= 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' }


== 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
