From: Ben Pfaff Date: Fri, 29 May 2009 23:00:57 +0000 (-0700) Subject: Add coding style document. X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4acf233acf6533f001fa6c9d77743f4019c6712d;p=openvswitch Add coding style document. This document is mainly a description of the rules of thumb that I have followed while writing Open vSwitch userspace code. If you don't like some of the rules, please talk to me. --- diff --git a/CodingStyle b/CodingStyle new file mode 100644 index 00000000..126b45a8 --- /dev/null +++ b/CodingStyle @@ -0,0 +1,504 @@ + Open vSwitch Coding Style + ========================= + +This file describes the coding style used in most C files in the Open +vSwitch distribution. However, Linux kernel code datapath directory +follows the Linux kernel's established coding conventions. + +BASICS + + Limit lines to 79 characters. + + Use form feeds (control+L) to divide long source files into logical +pieces. A form feed should appear as the only character on a line. + + Do not use tabs for indentation. + + Avoid trailing spaces on lines. + + +NAMING + + Use names that explain the purpose of a function or object. + + Use underscores to separate words in an identifier: multi_word_name. + + Use lowercase for most names. Use uppercase for macros, macro +parameters, and members of enumerations. + + Give arrays names that are plural. + + Pick a unique name prefix (ending with an underscore) for each +module, and apply that prefix to all of that module's externally +visible names. Names of macro parameters, struct and union members, +and parameters in function prototypes are not considered externally +visible for this purpose. + + Do not use names that begin with _. If you need a name for +"internal use only", use __ as a suffix instead of a prefix. + + Avoid negative names: "found" is a better name than "not_found". + + In names, a "size" is a count of bytes, a "length" is a count of +characters. A buffer has size, but a string has length. The length +of a string does not include the null terminator, but the size of the +buffer that contains the string does. + + +COMMENTS + + Comments should be written as full sentences that start with a +capital letter and end with a period. Put two spaces between +sentences. + + Write block comments as shown below. You may put the /* and */ on +the same line as comment text if you prefer. + + /* + * We redirect stderr to /dev/null because we often want to remove all + * traffic control configuration on a port so its in a known state. If + * this done when there is no such configuration, tc complains, so we just + * always ignore it. + */ + + Each function and each variable declared outside a function, and +each struct, union, and typedef declaration should be preceded by a +comment. See FUNCTION DEFINITIONS below for function comment +guidelines. + + Each struct and union member should each have an inline comment that +explains its meaning. structs and unions with many members should be +additionally divided into logical groups of members by block comments, +e.g.: + + /* An event that will wake the following call to poll_block(). */ + struct poll_waiter { + /* Set when the waiter is created. */ + struct list node; /* Element in global waiters list. */ + int fd; /* File descriptor. */ + short int events; /* Events to wait for (POLLIN, POLLOUT). */ + poll_fd_func *function; /* Callback function, if any, or null. */ + void *aux; /* Argument to callback function. */ + struct backtrace *backtrace; /* Event that created waiter, or null. */ + + /* Set only when poll_block() is called. */ + struct pollfd *pollfd; /* Pointer to element of the pollfds array + (null if added from a callback). */ + }; + + Use XXX or FIXME comments to mark code that needs work. + + Don't use // comments. + + Don't comment out or #if 0 out code. Just remove it. The code that +was there will still be in version control history. + + +FUNCTIONS + + Put the return type, function name, and the braces that surround the +function's code on separate lines, all starting in column 0. + + Before each function definition, write a comment that describes the +function's purpose, including each parameter, the return value, and +side effects. References to argument names should be given in +single-quotes, e.g. 'arg'. The comment should not include the +function name, nor need it follow any formal structure. The comment +does not need to describe how a function does its work, unless this +information is needed to use the function correctly (this is often +better done with comments *inside* the function). + + Simple static functions do not need a comment. + + Within a file, non-static functions should come first, in the order +that they are declared in the header file, followed by static +functions. Static functions should be in one or more separate pages +(separated by form feed characters) in logical groups. A commonly +useful way to divide groups is by "level", with high-level functions +first, followed by groups of progressively lower-level functions. +This makes it easy for the program's reader to see the top-down +structure by reading from top to bottom. + + All function declarations and definitions should include a +prototype. Empty parentheses, e.g. "int foo();", do not include a +prototype (they state that the function's parameters are unknown); +write "void" in parentheses instead, e.g. "int foo(void);". + + Prototypes for static functions should either all go at the top of +the file, separated into groups by blank lines, or they should appear +at the top of each page of functions. Don't comment individual +prototypes, but a comment on each group of prototypes is often +appropriate. + + In the absence of good reasons for another order, the following +parameter order is preferred. One notable exception is that data +parameters and their corresponding size parameters should be paired. + + 1. The primary object being manipulated, if any (equivalent to the + "this" pointer in C++). + 2. Input-only parameters. + 3. Input/output parameters. + 4. Output-only parameters. + 5. Status parameter. + + Example: + + /* Stores the features supported by 'netdev' into each of '*current', + * '*advertised', '*supported', and '*peer' that are non-null. Each value + * is a bitmap of "enum ofp_port_features" bits, in host byte order. + * Returns 0 if successful, otherwise a positive errno value. On failure, + * all of the passed-in values are set to 0. */ + int + netdev_get_features(struct netdev *netdev, + uint32_t *current, uint32_t *advertised, + uint32_t *supported, uint32_t *peer) + { + ... + } + + +FUNCTION PROTOTYPES + + Put the return type and function name on the same line in a function +prototype: + + static const struct option_class *get_option_class(int code); + + + Omit parameter names from function prototypes when the names do not +give useful information, e.g.: + + int netdev_get_mtu(const struct netdev *); + + +STATEMENTS + + Indent each level of code with 4 spaces. Use BSD-style brace +placement: + + if (a()) { + b(); + d(); + } + + Put a space between "if", "while", "for", etc. and the expressions +that follow them. + + Enclose single statements in braces: + + if (a > b) { + return a; + } else { + return b; + } + + Use comments and blank lines to divide long functions into logical +groups of statements. + + Avoid assignments inside "if" and "while" conditions. + + Do not put gratuitous parentheses around the expression in a return +statement, that is, write "return 0;" and not "return(0);" + + Write only one statement per line. + + Indent "switch" statements like this: + + switch (conn->state) { + case S_RECV: + error = run_connection_input(conn); + break; + + case S_PROCESS: + error = 0; + break; + + case S_SEND: + error = run_connection_output(conn); + break; + + default: + NOT_REACHED(); + } + + "switch" statements with very short, uniform cases may use an +abbreviated style: + + switch (code) { + case 200: return "OK"; + case 201: return "Created"; + case 202: return "Accepted"; + case 204: return "No Content"; + default: return "Unknown"; + } + + Use "for (;;)" to write an infinite loop. + + In an if/else construct where one branch is the "normal" or "common" +case and the other branch is the "uncommon" or "error" case, put the +common case after the "if", not the "else". This is a form of +documentation. It also places the most important code in sequential +order without forcing the reader to visually skip past less important +details. (Some compilers also assume that the "if" branch is the more +common case, so this can be a real form of optimization as well.) + + +MACROS + + Don't define an object-like macro if an enum can be used instead. + + Don't define a function-like macro if a "static inline" function +can be used instead. + + If a macro's definition contains multiple statements, enclose them +with "do { ... } while (0)" to allow them to work properly in all +syntactic circumstances. + + Do use macros to eliminate the need to update different parts of a +single file in parallel, e.g. a list of enums and an array that gives +the name of each enum. For example: + + /* Logging importance levels. */ + #define VLOG_LEVELS \ + VLOG_LEVEL(EMER, LOG_ALERT) \ + VLOG_LEVEL(ERR, LOG_ERR) \ + VLOG_LEVEL(WARN, LOG_WARNING) \ + VLOG_LEVEL(INFO, LOG_NOTICE) \ + VLOG_LEVEL(DBG, LOG_DEBUG) + enum vlog_level { + #define VLOG_LEVEL(NAME, SYSLOG_LEVEL) VLL_##NAME, + VLOG_LEVELS + #undef VLOG_LEVEL + VLL_N_LEVELS + }; + + /* Name for each logging level. */ + static const char *level_names[VLL_N_LEVELS] = { + #define VLOG_LEVEL(NAME, SYSLOG_LEVEL) #NAME, + VLOG_LEVELS + #undef VLOG_LEVEL + }; + + +SOURCE FILES + + Each source file should state its license in a comment at the very +top, followed by a comment explaining the purpose of the code that is +in that file. The comment should explain how the code in the file +relates to code in other files. The goal is to allow a programmer to +quickly figure out where a given module fits into the larger system. + + The first non-comment line in a .c source file should be: + + #include + +#include directives should appear in the following order: + + 1. #include + + 2. The module's own headers, if any. Including this before any + other header (besides ) ensures that the module's + header file is self-contained (see HEADER FILES) below. + + 3. Standard C library headers and other system headers, preferably + in alphabetical order. (Occasionally one encounters a set of + system headers that must be included in a particular order, in + which case that order must take precedence.) + + 4. Open vSwitch headers, in alphabetical order. Use "", not <>, + to specify Open vSwitch header names. + + +HEADER FILES + + Each header file should start with its license, as described under +SOURCE FILES above, followed by a "header guard" to make the header +file idempotent, like so: + + #ifndef NETDEV_H + #define NETDEV_H 1 + + ... + + #endif /* netdev.h */ + + Header files should be self-contained; that is, they should #include +whatever additional headers are required, without requiring the client +to #include them for it. + + Don't define the members of a struct or union in a header file, +unless client code is actually intended to access them directly or if +the definition is otherwise actually needed (e.g. inline functions +defined in the header need them). + + Similarly, don't #include a header file just for the declaration of +a struct or union tag (e.g. just for "struct ;"). Just declare +the tag yourself. This reduces the number of header file +dependencies. + + +TYPES + + Use typedefs sparingly. Code is clearer if the actual type is +visible at the point of declaration. Do not, in general, declare a +typedef for a struct, union, or enum. Do not declare a typedef for a +pointer type, because this can be very confusing to the reader. + + A function type is a good use for a typedef because it can clarify +code. The type should be a function type, not a pointer-to-function +type. That way, the typedef name can be used to declare function +prototypes. (It cannot be used for function definitions, because that +is explicitly prohibited by C89 and C99.) + + You may assume that "char" is exactly 8 bits and that "int" and +"long" are at least 32 bits. + + Don't assume that "long" is big enough to hold a pointer. If you +need to cast a pointer to an integer, use "intptr_t" or "uintptr_t" +from . + + Use the int_t and uint_t types from for exact-width +integer types. Use the PRId, PRIu, and PRIx macros from + for formatting them with printf() and related functions. + + Use %zu to format size_t with printf(). + + Use bit-fields sparingly. Do not use bit-fields for layout of +network protocol fields or in other circumstances where the exact +format is important. + + Declare bit-fields to be type "unsigned int" or "signed int". Do +*not* declare bit-fields of type "int": C89 allows these to be either +signed or unsigned according to the compiler's whim. (A 1-bit +bit-field of type "int" may have a range of -1...0!) Do not declare +bit-fields of type _Bool or enum or any other type, because these are +not portable. + + Try to order structure members such that they pack well on a system +with 2-byte "short", 4-byte "int", and 4- or 8-byte "long" and pointer +types. Prefer clear organization over size optimization unless you +are convinced there is a size or speed benefit. + + Pointer declarators bind to the variable name, not the type name. +Write "int *x", not "int* x" and definitely not "int * x". + + +EXPRESSIONS + + Put one space on each side of infix binary and ternary operators: + + * / % + + - + << >> + < <= > >= + == != + & + ^ + | + && + || + ?: + = += -= *= /= %= &= ^= |= <<= >>= + + Avoid comma operators. + + Do not put any white space around postfix, prefix, or grouping +operators: + + () [] -> . + ! ~ ++ -- + - * & + +Exception 1: Put a space after (but not before) the "sizeof" keyword. +Exception 2: Put a space between the () used in a cast and the +expression whose type is cast: (void *) 0. + + Break long lines before binary operators and the ternary operators ? +and :, rather than after them, e.g. + + if (first_long_condition() || second_long_condition() + || third_long_condition()) + +and + + return (out_port != VIGP_CONTROL_PATH + ? alpheus_output_port(dp, skb, out_port) + : alpheus_output_control(dp, skb, fwd_save_skb(skb), + VIGR_ACTION)); + + + Do not parenthesize the operands of && and || unless operator +precedence makes it necessary, or unless the operands are themselves +expressions that use && and ||. Thus: + + if (!isdigit(s[0]) || !isdigit(s[1]) || !isdigit(s[2])) { + printf("string %s does not start with 3-digit code\n", s); + } + +but + + if (rule && (!best || rule->priority > best->priority)) { + best = rule; + } + + Do parenthesize a subexpression that must be split across more than +one line, e.g.: + + *idxp = ((l1_idx << PORT_ARRAY_L1_SHIFT) + | (l2_idx << PORT_ARRAY_L2_SHIFT) + | (l3_idx << PORT_ARRAY_L3_SHIFT)); + + Try to avoid casts. Don't cast the return value of malloc(). + + The "sizeof" operator is unique among C operators in that it accepts +two very different kinds of operands: an expression or a type. In +general, prefer to specify an expression, e.g. "int *x = +xmalloc(sizeof *x);". When the operand of sizeof is an expression, +there is no need to parenthesize that operand, and please don't. + + Use the ARRAY_SIZE macro from lib/util.h to calculate the number of +elements in an array. + + When using a relational operator like "<" or "==", put an expression +or variable argument on the left and a constant argument on the +right, e.g. "x == 0", *not* "0 == x". + + +BLANK LINES + + Put one blank line between top-level definitions of functions and +global variables. + + +C DIALECT + + Try to avoid using GCC extensions where possible. + + Some C99 extensions are OK: + + * Flexible array members (e.g. struct { int foo[]; }). + + * "static inline" functions (but no other forms of "inline", for + which GCC and C99 have differing interpretations). + + * "long long" + + * and . + + * bool and , but don't assume that bool or _Bool can + only take on the values 0 or 1, because this behavior can't be + simulated on C89 compilers. + + Don't use other C99 extensions, and especially: + + * Don't use // comments. + + * Don't use designated initializers (e.g. don't write "struct foo + foo = {.a = 1};" or "int a[] = {[2] = 5};"). + + * Don't mix declarations and code within a block. + + * Don't use declarations in iteration statements (e.g. don't write + "for (int i = 0; i < 10; i++)"). + + * Don't put a trailing comma in an enum declaration (e.g. don't + write "enum { x = 1, };").