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STYLE(9)		 BSD 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 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 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 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 pro‐
     gram 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 paren‐
     theses.  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 encap‐
     sulates a compound statement, enclose it in a do loop, so that it can
     safely be used in if statements.  Any final statement-terminating semi‐
     colon 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 possi‐
     ble.  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.	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 conven‐
     tion requires a typedef, make its name match the struct tag.  Avoid type‐
     defs ending in “_t”, except as specified in Standard C or by POSIX.

     /* 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 else‐
     where) 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 rele‐
     vant include file.

     Functions that are used locally in more than one module go into a sepa‐
     rate 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 ker‐
     nel 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 under‐
     score) or no names with the types.	 It is preferable to use protected
     names.  E.g., use:

     void    function(int);


     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.
     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 com‐

	     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;
		     case 'n':
			     num = strtol(optarg, &ep, 10);
			     if (num <= 0 || *ep != '\0') {
				     warnx("illegal number, -n argument -- %s",
		     case '?':
			     /* 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 permit‐
     ted.  Forever loops are done with for's, not while's.

	     for (p = buf; *p != '\0'; ++p)
		     ;	     /* nothing */
	     for (;;)
	     for (;;) {
		     z = a + really + long + statement + that + needs +
			 two lines + gets + indented + four + spaces +
			 on + the + second + and + subsequent + lines;
	     for (;;) {
		     if (cond)
	     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++) {

     Indentation is an 8 character tab.	 Second level indents are four spaces.
     If you have to wrap a long statement, put the operator at the end of the

	     while (cnt < 20 && this_variable_name_is_too_long_for_its_own_good &&
		 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)
	     else if (bar) {
	     } else

     No spaces after function names.  Commas have a space after them.  No spa‐
     ces after ‘(’ or ‘[’ or preceding ‘]’ or ‘)’ characters.

	     error = function(a1, a2);
	     if (error != 0)

     Unary operators do not require spaces, binary operators do.  Do not use
     parentheses unless they are required for precedence or unless the state‐
     ment 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

	     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 over‐
     flows 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 declara‐
     tion.  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 variadic args and is neces‐
     sary for other args if the function prototype might not be in scope.)
     Test pointers against NULL, e.g., use:

     (p = f()) == NULL


     !(p = f())

     Do not use ! for tests unless it is a boolean, e.g. use

     if (*p == '\0')


     if (!*p)

     Routines returning void * should not have their return values cast to any
     pointer type.

     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);

     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 compatibil‐
     ity.  Long parameter lists are wrapped with a normal four space indent.

     Variable numbers of arguments should look like this.

     #include <stdarg.h>

     vaf(const char *fmt, ...)
	     va_list ap;

	     va_start(ap, fmt);
	     /* No return needed for void functions. */

     static void
	     /* Insert an empty line if the function has no local variables. */

     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");

     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 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.

     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.

     Whenever possible, code should be run through a code checker (e.g.,
     lint(1) or gcc -Wall) and produce minimal warnings.

     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.

BSD			       December 7, 2001				   BSD

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