STYLE(9)                     OpenBSD Kernel Manual                    STYLE(9)

NNAAMMEE
     ssttyyllee - Kernel source file style guide (KNF)

DDEESSCCRRIIPPTTIIOONN
     This file specifies the preferred style for kernel source files in the
     OpenBSD source tree.  It is also a guide for preferred user land code
     style.  These guidelines should be followed for all new code.  In gener-
     al, 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.

     /*
      * Style guide for the OpenBSD 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 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 _/_u_s_r include files.  The _/_u_s_r
     include files should be sorted!

     #include <stdio.h>

     Global pathnames are defined in _/_u_s_r_/_i_n_c_l_u_d_e_/_p_a_t_h_s_._h. Pathnames local to
     the program go in _p_a_t_h_n_a_m_e_s_._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. */

     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.  In user land, functions
     local to one source module should be declared `static'. This should not
     be done in kernel land since it makes it impossible to use the kernel de-
     bugger.

     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., _e_x_t_e_r_n_._h.

     Use of the __P macro has been deprecated.  It is allowed in code imported
     from other sources but should not be used in native OpenBSD code.  Proto-
     types should not have variable names associated with the types; i.e.:
     void    function(int);

     not:
     void    function(int a);

     Prototypes may have an extra space after a tab to enable function names
     to line up:

     static char     *function(int, const char *);
     static void      usage(void);

     There should be no space between the function name and the argument list.

     Use __dead from <sys/cdefs.h> for functions that don't return, i.e.,

     __dead void     abort(void);

     In header files, put function prototypes within __BEGIN_DECLS /
     __END_DECLS matching pairs.  This makes the header file usable from C++.

     Macros are capitalized and 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 back-
     slashes, as the resulting definition is easier to read.  If the macro en-
     capsulates 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 (largest to smallest), 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.
     Don't use the same name for a struct tag and a typedef, as this makes the
     code unusable from C++.

     /* Make the structure name match the typedef. */
     typedef struct _bar {
             int     level;
     } BAR;

     /*
      * 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 com-
     ment,

             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;


     Use 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, not while.

             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;
             }

     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, unless they cause a compiler warning.

             if (test)
                     stmt;
             else if (bar) {
                     stmt;
                     stmt;
             } else
                     stmt;

     Do not use spaces after function names.  Commas have a space after them.
     Do not use 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, the statement is con-
     fusing without them, or the compiler generates a warning without them.
     Remember that other people may be confused more easily 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 non-zero for errors.

             exit(0);        /*
                              * 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
     (largest to smallest), then in alphabetical order; multiple ones per line
     are okay.  Old style function declarations should be avoided.  ANSI style
     function declarations should go in an include file such as ``_e_x_t_e_r_n_._h''.
     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, such as
     ``gcc -Wtraditional''.

     Casts and ssiizzeeooff() calls are not followed by a space.  Note that in-
     dent(1) does not understand this rule.

     Use of the ``register'' specifier is discouraged in new code.  Optimizing
     compilers such as gcc can generally do a better job of choosing which
     variables to be placed in registers to improve code performance.  The ex-
     ception to this is in functions containing assembly code where the
     ``register'' specifier is required for proper code generation in the ab-
     sence of compiler optimization.

     When using lloonnggjjmmpp() or vvffoorrkk() in a program, the --WW or --WWaallll flag should
     be used to verify that that compiler does not generate warnings such as

     warning: variable `foo' might be clobbered by `longjmp' or `vfork'.

     If any warnings of this type occur you must apply the ``volatile'' speci-
     fier to the variable in question.  Failure to do so may result in improp-
     er code generation when optimization is enabled.  Note that for pointers,
     the ``volatile'' should go after to the type, not before.  Use

     char * volatile foo;

     not:

     volatile char *foo;

     Global flags set inside signal handlers should be of type ``volatile
     sig_atomic_t'' if possible.  This guarantees that the variable may be ac-
     cessed as an atomic entity, even when a signal has been delivered.  Glob-
     al variables of other types (such as structures) are not guaranteed to
     have consistent values when accessed via a signal handler.

     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, i.e., use:

     (p = f()) == NULL

     not:

     !(p = f())

     Don't use `!' for tests unless it's a boolean, i.e., 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.  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(void)
     {
             extern char *__progname;        /* from crt0.o */

     Usage statements should look like the manual pages.  Options without
     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 (`|') sepa-
     rates 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"

     The __progname string, should be used instead of hard-coding the program
     name.

             (void)fprintf(stderr, "usage: %s [-ab]\n", __progname);
             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 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 KNF compliant in the repository must not diverge from compli-
     ance.

     Whenever possible, code should be run through a code checker (e.g., ``gcc
     -Wall -W -Wtraditional -Wpointer-arith -Wbad-function-cast ...'', lint(1)
     or lclint from the ports tree) and produce minimal warnings.

SSEEEE AALLSSOO
     indent(1), err(3), queue(3), warn(3)

HHIISSTTOORRYY
     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 OpenBSD project.

OpenBSD 3.2                      June 18, 2001                               7
