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