Provided by: freebsd-manpages_12.2-1_all 
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: releng/12.2/share/man/man9/style.9 359375 2020-03-27 19:08:10Z emaste $
*/
/*
* 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:
/*-
* SPDX-License-Identifier: BSD-2-Clause-FreeBSD
*
* 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. Use the appropriate SPDX-License-Identifier line before the
copyright.
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.
/* From: @(#)style 1.14 (Berkeley) 4/28/95 */
#include <sys/cdefs.h>
__FBSDID("$FreeBSD: releng/12.2/share/man/man9/style.9 359375 2020-03-27 19:08:10Z emaste $");
Leave one blank line before the header files.
Kernel include files (sys/*.h) come first. If <sys/cdefs.h> is needed for __FBSDID(),
include it first. If either <sys/types.h> or <sys/param.h> is needed, include it before
other include files. (<sys/param.h> includes <sys/types.h>; do not include both.) The
remaining kernel headers should be sorted alphabetically.
#include <sys/types.h> /* Non-local includes in angle brackets. */
#include <sys/endian.h>
#include <sys/lock.h>
#include <sys/queue.h>
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 include files from /usr/include 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 <paths.h>. Pathnames local to the program go in
"pathnames.h" in the local directory.
#include <paths.h>
Leave another blank line before the local 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. 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 prefers the use of ISO/IEC 9899:1999 (“ISO C99”) unsigned integer identifiers of
the form uintXX_t rather than 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.
Similarly, the project prefers the use of ISO C99 bool rather than the older int or
boolean_t. New code should use bool, and old code may be converted if it is reasonable to
do so. Literal values are named true and false. These are preferred to the old spellings
TRUE and FALSE. Userspace code should include <stdbool.h>, while kernel code should include
<sys/types.h>.
Likewise, the project prefers ISO C99 designated initializers when it makes sense to do so.
Enumeration values are all uppercase.
enum enumtype { ONE, TWO } et;
The use of internal_underscores in identifiers is preferred over camelCase or TitleCase.
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; /* Does not 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; /* Does not 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:
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 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 which is unreachable for non-obvious reasons may be marked /*
NOTREACHED */.
while ((ch = getopt(argc, argv, "abNn:")) != -1)
switch (ch) { /* Indent the switch. */
case 'a': /* Do not 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();
}
argc -= optind;
argv += optind;
Space after keywords (if, while, for, return, switch). Two styles of braces (‘{’ and ‘}’)
are allowed for single line statements. Either they are used for all single statements, or
they are used only where needed for clarity. Usage within a function should be consistent.
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:
(p = f()) == NULL
not:
!(p = f())
Do not use ! for tests unless it is a boolean, e.g., use:
if (*p == '\0')
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);
}
When converting K&R style declarations to ANSI style, preserve any comments about
parameters.
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 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]]\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., various static analyzers
or cc -Wall) and produce minimal warnings.
FILES
/usr/src/tools/tools/editing/freebsd.el
An Emacs plugin to follow the FreeBSD style indentation rules.
/usr/src/tools/tools/editing/freebsd.vim
A Vim plugin to follow the FreeBSD style indentation rules.
SEE ALSO
indent(1), err(3), warn(3), style.Makefile(5), style.lua(9)
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.