STYLE(9)

🌸 NAME 🌸

style — kernel source file style guide

✨ DESCRIPTION ✨

Okay so bestie, this file specifies the preferred style for kernel source files in the FreeBSD source tree. It is also a guide for the preferred userland code style. Many of the style rules are implicit in the examples. Be careful to check the examples before assuming that style is silent on an issue. The way that the examples are literally carrying the whole vibe? Obsessed. 💅

/*
 * Style guide for FreeBSD.  Based on the CSRG's KNF (Kernel Normal Form).
 *
 *      @(#)style       1.14 (Berkeley) 4/28/95
 * $FreeBSD: releng/12.2/share/man/man9/style.9 359375 2020-03-27 19:08:10Z emaste $
 */

Not me showing you the comment lore. Single-line comments have levels, iykyk. 🎀

/*
 * 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.
 */

The copyright header should be a multi-line comment, with the first line of the comment having a dash after the star like so. It's giving very official. ✨

/*-
 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
 *
 * Copyright (c) 1984-2025 John Q. Public
 * All rights reserved.
 *
 * Long, boring license goes here, but trimmed for brevity
 */

An automatic script collects license information from the tree for all comments that start in the first column with /*-. If you desire to flag indent(1) to not reformat a comment that starts in the first column which is not a license or copyright notice, change the dash to a star for those comments. Comments starting in columns other than the first are never considered license statements. Use the appropriate SPDX-License-Identifier line before the copyright. Fr fr, don't skip the SPDX line, it's the main character of license headers. 👑

After any copyright header, there is a blank line, and the $FreeBSD$ for non C/C++ language source files. Version control system ID tags should only exist once in a file (unlike in 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 in files obtained from elsewhere should be maintained, including, where applicable, multiple IDs showing a file's history. In general, do not edit foreign IDs or their infrastructure. Unless otherwise wrapped (such as #if defined(LIBC_SCCS)), enclose both in #if 0 ... #endif to hide any uncompilable bits and to keep the IDs out of object files. Only add From: in front of foreign VCS IDs if the file is renamed. Lowkey a lot of rules but highkey important. 💀

/* From: @(#)style      1.14 (Berkeley) 4/28/95 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD: releng/12.2/share/man/man9/style.9 359375 2020-03-27 19:08:10Z emaste $");

Leave one blank line before the header files. Blank lines are the punctuation of code, bestie. 🫶

Kernel include files (sys/*.h) come first. If <sys/cdefs.h> is needed for __FBSDID(), include it first. If either <sys/types.h> or <sys/param.h> is needed, include it before other include files. (<sys/param.h> includes <sys/types.h>; do not include both.) The remaining kernel headers should be sorted alphabetically. Alphabetical order? Slay. 🦋

#include <sys/types.h>  /* Non-local includes in angle brackets. */
#include <sys/endian.h>
#include <sys/lock.h>
#include <sys/queue.h>

For a network program, put the network include files next. Networking besties, this is your moment. ✨

#include <net/if.h>
#include <net/if_dl.h>
#include <net/route.h>
#include <netinet/in.h>
#include <protocols/rwhod.h>

Do not include files from /usr/include in the kernel. Just don't. It's not the vibe. Periodt. 💅

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 local 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. Periodt. 👑

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. 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. The backslash alignment? It's giving attention to detail. 💅

#define MACRO(x, y) do {                                                \
        variable = (x) + (y);                                           \
        (y) += 2;                                                       \
} while (0)

When code is conditionally compiled using #ifdef or #if, a comment may be added following the matching #endif or #else to permit the reader to easily discern where conditionally compiled code regions end. This comment should be used only for (subjectively) long regions, regions greater than 20 lines, or where a series of nested #ifdef 's may be confusing to the reader. The comment should be separated from the #endif or #else by a single space. For short conditionally compiled regions, a closing comment should not be used. Don't over-comment, bestie, it's not cute. 🙈

The comment for #endif should match the expression used in the corresponding #if or #ifdef. The comment for #else and #elif should match the inverse of the expression(s) used in the preceding #if and/or #elif statements. In the comments, the subexpression defined(FOO) is abbreviated as FOO. For the purposes of comments, #ifndef FOO is treated as #if !defined(FOO).

#ifdef KTRACE
#include <sys/ktrace.h>
#endif

#ifdef COMPAT_43
/* A large region here, or other conditional code. */
#else /* !COMPAT_43 */
/* Or here. */
#endif /* COMPAT_43 */

#ifndef COMPAT_43
/* Yet another large region here, or other conditional code. */
#else /* COMPAT_43 */
/* Or here. */
#endif /* !COMPAT_43 */

The project prefers the use of ISO/IEC 9899:1999 (“ISO C99”) unsigned integer identifiers of the form uintXX_t rather than the older BSD-style integer identifiers of the form u_intXX_t. New code should use the former, and old code should be converted to the new form if other major work is being done in that area and there is no overriding reason to prefer the older BSD-style. Like white-space commits, care should be taken in making uintXX_t only commits. So no mixing, okay? Okay? 🌈

Similarly, the project prefers the use of ISO C99 bool rather than the older int or boolean_t. New code should use bool, and old code may be converted if it is reasonable to do so. Literal values are named true and false. These are preferred to the old spellings TRUE and FALSE. Userspace code should include <stdbool.h>, while kernel code should include <sys/types.h>. Bool is the moment. 💜

Likewise, the project prefers ISO C99 designated initializers when it makes sense to do so. If it makes sense, do it. If not, don't. Vibe check. ⭐

Enumeration values are all uppercase. Uppercase only, no lowercase energy. 🔥

enum enumtype { ONE, TWO } et;

The use of internal_underscores in identifiers is preferred over camelCase or TitleCase. Underscores are lowkey iconic. 🧚

In declarations, do not put any whitespace between asterisks and adjacent tokens, except for tokens that are identifiers related to types. (These identifiers are the names of basic types, type qualifiers, and typedef-names other than the one being declared.) Separate these identifiers from asterisks using a single space. The asterisk placement? It's giving precision. ✨

When declaring variables in structures, declare them sorted by use, then by size (largest to smallest), 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 only if it suffices to align at least 90% of the member names. Names following extremely long types should be separated by a single space. 90%? The math is mathing. 😭

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. Top of the file, bestie. 🎀

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;   /* Does not 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 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;   /* Does not 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. Typedefs are highkey problematic, bestie. 💀

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;
typedef int             foo;            /* This is foo. */
typedef const long      baz;            /* This is baz. */

All functions are prototyped somewhere. No exceptions. 👑

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.

okay bestie we're back and we're DEEP in the style lore now 💖 no thoughts, just vibes and tabs. let's go.

---

So like, functions that get used by other parts of the kernel? They get prototyped in the relevant include file. And bestie, list those prototypes in a logical order — preferably alphabetical, unless you have a genuinely compelling reason to do it different. we don't do chaos here 🎀

Functions that get used locally in more than one module? They go in their own separate header file, e.g. extern.h. separate little home for them, iconic.

Do not use the __P macro. just. don't. 💀

Okay and here's the thing about what counts as "new code": if it makes up about 50% or more of the file(s) involved, congrats, it's new code. That's enough to break precedents in the existing code and use the current style guidelines. main character energy, honestly. ✨

---

The kernel has a name associated with parameter types, so like, in the kernel you use:

void    function(int fd);

But in header files that are visible to userland applications, any prototype that's visible has to use either "protected" names (ones that start with an underscore) or no names with the types at all. And it is preferable to use protected names. So like, either:

void    function(int);

or:

void    function(int _fd);

Prototypes may also have an extra space after a tab so the function names line up all pretty 🌸:

static char    *function(int _arg, const char *_arg2, struct foo *_arg3,
            struct bar *_arg4);
static void     usage(void);

---

And okay, this is important — all major routines should have a comment briefly describing what they do. The comment before the "main" routine should describe what the program does. no cap.

/*
 * 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[])
{
    char *ep;
    long num;
    int ch;

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. And code that's unreachable for non-obvious reasons may be marked /* NOTREACHED */.

    while ((ch = getopt(argc, argv, "abNn:")) != -1)
        switch (ch) {        /* Indent the switch. */
        case 'a':        /* Do not indent the case. */
            aflag = 1;    /* Indent case body one tab. */
            /* FALLTHROUGH */
        case 'b':
            bflag = 1;
            break;
        case 'N':
            Nflag = 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();
        }
    argc -= optind;
    argv += optind;

---

Space after keywords (if, while, for, return, switch). Two styles of braces ({ and }) are allowed for single line statements: either they're used for all single statements, or they're used only where needed for clarity. And whatever you pick, usage within a function should be consistent. Forever loops are done with for's, not while's. periodt.

    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. And do not put declarations inside blocks unless the routine is unusually complicated. like, unusually. 🫠

    for (; cnt < 15; cnt++) {
        stmt1;
        stmt2;
    }

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

    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. it's giving order, it's giving discipline 💅

Closing and opening braces go on the same line as the else. And 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. 😌

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

Unary operators do not require spaces, binary operators do. Do not use parentheses unless they're required for precedence or unless the statement is confusing without them. And remember — other people may confuse easier than you. Do YOU understand the following? be honest. 🙈

    a = b->c[0] + ~d == (e || f) || g && h ? i : j >> 1;
    k = !(l & FLAGS);

---

Exits should be 0 on success, or 1 on failure. simple as.

    exit(0);    /*
             * Avoid obvious comments such as
             * "Exit 0 on success."
             */
}

---

The function type should be on a line by itself preceding the function. And the opening brace of the function body should be on a line by itself too. two little solo moments ✨

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. And if a line overflows, just reuse the type keyword. recycling, we love her ♻️

Be careful not to obfuscate the code by initializing variables in the declarations. Use this feature only thoughtfully. And DO NOT use function calls in initializers. nope nope nope.

    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 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. and bestie, you do not want a compiler coming for you 💀

Casts and sizeof's are not followed by a space. Note that indent(1) does not understand this rule. sizeof's are written with parenthesis always. And the redundant parenthesis rules do not apply to sizeof(var) instances.

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'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.

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);
}

---

When converting K&R style declarations to ANSI style, preserve any comments about parameters. those comments matter, keep them 🥺

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's faster and usually cleaner, not to mention avoiding stupid bugs. we're skipping the stupid bugs entirely, thank u 🎀

---

Usage statements should look like the manual pages "SYNOPSIS". And the usage statement should be structured in this exact 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(1);
}

---

And note — 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. And the alphabetical ordering should take into account the case ordering shown above. no shortcuts, no vibes-based ordering, actual alphabetical. 😤

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 they should be internally consistent with their style. consistency is the whole point bestie.

Stylistic changes (including whitespace changes) are hard on the source repository and are to be avoided without good reason. And code that's approximately FreeBSD KNF style compliant in the repository must not diverge from compliance. don't touch it if it's already right. 🙈

Whenever possible, code should be run through a code checker (e.g., various static analyzers or cc -Wall) and produce minimal warnings. clean warnings, clean conscience, glowing skin. ✨

🎀 FILES 🎀

bestie, if you want your editor to do the style rules for you instead of you crying over a diff at 3am, they got you 💅

• /usr/src/tools/tools/editing/freebsd.el — an Emacs plugin that follows the FreeBSD style indentation rules. yes, Emacs girlies ate with this one 🫶
• /usr/src/tools/tools/editing/freebsd.vim — a Vim plugin that follows the FreeBSD style indentation rules. Vim stans, this is your moment, go off 👑

---

🌈 SEE ALSO 🌈

the friend group of this page, fr fr:

• indent(1)
• err(3)
• warn(3)
• style.Makefile(5)
• style.lua(9)

go read them, they're iconic 💖

---

📜 HISTORY 📜

okay so this whole manual page is largely based on the src/admin/style/style file from the 4.4BSD-Lite2 release, with occasional updates so it keeps reflecting the current practice and desire of the FreeBSD project ✨

and like, the lore?? src/admin/style/style is a codification by the CSRG of the programming style of Ken Thompson and Dennis Ritchie in Version 6 AT&T UNIX 🌸

so when you're arguing about tabs vs spaces in a code review, you're literally arguing with the ghosts of the people who built the whole thing. main character energy tbh 👑

---

and that's the page, bestie, we're done 🦋 this one's from Debian, dated January 25, 2018, and it lives in section STYLE(9) — go format some code, you got this 💜