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STYLE(9) Kernel Developer's Manual STYLE(9)


stylekernel source file style guide


This file specifies the preferred style for kernel source files in the FreeBSD source tree. It is also a guide for the preferred userland code style. Many of the style rules are implicit in the examples. Be careful to check the examples before assuming that style is silent on an issue.

 * Style guide for FreeBSD.  Based on the CSRG's KNF (Kernel Normal Form).
 *	@(#)style	1.14 (Berkeley) 4/28/95
 * $FreeBSD: releng/12.2/share/man/man9/style.9 359375 2020-03-27 19:08:10Z emaste $

 * VERY important single-line comments look like this.

/* Most single-line comments look like this. */

 * Multi-line comments look like this.  Make them real sentences.  Fill
 * them so they look like real paragraphs.

The copyright header should be a multi-line comment, with the first line of the comment having a dash after the star like so:

 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
 * Copyright (c) 1984-2025 John Q. Public
 * All rights reserved.
 * Long, boring license goes here, but trimmed for brevity

An automatic script collects license information from the tree for all comments that start in the first column with “/*-”. If you desire to flag indent(1) to not reformat a comment that starts in the first column which is not a license or copyright notice, change the dash to a star for those comments. Comments starting in columns other than the first are never considered license statements. Use the appropriate SPDX-License-Identifier line before the copyright.

After any copyright header, there is a blank line, and the $FreeBSD$ for non C/C++ language source files. Version control system ID tags should only exist once in a file (unlike in this one). Non-C/C++ source files follow the example above, while C/C++ source files follow the one below. All VCS (version control system) revision identification in files obtained from elsewhere should be maintained, including, where applicable, multiple IDs showing a file's history. In general, do not edit foreign IDs or their infrastructure. Unless otherwise wrapped (such as “#if defined(LIBC_SCCS)”), enclose both in “#if 0 ... #endif” to hide any uncompilable bits and to keep the IDs out of object files. Only add “From: ” in front of foreign VCS IDs if the file is renamed.

/* From: @(#)style	1.14 (Berkeley) 4/28/95 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD: releng/12.2/share/man/man9/style.9 359375 2020-03-27 19:08:10Z emaste $");

Leave one blank line before the header files.

Kernel include files (sys/*.h) come first. If <sys/cdefs.h> is needed for (), include it first. If either <sys/types.h> or <sys/param.h> is needed, include it before other include files. (<sys/param.h> includes <sys/types.h>; do not include both.) The remaining kernel headers should be sorted alphabetically.

#include <sys/types.h>	/* Non-local includes in angle brackets. */
#include <sys/endian.h>
#include <sys/lock.h>
#include <sys/queue.h>

For a network program, put the network include files next.

#include <net/if.h>
#include <net/if_dl.h>
#include <net/route.h>
#include <netinet/in.h>
#include <protocols/rwhod.h>

Do not include files from /usr/include in the kernel.

Leave a blank line before the next group, the /usr/include files, which should be sorted alphabetically by name.

#include <stdio.h>

Global pathnames are defined in <paths.h>. Pathnames local to the program go in "pathnames.h" in the local directory.

#include <paths.h>

Leave another blank line before the local include files.

#include "pathnames.h"		/* Local includes in double quotes. */

Do not #define or declare names in the implementation namespace except for implementing application interfaces.

The names of “unsafe” macros (ones that have side effects), and the names of macros for manifest constants, are all in uppercase. The expansions of expression-like macros are either a single token or have outer parentheses. Put a single tab character between the #define and the macro name. If a macro is an inline expansion of a function, the function name is all in lowercase and the macro has the same name all in uppercase. Right-justify the backslashes; it makes it easier to read. If the macro encapsulates a compound statement, enclose it in a do loop, so that it can safely be used in if statements. Any final statement-terminating semicolon should be supplied by the macro invocation rather than the macro, to make parsing easier for pretty-printers and editors.

#define	MACRO(x, y) do {						\
	variable = (x) + (y);						\
	(y) += 2;							\
} while (0)

When code is conditionally compiled using #ifdef or #if, a comment may be added following the matching #endif or #else to permit the reader to easily discern where conditionally compiled code regions end. This comment should be used only for (subjectively) long regions, regions greater than 20 lines, or where a series of nested #ifdef 's may be confusing to the reader. The comment should be separated from the #endif or #else by a single space. For short conditionally compiled regions, a closing comment should not be used.

The comment for #endif should match the expression used in the corresponding #if or #ifdef. The comment for #else and #elif should match the inverse of the expression(s) used in the preceding #if and/or #elif statements. In the comments, the subexpression “defined(FOO)” is abbreviated as “FOO”. For the purposes of comments, “#ifndef FOO” is treated as “#if !defined(FOO)”.

#ifdef KTRACE
#include <sys/ktrace.h>

#ifdef COMPAT_43
/* A large region here, or other conditional code. */
#else /* !COMPAT_43 */
/* Or here. */
#endif /* COMPAT_43 */

#ifndef COMPAT_43
/* Yet another large region here, or other conditional code. */
#else /* COMPAT_43 */
/* Or here. */
#endif /* !COMPAT_43 */

The project prefers the use of ISO/IEC 9899:1999 (“ISO C99”) unsigned integer identifiers of the form uintXX_t rather than the older BSD-style integer identifiers of the form u_intXX_t. New code should use the former, and old code should be converted to the new form if other major work is being done in that area and there is no overriding reason to prefer the older BSD-style. Like white-space commits, care should be taken in making uintXX_t only commits.

Similarly, the project prefers the use of ISO C99 bool rather than the older int or boolean_t. New code should use bool, and old code may be converted if it is reasonable to do so. Literal values are named true and false. These are preferred to the old spellings TRUE and FALSE. Userspace code should include <stdbool.h>, while kernel code should include <sys/types.h>.

Likewise, the project prefers ISO C99 designated initializers when it makes sense to do so.

Enumeration values are all uppercase.

enum enumtype { ONE, TWO } et;

The use of internal_underscores in identifiers is preferred over camelCase or TitleCase.

In declarations, do not put any whitespace between asterisks and adjacent tokens, except for tokens that are identifiers related to types. (These identifiers are the names of basic types, type qualifiers, and typedef-names other than the one being declared.) Separate these identifiers from asterisks using a single space.

When declaring variables in structures, declare them sorted by use, then by size (largest to smallest), and then in alphabetical order. The first category normally does not apply, but there are exceptions. Each one gets its own line. Try to make the structure readable by aligning the member names using either one or two tabs depending upon your judgment. You should use one tab only if it suffices to align at least 90% of the member names. Names following extremely long types should be separated by a single space.

Major structures should be declared at the top of the file in which they are used, or in separate header files if they are used in multiple source files. Use of the structures should be by separate declarations and should be extern if they are declared in a header file.

struct foo {
	struct foo	*next;		/* List of active foo. */
	struct mumble	amumble;	/* Comment for mumble. */
	int		bar;		/* Try to align the comments. */
	struct verylongtypename *baz;	/* Does not fit in 2 tabs. */
struct foo *foohead;			/* Head of global foo list. */

Use queue(3) macros rather than rolling your own lists, whenever possible. Thus, the previous example would be better written:

#include <sys/queue.h>

struct foo {
	LIST_ENTRY(foo)	link;		/* Use queue macros for foo lists. */
	struct mumble	amumble;	/* Comment for mumble. */
	int		bar;		/* Try to align the comments. */
	struct verylongtypename *baz;	/* Does not fit in 2 tabs. */
LIST_HEAD(, foo) foohead;		/* Head of global foo list. */

Avoid using typedefs for structure types. Typedefs are problematic because they do not properly hide their underlying type; for example you need to know if the typedef is the structure itself or a pointer to the structure. In addition they must be declared exactly once, whereas an incomplete structure type can be mentioned as many times as necessary. Typedefs are difficult to use in stand-alone header files: the header that defines the typedef must be included before the header that uses it, or by the header that uses it (which causes namespace pollution), or there must be a back-door mechanism for obtaining the typedef.

When convention requires a typedef, make its name match the struct tag. Avoid typedefs ending in “_t”, except as specified in Standard C or by POSIX.

/* Make the structure name match the typedef. */
typedef	struct bar {
	int	level;
} BAR;
typedef	int		foo;		/* This is foo. */
typedef	const long	baz;		/* This is baz. */

All functions are prototyped somewhere.

Function prototypes for private functions (i.e., functions not used elsewhere) go at the top of the first source module. Functions local to one source module should be declared static.

Functions used from other parts of the kernel are prototyped in the relevant include file. Function prototypes should be listed in a logical order, preferably alphabetical unless there is a compelling reason to use a different ordering.

Functions that are used locally in more than one module go into a separate header file, e.g., "extern.h".

Do not use the __P macro.

In general code can be considered “new code” when it makes up about 50% or more of the file(s) involved. This is enough to break precedents in the existing code and use the current style guidelines.

The kernel has a name associated with parameter types, e.g., in the kernel use:

void	function(int fd);

In header files visible to userland applications, prototypes that are visible must use either “protected” names (ones beginning with an underscore) or no names with the types. It is preferable to use protected names. E.g., use:

void	function(int);


void	function(int _fd);

Prototypes may have an extra space after a tab to enable function names to line up:

static char	*function(int _arg, const char *_arg2, struct foo *_arg3,
		    struct bar *_arg4);
static void	 usage(void);

 * All major routines should have a comment briefly describing what
 * they do.  The comment before the "main" routine should describe
 * what the program does.
main(int argc, char *argv[])
	char *ep;
	long num;
	int ch;

For consistency, getopt(3) should be used to parse options. Options should be sorted in the getopt(3) call and the switch statement, unless parts of the switch cascade. Elements in a switch statement that cascade should have a FALLTHROUGH comment. Numerical arguments should be checked for accuracy. Code which is unreachable for non-obvious reasons may be marked /* NOTREACHED */.

	while ((ch = getopt(argc, argv, "abNn:")) != -1)
		switch (ch) {		/* Indent the switch. */
		case 'a':		/* Do not indent the case. */
			aflag = 1;	/* Indent case body one tab. */
		case 'b':
			bflag = 1;
		case 'N':
			Nflag = 1;
		case 'n':
			num = strtol(optarg, &ep, 10);
			if (num <= 0 || *ep != '\0') {
				warnx("illegal number, -n argument -- %s",
		case '?':
	argc -= optind;
	argv += optind;

Space after keywords (if, while, for, return, switch). Two styles of braces (‘{’ and ‘}’) are allowed for single line statements. Either they are used for all single statements, or they are used only where needed for clarity. Usage within a function should be consistent. Forever loops are done with for's, not while's.

	for (p = buf; *p != '\0'; ++p)
		;	/* nothing */
	for (;;)
	for (;;) {
		z = a + really + long + statement + that + needs +
		    two + lines + gets + indented + four + spaces +
		    on + the + second + and + subsequent + lines;
	for (;;) {
		if (cond)
	if (val != NULL)
		val = realloc(val, newsize);

Parts of a for loop may be left empty. Do not put declarations inside blocks unless the routine is unusually complicated.

	for (; cnt < 15; cnt++) {

Indentation is an 8 character tab. Second level indents are four spaces. If you have to wrap a long statement, put the operator at the end of the line.

	while (cnt < 20 && this_variable_name_is_too_long &&
	    ep != NULL)
		z = a + really + long + statement + that + needs +
		    two + lines + gets + indented + four + spaces +
		    on + the + second + and + subsequent + lines;

Do not add whitespace at the end of a line, and only use tabs followed by spaces to form the indentation. Do not use more spaces than a tab will produce and do not use spaces in front of tabs.

Closing and opening braces go on the same line as the else. Braces that are not necessary may be left out.

	if (test)
	else if (bar) {
	} else

No spaces after function names. Commas have a space after them. No spaces after ‘(’ or ‘[’ or preceding ‘]’ or ‘)’ characters.

	error = function(a1, a2);
	if (error != 0)

Unary operators do not require spaces, binary operators do. Do not use parentheses unless they are required for precedence or unless the statement is confusing without them. Remember that other people may confuse easier than you. Do YOU understand the following?

	a = b->c[0] + ~d == (e || f) || g && h ? i : j >> 1;
	k = !(l & FLAGS);

Exits should be 0 on success, or 1 on failure.

	exit(0);	/*
			 * Avoid obvious comments such as
			 * "Exit 0 on success."

The function type should be on a line by itself preceding the function. The opening brace of the function body should be on a line by itself.

static char *
function(int a1, int a2, float fl, int a4)

When declaring variables in functions declare them sorted by size, then in alphabetical order; multiple ones per line are okay. If a line overflows reuse the type keyword.

Be careful to not obfuscate the code by initializing variables in the declarations. Use this feature only thoughtfully. DO NOT use function calls in initializers.

	struct foo one, *two;
	double three;
	int *four, five;
	char *six, seven, eight, nine, ten, eleven, twelve;

	four = myfunction();

Do not declare functions inside other functions; ANSI C says that such declarations have file scope regardless of the nesting of the declaration. Hiding file declarations in what appears to be a local scope is undesirable and will elicit complaints from a good compiler.

Casts and sizeof's are not followed by a space. Note that indent(1) does not understand this rule. sizeof's are written with parenthesis always. The redundant parenthesis rules do not apply to (var) instances.

NULL is the preferred null pointer constant. Use NULL instead of (type *)0 or (type *)NULL in contexts where the compiler knows the type, e.g., in assignments. Use (type *)NULL in other contexts, in particular for all function args. (Casting is essential for variadic args and is necessary for other args if the function prototype might not be in scope.) Test pointers against NULL, e.g., use:

(p = f()) == NULL


!(p = f())

Do not use ! for tests unless it is a boolean, e.g., use:

if (*p == '\0')


if (!*p)

Routines returning void * should not have their return values cast to any pointer type.

Values in return statements should be enclosed in parentheses.

Use err(3) or warn(3), do not roll your own.

	if ((four = malloc(sizeof(struct foo))) == NULL)
		err(1, (char *)NULL);
	if ((six = (int *)overflow()) == NULL)
		errx(1, "number overflowed");
	return (eight);

When converting K&R style declarations to ANSI style, preserve any comments about parameters.

Long parameter lists are wrapped with a normal four space indent.

Variable numbers of arguments should look like this:

#include <stdarg.h>

vaf(const char *fmt, ...)
	va_list ap;

	va_start(ap, fmt);
	/* No return needed for void functions. */

static void
	/* Insert an empty line if the function has no local variables. */

Use printf(3), not fputs(3), puts(3), putchar(3), whatever; it is faster and usually cleaner, not to mention avoiding stupid bugs.

Usage statements should look like the manual pages SYNOPSIS. The usage statement should be structured in the following order:

  1. Options without operands come first, in alphabetical order, inside a single set of brackets (‘[’ and ‘]’).
  2. Options with operands come next, also in alphabetical order, with each option and its argument inside its own pair of brackets.
  3. Required arguments (if any) are next, listed in the order they should be specified on the command line.
  4. Finally, any optional arguments should be listed, listed in the order they should be specified, and all inside brackets.

A bar (‘|’) separates “either-or” options/arguments, and multiple options/arguments which are specified together are placed in a single set of brackets.

"usage: f [-aDde] [-b b_arg] [-m m_arg] req1 req2 [opt1 [opt2]]\n"
"usage: f [-a | -b] [-c [-dEe] [-n number]]\n"
	(void)fprintf(stderr, "usage: f [-ab]\n");

Note that the manual page options description should list the options in pure alphabetical order. That is, without regard to whether an option takes arguments or not. The alphabetical ordering should take into account the case ordering shown above.

New core kernel code should be reasonably compliant with the style guides. The guidelines for third-party maintained modules and device drivers are more relaxed but at a minimum should be internally consistent with their style.

Stylistic changes (including whitespace changes) are hard on the source repository and are to be avoided without good reason. Code that is approximately FreeBSD KNF style compliant in the repository must not diverge from compliance.

Whenever possible, code should be run through a code checker (e.g., various static analyzers or cc -Wall) and produce minimal warnings.


An Emacs plugin to follow the FreeBSD style indentation rules.
A Vim plugin to follow the FreeBSD style indentation rules.


indent(1), err(3), warn(3), style.Makefile(5), style.lua(9)


This manual page is largely based on the src/admin/style/style file from the 4.4BSD-Lite2 release, with occasional updates to reflect the current practice and desire of the FreeBSD project. src/admin/style/style is a codification by the CSRG of the programming style of Ken Thompson and Dennis Ritchie in Version 6 AT&T UNIX.

January 25, 2018 Debian