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 exam

ples. 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 1.121 2005/06/28

20:15:18 hmp 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 sen

tences. 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:

/*

served.

* Long, boring license goes here, but redacted 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 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 re

named.

#if 0 #ifndef lint static char sccsid[] = "@(#)style 1.14 (Berkeley)

4/28/95"; #endif /* not lint */ #endif

#include <sys/cdefs.h> __FBSDID("$FreeBSD: src/share/man/man9/style.9,v 1.121

2005/06/28 20:15:18 hmp Exp $");

Leave another blank line before the header files.

Kernel include files (i.e. sys/*.h) come first; normally,

include
#include <sys/types.h>
OR but not both. #include <sys/types.h> includes 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 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 names

pace except for implementing application interfaces.

The names of ``unsafe'' macros (ones that have side ef

fects), and the names of macros for manifest constants, are all in upper

case. The expansions 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 func

tion, the function name is all in lowercase and the macro has the same

name all in uppercase. Right-justify the backslashes; it makes it easi

er 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 invoca

tion 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 easily 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 con

fusing to the reader. Exceptions may be made for cases where code is con

ditionally not compiled for the purposes of lint(1), even though the uncom

piled region may be small. The comment should be separated from the

#endif or #else
by a single space. For short conditionally compiled re

gions, a closing comment should not be used.

The comment for #endif should match the expression used in

the corresponding #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 ``de

fined(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 prefer

ence 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 com

mits, care
should be taken in making uintXX_t only commits.

Enumeration values are all uppercase.

enum enumtype { ONE, TWO } et;

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 identifiers 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 or

der. 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 declara

tions 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 mum

ble. */

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 possible. 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 mum

ble. */

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 pollu

tion), or there must be a back-door mechanism for obtaining the type

def.

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 elsewhere) 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 relevant 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 separate 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 kernel 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 protected 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 describ

ing 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 state

ment, 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.

while ((ch = getopt(argc, argv, "abNn:")) != -1)

switch (ch) { /* Indent the

switch. */

case 'a': /* Don't indent the

case. */

aflag = 1;

/* FALLTHROUGH */

case 'b':

bflag = 1;

break;

case 'N':

Nflag = 1;

break;

case 'n':

num = strtol(optarg, &ep, 10);

if (num <= 0 || *ep != ' ') {

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 (`{' 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 != ' '; ++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 declara

tions 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;

} else

stmt;

No spaces after function names. Commas have a space after

them. No spaces 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 un

less the statement 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.

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 overflows reuse the type keyword.

Be careful to not obfuscate the code by initializing vari

ables 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 declaration. Hiding file declarations in what appears to be a lo

cal 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 paren

thesis always.
The redundant parenthesis rules do not apply to sizeof(var)

instances.

NULL is the preferred null pointer constant. Use NULL in

stead 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 par

ticular 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 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 == ' ')

not:

if (!*p)

Routines returning void * should not have their return val

ues cast to any pointer type.

Values in return statements should be enclosed in parenthe

ses.

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 compatibility. 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 or

der they should be specified on the command line.

4. Finally, any optional arguments should be listed, list

ed in theorder they should be specified, and all inside brack

ets.

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]]0 "usage: f [-a | -b] [-c [-dEe] [-n number]]0

(void)fprintf(stderr, "usage: f [-ab]0);

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 inter

nally 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.

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.
BSD February 10, 2005

docs.sk

comprehensive documentation repository

In section

See also

style(9)

NAME

DESCRIPTION

SEE ALSO

HISTORY