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diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle index 9c61c039ccd9..320983ca114e 100644 --- a/Documentation/CodingStyle +++ b/Documentation/CodingStyle @@ -1,1062 +1 @@ -.. _codingstyle: - -Linux kernel coding style -========================= - -This is a short document describing the preferred coding style for the -linux kernel. Coding style is very personal, and I won't **force** my -views on anybody, but this is what goes for anything that I have to be -able to maintain, and I'd prefer it for most other things too. Please -at least consider the points made here. - -First off, I'd suggest printing out a copy of the GNU coding standards, -and NOT read it. Burn them, it's a great symbolic gesture. - -Anyway, here goes: - - -1) Indentation --------------- - -Tabs are 8 characters, and thus indentations are also 8 characters. -There are heretic movements that try to make indentations 4 (or even 2!) -characters deep, and that is akin to trying to define the value of PI to -be 3. - -Rationale: The whole idea behind indentation is to clearly define where -a block of control starts and ends. Especially when you've been looking -at your screen for 20 straight hours, you'll find it a lot easier to see -how the indentation works if you have large indentations. - -Now, some people will claim that having 8-character indentations makes -the code move too far to the right, and makes it hard to read on a -80-character terminal screen. The answer to that is that if you need -more than 3 levels of indentation, you're screwed anyway, and should fix -your program. - -In short, 8-char indents make things easier to read, and have the added -benefit of warning you when you're nesting your functions too deep. -Heed that warning. - -The preferred way to ease multiple indentation levels in a switch statement is -to align the ``switch`` and its subordinate ``case`` labels in the same column -instead of ``double-indenting`` the ``case`` labels. E.g.: - -.. code-block:: c - - switch (suffix) { - case 'G': - case 'g': - mem <<= 30; - break; - case 'M': - case 'm': - mem <<= 20; - break; - case 'K': - case 'k': - mem <<= 10; - /* fall through */ - default: - break; - } - -Don't put multiple statements on a single line unless you have -something to hide: - -.. code-block:: c - - if (condition) do_this; - do_something_everytime; - -Don't put multiple assignments on a single line either. Kernel coding style -is super simple. Avoid tricky expressions. - -Outside of comments, documentation and except in Kconfig, spaces are never -used for indentation, and the above example is deliberately broken. - -Get a decent editor and don't leave whitespace at the end of lines. - - -2) Breaking long lines and strings ----------------------------------- - -Coding style is all about readability and maintainability using commonly -available tools. - -The limit on the length of lines is 80 columns and this is a strongly -preferred limit. - -Statements longer than 80 columns will be broken into sensible chunks, unless -exceeding 80 columns significantly increases readability and does not hide -information. Descendants are always substantially shorter than the parent and -are placed substantially to the right. The same applies to function headers -with a long argument list. However, never break user-visible strings such as -printk messages, because that breaks the ability to grep for them. - - -3) Placing Braces and Spaces ----------------------------- - -The other issue that always comes up in C styling is the placement of -braces. Unlike the indent size, there are few technical reasons to -choose one placement strategy over the other, but the preferred way, as -shown to us by the prophets Kernighan and Ritchie, is to put the opening -brace last on the line, and put the closing brace first, thusly: - -.. code-block:: c - - if (x is true) { - we do y - } - -This applies to all non-function statement blocks (if, switch, for, -while, do). E.g.: - -.. code-block:: c - - switch (action) { - case KOBJ_ADD: - return "add"; - case KOBJ_REMOVE: - return "remove"; - case KOBJ_CHANGE: - return "change"; - default: - return NULL; - } - -However, there is one special case, namely functions: they have the -opening brace at the beginning of the next line, thus: - -.. code-block:: c - - int function(int x) - { - body of function - } - -Heretic people all over the world have claimed that this inconsistency -is ... well ... inconsistent, but all right-thinking people know that -(a) K&R are **right** and (b) K&R are right. Besides, functions are -special anyway (you can't nest them in C). - -Note that the closing brace is empty on a line of its own, **except** in -the cases where it is followed by a continuation of the same statement, -ie a ``while`` in a do-statement or an ``else`` in an if-statement, like -this: - -.. code-block:: c - - do { - body of do-loop - } while (condition); - -and - -.. code-block:: c - - if (x == y) { - .. - } else if (x > y) { - ... - } else { - .... - } - -Rationale: K&R. - -Also, note that this brace-placement also minimizes the number of empty -(or almost empty) lines, without any loss of readability. Thus, as the -supply of new-lines on your screen is not a renewable resource (think -25-line terminal screens here), you have more empty lines to put -comments on. - -Do not unnecessarily use braces where a single statement will do. - -.. code-block:: c - - if (condition) - action(); - -and - -.. code-block:: none - - if (condition) - do_this(); - else - do_that(); - -This does not apply if only one branch of a conditional statement is a single -statement; in the latter case use braces in both branches: - -.. code-block:: c - - if (condition) { - do_this(); - do_that(); - } else { - otherwise(); - } - -3.1) Spaces -*********** - -Linux kernel style for use of spaces depends (mostly) on -function-versus-keyword usage. Use a space after (most) keywords. The -notable exceptions are sizeof, typeof, alignof, and __attribute__, which look -somewhat like functions (and are usually used with parentheses in Linux, -although they are not required in the language, as in: ``sizeof info`` after -``struct fileinfo info;`` is declared). - -So use a space after these keywords:: - - if, switch, case, for, do, while - -but not with sizeof, typeof, alignof, or __attribute__. E.g., - -.. code-block:: c - - - s = sizeof(struct file); - -Do not add spaces around (inside) parenthesized expressions. This example is -**bad**: - -.. code-block:: c - - - s = sizeof( struct file ); - -When declaring pointer data or a function that returns a pointer type, the -preferred use of ``*`` is adjacent to the data name or function name and not -adjacent to the type name. Examples: - -.. code-block:: c - - - char *linux_banner; - unsigned long long memparse(char *ptr, char **retptr); - char *match_strdup(substring_t *s); - -Use one space around (on each side of) most binary and ternary operators, -such as any of these:: - - = + - < > * / % | & ^ <= >= == != ? : - -but no space after unary operators:: - - & * + - ~ ! sizeof typeof alignof __attribute__ defined - -no space before the postfix increment & decrement unary operators:: - - ++ -- - -no space after the prefix increment & decrement unary operators:: - - ++ -- - -and no space around the ``.`` and ``->`` structure member operators. - -Do not leave trailing whitespace at the ends of lines. Some editors with -``smart`` indentation will insert whitespace at the beginning of new lines as -appropriate, so you can start typing the next line of code right away. -However, some such editors do not remove the whitespace if you end up not -putting a line of code there, such as if you leave a blank line. As a result, -you end up with lines containing trailing whitespace. - -Git will warn you about patches that introduce trailing whitespace, and can -optionally strip the trailing whitespace for you; however, if applying a series -of patches, this may make later patches in the series fail by changing their -context lines. - - -4) Naming ---------- - -C is a Spartan language, and so should your naming be. Unlike Modula-2 -and Pascal programmers, C programmers do not use cute names like -ThisVariableIsATemporaryCounter. A C programmer would call that -variable ``tmp``, which is much easier to write, and not the least more -difficult to understand. - -HOWEVER, while mixed-case names are frowned upon, descriptive names for -global variables are a must. To call a global function ``foo`` is a -shooting offense. - -GLOBAL variables (to be used only if you **really** need them) need to -have descriptive names, as do global functions. If you have a function -that counts the number of active users, you should call that -``count_active_users()`` or similar, you should **not** call it ``cntusr()``. - -Encoding the type of a function into the name (so-called Hungarian -notation) is brain damaged - the compiler knows the types anyway and can -check those, and it only confuses the programmer. No wonder MicroSoft -makes buggy programs. - -LOCAL variable names should be short, and to the point. If you have -some random integer loop counter, it should probably be called ``i``. -Calling it ``loop_counter`` is non-productive, if there is no chance of it -being mis-understood. Similarly, ``tmp`` can be just about any type of -variable that is used to hold a temporary value. - -If you are afraid to mix up your local variable names, you have another -problem, which is called the function-growth-hormone-imbalance syndrome. -See chapter 6 (Functions). - - -5) Typedefs ------------ - -Please don't use things like ``vps_t``. -It's a **mistake** to use typedef for structures and pointers. When you see a - -.. code-block:: c - - - vps_t a; - -in the source, what does it mean? -In contrast, if it says - -.. code-block:: c - - struct virtual_container *a; - -you can actually tell what ``a`` is. - -Lots of people think that typedefs ``help readability``. Not so. They are -useful only for: - - (a) totally opaque objects (where the typedef is actively used to **hide** - what the object is). - - Example: ``pte_t`` etc. opaque objects that you can only access using - the proper accessor functions. - - .. note:: - - Opaqueness and ``accessor functions`` are not good in themselves. - The reason we have them for things like pte_t etc. is that there - really is absolutely **zero** portably accessible information there. - - (b) Clear integer types, where the abstraction **helps** avoid confusion - whether it is ``int`` or ``long``. - - u8/u16/u32 are perfectly fine typedefs, although they fit into - category (d) better than here. - - .. note:: - - Again - there needs to be a **reason** for this. If something is - ``unsigned long``, then there's no reason to do - - typedef unsigned long myflags_t; - - but if there is a clear reason for why it under certain circumstances - might be an ``unsigned int`` and under other configurations might be - ``unsigned long``, then by all means go ahead and use a typedef. - - (c) when you use sparse to literally create a **new** type for - type-checking. - - (d) New types which are identical to standard C99 types, in certain - exceptional circumstances. - - Although it would only take a short amount of time for the eyes and - brain to become accustomed to the standard types like ``uint32_t``, - some people object to their use anyway. - - Therefore, the Linux-specific ``u8/u16/u32/u64`` types and their - signed equivalents which are identical to standard types are - permitted -- although they are not mandatory in new code of your - own. - - When editing existing code which already uses one or the other set - of types, you should conform to the existing choices in that code. - - (e) Types safe for use in userspace. - - In certain structures which are visible to userspace, we cannot - require C99 types and cannot use the ``u32`` form above. Thus, we - use __u32 and similar types in all structures which are shared - with userspace. - -Maybe there are other cases too, but the rule should basically be to NEVER -EVER use a typedef unless you can clearly match one of those rules. - -In general, a pointer, or a struct that has elements that can reasonably -be directly accessed should **never** be a typedef. - - -6) Functions ------------- - -Functions should be short and sweet, and do just one thing. They should -fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24, -as we all know), and do one thing and do that well. - -The maximum length of a function is inversely proportional to the -complexity and indentation level of that function. So, if you have a -conceptually simple function that is just one long (but simple) -case-statement, where you have to do lots of small things for a lot of -different cases, it's OK to have a longer function. - -However, if you have a complex function, and you suspect that a -less-than-gifted first-year high-school student might not even -understand what the function is all about, you should adhere to the -maximum limits all the more closely. Use helper functions with -descriptive names (you can ask the compiler to in-line them if you think -it's performance-critical, and it will probably do a better job of it -than you would have done). - -Another measure of the function is the number of local variables. They -shouldn't exceed 5-10, or you're doing something wrong. Re-think the -function, and split it into smaller pieces. A human brain can -generally easily keep track of about 7 different things, anything more -and it gets confused. You know you're brilliant, but maybe you'd like -to understand what you did 2 weeks from now. - -In source files, separate functions with one blank line. If the function is -exported, the **EXPORT** macro for it should follow immediately after the -closing function brace line. E.g.: - -.. code-block:: c - - int system_is_up(void) - { - return system_state == SYSTEM_RUNNING; - } - EXPORT_SYMBOL(system_is_up); - -In function prototypes, include parameter names with their data types. -Although this is not required by the C language, it is preferred in Linux -because it is a simple way to add valuable information for the reader. - - -7) Centralized exiting of functions ------------------------------------ - -Albeit deprecated by some people, the equivalent of the goto statement is -used frequently by compilers in form of the unconditional jump instruction. - -The goto statement comes in handy when a function exits from multiple -locations and some common work such as cleanup has to be done. If there is no -cleanup needed then just return directly. - -Choose label names which say what the goto does or why the goto exists. An -example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``. -Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to -renumber them if you ever add or remove exit paths, and they make correctness -difficult to verify anyway. - -The rationale for using gotos is: - -- unconditional statements are easier to understand and follow -- nesting is reduced -- errors by not updating individual exit points when making - modifications are prevented -- saves the compiler work to optimize redundant code away ;) - -.. code-block:: c - - int fun(int a) - { - int result = 0; - char *buffer; - - buffer = kmalloc(SIZE, GFP_KERNEL); - if (!buffer) - return -ENOMEM; - - if (condition1) { - while (loop1) { - ... - } - result = 1; - goto out_buffer; - } - ... - out_free_buffer: - kfree(buffer); - return result; - } - -A common type of bug to be aware of is ``one err bugs`` which look like this: - -.. code-block:: c - - err: - kfree(foo->bar); - kfree(foo); - return ret; - -The bug in this code is that on some exit paths ``foo`` is NULL. Normally the -fix for this is to split it up into two error labels ``err_free_bar:`` and -``err_free_foo:``: - -.. code-block:: c - - err_free_bar: - kfree(foo->bar); - err_free_foo: - kfree(foo); - return ret; - -Ideally you should simulate errors to test all exit paths. - - -8) Commenting -------------- - -Comments are good, but there is also a danger of over-commenting. NEVER -try to explain HOW your code works in a comment: it's much better to -write the code so that the **working** is obvious, and it's a waste of -time to explain badly written code. - -Generally, you want your comments to tell WHAT your code does, not HOW. -Also, try to avoid putting comments inside a function body: if the -function is so complex that you need to separately comment parts of it, -you should probably go back to chapter 6 for a while. You can make -small comments to note or warn about something particularly clever (or -ugly), but try to avoid excess. Instead, put the comments at the head -of the function, telling people what it does, and possibly WHY it does -it. - -When commenting the kernel API functions, please use the kernel-doc format. -See the files Documentation/kernel-documentation.rst and scripts/kernel-doc -for details. - -The preferred style for long (multi-line) comments is: - -.. code-block:: c - - /* - * This is the preferred style for multi-line - * comments in the Linux kernel source code. - * Please use it consistently. - * - * Description: A column of asterisks on the left side, - * with beginning and ending almost-blank lines. - */ - -For files in net/ and drivers/net/ the preferred style for long (multi-line) -comments is a little different. - -.. code-block:: c - - /* The preferred comment style for files in net/ and drivers/net - * looks like this. - * - * It is nearly the same as the generally preferred comment style, - * but there is no initial almost-blank line. - */ - -It's also important to comment data, whether they are basic types or derived -types. To this end, use just one data declaration per line (no commas for -multiple data declarations). This leaves you room for a small comment on each -item, explaining its use. - - -9) You've made a mess of it ---------------------------- - -That's OK, we all do. You've probably been told by your long-time Unix -user helper that ``GNU emacs`` automatically formats the C sources for -you, and you've noticed that yes, it does do that, but the defaults it -uses are less than desirable (in fact, they are worse than random -typing - an infinite number of monkeys typing into GNU emacs would never -make a good program). - -So, you can either get rid of GNU emacs, or change it to use saner -values. To do the latter, you can stick the following in your .emacs file: - -.. code-block:: none - - (defun c-lineup-arglist-tabs-only (ignored) - "Line up argument lists by tabs, not spaces" - (let* ((anchor (c-langelem-pos c-syntactic-element)) - (column (c-langelem-2nd-pos c-syntactic-element)) - (offset (- (1+ column) anchor)) - (steps (floor offset c-basic-offset))) - (* (max steps 1) - c-basic-offset))) - - (add-hook 'c-mode-common-hook - (lambda () - ;; Add kernel style - (c-add-style - "linux-tabs-only" - '("linux" (c-offsets-alist - (arglist-cont-nonempty - c-lineup-gcc-asm-reg - c-lineup-arglist-tabs-only)))))) - - (add-hook 'c-mode-hook - (lambda () - (let ((filename (buffer-file-name))) - ;; Enable kernel mode for the appropriate files - (when (and filename - (string-match (expand-file-name "~/src/linux-trees") - filename)) - (setq indent-tabs-mode t) - (setq show-trailing-whitespace t) - (c-set-style "linux-tabs-only"))))) - -This will make emacs go better with the kernel coding style for C -files below ``~/src/linux-trees``. - -But even if you fail in getting emacs to do sane formatting, not -everything is lost: use ``indent``. - -Now, again, GNU indent has the same brain-dead settings that GNU emacs -has, which is why you need to give it a few command line options. -However, that's not too bad, because even the makers of GNU indent -recognize the authority of K&R (the GNU people aren't evil, they are -just severely misguided in this matter), so you just give indent the -options ``-kr -i8`` (stands for ``K&R, 8 character indents``), or use -``scripts/Lindent``, which indents in the latest style. - -``indent`` has a lot of options, and especially when it comes to comment -re-formatting you may want to take a look at the man page. But -remember: ``indent`` is not a fix for bad programming. - - -10) Kconfig configuration files -------------------------------- - -For all of the Kconfig* configuration files throughout the source tree, -the indentation is somewhat different. Lines under a ``config`` definition -are indented with one tab, while help text is indented an additional two -spaces. Example:: - - config AUDIT - bool "Auditing support" - depends on NET - help - Enable auditing infrastructure that can be used with another - kernel subsystem, such as SELinux (which requires this for - logging of avc messages output). Does not do system-call - auditing without CONFIG_AUDITSYSCALL. - -Seriously dangerous features (such as write support for certain -filesystems) should advertise this prominently in their prompt string:: - - config ADFS_FS_RW - bool "ADFS write support (DANGEROUS)" - depends on ADFS_FS - ... - -For full documentation on the configuration files, see the file -Documentation/kbuild/kconfig-language.txt. - - -11) Data structures -------------------- - -Data structures that have visibility outside the single-threaded -environment they are created and destroyed in should always have -reference counts. In the kernel, garbage collection doesn't exist (and -outside the kernel garbage collection is slow and inefficient), which -means that you absolutely **have** to reference count all your uses. - -Reference counting means that you can avoid locking, and allows multiple -users to have access to the data structure in parallel - and not having -to worry about the structure suddenly going away from under them just -because they slept or did something else for a while. - -Note that locking is **not** a replacement for reference counting. -Locking is used to keep data structures coherent, while reference -counting is a memory management technique. Usually both are needed, and -they are not to be confused with each other. - -Many data structures can indeed have two levels of reference counting, -when there are users of different ``classes``. The subclass count counts -the number of subclass users, and decrements the global count just once -when the subclass count goes to zero. - -Examples of this kind of ``multi-level-reference-counting`` can be found in -memory management (``struct mm_struct``: mm_users and mm_count), and in -filesystem code (``struct super_block``: s_count and s_active). - -Remember: if another thread can find your data structure, and you don't -have a reference count on it, you almost certainly have a bug. - - -12) Macros, Enums and RTL -------------------------- - -Names of macros defining constants and labels in enums are capitalized. - -.. code-block:: c - - #define CONSTANT 0x12345 - -Enums are preferred when defining several related constants. - -CAPITALIZED macro names are appreciated but macros resembling functions -may be named in lower case. - -Generally, inline functions are preferable to macros resembling functions. - -Macros with multiple statements should be enclosed in a do - while block: - -.. code-block:: c - - #define macrofun(a, b, c) \ - do { \ - if (a == 5) \ - do_this(b, c); \ - } while (0) - -Things to avoid when using macros: - -1) macros that affect control flow: - -.. code-block:: c - - #define FOO(x) \ - do { \ - if (blah(x) < 0) \ - return -EBUGGERED; \ - } while (0) - -is a **very** bad idea. It looks like a function call but exits the ``calling`` -function; don't break the internal parsers of those who will read the code. - -2) macros that depend on having a local variable with a magic name: - -.. code-block:: c - - #define FOO(val) bar(index, val) - -might look like a good thing, but it's confusing as hell when one reads the -code and it's prone to breakage from seemingly innocent changes. - -3) macros with arguments that are used as l-values: FOO(x) = y; will -bite you if somebody e.g. turns FOO into an inline function. - -4) forgetting about precedence: macros defining constants using expressions -must enclose the expression in parentheses. Beware of similar issues with -macros using parameters. - -.. code-block:: c - - #define CONSTANT 0x4000 - #define CONSTEXP (CONSTANT | 3) - -5) namespace collisions when defining local variables in macros resembling -functions: - -.. code-block:: c - - #define FOO(x) \ - ({ \ - typeof(x) ret; \ - ret = calc_ret(x); \ - (ret); \ - }) - -ret is a common name for a local variable - __foo_ret is less likely -to collide with an existing variable. - -The cpp manual deals with macros exhaustively. The gcc internals manual also -covers RTL which is used frequently with assembly language in the kernel. - - -13) Printing kernel messages ----------------------------- - -Kernel developers like to be seen as literate. Do mind the spelling -of kernel messages to make a good impression. Do not use crippled -words like ``dont``; use ``do not`` or ``don't`` instead. Make the messages -concise, clear, and unambiguous. - -Kernel messages do not have to be terminated with a period. - -Printing numbers in parentheses (%d) adds no value and should be avoided. - -There are a number of driver model diagnostic macros in <linux/device.h> -which you should use to make sure messages are matched to the right device -and driver, and are tagged with the right level: dev_err(), dev_warn(), -dev_info(), and so forth. For messages that aren't associated with a -particular device, <linux/printk.h> defines pr_notice(), pr_info(), -pr_warn(), pr_err(), etc. - -Coming up with good debugging messages can be quite a challenge; and once -you have them, they can be a huge help for remote troubleshooting. However -debug message printing is handled differently than printing other non-debug -messages. While the other pr_XXX() functions print unconditionally, -pr_debug() does not; it is compiled out by default, unless either DEBUG is -defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also, -and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to -the ones already enabled by DEBUG. - -Many subsystems have Kconfig debug options to turn on -DDEBUG in the -corresponding Makefile; in other cases specific files #define DEBUG. And -when a debug message should be unconditionally printed, such as if it is -already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be -used. - - -14) Allocating memory ---------------------- - -The kernel provides the following general purpose memory allocators: -kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and -vzalloc(). Please refer to the API documentation for further information -about them. - -The preferred form for passing a size of a struct is the following: - -.. code-block:: c - - p = kmalloc(sizeof(*p), ...); - -The alternative form where struct name is spelled out hurts readability and -introduces an opportunity for a bug when the pointer variable type is changed -but the corresponding sizeof that is passed to a memory allocator is not. - -Casting the return value which is a void pointer is redundant. The conversion -from void pointer to any other pointer type is guaranteed by the C programming -language. - -The preferred form for allocating an array is the following: - -.. code-block:: c - - p = kmalloc_array(n, sizeof(...), ...); - -The preferred form for allocating a zeroed array is the following: - -.. code-block:: c - - p = kcalloc(n, sizeof(...), ...); - -Both forms check for overflow on the allocation size n * sizeof(...), -and return NULL if that occurred. - - -15) The inline disease ----------------------- - -There appears to be a common misperception that gcc has a magic "make me -faster" speedup option called ``inline``. While the use of inlines can be -appropriate (for example as a means of replacing macros, see Chapter 12), it -very often is not. Abundant use of the inline keyword leads to a much bigger -kernel, which in turn slows the system as a whole down, due to a bigger -icache footprint for the CPU and simply because there is less memory -available for the pagecache. Just think about it; a pagecache miss causes a -disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles -that can go into these 5 milliseconds. - -A reasonable rule of thumb is to not put inline at functions that have more -than 3 lines of code in them. An exception to this rule are the cases where -a parameter is known to be a compiletime constant, and as a result of this -constantness you *know* the compiler will be able to optimize most of your -function away at compile time. For a good example of this later case, see -the kmalloc() inline function. - -Often people argue that adding inline to functions that are static and used -only once is always a win since there is no space tradeoff. While this is -technically correct, gcc is capable of inlining these automatically without -help, and the maintenance issue of removing the inline when a second user -appears outweighs the potential value of the hint that tells gcc to do -something it would have done anyway. - - -16) Function return values and names ------------------------------------- - -Functions can return values of many different kinds, and one of the -most common is a value indicating whether the function succeeded or -failed. Such a value can be represented as an error-code integer -(-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure, -non-zero = success). - -Mixing up these two sorts of representations is a fertile source of -difficult-to-find bugs. If the C language included a strong distinction -between integers and booleans then the compiler would find these mistakes -for us... but it doesn't. To help prevent such bugs, always follow this -convention:: - - If the name of a function is an action or an imperative command, - the function should return an error-code integer. If the name - is a predicate, the function should return a "succeeded" boolean. - -For example, ``add work`` is a command, and the add_work() function returns 0 -for success or -EBUSY for failure. In the same way, ``PCI device present`` is -a predicate, and the pci_dev_present() function returns 1 if it succeeds in -finding a matching device or 0 if it doesn't. - -All EXPORTed functions must respect this convention, and so should all -public functions. Private (static) functions need not, but it is -recommended that they do. - -Functions whose return value is the actual result of a computation, rather -than an indication of whether the computation succeeded, are not subject to -this rule. Generally they indicate failure by returning some out-of-range -result. Typical examples would be functions that return pointers; they use -NULL or the ERR_PTR mechanism to report failure. - - -17) Don't re-invent the kernel macros -------------------------------------- - -The header file include/linux/kernel.h contains a number of macros that -you should use, rather than explicitly coding some variant of them yourself. -For example, if you need to calculate the length of an array, take advantage -of the macro - -.. code-block:: c - - #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0])) - -Similarly, if you need to calculate the size of some structure member, use - -.. code-block:: c - - #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f)) - -There are also min() and max() macros that do strict type checking if you -need them. Feel free to peruse that header file to see what else is already -defined that you shouldn't reproduce in your code. - - -18) Editor modelines and other cruft ------------------------------------- - -Some editors can interpret configuration information embedded in source files, -indicated with special markers. For example, emacs interprets lines marked -like this: - -.. code-block:: c - - -*- mode: c -*- - -Or like this: - -.. code-block:: c - - /* - Local Variables: - compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c" - End: - */ - -Vim interprets markers that look like this: - -.. code-block:: c - - /* vim:set sw=8 noet */ - -Do not include any of these in source files. People have their own personal -editor configurations, and your source files should not override them. This -includes markers for indentation and mode configuration. People may use their -own custom mode, or may have some other magic method for making indentation -work correctly. - - -19) Inline assembly -------------------- - -In architecture-specific code, you may need to use inline assembly to interface -with CPU or platform functionality. Don't hesitate to do so when necessary. -However, don't use inline assembly gratuitously when C can do the job. You can -and should poke hardware from C when possible. - -Consider writing simple helper functions that wrap common bits of inline -assembly, rather than repeatedly writing them with slight variations. Remember -that inline assembly can use C parameters. - -Large, non-trivial assembly functions should go in .S files, with corresponding -C prototypes defined in C header files. The C prototypes for assembly -functions should use ``asmlinkage``. - -You may need to mark your asm statement as volatile, to prevent GCC from -removing it if GCC doesn't notice any side effects. You don't always need to -do so, though, and doing so unnecessarily can limit optimization. - -When writing a single inline assembly statement containing multiple -instructions, put each instruction on a separate line in a separate quoted -string, and end each string except the last with \n\t to properly indent the -next instruction in the assembly output: - -.. code-block:: c - - asm ("magic %reg1, #42\n\t" - "more_magic %reg2, %reg3" - : /* outputs */ : /* inputs */ : /* clobbers */); - - -20) Conditional Compilation ---------------------------- - -Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c -files; doing so makes code harder to read and logic harder to follow. Instead, -use such conditionals in a header file defining functions for use in those .c -files, providing no-op stub versions in the #else case, and then call those -functions unconditionally from .c files. The compiler will avoid generating -any code for the stub calls, producing identical results, but the logic will -remain easy to follow. - -Prefer to compile out entire functions, rather than portions of functions or -portions of expressions. Rather than putting an ifdef in an expression, factor -out part or all of the expression into a separate helper function and apply the -conditional to that function. - -If you have a function or variable which may potentially go unused in a -particular configuration, and the compiler would warn about its definition -going unused, mark the definition as __maybe_unused rather than wrapping it in -a preprocessor conditional. (However, if a function or variable *always* goes -unused, delete it.) - -Within code, where possible, use the IS_ENABLED macro to convert a Kconfig -symbol into a C boolean expression, and use it in a normal C conditional: - -.. code-block:: c - - if (IS_ENABLED(CONFIG_SOMETHING)) { - ... - } - -The compiler will constant-fold the conditional away, and include or exclude -the block of code just as with an #ifdef, so this will not add any runtime -overhead. However, this approach still allows the C compiler to see the code -inside the block, and check it for correctness (syntax, types, symbol -references, etc). Thus, you still have to use an #ifdef if the code inside the -block references symbols that will not exist if the condition is not met. - -At the end of any non-trivial #if or #ifdef block (more than a few lines), -place a comment after the #endif on the same line, noting the conditional -expression used. For instance: - -.. code-block:: c - - #ifdef CONFIG_SOMETHING - ... - #endif /* CONFIG_SOMETHING */ - - -Appendix I) References ----------------------- - -The C Programming Language, Second Edition -by Brian W. Kernighan and Dennis M. Ritchie. -Prentice Hall, Inc., 1988. -ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback). - -The Practice of Programming -by Brian W. Kernighan and Rob Pike. -Addison-Wesley, Inc., 1999. -ISBN 0-201-61586-X. - -GNU manuals - where in compliance with K&R and this text - for cpp, gcc, -gcc internals and indent, all available from http://www.gnu.org/manual/ - -WG14 is the international standardization working group for the programming -language C, URL: http://www.open-std.org/JTC1/SC22/WG14/ - -Kernel CodingStyle, by greg@kroah.com at OLS 2002: -http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/ +This file has moved to process/coding-style.rst |