[qemu.git] / docs / qapi-code-gen.txt

= 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 Client JSON Protocol interfaces to the native C QAPI
implementations, a JSON-based schema is used to define types and
function signatures, and a set of scripts is used to generate types,
signatures, and marshaling/dispatch code. This document will describe
how the schemas, scripts, and resulting code are used.


== QMP/Guest agent schema ==

A QAPI schema file is designed to be loosely based on JSON
(http://www.ietf.org/rfc/rfc7159.txt) with changes for quoting style
and the use of comments; a QAPI schema file is then parsed by a python
code generation program.  A valid QAPI schema consists of a series of
top-level expressions, with no commas between them.  Where
dictionaries (JSON objects) are used, they are parsed as python
OrderedDicts so that ordering is preserved (for predictable layout of
generated C structs and parameter lists).  Ordering doesn't matter
between top-level expressions or the keys within an expression, but
does matter within dictionary values for 'data' and 'returns' members
of a single expression.  QAPI schema input is written using 'single
quotes' instead of JSON's "double quotes" (in contrast, Client JSON
Protocol uses no comments, and while input accepts 'single quotes' as
an extension, output is strict JSON using only "double quotes").  As
in JSON, trailing commas are not permitted in arrays or dictionaries.
Input must be ASCII (although QMP supports full Unicode strings, the
QAPI parser does not).  At present, there is no place where a QAPI
schema requires the use of JSON numbers or null.

Comments are allowed; anything between an unquoted # and the following
newline is ignored.  Although there is not yet a documentation
generator, a form of stylized comments has developed for consistently
documenting details about an expression and when it was added to the
schema.  The documentation is delimited between two lines of ##, then
the first line names the expression, an optional overview is provided,
then individual documentation about each member of 'data' is provided,
and finally, a 'Since: x.y.z' tag lists the release that introduced
the expression.  Optional members are tagged with the phrase
'#optional', often with their default value; and extensions added
after the expression was first released are also given a '(since
x.y.z)' comment.  For example:

    ##
    # @BlockStats:
    #
    # Statistics of a virtual block device or a block backing device.
    #
    # @device: #optional If the stats are for a virtual block device, the name
    #          corresponding to the virtual block device.
    #
    # @stats:  A @BlockDeviceStats for the device.
    #
    # @parent: #optional This describes the file block device if it has one.
    #
    # @backing: #optional This describes the backing block device if it has one.
    #           (Since 2.0)
    #
    # Since: 0.14.0
    ##
    { 'struct': 'BlockStats',
      'data': {'*device': 'str', 'stats': 'BlockDeviceStats',
               '*parent': 'BlockStats',
               '*backing': 'BlockStats'} }

The schema sets up a series of types, as well as commands and events
that will use those types.  Forward references are allowed: the parser
scans in two passes, where the first pass learns all type names, and
the second validates the schema and generates the code.  This allows
the definition of complex structs that can have mutually recursive
types, and allows for indefinite nesting of Client JSON Protocol that
satisfies the schema.  A type name should not be defined more than
once.  It is permissible for the schema to contain additional types
not used by any commands or events in the Client JSON Protocol, for
the side effect of generated C code used internally.

There are seven top-level expressions recognized by the parser:
'include', 'command', 'struct', 'enum', 'union', 'alternate', and
'event'.  There are several groups of types: simple types (a number of
built-in types, such as 'int' and 'str'; as well as enumerations),
complex types (structs and two flavors of unions), and alternate types
(a choice between other types).  The 'command' and 'event' expressions
can refer to existing types by name, or list an anonymous type as a
dictionary. Listing a type name inside an array refers to a
single-dimension array of that type; multi-dimension arrays are not
directly supported (although an array of a complex struct that
contains an array member is possible).

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
definitions should not end in 'Kind', as this namespace is used for
creating implicit C enums for visiting union types, or in 'List', as
this namespace is used for creating array types.  Command names,
and member names within a type, should be all lower case with words
separated by a hyphen.  However, some existing older commands and
complex types use underscore; when extending such expressions,
consistency is preferred over blindly avoiding underscore.  Event
names should be ALL_CAPS with words separated by underscore.  Member
names cannot start with 'has-' or 'has_', as this is reserved for
tracking optional members.

Any name (command, event, type, member, or enum value) beginning with
"x-" is marked experimental, and may be withdrawn or changed
incompatibly in a future release.  All names must begin with a letter,
and contain only ASCII letters, digits, dash, and underscore.  There
are two exceptions: enum values may start with a digit, and any
extensions added by downstream vendors should start with a prefix
matching "__RFQDN_" (for the reverse-fully-qualified-domain-name of
the vendor), even if the rest of the name uses dash (example:
__com.redhat_drive-mirror).  Names beginning with 'q_' are reserved
for the generator: QMP names that resemble C keywords or other
problematic strings will be munged in C to use this prefix.  For
example, a member named "default" in qapi becomes "q_default" in the
generated C code.

In the rest of this document, usage lines are given for each
expression type, with literal strings written in lower case and
placeholders written in capitals.  If a literal string includes a
prefix of '*', that key/value pair can be omitted from the expression.
For example, a usage statement that includes '*base':STRUCT-NAME
means that an expression has an optional key 'base', which if present
must have a value that forms a struct name.


=== 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
  any       QObject *  any JSON value
  QType     QType      JSON string matching enum QType values


=== Includes ===

Usage: { '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.  No other keys should appear in the expression, and the include
value should be a string.

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.


=== Struct types ===

Usage: { 'struct': STRING, 'data': DICT, '*base': STRUCT-NAME }

A struct is a dictionary containing a single 'data' key whose value is
a dictionary; the dictionary may be empty.  This corresponds to a
struct in C or an Object in JSON. Each value of the 'data' dictionary
must be the name of a type, or a one-element array containing a type
name.  An example of a struct is:

 { 'struct': 'MyType',
   'data': { 'member1': 'str', 'member2': 'int', '*member3': 'str' } }

The use of '*' as a prefix to the name means the member is optional in
the corresponding JSON protocol usage.

The default initialization value of an optional argument should not be changed
between versions of QEMU unless the new default maintains backward
compatibility to the user-visible behavior of the old default.

With proper documentation, this policy still allows some flexibility; for
example, documenting that a default of 0 picks an optimal buffer size allows
one release to declare the optimal size at 512 while another release declares
the optimal size at 4096 - the user-visible behavior is not the bytes used by
the buffer, but the fact that the buffer was optimal size.

On input structures (only mentioned in the 'data' side of a command), changing
from mandatory to optional is safe (older clients will supply the option, and
newer clients can benefit from the default); changing from optional to
mandatory is backwards incompatible (older clients may be omitting the option,
and must continue to work).

On output structures (only mentioned in the 'returns' side of a command),
changing from mandatory to optional is in general unsafe (older clients may be
expecting the member, and could crash if it is missing), although it
can be done if the only way that the optional argument will be omitted
is when it is triggered by the presence of a new input flag to the
command that older clients don't know to send.  Changing from optional
to mandatory is safe.

A structure that is used in both input and output of various commands
must consider the backwards compatibility constraints of both directions
of use.

A struct definition can specify another struct as its base.
In this case, the members of the base type are included as top-level members
of the new struct's dictionary in the Client JSON Protocol wire
format. An example definition is:

 { '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" }


=== Enumeration types ===

Usage: { 'enum': STRING, 'data': ARRAY-OF-STRING }
       { 'enum': STRING, '*prefix': STRING, 'data': ARRAY-OF-STRING }

An enumeration type is a dictionary containing a single 'data' key
whose value is a list of strings.  An example enumeration is:

 { 'enum': 'MyEnum', 'data': [ 'value1', 'value2', 'value3' ] }

Nothing prevents an empty enumeration, although it is probably not
useful.  The list of strings should be lower case; if an enum name
represents multiple words, use '-' between words.  The string 'max' is
not allowed as an enum value, and values should not be repeated.

The enum constants will be named by using a heuristic to turn the
type name into a set of underscore separated words. For the example
above, 'MyEnum' will turn into 'MY_ENUM' giving a constant name
of 'MY_ENUM_VALUE1' for the first value. If the default heuristic
does not result in a desirable name, the optional 'prefix' member
can be used when defining the enum.

The enumeration values are passed as strings over the Client JSON
Protocol, but are encoded as C enum integral values in generated code.
While the C code starts numbering at 0, it is better to use explicit
comparisons to enum values than implicit comparisons to 0; the C code
will also include a generated enum member ending in _MAX for tracking
the size of the enum, useful when using common functions for
converting between strings and enum values.  Since the wire format
always passes by name, it is acceptable to reorder or add new
enumeration members in any location without breaking clients of Client
JSON Protocol; however, removing enum values would break
compatibility.  For any struct that has a member that will only contain
a finite set of string values, using an enum type for that member is
better than open-coding the member to be type 'str'.


=== Union types ===

Usage: { 'union': STRING, 'data': DICT }
or:    { 'union': STRING, 'data': DICT, 'base': STRUCT-NAME-OR-DICT,
         'discriminator': ENUM-MEMBER-OF-BASE }

Union types are used to let the user choose between several different
variants for an object.  There are two flavors: simple (no
discriminator or base), and flat (both discriminator and base).  A union
type is defined using a data dictionary as explained in the following
paragraphs.  The data dictionary for either type of union must not
be empty.

A simple union type defines a mapping from automatic discriminator
values to data types like in this example:

 { 'struct': 'BlockdevOptionsFile', 'data': { 'filename': 'str' } }
 { 'struct': 'BlockdevOptionsQcow2',
   'data': { 'backing': 'str', '*lazy-refcounts': 'bool' } }

 { 'union': 'BlockdevOptionsSimple',
   'data': { 'file': 'BlockdevOptionsFile',
             'qcow2': 'BlockdevOptionsQcow2' } }

In the Client JSON Protocol, a simple union is represented by a
dictionary that contains the 'type' member as a discriminator, and a
'data' member that is of the specified data type corresponding to the
discriminator value, as in these examples:

 { "type": "file", "data": { "filename": "/some/place/my-image" } }
 { "type": "qcow2", "data": { "backing": "/some/place/my-image",
                              "lazy-refcounts": true } }

The generated C code uses a struct containing a union. Additionally,
an implicit C enum 'NameKind' is created, corresponding to the union
'Name', for accessing the various branches of the union.  No branch of
the union can be named 'max', as this would collide with the implicit
enum.  The value for each branch can be of any type.

A flat union definition avoids nesting on the wire, and specifies a
set of common members that occur in all variants of the union.  The
'base' key must specify either a type name (the type must be a
struct, not a union), or a dictionary representing an anonymous type.
All branches of the union must be complex types, and the top-level
members of the union dictionary on the wire will be combination of
members from both the base type and the appropriate branch type (when
merging two dictionaries, there must be no keys in common).  The
'discriminator' member must be the name of a non-optional enum-typed
member of the base struct.

The following example enhances the above simple union example by
adding an optional common member 'read-only', renaming the
discriminator to something more applicable than the simple union's
default of 'type', and reducing the number of {} required on the wire:

 { '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 }

Notice that in a flat union, the discriminator name is controlled by
the user, but because it must map to a base member with enum type, the
code generator can ensure that branches exist for all values of the
enum (although the order of the keys need not match the declaration of
the enum).  In the resulting generated C data types, a flat union is
represented as a struct with the base members included directly, and
then a union of structures for each branch of the struct.

A simple union can always be re-written as a flat union where the base
class has a single member named 'type', and where each branch of the
union has a struct with a single member named 'data'.  That is,

 { 'union': 'Simple', 'data': { 'one': 'str', 'two': 'int' } }

is identical on the wire to:

 { 'enum': 'Enum', 'data': ['one', 'two'] }
 { 'struct': 'Branch1', 'data': { 'data': 'str' } }
 { 'struct': 'Branch2', 'data': { 'data': 'int' } }
 { 'union': 'Flat': 'base': { 'type': 'Enum' }, 'discriminator': 'type',
   'data': { 'one': 'Branch1', 'two': 'Branch2' } }


=== Alternate types ===

Usage: { 'alternate': STRING, 'data': DICT }

An alternate type is one that allows a choice between two or more JSON
data types (string, integer, number, or object, but currently not
array) on the wire.  The definition is similar to a simple union type,
where each branch of the union names a QAPI type.  For example:

 { 'alternate': 'BlockdevRef',
   'data': { 'definition': 'BlockdevOptions',
             'reference': 'str' } }

Unlike a union, the discriminator string is never passed on the wire
for the Client JSON Protocol.  Instead, the value's JSON type serves
as an implicit discriminator, which in turn means that an alternate
can only express a choice between types represented differently in
JSON.  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; and if it is
typed as a complex type (struct or union), it accepts a JSON object.
Two different complex types, for instance, aren't permitted, because
both are represented as 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" } }


=== Commands ===

Usage: { 'command': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT,
         '*returns': TYPE-NAME, '*boxed': true,
         '*gen': false, '*success-response': false }

Commands are defined by using a dictionary containing several members,
where three members are most common.  The 'command' member is a
mandatory string, and determines the "execute" value passed in a
Client JSON Protocol command exchange.

The 'data' argument maps to the "arguments" dictionary passed in as
part of a Client JSON Protocol command.  The 'data' member is optional
and defaults to {} (an empty dictionary).  If present, it must be the
string name of a complex type, or a dictionary that declares an
anonymous type with the same semantics as a 'struct' expression, with
one exception noted below when 'gen' is used.

The 'returns' member describes what will appear in the "return" member
of a Client JSON Protocol reply on successful completion of a command.
The member is optional from the command declaration; if absent, the
"return" member will be an empty dictionary.  If 'returns' is present,
it must be the string name of a complex or built-in type, a
one-element array containing the name of a complex or built-in type,
with one exception noted below when 'gen' is used.  Although it is
permitted to have the 'returns' member name a built-in type or an
array of built-in types, any command that does this cannot be extended
to return additional information in the future; thus, new commands
should strongly consider returning a dictionary-based type or an array
of dictionaries, even if the dictionary only contains one member at the
present.

All commands in Client JSON Protocol use a dictionary to report
failure, with no way to specify that in QAPI.  Where the error return
is different than the usual GenericError class in order to help the
client react differently to certain error conditions, it is worth
documenting this in the comments before the command declaration.

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 user's function implementing
the command.  Normally, 'data' is a dictionary for an anonymous type,
or names a struct type (possibly empty, but not a union), and its
members are passed as separate arguments to this function.  If the
command definition includes a key 'boxed' with the boolean value true,
then 'data' is instead the name of any non-empty complex type
(struct, union, or alternate), and a pointer to that QAPI type is
passed as a single argument.

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.

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 key 'gen' with
boolean value false, and instead write your own function.  Please try
to avoid adding new commands that rely on this, and instead use
type-safe unions.  For an example of this usage:

 { 'command': 'netdev_add',
   'data': {'type': 'str', 'id': 'str'},
   'gen': false }

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 a
normal dictionary error on failure).  When a successful reply is not
possible, the command expression should include the optional key
'success-response' with boolean value false.  So far, only QGA makes
use of this member.


=== Events ===

Usage: { 'event': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT,
         '*boxed': true }

Events are defined with the keyword 'event'.  It is not allowed to
name an event 'MAX', since the generator also produces a C enumeration
of all event names with a generated _MAX value at the end.  When
'data' is also specified, additional info will be included in the
event, with similar semantics to a 'struct' expression.  Finally there
will be C API generated in qapi-event.h; when called by QEMU code, a
message with timestamp will be emitted on the wire.

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.  Normally, 'data' is
a dictionary for an anonymous type, or names a struct type (possibly
empty, but not a union), and its members are passed as separate
arguments to this function.  If the event definition includes a key
'boxed' with the boolean value true, then 'data' is instead the name of
any non-empty complex type (struct, union, or alternate), and a
pointer to that QAPI type is passed as a single argument.


== 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" and "meta-type", 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.

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" and "ret-type".  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".

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 dictionary-valued 'data' 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), and optionally "default".  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 } ] }

"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 flat 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.

A simple union implicitly defines an enumeration type for its implicit
discriminator (called "type" on the wire, see section Union types).

A simple union implicitly defines an object type for each of its
variants.

Example: the SchemaInfo for simple union BlockdevOptionsSimple from section
Union types

    { "name": "BlockdevOptionsSimple", "meta-type": "object",
      "members": [
          { "name": "type", "type": "BlockdevOptionsSimpleKind" } ],
      "tag": "type",
      "variants": [
          { "case": "file", "type": "q_obj-BlockdevOptionsFile-wrapper" },
          { "case": "qcow2", "type": "q_obj-BlockdevOptionsQcow2-wrapper" } ] }

    Enumeration type "BlockdevOptionsSimpleKind" and the object types
    "q_obj-BlockdevOptionsFile-wrapper", "q_obj-BlockdevOptionsQcow2-wrapper"
    are implicitly defined.

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 "values".  The values are listed in no particular
order; clients must search the entire enum when learning whether a
particular value is supported.

Example: the SchemaInfo for MyEnum from section Enumeration types

    { "name": "MyEnum", "meta-type": "enum",
      "values": [ "value1", "value2", "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.


== Code generation ==

Schemas are fed into five scripts to generate all the code/files that,
paired with the core QAPI libraries, comprise 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' }

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'.

=== scripts/qapi-types.py ===

Used to generate the C types defined by a schema, along with
supporting code. The following files are created:

$(prefix)qapi-types.h - C types corresponding to types defined in
                        the schema you pass in
$(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:

    $ python scripts/qapi-types.py --output-dir="qapi-generated" \
    --prefix="example-" example-schema.json
    $ cat qapi-generated/example-qapi-types.h
[Uninteresting stuff omitted...]

    #ifndef EXAMPLE_QAPI_TYPES_H
    #define EXAMPLE_QAPI_TYPES_H

[Built-in types omitted...]

    typedef struct UserDefOne UserDefOne;

    typedef struct UserDefOneList UserDefOneList;

    struct UserDefOne {
        int64_t integer;
        bool has_string;
        char *string;
    };

    void qapi_free_UserDefOne(UserDefOne *obj);

    struct UserDefOneList {
        UserDefOneList *next;
        UserDefOne *value;
    };

    void qapi_free_UserDefOneList(UserDefOneList *obj);

    #endif
    $ 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);
    }

=== scripts/qapi-visit.py ===

Used to generate 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:

    $ python scripts/qapi-visit.py --output-dir="qapi-generated"
    --prefix="example-" example-schema.json
    $ cat qapi-generated/example-qapi-visit.h
[Uninteresting stuff omitted...]

    #ifndef EXAMPLE_QAPI_VISIT_H
    #define EXAMPLE_QAPI_VISIT_H

[Visitors for built-in types omitted...]

    void visit_type_UserDefOne_members(Visitor *v, UserDefOne *obj, Error **errp);
    void visit_type_UserDefOne(Visitor *v, const char *name, UserDefOne **obj, Error **errp);
    void visit_type_UserDefOneList(Visitor *v, const char *name, UserDefOneList **obj, Error **errp);

    #endif
    $ cat qapi-generated/example-qapi-visit.c
[Uninteresting stuff omitted...]

    void visit_type_UserDefOne_members(Visitor *v, UserDefOne *obj, Error **errp)
    {
        Error *err = NULL;

        visit_type_int(v, "integer", &obj->integer, &err);
        if (err) {
            goto out;
        }
        if (visit_optional(v, "string", &obj->has_string)) {
            visit_type_str(v, "string", &obj->string, &err);
            if (err) {
                goto out;
            }
        }

    out:
        error_propagate(errp, err);
    }

    void visit_type_UserDefOne(Visitor *v, const char *name, UserDefOne **obj, Error **errp)
    {
        Error *err = NULL;

        visit_start_struct(v, name, (void **)obj, sizeof(UserDefOne), &err);
        if (err) {
            goto out;
        }
        if (!*obj) {
            goto out_obj;
        }
        visit_type_UserDefOne_members(v, *obj, &err);
        if (err) {
            goto out_obj;
        }
        visit_check_struct(v, &err);
    out_obj:
        visit_end_struct(v, (void **)obj);
        if (err && visit_is_input(v)) {
            qapi_free_UserDefOne(*obj);
            *obj = NULL;
        }
    out:
        error_propagate(errp, err);
    }

    void visit_type_UserDefOneList(Visitor *v, const char *name, UserDefOneList **obj, Error **errp)
    {
        Error *err = NULL;
        UserDefOneList *tail;
        size_t size = sizeof(**obj);

        visit_start_list(v, name, (GenericList **)obj, size, &err);
        if (err) {
            goto out;
        }

        for (tail = *obj; tail;
             tail = (UserDefOneList *)visit_next_list(v, (GenericList *)tail, size)) {
            visit_type_UserDefOne(v, NULL, &tail->value, &err);
            if (err) {
                break;
            }
        }

        visit_end_list(v, (void **)obj);
        if (err && visit_is_input(v)) {
            qapi_free_UserDefOneList(*obj);
            *obj = NULL;
        }
    out:
        error_propagate(errp, err);
    }

=== scripts/qapi-commands.py ===

Used to generate the marshaling/dispatch functions for the commands
defined in the schema. The generated code implements
qmp_marshal_COMMAND() (registered automatically), and declares
qmp_COMMAND() that the user must implement.  The following files are
generated:

$(prefix)qmp-marshal.c: command marshal/dispatch functions for each
                        QMP command defined in the schema. Functions
                        generated by qapi-visit.py are used to
                        convert QObjects received from the wire into
                        function parameters, and uses the same
                        visitor functions to convert native C return
                        values to QObjects from transmission back
                        over the wire.

$(prefix)qmp-commands.h: Function prototypes for the QMP commands
                         specified in the schema.

Example:

    $ python scripts/qapi-commands.py --output-dir="qapi-generated"
    --prefix="example-" example-schema.json
    $ cat qapi-generated/example-qmp-commands.h
[Uninteresting stuff omitted...]

    #ifndef EXAMPLE_QMP_COMMANDS_H
    #define EXAMPLE_QMP_COMMANDS_H

    #include "example-qapi-types.h"
    #include "qapi/qmp/qdict.h"
    #include "qapi/error.h"

    UserDefOne *qmp_my_command(UserDefOneList *arg1, Error **errp);

    #endif
    $ cat qapi-generated/example-qmp-marshal.c
[Uninteresting stuff omitted...]

    static void qmp_marshal_output_UserDefOne(UserDefOne *ret_in, QObject **ret_out, Error **errp)
    {
        Error *err = NULL;
        Visitor *v;

        v = qobject_output_visitor_new(ret_out);
        visit_type_UserDefOne(v, "unused", &ret_in, &err);
        if (!err) {
            visit_complete(v, ret_out);
        }
        error_propagate(errp, err);
        visit_free(v);
        v = qapi_dealloc_visitor_new();
        visit_type_UserDefOne(v, "unused", &ret_in, NULL);
        visit_free(v);
    }

    static void qmp_marshal_my_command(QDict *args, QObject **ret, Error **errp)
    {
        Error *err = NULL;
        UserDefOne *retval;
        Visitor *v;
        UserDefOneList *arg1 = NULL;

        v = qobject_input_visitor_new(QOBJECT(args), true);
        visit_start_struct(v, NULL, NULL, 0, &err);
        if (err) {
            goto out;
        }
        visit_type_UserDefOneList(v, "arg1", &arg1, &err);
        if (!err) {
            visit_check_struct(v, &err);
        }
        visit_end_struct(v, NULL);
        if (err) {
            goto out;
        }

        retval = qmp_my_command(arg1, &err);
        if (err) {
            goto out;
        }

        qmp_marshal_output_UserDefOne(retval, ret, &err);

    out:
        error_propagate(errp, err);
        visit_free(v);
        v = qapi_dealloc_visitor_new();
        visit_start_struct(v, NULL, NULL, 0, NULL);
        visit_type_UserDefOneList(v, "arg1", &arg1, NULL);
        visit_end_struct(v, NULL);
        visit_free(v);
    }

    static void qmp_init_marshal(void)
    {
        qmp_register_command("my-command", qmp_marshal_my_command, QCO_NO_OPTIONS);
    }

    qapi_init(qmp_init_marshal);

=== scripts/qapi-event.py ===

Used to generate the event-related C code defined by a schema, with
implementations for qapi_event_send_FOO(). The following files are
created:

$(prefix)qapi-event.h - Function prototypes for each event type, plus an
                        enumeration of all event names
$(prefix)qapi-event.c - Implementation of functions to send an event

Example:

    $ python scripts/qapi-event.py --output-dir="qapi-generated"
    --prefix="example-" example-schema.json
    $ cat qapi-generated/example-qapi-event.h
[Uninteresting stuff omitted...]

    #ifndef EXAMPLE_QAPI_EVENT_H
    #define EXAMPLE_QAPI_EVENT_H

    #include "qapi/error.h"
    #include "qapi/qmp/qdict.h"
    #include "example-qapi-types.h"


    void qapi_event_send_my_event(Error **errp);

    typedef enum example_QAPIEvent {
        EXAMPLE_QAPI_EVENT_MY_EVENT = 0,
        EXAMPLE_QAPI_EVENT__MAX = 1,
    } example_QAPIEvent;

    extern const char *const example_QAPIEvent_lookup[];

    #endif
    $ cat qapi-generated/example-qapi-event.c
[Uninteresting stuff omitted...]

    void qapi_event_send_my_event(Error **errp)
    {
        QDict *qmp;
        Error *err = NULL;
        QMPEventFuncEmit emit;
        emit = qmp_event_get_func_emit();
        if (!emit) {
            return;
        }

        qmp = qmp_event_build_dict("MY_EVENT");

        emit(EXAMPLE_QAPI_EVENT_MY_EVENT, qmp, &err);

        error_propagate(errp, err);
        QDECREF(qmp);
    }

    const char *const example_QAPIEvent_lookup[] = {
        [EXAMPLE_QAPI_EVENT_MY_EVENT] = "MY_EVENT",
        [EXAMPLE_QAPI_EVENT__MAX] = NULL,
    };

=== scripts/qapi-introspect.py ===

Used to generate the introspection C code for a schema. The following
files are created:

$(prefix)qmp-introspect.c - Defines a string holding a JSON
                            description of the schema.
$(prefix)qmp-introspect.h - Declares the above string.

Example:

    $ python scripts/qapi-introspect.py --output-dir="qapi-generated"
    --prefix="example-" example-schema.json
    $ cat qapi-generated/example-qmp-introspect.h
[Uninteresting stuff omitted...]

    #ifndef EXAMPLE_QMP_INTROSPECT_H
    #define EXAMPLE_QMP_INTROSPECT_H

    extern const char example_qmp_schema_json[];

    #endif
    $ cat qapi-generated/example-qmp-introspect.c
[Uninteresting stuff omitted...]

    const char example_qmp_schema_json[] = "["
        "{\"arg-type\": \"0\", \"meta-type\": \"event\", \"name\": \"MY_EVENT\"}, "
        "{\"arg-type\": \"1\", \"meta-type\": \"command\", \"name\": \"my-command\", \"ret-type\": \"2\"}, "
        "{\"members\": [], \"meta-type\": \"object\", \"name\": \"0\"}, "
        "{\"members\": [{\"name\": \"arg1\", \"type\": \"[2]\"}], \"meta-type\": \"object\", \"name\": \"1\"}, "
        "{\"members\": [{\"name\": \"integer\", \"type\": \"int\"}, {\"default\": null, \"name\": \"string\", \"type\": \"str\"}], \"meta-type\": \"object\", \"name\": \"2\"}, "
        "{\"element-type\": \"2\", \"meta-type\": \"array\", \"name\": \"[2]\"}, "
        "{\"json-type\": \"int\", \"meta-type\": \"builtin\", \"name\": \"int\"}, "
        "{\"json-type\": \"string\", \"meta-type\": \"builtin\", \"name\": \"str\"}]";