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 Windows Datapath Coding Style.

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


  • 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.


  • 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 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 functions 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.


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.


/* 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. */
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 compiler 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);


Indent each level of code with 4 spaces. Use BSD-style brace placement:

if (a()) {

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);

    error = 0;

case S_SEND:
    error = run_connection_output(conn);


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.


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)                \
enum vlog_level {

/* Name for each logging level. */
static const char *level_names[VLL_N_LEVELS] = {

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 ) 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.


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.

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.


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),

Parenthesize the operands of && and || if operator precedence makes it necessary, or if the operands are themselves expressions that use && and ||, but not otherwise. Thus:

if (rule && (!best || rule->priority > best->priority)) {
    best = rule;


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);

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));

Breaking a long line after a binary operator gives its operands a more consistent look, since each operand has the same horizontal position. This makes the end-of-line position a good choice when the operands naturally resemble each other, as in the previous two examples. On the other hand, breaking before a binary operator better draws the eye to the operator, which can help clarify code by making it more obvious what’s happening, such as in the following example:

if (!ctx.freezing
    && xbridge->has_in_band
    && in_band_must_output_to_local_port(flow)
    && !actions_output_to_local_port(&ctx)) {

Thus, decide whether to break before or after a binary operator separately in each situation, based on which of these factors appear to be more important.

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.


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

  • 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. Favor positioning that allows variables to be initialized at their point of declaration.

  • 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.


When introducing new Python code, try to follow Python’s PEP 8 style. Consider running the pep8 or flake8 tool against your code to find issues.


When introducing a new library, follow Open vSwitch Library ABI guide