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. The preferred line width is 80 characters, but some exceptions
are made when a slightly longer line is clearer or easier to read.
Anything that is frequently grepped for, such as diagnostic, error, or
panic messages, should not be broken up over multiple lines despite this
rule. 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).
*/
/*
* VERY important single-line comments look like this.
*/
/* Most single-line comments look like this. */
// Although they may look like this.
/*
* Multi-line comments look like this. Make them real sentences. Fill
* them so they look like real paragraphs.
*/
C++ comments may be used in C and C++ code. Single-line comments should
be consistently either C or C++ within a file. Multi-line comments
should also be consistently either C or C++, but may differ from single-
line comments.
The copyright header should be a multi-line comment like so:
/*
* Copyright (c) 1984-2025 John Q. Public
*
* SPDX-License-Identifier: BSD-2-Clause
*/
Comments starting in columns other than the first are never considered
license statements. Write the copyright lines before the appropriate
SPDX-License-Identifier. If the copyright assertion contains the phrase
"All Rights Reserved" that should be on the same line as the word
"Copyright". You should not insert a new copyright line between an old
copyright line and this phrase. Instead, you should insert a new
copyright phrase after a pre-existing "All Rights Reserved" line. When
making changes, it is acceptable to fold an "All Rights Reserved" line
with each of the "Copyright" lines. For files that have the "All Rights
Reserved" line on the same line(s) as the word "Copyright", new copyright
assertions should be added last. New "Copyright" lines should only be
added when making substantial changes to the file, not for trivial
changes.
After any copyright and license comment, there is a blank line. Non-
C/C++ source files follow the example above, while C/C++ source files
follow the one below. Version control system ID tags should only exist
once in a file (unlike in this one). 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. Add "From: " and the FreeBSD git hash with full
path name if the file was derived from another FreeBSD file and include
relevant copyright info from the original file.
Leave one blank line before the header files.
Kernel include files (sys/*.h) come 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.) Next,
include <sys/systm.h>, if needed. The remaining kernel headers should be
sorted alphabetically.
#include <sys/types.h> /* Non-local includes in angle brackets. */
#include <sys/systm.h>
#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 space or tab character between the #define and
the macro name, but be consistent within a file. 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".
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 execute
some code and then cascade to the next case 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.
for (; cnt < 15; cnt++) {
stmt1;
stmt2;
}
A for loop may declare and initialize its counting variable.
for (int i = 0; i < 15; i++) {
stmt1;
}
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, struct bar *bar)
{
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. Variables may be initialized where
declared especially when they are constant for the rest of the scope.
Declarations may be in any block, but must be placed before statements.
Calls to complicated functions should be avoided when initializing
variables.
struct foo one, *two;
struct baz *three = bar_get_baz(bar);
double four;
int *five, six;
char *seven, eight, nine, ten, eleven, twelve;
four = my_complicated_function(a1, f1, a4);
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. 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 test without a comparison, or with unary ! (except for booleans).
For example, use:
if (*p == '\0')
not:
if (!*p)
Prefer:
if (count != 0)
over:
if (count)
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);
}
Do not use K&R style declarations or definitions, they are obsolete and
are forbidden in C23. Compilers warn of their use and some treat them as
an error by default. When converting K&R style definitions 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(void)
{
Functions should have local variable declarations first, followed by one
blank line, followed by the first statement. If no local variables are
declared, the first line should be a statement. Older versions of this
style document required a blank line before code. Such lines should be
removed when significant changes are made to the code.
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.
Whenever possible, code should be run through a code checker (e.g.,
various static analyzers or cc -Wall) and produce minimal warnings.
New code should use _Static_assert() instead of the older CTASSERT().
__predict_true() and __predict_false() should only be used in frequently
executed code when it makes the code measurably faster. It is wasteful
to make predictions for infrequently run code, like subsystem
initialization. When using branch prediction hints, atypical error
conditions should use __predict_false() (document the exceptions).
Operations that almost always succeed use __predict_true(). Only use the
annotation for the entire if statement, rather than individual clauses.
Do not add these annotations without empirical evidence of the likelihood
of the branch.
New core kernel code should be compliant with the style guides. The
guidelines for third-party maintained modules and device drivers are more
relaxed. Their code is expected to at least be internally consistent
with their style.
Stylistic changes, including whitespace ones, complicate the work of
downstream consumers and may impair developers' ability to trace the
history of some changes. Such standalone changes must be avoided, and
should not span unrelated directories as this increases the chances of
conflicts when merging to stable and release branches (MFCs). On the
other hand, when a significant portion, usually about a half, of some
logical unit of code, be it a function, group of functions, file or group
of files, is going to be modified, developers are encouraged to amend the
style of the whole unit as described in this document. In this case,
style changes to otherwise unmodified code should be committed
separately. Style-only commits should be added to the file
.git-blame-ignore-revs at the top of the source repository to hide them
from `git blame'. Comments in this file indicate how to use it. Code
that is approximately FreeBSD KNF style compliant in the repository must
not diverge from compliance.
C++
KNF style was originally defined as a style for C. C++ introduces
several new idioms which do not have an existing corollary in KNF C such
as inline function definitions in classes. C++ is also not always
compatible with some KNF guidelines such as enclosing return values in
parentheses. For C++ code, FreeBSD aims to follow broadly accepted C++
practices while also following the general shape of KNF. This section
enumerates C++ specific guidelines that differ from KNF C.
The preferred suffixes for C++ source files are ".cc" and ".hh". Header
files should always use a suffix, unlike headers from the C++ standard
library.
Return values should not be enclosed in parentheses. When converting
existing C code to C++, existing return values may remain in parentheses.
The opening curly brace for namespace declarations should be on the first
line similar to structure and class definitions. Nested namespaces
should be declared using a single namespace declaration.
namespace foo::bar {
}
Member function declarations should follow the same style used for
standalone function prototypes except that a space should be used between
a function's return type and name.
Function definitions at the top level should use a newline after the
function type similar to C function definitions.
Nested member function definitions inside of a class, structure, or union
should not use a newline after the function type. Instead, these should
follow the style of member function declarations. This is more common
C++ style and is more compact for small methods such as getters and
setters.
Inline functions whose body consists of a single statement may use a
single line for the function body. Inline functions with an empty body
should always use a single line.
struct widget {
int foo() { return 4; }
int bar();
};
int
widget::bar()
{
return 6;
}
Default and deleted methods should be declared as a single line.
class box {
~box() = default;
};
In template declarations, the template keyword and list of template
parameters should be followed by a newline before the templated
declaration.
template <typename T>
class box {
T data;
};
The & for reference variables should be placed on the variable name
rather than the type similar to the style used with * for pointers.
int x;
int &xp = x;
Variables may be declared at any point within a function, not just at the
start of blocks.
Standard library containers should be used in preference to queue(3) or
tree(3) macros.
nullptr should be used instead of NULL or 0.
Use standard library types for managing strings such as std::string and
std::string_view rather than char * and const char *. C types may be
used when interfacing with C code.
The auto keyword can be used in various contexts which improve
readability. Examples include iterators, non-trivial types of ranged-for
values, and return values of obvious types, such as static_cast or
std::make_unique(). Place any qualifiers before auto, for example: const
auto.
Use the std::unique_ptr and std::shared_ptr smart pointers to manage the
lifetime of dynamically allocated objects instead of new and delete.
Construct smart pointers with std::make_unique() or std::make_shared().
Do not use malloc(3) except when necessary to interface with C code.
Do not import any namespaces with using at global scope in header files.
Namespaces other than the std namespace (for example, std::literals) may
be imported in source files and in function scope in header files.
Define type aliases using using instead of typedef.
FILES
/usr/src/tools/build/checkstyle9.pl
A script to check for violations of style in a source file.
/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.mdoc(5),
style.lua(9)
HISTORY
This manual page was originally based on the src/admin/style/style file
from the 4.4BSD-Lite2 release, with frequent 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.
FreeBSD 15.1-STABLE-HBSD July 28, 2025 STYLE(9)