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

NAME
     style -- kernel source file style guide

DESCRIPTION
     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: release/10.0.0/share/man/man9/style.9	217087 2011-01-07 08:34:12Z trasz $
      */

     /*
      *	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:

     /*-
      *	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.

     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 applica-
     ble, multiple IDs showing a file's	history.  In general, do not edit for-
     eign 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.

     #if 0
     #ifndef lint
     static char sccsid[] = "@(#)style	     1.14 (Berkeley) 4/28/95";
     #endif /* not lint	*/
     #endif

     #include <sys/cdefs.h>
     __FBSDID("$FreeBSD: release/10.0.0/share/man/man9/style.9 217087 2011-01-07 08:34:12Z trasz $");

     Leave another blank line before the header	files.

     Kernel include files (i.e.	sys/*.h) come first; normally, include
     <sys/types.h> OR <sys/param.h>, but not both.  <sys/types.h> includes
     <sys/cdefs.h>, and	it is okay to depend on	that.

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

     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 use	files in /usr/include for files	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 user 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	expan-
     sions 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 func-
     tion 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-ter-
     minating 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.  Exceptions may be	made for cases where code is conditionally not
     compiled for the purposes of lint(1), even	though the uncompiled region
     may be small.  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 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
     abbreviated as ``FOO''.  For the purposes of comments, ``#ifndef FOO'' is
     treated as	``#if !defined(FOO)''.

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

     #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 is slowly moving to use the ISO/IEC 9899:1999 (``ISO C99'')
     unsigned integer identifiers of the form uintXX_t in preference to	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.

     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;   /*	Won't 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;   /*	Won't 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 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
     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 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
     underscore) or no names with the types.  It is preferable to use pro-
     tected names.  E.g., use:

     void    function(int);

     or:

     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.
      */
     int
     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':		     /*	Don't indent the case. */
			     aflag = 1;	     /*	Indent case body one tab. */
			     /*	FALLTHROUGH */
		     case 'b':
			     bflag = 1;
			     break;
		     case 'N':
			     Nflag = 1;
			     break;
		     case 'n':
			     num = strtol(optarg, &ep, 10);
			     if	(num <=	0 || *ep != '\0') {
				     warnx("illegal number, -n argument	-- %s",
					 optarg);
				     usage();
			     }
			     break;
		     case '?':
		     default:
			     usage();
		     }
	     argc -= optind;
	     argv += optind;

     Space after keywords (if, while, for, return, switch).  No	braces (`{'
     and `}') are used for control statements with zero	or only	a single
     statement unless that statement is	more than a single line	in which case
     they are permitted.  Forever loops	are done with for's, not while's.

	     for (p = buf; *p != '\0'; ++p)
		     ;	     /*	nothing	*/
	     for (;;)
		     stmt;
	     for (;;) {
		     z = a + really + long + statement + that +	needs +
			 two + lines + gets + indented + four +	spaces +
			 on + the + second + and + subsequent +	lines;
	     }
	     for (;;) {
		     if	(cond)
			     stmt;
	     }
	     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++) {
		     stmt1;
		     stmt2;
	     }

     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)
		     stmt;
	     else if (bar) {
		     stmt;
		     stmt;
	     } else
		     stmt;

     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)
		     exit(error);

     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)
     {

     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.

     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 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.  Note that	indent(1) does
     not understand this rule.	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

     not:

     !(p = f())

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

     if	(*p == '\0')

     not:

     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);
     }

     Old-style function	declarations look like this:

     static char *
     function(a1, a2, fl, a4)
	     int a1, a2;     /*	Declare	ints, too, don't default them. */
	     float fl;	     /*	Beware double vs. float	prototype differences. */
	     int a4;	     /*	List in	order declared.	*/
     {

     Use ANSI function declarations unless you explicitly need K&R compatibil-
     ity.  Long	parameter lists	are wrapped with a normal four space indent.

     Variable numbers of arguments should look like this:

     #include <stdarg.h>

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

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

     static void
     usage()
     {
	     /*	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");
	     exit(1);
     }

     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.,
     lint(1) or	gcc -Wall) and produce minimal warnings.

SEE ALSO
     indent(1),	lint(1), err(3), warn(3), style.Makefile(5)

HISTORY
     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	10.1			January	7, 2010			  FreeBSD 10.1

NAME | DESCRIPTION | SEE ALSO | HISTORY

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