docs: Document QAPI union types

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

= 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.

=== 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.  Optional
members should always be added to the end of the dictionary to preserve
backwards compatibility.

=== 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 } }

=== 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:

    mdroth@illuin:~/w/qemu2.git$ cat example-schema.json
    { 'type': 'UserDefOne',
      'data': { 'integer': 'int', 'string': 'str' } }

    { 'command': 'my-command',
      'data':    {'arg1': 'UserDefOne'},
      'returns': 'UserDefOne' }
    mdroth@illuin:~/w/qemu2.git$

=== 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:

    mdroth@illuin:~/w/qemu2.git$ python scripts/qapi-types.py \
      --output-dir="qapi-generated" --prefix="example-" < example-schema.json
    mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-types.c
    /* AUTOMATICALLY GENERATED, DO NOT MODIFY */

    #include "qapi/qapi-dealloc-visitor.h"
    #include "example-qapi-types.h"
    #include "example-qapi-visit.h"

    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);
    }

    mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-types.h
    /* AUTOMATICALLY GENERATED, DO NOT MODIFY */
    #ifndef QAPI_GENERATED_EXAMPLE_QAPI_TYPES
    #define QAPI_GENERATED_EXAMPLE_QAPI_TYPES

    #include "qapi/qapi-types-core.h"

    typedef struct UserDefOne UserDefOne;

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

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

    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:

    mdroth@illuin:~/w/qemu2.git$ python scripts/qapi-visit.py \
        --output-dir="qapi-generated" --prefix="example-" < example-schema.json
    mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-visit.c
    /* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */

    #include "example-qapi-visit.h"

    void visit_type_UserDefOne(Visitor *m, UserDefOne ** obj, const char *name, Error **errp)
    {
        visit_start_struct(m, (void **)obj, "UserDefOne", name, sizeof(UserDefOne), errp);
        visit_type_int(m, (obj && *obj) ? &(*obj)->integer : NULL, "integer", errp);
        visit_type_str(m, (obj && *obj) ? &(*obj)->string : NULL, "string", errp);
        visit_end_struct(m, errp);
    }

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

        visit_start_list(m, name, errp);

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

        visit_end_list(m, errp);
    }
    mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-visit.h
    /* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */

    #ifndef QAPI_GENERATED_EXAMPLE_QAPI_VISIT
    #define QAPI_GENERATED_EXAMPLE_QAPI_VISIT

    #include "qapi/qapi-visit-core.h"
    #include "example-qapi-types.h"

    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
    mdroth@illuin:~/w/qemu2.git$

(The actual structure of the visit_type_* functions is a bit more complex
in order to propagate errors correctly and avoid leaking memory).

=== 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:

    mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qmp-marshal.c
    /* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */

    #include "qemu-objects.h"
    #include "qapi/qmp-core.h"
    #include "qapi/qapi-visit-core.h"
    #include "qapi/qmp-output-visitor.h"
    #include "qapi/qmp-input-visitor.h"
    #include "qapi/qapi-dealloc-visitor.h"
    #include "example-qapi-types.h"
    #include "example-qapi-visit.h"

    #include "example-qmp-commands.h"
    static void qmp_marshal_output_my_command(UserDefOne * ret_in, QObject **ret_out, Error **errp)
    {
        QapiDeallocVisitor *md = qapi_dealloc_visitor_new();
        QmpOutputVisitor *mo = qmp_output_visitor_new();
        Visitor *v;

        v = qmp_output_get_visitor(mo);
        visit_type_UserDefOne(v, &ret_in, "unused", errp);
        v = qapi_dealloc_get_visitor(md);
        visit_type_UserDefOne(v, &ret_in, "unused", errp);
        qapi_dealloc_visitor_cleanup(md);


        *ret_out = qmp_output_get_qobject(mo);
    }

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

        mi = qmp_input_visitor_new(QOBJECT(args));
        v = qmp_input_get_visitor(mi);
        visit_type_UserDefOne(v, &arg1, "arg1", errp);

        if (error_is_set(errp)) {
            goto out;
        }
        retval = qmp_my_command(arg1, errp);
        qmp_marshal_output_my_command(retval, ret, errp);

    out:
        md = qapi_dealloc_visitor_new();
        v = qapi_dealloc_get_visitor(md);
        visit_type_UserDefOne(v, &arg1, "arg1", errp);
        qapi_dealloc_visitor_cleanup(md);
        return;
    }

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

    qapi_init(qmp_init_marshal);
    mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qmp-commands.h
    /* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */

    #ifndef QAPI_GENERATED_EXAMPLE_QMP_COMMANDS
    #define QAPI_GENERATED_EXAMPLE_QMP_COMMANDS

    #include "example-qapi-types.h"
    #include "error.h"

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

    #endif
    mdroth@illuin:~/w/qemu2.git$