man/man9/style.9

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.\"	$OpenBSD: style.9,v 1.76 2020/05/10 00:56:06 guenther Exp $
.\"
.Dd $Mdocdate: May 10 2020 $
.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 userland 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 indent
/*
 * 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.,
.In sys/*.h )
come first; normally, you'll need
.In sys/types.h
OR
.In sys/param.h ,
but not both!
.In sys/types.h
includes
.In sys/cdefs.h ,
and it's okay to depend on that.
.Bd -literal -offset indent
#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 indent
#include <net/if.h>
#include <net/if_dl.h>
#include <net/route.h>
#include <netinet/in.h>
.Ed
.Pp
Then there's a blank line, followed by the
.Pa /usr/include
files.
The
.Pa /usr/include
files, for the most part, should be sorted.
.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 indent
#include <paths.h>
.Ed
.Pp
Then there's a blank line, and the user include files.
.Bd -literal -offset indent
#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 userland, 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 files 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
Prototypes should not have variable names associated with the types; i.e.,
.Bd -literal -offset indent
void	function(int);
.Ed
not:
.Bd -literal -offset indent -compact
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 indent
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
.In sys/cdefs.h
for functions that don't return, i.e.,
.Bd -literal -offset indent
__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 indent
#define	MACRO(x, y) do {					\e
	variable = (x) + (y);					\e
	(y) += 2;						\e
} while (0)
.Ed
.Pp
If a macro with arguments declares local variables,
those variables should use identifiers beginning with two underscores.
This is required for macros implementing C and POSIX interfaces
and recommended for all macros for consistency.
.Pp
Enumeration values are all uppercase.
.Bd -literal -offset indent
enum enumtype { ONE, TWO } et;
.Ed
.Pp
When defining unsigned integers use
.Dq "unsigned int"
rather than just
.Dq "unsigned" ;
the latter has been a source of confusion in the past.
.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 indent
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 indent
#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 POSIX.
.Bd -literal -offset indent
/*
 * 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;
.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.
.Bd -literal -offset indent
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();
	}
}
argc -= optind;
argv += optind;
.Ed
.Pp
Use a 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.
.Bd -literal -offset indent
for (p = buf; *p != '\e0'; ++p)
	continue;
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.
.Bd -literal -offset indent
for (; cnt < 15; cnt++) {
	stmt1;
	stmt2;
}
.Ed
.Pp
Indentation is an 8 character tab.
Second level indents are four spaces.
All code should fit in 80 columns.
.Bd -literal -offset indent
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 indent
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 indent
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 indent
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 indent
/*
 * Avoid obvious comments such as
 * "Exit 0 on success."
 */
exit(0);
.Ed
.Pp
The function type should be on a line by itself
preceding the function.
.Bd -literal -offset indent
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.
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 indent
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.
.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 indent
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
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
.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 indent
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 indent
volatile char *foo;
.Ed
.Pp
If both the pointer and the thing pointed to are volatile use
.Bd -literal -offset indent
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 indent
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 all cases except for arguments to variadic
functions where the compiler does not know the type.
.Pp
Don't use
.Ql \&!
for tests unless it's a boolean, i.e., use
.Bd -literal -offset indent
if (*p == '\e0')
.Ed
not
.Bd -literal -offset indent -compact
if (!*p)
.Ed
.Pp
Routines returning
.Li void *
should not have their return values cast to any pointer type.
.Pp
Use the
.Xr err 3
and
.Xr warn 3
family of functions.
Don't roll your own!
.Bd -literal -offset indent
if ((four = malloc(sizeof(struct foo))) == NULL)
	err(1, NULL);
if ((six = (int *)overflow()) == NULL)
	errx(1, "Number overflowed.");
return eight;
.Ed
.Pp
Always use ANSI function definitions.
Long parameter lists are wrapped with a normal four space indent.
.Pp
Variable numbers of arguments should look like this:
.Bd -literal -offset indent
#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)
{
.Ed
.Pp
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.
.Pp
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
If numbers are used as options, they should be placed first,
as shown in the example below.
Uppercase letters take precedence over lowercase.
.Bd -literal -offset indent
"usage: f [-12aDde] [-b b_arg] [-m m_arg] req1 req2 [opt1 [opt2]]\en"
"usage: f [-a | -b] [-c [-de] [-n number]]\en"
.Ed
.Pp
The
.Xr getprogname 3
function may be used instead of hard-coding the program name.
.Bd -literal -offset indent
fprintf(stderr, "usage: %s [-ab]\en", getprogname());
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
Whenever possible, code should be run through a code checker
(e.g.,
.Dq Li gcc -Wall -W -Wpointer-arith -Wbad-function-cast ...\&
or splint from the ports tree) and produce minimal warnings.
Since lint has been removed, the only lint-style comment that should
be used is FALLTHROUGH, as it's useful to humans.
Other lint-style comments such as ARGSUSED, LINTED, and NOTREACHED
may be deleted.
.Pp
Note that documentation follows its own style guide,
as documented in
.Xr mdoc 7 .
.Sh FILES
.Bl -tag -width "/usr/share/misc/license.template " -compact
.It Pa /usr/share/misc/license.template
Example license for new code.
.El
.Sh SEE ALSO
.Xr indent 1 ,
.Xr err 3 ,
.Xr queue 3 ,
.Xr warn 3 ,
.Xr mdoc 7
.Sh HISTORY
This man page is largely based on the src/admin/style/style file from the
.Bx 4.4 Lite2
release, with updates to reflect the current practice and
desire of the
.Ox
project.