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.:
+
+ 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:
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.
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 hard limit.
+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.
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. Long strings are as well broken into shorter strings.
+argument list. Long strings are as well broken into shorter strings. The
+only exception to this is where exceeding 80 columns significantly increases
+readability and does not hide information.
void fun(int a, int b, int c)
{
next_statement;
}
- Chapter 3: Placing Braces
+ Chapter 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
we do y
}
+This applies to all non-function statement blocks (if, switch, for,
+while, do). E.g.:
+
+ 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:
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.
+
+if (condition)
+ action();
+
+This does not apply if one branch of a conditional statement is a single
+statement. Use braces in both branches.
+
+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.,
+ s = sizeof(struct file);
+
+Do not add spaces around (inside) parenthesized expressions. This example is
+*bad*:
+
+ 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:
+
+ 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.
+
Chapter 4: Naming
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 next chapter.
+See chapter 6 (Functions).
+
+
+ Chapter 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
- Chapter 5: Functions
+ vps_t a;
+
+in the source, what does it mean?
+
+In contrast, if it says
+
+ 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.
+
+
+ Chapter 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,
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.:
+
+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.
+
- Chapter 6: Centralized exiting of functions
+ Chapter 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.
modifications are prevented
- saves the compiler work to optimize redundant code away ;)
-int fun(int )
+int fun(int a)
{
int result = 0;
char *buffer = kmalloc(SIZE);
return result;
}
- Chapter 7: Commenting
+ Chapter 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
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 5 for a while. You can make
+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-doc-nano-HOWTO.txt and scripts/kernel-doc
+for details.
- Chapter 8: You've made a mess of it
+Linux style for comments is the C89 "/* ... */" style.
+Don't use C99-style "// ..." comments.
+
+The preferred style for long (multi-line) comments is:
+
+ /*
+ * 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.
+ */
+
+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.
+
+
+ Chapter 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
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:
-(defun linux-c-mode ()
- "C mode with adjusted defaults for use with the Linux kernel."
- (interactive)
- (c-mode)
- (c-set-style "K&R")
- (setq tab-width 8)
- (setq indent-tabs-mode t)
- (setq c-basic-offset 8))
-
-This will define the M-x linux-c-mode command. When hacking on a
-module, if you put the string -*- linux-c -*- somewhere on the first
-two lines, this mode will be automatically invoked. Also, you may want
-to add
-
-(setq auto-mode-alist (cons '("/usr/src/linux.*/.*\\.[ch]$" . linux-c-mode)
- auto-mode-alist))
-
-to your .emacs file if you want to have linux-c-mode switched on
-automagically when you edit source files under /usr/src/linux.
+(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)
+ (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".
remember: "indent" is not a fix for bad programming.
- Chapter 9: Configuration-files
+ Chapter 10: Kconfig configuration files
-For configuration options (arch/xxx/Kconfig, and all the Kconfig files),
-somewhat different indentation is used.
+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:
-Help text is indented with 2 spaces.
-
-if CONFIG_EXPERIMENTAL
- tristate CONFIG_BOOM
- default n
- help
- Apply nitroglycerine inside the keyboard (DANGEROUS)
- bool CONFIG_CHEER
- depends on CONFIG_BOOM
- default y
+config AUDIT
+ bool "Auditing support"
+ depends on NET
help
- Output nice messages when you explode
-endif
+ 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.
-Generally, CONFIG_EXPERIMENTAL should surround all options not considered
-stable. All options that are known to trash data (experimental write-
-support for file-systems, for instance) should be denoted (DANGEROUS), other
-experimental options should be denoted (EXPERIMENTAL).
+Features that might still be considered unstable should be defined as
+dependent on "EXPERIMENTAL":
+
+config SLUB
+ depends on EXPERIMENTAL && !ARCH_USES_SLAB_PAGE_STRUCT
+ bool "SLUB (Unqueued Allocator)"
+ ...
+
+while 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.
- Chapter 10: Data structures
+
+ Chapter 11: Data structures
Data structures that have visibility outside the single-threaded
environment they are created and destroyed in should always have
have a reference count on it, you almost certainly have a bug.
- Chapter 11: Macros, Enums, Inline functions and RTL
+ Chapter 12: Macros, Enums and RTL
Names of macros defining constants and labels in enums are capitalized.
covers RTL which is used frequently with assembly language in the kernel.
- Chapter 12: Printing kernel messages
+ Chapter 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" and use "do not" or "don't" instead.
+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/kernel.h> defines pr_debug() and pr_info().
+
+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. Such
+messages should be compiled out when the DEBUG symbol is not defined (that
+is, by default they are not included). When you use dev_dbg() or pr_debug(),
+that's automatic. Many subsystems have Kconfig options to turn on -DDEBUG.
+A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the
+ones already enabled by DEBUG.
+
+
+ Chapter 14: Allocating memory
+
+The kernel provides the following general purpose memory allocators:
+kmalloc(), kzalloc(), kcalloc(), and vmalloc(). Please refer to the API
+documentation for further information about them.
+
+The preferred form for passing a size of a struct is the following:
+
+ 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.
+
+
+ Chapter 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.
- Chapter 13: References
+
+ Chapter 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.
+
+
+ Chapter 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
+
+ #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+
+Similarly, if you need to calculate the size of some structure member, use
+
+ #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.
+
+
+ Chapter 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:
+
+-*- mode: c -*-
+
+Or like this:
+
+/*
+Local Variables:
+compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
+End:
+*/
+
+Vim interprets markers that look like this:
+
+/* 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.
+
+
+
+ Appendix I: References
The C Programming Language, Second Edition
by Brian W. Kernighan and Dennis M. Ritchie.
URL: http://cm.bell-labs.com/cm/cs/tpop/
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
+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://std.dkuug.dk/JTC1/SC22/WG14/
+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/
--
-Last updated on 16 February 2004 by a community effort on LKML.
+Last updated on 2007-July-13.
+
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff