NAME
ssttyyllee - 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 codestyle.
/* * 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 afile (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'shistory. In general, keep the IDs intact, including any `$'s. There is
no reason to add "From" in front of foreign VCS IDs. Most non-FreeBSD
VCS IDs should be indented by a tab if in a comment.#include
RCSID("@(#)style 1.14 (Berkeley) 4/28/95");
FBSDID("$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, includeOR , but not both. includes , and it is okay to depend on that. #include
For a network program, put the network include files next./* Non-local includes in angle brackets. */ #include
#include
#include
#include
#include
Leave a blank line before the next group, the /usr include files, which should be sorted alphabetically by name.#include
Global pathnames are defined in
gram go in "pathnames.h" in the local directory.. Pathnames local to the pro- #include
Leave another blank line before the user include files.#include "pathnames.h" /* Local includes in double quotes. */
Do not ##ddeeffiinnee or declare names in the implementation namespace except
for implementing application interfaces. The names of ``unsafe'' macros (ones that have side effects), and thenames 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 ##ddeeffiinnee 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 ddoo loop, so thatit can safely be used in iiff statements. Any final statement-terminating
semicolon should be supplied by the macro invocation rather than themacro, 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 eexxtteerrnn 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
struct foo { LISTENTRY(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. */ }; LISTHEAD(, 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 bothpossible and beneficial when using an ordinary struct tag. When conven-
tion requires a ttyyppeeddeeff, 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 ssttaattiicc.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 fileif 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 ssttyyllee 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 anunderscore) or no names with the types. It is preferable to use pro-
tected 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 sswwiittcchh statement, unless parts of the sswwiittcchh cascade. Elements in a sswwiittcchh statement that cascade should have a FALLTHROUGH comment. Numerical arguments should be checkedfor 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; Space after keywords (iiff, wwhhiillee, ffoorr, rreettuurrnn, sswwiittcchh). No braces are used for control statements with zero or only a single statement unlessthat statement is more than a single line in which case they are permit-
ted. Forever loops are done with ffoorr's, not wwhhiillee'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 ffoorr 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 && thisvariablenameistoolongforitsowngood && 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 eellssee. 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 spa-
ces 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 useparentheses 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 sysexits(3). exit(EXOK); /* * 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, thenin 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 suchdeclarations 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 ssiizzeeooff'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 forall 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()) 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 compatibil-
ity. Long parameter lists are wrapped with a normal four space indent. Variable numbers of arguments should look like this.#include
void vaf(const char *fmt, ...) { valist ap; vastart(ap, fmt); STUFF; vaend(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 barg] [-m marg] req1 req2 [opt1 [opt2]]\n"
"usage: f [-a | -b] [-c [-dEe] [-n number]]\n"
(void)fprintf(stderr, "usage: f [-ab]\n");
exit(EXUSAGE);
} 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 reasonably compliant with the ssttyylleeguides. The guidelines for third-party maintained modules and device
drivers are more relaxed but at a minimum should be internally consistentwith 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 ssttyyllee compliant in the repository must not diverge from compliance. Whenever possible, code should be run through a code checker (e.g.,lint(1) or ggcccc -WWaallll) and produce minimal warnings.
SEE ALSO
indent(1), lint(1), err(3), sysexits(3), warn(3) HISTORYThis 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