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

     style -- kernel 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
      *	$FreeBSD$

      *	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
      *	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.  If the copyright assertion con-
     tains the phrase "All Rights Reserved" that should	be on the same line as
     the word "Copyright".  You	should not insert a new	copyright line between
     an	old copyright line and this phrase.  Instead, you should insert	a new
     copyright phrase after a pre-existing "All	Rights Reserved" line.	When
     making changes, it	is acceptable to fold an "All Rights Reserved" line
     with each of the "Copyright" lines.  For files that have the "All Rights
     Reserved" line on the same	line(s)	as the word "Copyright", new copyright
     assertions	should be added	last.  New "Copyright" lines should only be
     added when	making substantial changes to the file,	not for	trivial

     After any copyright and license comment, 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>

     Leave one blank line before the header files.

     Kernel include files (sys/*.h) come first.	 If <sys/cdefs.h> is needed
     for __FBSDID(), 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.)  Next, in-
     clude <sys/systm.h>, if needed.  The remaining kernel headers should be
     sorted alphabetically.

     #include <sys/types.h>  /*	Non-local includes in angle brackets. */
     #include <sys/systm.h>
     #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	pro-
     gram 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 paren-
     theses.  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 eas-
     ily 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	com-
     ment should not be	used.

     The comment for #endif should match the expression	used in	the corre-
     sponding #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 abbre-
     viated 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 in-
     teger 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 for-
     mer, 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 in-
     clude <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 camel-
     Case 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 identi-
     fiers 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 possi-
     ble.  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 be-
     cause 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

     /*	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 else-
     where) 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 rele-
     vant include file.	 Function prototypes should be listed in a logical or-
     der, 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 sepa-
     rate 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 ker-
     nel 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 under-
     score) 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. */
			     /*	FALLTHROUGH */
		     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.

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

     A for loop	may declare and	initialize its counting	variable.

	     for (int i	= 0; i < 15; i++) {

     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

	     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 spa-
     ces 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 state-
     ment 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,	struct bar *bar)

     When declaring variables in functions declare them	sorted by size,	then
     in	alphabetical order; multiple ones per line are okay.  If a line	over-
     flows reuse the type keyword.  Variables may be initialized where de-
     clared especially when they are constant for the rest of the scope.  Dec-
     larations may be placed before executable lines at	the start of any
     block.  Calls to complicated functions should be avoided when initializ-
     ing variables.

	     struct foo	one, *two;
	     struct baz	*three = bar_get_baz(bar);
	     double four;
	     int *five,	six;
	     char *seven, eight, nine, ten, eleven, twelve;

	     four = my_complicated_function(a1,	f1, a4);

     Do	not declare functions inside other functions; ANSI C says that such
     declarations have file scope regardless of	the nesting of the declara-
     tion.  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.  sizeof's are written
     with parenthesis always.  The redundant parenthesis rules do not apply to
     sizeof(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	neces-
     sary 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	com-
     ments 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
	     /*	Optional blank line goes here. */

     Optionally, insert	a blank	line at	the beginning of functions with	no lo-
     cal variables.  Older versions of this style document required the	blank
     line convention, so it is widely used in existing code.

     Do	not insert a blank line	at the beginning of functions with local vari-
     ables.  Instead, these should have	local variable declarations first,
     followed by one blank line, followed by the first statement.

     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 or-
	  der they should be specified,	and all	inside brackets.

     A bar (`|') separates "either-or" options/arguments, and multiple op-
     tions/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 ac-
     count 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 ap-
     proximately FreeBSD KNF style compliant in	the repository must not	di-
     verge from	compliance.

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

     New code should use _Static_assert() instead of the older CTASSERT().

	     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.mdoc(5),

     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.

FreeBSD	13.0		       October 28, 2020			  FreeBSD 13.0


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