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$FreeBSD: head/share/man/man9/style.9 63324 2000-07-17 19:05:57Z ben $
.Dd December 14, 1995 .Dt STYLE 9 .Os FreeBSD .Sh NAME .Nm style .Nd "Kernel source file style guide" .Sh DESCRIPTION This file specifies the preferred style for kernel source files in the .Tn FreeBSD source tree. It is also a guide for preferred userland code style. d -literal -offset 0i /* * 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. */ .Ed
p 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. d -literal -offset 0i #include <sys/types.h> /* Non-local includes in angle brackets. */ .Ed
p If it's a network program, put the network include files next. d -literal -offset 0i #include <net/if.h> #include <net/if_dl.h> #include <net/route.h> #include <netinet/in.h> #include <protocols/rwhod.h> .Ed
p Then there's a blank line, followed by the /usr include files. The /usr include files should be sorted! d -literal -offset 0i #include <stdio.h> .Ed
p Global pathnames are defined in /usr/include/paths.h. Pathnames local to the program go in pathnames.h in the local directory. d -literal -offset 0i #include <paths.h> .Ed
p Then, there's a blank line, and the user include files. d -literal -offset 0i #include "pathnames.h" /* Local includes in double quotes. */ .Ed
p Macros are capitalized, parenthesized, and should avoid side-effects. Put a single tab character between the .Ql #define and the macro name. 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 backslashes; it makes it easier to read. If the macro encapsulates a compound statement, enclose it in a .Dq Li do loop, so that it can safely be used in .Dq Li 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. d -literal -offset 0i #define MACRO(x, y) do { \e variable = (x) + (y); \e (y) += 2; \e } while(0) .Ed
p Enumeration values are all uppercase. d -literal -offset 0i enum enumtype { ONE, TWO } et; .Ed
p 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 .Ql int^Ix; and .Ql struct^Ifoo *x; .
p 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. d -literal -offset 0i 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 */ .Ed
p Use .Xr queue 3 macros rather than rolling your own lists, whenever possible. Thus, the previous example would be better written: d -literal -offset 0i #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 */ .Ed
p 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 convention requires a typedef, make its name match the struct tag. Avoid typedefs ending in .Dq Li _t , except as specified in Standard C or by .Tn POSIX . d -literal -offset 0i /* Make the structure name match the typedef. */ typedef struct _bar { int level; } BAR; .Ed
p All functions are prototyped somewhere.
p 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 .Ql static .
p Functions used from other parts of the kernel are prototyped in the relevant include file.
p Functions that are used locally in more than one module go into a separate header file, e.g.
a extern.h .
p 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.
p In general code can be considered .Dq 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.
p The kernel has a name associated with parameter types, e.g., in the kernel use: d -literal -offset 0i void function(int fd); .Ed
p 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: d -literal -offset 0i void function(int); .Ed
p or: d -literal -offset 0i void function(int _fd); .Ed
p Prototypes may have an extra space after a tab to enable function names to line up: d -literal -offset 0i 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; .Ed
p 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. d -literal -offset 0i 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 != '\e0') { warnx("illegal number, -n argument -- %s", optarg); usage(); } break; case '?': default: usage(); /* NOTREACHED */ } argc -= optind; argv += optind; .Ed
p 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 permitted. Forever loops are done with for's, not while's. d -literal -offset 0i for (p = buf; *p != '\e0'; ++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); .Ed
p Parts of a for loop may be left empty. Don't put declarations inside blocks unless the routine is unusually complicated. d -literal -offset 0i for (; cnt < 15; cnt++) { stmt1; stmt2; } .Ed
p Indentation is an 8 character tab. Second level indents are four spaces. d -literal -offset 0i while (cnt < 20) z = a + really + long + statement + that + needs + two lines + gets + indented + four + spaces + on + the + second + and + subsequent + lines. .Ed
p 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.
p Closing and opening braces go on the same line as the else. Braces that aren't necessary may be left out. d -literal -offset 0i if (test) stmt; else if (bar) { stmt; stmt; } else stmt; .Ed
p No spaces after function names. Commas have a space after them. No spaces after .Sq ( or .Sq [ or preceding .Sq ] or .Sq ) characters. d -literal -offset 0i if (error = function(a1, a2)) exit(error); .Ed
p Unary operators don't require spaces, binary operators do. Don't use parentheses unless they're required for precedence or unless the statement is confusing without them. Remember that other people may confuse easier than you. Do YOU understand the following? d -literal -offset 0i a = b->c[0] + ~d == (e || f) || g && h ? i : j >> 1; k = !(l & FLAGS); .Ed
p Exits should be 0 on success, or according to the predefined values in .Xr sysexits 3 . d -literal -offset 0i exit(EX_OK); /* * Avoid obvious comments such as * "Exit 0 on success." */ } .Ed
p The function type should be on a line by itself preceding the function. d -literal -offset 0i static char * function(int a1, int a2, float fl, int a4) { .Ed
p When declaring variables in functions declare them sorted by size, then in alphabetical order; multiple ones per line are okay. Declaring functions inside functions is not recommended, since their linkage scope is always global. If a line overflows reuse the type keyword.
p 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! d -literal -offset 0i struct foo one, *two; double three; int *four, five; char *six, seven, eight, nine, ten, eleven, twelve; four = myfunction(); .Ed
p Do not declare functions inside other functions; ANSI C says that 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.
p Casts and sizeof's are not followed by a space. Note that .Xr indent 1 does not understand this rule.
p 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: d -literal -offset 0i (p = f()) == NULL .Ed
p not: d -literal -offset 0i !(p = f()) .Ed
p Don't use '!' for tests unless it's a boolean, e.g. use d -literal -offset 0i if (*p == '\e0') .Ed
p not d -literal -offset 0i if (!*p) .Ed
p Routines returning void * should not have their return values cast to any pointer type.
p Use .Xr err 3 or .Xr warn 3 , don't roll your own! d -literal -offset 0i if ((four = malloc(sizeof(struct foo))) == NULL) err(1, (char *)NULL); if ((six = (int *)overflow()) == NULL) errx(1, "Number overflowed."); return (eight); } .Ed
p Old-style function declarations look like this: d -literal -offset 0i 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. */ { .Ed
p Use ANSI function declarations unless you explicitly need K&R compatibility.
p Variable numbers of arguments should look like this. d -literal -offset 0i #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. */ .Ed
p Use .Xr printf 3 , not fputs/puts/putchar/whatever; it's faster and usually cleaner, not to mention avoiding stupid bugs.
p Usage statements should look like the manual pages synopsis. The usage statement should be structured in the following order:
p l -enum -compat t Options without operands come first, in alphabetical order, inside a single set of brackets. t Options with operands come next, also in alphabetical order, with each option and its argument inside its own pair of brackets. t Required arguments
q if any are next, listed in the order they should be specified in the command line. t Finally, any optional arguments should be listed, listed in the order they should be specified, and all inside brackets. .El
p A bar
q Sq | separates either-or options/arguments, and multiple options/arguments which are specified together are placed in a single set of brackets.
p d -ragged -offset 0.3i "usage: f [-aDde] [-b b_arg] [-m m_arg] req1 req2 [opt1 [opt2]]\en" "usage: f [-a | -b] [-c [-dEe] [-n number]]\en" .Ed d -literal -offset 0i (void)fprintf(stderr, "usage: f [-ab]\en"); exit(1); } .Ed
p 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.
p Stylistic changes (including whitespace changes) are hard on the source repository and are to be avoided without good reason. Code that is approximately .Xr style 9 compliant in the repository must not diverge from compliance.
p Whenever possible, code should be run through a code checker (e.g., "gcc -Wall" or xlint(1)) and produce minimal warnings. .Sh SEE ALSO .Xr indent 1 , .Xr err 3 , .Xr sysexits 3 , .Xr warn 3 .Sh HISTORY This man page is largely based on the src/admin/style/style file from the .Tn BSD 4.4-Lite2 release, with updates to reflect the current practice and desire of the .Tn FreeBSD project.