path: root/share/man/man9/style.9

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.\"	$OpenBSD: style.9,v 1.34 2003/09/29 11:29:20 jmc Exp $
.\"
.Dd June 18, 2001
.Dt STYLE 9
.Os
.Sh NAME
.Nm style
.Nd "Kernel source file style guide (KNF)"
.Sh DESCRIPTION
This file specifies the preferred style for kernel source files in the
.Ox
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
.Dq 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.
.Bd -literal -offset 0i
/*
 * 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.
 */
.Ed
.Pp
Kernel include files (i.e.,
.Aq Pa sys/*.h )
come first; normally, you'll need
.Aq Pa sys/types.h
OR
.Aq Pa sys/param.h ,
but not both!
.Aq Pa sys/types.h
includes
.Aq Pa sys/cdefs.h ,
and it's okay to depend on that.
.Bd -literal -offset 0i
#include <sys/types.h>		/* Non-local includes in brackets. */
.Ed
.Pp
If it's a network program, put the network include files next.
.Bd -literal -offset 0i
#include <net/if.h>
#include <net/if_dl.h>
#include <net/route.h>
#include <netinet/in.h>
#include <protocols/rwhod.h>
.Ed
.Pp
Then there's a blank line, followed by the
.Pa /usr/include
files.
The
.Pa /usr/include
files should be sorted!
.Bd -literal -offset 0i
#include <stdio.h>
.Ed
.Pp
Global pathnames are defined in
.Pa /usr/include/paths.h .
Pathnames local to the program go in
.Pa pathnames.h
in the local directory.
.Bd -literal -offset 0i
#include <paths.h>
.Ed
.Pp
Then, there's a blank line, and the user include files.
.Bd -literal -offset 0i
#include "pathnames.h"		/* Local includes in double quotes. */
.Ed
.Pp
All functions are prototyped somewhere.
.Pp
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
.Ql static .
This should not be done in kernel land since it makes it impossible
to use the kernel debugger.
.Pp
Functions used from other parts of the kernel are prototyped in the
relevant include file.
.Pp
Functions that are used locally in more than one module go into a
separate header file, e.g.,
.Pa extern.h .
.Pp
Use of the
.Li __P
macro has been deprecated.
It is allowed in code imported from other sources but should not be
used in native
.Ox
code.
Prototypes should not have variable names associated with the types; i.e.:
.Bd -literal -compact -offset 0i
void	function(int);
.Ed
.Pp
not:
.Bd -literal -compact -offset 0i
void	function(int a);
.Ed
.Pp
Prototypes may have an extra space after a tab to enable function names
to line up:
.Bd -literal -offset 0i
static char	*function(int, const char *);
static void	 usage(void);
.Ed
.Pp
There should be no space between the function name and the argument list.
.Pp
Use
.Li __dead
from
.Aq Pa sys/cdefs.h
for functions that don't return, i.e.,
.Bd -literal -offset 0i
__dead void	abort(void);
.Ed
.Pp
In header files, put function prototypes within
.Dv __BEGIN_DECLS / __END_DECLS
matching pairs.
This makes the header file usable from C++.
.Pp
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
.Dq Li do
loop,
so that it can safely be used in
.Dq Li 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.
.Bd -literal -offset 0i
#define	MACRO(x, y) do {						\e
	variable = (x) + (y);						\e
	(y) += 2;							\e
} while (0)
.Ed
.Pp
Enumeration values are all uppercase.
.Bd -literal -offset 0i
enum enumtype { ONE, TWO } et;
.Ed
.Pp
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
.Ql int^Ix;
and
.Ql struct^Ifoo *x; .
.Pp
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
.Dq Li extern
if they are declared in a header file.
.Bd -literal -offset 0i
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 */
.Ed
.Pp
Use
.Xr queue 3
macros rather than rolling your own lists, whenever possible.
Thus, the previous example would be better written:
.Bd -literal -offset 0i
#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 */
.Ed
.Pp
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
.Dq Li \&_t ,
except as specified in Standard C or by
.Tn POSIX .
Don't use the same name for a struct tag and a typedef, as this makes
the code unusable from C++.
.Bd -literal -offset 0i
/* Make the structure name match the typedef. */
typedef struct _bar {
	int	level;
} BAR;
.Ed
.Pp
.Bd -literal -offset 0i
/*
 * 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[])
{
	long num;
	int ch;
	char *ep;

.Ed
.Pp
For consistency,
.Xr getopt 3
should be used to parse options.
Options should be sorted in the
.Xr 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.
.Bd -literal -offset 0i
	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 = strtol(optarg, &ep, 10);
			if (num <= 0 || *ep != '\e0') {
				warnx("illegal number, -n argument -- %s",
				    optarg);
				usage();
			}
			break;
		case '?':
		default:
			usage();
			/* NOTREACHED */
		}
	argc -= optind;
	argv += optind;

.Ed
.Pp
Use space after keywords
.Pf ( Li if ,
.Li while ,
.Li for ,
.Li return ,
.Li 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.
Forever loops are done with
.Li for ,
not
.Li while .
.Bd -literal -offset 0i
	for (p = buf; *p != '\e0'; ++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;
	}
.Ed
.Pp
Parts of a for loop may be left empty.
Don't put declarations inside blocks unless the routine is
unusually complicated.
.Bd -literal -offset 0i
	for (; cnt < 15; cnt++) {
		stmt1;
		stmt2;
	}
.Ed
.Pp
Indentation is an 8 character tab.
Second level indents are four spaces.
.Bd -literal -offset 0i
	while (cnt < 20)
		z = a + really + long + statement + that + needs +
		    two lines + gets + indented + four + spaces +
		    on + the + second + and + subsequent + lines;
.Ed
.Pp
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.
.Pp
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.
.Bd -literal -offset 0i
	if (test)
		stmt;
	else if (bar) {
		stmt;
		stmt;
	} else
		stmt;
.Ed
.Pp
Do not use spaces after function names.
Commas have a space after them.
Do not use spaces after
.Sq \&(
or
.Sq \&[
or preceding
.Sq \&]
or
.Sq \&)
characters.
.Bd -literal -offset 0i
	if ((error = function(a1, a2)))
		exit(error);
.Ed
.Pp
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?
.Bd -literal -offset 0i
	a = b->c[0] + ~d == (e || f) || g && h ? i : j >> 1;
	k = !(l & FLAGS);
.Ed
.Pp
Exits should be 0 on success, or non-zero for errors.
.Bd -literal -offset 0i
	exit(0);	/*
			 * Avoid obvious comments such as
			 * "Exit 0 on success."
			 */
}
.Ed
.Pp
The function type should be on a line by itself
preceding the function.
.Bd -literal -offset 0i
static char *
function(int a1, int a2, float fl, int a4)
{
.Ed
.Pp
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
.Dq Pa extern.h .
If a line overflows reuse the type keyword.
.Pp
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!
.Bd -literal -offset 0i
	struct foo one, *two;
	double three;
	int *four, five;
	char *six, seven, eight, nine, ten, eleven, twelve;

	four = myfunction();
.Ed
.Pp
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, such as
.Dq Li gcc -Wtraditional .
.Pp
Casts and
.Fn sizeof
calls are not followed by a space.
Note that
.Xr indent 1
does not understand this rule.
.Pp
Use of the
.Dq 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
.Dq register
specifier is required for proper code generation in the absence of
compiler optimization.
.Pp
When using
.Fn longjmp
or
.Fn vfork
in a program, the
.Fl W
or
.Fl Wall
flag should be used to verify that the compiler does not generate
warnings such as
.Bd -literal -offset 0i
warning: variable `foo' might be clobbered by `longjmp' or `vfork'.
.Ed
.Pp
If any warnings of this type occur you must apply the
.Dq volatile
specifier 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
.Dq volatile
specifies if the type-qualifier applies to the pointer, or the thing being
pointed to.
A volatile pointer is declared with
.Dq volatile
to the right of the
.Dq * .
Example:
.Bd -literal -offset 0i
char *volatile foo;
.Ed
.Pp
says that
.Dq foo
is volatile, but
.Dq *foo
is not.
To make
.Dq *foo
volatile use the syntax
.Bd -literal -offset 0i
volatile char *foo;
.Ed
.Pp
If both the pointer and the thing pointed to are volatile use
.Bd -literal -offset 0i
volatile char *volatile foo;
.Ed
.Pp
.Dq const
is also a type-qualifier and the same rules apply.
The description of a read-only hardware register might look something like:
.Bd -literal -offset 0i
const volatile char *reg;
.Ed
.Pp
Global flags set inside signal handlers should be of type
.Dq 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.
.Pp
.Dv NULL
is the preferred null pointer constant.
Use
.Dv 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
.Dv NULL ,
i.e., use:
.Bd -literal -offset 0i
(p = f()) == NULL
.Ed
.Pp
not:
.Bd -literal -offset 0i
!(p = f())
.Ed
.Pp
Don't use
.Ql \&!
for tests unless it's a boolean, i.e., use
.Bd -literal -offset 0i
if (*p == '\e0')
.Ed
.Pp
not
.Bd -literal -offset 0i
if (!*p)
.Ed
.Pp
Routines returning
.Li void *
should not have their return values cast to any pointer type.
.Pp
Use
.Xr err 3
or
.Xr warn 3 ,
don't roll your own!
.Bd -literal -offset 0i
	if ((four = malloc(sizeof(struct foo))) == NULL)
		err(1, (char *)NULL);
	if ((six = (int *)overflow()) == NULL)
		errx(1, "Number overflowed.");
	return (eight);
}
.Ed
.Pp
Old-style function declarations look like this:
.Bd -literal -offset 0i
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. */
{
	...
}
.Ed
.Pp
Use ANSI function declarations unless you explicitly need K&R compatibility.
Long parameter lists are wrapped with a normal four space indent.
.Pp
Variable numbers of arguments should look like this:
.Bd -literal -offset 0i
#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 */
.Ed
.Pp
Usage statements should look like the 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
.Pq Sq \&|
separates either-or options/arguments,
and multiple options/arguments which are specified together are
placed in a single set of braces.
.Pp
.Bd -ragged -offset 0.3i
"usage: f [-ade] [-b b_arg] [-m m_arg] req1 req2 [opt1 [opt2]]\en"
"usage: f [-a | -b] [-c [-de] [-n number]]\en"
.Ed
.Pp
The
.Li __progname
string, should be used instead of hard-coding the program name.
.Bd -literal -offset 0i
	(void)fprintf(stderr, "usage: %s [-ab]\en", __progname);
	exit(1);
}
.Ed
.Pp
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.
.Pp
Stylistic changes (including whitespace changes) are hard on the source
repository and are to be avoided without good reason.
Code that is approximately KNF compliant in the repository must not diverge
from compliance.
.Pp
Whenever possible, code should be run through a code checker
(e.g.,
.Dq Li gcc -Wall -W -Wtraditional -Wpointer-arith -Wbad-function-cast ... ,
.Xr lint 1
or lclint from the ports tree) and produce minimal warnings.
.Sh SEE ALSO
.Xr indent 1 ,
.Xr err 3 ,
.Xr queue 3 ,
.Xr warn 3
.Sh HISTORY
This man page is largely based on the src/admin/style/style file from
the
.Tn BSD
4.4-Lite2 release, with updates to reflect the current practice and
desire of the
.Ox
project.