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diff --git a/src/docs/qapi-code-gen.txt b/src/docs/qapi-code-gen.txt new file mode 100644 index 0000000..ceb9a78 --- /dev/null +++ b/src/docs/qapi-code-gen.txt @@ -0,0 +1,1112 @@ += How to use the QAPI code generator = + +Copyright IBM Corp. 2011 +Copyright (C) 2012-2015 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 fields 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 field 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. Field +names cannot start with 'has-' or 'has_', as this is reserved for +tracking optional fields. + +Any name (command, event, type, field, or enum value) beginning with +"x-" is marked experimental, and may be withdrawn or changed +incompatibly in a future release. Downstream vendors may add +extensions; such extensions should begin 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). Other than downstream extensions (with +leading underscore and the use of dots), all names should begin with a +letter, and contain only ASCII letters, digits, dash, and underscore. +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 field 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 + + +=== 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. 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 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 struct definition can specify another struct as its base. +In this case, the fields of the base type are included as top-level fields +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 fields 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' field +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 field that will only contain +a finite set of string values, using an enum type for that field is +better than open-coding the field to be type 'str'. + + +=== Union types === + +Usage: { 'union': STRING, 'data': DICT } +or: { 'union': STRING, 'data': DICT, 'base': STRUCT-NAME, + '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), flat (both discriminator and base). A union +type is defined using a data dictionary as explained in the following +paragraphs. + +A simple union type defines a mapping from automatic discriminator +values to data types like in this example: + + { 'struct': 'FileOptions', 'data': { 'filename': 'str' } } + { 'struct': 'Qcow2Options', + 'data': { 'backing-file': 'str', 'lazy-refcounts': 'bool' } } + + { 'union': 'BlockdevOptions', + 'data': { 'file': 'FileOptions', + 'qcow2': 'Qcow2Options' } } + +In the Client JSON Protocol, 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, as in these examples: + + { "type": "file", "data" : { "filename": "/some/place/my-image" } } + { "type": "qcow2", "data" : { "backing-file": "/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 specifies a struct as its base, and +avoids nesting on the wire. All branches of the union must be +complex types, and the top-level fields of the union dictionary on +the wire will be combination of fields from both the base type and the +appropriate branch type (when merging two dictionaries, there must be +no keys in common). The 'discriminator' field must be the name of an +enum-typed member of the base struct. + +The following example enhances the above simple union example by +adding a common field 'readonly', renaming the discriminator to +something more applicable, and reducing the number of {} required on +the wire: + + { 'enum': 'BlockdevDriver', 'data': [ 'file', 'qcow2' ] } + { 'struct': 'BlockdevCommonOptions', + 'data': { 'driver': 'BlockdevDriver', 'readonly': 'bool' } } + { 'union': 'BlockdevOptions', + 'base': 'BlockdevCommonOptions', + 'discriminator': 'driver', + 'data': { 'file': 'FileOptions', + 'qcow2': 'Qcow2Options' } } + +Resulting in these JSON objects: + + { "driver": "file", "readonly": true, + "filename": "/some/place/my-image" } + { "driver": "qcow2", "readonly": false, + "backing-file": "/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 member fields 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': 'Base', 'data': { 'type': 'Enum' } } + { 'struct': 'Branch1', 'data': { 'data': 'str' } } + { 'struct': 'Branch2', 'data': { 'data': 'int' } } + { 'union': 'Flat', 'base': 'Base', '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': 'BlockRef', + 'data': { 'definition': 'BlockdevOptions', + 'reference': 'str' } } + +Just like for a simple union, an implicit C enum 'NameKind' is created +to enumerate the branches for the alternate 'Name'. + +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", + "readonly": false, + "filename": "/tmp/mydisk.qcow2" } } + + +=== Commands === + +Usage: { 'command': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT, + '*returns': TYPE-NAME, + '*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" field +of a Client JSON Protocol reply on successful completion of a command. +The member is optional from the command declaration; if absent, the +"return" field 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 field 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" }, { } ] } + +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 field. + + +=== Events === + +Usage: { 'event': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT } + +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 } } + + +== 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": ":empty", "ret-type": "SchemaInfoList" } + + Type ":empty" is an 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": ":obj-EVENT_C-arg" } + + Type ":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": "readonly", "type": "bool"} ], + "tag": "driver", + "variants": [ + { "case": "file", "type": "FileOptions" }, + { "case": "qcow2", "type": "Qcow2Options" } ] } + +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 BlockdevOptions from section +Union types + + { "name": "BlockdevOptions", "meta-type": "object", + "members": [ + { "name": "kind", "type": "BlockdevOptionsKind" } ], + "tag": "type", + "variants": [ + { "case": "file", "type": ":obj-FileOptions-wrapper" }, + { "case": "qcow2", "type": ":obj-Qcow2Options-wrapper" } ] } + + Enumeration type "BlockdevOptionsKind" and the object types + ":obj-FileOptions-wrapper", ":obj-Qcow2Options-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 BlockRef from section Alternate types + + { "name": "BlockRef", "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 four 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, and map the response back to a Client JSON Protocol +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 + { 'struct': 'UserDefOne', + 'data': { 'integer': 'int', 'string': 'str' } } + + { 'command': 'my-command', + 'data': {'arg1': 'UserDefOne'}, + 'returns': 'UserDefOne' } + + { 'event': 'MY_EVENT' } + +=== 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-" example-schema.json + $ cat qapi-generated/example-qapi-types.c +[Uninteresting stuff omitted...] + + void qapi_free_UserDefOne(UserDefOne *obj) + { + QapiDeallocVisitor *qdv; + Visitor *v; + + if (!obj) { + return; + } + + qdv = qapi_dealloc_visitor_new(); + v = qapi_dealloc_get_visitor(qdv); + visit_type_UserDefOne(v, &obj, NULL, NULL); + qapi_dealloc_visitor_cleanup(qdv); + } + + void qapi_free_UserDefOneList(UserDefOneList *obj) + { + QapiDeallocVisitor *qdv; + Visitor *v; + + if (!obj) { + return; + } + + qdv = qapi_dealloc_visitor_new(); + v = qapi_dealloc_get_visitor(qdv); + visit_type_UserDefOneList(v, &obj, NULL, NULL); + qapi_dealloc_visitor_cleanup(qdv); + } + $ 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; + char *string; + }; + + void qapi_free_UserDefOne(UserDefOne *obj); + + struct UserDefOneList { + union { + UserDefOne *value; + uint64_t padding; + }; + UserDefOneList *next; + }; + + void qapi_free_UserDefOneList(UserDefOneList *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-" example-schema.json + $ cat qapi-generated/example-qapi-visit.c +[Uninteresting stuff omitted...] + + static void visit_type_UserDefOne_fields(Visitor *v, UserDefOne **obj, Error **errp) + { + Error *err = NULL; + + visit_type_int(v, &(*obj)->integer, "integer", &err); + if (err) { + goto out; + } + visit_type_str(v, &(*obj)->string, "string", &err); + if (err) { + goto out; + } + + out: + error_propagate(errp, err); + } + + void visit_type_UserDefOne(Visitor *v, UserDefOne **obj, const char *name, Error **errp) + { + Error *err = NULL; + + visit_start_struct(v, (void **)obj, "UserDefOne", name, sizeof(UserDefOne), &err); + if (!err) { + if (*obj) { + visit_type_UserDefOne_fields(v, obj, errp); + } + visit_end_struct(v, &err); + } + error_propagate(errp, err); + } + + void visit_type_UserDefOneList(Visitor *v, UserDefOneList **obj, const char *name, Error **errp) + { + Error *err = NULL; + GenericList *i, **prev; + + visit_start_list(v, name, &err); + if (err) { + goto out; + } + + for (prev = (GenericList **)obj; + !err && (i = visit_next_list(v, prev, &err)) != NULL; + prev = &i) { + UserDefOneList *native_i = (UserDefOneList *)i; + visit_type_UserDefOne(v, &native_i->value, NULL, &err); + } + + error_propagate(errp, err); + err = NULL; + visit_end_list(v, &err); + out: + error_propagate(errp, err); + } + $ 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(Visitor *v, UserDefOne **obj, const char *name, Error **errp); + void visit_type_UserDefOneList(Visitor *v, 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: + + $ python scripts/qapi-commands.py --output-dir="qapi-generated" + --prefix="example-" example-schema.json + $ 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; + QmpOutputVisitor *qov = qmp_output_visitor_new(); + QapiDeallocVisitor *qdv; + Visitor *v; + + v = qmp_output_get_visitor(qov); + visit_type_UserDefOne(v, &ret_in, "unused", &err); + if (err) { + goto out; + } + *ret_out = qmp_output_get_qobject(qov); + + out: + error_propagate(errp, err); + qmp_output_visitor_cleanup(qov); + qdv = qapi_dealloc_visitor_new(); + v = qapi_dealloc_get_visitor(qdv); + visit_type_UserDefOne(v, &ret_in, "unused", NULL); + qapi_dealloc_visitor_cleanup(qdv); + } + + static void qmp_marshal_my_command(QDict *args, QObject **ret, Error **errp) + { + Error *err = NULL; + UserDefOne *retval; + QmpInputVisitor *qiv = qmp_input_visitor_new_strict(QOBJECT(args)); + QapiDeallocVisitor *qdv; + Visitor *v; + UserDefOne *arg1 = NULL; + + v = qmp_input_get_visitor(qiv); + visit_type_UserDefOne(v, &arg1, "arg1", &err); + 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); + qmp_input_visitor_cleanup(qiv); + qdv = qapi_dealloc_visitor_new(); + v = qapi_dealloc_get_visitor(qdv); + visit_type_UserDefOne(v, &arg1, "arg1", NULL); + qapi_dealloc_visitor_cleanup(qdv); + } + + static void qmp_init_marshal(void) + { + qmp_register_command("my-command", qmp_marshal_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 + +=== scripts/qapi-event.py === + +Used to generate the event-related C code defined by a schema. 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.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, + }; + $ 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 + +=== 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.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\"}, {\"name\": \"string\", \"type\": \"str\"}], \"meta-type\": \"object\", \"name\": \"2\"}, " + "{\"json-type\": \"int\", \"meta-type\": \"builtin\", \"name\": \"int\"}, " + "{\"json-type\": \"string\", \"meta-type\": \"builtin\", \"name\": \"str\"}]"; + $ 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 |
