man/man9/style.9

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.Dd December 14, 1995
.Dt STYLE 9
.Os FreeBSD 2.2
.Sh NAME
.Nm style
.Nd "Kernel source file style guide"
.Sh DESCRIPTION
This file contains an example of the preferred style for kernel source
files in the FreeBSD source tree.
.Bd -literal -offset 0i
/*
 * Style guide for the 4BSD KNF (Kernel Normal Form).
 *
 *	@(#)style	1.14 (Berkeley) 4/28/95
 *
 *	$Id$
 *
 */

/*
 * 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.
 */
.Ed
.Pp
Kernel include files come first; normally, you'll need <sys/types.h>
OR <sys/param.h>, but not both!  <sys/types.h> includes <sys/cdefs.h>,
and it's okay to depend on that.
.Bd -literal -offset 0i
#include <sys/types.h>		/* Non-local includes in brackets. */
.Ed
.Pp
If it's a network program, put the network include files next.
.Bd -literal -offset 0i
#include <net/if.h>
#include <net/if_dl.h>
#include <net/route.h>
#include <netinet/in.h>
#include <protocols/rwhod.h>
.Ed
.Pp
Then there's a blank line, followed by the /usr include files.
The /usr include files should be sorted!
.Bd -literal -offset 0i
#include <stdio.h>
.Ed
.Pp
Global pathnames are defined in /usr/include/paths.h.  Pathnames local
to the program go in pathnames.h in the local directory.
.Bd -literal -offset 0i
#include <paths.h>
.Ed
.Pp
Then, there's a blank line, and the user include files.
.Bd -literal -offset 0i
#include "pathnames.h"		/* Local includes in double quotes. */
.Ed
.Pp
Macros are capitalized, parenthesized, and should avoid side-effects.
If they are an inline expansion of a function, the function is defined
all in lowercase, the macro has the same name all in uppercase. If the
macro needs more than a single line, use braces.  Right-justify the
backslashes, it makes it easier to read.
.Bd -literal -offset 0i
#define	MACRO(x, y) {							\e
	variable = (x) + (y);						\e
	(y) += 2;							\e
}
.Ed
.Pp
Enum types are capitalized.
.Bd -literal -offset 0i
enum enumtype { ONE, TWO } et;
.Ed
.Pp
When declaring variables in structures, declare them sorted by use, then
by size, and then by alphabetical order.  The first category normally
doesn't apply, but there are exceptions.  Each one gets its own line.
Put a tab after the first word, i.e. use
.Ql int^Ix;
and
.Ql struct^Ifoo *x; .
.Pp
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.
.Bd -literal -offset 0i
struct foo {
	struct	foo *next;	/* List of active foo */
	struct	mumble amumble;	/* Comment for mumble */
	int	bar;
};
struct foo *foohead;		/* Head of global foo list */

/* Make the structure name match the typedef. */
typedef struct _bar {
	int	level;
} BAR;
.Ed
.Pp
All functions are prototyped somewhere.
.Pp
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
.Ql static .
.Pp
Functions used from other parts of the kernel are prototyped in the
relevant include file.
.Pp
Functions that are used locally in more than one module go into a
separate header file, e.g.
.Pa extern.h .
.Pp
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.
.Pp
Only the kernel has a name associated with the types, i.e. in the kernel
use:
.Bd -literal -offset 0i
void function __P((int fd));
.Ed
.Pp
in user land use:
.Bd -literal -offset 0i
	void function __P((int));

static char	*function __P((int, const char *));
static void	 usage __P((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(argc, argv)
	int argc;
	char *argv[];
{
	extern char *optarg;
	extern int optind;
	long num;
	int ch;
	char *ep;

.Ed
.Pp
For consistency, getopt should be used to parse options.  Options
should be sorted in the getopt 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 comment.
.Bd -literal -offset 0i
	while ((ch = getopt(argc, argv, "abn")) != EOF)
		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 != '\e0')
				err("illegal number -- %s", optarg);
			break;
		case '?':
		default:
			usage();
			/* NOTREACHED */
		}
	argc -= optind;
	argv += optind;

.Ed
.Pp
Space after keywords (while, for, return, switch).  No braces are
used for control statements with zero or only a single statement.
.Pp
Forever loops are done with for's, not while's.
.Bd -literal -offset 0i
	for (p = buf; *p != '\e0'; ++p);
	for (;;)
		stmt;

.Ed
.Pp
Parts of a for loop may be left empty.  Don't put declarations
inside blocks unless the routine is unusually complicated.
.Bd -literal -offset 0i
	for (; cnt < 15; cnt++) {
		stmt1;
		stmt2;
	}
.Ed
.Pp
Second level indents are four spaces.
.Bd -literal -offset 0i
	while (cnt < 20)
		z = a + really + long + statement + that + needs +
		    two lines + gets + indented + four + spaces +
		    on + the + second + and + subsequent + lines.
.Ed
.Pp
Closing and opening braces go on the same line as the else.
Don't add braces that aren't necessary.
.Bd -literal -offset 0i
	if (test)
		stmt;
	else if (bar) {
		stmt;
		stmt;
	} else
		stmt;
.Ed
.Pp
No spaces after function names.
.Bd -literal -offset 0i
	if (error = function(a1, a2))
		exit(error);
.Ed
.Pp
Unary operators don't require spaces, binary operators do. Don't
use parenthesis unless they're required for precedence, or the
statement is really confusing without them.
.Bd -literal -offset 0i
	a = b->c[0] + ~d == (e || f) || g && h ? i : j >> 1;
	k = !(l & FLAGS);
.Ed
.Pp
Exits should be 0 on success, or according to the predefined
values in
.Xr sysexits 3 .
.Bd -literal -offset 0i
	exit(EX_OK);	/*
			 * Avoid obvious comments such as
			 * "Exit 0 on success."
			 */
}
.Ed
.Pp
The function type should be on a line by itself
preceding the function.
.Bd -literal -offset 0i
static char *
function(a1, a2, fl, a4)
	int a1, a2, a4;	/* Declare ints, too, don't default them. */
	float fl;	/* List in order declared, as much as possible. */
{
.Ed
.Pp
When declaring variables in functions declare them sorted by size,
then in alphabetical order; multiple ones per line are okay.
Declaring functions inside functions is not recommendable, since their
linkage scope is always global.  If a line overflows reuse the type
keyword.
.Pp
Be careful to not obfuscate the code by initializing variables in
the declarations.  Use this feature only thoughtfully.
.Bd -literal -offset 0i
	extern u_char one;
	extern char two;
	struct foo three, *four;
	double five;
	int *six, seven, eight();
	char *nine, ten, eleven, twelve, thirteen, fourteen, fifteen;
	char *overflow __P((void));
	void *mymalloc __P((u_int));
.Ed
.Pp
Casts and sizeof's are not followed by a space.
.Pp
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
varadic args and is necessary for other args if the function prototype
might not be in scope; since we pretend to support K&R compilers,
most prototypes might not be in scope.)
Test pointers
against NULL, e.g., use:
.Bd -literal -offset 0i
(p = f()) == NULL
.Ed
.Pp
not:
.Bd -literal -offset 0i
!(p = f())
.Ed
.Pp
Don't use '!' for tests unless it's a boolean, e.g. use
.Bd -literal -offset 0i
if (*p == '\e0')
.Ed
.Pp
not
.Bd -literal -offset 0i
if (!*p)
.Ed
.Pp
Routines returning void * should not have their return values cast
to any pointer type.
.Pp
Use
.Xr err 3
or
.Xr warn 3 ,
don't roll your own!
.Bd -literal -offset 0i
	if ((four = malloc(sizeof(struct foo))) == NULL)
		err(1, (char *)NULL);
	if ((six = (int *)overflow()) == NULL)
		errx(1, "Number overflowed.");
	return (eight);
}
.Ed
.Pp
Don't use ANSI function declarations unless you absolutely have too,
i.e. you're declaring functions with variable numbers of arguments.
.Pp
ANSI function return values and braces look like regular functions.
.Bd -literal -offset 0i
int
function(int a1, int a2)
{
	...
}
.Ed
.Pp
Variable numbers of arguments should look like this.
.Bd -literal -offset 0i
#if __STDC__
#include <stdarg.h>
#else
#include <varargs.h>
#endif

void
#if __STDC__
vaf(const char *fmt, ...)
#else
vaf(fmt, va_alist)
	char *fmt;
	va_dcl
#endif
{
	va_list ap;
#if __STDC__
	va_start(ap, fmt);
#else
	va_start(ap);
#endif
	STUFF;

	va_end(ap);		/* No return needed for void functions. */
}

static void
usage()
{
	/* Insert an empty line if the function has no local variables. */
.Ed
.Pp
Use
.Xr printf 3 ,
not fputs/puts/putchar/whatever, it's faster and usually cleaner, not
to mention avoiding stupid bugs.
.Pp
Usage statements should look like the manual pages.  Options w/o
operands come first, in alphabetical order inside a single set of
braces.  Followed by options with operands, in alphabetical order,
each in braces.  Followed by required arguments in the order they
are specified, followed by optional arguments in the order they
are specified.  A bar
.Pq Sq \&|
separates either/or options/arguments,
and multiple options/arguments which are specified together are
placed in a single set of braces.
.Pp
.Bd -ragged -offset 0.3i
"usage: f [-ade] [-b b_arg] [-m m_arg] req1 req2 [opt1 [opt2]]\en"
"usage: f [-a | -b] [-c [-de] [-n number]]\en"
.Ed
.Bd -literal -offset 0i
	(void)fprintf(stderr, "usage: f [-ab]\en");
	exit(1);
}
.Ed
.Pp
Note that the policy regarding the usage of K&R versus ANSI function
definitions could not be commonly agreed to.  While keeping the old
form is more consistent with the existing code base, sticking to it
defeats the migration to the more modern ANSI style.  For new code,
chose what you feel is more important.  However, when modifying
existing subsystems or files, stick with the style that is already
there.
.Sh SEE ALSO
.Xr err 3 ,
.Xr sysexits 3 ,
.Xr warn 3
.Sh HISTORY
This man page is largely based on the src/admin/style/style file from
the BSD 4.4-Lite2 release, with a few updates to reflect the current
practice and desire of the FreeBSD project.