STYLE(9)

NAME
style - Kernel source file style guide

DESCRIPTION
This file contains an example of the preferred style for kernel source files in the FreeBSD source tree.

    /*
     * Style guide for the 4BSD KNF (Kernel Normal Form).
     *
     *	@(#)style	1.14 (Berkeley) 4/28/95
     *
     *	$Id: style.9,v 1.16 1997/03/21 20:14:15 mpp 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.
     */

Kernel include files 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. */

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.

    #define	MACRO(x, y) {							\
    	variable = (x) + (y);						\
    	(y) += 2;							\
    }

Enum types are capitalized.

    enum enumtype { ONE, TWO } et;

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 ‘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 */
    /* Make the structure name match the typedef. */
    typedef struct _bar {
    	int	level;
    } BAR;

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 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.

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.

Only the kernel has a name associated with the types, i.e. in the kernel use:

    void function __P((int fd));

in user land use:

    	void function __P((int));
    static char	*function __P((int, const char *));
    static void	 usage __P((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(argc, argv)
    	int argc;
    	char *argv[];
    {
    	extern char *optarg;
    	extern int optind;
    	long num;
    	int ch;
    	char *ep;

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.

    	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')
    				err("illegal number -- %s", optarg);
    			break;
    		case '?':
    		default:
    			usage();
    			/* NOTREACHED */
    		}
    	argc -= optind;
    	argv += optind;

Space after keywords (while, for, return, switch). No braces are used for control statements with zero or only a single statement.

Forever loops are done with for's, not while's.

    	for (p = buf; *p != '\0'; ++p);
    	for (;;)
    		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;
    	}

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.

Closing and opening braces go on the same line as the else. Don't add braces that aren't necessary.

    	if (test)
    		stmt;
    	else if (bar) {
    		stmt;
    		stmt;
    	} else
    		stmt;

No spaces after function names.

    	if (error = function(a1, a2))
    		exit(error);

Unary operators don't require spaces, binary operators do. Don't use parenthesis unless they're required for precedence, or the statement is really confusing without them.

    	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(a1, a2, fl, a4)
    	int a1, a2, a4;	/* Declare ints, too, don't default them. */
    	float fl;	/* List in order declared, as much as possible. */
    {

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 recommendable, since their linkage scope is always global. 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.

    	extern u_char one;
    	extern char two;
    	struct foo three, *four;
    	double five;
    	int *six, seven, eight();
    	char *nine, ten, eleven, twelve, thirteen, fourteen, fifteen;
    	char *overflow __P((void));
    	void *mymalloc __P((u_int));

Casts and sizeof's are not followed by a space.

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 varadic args and is necessary for other args if the function prototype might not be in scope; since we pretend to support K&R compilers, most prototypes might not be in scope.) Test pointers against NULL, e.g., use:

    (p = f()) == NULL

not:

    !(p = f())

Don't use '!' for tests unless it's 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.

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);
    }

Don't use ANSI function declarations unless you absolutely have too, i.e. you're declaring functions with variable numbers of arguments.

ANSI function return values and braces look like regular functions.

    int
    function(int a1, int a2)
    {
    	...
    }

Variable numbers of arguments should look like this.

    #if __STDC__
    #include <stdarg.h>
    #else
    #include <varargs.h>
    #endif
    void
    #if __STDC__
    vaf(const char *fmt, ...)
    #else
    vaf(fmt, va_alist)
    	char *fmt;
    	va_dcl
    #endif
    {
    	va_list ap;
    #if __STDC__
    	va_start(ap, fmt);
    #else
    	va_start(ap);
    #endif
    	STUFF;
    	va_end(ap);		/* No return needed for void functions. */
    }
    static void
    usage()
    {
    	/* Insert an empty line if the function has no local variables. */

Use printf(3), not fputs/puts/putchar/whatever, it's faster and usually cleaner, not to mention avoiding stupid bugs.

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 (‘|’) separates either/or options/arguments, and multiple options/arguments which are specified together are placed in a single set of braces.

"usage: f [-ade] [-b b_arg] [-m m_arg] req1 req2 [opt1 [opt2]]\n" "usage: f [-a | -b] [-c [-de] [-n number]]\n"

    	(void)fprintf(stderr, "usage: f [-ab]\n");
    	exit(1);
    }

Note that the policy regarding the usage of K&R versus ANSI function definitions could not be commonly agreed to. While keeping the old form is more consistent with the existing code base, sticking to it defeats the migration to the more modern ANSI style. For new code, chose what you feel is more important. However, when modifying existing subsystems or files, stick with the style that is already there.

SEE ALSO
err(3), sysexits(3), warn(3)

HISTORY
This man page is largely based on the src/admin/style/style file from the BSD 4.4-Lite2 release, with a few updates to reflect the current practice and desire of the FreeBSD project.