table of contents
STYLE(9) | Kernel Developer's Manual | STYLE(9) |
NAME¶
style
—
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 thatstyle
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/11.0/share/man/man9/style.9 291861 2015-12-05 17:01:38Z cem $ */ /* * 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: releng/11.0/share/man/man9/style.9 291861 2015-12-05 17:01:38Z cem $");
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 commits, care should be taken in making uintXX_t only commits.
Similarly, the project is slowly moving to use the
ISO/IEC 9899:1999 (“ISO C99”)
bool in preference to 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>
.
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; /* 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:
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 != '\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); }
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 cc
-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.December 5, 2015 | Linux 4.9.0-9-amd64 |