func() -> func(void) style.

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.\" Copyright (c) 1995-2001 FreeBSD Inc.
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\"
.Dd December 7, 2001
.Dt STYLE 9
.Os
.Sh NAME
.Nm style
.Nd "kernel source file style guide"
.Sh DESCRIPTION
This file specifies the preferred style for kernel source files in the
.Fx
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
.Nm
is silent on an issue.
.Bd -literal
/*
 * 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 $
 * $DragonFly: src/share/man/man9/style.9,v 1.4 2004/01/27 21:03:08 dillon 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.
 */
.Ed
.Pp
Version control system ID tags should only exist once in a file
(unlike this one).
All VCS (version control system) revision identification from files obtained
from elsewhere should be maintained in comments, including, where applicable,
multiple IDs showing a file's history.
In general, keep the IDs intact, including any
.So Li $ Sc Ns s .
There is no reason to add
.Qq Li "From"
in front of foreign VCS IDs.
All VCS IDs should generally be placed in comments somewhere near the
top of the source, typically either before or after the copyright message.
.Pp
Leave another blank line before the header files.
.Pp
Kernel include files (i.e.\&
.Pa sys/*.h )
come first; normally, include
.Aq Pa sys/types.h
OR
.Aq Pa sys/param.h ,
but not both.
.Aq Pa sys/types.h
includes
.Aq Pa sys/cdefs.h ,
and it is okay to depend on that.
.Bd -literal
#include <sys/types.h>	/* Non-local includes in angle brackets. */
.Ed
.Pp
For a network program, put the network include files next.
.Bd -literal
#include <net/if.h>
#include <net/if_dl.h>
#include <net/route.h>
#include <netinet/in.h>
#include <protocols/rwhod.h>
.Ed
.Pp
Do not use files in
.Pa /usr/include
for files in the kernel.
.Pp
Leave a blank line before the next group, the
.Pa /usr
include files,
which should be sorted alphabetically by name.
.Bd -literal
#include <stdio.h>
.Ed
.Pp
Global pathnames are defined in
.Aq Pa paths.h .
Pathnames local
to the program go in
.Qq Pa pathnames.h
in the local directory.
.Bd -literal
#include <paths.h>
.Ed
.Pp
Leave another blank line before the user include files.
.Bd -literal
#include "pathnames.h"		/* Local includes in double quotes. */
.Ed
.Pp
Do not
.Ic #define
or declare names in the implementation namespace except
for implementing application interfaces.
.Pp
The names of
.Dq 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
.Ic #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.
.\" XXX the above conflicts with ANSI style where the names are the
.\" same and you #undef the macro (if any) to get the function.
.\" It is not followed for MALLOC(), and not very common if inline
.\" functions are used.
If a
macro needs more than a single line, use braces
.Ql ( \&{
and
.Ql \&} ) .
Right-justify the
backslashes; it makes it easier to read.
If the macro encapsulates a compound statement, enclose it in a
.Ic do
loop,
so that it can safely be used in
.Ic 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.
.Bd -literal
#define	MACRO(x, y) do {						\e
	variable = (x) + (y);						\e
	(y) += 2;							\e
} while(0)
.Ed
.Pp
Enumeration values are all uppercase.
.Bd -literal
enum enumtype { ONE, TWO } et;
.Ed
.Pp
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.
.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
.Ic extern
if they are declared in a header file.
.Bd -literal
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. */
.Ed
.Pp
Use
.Xr queue 3
macros rather than rolling your own lists, whenever possible.
Thus,
the previous example would be better written:
.Bd -literal
#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. */
.Ed
.Pp
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 convention requires a
.Ic typedef ,
make its name match the struct tag.
Avoid typedefs ending in
.Dq Li _t ,
except as specified in Standard C or by \*[Px].
.Bd -literal
/* 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. */
.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
.Ic 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.\&
.Qq Pa extern.h .
.Pp
Only use the
.Dv __P
macro from the include file
.Aq Pa sys/cdefs.h
if the source
file in general is (to be) compilable with a K&R Old Testament compiler.
Use of the
.Dv __P
macro in new code is discouraged, although modifications
to existing files should be consistent with that file's conventions.
.Pp
In general code can be considered
.Dq "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
.Nm
guidelines.
.Pp
The kernel has a name associated with parameter types, e.g., in the kernel
use:
.Bd -literal
void	function(int fd);
.Ed
.Pp
In header files visible to userland applications, prototypes that are
visible must use either
.Dq protected
names (ones beginning with an underscore)
or no names with the types.
It is preferable to use protected names.
E.g., use:
.Bd -literal
void	function(int);
.Ed
.Pp
or:
.Bd -literal
void	function(int _fd);
.Ed
.Pp
Prototypes may have an extra space after a tab to enable function names
to line up:
.Bd -literal
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;

.Ed
.Pp
For consistency,
.Xr getopt 3
should be used to parse options.
Options
should be sorted in the
.Xr getopt 3
call and the
.Ic switch
statement, unless
parts of the
.Ic switch
cascade.
Elements in a
.Ic switch
statement that cascade should have a
.Li FALLTHROUGH
comment.
Numerical arguments should be checked for accuracy.
Code that cannot be reached should have a
.Li NOTREACHED
comment.
.Bd -literal
	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 != '\e0') {
				warnx("illegal number, -n argument -- %s",
				    optarg);
				usage();
			}
			break;
		case '?':
		default:
			usage();
			/* NOTREACHED */
		}
	argc -= optind;
	argv += optind;
.Ed
.Pp
Space after keywords
.Pq Ic 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 permitted.
Forever loops are done with
.Ic for Ns 's ,
not
.Ic while Ns 's .
.Bd -literal
	for (p = buf; *p != '\e0'; ++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);
.Ed
.Pp
Parts of a
.Ic for
loop may be left empty.
Do not put declarations
inside blocks unless the routine is unusually complicated.
.Bd -literal
	for (; cnt < 15; cnt++) {
		stmt1;
		stmt2;
	}
.Ed
.Pp
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.
.Bd -literal
	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;
.Ed
.Pp
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.
.Pp
Closing and opening braces go on the same line as the
.Ic else .
Braces that are not necessary may be left out.
.Bd -literal
	if (test)
		stmt;
	else if (bar) {
		stmt;
		stmt;
	} else
		stmt;
.Ed
.Pp
No spaces after function names.
Commas have a space after them.
No spaces
after
.Ql \&(
or
.Ql \&[
or preceding
.Ql \&]
or
.Ql \&)
characters.
.Bd -literal
	error = function(a1, a2);
	if (error != 0)
		exit(error);
.Ed
.Pp
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?
.Bd -literal
	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
	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
static char *
function(int a1, int a2, float fl, int a4)
{
.Ed
.Pp
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.
.Pp
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.
.Bd -literal
	struct foo one, *two;
	double three;
	int *four, five;
	char *six, seven, eight, nine, ten, eleven, twelve;

	four = myfunction();
.Ed
.Pp
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.
.Pp
Casts and
.Ic sizeof Ns 's
are not followed by a space.
Note that
.Xr indent 1
does not understand this rule.
.Pp
.Dv NULL
is the preferred null pointer constant.
Use
.Dv NULL
instead of
.Vt ( "type *" ) Ns 0
or
.Vt ( "type *" ) Ns Dv NULL
in contexts where the compiler knows the
type, e.g., in assignments.
Use
.Vt ( "type *" ) Ns Dv 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
.Dv NULL ,
e.g., use:
.Pp
.Bd -literal
(p = f()) == NULL
.Ed
.Pp
not:
.Bd -literal
!(p = f())
.Ed
.Pp
Do not use
.Ic \&!
for tests unless it is a boolean, e.g. use
.Bd -literal
if (*p == '\e0')
.Ed
.Pp
not
.Bd -literal
if (!*p)
.Ed
.Pp
Routines returning
.Vt "void *"
should not have their return values cast
to any pointer type.
.Pp
Use
.Xr err 3
or
.Xr warn 3 ,
do not roll your own.
.Bd -literal
	if ((four = malloc(sizeof(struct foo))) == NULL)
		err(1, (char *)NULL);
	if ((six = (int *)overflow()) == NULL)
		errx(1, "number overflowed");
	return (eight);
}
.Ed
.Pp
Old-style function declarations look like this:
.Bd -literal
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. */
{
.Ed
.Pp
Use ANSI function declarations unless you explicitly need K&R compatibility.
Long parameter lists are wrapped with a normal four space indent.
.Pp
Variable numbers of arguments should look like this.
.Bd -literal
#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(void)
{
	/* Insert an empty line if the function has no local variables. */
.Ed
.Pp
Use
.Xr printf 3 ,
not
.Xr fputs 3 ,
.Xr puts 3 ,
.Xr putchar 3 ,
whatever; it is faster and usually cleaner, not
to mention avoiding stupid bugs.
.Pp
Usage statements should look like the manual pages
.Sx SYNOPSIS .
The usage statement should be structured in the following order:
.Bl -enum
.It
Options without operands come first,
in alphabetical order,
inside a single set of brackets
.Ql ( \&[
and
.Ql \&] ) .
.It
Options with operands come next,
also in alphabetical order,
with each option and its argument inside its own pair of brackets.
.It
Required arguments
(if any)
are next,
listed in the order they should be specified on the command line.
.It
Finally,
any optional arguments should be listed,
listed in the order they should be specified,
and all inside brackets.
.El
.Pp
A bar
.Pq Ql \&|
separates
.Dq either-or
options/arguments,
and multiple options/arguments which are specified together are
placed in a single set of brackets.
.Bd -literal -offset 4n
"usage: f [-aDde] [-b b_arg] [-m m_arg] req1 req2 [opt1 [opt2]]\en"
"usage: f [-a | -b] [-c [-dEe] [-n number]]\en"
.Ed
.Bd -literal
	(void)fprintf(stderr, "usage: f [-ab]\en");
	exit(EX_USAGE);
}
.Ed
.Pp
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.
.Pp
New core kernel code should be reasonably compliant with the
.Nm
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.
.Pp
Stylistic changes (including whitespace changes) are hard on the source
repository and are to be avoided without good reason.
Code that is approximately
.Fx
KNF
.Nm
compliant in the repository must not diverge from compliance.
.Pp
Whenever possible, code should be run through a code checker
(e.g.,
.Xr lint 1
or
.Nm gcc Fl Wall )
and produce minimal warnings.
.Sh SEE ALSO
.Xr indent 1 ,
.Xr lint 1 ,
.Xr err 3 ,
.Xr sysexits 3 ,
.Xr warn 3
.Sh HISTORY
This man page is largely based on the
.Pa src/admin/style/style
file from the
.Bx 4.4 Lite2
release, with occasional updates to reflect the current practice and
desire of the
.Fx
project.