docs/qapi-code-gen.txt - qemu - Git at Google

 = How to use the QAPI code generator =

 * Note: as of this writing, QMP does not use QAPI. Eventually QMP
 commands will be converted to use QAPI internally. The following
 information describes QMP/QAPI as it will exist after the
 conversion.

 QAPI is a native C API within QEMU which provides management-level
 functionality to internal/external users. For external
 users/processes, this interface is made available by a JSON-based
 QEMU Monitor protocol that is provided by the QMP server.

 To map QMP-defined 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. The QEMU Guest Agent also uses these
 scripts, paired with a separate schema, to generate
 marshaling/dispatch code for the guest agent server running in the
 guest.

 This document will describe how the schemas, scripts, and resulting
 code is used.


 == QMP/Guest agent schema ==

 This file defines the types, commands, and events used by QMP.  It should
 fully describe the interface used by QMP.

 This file is designed to be loosely based on JSON although it's technically
 executable Python.  While dictionaries are used, they are parsed as
 OrderedDicts so that ordering is preserved.

 There are two basic syntaxes used, type definitions and command definitions.

 The first syntax defines a type and is represented by a dictionary.  There are
 three kinds of user-defined types that are supported: complex types,
 enumeration types and union types.

 Generally speaking, types definitions should always use CamelCase for the type
 names. Command names should be all lower case with words separated by a hyphen.


 === Includes ===

 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 safe.


 === Complex types ===

 A complex type is a dictionary containing a single key whose value is a
 dictionary.  This corresponds to a struct in C or an Object in JSON.  An
 example of a complex type is:

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

 The use of '*' as a prefix to the name means the member is optional.

 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 field, 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 complex type definition can specify another complex type as its base.
 In this case, the fields of the base type are included as top-level fields
 of the new complex type's dictionary in the QMP wire format. An example
 definition is:

  { 'type': 'BlockdevOptionsGenericFormat', 'data': { 'file': 'str' } }
  { 'type': 'BlockdevOptionsGenericCOWFormat',
    'base': 'BlockdevOptionsGenericFormat',
    'data': { '*backing': 'str' } }

 An example BlockdevOptionsGenericCOWFormat object on the wire could use
 both fields like this:

  { "file": "/some/place/my-image",
    "backing": "/some/place/my-backing-file" }

 === Enumeration types ===

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

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

 === Union types ===

 Union types are used to let the user choose between several different data
 types.  A union type is defined using a dictionary as explained in the
 following paragraphs.


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

  { 'type': 'FileOptions', 'data': { 'filename': 'str' } }
  { 'type': 'Qcow2Options',
    'data': { 'backing-file': 'str', 'lazy-refcounts': 'bool' } }

  { 'union': 'BlockdevOptions',
    'data': { 'file': 'FileOptions',
              'qcow2': 'Qcow2Options' } }

 In the QMP wire format, a simple union is represented by a dictionary that
 contains the 'type' field as a discriminator, and a 'data' field that is of the
 specified data type corresponding to the discriminator value:

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


 A union definition can specify a complex type as its base. In this case, the
 fields of the complex type are included as top-level fields of the union
 dictionary in the QMP wire format. An example definition is:

  { 'type': 'BlockdevCommonOptions', 'data': { 'readonly': 'bool' } }
  { 'union': 'BlockdevOptions',
    'base': 'BlockdevCommonOptions',
    'data': { 'raw': 'RawOptions',
              'qcow2': 'Qcow2Options' } }

 And it looks like this on the wire:

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


 Flat union types avoid the nesting on the wire. They are used whenever a
 specific field of the base type is declared as the discriminator ('type' is
 then no longer generated). The discriminator must be of enumeration type.
 The above example can then be modified as follows:

  { 'enum': 'BlockdevDriver', 'data': [ 'raw', 'qcow2' ] }
  { 'type': 'BlockdevCommonOptions',
    'data': { 'driver': 'BlockdevDriver', 'readonly': 'bool' } }
  { 'union': 'BlockdevOptions',
    'base': 'BlockdevCommonOptions',
    'discriminator': 'driver',
    'data': { 'raw': 'RawOptions',
              'qcow2': 'Qcow2Options' } }

 Resulting in this JSON object:

  { "driver": "qcow2",
    "readonly": false,
    "backing-file": "/some/place/my-image",
    "lazy-refcounts": true }


 A special type of unions are anonymous unions. They don't form a dictionary in
 the wire format but allow the direct use of different types in their place. As
 they aren't structured, they don't have any explicit discriminator but use
 the (QObject) data type of their value as an implicit discriminator. This means
 that they are restricted to using only one discriminator value per QObject
 type. For example, you cannot have two different complex types in an anonymous
 union, or two different integer types.

 Anonymous unions are declared using an empty dictionary as their discriminator.
 The discriminator values never appear on the wire, they are only used in the
 generated C code. Anonymous unions cannot have a base type.

  { 'union': 'BlockRef',
    'discriminator': {},
    'data': { 'definition': 'BlockdevOptions',
              'reference': 'str' } }

 This example allows using both of the following example objects:

  { "file": "my_existing_block_device_id" }
  { "file": { "driver": "file",
              "readonly": false,
              "filename": "/tmp/mydisk.qcow2" } }


 === Commands ===

 Commands are defined by using a list containing three members.  The first
 member is the command name, the second member is a dictionary containing
 arguments, and the third member is the return type.

 An example command is:

  { 'command': 'my-command',
    'data': { 'arg1': 'str', '*arg2': 'str' },
    'returns': 'str' }

 === Events ===

 Events are defined with the keyword 'event'.  When 'data' is also specified,
 additional info will be included in the event.  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.  If timestamp is -1, it means failure to retrieve
 host time.

 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 } }

 == Code generation ==

 Schemas are fed into 3 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 QMP/guest agent server, unmarshal the arguments into
 the underlying C types, call into the corresponding C function, and map the
 response back to a QMP/guest agent response to be returned to the user.

 As an example, we'll use the following schema, which describes a single
 complex user-defined type (which will produce a C struct, along with a list
 node structure that can be used to chain together a list of such types in
 case we want to accept/return a list of this type with a command), and a
 command which takes that type as a parameter and returns the same type:

     $ cat example-schema.json
     { 'type': 'UserDefOne',
       'data': { 'integer': 'int', 'string': 'str' } }

     { 'command': 'my-command',
       'data':    {'arg1': 'UserDefOne'},
       'returns': 'UserDefOne' }

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

 Used to generate the C types defined by a schema. 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-" --input-file=example-schema.json
     $ cat qapi-generated/example-qapi-types.c
 [Uninteresting stuff omitted...]

     void qapi_free_UserDefOneList(UserDefOneList * obj)
     {
         QapiDeallocVisitor *md;
         Visitor *v;

         if (!obj) {
             return;
         }

         md = qapi_dealloc_visitor_new();
         v = qapi_dealloc_get_visitor(md);
         visit_type_UserDefOneList(v, &obj, NULL, NULL);
         qapi_dealloc_visitor_cleanup(md);
     }

     void qapi_free_UserDefOne(UserDefOne * obj)
     {
         QapiDeallocVisitor *md;
         Visitor *v;

         if (!obj) {
             return;
         }

         md = qapi_dealloc_visitor_new();
         v = qapi_dealloc_get_visitor(md);
         visit_type_UserDefOne(v, &obj, NULL, NULL);
         qapi_dealloc_visitor_cleanup(md);
     }

     $ cat qapi-generated/example-qapi-types.h
 [Uninteresting stuff omitted...]

     #ifndef EXAMPLE_QAPI_TYPES_H
     #define EXAMPLE_QAPI_TYPES_H

 [Builtin types omitted...]

     typedef struct UserDefOne UserDefOne;

     typedef struct UserDefOneList
     {
         union {
             UserDefOne *value;
             uint64_t padding;
         };
         struct UserDefOneList *next;
     } UserDefOneList;

 [Functions on builtin types omitted...]

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

     void qapi_free_UserDefOneList(UserDefOneList * obj);
     void qapi_free_UserDefOne(UserDefOne * obj);

     #endif

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

 Used to generate the visitor functions used to walk through and convert
 a QObject (as provided by QMP) to a native C data structure and
 vice-versa, as well as the visitor function used to dealloc a complex
 schema-defined C type.

 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-" --input-file=example-schema.json
     $ cat qapi-generated/example-qapi-visit.c
 [Uninteresting stuff omitted...]

     static void visit_type_UserDefOne_fields(Visitor *m, UserDefOne ** obj, Error **errp)
     {
         Error *err = NULL;
         visit_type_int(m, &(*obj)->integer, "integer", &err);
         if (err) {
             goto out;
         }
         visit_type_str(m, &(*obj)->string, "string", &err);
         if (err) {
             goto out;
         }

     out:
         error_propagate(errp, err);
     }

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

         visit_start_struct(m, (void **)obj, "UserDefOne", name, sizeof(UserDefOne), &err);
         if (!err) {
             if (*obj) {
                 visit_type_UserDefOne_fields(m, obj, errp);
             }
             visit_end_struct(m, &err);
         }
         error_propagate(errp, err);
     }

     void visit_type_UserDefOneList(Visitor *m, UserDefOneList ** obj, const char *name, Error **errp)
     {
         Error *err = NULL;
         GenericList *i, **prev;

         visit_start_list(m, name, &err);
         if (err) {
             goto out;
         }

         for (prev = (GenericList **)obj;
              !err && (i = visit_next_list(m, prev, &err)) != NULL;
              prev = &i) {
             UserDefOneList *native_i = (UserDefOneList *)i;
             visit_type_UserDefOne(m, &native_i->value, NULL, &err);
         }

         error_propagate(errp, err);
         err = NULL;
         visit_end_list(m, &err);
     out:
         error_propagate(errp, err);
     }
     $ python scripts/qapi-commands.py --output-dir="qapi-generated" \
     --prefix="example-" --input-file=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 builtin types omitted...]

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

     #endif

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

 Used to generate the marshaling/dispatch functions for the commands defined
 in the schema. 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:

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

     static void qmp_marshal_output_my_command(UserDefOne * ret_in, QObject **ret_out, Error **errp)
     {
         Error *local_err = NULL;
         QmpOutputVisitor *mo = qmp_output_visitor_new();
         QapiDeallocVisitor *md;
         Visitor *v;

         v = qmp_output_get_visitor(mo);
         visit_type_UserDefOne(v, &ret_in, "unused", &local_err);
         if (local_err) {
             goto out;
         }
         *ret_out = qmp_output_get_qobject(mo);

     out:
         error_propagate(errp, local_err);
         qmp_output_visitor_cleanup(mo);
         md = qapi_dealloc_visitor_new();
         v = qapi_dealloc_get_visitor(md);
         visit_type_UserDefOne(v, &ret_in, "unused", NULL);
         qapi_dealloc_visitor_cleanup(md);
     }

     static void qmp_marshal_input_my_command(QDict *args, QObject **ret, Error **errp)
     {
         Error *local_err = NULL;
         UserDefOne * retval = NULL;
         QmpInputVisitor *mi = qmp_input_visitor_new_strict(QOBJECT(args));
         QapiDeallocVisitor *md;
         Visitor *v;
         UserDefOne * arg1 = NULL;

         v = qmp_input_get_visitor(mi);
         visit_type_UserDefOne(v, &arg1, "arg1", &local_err);
         if (local_err) {
             goto out;
         }

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

         qmp_marshal_output_my_command(retval, ret, &local_err);

     out:
         error_propagate(errp, local_err);
         qmp_input_visitor_cleanup(mi);
         md = qapi_dealloc_visitor_new();
         v = qapi_dealloc_get_visitor(md);
         visit_type_UserDefOne(v, &arg1, "arg1", NULL);
         qapi_dealloc_visitor_cleanup(md);
         return;
     }

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

     qapi_init(qmp_init_marshal);
     $ 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(UserDefOne * arg1, Error **errp);

     #endif
	= How to use the QAPI code generator =

	* Note: as of this writing, QMP does not use QAPI. Eventually QMP
	commands will be converted to use QAPI internally. The following
	information describes QMP/QAPI as it will exist after the
	conversion.

	QAPI is a native C API within QEMU which provides management-level
	functionality to internal/external users. For external
	users/processes, this interface is made available by a JSON-based
	QEMU Monitor protocol that is provided by the QMP server.

	To map QMP-defined 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. The QEMU Guest Agent also uses these
	scripts, paired with a separate schema, to generate
	marshaling/dispatch code for the guest agent server running in the
	guest.

	This document will describe how the schemas, scripts, and resulting
	code is used.


	== QMP/Guest agent schema ==

	This file defines the types, commands, and events used by QMP. It should
	fully describe the interface used by QMP.

	This file is designed to be loosely based on JSON although it's technically
	executable Python. While dictionaries are used, they are parsed as
	OrderedDicts so that ordering is preserved.

	There are two basic syntaxes used, type definitions and command definitions.

	The first syntax defines a type and is represented by a dictionary. There are
	three kinds of user-defined types that are supported: complex types,
	enumeration types and union types.

	Generally speaking, types definitions should always use CamelCase for the type
	names. Command names should be all lower case with words separated by a hyphen.


	=== Includes ===

	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 safe.


	=== Complex types ===

	A complex type is a dictionary containing a single key whose value is a
	dictionary. This corresponds to a struct in C or an Object in JSON. An
	example of a complex type is:

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

	The use of '*' as a prefix to the name means the member is optional.

	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 field, 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 complex type definition can specify another complex type as its base.
	In this case, the fields of the base type are included as top-level fields
	of the new complex type's dictionary in the QMP wire format. An example
	definition is:

	{ 'type': 'BlockdevOptionsGenericFormat', 'data': { 'file': 'str' } }
	{ 'type': 'BlockdevOptionsGenericCOWFormat',
	'base': 'BlockdevOptionsGenericFormat',
	'data': { '*backing': 'str' } }

	An example BlockdevOptionsGenericCOWFormat object on the wire could use
	both fields like this:

	{ "file": "/some/place/my-image",
	"backing": "/some/place/my-backing-file" }

	=== Enumeration types ===

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

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

	=== Union types ===

	Union types are used to let the user choose between several different data
	types. A union type is defined using a dictionary as explained in the
	following paragraphs.


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

	{ 'type': 'FileOptions', 'data': { 'filename': 'str' } }
	{ 'type': 'Qcow2Options',
	'data': { 'backing-file': 'str', 'lazy-refcounts': 'bool' } }

	{ 'union': 'BlockdevOptions',
	'data': { 'file': 'FileOptions',
	'qcow2': 'Qcow2Options' } }

	In the QMP wire format, a simple union is represented by a dictionary that
	contains the 'type' field as a discriminator, and a 'data' field that is of the
	specified data type corresponding to the discriminator value:

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


	A union definition can specify a complex type as its base. In this case, the
	fields of the complex type are included as top-level fields of the union
	dictionary in the QMP wire format. An example definition is:

	{ 'type': 'BlockdevCommonOptions', 'data': { 'readonly': 'bool' } }
	{ 'union': 'BlockdevOptions',
	'base': 'BlockdevCommonOptions',
	'data': { 'raw': 'RawOptions',
	'qcow2': 'Qcow2Options' } }

	And it looks like this on the wire:

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


	Flat union types avoid the nesting on the wire. They are used whenever a
	specific field of the base type is declared as the discriminator ('type' is
	then no longer generated). The discriminator must be of enumeration type.
	The above example can then be modified as follows:

	{ 'enum': 'BlockdevDriver', 'data': [ 'raw', 'qcow2' ] }
	{ 'type': 'BlockdevCommonOptions',
	'data': { 'driver': 'BlockdevDriver', 'readonly': 'bool' } }
	{ 'union': 'BlockdevOptions',
	'base': 'BlockdevCommonOptions',
	'discriminator': 'driver',
	'data': { 'raw': 'RawOptions',
	'qcow2': 'Qcow2Options' } }

	Resulting in this JSON object:

	{ "driver": "qcow2",
	"readonly": false,
	"backing-file": "/some/place/my-image",
	"lazy-refcounts": true }


	A special type of unions are anonymous unions. They don't form a dictionary in
	the wire format but allow the direct use of different types in their place. As
	they aren't structured, they don't have any explicit discriminator but use
	the (QObject) data type of their value as an implicit discriminator. This means
	that they are restricted to using only one discriminator value per QObject
	type. For example, you cannot have two different complex types in an anonymous
	union, or two different integer types.

	Anonymous unions are declared using an empty dictionary as their discriminator.
	The discriminator values never appear on the wire, they are only used in the
	generated C code. Anonymous unions cannot have a base type.

	{ 'union': 'BlockRef',
	'discriminator': {},
	'data': { 'definition': 'BlockdevOptions',
	'reference': 'str' } }

	This example allows using both of the following example objects:

	{ "file": "my_existing_block_device_id" }
	{ "file": { "driver": "file",
	"readonly": false,
	"filename": "/tmp/mydisk.qcow2" } }


	=== Commands ===

	Commands are defined by using a list containing three members. The first
	member is the command name, the second member is a dictionary containing
	arguments, and the third member is the return type.

	An example command is:

	{ 'command': 'my-command',
	'data': { 'arg1': 'str', '*arg2': 'str' },
	'returns': 'str' }

	=== Events ===

	Events are defined with the keyword 'event'. When 'data' is also specified,
	additional info will be included in the event. 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. If timestamp is -1, it means failure to retrieve
	host time.

	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 } }

	== Code generation ==

	Schemas are fed into 3 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 QMP/guest agent server, unmarshal the arguments into
	the underlying C types, call into the corresponding C function, and map the
	response back to a QMP/guest agent response to be returned to the user.

	As an example, we'll use the following schema, which describes a single
	complex user-defined type (which will produce a C struct, along with a list
	node structure that can be used to chain together a list of such types in
	case we want to accept/return a list of this type with a command), and a
	command which takes that type as a parameter and returns the same type:

	$ cat example-schema.json
	{ 'type': 'UserDefOne',
	'data': { 'integer': 'int', 'string': 'str' } }

	{ 'command': 'my-command',
	'data': {'arg1': 'UserDefOne'},
	'returns': 'UserDefOne' }

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

	Used to generate the C types defined by a schema. 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-" --input-file=example-schema.json
	$ cat qapi-generated/example-qapi-types.c
	[Uninteresting stuff omitted...]

	void qapi_free_UserDefOneList(UserDefOneList * obj)
	{
	QapiDeallocVisitor *md;
	Visitor *v;

	if (!obj) {
	return;
	}

	md = qapi_dealloc_visitor_new();
	v = qapi_dealloc_get_visitor(md);
	visit_type_UserDefOneList(v, &obj, NULL, NULL);
	qapi_dealloc_visitor_cleanup(md);
	}

	void qapi_free_UserDefOne(UserDefOne * obj)
	{
	QapiDeallocVisitor *md;
	Visitor *v;

	if (!obj) {
	return;
	}

	md = qapi_dealloc_visitor_new();
	v = qapi_dealloc_get_visitor(md);
	visit_type_UserDefOne(v, &obj, NULL, NULL);
	qapi_dealloc_visitor_cleanup(md);
	}

	$ cat qapi-generated/example-qapi-types.h
	[Uninteresting stuff omitted...]

	#ifndef EXAMPLE_QAPI_TYPES_H
	#define EXAMPLE_QAPI_TYPES_H

	[Builtin types omitted...]

	typedef struct UserDefOne UserDefOne;

	typedef struct UserDefOneList
	{
	union {
	UserDefOne *value;
	uint64_t padding;
	};
	struct UserDefOneList *next;
	} UserDefOneList;

	[Functions on builtin types omitted...]

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

	void qapi_free_UserDefOneList(UserDefOneList * obj);
	void qapi_free_UserDefOne(UserDefOne * obj);

	#endif

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

	Used to generate the visitor functions used to walk through and convert
	a QObject (as provided by QMP) to a native C data structure and
	vice-versa, as well as the visitor function used to dealloc a complex
	schema-defined C type.

	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-" --input-file=example-schema.json
	$ cat qapi-generated/example-qapi-visit.c
	[Uninteresting stuff omitted...]

	static void visit_type_UserDefOne_fields(Visitor m, UserDefOne * obj, Error **errp)
	{
	Error *err = NULL;
	visit_type_int(m, &(*obj)->integer, "integer", &err);
	if (err) {
	goto out;
	}
	visit_type_str(m, &(*obj)->string, "string", &err);
	if (err) {
	goto out;
	}

	out:
	error_propagate(errp, err);
	}

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

	visit_start_struct(m, (void **)obj, "UserDefOne", name, sizeof(UserDefOne), &err);
	if (!err) {
	if (*obj) {
	visit_type_UserDefOne_fields(m, obj, errp);
	}
	visit_end_struct(m, &err);
	}
	error_propagate(errp, err);
	}

	void visit_type_UserDefOneList(Visitor m, UserDefOneList * obj, const char name, Error *errp)
	{
	Error *err = NULL;
	GenericList i, *prev;

	visit_start_list(m, name, &err);
	if (err) {
	goto out;
	}

	for (prev = (GenericList **)obj;
	!err && (i = visit_next_list(m, prev, &err)) != NULL;
	prev = &i) {
	UserDefOneList native_i = (UserDefOneList )i;
	visit_type_UserDefOne(m, &native_i->value, NULL, &err);
	}

	error_propagate(errp, err);
	err = NULL;
	visit_end_list(m, &err);
	out:
	error_propagate(errp, err);
	}
	$ python scripts/qapi-commands.py --output-dir="qapi-generated" \
	--prefix="example-" --input-file=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 builtin types omitted...]

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

	#endif

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

	Used to generate the marshaling/dispatch functions for the commands defined
	in the schema. 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:

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

	static void qmp_marshal_output_my_command(UserDefOne * ret_in, QObject ret_out, Error errp)
	{
	Error *local_err = NULL;
	QmpOutputVisitor *mo = qmp_output_visitor_new();
	QapiDeallocVisitor *md;
	Visitor *v;

	v = qmp_output_get_visitor(mo);
	visit_type_UserDefOne(v, &ret_in, "unused", &local_err);
	if (local_err) {
	goto out;
	}
	*ret_out = qmp_output_get_qobject(mo);

	out:
	error_propagate(errp, local_err);
	qmp_output_visitor_cleanup(mo);
	md = qapi_dealloc_visitor_new();
	v = qapi_dealloc_get_visitor(md);
	visit_type_UserDefOne(v, &ret_in, "unused", NULL);
	qapi_dealloc_visitor_cleanup(md);
	}

	static void qmp_marshal_input_my_command(QDict args, QObject ret, Error *errp)
	{
	Error *local_err = NULL;
	UserDefOne * retval = NULL;
	QmpInputVisitor *mi = qmp_input_visitor_new_strict(QOBJECT(args));
	QapiDeallocVisitor *md;
	Visitor *v;
	UserDefOne * arg1 = NULL;

	v = qmp_input_get_visitor(mi);
	visit_type_UserDefOne(v, &arg1, "arg1", &local_err);
	if (local_err) {
	goto out;
	}

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

	qmp_marshal_output_my_command(retval, ret, &local_err);

	out:
	error_propagate(errp, local_err);
	qmp_input_visitor_cleanup(mi);
	md = qapi_dealloc_visitor_new();
	v = qapi_dealloc_get_visitor(md);
	visit_type_UserDefOne(v, &arg1, "arg1", NULL);
	qapi_dealloc_visitor_cleanup(md);
	return;
	}

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

	qapi_init(qmp_init_marshal);
	$ 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(UserDefOne * arg1, Error **errp);

	#endif