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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 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: src/share/man/man9/style.9,v 1.32.2.19 2002/04/14 19:28:03 asmodai Exp $
      */

     /*
      *	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.
      */

     After any copyright header, there is a blank line,	and the	rcsid for
     source files.  Version control system ID tags should only exist once in a
     file (unlike 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 from files	obtained from elsewhere	should
     be	maintained, including, where applicable, multiple IDs showing a	file's
     history.  In general, keep	the IDs	intact,	including any `$'s.  There is
     no	reason to add "From" in	front of foreign VCS IDs.  Most	non-FreeBSD
     VCS IDs should be indented	by a tab if in a comment.

     #include <sys/cdefs.h>
     __RCSID("@(#)style	     1.14 (Berkeley) 4/28/95");
     __FBSDID("$FreeBSD: src/share/man/man9/style.9,v 1.32.2.19	2002/04/14 19:28:03 asmodai Exp	$");

     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.	 If a macro needs more than a single line, use braces (`{' and
     `}').  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)

     Enumeration values	are all	uppercase.

     enum enumtype { ONE, TWO }	et;

     When declaring variables in structures, declare them sorted by use, then
     by	size, 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 if it suffices	to align most 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.	This makes it impossible for
     applications to use pointers to such a structure opaquely,	which is both
     possible and beneficial when using	an ordinary struct tag.	 When conven-
     tion requires a typedef, make its name match the struct tag.  Avoid type-
     defs 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.

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

     Only use the __P macro from the include file <sys/cdefs.h>	if the source
     file in general is	(to be)	compilable with	a K&R Old Testament compiler.
     Use of the	__P macro in new code is discouraged, although modifications
     to	existing files should be consistent with that file's conventions.

     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[])
     {
	     long num;
	     int ch;
	     char *ep;

     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 that cannot be	reached	should have a NOTREACHED com-
     ment.

	     while ((ch	= getopt(argc, argv, "abn:")) != -1)
		     switch (ch) {	     /*	Indent the switch. */
		     case 'a':		     /*	Don't indent the case. */
			     aflag = 1;
			     /*	FALLTHROUGH */
		     case 'b':
			     bflag = 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();
			     /*	NOTREACHED */
		     }
	     argc -= optind;
	     argv += optind;

     Space after keywords (if, while, for, return, switch).  No	braces 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 permit-
     ted.  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_for_its_own_good	&&
		 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 according	to the predefined values in
     sysexits(3).

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

     The function type should be on a line by itself preceding the function.

     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.

     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.

     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(EX_USAGE);
     }

     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), sysexits(3), warn(3)

HISTORY
     This man 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.

FreeBSD	10.1		       December	7, 2001			  FreeBSD 10.1

NAME | DESCRIPTION | SEE ALSO | HISTORY

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