STYLE(9) BSD 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: head/share/man/man9/style.9 217087 2011-01-07 08:34:12Z trasz $
*/
/*
* 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: head/share/man/man9/style.9 217087 2011-01-07 08:34:12Z trasz $");
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.
#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 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 com-
mits, care should be taken in making uintXX_t only commits.
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 iden-
tifiers 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 align-
ing 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 pollu-
tion), 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 prece-
dents 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': /* 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 != '