
160 160
161 Heretic people all over the world have claimed 161 Heretic people all over the world have claimed that this inconsistency
162 is ... well ... inconsistent, but all right- 162 is ... well ... inconsistent, but all right-thinking people know that
163 (a) K&R are **right** and (b) K&R are right. 163 (a) K&R are **right** and (b) K&R are right. Besides, functions are
164 special anyway (you can't nest them in C). 164 special anyway (you can't nest them in C).
165 165
166 Note that the closing brace is empty on a line 166 Note that the closing brace is empty on a line of its own, **except** in
167 the cases where it is followed by a continuati 167 the cases where it is followed by a continuation of the same statement,
168 ie a ``while`` in a do-statement or an ``else` 168 ie a ``while`` in a do-statement or an ``else`` in an if-statement, like
169 this: 169 this:
170 170
171 .. code-block:: c 171 .. code-block:: c
172 172
173 do { 173 do {
174 body of do-loop 174 body of do-loop
175 } while (condition); 175 } while (condition);
176 176
177 and 177 and
178 178
179 .. code-block:: c 179 .. code-block:: c
180 180
181 if (x == y) { 181 if (x == y) {
182 .. 182 ..
183 } else if (x > y) { 183 } else if (x > y) {
184 ... 184 ...
185 } else { 185 } else {
186 .... 186 ....
187 } 187 }
188 188
189 Rationale: K&R. 189 Rationale: K&R.
190 190
191 Also, note that this brace-placement also mini 191 Also, note that this brace-placement also minimizes the number of empty
192 (or almost empty) lines, without any loss of r 192 (or almost empty) lines, without any loss of readability. Thus, as the
193 supply of new-lines on your screen is not a re 193 supply of new-lines on your screen is not a renewable resource (think
194 25-line terminal screens here), you have more 194 25-line terminal screens here), you have more empty lines to put
195 comments on. 195 comments on.
196 196
197 Do not unnecessarily use braces where a single 197 Do not unnecessarily use braces where a single statement will do.
198 198
199 .. code-block:: c 199 .. code-block:: c
200 200
201 if (condition) 201 if (condition)
202 action(); 202 action();
203 203
204 and 204 and
205 205
206 .. code-block:: c !! 206 .. code-block:: none
207 207
208 if (condition) 208 if (condition)
209 do_this(); 209 do_this();
210 else 210 else
211 do_that(); 211 do_that();
212 212
213 This does not apply if only one branch of a co 213 This does not apply if only one branch of a conditional statement is a single
214 statement; in the latter case use braces in bo 214 statement; in the latter case use braces in both branches:
215 215
216 .. code-block:: c 216 .. code-block:: c
217 217
218 if (condition) { 218 if (condition) {
219 do_this(); 219 do_this();
220 do_that(); 220 do_that();
221 } else { 221 } else {
222 otherwise(); 222 otherwise();
223 } 223 }
224 224
225 Also, use braces when a loop contains more tha 225 Also, use braces when a loop contains more than a single simple statement:
226 226
227 .. code-block:: c 227 .. code-block:: c
228 228
229 while (condition) { 229 while (condition) {
230 if (test) 230 if (test)
231 do_something(); 231 do_something();
232 } 232 }
233 233
234 3.1) Spaces 234 3.1) Spaces
235 *********** 235 ***********
236 236
237 Linux kernel style for use of spaces depends ( 237 Linux kernel style for use of spaces depends (mostly) on
238 function-versus-keyword usage. Use a space af 238 function-versus-keyword usage. Use a space after (most) keywords. The
239 notable exceptions are sizeof, typeof, alignof 239 notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
240 somewhat like functions (and are usually used 240 somewhat like functions (and are usually used with parentheses in Linux,
241 although they are not required in the language 241 although they are not required in the language, as in: ``sizeof info`` after
242 ``struct fileinfo info;`` is declared). 242 ``struct fileinfo info;`` is declared).
243 243
244 So use a space after these keywords:: 244 So use a space after these keywords::
245 245
246 if, switch, case, for, do, while 246 if, switch, case, for, do, while
247 247
248 but not with sizeof, typeof, alignof, or __att 248 but not with sizeof, typeof, alignof, or __attribute__. E.g.,
249 249
250 .. code-block:: c 250 .. code-block:: c
251 251
252 252
253 s = sizeof(struct file); 253 s = sizeof(struct file);
254 254
255 Do not add spaces around (inside) parenthesize 255 Do not add spaces around (inside) parenthesized expressions. This example is
256 **bad**: 256 **bad**:
257 257
258 .. code-block:: c 258 .. code-block:: c
259 259
260 260
261 s = sizeof( struct file ); 261 s = sizeof( struct file );
262 262
263 When declaring pointer data or a function that 263 When declaring pointer data or a function that returns a pointer type, the
264 preferred use of ``*`` is adjacent to the data 264 preferred use of ``*`` is adjacent to the data name or function name and not
265 adjacent to the type name. Examples: 265 adjacent to the type name. Examples:
266 266
267 .. code-block:: c 267 .. code-block:: c
268 268
269 269
270 char *linux_banner; 270 char *linux_banner;
271 unsigned long long memparse(char *ptr, 271 unsigned long long memparse(char *ptr, char **retptr);
272 char *match_strdup(substring_t *s); 272 char *match_strdup(substring_t *s);
273 273
274 Use one space around (on each side of) most bi 274 Use one space around (on each side of) most binary and ternary operators,
275 such as any of these:: 275 such as any of these::
276 276
277 = + - < > * / % | & ^ <= > 277 = + - < > * / % | & ^ <= >= == != ? :
278 278
279 but no space after unary operators:: 279 but no space after unary operators::
280 280
281 & * + - ~ ! sizeof typeof alig 281 & * + - ~ ! sizeof typeof alignof __attribute__ defined
282 282
283 no space before the postfix increment & decrem 283 no space before the postfix increment & decrement unary operators::
284 284
285 ++ -- 285 ++ --
286 286
287 no space after the prefix increment & decremen 287 no space after the prefix increment & decrement unary operators::
288 288
289 ++ -- 289 ++ --
290 290
291 and no space around the ``.`` and ``->`` struc 291 and no space around the ``.`` and ``->`` structure member operators.
292 292
293 Do not leave trailing whitespace at the ends o 293 Do not leave trailing whitespace at the ends of lines. Some editors with
294 ``smart`` indentation will insert whitespace a 294 ``smart`` indentation will insert whitespace at the beginning of new lines as
295 appropriate, so you can start typing the next 295 appropriate, so you can start typing the next line of code right away.
296 However, some such editors do not remove the w 296 However, some such editors do not remove the whitespace if you end up not
297 putting a line of code there, such as if you l 297 putting a line of code there, such as if you leave a blank line. As a result,
298 you end up with lines containing trailing whit 298 you end up with lines containing trailing whitespace.
299 299
300 Git will warn you about patches that introduce 300 Git will warn you about patches that introduce trailing whitespace, and can
301 optionally strip the trailing whitespace for y 301 optionally strip the trailing whitespace for you; however, if applying a series
302 of patches, this may make later patches in the 302 of patches, this may make later patches in the series fail by changing their
303 context lines. 303 context lines.
304 304
305 305
306 4) Naming 306 4) Naming
307 --------- 307 ---------
308 308
309 C is a Spartan language, and your naming conve 309 C is a Spartan language, and your naming conventions should follow suit.
310 Unlike Modula-2 and Pascal programmers, C prog 310 Unlike Modula-2 and Pascal programmers, C programmers do not use cute
311 names like ThisVariableIsATemporaryCounter. A 311 names like ThisVariableIsATemporaryCounter. A C programmer would call that
312 variable ``tmp``, which is much easier to writ 312 variable ``tmp``, which is much easier to write, and not the least more
313 difficult to understand. 313 difficult to understand.
314 314
315 HOWEVER, while mixed-case names are frowned up 315 HOWEVER, while mixed-case names are frowned upon, descriptive names for
316 global variables are a must. To call a global 316 global variables are a must. To call a global function ``foo`` is a
317 shooting offense. 317 shooting offense.
318 318
319 GLOBAL variables (to be used only if you **rea 319 GLOBAL variables (to be used only if you **really** need them) need to
320 have descriptive names, as do global functions 320 have descriptive names, as do global functions. If you have a function
321 that counts the number of active users, you sh 321 that counts the number of active users, you should call that
322 ``count_active_users()`` or similar, you shoul 322 ``count_active_users()`` or similar, you should **not** call it ``cntusr()``.
323 323
324 Encoding the type of a function into the name 324 Encoding the type of a function into the name (so-called Hungarian
325 notation) is asinine - the compiler knows the 325 notation) is asinine - the compiler knows the types anyway and can check
326 those, and it only confuses the programmer. 326 those, and it only confuses the programmer.
327 327
328 LOCAL variable names should be short, and to t 328 LOCAL variable names should be short, and to the point. If you have
329 some random integer loop counter, it should pr 329 some random integer loop counter, it should probably be called ``i``.
330 Calling it ``loop_counter`` is non-productive, 330 Calling it ``loop_counter`` is non-productive, if there is no chance of it
331 being mis-understood. Similarly, ``tmp`` can 331 being mis-understood. Similarly, ``tmp`` can be just about any type of
332 variable that is used to hold a temporary valu 332 variable that is used to hold a temporary value.
333 333
334 If you are afraid to mix up your local variabl 334 If you are afraid to mix up your local variable names, you have another
335 problem, which is called the function-growth-h 335 problem, which is called the function-growth-hormone-imbalance syndrome.
336 See chapter 6 (Functions). 336 See chapter 6 (Functions).
337 337
338 For symbol names and documentation, avoid intr 338 For symbol names and documentation, avoid introducing new usage of
339 'master / slave' (or 'slave' independent of 'm 339 'master / slave' (or 'slave' independent of 'master') and 'blacklist /
340 whitelist'. 340 whitelist'.
341 341
342 Recommended replacements for 'master / slave' 342 Recommended replacements for 'master / slave' are:
343 '{primary,main} / {secondary,replica,subor 343 '{primary,main} / {secondary,replica,subordinate}'
344 '{initiator,requester} / {target,responder 344 '{initiator,requester} / {target,responder}'
345 '{controller,host} / {device,worker,proxy} 345 '{controller,host} / {device,worker,proxy}'
346 'leader / follower' 346 'leader / follower'
347 'director / performer' 347 'director / performer'
348 348
349 Recommended replacements for 'blacklist/whitel 349 Recommended replacements for 'blacklist/whitelist' are:
350 'denylist / allowlist' 350 'denylist / allowlist'
351 'blocklist / passlist' 351 'blocklist / passlist'
352 352
353 Exceptions for introducing new usage is to mai 353 Exceptions for introducing new usage is to maintain a userspace ABI/API,
354 or when updating code for an existing (as of 2 354 or when updating code for an existing (as of 2020) hardware or protocol
355 specification that mandates those terms. For n 355 specification that mandates those terms. For new specifications
356 translate specification usage of the terminolo 356 translate specification usage of the terminology to the kernel coding
357 standard where possible. 357 standard where possible.
358 358
359 5) Typedefs 359 5) Typedefs
360 ----------- 360 -----------
361 361
362 Please don't use things like ``vps_t``. 362 Please don't use things like ``vps_t``.
363 It's a **mistake** to use typedef for structur 363 It's a **mistake** to use typedef for structures and pointers. When you see a
364 364
365 .. code-block:: c 365 .. code-block:: c
366 366
367 367
368 vps_t a; 368 vps_t a;
369 369
370 in the source, what does it mean? 370 in the source, what does it mean?
371 In contrast, if it says 371 In contrast, if it says
372 372
373 .. code-block:: c 373 .. code-block:: c
374 374
375 struct virtual_container *a; 375 struct virtual_container *a;
376 376
377 you can actually tell what ``a`` is. 377 you can actually tell what ``a`` is.
378 378
379 Lots of people think that typedefs ``help read 379 Lots of people think that typedefs ``help readability``. Not so. They are
380 useful only for: 380 useful only for:
381 381
382 (a) totally opaque objects (where the typedef 382 (a) totally opaque objects (where the typedef is actively used to **hide**
383 what the object is). 383 what the object is).
384 384
385 Example: ``pte_t`` etc. opaque objects th 385 Example: ``pte_t`` etc. opaque objects that you can only access using
386 the proper accessor functions. 386 the proper accessor functions.
387 387
388 .. note:: 388 .. note::
389 389
390 Opaqueness and ``accessor functions`` a 390 Opaqueness and ``accessor functions`` are not good in themselves.
391 The reason we have them for things like 391 The reason we have them for things like pte_t etc. is that there
392 really is absolutely **zero** portably 392 really is absolutely **zero** portably accessible information there.
393 393
394 (b) Clear integer types, where the abstractio 394 (b) Clear integer types, where the abstraction **helps** avoid confusion
395 whether it is ``int`` or ``long``. 395 whether it is ``int`` or ``long``.
396 396
397 u8/u16/u32 are perfectly fine typedefs, a 397 u8/u16/u32 are perfectly fine typedefs, although they fit into
398 category (d) better than here. 398 category (d) better than here.
399 399
400 .. note:: 400 .. note::
401 401
402 Again - there needs to be a **reason** 402 Again - there needs to be a **reason** for this. If something is
403 ``unsigned long``, then there's no reas 403 ``unsigned long``, then there's no reason to do
404 404
405 typedef unsigned long myflags_t; 405 typedef unsigned long myflags_t;
406 406
407 but if there is a clear reason for why it 407 but if there is a clear reason for why it under certain circumstances
408 might be an ``unsigned int`` and under ot 408 might be an ``unsigned int`` and under other configurations might be
409 ``unsigned long``, then by all means go a 409 ``unsigned long``, then by all means go ahead and use a typedef.
410 410
411 (c) when you use sparse to literally create a 411 (c) when you use sparse to literally create a **new** type for
412 type-checking. 412 type-checking.
413 413
414 (d) New types which are identical to standard 414 (d) New types which are identical to standard C99 types, in certain
415 exceptional circumstances. 415 exceptional circumstances.
416 416
417 Although it would only take a short amoun 417 Although it would only take a short amount of time for the eyes and
418 brain to become accustomed to the standar 418 brain to become accustomed to the standard types like ``uint32_t``,
419 some people object to their use anyway. 419 some people object to their use anyway.
420 420
421 Therefore, the Linux-specific ``u8/u16/u3 421 Therefore, the Linux-specific ``u8/u16/u32/u64`` types and their
422 signed equivalents which are identical to 422 signed equivalents which are identical to standard types are
423 permitted -- although they are not mandat 423 permitted -- although they are not mandatory in new code of your
424 own. 424 own.
425 425
426 When editing existing code which already 426 When editing existing code which already uses one or the other set
427 of types, you should conform to the exist 427 of types, you should conform to the existing choices in that code.
428 428
429 (e) Types safe for use in userspace. 429 (e) Types safe for use in userspace.
430 430
431 In certain structures which are visible t 431 In certain structures which are visible to userspace, we cannot
432 require C99 types and cannot use the ``u3 432 require C99 types and cannot use the ``u32`` form above. Thus, we
433 use __u32 and similar types in all struct 433 use __u32 and similar types in all structures which are shared
434 with userspace. 434 with userspace.
435 435
436 Maybe there are other cases too, but the rule 436 Maybe there are other cases too, but the rule should basically be to NEVER
437 EVER use a typedef unless you can clearly matc 437 EVER use a typedef unless you can clearly match one of those rules.
438 438
439 In general, a pointer, or a struct that has el 439 In general, a pointer, or a struct that has elements that can reasonably
440 be directly accessed should **never** be a typ 440 be directly accessed should **never** be a typedef.
441 441
442 442
443 6) Functions 443 6) Functions
444 ------------ 444 ------------
445 445
446 Functions should be short and sweet, and do ju 446 Functions should be short and sweet, and do just one thing. They should
447 fit on one or two screenfuls of text (the ISO/ 447 fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
448 as we all know), and do one thing and do that 448 as we all know), and do one thing and do that well.
449 449
450 The maximum length of a function is inversely 450 The maximum length of a function is inversely proportional to the
451 complexity and indentation level of that funct 451 complexity and indentation level of that function. So, if you have a
452 conceptually simple function that is just one 452 conceptually simple function that is just one long (but simple)
453 case-statement, where you have to do lots of s 453 case-statement, where you have to do lots of small things for a lot of
454 different cases, it's OK to have a longer func 454 different cases, it's OK to have a longer function.
455 455
456 However, if you have a complex function, and y 456 However, if you have a complex function, and you suspect that a
457 less-than-gifted first-year high-school studen 457 less-than-gifted first-year high-school student might not even
458 understand what the function is all about, you 458 understand what the function is all about, you should adhere to the
459 maximum limits all the more closely. Use help 459 maximum limits all the more closely. Use helper functions with
460 descriptive names (you can ask the compiler to 460 descriptive names (you can ask the compiler to in-line them if you think
461 it's performance-critical, and it will probabl 461 it's performance-critical, and it will probably do a better job of it
462 than you would have done). 462 than you would have done).
463 463
464 Another measure of the function is the number 464 Another measure of the function is the number of local variables. They
465 shouldn't exceed 5-10, or you're doing somethi 465 shouldn't exceed 5-10, or you're doing something wrong. Re-think the
466 function, and split it into smaller pieces. A 466 function, and split it into smaller pieces. A human brain can
467 generally easily keep track of about 7 differe 467 generally easily keep track of about 7 different things, anything more
468 and it gets confused. You know you're brillia 468 and it gets confused. You know you're brilliant, but maybe you'd like
469 to understand what you did 2 weeks from now. 469 to understand what you did 2 weeks from now.
470 470
471 In source files, separate functions with one b 471 In source files, separate functions with one blank line. If the function is
472 exported, the **EXPORT** macro for it should f 472 exported, the **EXPORT** macro for it should follow immediately after the
473 closing function brace line. E.g.: 473 closing function brace line. E.g.:
474 474
475 .. code-block:: c 475 .. code-block:: c
476 476
477 int system_is_up(void) 477 int system_is_up(void)
478 { 478 {
479 return system_state == SYSTEM_ 479 return system_state == SYSTEM_RUNNING;
480 } 480 }
481 EXPORT_SYMBOL(system_is_up); 481 EXPORT_SYMBOL(system_is_up);
482 482
483 6.1) Function prototypes 483 6.1) Function prototypes
484 ************************ 484 ************************
485 485
486 In function prototypes, include parameter name 486 In function prototypes, include parameter names with their data types.
487 Although this is not required by the C languag 487 Although this is not required by the C language, it is preferred in Linux
488 because it is a simple way to add valuable inf 488 because it is a simple way to add valuable information for the reader.
489 489
490 Do not use the ``extern`` keyword with functio 490 Do not use the ``extern`` keyword with function declarations as this makes
491 lines longer and isn't strictly necessary. 491 lines longer and isn't strictly necessary.
492 492
493 When writing function prototypes, please keep 493 When writing function prototypes, please keep the `order of elements regular
494 <https://lore.kernel.org/mm-commits/CAHk-=wiOCL 494 <https://lore.kernel.org/mm-commits/CAHk-=wiOCLRny5aifWNhr621kYrJwhfURsa0vFPeUEm8mF0ufg@mail.gmail.com/">https://lore.kernel.org/mm-commits/CAHk-=wiOCLRny5aifWNhr621kYrJwhfURsa0vFPeUEm8mF0ufg@mail.gmail.com/>`_.
495 For example, using this function declaration e 495 For example, using this function declaration example::
496 496
497 __init void * __must_check action(enum magic 497 __init void * __must_check action(enum magic value, size_t size, u8 count,
498 char *fmt, 498 char *fmt, ...) __printf(4, 5) __malloc;
499 499
500 The preferred order of elements for a function 500 The preferred order of elements for a function prototype is:
501 501
502 - storage class (below, ``static __always_inli 502 - storage class (below, ``static __always_inline``, noting that ``__always_inline``
503 is technically an attribute but is treated l 503 is technically an attribute but is treated like ``inline``)
504 - storage class attributes (here, ``__init`` - 504 - storage class attributes (here, ``__init`` -- i.e. section declarations, but also
505 things like ``__cold``) 505 things like ``__cold``)
506 - return type (here, ``void *``) 506 - return type (here, ``void *``)
507 - return type attributes (here, ``__must_check 507 - return type attributes (here, ``__must_check``)
508 - function name (here, ``action``) 508 - function name (here, ``action``)
509 - function parameters (here, ``(enum magic val 509 - function parameters (here, ``(enum magic value, size_t size, u8 count, char *fmt, ...)``,
510 noting that parameter names should always be 510 noting that parameter names should always be included)
511 - function parameter attributes (here, ``__pri 511 - function parameter attributes (here, ``__printf(4, 5)``)
512 - function behavior attributes (here, ``__mall 512 - function behavior attributes (here, ``__malloc``)
513 513
514 Note that for a function **definition** (i.e. 514 Note that for a function **definition** (i.e. the actual function body),
515 the compiler does not allow function parameter 515 the compiler does not allow function parameter attributes after the
516 function parameters. In these cases, they shou 516 function parameters. In these cases, they should go after the storage
517 class attributes (e.g. note the changed positi 517 class attributes (e.g. note the changed position of ``__printf(4, 5)``
518 below, compared to the **declaration** example 518 below, compared to the **declaration** example above)::
519 519
520 static __always_inline __init __printf(4, 5) 520 static __always_inline __init __printf(4, 5) void * __must_check action(enum magic value,
521 size_t size, u8 count, char *f 521 size_t size, u8 count, char *fmt, ...) __malloc
522 { 522 {
523 ... 523 ...
524 } 524 }
525 525
526 7) Centralized exiting of functions 526 7) Centralized exiting of functions
527 ----------------------------------- 527 -----------------------------------
528 528
529 Albeit deprecated by some people, the equivale 529 Albeit deprecated by some people, the equivalent of the goto statement is
530 used frequently by compilers in form of the un 530 used frequently by compilers in form of the unconditional jump instruction.
531 531
532 The goto statement comes in handy when a funct 532 The goto statement comes in handy when a function exits from multiple
533 locations and some common work such as cleanup 533 locations and some common work such as cleanup has to be done. If there is no
534 cleanup needed then just return directly. 534 cleanup needed then just return directly.
535 535
536 Choose label names which say what the goto doe 536 Choose label names which say what the goto does or why the goto exists. An
537 example of a good name could be ``out_free_buf 537 example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``.
538 Avoid using GW-BASIC names like ``err1:`` and 538 Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to
539 renumber them if you ever add or remove exit p 539 renumber them if you ever add or remove exit paths, and they make correctness
540 difficult to verify anyway. 540 difficult to verify anyway.
541 541
542 The rationale for using gotos is: 542 The rationale for using gotos is:
543 543
544 - unconditional statements are easier to under 544 - unconditional statements are easier to understand and follow
545 - nesting is reduced 545 - nesting is reduced
546 - errors by not updating individual exit point 546 - errors by not updating individual exit points when making
547 modifications are prevented 547 modifications are prevented
548 - saves the compiler work to optimize redundan 548 - saves the compiler work to optimize redundant code away ;)
549 549
550 .. code-block:: c 550 .. code-block:: c
551 551
552 int fun(int a) 552 int fun(int a)
553 { 553 {
554 int result = 0; 554 int result = 0;
555 char *buffer; 555 char *buffer;
556 556
557 buffer = kmalloc(SIZE, GFP_KER 557 buffer = kmalloc(SIZE, GFP_KERNEL);
558 if (!buffer) 558 if (!buffer)
559 return -ENOMEM; 559 return -ENOMEM;
560 560
561 if (condition1) { 561 if (condition1) {
562 while (loop1) { 562 while (loop1) {
563 ... 563 ...
564 } 564 }
565 result = 1; 565 result = 1;
566 goto out_free_buffer; 566 goto out_free_buffer;
567 } 567 }
568 ... 568 ...
569 out_free_buffer: 569 out_free_buffer:
570 kfree(buffer); 570 kfree(buffer);
571 return result; 571 return result;
572 } 572 }
573 573
574 A common type of bug to be aware of is ``one e 574 A common type of bug to be aware of is ``one err bugs`` which look like this:
575 575
576 .. code-block:: c 576 .. code-block:: c
577 577
578 err: 578 err:
579 kfree(foo->bar); 579 kfree(foo->bar);
580 kfree(foo); 580 kfree(foo);
581 return ret; 581 return ret;
582 582
583 The bug in this code is that on some exit path 583 The bug in this code is that on some exit paths ``foo`` is NULL. Normally the
584 fix for this is to split it up into two error 584 fix for this is to split it up into two error labels ``err_free_bar:`` and
585 ``err_free_foo:``: 585 ``err_free_foo:``:
586 586
587 .. code-block:: c 587 .. code-block:: c
588 588
589 err_free_bar: !! 589 err_free_bar:
590 kfree(foo->bar); 590 kfree(foo->bar);
591 err_free_foo: !! 591 err_free_foo:
592 kfree(foo); 592 kfree(foo);
593 return ret; 593 return ret;
594 594
595 Ideally you should simulate errors to test all 595 Ideally you should simulate errors to test all exit paths.
596 596
597 597
598 8) Commenting 598 8) Commenting
599 ------------- 599 -------------
600 600
601 Comments are good, but there is also a danger 601 Comments are good, but there is also a danger of over-commenting. NEVER
602 try to explain HOW your code works in a commen 602 try to explain HOW your code works in a comment: it's much better to
603 write the code so that the **working** is obvi 603 write the code so that the **working** is obvious, and it's a waste of
604 time to explain badly written code. 604 time to explain badly written code.
605 605
606 Generally, you want your comments to tell WHAT 606 Generally, you want your comments to tell WHAT your code does, not HOW.
607 Also, try to avoid putting comments inside a f 607 Also, try to avoid putting comments inside a function body: if the
608 function is so complex that you need to separa 608 function is so complex that you need to separately comment parts of it,
609 you should probably go back to chapter 6 for a 609 you should probably go back to chapter 6 for a while. You can make
610 small comments to note or warn about something 610 small comments to note or warn about something particularly clever (or
611 ugly), but try to avoid excess. Instead, put 611 ugly), but try to avoid excess. Instead, put the comments at the head
612 of the function, telling people what it does, 612 of the function, telling people what it does, and possibly WHY it does
613 it. 613 it.
614 614
615 When commenting the kernel API functions, plea 615 When commenting the kernel API functions, please use the kernel-doc format.
616 See the files at :ref:`Documentation/doc-guide 616 See the files at :ref:`Documentation/doc-guide/ <doc_guide>` and
617 ``scripts/kernel-doc`` for details. 617 ``scripts/kernel-doc`` for details.
618 618
619 The preferred style for long (multi-line) comm 619 The preferred style for long (multi-line) comments is:
620 620
621 .. code-block:: c 621 .. code-block:: c
622 622
623 /* 623 /*
624 * This is the preferred style for mul 624 * This is the preferred style for multi-line
625 * comments in the Linux kernel source 625 * comments in the Linux kernel source code.
626 * Please use it consistently. 626 * Please use it consistently.
627 * 627 *
628 * Description: A column of asterisks 628 * Description: A column of asterisks on the left side,
629 * with beginning and ending almost-bl 629 * with beginning and ending almost-blank lines.
630 */ 630 */
631 631
>> 632 For files in net/ and drivers/net/ the preferred style for long (multi-line)
>> 633 comments is a little different.
>> 634
>> 635 .. code-block:: c
>> 636
>> 637 /* The preferred comment style for files in net/ and drivers/net
>> 638 * looks like this.
>> 639 *
>> 640 * It is nearly the same as the generally preferred comment style,
>> 641 * but there is no initial almost-blank line.
>> 642 */
>> 643
632 It's also important to comment data, whether t 644 It's also important to comment data, whether they are basic types or derived
633 types. To this end, use just one data declara 645 types. To this end, use just one data declaration per line (no commas for
634 multiple data declarations). This leaves you 646 multiple data declarations). This leaves you room for a small comment on each
635 item, explaining its use. 647 item, explaining its use.
636 648
637 649
638 9) You've made a mess of it 650 9) You've made a mess of it
639 --------------------------- 651 ---------------------------
640 652
641 That's OK, we all do. You've probably been to 653 That's OK, we all do. You've probably been told by your long-time Unix
642 user helper that ``GNU emacs`` automatically f 654 user helper that ``GNU emacs`` automatically formats the C sources for
643 you, and you've noticed that yes, it does do t 655 you, and you've noticed that yes, it does do that, but the defaults it
644 uses are less than desirable (in fact, they ar 656 uses are less than desirable (in fact, they are worse than random
645 typing - an infinite number of monkeys typing 657 typing - an infinite number of monkeys typing into GNU emacs would never
646 make a good program). 658 make a good program).
647 659
648 So, you can either get rid of GNU emacs, or ch 660 So, you can either get rid of GNU emacs, or change it to use saner
649 values. To do the latter, you can stick the f 661 values. To do the latter, you can stick the following in your .emacs file:
650 662
651 .. code-block:: elisp !! 663 .. code-block:: none
652 664
653 (defun c-lineup-arglist-tabs-only (ignored) 665 (defun c-lineup-arglist-tabs-only (ignored)
654 "Line up argument lists by tabs, not space 666 "Line up argument lists by tabs, not spaces"
655 (let* ((anchor (c-langelem-pos c-syntactic 667 (let* ((anchor (c-langelem-pos c-syntactic-element))
656 (column (c-langelem-2nd-pos c-synta 668 (column (c-langelem-2nd-pos c-syntactic-element))
657 (offset (- (1+ column) anchor)) 669 (offset (- (1+ column) anchor))
658 (steps (floor offset c-basic-offset 670 (steps (floor offset c-basic-offset)))
659 (* (max steps 1) 671 (* (max steps 1)
660 c-basic-offset))) 672 c-basic-offset)))
661 673
662 (dir-locals-set-class-variables 674 (dir-locals-set-class-variables
663 'linux-kernel 675 'linux-kernel
664 '((c-mode . ( 676 '((c-mode . (
665 (c-basic-offset . 8) 677 (c-basic-offset . 8)
666 (c-label-minimum-indentation . 0) 678 (c-label-minimum-indentation . 0)
667 (c-offsets-alist . ( 679 (c-offsets-alist . (
668 (arglist-close . c-l 680 (arglist-close . c-lineup-arglist-tabs-only)
669 (arglist-cont-nonempty . 681 (arglist-cont-nonempty .
670 (c-lineup-gcc-asm-reg c- !! 682 (c-lineup-gcc-asm-reg c-lineup-arglist-tabs-only))
671 (arglist-intro . +) 683 (arglist-intro . +)
672 (brace-list-intro . +) 684 (brace-list-intro . +)
673 (c . c-l 685 (c . c-lineup-C-comments)
674 (case-label . 0) 686 (case-label . 0)
675 (comment-intro . c-l 687 (comment-intro . c-lineup-comment)
676 (cpp-define-intro . +) 688 (cpp-define-intro . +)
677 (cpp-macro . -10 689 (cpp-macro . -1000)
678 (cpp-macro-cont . +) 690 (cpp-macro-cont . +)
679 (defun-block-intro . +) 691 (defun-block-intro . +)
680 (else-clause . 0) 692 (else-clause . 0)
681 (func-decl-cont . +) 693 (func-decl-cont . +)
682 (inclass . +) 694 (inclass . +)
683 (inher-cont . c-l 695 (inher-cont . c-lineup-multi-inher)
684 (knr-argdecl-intro . 0) 696 (knr-argdecl-intro . 0)
685 (label . -10 697 (label . -1000)
686 (statement . 0) 698 (statement . 0)
687 (statement-block-intro . +) 699 (statement-block-intro . +)
688 (statement-case-intro . +) 700 (statement-case-intro . +)
689 (statement-cont . +) 701 (statement-cont . +)
690 (substatement . +) 702 (substatement . +)
691 )) 703 ))
692 (indent-tabs-mode . t) 704 (indent-tabs-mode . t)
693 (show-trailing-whitespace . t) 705 (show-trailing-whitespace . t)
694 )))) 706 ))))
695 707
696 (dir-locals-set-directory-class 708 (dir-locals-set-directory-class
697 (expand-file-name "~/src/linux-trees") 709 (expand-file-name "~/src/linux-trees")
698 'linux-kernel) 710 'linux-kernel)
699 711
700 This will make emacs go better with the kernel 712 This will make emacs go better with the kernel coding style for C
701 files below ``~/src/linux-trees``. 713 files below ``~/src/linux-trees``.
702 714
703 But even if you fail in getting emacs to do sa 715 But even if you fail in getting emacs to do sane formatting, not
704 everything is lost: use ``indent``. 716 everything is lost: use ``indent``.
705 717
706 Now, again, GNU indent has the same brain-dead 718 Now, again, GNU indent has the same brain-dead settings that GNU emacs
707 has, which is why you need to give it a few co 719 has, which is why you need to give it a few command line options.
708 However, that's not too bad, because even the 720 However, that's not too bad, because even the makers of GNU indent
709 recognize the authority of K&R (the GNU people 721 recognize the authority of K&R (the GNU people aren't evil, they are
710 just severely misguided in this matter), so yo 722 just severely misguided in this matter), so you just give indent the
711 options ``-kr -i8`` (stands for ``K&R, 8 chara 723 options ``-kr -i8`` (stands for ``K&R, 8 character indents``), or use
712 ``scripts/Lindent``, which indents in the late 724 ``scripts/Lindent``, which indents in the latest style.
713 725
714 ``indent`` has a lot of options, and especiall 726 ``indent`` has a lot of options, and especially when it comes to comment
715 re-formatting you may want to take a look at t 727 re-formatting you may want to take a look at the man page. But
716 remember: ``indent`` is not a fix for bad prog 728 remember: ``indent`` is not a fix for bad programming.
717 729
718 Note that you can also use the ``clang-format` 730 Note that you can also use the ``clang-format`` tool to help you with
719 these rules, to quickly re-format parts of you 731 these rules, to quickly re-format parts of your code automatically,
720 and to review full files in order to spot codi 732 and to review full files in order to spot coding style mistakes,
721 typos and possible improvements. It is also ha 733 typos and possible improvements. It is also handy for sorting ``#includes``,
722 for aligning variables/macros, for reflowing t 734 for aligning variables/macros, for reflowing text and other similar tasks.
723 See the file :ref:`Documentation/dev-tools/cla !! 735 See the file :ref:`Documentation/process/clang-format.rst <clangformat>`
724 for more details. 736 for more details.
725 737
726 Some basic editor settings, such as indentatio <<
727 set automatically if you are using an editor t <<
728 EditorConfig. See the official EditorConfig we <<
729 https://editorconfig.org/ <<
730 738
731 10) Kconfig configuration files 739 10) Kconfig configuration files
732 ------------------------------- 740 -------------------------------
733 741
734 For all of the Kconfig* configuration files th 742 For all of the Kconfig* configuration files throughout the source tree,
735 the indentation is somewhat different. Lines 743 the indentation is somewhat different. Lines under a ``config`` definition
736 are indented with one tab, while help text is 744 are indented with one tab, while help text is indented an additional two
737 spaces. Example:: 745 spaces. Example::
738 746
739 config AUDIT 747 config AUDIT
740 bool "Auditing support" 748 bool "Auditing support"
741 depends on NET 749 depends on NET
742 help 750 help
743 Enable auditing infrastructure that 751 Enable auditing infrastructure that can be used with another
744 kernel subsystem, such as SELinux (w 752 kernel subsystem, such as SELinux (which requires this for
745 logging of avc messages output). Do 753 logging of avc messages output). Does not do system-call
746 auditing without CONFIG_AUDITSYSCALL 754 auditing without CONFIG_AUDITSYSCALL.
747 755
748 Seriously dangerous features (such as write su 756 Seriously dangerous features (such as write support for certain
749 filesystems) should advertise this prominently 757 filesystems) should advertise this prominently in their prompt string::
750 758
751 config ADFS_FS_RW 759 config ADFS_FS_RW
752 bool "ADFS write support (DANGEROUS)" 760 bool "ADFS write support (DANGEROUS)"
753 depends on ADFS_FS 761 depends on ADFS_FS
754 ... 762 ...
755 763
756 For full documentation on the configuration fi 764 For full documentation on the configuration files, see the file
757 Documentation/kbuild/kconfig-language.rst. 765 Documentation/kbuild/kconfig-language.rst.
758 766
759 767
760 11) Data structures 768 11) Data structures
761 ------------------- 769 -------------------
762 770
763 Data structures that have visibility outside t 771 Data structures that have visibility outside the single-threaded
764 environment they are created and destroyed in 772 environment they are created and destroyed in should always have
765 reference counts. In the kernel, garbage coll 773 reference counts. In the kernel, garbage collection doesn't exist (and
766 outside the kernel garbage collection is slow 774 outside the kernel garbage collection is slow and inefficient), which
767 means that you absolutely **have** to referenc 775 means that you absolutely **have** to reference count all your uses.
768 776
769 Reference counting means that you can avoid lo 777 Reference counting means that you can avoid locking, and allows multiple
770 users to have access to the data structure in 778 users to have access to the data structure in parallel - and not having
771 to worry about the structure suddenly going aw 779 to worry about the structure suddenly going away from under them just
772 because they slept or did something else for a 780 because they slept or did something else for a while.
773 781
774 Note that locking is **not** a replacement for 782 Note that locking is **not** a replacement for reference counting.
775 Locking is used to keep data structures cohere 783 Locking is used to keep data structures coherent, while reference
776 counting is a memory management technique. Us 784 counting is a memory management technique. Usually both are needed, and
777 they are not to be confused with each other. 785 they are not to be confused with each other.
778 786
779 Many data structures can indeed have two level 787 Many data structures can indeed have two levels of reference counting,
780 when there are users of different ``classes``. 788 when there are users of different ``classes``. The subclass count counts
781 the number of subclass users, and decrements t 789 the number of subclass users, and decrements the global count just once
782 when the subclass count goes to zero. 790 when the subclass count goes to zero.
783 791
784 Examples of this kind of ``multi-level-referen 792 Examples of this kind of ``multi-level-reference-counting`` can be found in
785 memory management (``struct mm_struct``: mm_us 793 memory management (``struct mm_struct``: mm_users and mm_count), and in
786 filesystem code (``struct super_block``: s_cou 794 filesystem code (``struct super_block``: s_count and s_active).
787 795
788 Remember: if another thread can find your data 796 Remember: if another thread can find your data structure, and you don't
789 have a reference count on it, you almost certa 797 have a reference count on it, you almost certainly have a bug.
790 798
791 799
792 12) Macros, Enums and RTL 800 12) Macros, Enums and RTL
793 ------------------------- 801 -------------------------
794 802
795 Names of macros defining constants and labels 803 Names of macros defining constants and labels in enums are capitalized.
796 804
797 .. code-block:: c 805 .. code-block:: c
798 806
799 #define CONSTANT 0x12345 807 #define CONSTANT 0x12345
800 808
801 Enums are preferred when defining several rela 809 Enums are preferred when defining several related constants.
802 810
803 CAPITALIZED macro names are appreciated but ma 811 CAPITALIZED macro names are appreciated but macros resembling functions
804 may be named in lower case. 812 may be named in lower case.
805 813
806 Generally, inline functions are preferable to 814 Generally, inline functions are preferable to macros resembling functions.
807 815
808 Macros with multiple statements should be encl 816 Macros with multiple statements should be enclosed in a do - while block:
809 817
810 .. code-block:: c 818 .. code-block:: c
811 819
812 #define macrofun(a, b, c) 820 #define macrofun(a, b, c) \
813 do { 821 do { \
814 if (a == 5) 822 if (a == 5) \
815 do_this(b, c); 823 do_this(b, c); \
816 } while (0) 824 } while (0)
817 825
818 Function-like macros with unused parameters sh <<
819 inline functions to avoid the issue of unused <<
820 <<
821 .. code-block:: c <<
822 <<
823 static inline void fun(struct foo *foo <<
824 { <<
825 } <<
826 <<
827 Due to historical practices, many files still <<
828 approach to evaluate parameters. However, this <<
829 Inline functions address the issue of "express <<
830 evaluated more than once", circumvent unused-v <<
831 are generally better documented than macros fo <<
832 <<
833 .. code-block:: c <<
834 <<
835 /* <<
836 * Avoid doing this whenever possible <<
837 * inline functions <<
838 */ <<
839 #define macrofun(foo) do { (void) (foo <<
840 <<
841 Things to avoid when using macros: 826 Things to avoid when using macros:
842 827
843 1) macros that affect control flow: 828 1) macros that affect control flow:
844 829
845 .. code-block:: c 830 .. code-block:: c
846 831
847 #define FOO(x) 832 #define FOO(x) \
848 do { 833 do { \
849 if (blah(x) < 0) 834 if (blah(x) < 0) \
850 return -EBUGGE 835 return -EBUGGERED; \
851 } while (0) 836 } while (0)
852 837
853 is a **very** bad idea. It looks like a funct 838 is a **very** bad idea. It looks like a function call but exits the ``calling``
854 function; don't break the internal parsers of 839 function; don't break the internal parsers of those who will read the code.
855 840
856 2) macros that depend on having a local variab 841 2) macros that depend on having a local variable with a magic name:
857 842
858 .. code-block:: c 843 .. code-block:: c
859 844
860 #define FOO(val) bar(index, val) 845 #define FOO(val) bar(index, val)
861 846
862 might look like a good thing, but it's confusi 847 might look like a good thing, but it's confusing as hell when one reads the
863 code and it's prone to breakage from seemingly 848 code and it's prone to breakage from seemingly innocent changes.
864 849
865 3) macros with arguments that are used as l-va 850 3) macros with arguments that are used as l-values: FOO(x) = y; will
866 bite you if somebody e.g. turns FOO into an in 851 bite you if somebody e.g. turns FOO into an inline function.
867 852
868 4) forgetting about precedence: macros definin 853 4) forgetting about precedence: macros defining constants using expressions
869 must enclose the expression in parentheses. Be 854 must enclose the expression in parentheses. Beware of similar issues with
870 macros using parameters. 855 macros using parameters.
871 856
872 .. code-block:: c 857 .. code-block:: c
873 858
874 #define CONSTANT 0x4000 859 #define CONSTANT 0x4000
875 #define CONSTEXP (CONSTANT | 3) 860 #define CONSTEXP (CONSTANT | 3)
876 861
877 5) namespace collisions when defining local va 862 5) namespace collisions when defining local variables in macros resembling
878 functions: 863 functions:
879 864
880 .. code-block:: c 865 .. code-block:: c
881 866
882 #define FOO(x) 867 #define FOO(x) \
883 ({ 868 ({ \
884 typeof(x) ret; 869 typeof(x) ret; \
885 ret = calc_ret(x); 870 ret = calc_ret(x); \
886 (ret); 871 (ret); \
887 }) 872 })
888 873
889 ret is a common name for a local variable - __ 874 ret is a common name for a local variable - __foo_ret is less likely
890 to collide with an existing variable. 875 to collide with an existing variable.
891 876
892 The cpp manual deals with macros exhaustively. 877 The cpp manual deals with macros exhaustively. The gcc internals manual also
893 covers RTL which is used frequently with assem 878 covers RTL which is used frequently with assembly language in the kernel.
894 879
895 880
896 13) Printing kernel messages 881 13) Printing kernel messages
897 ---------------------------- 882 ----------------------------
898 883
899 Kernel developers like to be seen as literate. 884 Kernel developers like to be seen as literate. Do mind the spelling
900 of kernel messages to make a good impression. 885 of kernel messages to make a good impression. Do not use incorrect
901 contractions like ``dont``; use ``do not`` or 886 contractions like ``dont``; use ``do not`` or ``don't`` instead. Make the
902 messages concise, clear, and unambiguous. 887 messages concise, clear, and unambiguous.
903 888
904 Kernel messages do not have to be terminated w 889 Kernel messages do not have to be terminated with a period.
905 890
906 Printing numbers in parentheses (%d) adds no v 891 Printing numbers in parentheses (%d) adds no value and should be avoided.
907 892
908 There are a number of driver model diagnostic 893 There are a number of driver model diagnostic macros in <linux/dev_printk.h>
909 which you should use to make sure messages are 894 which you should use to make sure messages are matched to the right device
910 and driver, and are tagged with the right leve 895 and driver, and are tagged with the right level: dev_err(), dev_warn(),
911 dev_info(), and so forth. For messages that a 896 dev_info(), and so forth. For messages that aren't associated with a
912 particular device, <linux/printk.h> defines pr 897 particular device, <linux/printk.h> defines pr_notice(), pr_info(),
913 pr_warn(), pr_err(), etc. When drivers are wor !! 898 pr_warn(), pr_err(), etc.
914 so prefer to use dev_dbg/pr_debug unless somet <<
915 899
916 Coming up with good debugging messages can be 900 Coming up with good debugging messages can be quite a challenge; and once
917 you have them, they can be a huge help for rem 901 you have them, they can be a huge help for remote troubleshooting. However
918 debug message printing is handled differently 902 debug message printing is handled differently than printing other non-debug
919 messages. While the other pr_XXX() functions 903 messages. While the other pr_XXX() functions print unconditionally,
920 pr_debug() does not; it is compiled out by def 904 pr_debug() does not; it is compiled out by default, unless either DEBUG is
921 defined or CONFIG_DYNAMIC_DEBUG is set. That 905 defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also,
922 and a related convention uses VERBOSE_DEBUG to 906 and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
923 the ones already enabled by DEBUG. 907 the ones already enabled by DEBUG.
924 908
925 Many subsystems have Kconfig debug options to 909 Many subsystems have Kconfig debug options to turn on -DDEBUG in the
926 corresponding Makefile; in other cases specifi 910 corresponding Makefile; in other cases specific files #define DEBUG. And
927 when a debug message should be unconditionally 911 when a debug message should be unconditionally printed, such as if it is
928 already inside a debug-related #ifdef section, 912 already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
929 used. 913 used.
930 914
931 915
932 14) Allocating memory 916 14) Allocating memory
933 --------------------- 917 ---------------------
934 918
935 The kernel provides the following general purp 919 The kernel provides the following general purpose memory allocators:
936 kmalloc(), kzalloc(), kmalloc_array(), kcalloc 920 kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
937 vzalloc(). Please refer to the API documentat 921 vzalloc(). Please refer to the API documentation for further information
938 about them. :ref:`Documentation/core-api/memo 922 about them. :ref:`Documentation/core-api/memory-allocation.rst
939 <memory_allocation>` 923 <memory_allocation>`
940 924
941 The preferred form for passing a size of a str 925 The preferred form for passing a size of a struct is the following:
942 926
943 .. code-block:: c 927 .. code-block:: c
944 928
945 p = kmalloc(sizeof(*p), ...); 929 p = kmalloc(sizeof(*p), ...);
946 930
947 The alternative form where struct name is spel 931 The alternative form where struct name is spelled out hurts readability and
948 introduces an opportunity for a bug when the p 932 introduces an opportunity for a bug when the pointer variable type is changed
949 but the corresponding sizeof that is passed to 933 but the corresponding sizeof that is passed to a memory allocator is not.
950 934
951 Casting the return value which is a void point 935 Casting the return value which is a void pointer is redundant. The conversion
952 from void pointer to any other pointer type is 936 from void pointer to any other pointer type is guaranteed by the C programming
953 language. 937 language.
954 938
955 The preferred form for allocating an array is 939 The preferred form for allocating an array is the following:
956 940
957 .. code-block:: c 941 .. code-block:: c
958 942
959 p = kmalloc_array(n, sizeof(...), ...) 943 p = kmalloc_array(n, sizeof(...), ...);
960 944
961 The preferred form for allocating a zeroed arr 945 The preferred form for allocating a zeroed array is the following:
962 946
963 .. code-block:: c 947 .. code-block:: c
964 948
965 p = kcalloc(n, sizeof(...), ...); 949 p = kcalloc(n, sizeof(...), ...);
966 950
967 Both forms check for overflow on the allocatio 951 Both forms check for overflow on the allocation size n * sizeof(...),
968 and return NULL if that occurred. 952 and return NULL if that occurred.
969 953
970 These generic allocation functions all emit a 954 These generic allocation functions all emit a stack dump on failure when used
971 without __GFP_NOWARN so there is no use in emi 955 without __GFP_NOWARN so there is no use in emitting an additional failure
972 message when NULL is returned. 956 message when NULL is returned.
973 957
974 15) The inline disease 958 15) The inline disease
975 ---------------------- 959 ----------------------
976 960
977 There appears to be a common misperception tha 961 There appears to be a common misperception that gcc has a magic "make me
978 faster" speedup option called ``inline``. Whil 962 faster" speedup option called ``inline``. While the use of inlines can be
979 appropriate (for example as a means of replaci 963 appropriate (for example as a means of replacing macros, see Chapter 12), it
980 very often is not. Abundant use of the inline 964 very often is not. Abundant use of the inline keyword leads to a much bigger
981 kernel, which in turn slows the system as a wh 965 kernel, which in turn slows the system as a whole down, due to a bigger
982 icache footprint for the CPU and simply becaus 966 icache footprint for the CPU and simply because there is less memory
983 available for the pagecache. Just think about 967 available for the pagecache. Just think about it; a pagecache miss causes a
984 disk seek, which easily takes 5 milliseconds. 968 disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
985 that can go into these 5 milliseconds. 969 that can go into these 5 milliseconds.
986 970
987 A reasonable rule of thumb is to not put inlin 971 A reasonable rule of thumb is to not put inline at functions that have more
988 than 3 lines of code in them. An exception to 972 than 3 lines of code in them. An exception to this rule are the cases where
989 a parameter is known to be a compiletime const 973 a parameter is known to be a compiletime constant, and as a result of this
990 constantness you *know* the compiler will be a 974 constantness you *know* the compiler will be able to optimize most of your
991 function away at compile time. For a good exam 975 function away at compile time. For a good example of this later case, see
992 the kmalloc() inline function. 976 the kmalloc() inline function.
993 977
994 Often people argue that adding inline to funct 978 Often people argue that adding inline to functions that are static and used
995 only once is always a win since there is no sp 979 only once is always a win since there is no space tradeoff. While this is
996 technically correct, gcc is capable of inlinin 980 technically correct, gcc is capable of inlining these automatically without
997 help, and the maintenance issue of removing th 981 help, and the maintenance issue of removing the inline when a second user
998 appears outweighs the potential value of the h 982 appears outweighs the potential value of the hint that tells gcc to do
999 something it would have done anyway. 983 something it would have done anyway.
1000 984
1001 985
1002 16) Function return values and names 986 16) Function return values and names
1003 ------------------------------------ 987 ------------------------------------
1004 988
1005 Functions can return values of many different 989 Functions can return values of many different kinds, and one of the
1006 most common is a value indicating whether the 990 most common is a value indicating whether the function succeeded or
1007 failed. Such a value can be represented as a 991 failed. Such a value can be represented as an error-code integer
1008 (-Exxx = failure, 0 = success) or a ``succeed 992 (-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure,
1009 non-zero = success). 993 non-zero = success).
1010 994
1011 Mixing up these two sorts of representations 995 Mixing up these two sorts of representations is a fertile source of
1012 difficult-to-find bugs. If the C language in 996 difficult-to-find bugs. If the C language included a strong distinction
1013 between integers and booleans then the compil 997 between integers and booleans then the compiler would find these mistakes
1014 for us... but it doesn't. To help prevent su 998 for us... but it doesn't. To help prevent such bugs, always follow this
1015 convention:: 999 convention::
1016 1000
1017 If the name of a function is an actio 1001 If the name of a function is an action or an imperative command,
1018 the function should return an error-c 1002 the function should return an error-code integer. If the name
1019 is a predicate, the function should r 1003 is a predicate, the function should return a "succeeded" boolean.
1020 1004
1021 For example, ``add work`` is a command, and t 1005 For example, ``add work`` is a command, and the add_work() function returns 0
1022 for success or -EBUSY for failure. In the sa 1006 for success or -EBUSY for failure. In the same way, ``PCI device present`` is
1023 a predicate, and the pci_dev_present() functi 1007 a predicate, and the pci_dev_present() function returns 1 if it succeeds in
1024 finding a matching device or 0 if it doesn't. 1008 finding a matching device or 0 if it doesn't.
1025 1009
1026 All EXPORTed functions must respect this conv 1010 All EXPORTed functions must respect this convention, and so should all
1027 public functions. Private (static) functions 1011 public functions. Private (static) functions need not, but it is
1028 recommended that they do. 1012 recommended that they do.
1029 1013
1030 Functions whose return value is the actual re 1014 Functions whose return value is the actual result of a computation, rather
1031 than an indication of whether the computation 1015 than an indication of whether the computation succeeded, are not subject to
1032 this rule. Generally they indicate failure b 1016 this rule. Generally they indicate failure by returning some out-of-range
1033 result. Typical examples would be functions 1017 result. Typical examples would be functions that return pointers; they use
1034 NULL or the ERR_PTR mechanism to report failu 1018 NULL or the ERR_PTR mechanism to report failure.
1035 1019
1036 1020
1037 17) Using bool 1021 17) Using bool
1038 -------------- 1022 --------------
1039 1023
1040 The Linux kernel bool type is an alias for th 1024 The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
1041 only evaluate to 0 or 1, and implicit or expl 1025 only evaluate to 0 or 1, and implicit or explicit conversion to bool
1042 automatically converts the value to true or f 1026 automatically converts the value to true or false. When using bool types the
1043 !! construction is not needed, which eliminat 1027 !! construction is not needed, which eliminates a class of bugs.
1044 1028
1045 When working with bool values the true and fa 1029 When working with bool values the true and false definitions should be used
1046 instead of 1 and 0. 1030 instead of 1 and 0.
1047 1031
1048 bool function return types and stack variable 1032 bool function return types and stack variables are always fine to use whenever
1049 appropriate. Use of bool is encouraged to imp 1033 appropriate. Use of bool is encouraged to improve readability and is often a
1050 better option than 'int' for storing boolean 1034 better option than 'int' for storing boolean values.
1051 1035
1052 Do not use bool if cache line layout or size 1036 Do not use bool if cache line layout or size of the value matters, as its size
1053 and alignment varies based on the compiled ar 1037 and alignment varies based on the compiled architecture. Structures that are
1054 optimized for alignment and size should not u 1038 optimized for alignment and size should not use bool.
1055 1039
1056 If a structure has many true/false values, co 1040 If a structure has many true/false values, consider consolidating them into a
1057 bitfield with 1 bit members, or using an appr 1041 bitfield with 1 bit members, or using an appropriate fixed width type, such as
1058 u8. 1042 u8.
1059 1043
1060 Similarly for function arguments, many true/f 1044 Similarly for function arguments, many true/false values can be consolidated
1061 into a single bitwise 'flags' argument and 'f 1045 into a single bitwise 'flags' argument and 'flags' can often be a more
1062 readable alternative if the call-sites have n 1046 readable alternative if the call-sites have naked true/false constants.
1063 1047
1064 Otherwise limited use of bool in structures a 1048 Otherwise limited use of bool in structures and arguments can improve
1065 readability. 1049 readability.
1066 1050
1067 18) Don't re-invent the kernel macros 1051 18) Don't re-invent the kernel macros
1068 ------------------------------------- 1052 -------------------------------------
1069 1053
1070 The header file include/linux/kernel.h contai 1054 The header file include/linux/kernel.h contains a number of macros that
1071 you should use, rather than explicitly coding 1055 you should use, rather than explicitly coding some variant of them yourself.
1072 For example, if you need to calculate the len 1056 For example, if you need to calculate the length of an array, take advantage
1073 of the macro 1057 of the macro
1074 1058
1075 .. code-block:: c 1059 .. code-block:: c
1076 1060
1077 #define ARRAY_SIZE(x) (sizeof(x) / si 1061 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
1078 1062
1079 Similarly, if you need to calculate the size 1063 Similarly, if you need to calculate the size of some structure member, use
1080 1064
1081 .. code-block:: c 1065 .. code-block:: c
1082 1066
1083 #define sizeof_field(t, f) (sizeof((( 1067 #define sizeof_field(t, f) (sizeof(((t*)0)->f))
1084 1068
1085 There are also min() and max() macros that do 1069 There are also min() and max() macros that do strict type checking if you
1086 need them. Feel free to peruse that header f 1070 need them. Feel free to peruse that header file to see what else is already
1087 defined that you shouldn't reproduce in your 1071 defined that you shouldn't reproduce in your code.
1088 1072
1089 1073
1090 19) Editor modelines and other cruft 1074 19) Editor modelines and other cruft
1091 ------------------------------------ 1075 ------------------------------------
1092 1076
1093 Some editors can interpret configuration info 1077 Some editors can interpret configuration information embedded in source files,
1094 indicated with special markers. For example, 1078 indicated with special markers. For example, emacs interprets lines marked
1095 like this: 1079 like this:
1096 1080
1097 .. code-block:: c 1081 .. code-block:: c
1098 1082
1099 -*- mode: c -*- 1083 -*- mode: c -*-
1100 1084
1101 Or like this: 1085 Or like this:
1102 1086
1103 .. code-block:: c 1087 .. code-block:: c
1104 1088
1105 /* 1089 /*
1106 Local Variables: 1090 Local Variables:
1107 compile-command: "gcc -DMAGIC_DEBUG_F 1091 compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
1108 End: 1092 End:
1109 */ 1093 */
1110 1094
1111 Vim interprets markers that look like this: 1095 Vim interprets markers that look like this:
1112 1096
1113 .. code-block:: c 1097 .. code-block:: c
1114 1098
1115 /* vim:set sw=8 noet */ 1099 /* vim:set sw=8 noet */
1116 1100
1117 Do not include any of these in source files. 1101 Do not include any of these in source files. People have their own personal
1118 editor configurations, and your source files 1102 editor configurations, and your source files should not override them. This
1119 includes markers for indentation and mode con 1103 includes markers for indentation and mode configuration. People may use their
1120 own custom mode, or may have some other magic 1104 own custom mode, or may have some other magic method for making indentation
1121 work correctly. 1105 work correctly.
1122 1106
1123 1107
1124 20) Inline assembly 1108 20) Inline assembly
1125 ------------------- 1109 -------------------
1126 1110
1127 In architecture-specific code, you may need t 1111 In architecture-specific code, you may need to use inline assembly to interface
1128 with CPU or platform functionality. Don't he 1112 with CPU or platform functionality. Don't hesitate to do so when necessary.
1129 However, don't use inline assembly gratuitous 1113 However, don't use inline assembly gratuitously when C can do the job. You can
1130 and should poke hardware from C when possible 1114 and should poke hardware from C when possible.
1131 1115
1132 Consider writing simple helper functions that 1116 Consider writing simple helper functions that wrap common bits of inline
1133 assembly, rather than repeatedly writing them 1117 assembly, rather than repeatedly writing them with slight variations. Remember
1134 that inline assembly can use C parameters. 1118 that inline assembly can use C parameters.
1135 1119
1136 Large, non-trivial assembly functions should 1120 Large, non-trivial assembly functions should go in .S files, with corresponding
1137 C prototypes defined in C header files. The 1121 C prototypes defined in C header files. The C prototypes for assembly
1138 functions should use ``asmlinkage``. 1122 functions should use ``asmlinkage``.
1139 1123
1140 You may need to mark your asm statement as vo 1124 You may need to mark your asm statement as volatile, to prevent GCC from
1141 removing it if GCC doesn't notice any side ef 1125 removing it if GCC doesn't notice any side effects. You don't always need to
1142 do so, though, and doing so unnecessarily can 1126 do so, though, and doing so unnecessarily can limit optimization.
1143 1127
1144 When writing a single inline assembly stateme 1128 When writing a single inline assembly statement containing multiple
1145 instructions, put each instruction on a separ 1129 instructions, put each instruction on a separate line in a separate quoted
1146 string, and end each string except the last w 1130 string, and end each string except the last with ``\n\t`` to properly indent
1147 the next instruction in the assembly output: 1131 the next instruction in the assembly output:
1148 1132
1149 .. code-block:: c 1133 .. code-block:: c
1150 1134
1151 asm ("magic %reg1, #42\n\t" 1135 asm ("magic %reg1, #42\n\t"
1152 "more_magic %reg2, %reg3" 1136 "more_magic %reg2, %reg3"
1153 : /* outputs */ : /* inputs */ : 1137 : /* outputs */ : /* inputs */ : /* clobbers */);
1154 1138
1155 1139
1156 21) Conditional Compilation 1140 21) Conditional Compilation
1157 --------------------------- 1141 ---------------------------
1158 1142
1159 Wherever possible, don't use preprocessor con 1143 Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
1160 files; doing so makes code harder to read and 1144 files; doing so makes code harder to read and logic harder to follow. Instead,
1161 use such conditionals in a header file defini 1145 use such conditionals in a header file defining functions for use in those .c
1162 files, providing no-op stub versions in the # 1146 files, providing no-op stub versions in the #else case, and then call those
1163 functions unconditionally from .c files. The 1147 functions unconditionally from .c files. The compiler will avoid generating
1164 any code for the stub calls, producing identi 1148 any code for the stub calls, producing identical results, but the logic will
1165 remain easy to follow. 1149 remain easy to follow.
1166 1150
1167 Prefer to compile out entire functions, rathe 1151 Prefer to compile out entire functions, rather than portions of functions or
1168 portions of expressions. Rather than putting 1152 portions of expressions. Rather than putting an ifdef in an expression, factor
1169 out part or all of the expression into a sepa 1153 out part or all of the expression into a separate helper function and apply the
1170 conditional to that function. 1154 conditional to that function.
1171 1155
1172 If you have a function or variable which may 1156 If you have a function or variable which may potentially go unused in a
1173 particular configuration, and the compiler wo 1157 particular configuration, and the compiler would warn about its definition
1174 going unused, mark the definition as __maybe_ 1158 going unused, mark the definition as __maybe_unused rather than wrapping it in
1175 a preprocessor conditional. (However, if a f 1159 a preprocessor conditional. (However, if a function or variable *always* goes
1176 unused, delete it.) 1160 unused, delete it.)
1177 1161
1178 Within code, where possible, use the IS_ENABL 1162 Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
1179 symbol into a C boolean expression, and use i 1163 symbol into a C boolean expression, and use it in a normal C conditional:
1180 1164
1181 .. code-block:: c 1165 .. code-block:: c
1182 1166
1183 if (IS_ENABLED(CONFIG_SOMETHING)) { 1167 if (IS_ENABLED(CONFIG_SOMETHING)) {
1184 ... 1168 ...
1185 } 1169 }
1186 1170
1187 The compiler will constant-fold the condition 1171 The compiler will constant-fold the conditional away, and include or exclude
1188 the block of code just as with an #ifdef, so 1172 the block of code just as with an #ifdef, so this will not add any runtime
1189 overhead. However, this approach still allow 1173 overhead. However, this approach still allows the C compiler to see the code
1190 inside the block, and check it for correctnes 1174 inside the block, and check it for correctness (syntax, types, symbol
1191 references, etc). Thus, you still have to us 1175 references, etc). Thus, you still have to use an #ifdef if the code inside the
1192 block references symbols that will not exist 1176 block references symbols that will not exist if the condition is not met.
1193 1177
1194 At the end of any non-trivial #if or #ifdef b 1178 At the end of any non-trivial #if or #ifdef block (more than a few lines),
1195 place a comment after the #endif on the same 1179 place a comment after the #endif on the same line, noting the conditional
1196 expression used. For instance: 1180 expression used. For instance:
1197 1181
1198 .. code-block:: c 1182 .. code-block:: c
1199 1183
1200 #ifdef CONFIG_SOMETHING 1184 #ifdef CONFIG_SOMETHING
1201 ... 1185 ...
1202 #endif /* CONFIG_SOMETHING */ 1186 #endif /* CONFIG_SOMETHING */
1203 1187
1204 1188
1205 22) Do not crash the kernel 1189 22) Do not crash the kernel
1206 --------------------------- 1190 ---------------------------
1207 1191
1208 In general, the decision to crash the kernel 1192 In general, the decision to crash the kernel belongs to the user, rather
1209 than to the kernel developer. 1193 than to the kernel developer.
1210 1194
1211 Avoid panic() 1195 Avoid panic()
1212 ************* 1196 *************
1213 1197
1214 panic() should be used with care and primaril 1198 panic() should be used with care and primarily only during system boot.
1215 panic() is, for example, acceptable when runn 1199 panic() is, for example, acceptable when running out of memory during boot and
1216 not being able to continue. 1200 not being able to continue.
1217 1201
1218 Use WARN() rather than BUG() 1202 Use WARN() rather than BUG()
1219 **************************** 1203 ****************************
1220 1204
1221 Do not add new code that uses any of the BUG( 1205 Do not add new code that uses any of the BUG() variants, such as BUG(),
1222 BUG_ON(), or VM_BUG_ON(). Instead, use a WARN 1206 BUG_ON(), or VM_BUG_ON(). Instead, use a WARN*() variant, preferably
1223 WARN_ON_ONCE(), and possibly with recovery co 1207 WARN_ON_ONCE(), and possibly with recovery code. Recovery code is not
1224 required if there is no reasonable way to at 1208 required if there is no reasonable way to at least partially recover.
1225 1209
1226 "I'm too lazy to do error handling" is not an 1210 "I'm too lazy to do error handling" is not an excuse for using BUG(). Major
1227 internal corruptions with no way of continuin 1211 internal corruptions with no way of continuing may still use BUG(), but need
1228 good justification. 1212 good justification.
1229 1213
1230 Use WARN_ON_ONCE() rather than WARN() or WARN 1214 Use WARN_ON_ONCE() rather than WARN() or WARN_ON()
1231 ********************************************* 1215 **************************************************
1232 1216
1233 WARN_ON_ONCE() is generally preferred over WA 1217 WARN_ON_ONCE() is generally preferred over WARN() or WARN_ON(), because it
1234 is common for a given warning condition, if i 1218 is common for a given warning condition, if it occurs at all, to occur
1235 multiple times. This can fill up and wrap the 1219 multiple times. This can fill up and wrap the kernel log, and can even slow
1236 the system enough that the excessive logging 1220 the system enough that the excessive logging turns into its own, additional
1237 problem. 1221 problem.
1238 1222
1239 Do not WARN lightly 1223 Do not WARN lightly
1240 ******************* 1224 *******************
1241 1225
1242 WARN*() is intended for unexpected, this-shou 1226 WARN*() is intended for unexpected, this-should-never-happen situations.
1243 WARN*() macros are not to be used for anythin 1227 WARN*() macros are not to be used for anything that is expected to happen
1244 during normal operation. These are not pre- o 1228 during normal operation. These are not pre- or post-condition asserts, for
1245 example. Again: WARN*() must not be used for 1229 example. Again: WARN*() must not be used for a condition that is expected
1246 to trigger easily, for example, by user space 1230 to trigger easily, for example, by user space actions. pr_warn_once() is a
1247 possible alternative, if you need to notify t 1231 possible alternative, if you need to notify the user of a problem.
1248 1232
1249 Do not worry about panic_on_warn users 1233 Do not worry about panic_on_warn users
1250 ************************************** 1234 **************************************
1251 1235
1252 A few more words about panic_on_warn: Remembe 1236 A few more words about panic_on_warn: Remember that ``panic_on_warn`` is an
1253 available kernel option, and that many users 1237 available kernel option, and that many users set this option. This is why
1254 there is a "Do not WARN lightly" writeup, abo 1238 there is a "Do not WARN lightly" writeup, above. However, the existence of
1255 panic_on_warn users is not a valid reason to 1239 panic_on_warn users is not a valid reason to avoid the judicious use
1256 WARN*(). That is because, whoever enables pan 1240 WARN*(). That is because, whoever enables panic_on_warn has explicitly
1257 asked the kernel to crash if a WARN*() fires, 1241 asked the kernel to crash if a WARN*() fires, and such users must be
1258 prepared to deal with the consequences of a s 1242 prepared to deal with the consequences of a system that is somewhat more
1259 likely to crash. 1243 likely to crash.
1260 1244
1261 Use BUILD_BUG_ON() for compile-time assertion 1245 Use BUILD_BUG_ON() for compile-time assertions
1262 ********************************************* 1246 **********************************************
1263 1247
1264 The use of BUILD_BUG_ON() is acceptable and e 1248 The use of BUILD_BUG_ON() is acceptable and encouraged, because it is a
1265 compile-time assertion that has no effect at 1249 compile-time assertion that has no effect at runtime.
1266 1250
1267 Appendix I) References 1251 Appendix I) References
1268 ---------------------- 1252 ----------------------
1269 1253
1270 The C Programming Language, Second Edition 1254 The C Programming Language, Second Edition
1271 by Brian W. Kernighan and Dennis M. Ritchie. 1255 by Brian W. Kernighan and Dennis M. Ritchie.
1272 Prentice Hall, Inc., 1988. 1256 Prentice Hall, Inc., 1988.
1273 ISBN 0-13-110362-8 (paperback), 0-13-110370-9 1257 ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
1274 1258
1275 The Practice of Programming 1259 The Practice of Programming
1276 by Brian W. Kernighan and Rob Pike. 1260 by Brian W. Kernighan and Rob Pike.
1277 Addison-Wesley, Inc., 1999. 1261 Addison-Wesley, Inc., 1999.
1278 ISBN 0-201-61586-X. 1262 ISBN 0-201-61586-X.
1279 1263
1280 GNU manuals - where in compliance with K&R an 1264 GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
1281 gcc internals and indent, all available from 1265 gcc internals and indent, all available from https://www.gnu.org/manual/
1282 1266
1283 WG14 is the international standardization wor 1267 WG14 is the international standardization working group for the programming
1284 language C, URL: http://www.open-std.org/JTC1 1268 language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
1285 1269
1286 Kernel CodingStyle, by greg@kroah.com at OLS 1270 Kernel CodingStyle, by greg@kroah.com at OLS 2002:
1287 http://www.kroah.com/linux/talks/ols_2002_ker 1271 http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.