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C — Coding Style

Note: This is taken from Linux Kernel Coding Style document. The document is actually for Linux Kernel Style, but is also a good document for C coding style. So, I took parts of it which are general enough to be used in C coding.

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.

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.

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();
}

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.

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).

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:

  1. 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.

  2. 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.

  3. when you use sparse to literally create a new type for type-checking.

  4. 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.

  5. 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.

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.

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:

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_free_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:

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::

 err_free_bar:
	kfree(foo->bar);
 err_free_foo:
	kfree(foo);
	return ret;

Ideally you should simulate errors to test all exit paths.

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 at Documentation/doc-guide/ and scripts/kernel-doc for details.

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.

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)
    
  5. namespace collisions when defining local variables in macros resembling functions:

     #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.

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.

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.

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:

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:

#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 here.

WG14 is the international standardization working group for the programming language C, URL: Link.

Kernel process/coding-style.rst, by greg@kroah.com at OLS 2002: Link.

License

Contents of this page is taken from the Linux Kernel, which itself if Licensed under GPL v2 license. Original License can be found here.

See Also