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STYLE(9)			  BSD Kernel Developer's Manual 			 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 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 gen-
     eral, 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 excep-
     tions.  Each one gets its own line.  Try to make the structure readable by aligning the mem-
     ber 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 previ-
     ous 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 convention requires a typedef, make its name match the struct
     tag.  Avoid typedefs 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 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 cur-
     rent 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;
	     }

     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 line.

	     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)
		     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.  Remem-
     ber 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.

     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.

     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 compatibility.  Long parameter
     lists are wrapped with a normal four space indent.

     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(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 alphabeti-
     cal 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.

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

HISTORY
     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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