qemu

qapi-code-gen.rst (65144B)

---

 ==================================
 How to use the QAPI code generator
 ==================================
 
 ..
    Copyright IBM Corp. 2011
    Copyright (C) 2012-2016 Red Hat, Inc.
 
    This work is licensed under the terms of the GNU GPL, version 2 or
    later.  See the COPYING file in the top-level directory.
 
 
 Introduction
 ============
 
 QAPI is a native C API within QEMU which provides management-level
 functionality to internal and external users.  For external
 users/processes, this interface is made available by a JSON-based wire
 format for the QEMU Monitor Protocol (QMP) for controlling qemu, as
 well as the QEMU Guest Agent (QGA) for communicating with the guest.
 The remainder of this document uses "Client JSON Protocol" when
 referring to the wire contents of a QMP or QGA connection.
 
 To map between Client JSON Protocol interfaces and the native C API,
 we generate C code from a QAPI schema.  This document describes the
 QAPI schema language, and how it gets mapped to the Client JSON
 Protocol and to C.  It additionally provides guidance on maintaining
 Client JSON Protocol compatibility.
 
 
 The QAPI schema language
 ========================
 
 The QAPI schema defines the Client JSON Protocol's commands and
 events, as well as types used by them.  Forward references are
 allowed.
 
 It is permissible for the schema to contain additional types not used
 by any commands or events, for the side effect of generated C code
 used internally.
 
 There are several kinds of types: simple types (a number of built-in
 types, such as ``int`` and ``str``; as well as enumerations), arrays,
 complex types (structs and unions), and alternate types (a choice
 between other types).
 
 
 Schema syntax
 -------------
 
 Syntax is loosely based on `JSON <http://www.ietf.org/rfc/rfc8259.txt>`_.
 Differences:
 
 * Comments: start with a hash character (``#``) that is not part of a
   string, and extend to the end of the line.
 
 * Strings are enclosed in ``'single quotes'``, not ``"double quotes"``.
 
 * Strings are restricted to printable ASCII, and escape sequences to
   just ``\\``.
 
 * Numbers and ``null`` are not supported.
 
 A second layer of syntax defines the sequences of JSON texts that are
 a correctly structured QAPI schema.  We provide a grammar for this
 syntax in an EBNF-like notation:
 
 * Production rules look like ``non-terminal = expression``
 * Concatenation: expression ``A B`` matches expression ``A``, then ``B``
 * Alternation: expression ``A | B`` matches expression ``A`` or ``B``
 * Repetition: expression ``A...`` matches zero or more occurrences of
   expression ``A``
 * Repetition: expression ``A, ...`` matches zero or more occurrences of
   expression ``A`` separated by ``,``
 * Grouping: expression ``( A )`` matches expression ``A``
 * JSON's structural characters are terminals: ``{ } [ ] : ,``
 * JSON's literal names are terminals: ``false true``
 * String literals enclosed in ``'single quotes'`` are terminal, and match
   this JSON string, with a leading ``*`` stripped off
 * When JSON object member's name starts with ``*``, the member is
   optional.
 * The symbol ``STRING`` is a terminal, and matches any JSON string
 * The symbol ``BOOL`` is a terminal, and matches JSON ``false`` or ``true``
 * ALL-CAPS words other than ``STRING`` are non-terminals
 
 The order of members within JSON objects does not matter unless
 explicitly noted.
 
 A QAPI schema consists of a series of top-level expressions::
 
     SCHEMA = TOP-LEVEL-EXPR...
 
 The top-level expressions are all JSON objects.  Code and
 documentation is generated in schema definition order.  Code order
 should not matter.
 
 A top-level expressions is either a directive or a definition::
 
     TOP-LEVEL-EXPR = DIRECTIVE | DEFINITION
 
 There are two kinds of directives and six kinds of definitions::
 
     DIRECTIVE = INCLUDE | PRAGMA
     DEFINITION = ENUM | STRUCT | UNION | ALTERNATE | COMMAND | EVENT
 
 These are discussed in detail below.
 
 
 Built-in Types
 --------------
 
 The following types are predefined, and map to C as follows:
 
   ============= ============== ============================================
   Schema        C              JSON
   ============= ============== ============================================
   ``str``       ``char *``     any JSON string, UTF-8
   ``number``    ``double``     any JSON number
   ``int``       ``int64_t``    a JSON number without fractional part
                                that fits into the C integer type
   ``int8``      ``int8_t``     likewise
   ``int16``     ``int16_t``    likewise
   ``int32``     ``int32_t``    likewise
   ``int64``     ``int64_t``    likewise
   ``uint8``     ``uint8_t``    likewise
   ``uint16``    ``uint16_t``   likewise
   ``uint32``    ``uint32_t``   likewise
   ``uint64``    ``uint64_t``   likewise
   ``size``      ``uint64_t``   like ``uint64_t``, except
                                ``StringInputVisitor`` accepts size suffixes
   ``bool``      ``bool``       JSON ``true`` or ``false``
   ``null``      ``QNull *``    JSON ``null``
   ``any``       ``QObject *``  any JSON value
   ``QType``     ``QType``      JSON string matching enum ``QType`` values
   ============= ============== ============================================
 
 
 Include directives
 ------------------
 
 Syntax::
 
     INCLUDE = { 'include': STRING }
 
 The QAPI schema definitions can be modularized using the 'include' directive::
 
  { 'include': 'path/to/file.json' }
 
 The directive is evaluated recursively, and include paths are relative
 to the file using the directive.  Multiple includes of the same file
 are idempotent.
 
 As a matter of style, it is a good idea to have all files be
 self-contained, but at the moment, nothing prevents an included file
 from making a forward reference to a type that is only introduced by
 an outer file.  The parser may be made stricter in the future to
 prevent incomplete include files.
 
 .. _pragma:
 
 Pragma directives
 -----------------
 
 Syntax::
 
     PRAGMA = { 'pragma': {
                    '*doc-required': BOOL,
                    '*command-name-exceptions': [ STRING, ... ],
                    '*command-returns-exceptions': [ STRING, ... ],
                    '*member-name-exceptions': [ STRING, ... ] } }
 
 The pragma directive lets you control optional generator behavior.
 
 Pragma's scope is currently the complete schema.  Setting the same
 pragma to different values in parts of the schema doesn't work.
 
 Pragma 'doc-required' takes a boolean value.  If true, documentation
 is required.  Default is false.
 
 Pragma 'command-name-exceptions' takes a list of commands whose names
 may contain ``"_"`` instead of ``"-"``.  Default is none.
 
 Pragma 'command-returns-exceptions' takes a list of commands that may
 violate the rules on permitted return types.  Default is none.
 
 Pragma 'member-name-exceptions' takes a list of types whose member
 names may contain uppercase letters, and ``"_"`` instead of ``"-"``.
 Default is none.
 
 .. _ENUM-VALUE:
 
 Enumeration types
 -----------------
 
 Syntax::
 
     ENUM = { 'enum': STRING,
              'data': [ ENUM-VALUE, ... ],
              '*prefix': STRING,
              '*if': COND,
              '*features': FEATURES }
     ENUM-VALUE = STRING
                | { 'name': STRING,
                    '*if': COND,
                    '*features': FEATURES }
 
 Member 'enum' names the enum type.
 
 Each member of the 'data' array defines a value of the enumeration
 type.  The form STRING is shorthand for :code:`{ 'name': STRING }`.  The
 'name' values must be be distinct.
 
 Example::
 
  { 'enum': 'MyEnum', 'data': [ 'value1', 'value2', 'value3' ] }
 
 Nothing prevents an empty enumeration, although it is probably not
 useful.
 
 On the wire, an enumeration type's value is represented by its
 (string) name.  In C, it's represented by an enumeration constant.
 These are of the form PREFIX_NAME, where PREFIX is derived from the
 enumeration type's name, and NAME from the value's name.  For the
 example above, the generator maps 'MyEnum' to MY_ENUM and 'value1' to
 VALUE1, resulting in the enumeration constant MY_ENUM_VALUE1.  The
 optional 'prefix' member overrides PREFIX.
 
 The generated C enumeration constants have values 0, 1, ..., N-1 (in
 QAPI schema order), where N is the number of values.  There is an
 additional enumeration constant PREFIX__MAX with value N.
 
 Do not use string or an integer type when an enumeration type can do
 the job satisfactorily.
 
 The optional 'if' member specifies a conditional.  See `Configuring the
 schema`_ below for more on this.
 
 The optional 'features' member specifies features.  See Features_
 below for more on this.
 
 
 .. _TYPE-REF:
 
 Type references and array types
 -------------------------------
 
 Syntax::
 
     TYPE-REF = STRING | ARRAY-TYPE
     ARRAY-TYPE = [ STRING ]
 
 A string denotes the type named by the string.
 
 A one-element array containing a string denotes an array of the type
 named by the string.  Example: ``['int']`` denotes an array of ``int``.
 
 
 Struct types
 ------------
 
 Syntax::
 
     STRUCT = { 'struct': STRING,
                'data': MEMBERS,
                '*base': STRING,
                '*if': COND,
                '*features': FEATURES }
     MEMBERS = { MEMBER, ... }
     MEMBER = STRING : TYPE-REF
            | STRING : { 'type': TYPE-REF,
                         '*if': COND,
                         '*features': FEATURES }
 
 Member 'struct' names the struct type.
 
 Each MEMBER of the 'data' object defines a member of the struct type.
 
 .. _MEMBERS:
 
 The MEMBER's STRING name consists of an optional ``*`` prefix and the
 struct member name.  If ``*`` is present, the member is optional.
 
 The MEMBER's value defines its properties, in particular its type.
 The form TYPE-REF_ is shorthand for :code:`{ 'type': TYPE-REF }`.
 
 Example::
 
  { 'struct': 'MyType',
    'data': { 'member1': 'str', 'member2': ['int'], '*member3': 'str' } }
 
 A struct type corresponds to a struct in C, and an object in JSON.
 The C struct's members are generated in QAPI schema order.
 
 The optional 'base' member names a struct type whose members are to be
 included in this type.  They go first in the C struct.
 
 Example::
 
  { 'struct': 'BlockdevOptionsGenericFormat',
    'data': { 'file': 'str' } }
  { 'struct': 'BlockdevOptionsGenericCOWFormat',
    'base': 'BlockdevOptionsGenericFormat',
    'data': { '*backing': 'str' } }
 
 An example BlockdevOptionsGenericCOWFormat object on the wire could use
 both members like this::
 
  { "file": "/some/place/my-image",
    "backing": "/some/place/my-backing-file" }
 
 The optional 'if' member specifies a conditional.  See `Configuring
 the schema`_ below for more on this.
 
 The optional 'features' member specifies features.  See Features_
 below for more on this.
 
 
 Union types
 -----------
 
 Syntax::
 
     UNION = { 'union': STRING,
               'base': ( MEMBERS | STRING ),
               'discriminator': STRING,
               'data': BRANCHES,
               '*if': COND,
               '*features': FEATURES }
     BRANCHES = { BRANCH, ... }
     BRANCH = STRING : TYPE-REF
            | STRING : { 'type': TYPE-REF, '*if': COND }
 
 Member 'union' names the union type.
 
 The 'base' member defines the common members.  If it is a MEMBERS_
 object, it defines common members just like a struct type's 'data'
 member defines struct type members.  If it is a STRING, it names a
 struct type whose members are the common members.
 
 Member 'discriminator' must name a non-optional enum-typed member of
 the base struct.  That member's value selects a branch by its name.
 If no such branch exists, an empty branch is assumed.
 
 Each BRANCH of the 'data' object defines a branch of the union.  A
 union must have at least one branch.
 
 The BRANCH's STRING name is the branch name.  It must be a value of
 the discriminator enum type.
 
 The BRANCH's value defines the branch's properties, in particular its
 type.  The type must a struct type.  The form TYPE-REF_ is shorthand
 for :code:`{ 'type': TYPE-REF }`.
 
 In the Client JSON Protocol, a union is represented by an object with
 the common members (from the base type) and the selected branch's
 members.  The two sets of member names must be disjoint.
 
 Example::
 
  { 'enum': 'BlockdevDriver', 'data': [ 'file', 'qcow2' ] }
  { 'union': 'BlockdevOptions',
    'base': { 'driver': 'BlockdevDriver', '*read-only': 'bool' },
    'discriminator': 'driver',
    'data': { 'file': 'BlockdevOptionsFile',
              'qcow2': 'BlockdevOptionsQcow2' } }
 
 Resulting in these JSON objects::
 
  { "driver": "file", "read-only": true,
    "filename": "/some/place/my-image" }
  { "driver": "qcow2", "read-only": false,
    "backing": "/some/place/my-image", "lazy-refcounts": true }
 
 The order of branches need not match the order of the enum values.
 The branches need not cover all possible enum values.  In the
 resulting generated C data types, a union is represented as a struct
 with the base members in QAPI schema order, and then a union of
 structures for each branch of the struct.
 
 The optional 'if' member specifies a conditional.  See `Configuring
 the schema`_ below for more on this.
 
 The optional 'features' member specifies features.  See Features_
 below for more on this.
 
 
 Alternate types
 ---------------
 
 Syntax::
 
     ALTERNATE = { 'alternate': STRING,
                   'data': ALTERNATIVES,
                   '*if': COND,
                   '*features': FEATURES }
     ALTERNATIVES = { ALTERNATIVE, ... }
     ALTERNATIVE = STRING : STRING
                 | STRING : { 'type': STRING, '*if': COND }
 
 Member 'alternate' names the alternate type.
 
 Each ALTERNATIVE of the 'data' object defines a branch of the
 alternate.  An alternate must have at least one branch.
 
 The ALTERNATIVE's STRING name is the branch name.
 
 The ALTERNATIVE's value defines the branch's properties, in particular
 its type.  The form STRING is shorthand for :code:`{ 'type': STRING }`.
 
 Example::
 
  { 'alternate': 'BlockdevRef',
    'data': { 'definition': 'BlockdevOptions',
              'reference': 'str' } }
 
 An alternate type is like a union type, except there is no
 discriminator on the wire.  Instead, the branch to use is inferred
 from the value.  An alternate can only express a choice between types
 represented differently on the wire.
 
 If a branch is typed as the 'bool' built-in, the alternate accepts
 true and false; if it is typed as any of the various numeric
 built-ins, it accepts a JSON number; if it is typed as a 'str'
 built-in or named enum type, it accepts a JSON string; if it is typed
 as the 'null' built-in, it accepts JSON null; and if it is typed as a
 complex type (struct or union), it accepts a JSON object.
 
 The example alternate declaration above allows using both of the
 following example objects::
 
  { "file": "my_existing_block_device_id" }
  { "file": { "driver": "file",
              "read-only": false,
              "filename": "/tmp/mydisk.qcow2" } }
 
 The optional 'if' member specifies a conditional.  See `Configuring
 the schema`_ below for more on this.
 
 The optional 'features' member specifies features.  See Features_
 below for more on this.
 
 
 Commands
 --------
 
 Syntax::
 
     COMMAND = { 'command': STRING,
                 (
                 '*data': ( MEMBERS | STRING ),
                 |
                 'data': STRING,
                 'boxed': true,
                 )
                 '*returns': TYPE-REF,
                 '*success-response': false,
                 '*gen': false,
                 '*allow-oob': true,
                 '*allow-preconfig': true,
                 '*coroutine': true,
                 '*if': COND,
                 '*features': FEATURES }
 
 Member 'command' names the command.
 
 Member 'data' defines the arguments.  It defaults to an empty MEMBERS_
 object.
 
 If 'data' is a MEMBERS_ object, then MEMBERS defines arguments just
 like a struct type's 'data' defines struct type members.
 
 If 'data' is a STRING, then STRING names a complex type whose members
 are the arguments.  A union type requires ``'boxed': true``.
 
 Member 'returns' defines the command's return type.  It defaults to an
 empty struct type.  It must normally be a complex type or an array of
 a complex type.  To return anything else, the command must be listed
 in pragma 'commands-returns-exceptions'.  If you do this, extending
 the command to return additional information will be harder.  Use of
 the pragma for new commands is strongly discouraged.
 
 A command's error responses are not specified in the QAPI schema.
 Error conditions should be documented in comments.
 
 In the Client JSON Protocol, the value of the "execute" or "exec-oob"
 member is the command name.  The value of the "arguments" member then
 has to conform to the arguments, and the value of the success
 response's "return" member will conform to the return type.
 
 Some example commands::
 
  { 'command': 'my-first-command',
    'data': { 'arg1': 'str', '*arg2': 'str' } }
  { 'struct': 'MyType', 'data': { '*value': 'str' } }
  { 'command': 'my-second-command',
    'returns': [ 'MyType' ] }
 
 which would validate this Client JSON Protocol transaction::
 
  => { "execute": "my-first-command",
       "arguments": { "arg1": "hello" } }
  <= { "return": { } }
  => { "execute": "my-second-command" }
  <= { "return": [ { "value": "one" }, { } ] }
 
 The generator emits a prototype for the C function implementing the
 command.  The function itself needs to be written by hand.  See
 section `Code generated for commands`_ for examples.
 
 The function returns the return type.  When member 'boxed' is absent,
 it takes the command arguments as arguments one by one, in QAPI schema
 order.  Else it takes them wrapped in the C struct generated for the
 complex argument type.  It takes an additional ``Error **`` argument in
 either case.
 
 The generator also emits a marshalling function that extracts
 arguments for the user's function out of an input QDict, calls the
 user's function, and if it succeeded, builds an output QObject from
 its return value.  This is for use by the QMP monitor core.
 
 In rare cases, QAPI cannot express a type-safe representation of a
 corresponding Client JSON Protocol command.  You then have to suppress
 generation of a marshalling function by including a member 'gen' with
 boolean value false, and instead write your own function.  For
 example::
 
  { 'command': 'netdev_add',
    'data': {'type': 'str', 'id': 'str'},
    'gen': false }
 
 Please try to avoid adding new commands that rely on this, and instead
 use type-safe unions.
 
 Normally, the QAPI schema is used to describe synchronous exchanges,
 where a response is expected.  But in some cases, the action of a
 command is expected to change state in a way that a successful
 response is not possible (although the command will still return an
 error object on failure).  When a successful reply is not possible,
 the command definition includes the optional member 'success-response'
 with boolean value false.  So far, only QGA makes use of this member.
 
 Member 'allow-oob' declares whether the command supports out-of-band
 (OOB) execution.  It defaults to false.  For example::
 
  { 'command': 'migrate_recover',
    'data': { 'uri': 'str' }, 'allow-oob': true }
 
 See qmp-spec.txt for out-of-band execution syntax and semantics.
 
 Commands supporting out-of-band execution can still be executed
 in-band.
 
 When a command is executed in-band, its handler runs in the main
 thread with the BQL held.
 
 When a command is executed out-of-band, its handler runs in a
 dedicated monitor I/O thread with the BQL *not* held.
 
 An OOB-capable command handler must satisfy the following conditions:
 
 - It terminates quickly.
 - It does not invoke system calls that may block.
 - It does not access guest RAM that may block when userfaultfd is
   enabled for postcopy live migration.
 - It takes only "fast" locks, i.e. all critical sections protected by
   any lock it takes also satisfy the conditions for OOB command
   handler code.
 
 The restrictions on locking limit access to shared state.  Such access
 requires synchronization, but OOB commands can't take the BQL or any
 other "slow" lock.
 
 When in doubt, do not implement OOB execution support.
 
 Member 'allow-preconfig' declares whether the command is available
 before the machine is built.  It defaults to false.  For example::
 
  { 'enum': 'QMPCapability',
    'data': [ 'oob' ] }
  { 'command': 'qmp_capabilities',
    'data': { '*enable': [ 'QMPCapability' ] },
    'allow-preconfig': true }
 
 QMP is available before the machine is built only when QEMU was
 started with --preconfig.
 
 Member 'coroutine' tells the QMP dispatcher whether the command handler
 is safe to be run in a coroutine.  It defaults to false.  If it is true,
 the command handler is called from coroutine context and may yield while
 waiting for an external event (such as I/O completion) in order to avoid
 blocking the guest and other background operations.
 
 Coroutine safety can be hard to prove, similar to thread safety.  Common
 pitfalls are:
 
 - The global mutex isn't held across ``qemu_coroutine_yield()``, so
   operations that used to assume that they execute atomically may have
   to be more careful to protect against changes in the global state.
 
 - Nested event loops (``AIO_WAIT_WHILE()`` etc.) are problematic in
   coroutine context and can easily lead to deadlocks.  They should be
   replaced by yielding and reentering the coroutine when the condition
   becomes false.
 
 Since the command handler may assume coroutine context, any callers
 other than the QMP dispatcher must also call it in coroutine context.
 In particular, HMP commands calling such a QMP command handler must be
 marked ``.coroutine = true`` in hmp-commands.hx.
 
 It is an error to specify both ``'coroutine': true`` and ``'allow-oob': true``
 for a command.  We don't currently have a use case for both together and
 without a use case, it's not entirely clear what the semantics should
 be.
 
 The optional 'if' member specifies a conditional.  See `Configuring
 the schema`_ below for more on this.
 
 The optional 'features' member specifies features.  See Features_
 below for more on this.
 
 
 Events
 ------
 
 Syntax::
 
     EVENT = { 'event': STRING,
               (
               '*data': ( MEMBERS | STRING ),
               |
               'data': STRING,
               'boxed': true,
               )
               '*if': COND,
               '*features': FEATURES }
 
 Member 'event' names the event.  This is the event name used in the
 Client JSON Protocol.
 
 Member 'data' defines the event-specific data.  It defaults to an
 empty MEMBERS object.
 
 If 'data' is a MEMBERS object, then MEMBERS defines event-specific
 data just like a struct type's 'data' defines struct type members.
 
 If 'data' is a STRING, then STRING names a complex type whose members
 are the event-specific data.  A union type requires ``'boxed': true``.
 
 An example event is::
 
  { 'event': 'EVENT_C',
    'data': { '*a': 'int', 'b': 'str' } }
 
 Resulting in this JSON object::
 
  { "event": "EVENT_C",
    "data": { "b": "test string" },
    "timestamp": { "seconds": 1267020223, "microseconds": 435656 } }
 
 The generator emits a function to send the event.  When member 'boxed'
 is absent, it takes event-specific data one by one, in QAPI schema
 order.  Else it takes them wrapped in the C struct generated for the
 complex type.  See section `Code generated for events`_ for examples.
 
 The optional 'if' member specifies a conditional.  See `Configuring
 the schema`_ below for more on this.
 
 The optional 'features' member specifies features.  See Features_
 below for more on this.
 
 
 .. _FEATURE:
 
 Features
 --------
 
 Syntax::
 
     FEATURES = [ FEATURE, ... ]
     FEATURE = STRING
             | { 'name': STRING, '*if': COND }
 
 Sometimes, the behaviour of QEMU changes compatibly, but without a
 change in the QMP syntax (usually by allowing values or operations
 that previously resulted in an error).  QMP clients may still need to
 know whether the extension is available.
 
 For this purpose, a list of features can be specified for a command or
 struct type.  Each list member can either be ``{ 'name': STRING, '*if':
 COND }``, or STRING, which is shorthand for ``{ 'name': STRING }``.
 
 The optional 'if' member specifies a conditional.  See `Configuring
 the schema`_ below for more on this.
 
 Example::
 
  { 'struct': 'TestType',
    'data': { 'number': 'int' },
    'features': [ 'allow-negative-numbers' ] }
 
 The feature strings are exposed to clients in introspection, as
 explained in section `Client JSON Protocol introspection`_.
 
 Intended use is to have each feature string signal that this build of
 QEMU shows a certain behaviour.
 
 
 Special features
 ~~~~~~~~~~~~~~~~
 
 Feature "deprecated" marks a command, event, enum value, or struct
 member as deprecated.  It is not supported elsewhere so far.
 Interfaces so marked may be withdrawn in future releases in accordance
 with QEMU's deprecation policy.
 
 Feature "unstable" marks a command, event, enum value, or struct
 member as unstable.  It is not supported elsewhere so far.  Interfaces
 so marked may be withdrawn or changed incompatibly in future releases.
 
 
 Naming rules and reserved names
 -------------------------------
 
 All names must begin with a letter, and contain only ASCII letters,
 digits, hyphen, and underscore.  There are two exceptions: enum values
 may start with a digit, and names that are downstream extensions (see
 section `Downstream extensions`_) start with underscore.
 
 Names beginning with ``q_`` are reserved for the generator, which uses
 them for munging QMP names that resemble C keywords or other
 problematic strings.  For example, a member named ``default`` in qapi
 becomes ``q_default`` in the generated C code.
 
 Types, commands, and events share a common namespace.  Therefore,
 generally speaking, type definitions should always use CamelCase for
 user-defined type names, while built-in types are lowercase.
 
 Type names ending with ``Kind`` or ``List`` are reserved for the
 generator, which uses them for implicit union enums and array types,
 respectively.
 
 Command names, member names within a type, and feature names should be
 all lower case with words separated by a hyphen.  However, some
 existing older commands and complex types use underscore; when
 extending them, consistency is preferred over blindly avoiding
 underscore.
 
 Event names should be ALL_CAPS with words separated by underscore.
 
 Member name ``u`` and names starting with ``has-`` or ``has_`` are reserved
 for the generator, which uses them for unions and for tracking
 optional members.
 
 Names beginning with ``x-`` used to signify "experimental".  This
 convention has been replaced by special feature "unstable".
 
 Pragmas ``command-name-exceptions`` and ``member-name-exceptions`` let
 you violate naming rules.  Use for new code is strongly discouraged. See
 `Pragma directives`_ for details.
 
 
 Downstream extensions
 ---------------------
 
 QAPI schema names that are externally visible, say in the Client JSON
 Protocol, need to be managed with care.  Names starting with a
 downstream prefix of the form __RFQDN_ are reserved for the downstream
 who controls the valid, reverse fully qualified domain name RFQDN.
 RFQDN may only contain ASCII letters, digits, hyphen and period.
 
 Example: Red Hat, Inc. controls redhat.com, and may therefore add a
 downstream command ``__com.redhat_drive-mirror``.
 
 
 Configuring the schema
 ----------------------
 
 Syntax::
 
     COND = STRING
          | { 'all: [ COND, ... ] }
          | { 'any: [ COND, ... ] }
          | { 'not': COND }
 
 All definitions take an optional 'if' member.  Its value must be a
 string, or an object with a single member 'all', 'any' or 'not'.
 
 The C code generated for the definition will then be guarded by an #if
 preprocessing directive with an operand generated from that condition:
 
  * STRING will generate defined(STRING)
  * { 'all': [COND, ...] } will generate (COND && ...)
  * { 'any': [COND, ...] } will generate (COND || ...)
  * { 'not': COND } will generate !COND
 
 Example: a conditional struct ::
 
  { 'struct': 'IfStruct', 'data': { 'foo': 'int' },
    'if': { 'all': [ 'CONFIG_FOO', 'HAVE_BAR' ] } }
 
 gets its generated code guarded like this::
 
  #if defined(CONFIG_FOO) && defined(HAVE_BAR)
  ... generated code ...
  #endif /* defined(HAVE_BAR) && defined(CONFIG_FOO) */
 
 Individual members of complex types, commands arguments, and
 event-specific data can also be made conditional.  This requires the
 longhand form of MEMBER.
 
 Example: a struct type with unconditional member 'foo' and conditional
 member 'bar' ::
 
  { 'struct': 'IfStruct',
    'data': { 'foo': 'int',
              'bar': { 'type': 'int', 'if': 'IFCOND'} } }
 
 A union's discriminator may not be conditional.
 
 Likewise, individual enumeration values be conditional.  This requires
 the longhand form of ENUM-VALUE_.
 
 Example: an enum type with unconditional value 'foo' and conditional
 value 'bar' ::
 
  { 'enum': 'IfEnum',
    'data': [ 'foo',
              { 'name' : 'bar', 'if': 'IFCOND' } ] }
 
 Likewise, features can be conditional.  This requires the longhand
 form of FEATURE_.
 
 Example: a struct with conditional feature 'allow-negative-numbers' ::
 
  { 'struct': 'TestType',
    'data': { 'number': 'int' },
    'features': [ { 'name': 'allow-negative-numbers',
                    'if': 'IFCOND' } ] }
 
 Please note that you are responsible to ensure that the C code will
 compile with an arbitrary combination of conditions, since the
 generator is unable to check it at this point.
 
 The conditions apply to introspection as well, i.e. introspection
 shows a conditional entity only when the condition is satisfied in
 this particular build.
 
 
 Documentation comments
 ----------------------
 
 A multi-line comment that starts and ends with a ``##`` line is a
 documentation comment.
 
 If the documentation comment starts like ::
 
     ##
     # @SYMBOL:
 
 it documents the definition of SYMBOL, else it's free-form
 documentation.
 
 See below for more on `Definition documentation`_.
 
 Free-form documentation may be used to provide additional text and
 structuring content.
 
 
 Headings and subheadings
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 A free-form documentation comment containing a line which starts with
 some ``=`` symbols and then a space defines a section heading::
 
     ##
     # = This is a top level heading
     #
     # This is a free-form comment which will go under the
     # top level heading.
     ##
 
     ##
     # == This is a second level heading
     ##
 
 A heading line must be the first line of the documentation
 comment block.
 
 Section headings must always be correctly nested, so you can only
 define a third-level heading inside a second-level heading, and so on.
 
 
 Documentation markup
 ~~~~~~~~~~~~~~~~~~~~
 
 Documentation comments can use most rST markup.  In particular,
 a ``::`` literal block can be used for examples::
 
     # ::
     #
     #   Text of the example, may span
     #   multiple lines
 
 ``*`` starts an itemized list::
 
     # * First item, may span
     #   multiple lines
     # * Second item
 
 You can also use ``-`` instead of ``*``.
 
 A decimal number followed by ``.`` starts a numbered list::
 
     # 1. First item, may span
     #    multiple lines
     # 2. Second item
 
 The actual number doesn't matter.
 
 Lists of either kind must be preceded and followed by a blank line.
 If a list item's text spans multiple lines, then the second and
 subsequent lines must be correctly indented to line up with the
 first character of the first line.
 
 The usual ****strong****, *\*emphasized\** and ````literal```` markup
 should be used.  If you need a single literal ``*``, you will need to
 backslash-escape it.  As an extension beyond the usual rST syntax, you
 can also use ``@foo`` to reference a name in the schema; this is rendered
 the same way as ````foo````.
 
 Example::
 
  ##
  # Some text foo with **bold** and *emphasis*
  # 1. with a list
  # 2. like that
  #
  # And some code:
  #
  # ::
  #
  #   $ echo foo
  #   -> do this
  #   <- get that
  ##
 
 
 Definition documentation
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 Definition documentation, if present, must immediately precede the
 definition it documents.
 
 When documentation is required (see pragma_ 'doc-required'), every
 definition must have documentation.
 
 Definition documentation starts with a line naming the definition,
 followed by an optional overview, a description of each argument (for
 commands and events), member (for structs and unions), branch (for
 alternates), or value (for enums), a description of each feature (if
 any), and finally optional tagged sections.
 
 The description of an argument or feature 'name' starts with
 '\@name:'.  The description text can start on the line following the
 '\@name:', in which case it must not be indented at all.  It can also
 start on the same line as the '\@name:'.  In this case if it spans
 multiple lines then second and subsequent lines must be indented to
 line up with the first character of the first line of the
 description::
 
  # @argone:
  # This is a two line description
  # in the first style.
  #
  # @argtwo: This is a two line description
  #          in the second style.
 
 The number of spaces between the ':' and the text is not significant.
 
 .. admonition:: FIXME
 
    The parser accepts these things in almost any order.
 
 .. admonition:: FIXME
 
    union branches should be described, too.
 
 Extensions added after the definition was first released carry a
 '(since x.y.z)' comment.
 
 The feature descriptions must be preceded by a line "Features:", like
 this::
 
   # Features:
   # @feature: Description text
 
 A tagged section starts with one of the following words:
 "Note:"/"Notes:", "Since:", "Example"/"Examples", "Returns:", "TODO:".
 The section ends with the start of a new section.
 
 The text of a section can start on a new line, in
 which case it must not be indented at all.  It can also start
 on the same line as the 'Note:', 'Returns:', etc tag.  In this
 case if it spans multiple lines then second and subsequent
 lines must be indented to match the first, in the same way as
 multiline argument descriptions.
 
 A 'Since: x.y.z' tagged section lists the release that introduced the
 definition.
 
 An 'Example' or 'Examples' section is automatically rendered
 entirely as literal fixed-width text.  In other sections,
 the text is formatted, and rST markup can be used.
 
 For example::
 
  ##
  # @BlockStats:
  #
  # Statistics of a virtual block device or a block backing device.
  #
  # @device: If the stats are for a virtual block device, the name
  #          corresponding to the virtual block device.
  #
  # @node-name: The node name of the device. (since 2.3)
  #
  # ... more members ...
  #
  # Since: 0.14.0
  ##
  { 'struct': 'BlockStats',
    'data': {'*device': 'str', '*node-name': 'str',
             ... more members ... } }
 
  ##
  # @query-blockstats:
  #
  # Query the @BlockStats for all virtual block devices.
  #
  # @query-nodes: If true, the command will query all the
  #               block nodes ... explain, explain ...  (since 2.3)
  #
  # Returns: A list of @BlockStats for each virtual block devices.
  #
  # Since: 0.14.0
  #
  # Example:
  #
  # -> { "execute": "query-blockstats" }
  # <- {
  #      ... lots of output ...
  #    }
  #
  ##
  { 'command': 'query-blockstats',
    'data': { '*query-nodes': 'bool' },
    'returns': ['BlockStats'] }
 
 
 Client JSON Protocol introspection
 ==================================
 
 Clients of a Client JSON Protocol commonly need to figure out what
 exactly the server (QEMU) supports.
 
 For this purpose, QMP provides introspection via command
 query-qmp-schema.  QGA currently doesn't support introspection.
 
 While Client JSON Protocol wire compatibility should be maintained
 between qemu versions, we cannot make the same guarantees for
 introspection stability.  For example, one version of qemu may provide
 a non-variant optional member of a struct, and a later version rework
 the member to instead be non-optional and associated with a variant.
 Likewise, one version of qemu may list a member with open-ended type
 'str', and a later version could convert it to a finite set of strings
 via an enum type; or a member may be converted from a specific type to
 an alternate that represents a choice between the original type and
 something else.
 
 query-qmp-schema returns a JSON array of SchemaInfo objects.  These
 objects together describe the wire ABI, as defined in the QAPI schema.
 There is no specified order to the SchemaInfo objects returned; a
 client must search for a particular name throughout the entire array
 to learn more about that name, but is at least guaranteed that there
 will be no collisions between type, command, and event names.
 
 However, the SchemaInfo can't reflect all the rules and restrictions
 that apply to QMP.  It's interface introspection (figuring out what's
 there), not interface specification.  The specification is in the QAPI
 schema.  To understand how QMP is to be used, you need to study the
 QAPI schema.
 
 Like any other command, query-qmp-schema is itself defined in the QAPI
 schema, along with the SchemaInfo type.  This text attempts to give an
 overview how things work.  For details you need to consult the QAPI
 schema.
 
 SchemaInfo objects have common members "name", "meta-type",
 "features", and additional variant members depending on the value of
 meta-type.
 
 Each SchemaInfo object describes a wire ABI entity of a certain
 meta-type: a command, event or one of several kinds of type.
 
 SchemaInfo for commands and events have the same name as in the QAPI
 schema.
 
 Command and event names are part of the wire ABI, but type names are
 not.  Therefore, the SchemaInfo for types have auto-generated
 meaningless names.  For readability, the examples in this section use
 meaningful type names instead.
 
 Optional member "features" exposes the entity's feature strings as a
 JSON array of strings.
 
 To examine a type, start with a command or event using it, then follow
 references by name.
 
 QAPI schema definitions not reachable that way are omitted.
 
 The SchemaInfo for a command has meta-type "command", and variant
 members "arg-type", "ret-type" and "allow-oob".  On the wire, the
 "arguments" member of a client's "execute" command must conform to the
 object type named by "arg-type".  The "return" member that the server
 passes in a success response conforms to the type named by "ret-type".
 When "allow-oob" is true, it means the command supports out-of-band
 execution.  It defaults to false.
 
 If the command takes no arguments, "arg-type" names an object type
 without members.  Likewise, if the command returns nothing, "ret-type"
 names an object type without members.
 
 Example: the SchemaInfo for command query-qmp-schema ::
 
  { "name": "query-qmp-schema", "meta-type": "command",
    "arg-type": "q_empty", "ret-type": "SchemaInfoList" }
 
    Type "q_empty" is an automatic object type without members, and type
    "SchemaInfoList" is the array of SchemaInfo type.
 
 The SchemaInfo for an event has meta-type "event", and variant member
 "arg-type".  On the wire, a "data" member that the server passes in an
 event conforms to the object type named by "arg-type".
 
 If the event carries no additional information, "arg-type" names an
 object type without members.  The event may not have a data member on
 the wire then.
 
 Each command or event defined with 'data' as MEMBERS object in the
 QAPI schema implicitly defines an object type.
 
 Example: the SchemaInfo for EVENT_C from section Events_ ::
 
     { "name": "EVENT_C", "meta-type": "event",
       "arg-type": "q_obj-EVENT_C-arg" }
 
     Type "q_obj-EVENT_C-arg" is an implicitly defined object type with
     the two members from the event's definition.
 
 The SchemaInfo for struct and union types has meta-type "object".
 
 The SchemaInfo for a struct type has variant member "members".
 
 The SchemaInfo for a union type additionally has variant members "tag"
 and "variants".
 
 "members" is a JSON array describing the object's common members, if
 any.  Each element is a JSON object with members "name" (the member's
 name), "type" (the name of its type), "features" (a JSON array of
 feature strings), and "default".  The latter two are optional.  The
 member is optional if "default" is present.  Currently, "default" can
 only have value null.  Other values are reserved for future
 extensions.  The "members" array is in no particular order; clients
 must search the entire object when learning whether a particular
 member is supported.
 
 Example: the SchemaInfo for MyType from section `Struct types`_ ::
 
     { "name": "MyType", "meta-type": "object",
       "members": [
           { "name": "member1", "type": "str" },
           { "name": "member2", "type": "int" },
           { "name": "member3", "type": "str", "default": null } ] }
 
 "features" exposes the command's feature strings as a JSON array of
 strings.
 
 Example: the SchemaInfo for TestType from section Features_::
 
     { "name": "TestType", "meta-type": "object",
       "members": [
           { "name": "number", "type": "int" } ],
       "features": ["allow-negative-numbers"] }
 
 "tag" is the name of the common member serving as type tag.
 "variants" is a JSON array describing the object's variant members.
 Each element is a JSON object with members "case" (the value of type
 tag this element applies to) and "type" (the name of an object type
 that provides the variant members for this type tag value).  The
 "variants" array is in no particular order, and is not guaranteed to
 list cases in the same order as the corresponding "tag" enum type.
 
 Example: the SchemaInfo for union BlockdevOptions from section
 `Union types`_ ::
 
     { "name": "BlockdevOptions", "meta-type": "object",
       "members": [
           { "name": "driver", "type": "BlockdevDriver" },
           { "name": "read-only", "type": "bool", "default": null } ],
       "tag": "driver",
       "variants": [
           { "case": "file", "type": "BlockdevOptionsFile" },
           { "case": "qcow2", "type": "BlockdevOptionsQcow2" } ] }
 
 Note that base types are "flattened": its members are included in the
 "members" array.
 
 The SchemaInfo for an alternate type has meta-type "alternate", and
 variant member "members".  "members" is a JSON array.  Each element is
 a JSON object with member "type", which names a type.  Values of the
 alternate type conform to exactly one of its member types.  There is
 no guarantee on the order in which "members" will be listed.
 
 Example: the SchemaInfo for BlockdevRef from section `Alternate types`_ ::
 
     { "name": "BlockdevRef", "meta-type": "alternate",
       "members": [
           { "type": "BlockdevOptions" },
           { "type": "str" } ] }
 
 The SchemaInfo for an array type has meta-type "array", and variant
 member "element-type", which names the array's element type.  Array
 types are implicitly defined.  For convenience, the array's name may
 resemble the element type; however, clients should examine member
 "element-type" instead of making assumptions based on parsing member
 "name".
 
 Example: the SchemaInfo for ['str'] ::
 
     { "name": "[str]", "meta-type": "array",
       "element-type": "str" }
 
 The SchemaInfo for an enumeration type has meta-type "enum" and
 variant member "members".
 
 "members" is a JSON array describing the enumeration values.  Each
 element is a JSON object with member "name" (the member's name), and
 optionally "features" (a JSON array of feature strings).  The
 "members" array is in no particular order; clients must search the
 entire array when learning whether a particular value is supported.
 
 Example: the SchemaInfo for MyEnum from section `Enumeration types`_ ::
 
     { "name": "MyEnum", "meta-type": "enum",
       "members": [
         { "name": "value1" },
         { "name": "value2" },
         { "name": "value3" }
       ] }
 
 The SchemaInfo for a built-in type has the same name as the type in
 the QAPI schema (see section `Built-in Types`_), with one exception
 detailed below.  It has variant member "json-type" that shows how
 values of this type are encoded on the wire.
 
 Example: the SchemaInfo for str ::
 
     { "name": "str", "meta-type": "builtin", "json-type": "string" }
 
 The QAPI schema supports a number of integer types that only differ in
 how they map to C.  They are identical as far as SchemaInfo is
 concerned.  Therefore, they get all mapped to a single type "int" in
 SchemaInfo.
 
 As explained above, type names are not part of the wire ABI.  Not even
 the names of built-in types.  Clients should examine member
 "json-type" instead of hard-coding names of built-in types.
 
 
 Compatibility considerations
 ============================
 
 Maintaining backward compatibility at the Client JSON Protocol level
 while evolving the schema requires some care.  This section is about
 syntactic compatibility, which is necessary, but not sufficient, for
 actual compatibility.
 
 Clients send commands with argument data, and receive command
 responses with return data and events with event data.
 
 Adding opt-in functionality to the send direction is backwards
 compatible: adding commands, optional arguments, enumeration values,
 union and alternate branches; turning an argument type into an
 alternate of that type; making mandatory arguments optional.  Clients
 oblivious of the new functionality continue to work.
 
 Incompatible changes include removing commands, command arguments,
 enumeration values, union and alternate branches, adding mandatory
 command arguments, and making optional arguments mandatory.
 
 The specified behavior of an absent optional argument should remain
 the same.  With proper documentation, this policy still allows some
 flexibility; for example, when an optional 'buffer-size' argument is
 specified to default to a sensible buffer size, the actual default
 value can still be changed.  The specified default behavior is not the
 exact size of the buffer, only that the default size is sensible.
 
 Adding functionality to the receive direction is generally backwards
 compatible: adding events, adding return and event data members.
 Clients are expected to ignore the ones they don't know.
 
 Removing "unreachable" stuff like events that can't be triggered
 anymore, optional return or event data members that can't be sent
 anymore, and return or event data member (enumeration) values that
 can't be sent anymore makes no difference to clients, except for
 introspection.  The latter can conceivably confuse clients, so tread
 carefully.
 
 Incompatible changes include removing return and event data members.
 
 Any change to a command definition's 'data' or one of the types used
 there (recursively) needs to consider send direction compatibility.
 
 Any change to a command definition's 'return', an event definition's
 'data', or one of the types used there (recursively) needs to consider
 receive direction compatibility.
 
 Any change to types used in both contexts need to consider both.
 
 Enumeration type values and complex and alternate type members may be
 reordered freely.  For enumerations and alternate types, this doesn't
 affect the wire encoding.  For complex types, this might make the
 implementation emit JSON object members in a different order, which
 the Client JSON Protocol permits.
 
 Since type names are not visible in the Client JSON Protocol, types
 may be freely renamed.  Even certain refactorings are invisible, such
 as splitting members from one type into a common base type.
 
 
 Code generation
 ===============
 
 The QAPI code generator qapi-gen.py generates code and documentation
 from the schema.  Together with the core QAPI libraries, this code
 provides everything required to take JSON commands read in by a Client
 JSON Protocol server, unmarshal the arguments into the underlying C
 types, call into the corresponding C function, map the response back
 to a Client JSON Protocol response to be returned to the user, and
 introspect the commands.
 
 As an example, we'll use the following schema, which describes a
 single complex user-defined type, along with command which takes a
 list of that type as a parameter, and returns a single element of that
 type.  The user is responsible for writing the implementation of
 qmp_my_command(); everything else is produced by the generator. ::
 
     $ cat example-schema.json
     { 'struct': 'UserDefOne',
       'data': { 'integer': 'int', '*string': 'str' } }
 
     { 'command': 'my-command',
       'data': { 'arg1': ['UserDefOne'] },
       'returns': 'UserDefOne' }
 
     { 'event': 'MY_EVENT' }
 
 We run qapi-gen.py like this::
 
     $ python scripts/qapi-gen.py --output-dir="qapi-generated" \
     --prefix="example-" example-schema.json
 
 For a more thorough look at generated code, the testsuite includes
 tests/qapi-schema/qapi-schema-tests.json that covers more examples of
 what the generator will accept, and compiles the resulting C code as
 part of 'make check-unit'.
 
 
 Code generated for QAPI types
 -----------------------------
 
 The following files are created:
 
  ``$(prefix)qapi-types.h``
      C types corresponding to types defined in the schema
 
  ``$(prefix)qapi-types.c``
      Cleanup functions for the above C types
 
 The $(prefix) is an optional parameter used as a namespace to keep the
 generated code from one schema/code-generation separated from others so code
 can be generated/used from multiple schemas without clobbering previously
 created code.
 
 Example::
 
     $ cat qapi-generated/example-qapi-types.h
     [Uninteresting stuff omitted...]
 
     #ifndef EXAMPLE_QAPI_TYPES_H
     #define EXAMPLE_QAPI_TYPES_H
 
     #include "qapi/qapi-builtin-types.h"
 
     typedef struct UserDefOne UserDefOne;
 
     typedef struct UserDefOneList UserDefOneList;
 
     typedef struct q_obj_my_command_arg q_obj_my_command_arg;
 
     struct UserDefOne {
         int64_t integer;
         bool has_string;
         char *string;
     };
 
     void qapi_free_UserDefOne(UserDefOne *obj);
     G_DEFINE_AUTOPTR_CLEANUP_FUNC(UserDefOne, qapi_free_UserDefOne)
 
     struct UserDefOneList {
         UserDefOneList *next;
         UserDefOne *value;
     };
 
     void qapi_free_UserDefOneList(UserDefOneList *obj);
     G_DEFINE_AUTOPTR_CLEANUP_FUNC(UserDefOneList, qapi_free_UserDefOneList)
 
     struct q_obj_my_command_arg {
         UserDefOneList *arg1;
     };
 
     #endif /* EXAMPLE_QAPI_TYPES_H */
     $ cat qapi-generated/example-qapi-types.c
     [Uninteresting stuff omitted...]
 
     void qapi_free_UserDefOne(UserDefOne *obj)
     {
         Visitor *v;
 
         if (!obj) {
             return;
         }
 
         v = qapi_dealloc_visitor_new();
         visit_type_UserDefOne(v, NULL, &obj, NULL);
         visit_free(v);
     }
 
     void qapi_free_UserDefOneList(UserDefOneList *obj)
     {
         Visitor *v;
 
         if (!obj) {
             return;
         }
 
         v = qapi_dealloc_visitor_new();
         visit_type_UserDefOneList(v, NULL, &obj, NULL);
         visit_free(v);
     }
 
     [Uninteresting stuff omitted...]
 
 For a modular QAPI schema (see section `Include directives`_), code for
 each sub-module SUBDIR/SUBMODULE.json is actually generated into ::
 
  SUBDIR/$(prefix)qapi-types-SUBMODULE.h
  SUBDIR/$(prefix)qapi-types-SUBMODULE.c
 
 If qapi-gen.py is run with option --builtins, additional files are
 created:
 
  ``qapi-builtin-types.h``
      C types corresponding to built-in types
 
  ``qapi-builtin-types.c``
      Cleanup functions for the above C types
 
 
 Code generated for visiting QAPI types
 --------------------------------------
 
 These are the visitor functions used to walk through and convert
 between a native QAPI C data structure and some other format (such as
 QObject); the generated functions are named visit_type_FOO() and
 visit_type_FOO_members().
 
 The following files are generated:
 
  ``$(prefix)qapi-visit.c``
      Visitor function for a particular C type, used to automagically
      convert QObjects into the corresponding C type and vice-versa, as
      well as for deallocating memory for an existing C type
 
  ``$(prefix)qapi-visit.h``
      Declarations for previously mentioned visitor functions
 
 Example::
 
     $ cat qapi-generated/example-qapi-visit.h
     [Uninteresting stuff omitted...]
 
     #ifndef EXAMPLE_QAPI_VISIT_H
     #define EXAMPLE_QAPI_VISIT_H
 
     #include "qapi/qapi-builtin-visit.h"
     #include "example-qapi-types.h"
 
 
     bool visit_type_UserDefOne_members(Visitor *v, UserDefOne *obj, Error **errp);
 
     bool visit_type_UserDefOne(Visitor *v, const char *name,
                      UserDefOne **obj, Error **errp);
 
     bool visit_type_UserDefOneList(Visitor *v, const char *name,
                      UserDefOneList **obj, Error **errp);
 
     bool visit_type_q_obj_my_command_arg_members(Visitor *v, q_obj_my_command_arg *obj, Error **errp);
 
     #endif /* EXAMPLE_QAPI_VISIT_H */
     $ cat qapi-generated/example-qapi-visit.c
     [Uninteresting stuff omitted...]
 
     bool visit_type_UserDefOne_members(Visitor *v, UserDefOne *obj, Error **errp)
     {
         if (!visit_type_int(v, "integer", &obj->integer, errp)) {
             return false;
         }
         if (visit_optional(v, "string", &obj->has_string)) {
             if (!visit_type_str(v, "string", &obj->string, errp)) {
                 return false;
             }
         }
         return true;
     }
 
     bool visit_type_UserDefOne(Visitor *v, const char *name,
                      UserDefOne **obj, Error **errp)
     {
         bool ok = false;
 
         if (!visit_start_struct(v, name, (void **)obj, sizeof(UserDefOne), errp)) {
             return false;
         }
         if (!*obj) {
             /* incomplete */
             assert(visit_is_dealloc(v));
             ok = true;
             goto out_obj;
         }
         if (!visit_type_UserDefOne_members(v, *obj, errp)) {
             goto out_obj;
         }
         ok = visit_check_struct(v, errp);
     out_obj:
         visit_end_struct(v, (void **)obj);
         if (!ok && visit_is_input(v)) {
             qapi_free_UserDefOne(*obj);
             *obj = NULL;
         }
         return ok;
     }
 
     bool visit_type_UserDefOneList(Visitor *v, const char *name,
                      UserDefOneList **obj, Error **errp)
     {
         bool ok = false;
         UserDefOneList *tail;
         size_t size = sizeof(**obj);
 
         if (!visit_start_list(v, name, (GenericList **)obj, size, errp)) {
             return false;
         }
 
         for (tail = *obj; tail;
              tail = (UserDefOneList *)visit_next_list(v, (GenericList *)tail, size)) {
             if (!visit_type_UserDefOne(v, NULL, &tail->value, errp)) {
                 goto out_obj;
             }
         }
 
         ok = visit_check_list(v, errp);
     out_obj:
         visit_end_list(v, (void **)obj);
         if (!ok && visit_is_input(v)) {
             qapi_free_UserDefOneList(*obj);
             *obj = NULL;
         }
         return ok;
     }
 
     bool visit_type_q_obj_my_command_arg_members(Visitor *v, q_obj_my_command_arg *obj, Error **errp)
     {
         if (!visit_type_UserDefOneList(v, "arg1", &obj->arg1, errp)) {
             return false;
         }
         return true;
     }
 
     [Uninteresting stuff omitted...]
 
 For a modular QAPI schema (see section `Include directives`_), code for
 each sub-module SUBDIR/SUBMODULE.json is actually generated into ::
 
  SUBDIR/$(prefix)qapi-visit-SUBMODULE.h
  SUBDIR/$(prefix)qapi-visit-SUBMODULE.c
 
 If qapi-gen.py is run with option --builtins, additional files are
 created:
 
  ``qapi-builtin-visit.h``
      Visitor functions for built-in types
 
  ``qapi-builtin-visit.c``
      Declarations for these visitor functions
 
 
 Code generated for commands
 ---------------------------
 
 These are the marshaling/dispatch functions for the commands defined
 in the schema.  The generated code provides qmp_marshal_COMMAND(), and
 declares qmp_COMMAND() that the user must implement.
 
 The following files are generated:
 
  ``$(prefix)qapi-commands.c``
      Command marshal/dispatch functions for each QMP command defined in
      the schema
 
  ``$(prefix)qapi-commands.h``
      Function prototypes for the QMP commands specified in the schema
 
  ``$(prefix)qapi-commands.trace-events``
      Trace event declarations, see :ref:`tracing`.
 
  ``$(prefix)qapi-init-commands.h``
      Command initialization prototype
 
  ``$(prefix)qapi-init-commands.c``
      Command initialization code
 
 Example::
 
     $ cat qapi-generated/example-qapi-commands.h
     [Uninteresting stuff omitted...]
 
     #ifndef EXAMPLE_QAPI_COMMANDS_H
     #define EXAMPLE_QAPI_COMMANDS_H
 
     #include "example-qapi-types.h"
 
     UserDefOne *qmp_my_command(UserDefOneList *arg1, Error **errp);
     void qmp_marshal_my_command(QDict *args, QObject **ret, Error **errp);
 
     #endif /* EXAMPLE_QAPI_COMMANDS_H */
 
     $ cat qapi-generated/example-qapi-commands.trace-events
     # AUTOMATICALLY GENERATED, DO NOT MODIFY
 
     qmp_enter_my_command(const char *json) "%s"
     qmp_exit_my_command(const char *result, bool succeeded) "%s %d"
 
     $ cat qapi-generated/example-qapi-commands.c
     [Uninteresting stuff omitted...]
 
 
     static void qmp_marshal_output_UserDefOne(UserDefOne *ret_in,
                                     QObject **ret_out, Error **errp)
     {
         Visitor *v;
 
         v = qobject_output_visitor_new_qmp(ret_out);
         if (visit_type_UserDefOne(v, "unused", &ret_in, errp)) {
             visit_complete(v, ret_out);
         }
         visit_free(v);
         v = qapi_dealloc_visitor_new();
         visit_type_UserDefOne(v, "unused", &ret_in, NULL);
         visit_free(v);
     }
 
     void qmp_marshal_my_command(QDict *args, QObject **ret, Error **errp)
     {
         Error *err = NULL;
         bool ok = false;
         Visitor *v;
         UserDefOne *retval;
         q_obj_my_command_arg arg = {0};
 
         v = qobject_input_visitor_new_qmp(QOBJECT(args));
         if (!visit_start_struct(v, NULL, NULL, 0, errp)) {
             goto out;
         }
         if (visit_type_q_obj_my_command_arg_members(v, &arg, errp)) {
             ok = visit_check_struct(v, errp);
         }
         visit_end_struct(v, NULL);
         if (!ok) {
             goto out;
         }
 
         if (trace_event_get_state_backends(TRACE_QMP_ENTER_MY_COMMAND)) {
             g_autoptr(GString) req_json = qobject_to_json(QOBJECT(args));
 
             trace_qmp_enter_my_command(req_json->str);
         }
 
         retval = qmp_my_command(arg.arg1, &err);
         if (err) {
             trace_qmp_exit_my_command(error_get_pretty(err), false);
             error_propagate(errp, err);
             goto out;
         }
 
         qmp_marshal_output_UserDefOne(retval, ret, errp);
 
         if (trace_event_get_state_backends(TRACE_QMP_EXIT_MY_COMMAND)) {
             g_autoptr(GString) ret_json = qobject_to_json(*ret);
 
             trace_qmp_exit_my_command(ret_json->str, true);
         }
 
     out:
         visit_free(v);
         v = qapi_dealloc_visitor_new();
         visit_start_struct(v, NULL, NULL, 0, NULL);
         visit_type_q_obj_my_command_arg_members(v, &arg, NULL);
         visit_end_struct(v, NULL);
         visit_free(v);
     }
 
     [Uninteresting stuff omitted...]
     $ cat qapi-generated/example-qapi-init-commands.h
     [Uninteresting stuff omitted...]
     #ifndef EXAMPLE_QAPI_INIT_COMMANDS_H
     #define EXAMPLE_QAPI_INIT_COMMANDS_H
 
     #include "qapi/qmp/dispatch.h"
 
     void example_qmp_init_marshal(QmpCommandList *cmds);
 
     #endif /* EXAMPLE_QAPI_INIT_COMMANDS_H */
     $ cat qapi-generated/example-qapi-init-commands.c
     [Uninteresting stuff omitted...]
     void example_qmp_init_marshal(QmpCommandList *cmds)
     {
         QTAILQ_INIT(cmds);
 
         qmp_register_command(cmds, "my-command",
                              qmp_marshal_my_command, QCO_NO_OPTIONS);
     }
     [Uninteresting stuff omitted...]
 
 For a modular QAPI schema (see section `Include directives`_), code for
 each sub-module SUBDIR/SUBMODULE.json is actually generated into::
 
  SUBDIR/$(prefix)qapi-commands-SUBMODULE.h
  SUBDIR/$(prefix)qapi-commands-SUBMODULE.c
 
 
 Code generated for events
 -------------------------
 
 This is the code related to events defined in the schema, providing
 qapi_event_send_EVENT().
 
 The following files are created:
 
  ``$(prefix)qapi-events.h``
      Function prototypes for each event type
 
  ``$(prefix)qapi-events.c``
      Implementation of functions to send an event
 
  ``$(prefix)qapi-emit-events.h``
      Enumeration of all event names, and common event code declarations
 
  ``$(prefix)qapi-emit-events.c``
      Common event code definitions
 
 Example::
 
     $ cat qapi-generated/example-qapi-events.h
     [Uninteresting stuff omitted...]
 
     #ifndef EXAMPLE_QAPI_EVENTS_H
     #define EXAMPLE_QAPI_EVENTS_H
 
     #include "qapi/util.h"
     #include "example-qapi-types.h"
 
     void qapi_event_send_my_event(void);
 
     #endif /* EXAMPLE_QAPI_EVENTS_H */
     $ cat qapi-generated/example-qapi-events.c
     [Uninteresting stuff omitted...]
 
     void qapi_event_send_my_event(void)
     {
         QDict *qmp;
 
         qmp = qmp_event_build_dict("MY_EVENT");
 
         example_qapi_event_emit(EXAMPLE_QAPI_EVENT_MY_EVENT, qmp);
 
         qobject_unref(qmp);
     }
 
     [Uninteresting stuff omitted...]
     $ cat qapi-generated/example-qapi-emit-events.h
     [Uninteresting stuff omitted...]
 
     #ifndef EXAMPLE_QAPI_EMIT_EVENTS_H
     #define EXAMPLE_QAPI_EMIT_EVENTS_H
 
     #include "qapi/util.h"
 
     typedef enum example_QAPIEvent {
         EXAMPLE_QAPI_EVENT_MY_EVENT,
         EXAMPLE_QAPI_EVENT__MAX,
     } example_QAPIEvent;
 
     #define example_QAPIEvent_str(val) \
         qapi_enum_lookup(&example_QAPIEvent_lookup, (val))
 
     extern const QEnumLookup example_QAPIEvent_lookup;
 
     void example_qapi_event_emit(example_QAPIEvent event, QDict *qdict);
 
     #endif /* EXAMPLE_QAPI_EMIT_EVENTS_H */
     $ cat qapi-generated/example-qapi-emit-events.c
     [Uninteresting stuff omitted...]
 
     const QEnumLookup example_QAPIEvent_lookup = {
         .array = (const char *const[]) {
             [EXAMPLE_QAPI_EVENT_MY_EVENT] = "MY_EVENT",
         },
         .size = EXAMPLE_QAPI_EVENT__MAX
     };
 
     [Uninteresting stuff omitted...]
 
 For a modular QAPI schema (see section `Include directives`_), code for
 each sub-module SUBDIR/SUBMODULE.json is actually generated into ::
 
  SUBDIR/$(prefix)qapi-events-SUBMODULE.h
  SUBDIR/$(prefix)qapi-events-SUBMODULE.c
 
 
 Code generated for introspection
 --------------------------------
 
 The following files are created:
 
  ``$(prefix)qapi-introspect.c``
      Defines a string holding a JSON description of the schema
 
  ``$(prefix)qapi-introspect.h``
      Declares the above string
 
 Example::
 
     $ cat qapi-generated/example-qapi-introspect.h
     [Uninteresting stuff omitted...]
 
     #ifndef EXAMPLE_QAPI_INTROSPECT_H
     #define EXAMPLE_QAPI_INTROSPECT_H
 
     #include "qapi/qmp/qlit.h"
 
     extern const QLitObject example_qmp_schema_qlit;
 
     #endif /* EXAMPLE_QAPI_INTROSPECT_H */
     $ cat qapi-generated/example-qapi-introspect.c
     [Uninteresting stuff omitted...]
 
     const QLitObject example_qmp_schema_qlit = QLIT_QLIST(((QLitObject[]) {
         QLIT_QDICT(((QLitDictEntry[]) {
             { "arg-type", QLIT_QSTR("0"), },
             { "meta-type", QLIT_QSTR("command"), },
             { "name", QLIT_QSTR("my-command"), },
             { "ret-type", QLIT_QSTR("1"), },
             {}
         })),
         QLIT_QDICT(((QLitDictEntry[]) {
             { "arg-type", QLIT_QSTR("2"), },
             { "meta-type", QLIT_QSTR("event"), },
             { "name", QLIT_QSTR("MY_EVENT"), },
             {}
         })),
         /* "0" = q_obj_my-command-arg */
         QLIT_QDICT(((QLitDictEntry[]) {
             { "members", QLIT_QLIST(((QLitObject[]) {
                 QLIT_QDICT(((QLitDictEntry[]) {
                     { "name", QLIT_QSTR("arg1"), },
                     { "type", QLIT_QSTR("[1]"), },
                     {}
                 })),
                 {}
             })), },
             { "meta-type", QLIT_QSTR("object"), },
             { "name", QLIT_QSTR("0"), },
             {}
         })),
         /* "1" = UserDefOne */
         QLIT_QDICT(((QLitDictEntry[]) {
             { "members", QLIT_QLIST(((QLitObject[]) {
                 QLIT_QDICT(((QLitDictEntry[]) {
                     { "name", QLIT_QSTR("integer"), },
                     { "type", QLIT_QSTR("int"), },
                     {}
                 })),
                 QLIT_QDICT(((QLitDictEntry[]) {
                     { "default", QLIT_QNULL, },
                     { "name", QLIT_QSTR("string"), },
                     { "type", QLIT_QSTR("str"), },
                     {}
                 })),
                 {}
             })), },
             { "meta-type", QLIT_QSTR("object"), },
             { "name", QLIT_QSTR("1"), },
             {}
         })),
         /* "2" = q_empty */
         QLIT_QDICT(((QLitDictEntry[]) {
             { "members", QLIT_QLIST(((QLitObject[]) {
                 {}
             })), },
             { "meta-type", QLIT_QSTR("object"), },
             { "name", QLIT_QSTR("2"), },
             {}
         })),
         QLIT_QDICT(((QLitDictEntry[]) {
             { "element-type", QLIT_QSTR("1"), },
             { "meta-type", QLIT_QSTR("array"), },
             { "name", QLIT_QSTR("[1]"), },
             {}
         })),
         QLIT_QDICT(((QLitDictEntry[]) {
             { "json-type", QLIT_QSTR("int"), },
             { "meta-type", QLIT_QSTR("builtin"), },
             { "name", QLIT_QSTR("int"), },
             {}
         })),
         QLIT_QDICT(((QLitDictEntry[]) {
             { "json-type", QLIT_QSTR("string"), },
             { "meta-type", QLIT_QSTR("builtin"), },
             { "name", QLIT_QSTR("str"), },
             {}
         })),
         {}
     }));
 
     [Uninteresting stuff omitted...]