Root/Documentation/CodingStyle

1
2        Linux kernel coding style
3
4This is a short document describing the preferred coding style for the
5linux kernel. Coding style is very personal, and I won't _force_ my
6views on anybody, but this is what goes for anything that I have to be
7able to maintain, and I'd prefer it for most other things too. Please
8at least consider the points made here.
9
10First off, I'd suggest printing out a copy of the GNU coding standards,
11and NOT read it. Burn them, it's a great symbolic gesture.
12
13Anyway, here goes:
14
15
16         Chapter 1: Indentation
17
18Tabs are 8 characters, and thus indentations are also 8 characters.
19There are heretic movements that try to make indentations 4 (or even 2!)
20characters deep, and that is akin to trying to define the value of PI to
21be 3.
22
23Rationale: The whole idea behind indentation is to clearly define where
24a block of control starts and ends. Especially when you've been looking
25at your screen for 20 straight hours, you'll find it a lot easier to see
26how the indentation works if you have large indentations.
27
28Now, some people will claim that having 8-character indentations makes
29the code move too far to the right, and makes it hard to read on a
3080-character terminal screen. The answer to that is that if you need
31more than 3 levels of indentation, you're screwed anyway, and should fix
32your program.
33
34In short, 8-char indents make things easier to read, and have the added
35benefit of warning you when you're nesting your functions too deep.
36Heed that warning.
37
38The preferred way to ease multiple indentation levels in a switch statement is
39to align the "switch" and its subordinate "case" labels in the same column
40instead of "double-indenting" the "case" labels. E.g.:
41
42    switch (suffix) {
43    case 'G':
44    case 'g':
45        mem <<= 30;
46        break;
47    case 'M':
48    case 'm':
49        mem <<= 20;
50        break;
51    case 'K':
52    case 'k':
53        mem <<= 10;
54        /* fall through */
55    default:
56        break;
57    }
58
59
60Don't put multiple statements on a single line unless you have
61something to hide:
62
63    if (condition) do_this;
64      do_something_everytime;
65
66Don't put multiple assignments on a single line either. Kernel coding style
67is super simple. Avoid tricky expressions.
68
69Outside of comments, documentation and except in Kconfig, spaces are never
70used for indentation, and the above example is deliberately broken.
71
72Get a decent editor and don't leave whitespace at the end of lines.
73
74
75        Chapter 2: Breaking long lines and strings
76
77Coding style is all about readability and maintainability using commonly
78available tools.
79
80The limit on the length of lines is 80 columns and this is a strongly
81preferred limit.
82
83Statements longer than 80 columns will be broken into sensible chunks, unless
84exceeding 80 columns significantly increases readability and does not hide
85information. Descendants are always substantially shorter than the parent and
86are placed substantially to the right. The same applies to function headers
87with a long argument list. However, never break user-visible strings such as
88printk messages, because that breaks the ability to grep for them.
89
90
91        Chapter 3: Placing Braces and Spaces
92
93The other issue that always comes up in C styling is the placement of
94braces. Unlike the indent size, there are few technical reasons to
95choose one placement strategy over the other, but the preferred way, as
96shown to us by the prophets Kernighan and Ritchie, is to put the opening
97brace last on the line, and put the closing brace first, thusly:
98
99    if (x is true) {
100        we do y
101    }
102
103This applies to all non-function statement blocks (if, switch, for,
104while, do). E.g.:
105
106    switch (action) {
107    case KOBJ_ADD:
108        return "add";
109    case KOBJ_REMOVE:
110        return "remove";
111    case KOBJ_CHANGE:
112        return "change";
113    default:
114        return NULL;
115    }
116
117However, there is one special case, namely functions: they have the
118opening brace at the beginning of the next line, thus:
119
120    int function(int x)
121    {
122        body of function
123    }
124
125Heretic people all over the world have claimed that this inconsistency
126is ... well ... inconsistent, but all right-thinking people know that
127(a) K&R are _right_ and (b) K&R are right. Besides, functions are
128special anyway (you can't nest them in C).
129
130Note that the closing brace is empty on a line of its own, _except_ in
131the cases where it is followed by a continuation of the same statement,
132ie a "while" in a do-statement or an "else" in an if-statement, like
133this:
134
135    do {
136        body of do-loop
137    } while (condition);
138
139and
140
141    if (x == y) {
142        ..
143    } else if (x > y) {
144        ...
145    } else {
146        ....
147    }
148
149Rationale: K&R.
150
151Also, note that this brace-placement also minimizes the number of empty
152(or almost empty) lines, without any loss of readability. Thus, as the
153supply of new-lines on your screen is not a renewable resource (think
15425-line terminal screens here), you have more empty lines to put
155comments on.
156
157Do not unnecessarily use braces where a single statement will do.
158
159if (condition)
160    action();
161
162and
163
164if (condition)
165    do_this();
166else
167    do_that();
168
169This does not apply if one branch of a conditional statement is a single
170statement. Use braces in both branches.
171
172if (condition) {
173    do_this();
174    do_that();
175} else {
176    otherwise();
177}
178
179        3.1: Spaces
180
181Linux kernel style for use of spaces depends (mostly) on
182function-versus-keyword usage. Use a space after (most) keywords. The
183notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
184somewhat like functions (and are usually used with parentheses in Linux,
185although they are not required in the language, as in: "sizeof info" after
186"struct fileinfo info;" is declared).
187
188So use a space after these keywords:
189    if, switch, case, for, do, while
190but not with sizeof, typeof, alignof, or __attribute__. E.g.,
191    s = sizeof(struct file);
192
193Do not add spaces around (inside) parenthesized expressions. This example is
194*bad*:
195
196    s = sizeof( struct file );
197
198When declaring pointer data or a function that returns a pointer type, the
199preferred use of '*' is adjacent to the data name or function name and not
200adjacent to the type name. Examples:
201
202    char *linux_banner;
203    unsigned long long memparse(char *ptr, char **retptr);
204    char *match_strdup(substring_t *s);
205
206Use one space around (on each side of) most binary and ternary operators,
207such as any of these:
208
209    = + - < > * / % | & ^ <= >= == != ? :
210
211but no space after unary operators:
212    & * + - ~ ! sizeof typeof alignof __attribute__ defined
213
214no space before the postfix increment & decrement unary operators:
215    ++ --
216
217no space after the prefix increment & decrement unary operators:
218    ++ --
219
220and no space around the '.' and "->" structure member operators.
221
222Do not leave trailing whitespace at the ends of lines. Some editors with
223"smart" indentation will insert whitespace at the beginning of new lines as
224appropriate, so you can start typing the next line of code right away.
225However, some such editors do not remove the whitespace if you end up not
226putting a line of code there, such as if you leave a blank line. As a result,
227you end up with lines containing trailing whitespace.
228
229Git will warn you about patches that introduce trailing whitespace, and can
230optionally strip the trailing whitespace for you; however, if applying a series
231of patches, this may make later patches in the series fail by changing their
232context lines.
233
234
235        Chapter 4: Naming
236
237C is a Spartan language, and so should your naming be. Unlike Modula-2
238and Pascal programmers, C programmers do not use cute names like
239ThisVariableIsATemporaryCounter. A C programmer would call that
240variable "tmp", which is much easier to write, and not the least more
241difficult to understand.
242
243HOWEVER, while mixed-case names are frowned upon, descriptive names for
244global variables are a must. To call a global function "foo" is a
245shooting offense.
246
247GLOBAL variables (to be used only if you _really_ need them) need to
248have descriptive names, as do global functions. If you have a function
249that counts the number of active users, you should call that
250"count_active_users()" or similar, you should _not_ call it "cntusr()".
251
252Encoding the type of a function into the name (so-called Hungarian
253notation) is brain damaged - the compiler knows the types anyway and can
254check those, and it only confuses the programmer. No wonder MicroSoft
255makes buggy programs.
256
257LOCAL variable names should be short, and to the point. If you have
258some random integer loop counter, it should probably be called "i".
259Calling it "loop_counter" is non-productive, if there is no chance of it
260being mis-understood. Similarly, "tmp" can be just about any type of
261variable that is used to hold a temporary value.
262
263If you are afraid to mix up your local variable names, you have another
264problem, which is called the function-growth-hormone-imbalance syndrome.
265See chapter 6 (Functions).
266
267
268        Chapter 5: Typedefs
269
270Please don't use things like "vps_t".
271
272It's a _mistake_ to use typedef for structures and pointers. When you see a
273
274    vps_t a;
275
276in the source, what does it mean?
277
278In contrast, if it says
279
280    struct virtual_container *a;
281
282you can actually tell what "a" is.
283
284Lots of people think that typedefs "help readability". Not so. They are
285useful only for:
286
287 (a) totally opaque objects (where the typedef is actively used to _hide_
288     what the object is).
289
290     Example: "pte_t" etc. opaque objects that you can only access using
291     the proper accessor functions.
292
293     NOTE! Opaqueness and "accessor functions" are not good in themselves.
294     The reason we have them for things like pte_t etc. is that there
295     really is absolutely _zero_ portably accessible information there.
296
297 (b) Clear integer types, where the abstraction _helps_ avoid confusion
298     whether it is "int" or "long".
299
300     u8/u16/u32 are perfectly fine typedefs, although they fit into
301     category (d) better than here.
302
303     NOTE! Again - there needs to be a _reason_ for this. If something is
304     "unsigned long", then there's no reason to do
305
306    typedef unsigned long myflags_t;
307
308     but if there is a clear reason for why it under certain circumstances
309     might be an "unsigned int" and under other configurations might be
310     "unsigned long", then by all means go ahead and use a typedef.
311
312 (c) when you use sparse to literally create a _new_ type for
313     type-checking.
314
315 (d) New types which are identical to standard C99 types, in certain
316     exceptional circumstances.
317
318     Although it would only take a short amount of time for the eyes and
319     brain to become accustomed to the standard types like 'uint32_t',
320     some people object to their use anyway.
321
322     Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
323     signed equivalents which are identical to standard types are
324     permitted -- although they are not mandatory in new code of your
325     own.
326
327     When editing existing code which already uses one or the other set
328     of types, you should conform to the existing choices in that code.
329
330 (e) Types safe for use in userspace.
331
332     In certain structures which are visible to userspace, we cannot
333     require C99 types and cannot use the 'u32' form above. Thus, we
334     use __u32 and similar types in all structures which are shared
335     with userspace.
336
337Maybe there are other cases too, but the rule should basically be to NEVER
338EVER use a typedef unless you can clearly match one of those rules.
339
340In general, a pointer, or a struct that has elements that can reasonably
341be directly accessed should _never_ be a typedef.
342
343
344        Chapter 6: Functions
345
346Functions should be short and sweet, and do just one thing. They should
347fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
348as we all know), and do one thing and do that well.
349
350The maximum length of a function is inversely proportional to the
351complexity and indentation level of that function. So, if you have a
352conceptually simple function that is just one long (but simple)
353case-statement, where you have to do lots of small things for a lot of
354different cases, it's OK to have a longer function.
355
356However, if you have a complex function, and you suspect that a
357less-than-gifted first-year high-school student might not even
358understand what the function is all about, you should adhere to the
359maximum limits all the more closely. Use helper functions with
360descriptive names (you can ask the compiler to in-line them if you think
361it's performance-critical, and it will probably do a better job of it
362than you would have done).
363
364Another measure of the function is the number of local variables. They
365shouldn't exceed 5-10, or you're doing something wrong. Re-think the
366function, and split it into smaller pieces. A human brain can
367generally easily keep track of about 7 different things, anything more
368and it gets confused. You know you're brilliant, but maybe you'd like
369to understand what you did 2 weeks from now.
370
371In source files, separate functions with one blank line. If the function is
372exported, the EXPORT* macro for it should follow immediately after the closing
373function brace line. E.g.:
374
375int system_is_up(void)
376{
377    return system_state == SYSTEM_RUNNING;
378}
379EXPORT_SYMBOL(system_is_up);
380
381In function prototypes, include parameter names with their data types.
382Although this is not required by the C language, it is preferred in Linux
383because it is a simple way to add valuable information for the reader.
384
385
386        Chapter 7: Centralized exiting of functions
387
388Albeit deprecated by some people, the equivalent of the goto statement is
389used frequently by compilers in form of the unconditional jump instruction.
390
391The goto statement comes in handy when a function exits from multiple
392locations and some common work such as cleanup has to be done.
393
394The rationale is:
395
396- unconditional statements are easier to understand and follow
397- nesting is reduced
398- errors by not updating individual exit points when making
399    modifications are prevented
400- saves the compiler work to optimize redundant code away ;)
401
402int fun(int a)
403{
404    int result = 0;
405    char *buffer = kmalloc(SIZE);
406
407    if (buffer == NULL)
408        return -ENOMEM;
409
410    if (condition1) {
411        while (loop1) {
412            ...
413        }
414        result = 1;
415        goto out;
416    }
417    ...
418out:
419    kfree(buffer);
420    return result;
421}
422
423        Chapter 8: Commenting
424
425Comments are good, but there is also a danger of over-commenting. NEVER
426try to explain HOW your code works in a comment: it's much better to
427write the code so that the _working_ is obvious, and it's a waste of
428time to explain badly written code.
429
430Generally, you want your comments to tell WHAT your code does, not HOW.
431Also, try to avoid putting comments inside a function body: if the
432function is so complex that you need to separately comment parts of it,
433you should probably go back to chapter 6 for a while. You can make
434small comments to note or warn about something particularly clever (or
435ugly), but try to avoid excess. Instead, put the comments at the head
436of the function, telling people what it does, and possibly WHY it does
437it.
438
439When commenting the kernel API functions, please use the kernel-doc format.
440See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
441for details.
442
443Linux style for comments is the C89 "/* ... */" style.
444Don't use C99-style "// ..." comments.
445
446The preferred style for long (multi-line) comments is:
447
448    /*
449     * This is the preferred style for multi-line
450     * comments in the Linux kernel source code.
451     * Please use it consistently.
452     *
453     * Description: A column of asterisks on the left side,
454     * with beginning and ending almost-blank lines.
455     */
456
457It's also important to comment data, whether they are basic types or derived
458types. To this end, use just one data declaration per line (no commas for
459multiple data declarations). This leaves you room for a small comment on each
460item, explaining its use.
461
462
463        Chapter 9: You've made a mess of it
464
465That's OK, we all do. You've probably been told by your long-time Unix
466user helper that "GNU emacs" automatically formats the C sources for
467you, and you've noticed that yes, it does do that, but the defaults it
468uses are less than desirable (in fact, they are worse than random
469typing - an infinite number of monkeys typing into GNU emacs would never
470make a good program).
471
472So, you can either get rid of GNU emacs, or change it to use saner
473values. To do the latter, you can stick the following in your .emacs file:
474
475(defun c-lineup-arglist-tabs-only (ignored)
476  "Line up argument lists by tabs, not spaces"
477  (let* ((anchor (c-langelem-pos c-syntactic-element))
478     (column (c-langelem-2nd-pos c-syntactic-element))
479     (offset (- (1+ column) anchor))
480     (steps (floor offset c-basic-offset)))
481    (* (max steps 1)
482       c-basic-offset)))
483
484(add-hook 'c-mode-common-hook
485          (lambda ()
486            ;; Add kernel style
487            (c-add-style
488             "linux-tabs-only"
489             '("linux" (c-offsets-alist
490                        (arglist-cont-nonempty
491                         c-lineup-gcc-asm-reg
492                         c-lineup-arglist-tabs-only))))))
493
494(add-hook 'c-mode-hook
495          (lambda ()
496            (let ((filename (buffer-file-name)))
497              ;; Enable kernel mode for the appropriate files
498              (when (and filename
499                         (string-match (expand-file-name "~/src/linux-trees")
500                                       filename))
501                (setq indent-tabs-mode t)
502                (c-set-style "linux-tabs-only")))))
503
504This will make emacs go better with the kernel coding style for C
505files below ~/src/linux-trees.
506
507But even if you fail in getting emacs to do sane formatting, not
508everything is lost: use "indent".
509
510Now, again, GNU indent has the same brain-dead settings that GNU emacs
511has, which is why you need to give it a few command line options.
512However, that's not too bad, because even the makers of GNU indent
513recognize the authority of K&R (the GNU people aren't evil, they are
514just severely misguided in this matter), so you just give indent the
515options "-kr -i8" (stands for "K&R, 8 character indents"), or use
516"scripts/Lindent", which indents in the latest style.
517
518"indent" has a lot of options, and especially when it comes to comment
519re-formatting you may want to take a look at the man page. But
520remember: "indent" is not a fix for bad programming.
521
522
523        Chapter 10: Kconfig configuration files
524
525For all of the Kconfig* configuration files throughout the source tree,
526the indentation is somewhat different. Lines under a "config" definition
527are indented with one tab, while help text is indented an additional two
528spaces. Example:
529
530config AUDIT
531    bool "Auditing support"
532    depends on NET
533    help
534      Enable auditing infrastructure that can be used with another
535      kernel subsystem, such as SELinux (which requires this for
536      logging of avc messages output). Does not do system-call
537      auditing without CONFIG_AUDITSYSCALL.
538
539Features that might still be considered unstable should be defined as
540dependent on "EXPERIMENTAL":
541
542config SLUB
543    depends on EXPERIMENTAL && !ARCH_USES_SLAB_PAGE_STRUCT
544    bool "SLUB (Unqueued Allocator)"
545    ...
546
547while seriously dangerous features (such as write support for certain
548filesystems) should advertise this prominently in their prompt string:
549
550config ADFS_FS_RW
551    bool "ADFS write support (DANGEROUS)"
552    depends on ADFS_FS
553    ...
554
555For full documentation on the configuration files, see the file
556Documentation/kbuild/kconfig-language.txt.
557
558
559        Chapter 11: Data structures
560
561Data structures that have visibility outside the single-threaded
562environment they are created and destroyed in should always have
563reference counts. In the kernel, garbage collection doesn't exist (and
564outside the kernel garbage collection is slow and inefficient), which
565means that you absolutely _have_ to reference count all your uses.
566
567Reference counting means that you can avoid locking, and allows multiple
568users to have access to the data structure in parallel - and not having
569to worry about the structure suddenly going away from under them just
570because they slept or did something else for a while.
571
572Note that locking is _not_ a replacement for reference counting.
573Locking is used to keep data structures coherent, while reference
574counting is a memory management technique. Usually both are needed, and
575they are not to be confused with each other.
576
577Many data structures can indeed have two levels of reference counting,
578when there are users of different "classes". The subclass count counts
579the number of subclass users, and decrements the global count just once
580when the subclass count goes to zero.
581
582Examples of this kind of "multi-level-reference-counting" can be found in
583memory management ("struct mm_struct": mm_users and mm_count), and in
584filesystem code ("struct super_block": s_count and s_active).
585
586Remember: if another thread can find your data structure, and you don't
587have a reference count on it, you almost certainly have a bug.
588
589
590        Chapter 12: Macros, Enums and RTL
591
592Names of macros defining constants and labels in enums are capitalized.
593
594#define CONSTANT 0x12345
595
596Enums are preferred when defining several related constants.
597
598CAPITALIZED macro names are appreciated but macros resembling functions
599may be named in lower case.
600
601Generally, inline functions are preferable to macros resembling functions.
602
603Macros with multiple statements should be enclosed in a do - while block:
604
605#define macrofun(a, b, c) \
606    do { \
607        if (a == 5) \
608            do_this(b, c); \
609    } while (0)
610
611Things to avoid when using macros:
612
6131) macros that affect control flow:
614
615#define FOO(x) \
616    do { \
617        if (blah(x) < 0) \
618            return -EBUGGERED; \
619    } while(0)
620
621is a _very_ bad idea. It looks like a function call but exits the "calling"
622function; don't break the internal parsers of those who will read the code.
623
6242) macros that depend on having a local variable with a magic name:
625
626#define FOO(val) bar(index, val)
627
628might look like a good thing, but it's confusing as hell when one reads the
629code and it's prone to breakage from seemingly innocent changes.
630
6313) macros with arguments that are used as l-values: FOO(x) = y; will
632bite you if somebody e.g. turns FOO into an inline function.
633
6344) forgetting about precedence: macros defining constants using expressions
635must enclose the expression in parentheses. Beware of similar issues with
636macros using parameters.
637
638#define CONSTANT 0x4000
639#define CONSTEXP (CONSTANT | 3)
640
641The cpp manual deals with macros exhaustively. The gcc internals manual also
642covers RTL which is used frequently with assembly language in the kernel.
643
644
645        Chapter 13: Printing kernel messages
646
647Kernel developers like to be seen as literate. Do mind the spelling
648of kernel messages to make a good impression. Do not use crippled
649words like "dont"; use "do not" or "don't" instead. Make the messages
650concise, clear, and unambiguous.
651
652Kernel messages do not have to be terminated with a period.
653
654Printing numbers in parentheses (%d) adds no value and should be avoided.
655
656There are a number of driver model diagnostic macros in <linux/device.h>
657which you should use to make sure messages are matched to the right device
658and driver, and are tagged with the right level: dev_err(), dev_warn(),
659dev_info(), and so forth. For messages that aren't associated with a
660particular device, <linux/printk.h> defines pr_debug() and pr_info().
661
662Coming up with good debugging messages can be quite a challenge; and once
663you have them, they can be a huge help for remote troubleshooting. Such
664messages should be compiled out when the DEBUG symbol is not defined (that
665is, by default they are not included). When you use dev_dbg() or pr_debug(),
666that's automatic. Many subsystems have Kconfig options to turn on -DDEBUG.
667A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the
668ones already enabled by DEBUG.
669
670
671        Chapter 14: Allocating memory
672
673The kernel provides the following general purpose memory allocators:
674kmalloc(), kzalloc(), kcalloc(), vmalloc(), and vzalloc(). Please refer to
675the API documentation for further information about them.
676
677The preferred form for passing a size of a struct is the following:
678
679    p = kmalloc(sizeof(*p), ...);
680
681The alternative form where struct name is spelled out hurts readability and
682introduces an opportunity for a bug when the pointer variable type is changed
683but the corresponding sizeof that is passed to a memory allocator is not.
684
685Casting the return value which is a void pointer is redundant. The conversion
686from void pointer to any other pointer type is guaranteed by the C programming
687language.
688
689
690        Chapter 15: The inline disease
691
692There appears to be a common misperception that gcc has a magic "make me
693faster" speedup option called "inline". While the use of inlines can be
694appropriate (for example as a means of replacing macros, see Chapter 12), it
695very often is not. Abundant use of the inline keyword leads to a much bigger
696kernel, which in turn slows the system as a whole down, due to a bigger
697icache footprint for the CPU and simply because there is less memory
698available for the pagecache. Just think about it; a pagecache miss causes a
699disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
700that can go into these 5 milliseconds.
701
702A reasonable rule of thumb is to not put inline at functions that have more
703than 3 lines of code in them. An exception to this rule are the cases where
704a parameter is known to be a compiletime constant, and as a result of this
705constantness you *know* the compiler will be able to optimize most of your
706function away at compile time. For a good example of this later case, see
707the kmalloc() inline function.
708
709Often people argue that adding inline to functions that are static and used
710only once is always a win since there is no space tradeoff. While this is
711technically correct, gcc is capable of inlining these automatically without
712help, and the maintenance issue of removing the inline when a second user
713appears outweighs the potential value of the hint that tells gcc to do
714something it would have done anyway.
715
716
717        Chapter 16: Function return values and names
718
719Functions can return values of many different kinds, and one of the
720most common is a value indicating whether the function succeeded or
721failed. Such a value can be represented as an error-code integer
722(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
723non-zero = success).
724
725Mixing up these two sorts of representations is a fertile source of
726difficult-to-find bugs. If the C language included a strong distinction
727between integers and booleans then the compiler would find these mistakes
728for us... but it doesn't. To help prevent such bugs, always follow this
729convention:
730
731    If the name of a function is an action or an imperative command,
732    the function should return an error-code integer. If the name
733    is a predicate, the function should return a "succeeded" boolean.
734
735For example, "add work" is a command, and the add_work() function returns 0
736for success or -EBUSY for failure. In the same way, "PCI device present" is
737a predicate, and the pci_dev_present() function returns 1 if it succeeds in
738finding a matching device or 0 if it doesn't.
739
740All EXPORTed functions must respect this convention, and so should all
741public functions. Private (static) functions need not, but it is
742recommended that they do.
743
744Functions whose return value is the actual result of a computation, rather
745than an indication of whether the computation succeeded, are not subject to
746this rule. Generally they indicate failure by returning some out-of-range
747result. Typical examples would be functions that return pointers; they use
748NULL or the ERR_PTR mechanism to report failure.
749
750
751        Chapter 17: Don't re-invent the kernel macros
752
753The header file include/linux/kernel.h contains a number of macros that
754you should use, rather than explicitly coding some variant of them yourself.
755For example, if you need to calculate the length of an array, take advantage
756of the macro
757
758  #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
759
760Similarly, if you need to calculate the size of some structure member, use
761
762  #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
763
764There are also min() and max() macros that do strict type checking if you
765need them. Feel free to peruse that header file to see what else is already
766defined that you shouldn't reproduce in your code.
767
768
769        Chapter 18: Editor modelines and other cruft
770
771Some editors can interpret configuration information embedded in source files,
772indicated with special markers. For example, emacs interprets lines marked
773like this:
774
775-*- mode: c -*-
776
777Or like this:
778
779/*
780Local Variables:
781compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
782End:
783*/
784
785Vim interprets markers that look like this:
786
787/* vim:set sw=8 noet */
788
789Do not include any of these in source files. People have their own personal
790editor configurations, and your source files should not override them. This
791includes markers for indentation and mode configuration. People may use their
792own custom mode, or may have some other magic method for making indentation
793work correctly.
794
795
796
797        Appendix I: References
798
799The C Programming Language, Second Edition
800by Brian W. Kernighan and Dennis M. Ritchie.
801Prentice Hall, Inc., 1988.
802ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
803URL: http://cm.bell-labs.com/cm/cs/cbook/
804
805The Practice of Programming
806by Brian W. Kernighan and Rob Pike.
807Addison-Wesley, Inc., 1999.
808ISBN 0-201-61586-X.
809URL: http://cm.bell-labs.com/cm/cs/tpop/
810
811GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
812gcc internals and indent, all available from http://www.gnu.org/manual/
813
814WG14 is the international standardization working group for the programming
815language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
816
817Kernel CodingStyle, by greg@kroah.com at OLS 2002:
818http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
819
820

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