path: root/CodingStyle.md

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.  For the
Windows kernel datapath code, use the coding style described in
datapath-windows/CodingStyle.

The following GNU indent options approximate this style:

    -npro -bad -bap -bbb -br -blf -brs -cdw -ce -fca -cli0 -npcs -i4 -l79 \
    -lc79 -nbfda -nut -saf -sai -saw -sbi4 -sc -sob -st -ncdb -pi4 -cs -bs \
    -di1 -lp -il0 -hnl


## 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 ovs_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)
    {
        ...
    }
    ```

Functions that destroy an instance of a dynamically-allocated type
should accept and ignore a null pointer argument.  Code that calls
such a function (including the C standard library function free())
should omit a null-pointer check.  We find that this usually makes
code easier to read.

Functions in .c files should not normally be marked "inline", because
it does not usually help code generation and it does suppress
compilers warnings about unused functions.  (Functions defined in .h
usually should be marked inline.)


## 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 *, int *mtup);


## 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:
        OVS_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.)


## RETURN VALUES

  For functions that return a success or failure indication, prefer
one of the following return value conventions:

* An "int" where 0 indicates success and a positive errno value
  indicates a reason for failure.

* A "bool" where true indicates success and false indicates
  failure.


## 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
    };


## THREAD SAFETY ANNOTATIONS

  Use the macros in lib/compiler.h to annotate locking requirements.
For example:

    static struct ovs_mutex mutex = OVS_MUTEX_INITIALIZER;
    static struct ovs_rwlock rwlock = OVS_RWLOCK_INITIALIZER;

    void function_require_plain_mutex(void) OVS_REQUIRES(mutex);
    void function_require_rwlock(void) OVS_REQ_RDLOCK(rwlock);

  Pass lock objects, not their addresses, to the annotation macros.
(Thus we have OVS_REQUIRES(mutex) above, not OVS_REQUIRES(&mutex).)


## 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 <config.h>

`#include` directives should appear in the following order:

1. `#include <config.h>`

2. The module's own headers, if any.  Including this before any
   other header (besides <config.h>) 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 <name>;").  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 <stdint.h>.

  Use the int<N>_t and uint<N>_t types from <stdint.h> for exact-width
integer types.  Use the PRId<N>, PRIu<N>, and PRIx<N> macros from
<inttypes.h> for formatting them with printf() and related functions.

  For compatibility with antique printf() implementations:

  - Instead of "%zu", use "%"PRIuSIZE.

  - Instead of "%td", use "%"PRIdPTR.

  - Instead of "%ju", use "%"PRIuMAX.

Other variants exist for different radixes.  For example, use
"%"PRIxSIZE instead of "%zx" or "%x" instead of "%hhx".

  Also, instead of "%hhd", use "%d".  Be cautious substituting "%u",
"%x", and "%o" for the corresponding versions with "hh": cast the
argument to unsigned char if necessary, because printf("%hhu", -1)
prints 255 but printf("%u", -1) prints 4294967295.

  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 signed or unsigned integer types or _Bool
(aka bool).  Do *not* declare bit-fields of type "int": C99 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!)

  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 the ternary operators ? and :, rather than
after them, e.g.

    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((unsigned char)s[0])
        || !isdigit((unsigned char)s[1])
        || !isdigit((unsigned char)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

  Most C99 features are OK because they are widely implemented:

  * 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"

  * <stdint.h> and <inttypes.h>.

  * bool and <stdbool.h>, 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.
    Also, don't assume that a conversion to bool or _Bool follows
    C99 semantics.  I.e. use "(bool)(some_value != 0)" rather than
    "(bool)some_value".  The latter might produce unexpected results
    on non-C99 environments.  For example, if bool is implemented as
    a typedef of char and some_value = 0x10000000.

  * Designated initializers (e.g. "struct foo foo = {.a = 1};" and
    "int a[] = {[2] = 5};").

  * Mixing of declarations and code within a block.  Please use this
    judiciously; keep declarations nicely grouped together in the
    beginning of a block if possible.

  * Use of declarations in iteration statements (e.g.
    "for (int i = 0; i < 10; i++)").

  * Use of a trailing comma in an enum declaration (e.g.
    "enum { x = 1, };").

  As a matter of style, avoid // comments.

  Avoid using GCC or Clang extensions unless you also add a fallback
for other compilers.  You can, however, use C99 features or GCC
extensions also supported by Clang in code that compiles only on
GNU/Linux (such as lib/netdev-linux.c), because GCC is the system
compiler there.

## PYTHON

When introducing new Python code, try to follow Python's
[PEP 8](http://www.python.org/dev/peps/pep-0008/) style.
Consider running the `pep8` or `flake8` tool against your
code to find issues.