STYLE(9) Kernel Developer's Manual STYLE(9)

style — kernel source file style guide

This file specifies the preferred style for kernel source files in the FreeBSD source tree. It is also a guide for preferred userland code style.

/*
 * Style guide for FreeBSD.  Based on the CSRG's KNF (Kernel Normal Form).
 *
 *	@(#)style	1.14 (Berkeley) 4/28/95
 * $FreeBSD: src/share/man/man9/style.9,v 1.32.2.16 2001/12/17 11:30:19 ru Exp $
 */

/*
 * 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.
 */

After any copyright header, there is a blank line, and the rcsid for source files. Version control system ID tags should only exist once in a file (unlike 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 from files obtained from elsewhere should be maintained, including, where applicable, multiple IDs showing a file's history. In general, keep the IDs intact, including any ‘$’s. There is no reason to add "From" in front of foreign VCS IDs. Most non-FreeBSD VCS IDs should be indented by a tab if in a comment.

#include <sys/cdefs.h>
__RCSID("@(#)style	1.14 (Berkeley) 4/28/95");
__FBSDID("$FreeBSD: src/share/man/man9/style.9,v 1.32.2.16 2001/12/17 11:30:19 ru Exp $");

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>

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. If a macro needs more than a single line, use braces (‘{’ and ‘}’). 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)

Enumeration values are all uppercase.

enum enumtype { ONE, TWO } et;

When declaring variables in structures, declare them sorted by use, then by size, 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 if it suffices to align most 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.

/* Make the structure name match the typedef. */
typedef struct bar {
	int	level;
} bar_t;

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 that are not exported outside of the kernel should be declared __private_extern__.

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".

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[])
{
	long num;
	int ch;
	char *ep;

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 = strtol(optarg, &ep, 10);
			if (num <= 0 || *ep != '\0') {
				warnx("illegal number, -n argument -- %s",
				    optarg);
				usage();
			}
			break;
		case '?':
		default:
			usage();
			/* NOTREACHED */
		}
	argc -= optind;
	argv += optind;

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.

	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;
	}

Variable names should contain underscores to separate words. DO NOT use StudlyCaps.

Indentation is an 8 character tab. All code should fit in 80 columns. 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 according to the predefined values in sysexits(3).

	exit(EX_OK);	/*
			 * 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, 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 parentheses always.

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);
}

Use ANSI function declarations.

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. */
}

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:

  1. Options without operands come first, in alphabetical order, inside a single set of brackets (‘[’ and ‘]’).
  2. Options with operands come next, also in alphabetical order, with each option and its argument inside its own pair of brackets.
  3. Required arguments (if any) are next, listed in the order they should be specified on the command line.
  4. 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(EX_USAGE);
}

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 compliant with the style guides.

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.

Code should be run through a code checker (e.g., sparse or gcc -Wall -Werror ).

indent(1), lint(1), err(3), sysexits(3), warn(3)

This man 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.

December 7, 2001 macOS 15.0