STYLE(9) OpenBSD Kernel Manual STYLE(9)NAMEstyle - Kernel source file style guide (KNF)
DESCRIPTION
This file specifies the preferred style for kernel source files in the
OpenBSD source tree. It is also a guide for preferred user land code
style. These guidelines should be followed for all new code. 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.
/*
* Style guide for the OpenBSD KNF (Kernel Normal Form).
*/
/*
* 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.
*/
Kernel include files (i.e., <sys/*.h>) come first; normally, you'll need
<sys/types.h> OR <sys/param.h>, but not both! <sys/types.h> includes
<sys/cdefs.h>, and it's okay to depend on that.
#include <sys/types.h> /* Non-local includes in brackets. */
If it's 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>
Then there's a blank line, followed by the /usr/include files. The
/usr/include files should be sorted!
#include <stdio.h>
Global pathnames are defined in /usr/include/paths.h. Pathnames local to
the program go in pathnames.h in the local directory.
#include <paths.h>
Then there's a blank line, and the user include files.
#include "pathnames.h" /* Local includes in double quotes. */
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. In user land,
functions local to one source module should be declared `static'. This
should not be done in kernel land since it makes it impossible to use the
kernel debugger.
Functions used from other parts of the kernel are prototyped in the
relevant include file.
Functions that are used locally in more than one module go into a
separate header file, e.g., extern.h.
Use of the __P macro has been deprecated. It is allowed in code imported
from other sources but should not be used in native OpenBSD code.
Prototypes should not have variable names associated with the types;
i.e.,
void function(int);
not:
void function(int a);
Prototypes may have an extra space after a tab to enable function names
to line up:
static char *function(int, const char *);
static void usage(void);
There should be no space between the function name and the argument list.
Use __dead from <sys/cdefs.h> for functions that don't return, i.e.,
__dead void abort(void);
In header files, put function prototypes within __BEGIN_DECLS /
__END_DECLS matching pairs. This makes the header file usable from C++.
Macros are capitalized and parenthesized, and should avoid side-effects.
If they are an inline expansion of a function, the function is defined
all in lowercase; the macro has the same name all in uppercase. If the
macro needs more than a single line, use braces. Right-justify the
backslashes, as the resulting definition is 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)
Enumeration values are all uppercase.
enum enumtype { ONE, TWO } et;
When declaring variables in structures, declare them sorted by use, then
by size (largest to smallest), then by alphabetical order. The first
category normally doesn't apply, but there are exceptions. Each one gets
its own line. Put a tab after the first word, i.e., use `int^Ix;' and
`struct^Ifoo *x;'.
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;
};
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; /* Queue macro glue for foo lists */
struct mumble amumble; /* Comment for mumble */
int bar;
};
LIST_HEAD(, foo) foohead; /* Head of global foo list */
Avoid using typedefs for structure types. This makes it impossible for
applications to use pointers to such a structure opaquely, which is both
possible and beneficial when using an ordinary struct tag. 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.
Don't use the same name for a struct tag and a typedef, as this makes the
code unusable from C++.
/* Make the structure name match the typedef. */
typedef struct _bar {
int level;
} BAR;
/*
* 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[])
{
int aflag, bflag, ch, num;
const char *errstr;
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, "abn:")) != -1)
switch (ch) { /* Indent the switch. */
case 'a': /* Don't indent the case. */
aflag = 1;
/* FALLTHROUGH */
case 'b':
bflag = 1;
break;
case 'n':
num = strtonum(optarg, 0, INT_MAX, &errstr);
if (errstr) {
warnx("number is %s: %s", errstr, optarg);
usage();
}
break;
default:
usage();
/* NOTREACHED */
}
argc -= optind;
argv += optind;
Use a space after keywords (if, while, for, return, switch). No braces
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.
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;
}
Parts of a for loop may be left empty. Don't 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.
while (cnt < 20)
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
aren't necessary may be left out, unless they cause a compiler warning.
if (test)
stmt;
else if (bar) {
stmt;
stmt;
} else
stmt;
Do not use spaces after function names. Commas have a space after them.
Do not use spaces after `(' or `[' or preceding `]' or `)' characters.
if ((error = function(a1, a2)))
exit(error);
Unary operators don't require spaces; binary operators do. Don't use
parentheses unless they're required for precedence, the statement is
confusing without them, or the compiler generates a warning without them.
Remember that other people may be confused more easily 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 non-zero for errors.
exit(0); /*
* Avoid obvious comments such as
* "Exit 0 on success."
*/
}
The function type should be on a line by itself preceding the function.
static char *
function(int a1, int a2, float fl, int a4)
{
When declaring variables in functions, declare them sorted by size
(largest to smallest), then in alphabetical order; multiple ones per line
are okay. Old style function declarations should be avoided. ANSI style
function declarations should go in an include file such as ``extern.h''.
If a line overflows, reuse the type keyword.
Be careful not to 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.
Casts and sizeof() calls are not followed by a space. Note that
indent(1) does not understand this rule.
Use of the ``register'' specifier is discouraged in new code. Optimizing
compilers such as gcc can generally do a better job of choosing which
variables to place in registers to improve code performance. The
exception to this is in functions containing assembly code where the
``register'' specifier is required for proper code generation in the
absence of compiler optimization.
When using longjmp() or vfork() in a program, the -W or -Wall flag should
be used to verify that the compiler does not generate warnings such as
warning: variable `foo' might be clobbered by `longjmp' or `vfork'.
If any warnings of this type occur, you must apply the ``volatile'' type-
qualifier to the variable in question. Failure to do so may result in
improper code generation when optimization is enabled. Note that for
pointers, the location of ``volatile'' specifies if the type-qualifier
applies to the pointer, or the thing being pointed to. A volatile
pointer is declared with ``volatile'' to the right of the ``*''.
Example:
char *volatile foo;
says that ``foo'' is volatile, but ``*foo'' is not. To make ``*foo''
volatile use the syntax
volatile char *foo;
If both the pointer and the thing pointed to are volatile use
volatile char *volatile foo;
``const'' is also a type-qualifier and the same rules apply. The
description of a read-only hardware register might look something like:
const volatile char *reg;
Global flags set inside signal handlers should be of type ``volatile
sig_atomic_t'' if possible. This guarantees that the variable may be
accessed as an atomic entity, even when a signal has been delivered.
Global variables of other types (such as structures) are not guaranteed
to have consistent values when accessed via a signal handler.
NULL is the preferred null pointer constant. Use NULL instead of
(type *)0 or (type *)NULL in all cases except for arguments to variadic
functions where the compiler does not know the type.
Don't use `!' for tests unless it's a boolean, i.e., use
if (*p == '\0')
not
if (!*p)
Routines returning void * should not have their return values cast to any
pointer type.
Use err(3) or warn(3), don't 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(void)
{
extern char *__progname; /* from crt0.o */
Usage statements should take the same form as the synopsis in manual
pages. Options without operands come first, in alphabetical order inside
a single set of braces, followed by options with operands, in
alphabetical order, each in braces, followed by required arguments in the
order they are specified, followed by optional arguments in the order
they are specified.
A bar (`|') separates either-or options/arguments, and multiple
options/arguments which are specified together are placed in a single set
of braces.
If numbers are used as options, they should be placed first, as shown in
the example below. Uppercase letters take precedence over lowercase.
Note that the options list in manual pages should be purely alphabetical.
That is, with no regard to whether an option takes an argument.
"usage: f [-12aDde] [-b b_arg] [-m m_arg] req1 req2 [opt1 [opt2]]\n"
"usage: f [-a | -b] [-c [-de] [-n number]]\n"
The __progname string may be used instead of hard-coding the program
name.
(void)fprintf(stderr, "usage: %s [-ab]\n", __progname);
exit(1);
}
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.
Whenever possible, code should be run through a code checker (e.g., ``gcc
-Wall -W -Wpointer-arith -Wbad-function-cast ...'', lint(1) or splint
from the ports tree) and produce minimal warnings.
Note that documentation follows its own style guide, as documented in
mdoc.samples(7).
FILES
/usr/share/misc/license.template Example license for new code.
SEE ALSOindent(1), err(3), queue(3), warn(3), mdoc.samples(7)HISTORY
This man page is largely based on the src/admin/style/style file from the
BSD 4.4-Lite2 release, with updates to reflect the current practice and
desire of the OpenBSD project.
OpenBSD 4.9 November 14, 2008 OpenBSD 4.9
