doc: Convert docs to Markdown language

Converts the majority of docs over to use the Markdown language for
pretty printing on GitHub. It's a rough first convertion without
exploiting the full potential of Markdown at this point. Section
titles and indentation are fixed as needed. Minimal docs interlinking
is added.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>

1 files changed, 571 insertions, 0 deletions

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+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 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.