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  • linux c coding style

    		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:
    
    
    	 	Chapter 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.:
    
    	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.
    
    Get a decent editor and don't leave whitespace at the end of lines.
    
    
    		Chapter 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.
    
    
    		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
    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:
    
    	if (x is true) {
    		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:
    
    	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:
    
    	do {
    		body of do-loop
    	} while (condition);
    
    and
    
    	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.
    
    if (condition)
    	action();
    
    and
    
    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:
    
    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
    
    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).
    
    
    		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
    
    	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,
    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.:
    
    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 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.
    
    The rationale 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 ;)
    
    int fun(int a)
    {
    	int result = 0;
    	char *buffer = kmalloc(SIZE);
    
    	if (buffer == NULL)
    		return -ENOMEM;
    
    	if (condition1) {
    		while (loop1) {
    			...
    		}
    		result = 1;
    		goto out;
    	}
    	...
    out:
    	kfree(buffer);
    	return result;
    }
    
    		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
    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-doc-nano-HOWTO.txt and scripts/kernel-doc
    for details.
    
    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.
    	 */
    
    For files in net/ and drivers/net/ the preferred style for long (multi-line)
    comments is a little different.
    
    	/* 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.
    
    
    		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
    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:
    
    (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".
    
    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.
    
    
    		Chapter 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.
    
    
    		Chapter 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.
    
    
    		Chapter 12: Macros, Enums and RTL
    
    Names of macros defining constants and labels in enums are capitalized.
    
    #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:
    
    #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:
    
    #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:
    
    #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.
    
    #define CONSTANT 0x4000
    #define CONSTEXP (CONSTANT | 3)
    
    The cpp manual deals with macros exhaustively. The gcc internals manual also
    covers RTL which is used frequently with assembly language in the kernel.
    
    
    		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"; 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_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(), 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:
    
    	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:
    
    	p = kmalloc_array(n, sizeof(...), ...);
    
    The preferred form for allocating a zeroed array is the following:
    
    	p = kcalloc(n, sizeof(...), ...);
    
    Both forms check for overflow on the allocation size n * sizeof(...),
    and return NULL if that occurred.
    
    
    		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 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.
    
    
    		Chapter 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 
    	 to properly indent the
    next instruction in the assembly output:
    
    	asm ("magic %reg1, #42
    	"
    	     "more_magic %reg2, %reg3"
    	     : /* outputs */ : /* inputs */ : /* clobbers */);
    
    
    
    		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).
    URL: http://cm.bell-labs.com/cm/cs/cbook/
    
    The Practice of Programming
    by Brian W. Kernighan and Rob Pike.
    Addison-Wesley, Inc., 1999.
    ISBN 0-201-61586-X.
    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/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/
    
    
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