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STYLE(9)	       FreeBSD 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 the FreeBSD.  Based on 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 (i.e. sys/*.h) 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 angle 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. */

     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 expan­
     sions 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 func­
     tion name is all in lowercase and the macro has the same name all in up­
     percase.  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 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 */

     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;   /* Queue macro glue for foo lists */
	     struct  mumble amumble; /* Comment for mumble */
	     int     bar;
     };
     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 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') {
				     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 permit­
     ted.  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.  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 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
     aren't 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.

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

     Unary operators don't require spaces, binary operators do.  Don't use
     parentheses unless they're 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
     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 func­
     tions inside functions is not recommended, 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 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

     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 compatibil­
     ity.

     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 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 in the command line.

     4.   Finally, any optional arguments should be listed, listed in the or­
	  der they should be specified, and all inside brackets.

     A bar (`|') separates either-or options/arguments, and multiple op­
     tions/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);
     }

     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 ap­
     proximately style(9) compliant in the repository must not diverge from
     compliance.

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

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.

FreeBSD 		       December 14, 1995			     7

NAME | DESCRIPTION | SEE ALSO | HISTORY

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