style - kernel source file style guide


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:
* Copyright (c) 1984-2025 John Q. Public. All Rights Re
* 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
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
#if 0
#ifndef lint
static char sccsid[] = "@(#)style 1.14 (Berkeley)
#endif /* not lint */
#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 <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
#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>
#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,
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
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
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
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
/* 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);
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
* what the program does.
main(int argc, char *argv[])
char *ep;
long num;
int ch;
For consistency, getopt(3) should be used to parse 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

while ((ch = getopt(argc, argv, "abNn:")) != -1)
switch (ch) { /* Indent the
switch. */
case 'a': /* Don't indent the
case. */
aflag = 1;
case 'b':
bflag = 1;
case 'N':
Nflag = 1;
case 'n':
num = strtol(optarg, &ep, 10);
if (num <= 0 || *ep != ' ') {
warnx("illegal number, -n
argument -- %s",
case '?':
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
statement unless that statement is more than a single line
in which case
they are permitted. Forever loops are done with for's, not

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

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 +
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)
else if (bar) {
} else
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)
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

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)
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
!(p = f())
Do not use ! for tests unless it is a boolean, e.g. use:
if (*p == ' ')
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
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>
vaf(const char *fmt, ...)
va_list ap;
va_start(ap, fmt);
/* No return needed for void functions. */
static void
/* 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
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
A bar (`|') separates ``either-or'' options/arguments, and
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
"usage: f [-a | -b] [-c [-dEe] [-n number]]0

(void)fprintf(stderr, "usage: f [-ab]0);
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
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
lint(1) or gcc -Wall) and produce minimal warnings.


indent(1), lint(1), err(3), sysexits(3), warn(3),


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
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