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 1.130.2.1.6.1 2010/12/21 17:09:25 kensmith 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 trimmed 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 renamed.
#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 1.130.2.1.6.1 2010/12/21 17:09:25 kensmith Exp $");
Leave another blank line before the header files.
Kernel include files (i.e.
sys/*.h) come first; normally,
include
<sys/types.h> OR
<sys/param.h>, but not both.
<sys/types.h> includes
<sys/cdefs.h>, 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.
Global pathnames are defined in
<paths.h>. Pathnames local to the
program go in “
pathnames.h” in the local
directory.
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 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 function, the function 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-terminating 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
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
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
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
“
defined(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 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 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 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 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 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 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:
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:
or:
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
comment.
while ((ch = getopt(argc, argv, "abNn:")) != -1)
switch (ch) { /* Indent the switch. */
case 'a': /* Don't indent the case. */
aflag = 1; /* Indent case body one tab. */
/* FALLTHROUGH */
case 'b':
bflag = 1;
break;
case 'N':
Nflag = 1;
break;
case 'n':
num = strtol(optarg, &ep, 10);
if (num <= 0 || *ep != '\0') {
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 != '\0'; ++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 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 unless 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 1 on failure.
exit(0); /*
* 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 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 declaration.
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 necessary for
other args if the function prototype might not be in scope.) Test pointers
against
NULL
, e.g., use:
not:
Do not use
! for tests unless it is a boolean, e.g. use:
not:
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 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:
- Options without operands come first, in alphabetical
order, inside a single set of brackets
(‘
[
’ and
‘]
’).
- Options with operands come next, also in alphabetical
order, with each option and its argument inside its own pair of
brackets.
- Required arguments (if any) are next, listed in the order
they should be specified on the command line.
- 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]]\n"
"usage: f [-a | -b] [-c [-dEe] [-n number]]\n"
(void)fprintf(stderr, "usage: f [-ab]\n");
exit(1);
}
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),
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.
src/admin/style/style is a codification by the CSRG of the
programming style of Ken Thompson and Dennis Ritchie in
Version 6 AT&T UNIX.