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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 the 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 1.121 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 sentences. 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 Reserved.
*
* 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 applica-
ble,
multiple IDs showing a file’s history. In general, do not edit
for-
eign 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 renamed.
#if
0
#ifndef lint
static
char sccsid[] = "@(#)style 1.14 (Berkeley)
4/28/95";
#endif /* not lint
*/
#endif
#include
__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
OR , but not both.
includes
, and it is okay to depend on
that.
#include /* Non-local includes
in angle brackets. */
For a network program, put
the network include files next.
#include
#include
#include
#include
#include
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
Global pathnames are defined in .
Pathnames local to the pro-
gram go in "pathnames.h"
in the local directory.
#include
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.
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-ter-
minating 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)
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 eas-
ily 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 confusing to the
reader. Exceptions may be made for cases where code is conditionally
not
compiled for the purposes of lint(1), even
though the uncompiled region
may be small. The
comment should be separated from the #endif or #else
by a single space. For short conditionally compiled regions, a
closing
comment should not be
used.
The comment for #endif should match the
expression used in the corre-
sponding #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 “defined(FOO)” is
abbreviated as “FOO”. For the purposes of comments, “#ifndef FOO”
is
treated as “#if
!defined(FOO)”.
#ifdef
KTRACE
#include
#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 preference 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 commits, 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 identi-
fiers 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 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 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 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
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. 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 pollution), or
there must be a back-door
mechanism for obtaining the typedef.
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 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. 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 sepa-
rate 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 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[])
{
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 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, "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 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
&&
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.
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 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. sizeof’s are
written with parenthesis always.
The redundant
parenthesis rules do not apply to sizeof(var)
instances.
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 ==
”)
not:
if
(!*p)
Routines returning void * should not have
their return values cast to any
pointer
type.
Values in return statements should be
enclosed in parentheses.
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
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]]
"
"usage: f [-a | -b] [-c
[-dEe] [-n number]]
"
(void)fprintf(stderr, "usage: f [-ab]
");
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),
style.Makefile(5)
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.
FreeBSD
6.0
February 10,
2005
FreeBSD 6.0