STYLE(9)

NAME
style - Kernel source file style guide

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

    /*
     * 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.
     */

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. Use of the __P macro in new code is discouraged, although modifications to existing files should be consistent with that file's conventions.

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 user land 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:

    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 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 (if, 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)
    		;	/* nothing */
    	for (;;)
    		stmt;
    	if (val != NULL)
    		val = realloc(val, newsize);

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;
    	}

Indentation is an 8 character tab. 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.

Do not add whitespace at the end of a line, and only use tabs then 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. Don't add braces that aren't necessary.

    	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.

    	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(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. 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. 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.

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.) 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);
    }

Old-style function declarations look like this:

    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. */
    {

Use ANSI function declarations unless you explicitly need K&R compatability.

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. */
    }
    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);
    }

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 consistant with their style.

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.

SEE ALSO
indent(1), 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 updates to reflect the current practice and desire of the FreeBSD project.