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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 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
     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 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.  Typedefs are problematic
     because they do not properly hide their underlying type; for example you
     need to know if the typedef is the structure itself or a pointer to the
     structure.  In addition they must be declared exactly once, whereas an
     incomplete structure type can be mentioned as many times as necessary.
     Typedefs are difficult to use in stand-alone header files: the header
     that defines the typedef must be included before the header that uses it,
     or by the header that uses it (which causes namespace pollution), or
     there must be a back-door mechanism for obtaining the typedef.

     When convention requires a typedef, make its name match the struct tag.

     /* Make the structure name match the typedef. */
     typedef struct bar {
             int     level;
     } bar_t;

     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 that are not exported
     outside of the kernel should be declared __private_extern__.

     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".

     Do not use the __P macro.

     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
     underscore) or no names with the types.  It is preferable to use pro-
     tected 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++) {

     Variable names should contain underscores to separate words.  DO NOT use

     Indentation is an 8 character tab.  All code should fit in 80 columns.
     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 &&
                    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.  sizeof's are written with parentheses always.

     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.

     Values in return statements should be enclosed in parentheses.

     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);

     Use ANSI function declarations.

     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. */

     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 compliant with the style guides.

     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.

     Code should be run through a code checker (e.g., sparse or gcc -Wall
     -Werror ).


     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

Mac OS X 10.9 - Generated Fri Oct 18 15:45:17 CDT 2013
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