man/man9/style.9

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.Dd December 14, 1995
.Dt STYLE 9
.Os FreeBSD
.Sh NAME
.Nm style
.Nd "Kernel source file style guide"
.Sh DESCRIPTION
This file specifies the preferred style for kernel source files in the
.Tn FreeBSD
source tree.  It is also a guide for preferred userland code style.
.Bd -literal -offset 0i
/*
 * Style guide for the FreeBSD 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. 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.
.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 /usr include files.
The /usr include files should be sorted!
.Bd -literal -offset 0i
#include <stdio.h>
.Ed
.Pp
Global pathnames are defined in /usr/include/paths.h.  Pathnames local
to the program go in 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
Macros are capitalized, 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; it makes it 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, and 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 "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 .
.Bd -literal -offset 0i
/* Make the structure name match the typedef. */
typedef struct _bar {
	int	level;
} BAR;
.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.  Functions
local to one source module should be declared
.Ql static .
.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
Only use the __P macro from the include file <sys/cdefs.h> if the source
file in general is (to be) compilable with a K&R Old Testament compiler.
Use of the __P macro in new code is discouraged, although modifications
to existing files should be consistent with that file's conventions.
.Pp
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.
.Pp
The kernel has a name associated with parameter types, e.g., in the kernel
use:
.Bd -literal -offset 0i
void	function(int fd);
.Ed
.Pp
In header files visible to userland applications, prototypes that are
visible must use either protected names or no names with the types.  It
is preferable to use protected names.
e.g., use:
.Bd -literal -offset 0i
void	function(int);
.Ed
.Pp
or:
.Bd -literal -offset 0i
void	function(int _fd);
.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 _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[])
{
	long num;
	int ch;
	char *ep;

.Ed
.Pp
For consistency, getopt should be used to parse options.  Options
should be sorted in the getopt 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
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.
Forever loops are done with for's, not while's.
.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;
	}
	if (val != NULL)
		val = realloc(val, newsize);
.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.
.Bd -literal -offset 0i
	if (test)
		stmt;
	else if (bar) {
		stmt;
		stmt;
	} else
		stmt;
.Ed
.Pp
No spaces after function names.  Commas have a space after them.  No 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 or unless the
statement is confusing without them.  Remember that other people may
confuse easier 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 according to the predefined
values in
.Xr sysexits 3 .
.Bd -literal -offset 0i
	exit(EX_OK);	/*
			 * 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,
then in alphabetical order; multiple ones per line are okay.
Declaring functions inside functions is not recommended, since their
linkage scope is always global.  If a line overflows reuse the type
keyword.
.Pp
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!
.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.
.Pp
Casts and sizeof's are not followed by a space.  Note that
.Xr indent 1
does not understand this rule.
.Pp
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:
.Bd -literal -offset 0i
(p = f()) == NULL
.Ed
.Pp
not:
.Bd -literal -offset 0i
!(p = f())
.Ed
.Pp
Don't use '!' for tests unless it's a boolean, e.g. use
.Bd -literal -offset 0i
if (*p == '\e0')
.Ed
.Pp
not
.Bd -literal -offset 0i
if (!*p)
.Ed
.Pp
Routines returning 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.
.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()
{
	/* Insert an empty line if the function has no local variables. */
.Ed
.Pp
Use
.Xr printf 3 ,
not fputs/puts/putchar/whatever; it's faster and usually cleaner, not
to mention avoiding stupid bugs.
.Pp
Usage statements should look like the manual pages.  Options w/o
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
.Bd -literal -offset 0i
	(void)fprintf(stderr, "usage: f [-ab]\en");
	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., "gcc -Wall" or xlint(1)) and produce minimal warnings.

.Sh SEE ALSO
.Xr indent 1 ,
.Xr err 3 ,
.Xr sysexits 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
.Tn FreeBSD
project.