1274 lines
47 KiB
Plaintext
1274 lines
47 KiB
Plaintext
= How to use the QAPI code generator =
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Copyright IBM Corp. 2011
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Copyright (C) 2012-2016 Red Hat, Inc.
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This work is licensed under the terms of the GNU GPL, version 2 or
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later. See the COPYING file in the top-level directory.
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== Introduction ==
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QAPI is a native C API within QEMU which provides management-level
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functionality to internal and external users. For external
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users/processes, this interface is made available by a JSON-based wire
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format for the QEMU Monitor Protocol (QMP) for controlling qemu, as
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well as the QEMU Guest Agent (QGA) for communicating with the guest.
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The remainder of this document uses "Client JSON Protocol" when
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referring to the wire contents of a QMP or QGA connection.
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To map Client JSON Protocol interfaces to the native C QAPI
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implementations, a JSON-based schema is used to define types and
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function signatures, and a set of scripts is used to generate types,
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signatures, and marshaling/dispatch code. This document will describe
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how the schemas, scripts, and resulting code are used.
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== QMP/Guest agent schema ==
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A QAPI schema file is designed to be loosely based on JSON
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(http://www.ietf.org/rfc/rfc7159.txt) with changes for quoting style
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and the use of comments; a QAPI schema file is then parsed by a python
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code generation program. A valid QAPI schema consists of a series of
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top-level expressions, with no commas between them. Where
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dictionaries (JSON objects) are used, they are parsed as python
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OrderedDicts so that ordering is preserved (for predictable layout of
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generated C structs and parameter lists). Ordering doesn't matter
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between top-level expressions or the keys within an expression, but
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does matter within dictionary values for 'data' and 'returns' members
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of a single expression. QAPI schema input is written using 'single
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quotes' instead of JSON's "double quotes" (in contrast, Client JSON
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Protocol uses no comments, and while input accepts 'single quotes' as
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an extension, output is strict JSON using only "double quotes"). As
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in JSON, trailing commas are not permitted in arrays or dictionaries.
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Input must be ASCII (although QMP supports full Unicode strings, the
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QAPI parser does not). At present, there is no place where a QAPI
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schema requires the use of JSON numbers or null.
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=== Comments ===
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Comments are allowed; anything between an unquoted # and the following
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newline is ignored.
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A multi-line comment that starts and ends with a '##' line is a
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documentation comment. These are parsed by the documentation
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generator, which recognizes certain markup detailed below.
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==== Documentation markup ====
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Comment text starting with '=' is a section title:
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# = Section title
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Double the '=' for a subsection title:
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# == Subection title
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'|' denotes examples:
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# | Text of the example, may span
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# | multiple lines
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'*' starts an itemized list:
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# * First item, may span
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# multiple lines
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# * Second item
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You can also use '-' instead of '*'.
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A decimal number followed by '.' starts a numbered list:
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# 1. First item, may span
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# multiple lines
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# 2. Second item
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The actual number doesn't matter. You could even use '*' instead of
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'2.' for the second item.
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Lists can't be nested. Blank lines are currently not supported within
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lists.
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Additional whitespace between the initial '#' and the comment text is
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permitted.
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*foo* and _foo_ are for strong and emphasis styles respectively (they
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do not work over multiple lines). @foo is used to reference a name in
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the schema.
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Example:
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##
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# = Section
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# == Subsection
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#
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# Some text foo with *strong* and _emphasis_
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# 1. with a list
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# 2. like that
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#
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# And some code:
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# | $ echo foo
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# | -> do this
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# | <- get that
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#
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##
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==== Expression documentation ====
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Each expression that isn't an include directive must be preceded by a
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documentation block. Such blocks are called expression documentation
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blocks.
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The documentation block consists of a first line naming the
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expression, an optional overview, a description of each argument (for
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commands and events) or member (for structs, unions and alternates),
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and optional tagged sections.
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FIXME: the parser accepts these things in almost any order.
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Optional arguments / members are tagged with the phrase '#optional',
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often with their default value; and extensions added after the
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expression was first released are also given a '(since x.y.z)'
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comment.
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A tagged section starts with one of the following words:
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"Note:"/"Notes:", "Since:", "Example"/"Examples", "Returns:", "TODO:".
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The section ends with the start of a new section.
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A 'Since: x.y.z' tagged section lists the release that introduced the
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expression.
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For example:
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##
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# @BlockStats:
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#
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# Statistics of a virtual block device or a block backing device.
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#
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# @device: #optional If the stats are for a virtual block device, the name
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# corresponding to the virtual block device.
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#
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# @node-name: #optional The node name of the device. (since 2.3)
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#
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# ... more members ...
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#
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# Since: 0.14.0
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##
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{ 'struct': 'BlockStats',
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'data': {'*device': 'str', '*node-name': 'str',
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... more members ... } }
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##
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# @query-blockstats:
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#
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# Query the @BlockStats for all virtual block devices.
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#
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# @query-nodes: #optional If true, the command will query all the
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# block nodes ... explain, explain ... (since 2.3)
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#
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# Returns: A list of @BlockStats for each virtual block devices.
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#
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# Since: 0.14.0
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#
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# Example:
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#
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# -> { "execute": "query-blockstats" }
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# <- {
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# ... lots of output ...
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# }
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#
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##
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{ 'command': 'query-blockstats',
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'data': { '*query-nodes': 'bool' },
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'returns': ['BlockStats'] }
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==== Free-form documentation ====
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A documentation block that isn't an expression documentation block is
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a free-form documentation block. These may be used to provide
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additional text and structuring content.
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=== Schema overview ===
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The schema sets up a series of types, as well as commands and events
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that will use those types. Forward references are allowed: the parser
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scans in two passes, where the first pass learns all type names, and
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the second validates the schema and generates the code. This allows
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the definition of complex structs that can have mutually recursive
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types, and allows for indefinite nesting of Client JSON Protocol that
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satisfies the schema. A type name should not be defined more than
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once. It is permissible for the schema to contain additional types
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not used by any commands or events in the Client JSON Protocol, for
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the side effect of generated C code used internally.
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There are seven top-level expressions recognized by the parser:
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'include', 'command', 'struct', 'enum', 'union', 'alternate', and
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'event'. There are several groups of types: simple types (a number of
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built-in types, such as 'int' and 'str'; as well as enumerations),
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complex types (structs and two flavors of unions), and alternate types
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(a choice between other types). The 'command' and 'event' expressions
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can refer to existing types by name, or list an anonymous type as a
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dictionary. Listing a type name inside an array refers to a
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single-dimension array of that type; multi-dimension arrays are not
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directly supported (although an array of a complex struct that
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contains an array member is possible).
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Types, commands, and events share a common namespace. Therefore,
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generally speaking, type definitions should always use CamelCase for
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user-defined type names, while built-in types are lowercase. Type
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definitions should not end in 'Kind', as this namespace is used for
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creating implicit C enums for visiting union types, or in 'List', as
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this namespace is used for creating array types. Command names,
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and member names within a type, should be all lower case with words
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separated by a hyphen. However, some existing older commands and
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complex types use underscore; when extending such expressions,
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consistency is preferred over blindly avoiding underscore. Event
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names should be ALL_CAPS with words separated by underscore. Member
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names cannot start with 'has-' or 'has_', as this is reserved for
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tracking optional members.
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Any name (command, event, type, member, or enum value) beginning with
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"x-" is marked experimental, and may be withdrawn or changed
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incompatibly in a future release. All names must begin with a letter,
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and contain only ASCII letters, digits, dash, and underscore. There
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are two exceptions: enum values may start with a digit, and any
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extensions added by downstream vendors should start with a prefix
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matching "__RFQDN_" (for the reverse-fully-qualified-domain-name of
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the vendor), even if the rest of the name uses dash (example:
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__com.redhat_drive-mirror). Names beginning with 'q_' are reserved
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for the generator: QMP names that resemble C keywords or other
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problematic strings will be munged in C to use this prefix. For
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example, a member named "default" in qapi becomes "q_default" in the
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generated C code.
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In the rest of this document, usage lines are given for each
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expression type, with literal strings written in lower case and
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placeholders written in capitals. If a literal string includes a
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prefix of '*', that key/value pair can be omitted from the expression.
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For example, a usage statement that includes '*base':STRUCT-NAME
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means that an expression has an optional key 'base', which if present
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must have a value that forms a struct name.
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=== Built-in Types ===
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The following types are predefined, and map to C as follows:
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Schema C JSON
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str char * any JSON string, UTF-8
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number double any JSON number
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int int64_t a JSON number without fractional part
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that fits into the C integer type
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int8 int8_t likewise
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int16 int16_t likewise
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int32 int32_t likewise
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int64 int64_t likewise
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uint8 uint8_t likewise
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uint16 uint16_t likewise
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uint32 uint32_t likewise
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uint64 uint64_t likewise
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size uint64_t like uint64_t, except StringInputVisitor
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accepts size suffixes
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bool bool JSON true or false
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any QObject * any JSON value
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QType QType JSON string matching enum QType values
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=== Includes ===
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Usage: { 'include': STRING }
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The QAPI schema definitions can be modularized using the 'include' directive:
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{ 'include': 'path/to/file.json' }
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The directive is evaluated recursively, and include paths are relative to the
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file using the directive. Multiple includes of the same file are
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idempotent. No other keys should appear in the expression, and the include
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value should be a string.
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As a matter of style, it is a good idea to have all files be
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self-contained, but at the moment, nothing prevents an included file
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from making a forward reference to a type that is only introduced by
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an outer file. The parser may be made stricter in the future to
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prevent incomplete include files.
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=== Struct types ===
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Usage: { 'struct': STRING, 'data': DICT, '*base': STRUCT-NAME }
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A struct is a dictionary containing a single 'data' key whose value is
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a dictionary; the dictionary may be empty. This corresponds to a
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struct in C or an Object in JSON. Each value of the 'data' dictionary
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must be the name of a type, or a one-element array containing a type
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name. An example of a struct is:
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{ 'struct': 'MyType',
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'data': { 'member1': 'str', 'member2': 'int', '*member3': 'str' } }
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The use of '*' as a prefix to the name means the member is optional in
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the corresponding JSON protocol usage.
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The default initialization value of an optional argument should not be changed
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between versions of QEMU unless the new default maintains backward
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compatibility to the user-visible behavior of the old default.
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With proper documentation, this policy still allows some flexibility; for
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example, documenting that a default of 0 picks an optimal buffer size allows
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one release to declare the optimal size at 512 while another release declares
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the optimal size at 4096 - the user-visible behavior is not the bytes used by
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the buffer, but the fact that the buffer was optimal size.
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On input structures (only mentioned in the 'data' side of a command), changing
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from mandatory to optional is safe (older clients will supply the option, and
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newer clients can benefit from the default); changing from optional to
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mandatory is backwards incompatible (older clients may be omitting the option,
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and must continue to work).
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On output structures (only mentioned in the 'returns' side of a command),
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changing from mandatory to optional is in general unsafe (older clients may be
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expecting the member, and could crash if it is missing), although it
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can be done if the only way that the optional argument will be omitted
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is when it is triggered by the presence of a new input flag to the
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command that older clients don't know to send. Changing from optional
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to mandatory is safe.
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A structure that is used in both input and output of various commands
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must consider the backwards compatibility constraints of both directions
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of use.
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A struct definition can specify another struct as its base.
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In this case, the members of the base type are included as top-level members
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of the new struct's dictionary in the Client JSON Protocol wire
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format. An example definition is:
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{ 'struct': 'BlockdevOptionsGenericFormat', 'data': { 'file': 'str' } }
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{ 'struct': 'BlockdevOptionsGenericCOWFormat',
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'base': 'BlockdevOptionsGenericFormat',
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'data': { '*backing': 'str' } }
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An example BlockdevOptionsGenericCOWFormat object on the wire could use
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both members like this:
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{ "file": "/some/place/my-image",
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"backing": "/some/place/my-backing-file" }
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=== Enumeration types ===
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Usage: { 'enum': STRING, 'data': ARRAY-OF-STRING }
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{ 'enum': STRING, '*prefix': STRING, 'data': ARRAY-OF-STRING }
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An enumeration type is a dictionary containing a single 'data' key
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whose value is a list of strings. An example enumeration is:
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{ 'enum': 'MyEnum', 'data': [ 'value1', 'value2', 'value3' ] }
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Nothing prevents an empty enumeration, although it is probably not
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useful. The list of strings should be lower case; if an enum name
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represents multiple words, use '-' between words. The string 'max' is
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not allowed as an enum value, and values should not be repeated.
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The enum constants will be named by using a heuristic to turn the
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type name into a set of underscore separated words. For the example
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above, 'MyEnum' will turn into 'MY_ENUM' giving a constant name
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of 'MY_ENUM_VALUE1' for the first value. If the default heuristic
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does not result in a desirable name, the optional 'prefix' member
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can be used when defining the enum.
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The enumeration values are passed as strings over the Client JSON
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Protocol, but are encoded as C enum integral values in generated code.
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While the C code starts numbering at 0, it is better to use explicit
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comparisons to enum values than implicit comparisons to 0; the C code
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will also include a generated enum member ending in _MAX for tracking
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the size of the enum, useful when using common functions for
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converting between strings and enum values. Since the wire format
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always passes by name, it is acceptable to reorder or add new
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enumeration members in any location without breaking clients of Client
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JSON Protocol; however, removing enum values would break
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compatibility. For any struct that has a member that will only contain
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a finite set of string values, using an enum type for that member is
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better than open-coding the member to be type 'str'.
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=== Union types ===
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Usage: { 'union': STRING, 'data': DICT }
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or: { 'union': STRING, 'data': DICT, 'base': STRUCT-NAME-OR-DICT,
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'discriminator': ENUM-MEMBER-OF-BASE }
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Union types are used to let the user choose between several different
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variants for an object. There are two flavors: simple (no
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discriminator or base), and flat (both discriminator and base). A union
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type is defined using a data dictionary as explained in the following
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paragraphs. The data dictionary for either type of union must not
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be empty.
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A simple union type defines a mapping from automatic discriminator
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values to data types like in this example:
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{ 'struct': 'BlockdevOptionsFile', 'data': { 'filename': 'str' } }
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{ 'struct': 'BlockdevOptionsQcow2',
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'data': { 'backing': 'str', '*lazy-refcounts': 'bool' } }
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{ 'union': 'BlockdevOptionsSimple',
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'data': { 'file': 'BlockdevOptionsFile',
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'qcow2': 'BlockdevOptionsQcow2' } }
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In the Client JSON Protocol, a simple union is represented by a
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dictionary that contains the 'type' member as a discriminator, and a
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'data' member that is of the specified data type corresponding to the
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discriminator value, as in these examples:
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{ "type": "file", "data": { "filename": "/some/place/my-image" } }
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{ "type": "qcow2", "data": { "backing": "/some/place/my-image",
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"lazy-refcounts": true } }
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The generated C code uses a struct containing a union. Additionally,
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an implicit C enum 'NameKind' is created, corresponding to the union
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'Name', for accessing the various branches of the union. No branch of
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the union can be named 'max', as this would collide with the implicit
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enum. The value for each branch can be of any type.
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A flat union definition avoids nesting on the wire, and specifies a
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set of common members that occur in all variants of the union. The
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'base' key must specify either a type name (the type must be a
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struct, not a union), or a dictionary representing an anonymous type.
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All branches of the union must be complex types, and the top-level
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members of the union dictionary on the wire will be combination of
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members from both the base type and the appropriate branch type (when
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merging two dictionaries, there must be no keys in common). The
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'discriminator' member must be the name of a non-optional enum-typed
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member of the base struct.
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The following example enhances the above simple union example by
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adding an optional common member 'read-only', renaming the
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discriminator to something more applicable than the simple union's
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default of 'type', and reducing the number of {} required on the wire:
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{ 'enum': 'BlockdevDriver', 'data': [ 'file', 'qcow2' ] }
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{ 'union': 'BlockdevOptions',
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'base': { 'driver': 'BlockdevDriver', '*read-only': 'bool' },
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'discriminator': 'driver',
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'data': { 'file': 'BlockdevOptionsFile',
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'qcow2': 'BlockdevOptionsQcow2' } }
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Resulting in these JSON objects:
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{ "driver": "file", "read-only": true,
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"filename": "/some/place/my-image" }
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{ "driver": "qcow2", "read-only": false,
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"backing": "/some/place/my-image", "lazy-refcounts": true }
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Notice that in a flat union, the discriminator name is controlled by
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the user, but because it must map to a base member with enum type, the
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code generator can ensure that branches exist for all values of the
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enum (although the order of the keys need not match the declaration of
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the enum). In the resulting generated C data types, a flat union is
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represented as a struct with the base members included directly, and
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then a union of structures for each branch of the struct.
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A simple union can always be re-written as a flat union where the base
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class has a single member named 'type', and where each branch of the
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union has a struct with a single member named 'data'. That is,
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{ 'union': 'Simple', 'data': { 'one': 'str', 'two': 'int' } }
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is identical on the wire to:
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{ 'enum': 'Enum', 'data': ['one', 'two'] }
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{ 'struct': 'Branch1', 'data': { 'data': 'str' } }
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{ 'struct': 'Branch2', 'data': { 'data': 'int' } }
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{ 'union': 'Flat': 'base': { 'type': 'Enum' }, 'discriminator': 'type',
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'data': { 'one': 'Branch1', 'two': 'Branch2' } }
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|
|
=== Alternate types ===
|
|
|
|
Usage: { 'alternate': STRING, 'data': DICT }
|
|
|
|
An alternate type is one that allows a choice between two or more JSON
|
|
data types (string, integer, number, or object, but currently not
|
|
array) on the wire. The definition is similar to a simple union type,
|
|
where each branch of the union names a QAPI type. For example:
|
|
|
|
{ 'alternate': 'BlockdevRef',
|
|
'data': { 'definition': 'BlockdevOptions',
|
|
'reference': 'str' } }
|
|
|
|
Unlike a union, the discriminator string is never passed on the wire
|
|
for the Client JSON Protocol. Instead, the value's JSON type serves
|
|
as an implicit discriminator, which in turn means that an alternate
|
|
can only express a choice between types represented differently in
|
|
JSON. If a branch is typed as the 'bool' built-in, the alternate
|
|
accepts true and false; if it is typed as any of the various numeric
|
|
built-ins, it accepts a JSON number; if it is typed as a 'str'
|
|
built-in or named enum type, it accepts a JSON string; and if it is
|
|
typed as a complex type (struct or union), it accepts a JSON object.
|
|
Two different complex types, for instance, aren't permitted, because
|
|
both are represented as a JSON object.
|
|
|
|
The example alternate declaration above allows using both of the
|
|
following example objects:
|
|
|
|
{ "file": "my_existing_block_device_id" }
|
|
{ "file": { "driver": "file",
|
|
"read-only": false,
|
|
"filename": "/tmp/mydisk.qcow2" } }
|
|
|
|
|
|
=== Commands ===
|
|
|
|
Usage: { 'command': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT,
|
|
'*returns': TYPE-NAME, '*boxed': true,
|
|
'*gen': false, '*success-response': false }
|
|
|
|
Commands are defined by using a dictionary containing several members,
|
|
where three members are most common. The 'command' member is a
|
|
mandatory string, and determines the "execute" value passed in a
|
|
Client JSON Protocol command exchange.
|
|
|
|
The 'data' argument maps to the "arguments" dictionary passed in as
|
|
part of a Client JSON Protocol command. The 'data' member is optional
|
|
and defaults to {} (an empty dictionary). If present, it must be the
|
|
string name of a complex type, or a dictionary that declares an
|
|
anonymous type with the same semantics as a 'struct' expression, with
|
|
one exception noted below when 'gen' is used.
|
|
|
|
The 'returns' member describes what will appear in the "return" member
|
|
of a Client JSON Protocol reply on successful completion of a command.
|
|
The member is optional from the command declaration; if absent, the
|
|
"return" member will be an empty dictionary. If 'returns' is present,
|
|
it must be the string name of a complex or built-in type, a
|
|
one-element array containing the name of a complex or built-in type,
|
|
with one exception noted below when 'gen' is used. Although it is
|
|
permitted to have the 'returns' member name a built-in type or an
|
|
array of built-in types, any command that does this cannot be extended
|
|
to return additional information in the future; thus, new commands
|
|
should strongly consider returning a dictionary-based type or an array
|
|
of dictionaries, even if the dictionary only contains one member at the
|
|
present.
|
|
|
|
All commands in Client JSON Protocol use a dictionary to report
|
|
failure, with no way to specify that in QAPI. Where the error return
|
|
is different than the usual GenericError class in order to help the
|
|
client react differently to certain error conditions, it is worth
|
|
documenting this in the comments before the command declaration.
|
|
|
|
Some example commands:
|
|
|
|
{ 'command': 'my-first-command',
|
|
'data': { 'arg1': 'str', '*arg2': 'str' } }
|
|
{ 'struct': 'MyType', 'data': { '*value': 'str' } }
|
|
{ 'command': 'my-second-command',
|
|
'returns': [ 'MyType' ] }
|
|
|
|
which would validate this Client JSON Protocol transaction:
|
|
|
|
=> { "execute": "my-first-command",
|
|
"arguments": { "arg1": "hello" } }
|
|
<= { "return": { } }
|
|
=> { "execute": "my-second-command" }
|
|
<= { "return": [ { "value": "one" }, { } ] }
|
|
|
|
The generator emits a prototype for the user's function implementing
|
|
the command. Normally, 'data' is a dictionary for an anonymous type,
|
|
or names a struct type (possibly empty, but not a union), and its
|
|
members are passed as separate arguments to this function. If the
|
|
command definition includes a key 'boxed' with the boolean value true,
|
|
then 'data' is instead the name of any non-empty complex type
|
|
(struct, union, or alternate), and a pointer to that QAPI type is
|
|
passed as a single argument.
|
|
|
|
The generator also emits a marshalling function that extracts
|
|
arguments for the user's function out of an input QDict, calls the
|
|
user's function, and if it succeeded, builds an output QObject from
|
|
its return value.
|
|
|
|
In rare cases, QAPI cannot express a type-safe representation of a
|
|
corresponding Client JSON Protocol command. You then have to suppress
|
|
generation of a marshalling function by including a key 'gen' with
|
|
boolean value false, and instead write your own function. Please try
|
|
to avoid adding new commands that rely on this, and instead use
|
|
type-safe unions. For an example of this usage:
|
|
|
|
{ 'command': 'netdev_add',
|
|
'data': {'type': 'str', 'id': 'str'},
|
|
'gen': false }
|
|
|
|
Normally, the QAPI schema is used to describe synchronous exchanges,
|
|
where a response is expected. But in some cases, the action of a
|
|
command is expected to change state in a way that a successful
|
|
response is not possible (although the command will still return a
|
|
normal dictionary error on failure). When a successful reply is not
|
|
possible, the command expression should include the optional key
|
|
'success-response' with boolean value false. So far, only QGA makes
|
|
use of this member.
|
|
|
|
|
|
=== Events ===
|
|
|
|
Usage: { 'event': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT,
|
|
'*boxed': true }
|
|
|
|
Events are defined with the keyword 'event'. It is not allowed to
|
|
name an event 'MAX', since the generator also produces a C enumeration
|
|
of all event names with a generated _MAX value at the end. When
|
|
'data' is also specified, additional info will be included in the
|
|
event, with similar semantics to a 'struct' expression. Finally there
|
|
will be C API generated in qapi-event.h; when called by QEMU code, a
|
|
message with timestamp will be emitted on the wire.
|
|
|
|
An example event is:
|
|
|
|
{ 'event': 'EVENT_C',
|
|
'data': { '*a': 'int', 'b': 'str' } }
|
|
|
|
Resulting in this JSON object:
|
|
|
|
{ "event": "EVENT_C",
|
|
"data": { "b": "test string" },
|
|
"timestamp": { "seconds": 1267020223, "microseconds": 435656 } }
|
|
|
|
The generator emits a function to send the event. Normally, 'data' is
|
|
a dictionary for an anonymous type, or names a struct type (possibly
|
|
empty, but not a union), and its members are passed as separate
|
|
arguments to this function. If the event definition includes a key
|
|
'boxed' with the boolean value true, then 'data' is instead the name of
|
|
any non-empty complex type (struct, union, or alternate), and a
|
|
pointer to that QAPI type is passed as a single argument.
|
|
|
|
|
|
== Client JSON Protocol introspection ==
|
|
|
|
Clients of a Client JSON Protocol commonly need to figure out what
|
|
exactly the server (QEMU) supports.
|
|
|
|
For this purpose, QMP provides introspection via command
|
|
query-qmp-schema. QGA currently doesn't support introspection.
|
|
|
|
While Client JSON Protocol wire compatibility should be maintained
|
|
between qemu versions, we cannot make the same guarantees for
|
|
introspection stability. For example, one version of qemu may provide
|
|
a non-variant optional member of a struct, and a later version rework
|
|
the member to instead be non-optional and associated with a variant.
|
|
Likewise, one version of qemu may list a member with open-ended type
|
|
'str', and a later version could convert it to a finite set of strings
|
|
via an enum type; or a member may be converted from a specific type to
|
|
an alternate that represents a choice between the original type and
|
|
something else.
|
|
|
|
query-qmp-schema returns a JSON array of SchemaInfo objects. These
|
|
objects together describe the wire ABI, as defined in the QAPI schema.
|
|
There is no specified order to the SchemaInfo objects returned; a
|
|
client must search for a particular name throughout the entire array
|
|
to learn more about that name, but is at least guaranteed that there
|
|
will be no collisions between type, command, and event names.
|
|
|
|
However, the SchemaInfo can't reflect all the rules and restrictions
|
|
that apply to QMP. It's interface introspection (figuring out what's
|
|
there), not interface specification. The specification is in the QAPI
|
|
schema. To understand how QMP is to be used, you need to study the
|
|
QAPI schema.
|
|
|
|
Like any other command, query-qmp-schema is itself defined in the QAPI
|
|
schema, along with the SchemaInfo type. This text attempts to give an
|
|
overview how things work. For details you need to consult the QAPI
|
|
schema.
|
|
|
|
SchemaInfo objects have common members "name" and "meta-type", and
|
|
additional variant members depending on the value of meta-type.
|
|
|
|
Each SchemaInfo object describes a wire ABI entity of a certain
|
|
meta-type: a command, event or one of several kinds of type.
|
|
|
|
SchemaInfo for commands and events have the same name as in the QAPI
|
|
schema.
|
|
|
|
Command and event names are part of the wire ABI, but type names are
|
|
not. Therefore, the SchemaInfo for types have auto-generated
|
|
meaningless names. For readability, the examples in this section use
|
|
meaningful type names instead.
|
|
|
|
To examine a type, start with a command or event using it, then follow
|
|
references by name.
|
|
|
|
QAPI schema definitions not reachable that way are omitted.
|
|
|
|
The SchemaInfo for a command has meta-type "command", and variant
|
|
members "arg-type" and "ret-type". On the wire, the "arguments"
|
|
member of a client's "execute" command must conform to the object type
|
|
named by "arg-type". The "return" member that the server passes in a
|
|
success response conforms to the type named by "ret-type".
|
|
|
|
If the command takes no arguments, "arg-type" names an object type
|
|
without members. Likewise, if the command returns nothing, "ret-type"
|
|
names an object type without members.
|
|
|
|
Example: the SchemaInfo for command query-qmp-schema
|
|
|
|
{ "name": "query-qmp-schema", "meta-type": "command",
|
|
"arg-type": "q_empty", "ret-type": "SchemaInfoList" }
|
|
|
|
Type "q_empty" is an automatic object type without members, and type
|
|
"SchemaInfoList" is the array of SchemaInfo type.
|
|
|
|
The SchemaInfo for an event has meta-type "event", and variant member
|
|
"arg-type". On the wire, a "data" member that the server passes in an
|
|
event conforms to the object type named by "arg-type".
|
|
|
|
If the event carries no additional information, "arg-type" names an
|
|
object type without members. The event may not have a data member on
|
|
the wire then.
|
|
|
|
Each command or event defined with dictionary-valued 'data' in the
|
|
QAPI schema implicitly defines an object type.
|
|
|
|
Example: the SchemaInfo for EVENT_C from section Events
|
|
|
|
{ "name": "EVENT_C", "meta-type": "event",
|
|
"arg-type": "q_obj-EVENT_C-arg" }
|
|
|
|
Type "q_obj-EVENT_C-arg" is an implicitly defined object type with
|
|
the two members from the event's definition.
|
|
|
|
The SchemaInfo for struct and union types has meta-type "object".
|
|
|
|
The SchemaInfo for a struct type has variant member "members".
|
|
|
|
The SchemaInfo for a union type additionally has variant members "tag"
|
|
and "variants".
|
|
|
|
"members" is a JSON array describing the object's common members, if
|
|
any. Each element is a JSON object with members "name" (the member's
|
|
name), "type" (the name of its type), and optionally "default". The
|
|
member is optional if "default" is present. Currently, "default" can
|
|
only have value null. Other values are reserved for future
|
|
extensions. The "members" array is in no particular order; clients
|
|
must search the entire object when learning whether a particular
|
|
member is supported.
|
|
|
|
Example: the SchemaInfo for MyType from section Struct types
|
|
|
|
{ "name": "MyType", "meta-type": "object",
|
|
"members": [
|
|
{ "name": "member1", "type": "str" },
|
|
{ "name": "member2", "type": "int" },
|
|
{ "name": "member3", "type": "str", "default": null } ] }
|
|
|
|
"tag" is the name of the common member serving as type tag.
|
|
"variants" is a JSON array describing the object's variant members.
|
|
Each element is a JSON object with members "case" (the value of type
|
|
tag this element applies to) and "type" (the name of an object type
|
|
that provides the variant members for this type tag value). The
|
|
"variants" array is in no particular order, and is not guaranteed to
|
|
list cases in the same order as the corresponding "tag" enum type.
|
|
|
|
Example: the SchemaInfo for flat union BlockdevOptions from section
|
|
Union types
|
|
|
|
{ "name": "BlockdevOptions", "meta-type": "object",
|
|
"members": [
|
|
{ "name": "driver", "type": "BlockdevDriver" },
|
|
{ "name": "read-only", "type": "bool", "default": null } ],
|
|
"tag": "driver",
|
|
"variants": [
|
|
{ "case": "file", "type": "BlockdevOptionsFile" },
|
|
{ "case": "qcow2", "type": "BlockdevOptionsQcow2" } ] }
|
|
|
|
Note that base types are "flattened": its members are included in the
|
|
"members" array.
|
|
|
|
A simple union implicitly defines an enumeration type for its implicit
|
|
discriminator (called "type" on the wire, see section Union types).
|
|
|
|
A simple union implicitly defines an object type for each of its
|
|
variants.
|
|
|
|
Example: the SchemaInfo for simple union BlockdevOptionsSimple from section
|
|
Union types
|
|
|
|
{ "name": "BlockdevOptionsSimple", "meta-type": "object",
|
|
"members": [
|
|
{ "name": "type", "type": "BlockdevOptionsSimpleKind" } ],
|
|
"tag": "type",
|
|
"variants": [
|
|
{ "case": "file", "type": "q_obj-BlockdevOptionsFile-wrapper" },
|
|
{ "case": "qcow2", "type": "q_obj-BlockdevOptionsQcow2-wrapper" } ] }
|
|
|
|
Enumeration type "BlockdevOptionsSimpleKind" and the object types
|
|
"q_obj-BlockdevOptionsFile-wrapper", "q_obj-BlockdevOptionsQcow2-wrapper"
|
|
are implicitly defined.
|
|
|
|
The SchemaInfo for an alternate type has meta-type "alternate", and
|
|
variant member "members". "members" is a JSON array. Each element is
|
|
a JSON object with member "type", which names a type. Values of the
|
|
alternate type conform to exactly one of its member types. There is
|
|
no guarantee on the order in which "members" will be listed.
|
|
|
|
Example: the SchemaInfo for BlockdevRef from section Alternate types
|
|
|
|
{ "name": "BlockdevRef", "meta-type": "alternate",
|
|
"members": [
|
|
{ "type": "BlockdevOptions" },
|
|
{ "type": "str" } ] }
|
|
|
|
The SchemaInfo for an array type has meta-type "array", and variant
|
|
member "element-type", which names the array's element type. Array
|
|
types are implicitly defined. For convenience, the array's name may
|
|
resemble the element type; however, clients should examine member
|
|
"element-type" instead of making assumptions based on parsing member
|
|
"name".
|
|
|
|
Example: the SchemaInfo for ['str']
|
|
|
|
{ "name": "[str]", "meta-type": "array",
|
|
"element-type": "str" }
|
|
|
|
The SchemaInfo for an enumeration type has meta-type "enum" and
|
|
variant member "values". The values are listed in no particular
|
|
order; clients must search the entire enum when learning whether a
|
|
particular value is supported.
|
|
|
|
Example: the SchemaInfo for MyEnum from section Enumeration types
|
|
|
|
{ "name": "MyEnum", "meta-type": "enum",
|
|
"values": [ "value1", "value2", "value3" ] }
|
|
|
|
The SchemaInfo for a built-in type has the same name as the type in
|
|
the QAPI schema (see section Built-in Types), with one exception
|
|
detailed below. It has variant member "json-type" that shows how
|
|
values of this type are encoded on the wire.
|
|
|
|
Example: the SchemaInfo for str
|
|
|
|
{ "name": "str", "meta-type": "builtin", "json-type": "string" }
|
|
|
|
The QAPI schema supports a number of integer types that only differ in
|
|
how they map to C. They are identical as far as SchemaInfo is
|
|
concerned. Therefore, they get all mapped to a single type "int" in
|
|
SchemaInfo.
|
|
|
|
As explained above, type names are not part of the wire ABI. Not even
|
|
the names of built-in types. Clients should examine member
|
|
"json-type" instead of hard-coding names of built-in types.
|
|
|
|
|
|
== Code generation ==
|
|
|
|
Schemas are fed into five scripts to generate all the code/files that,
|
|
paired with the core QAPI libraries, comprise everything required to
|
|
take JSON commands read in by a Client JSON Protocol server, unmarshal
|
|
the arguments into the underlying C types, call into the corresponding
|
|
C function, map the response back to a Client JSON Protocol response
|
|
to be returned to the user, and introspect the commands.
|
|
|
|
As an example, we'll use the following schema, which describes a
|
|
single complex user-defined type, along with command which takes a
|
|
list of that type as a parameter, and returns a single element of that
|
|
type. The user is responsible for writing the implementation of
|
|
qmp_my_command(); everything else is produced by the generator.
|
|
|
|
$ cat example-schema.json
|
|
{ 'struct': 'UserDefOne',
|
|
'data': { 'integer': 'int', '*string': 'str' } }
|
|
|
|
{ 'command': 'my-command',
|
|
'data': { 'arg1': ['UserDefOne'] },
|
|
'returns': 'UserDefOne' }
|
|
|
|
{ 'event': 'MY_EVENT' }
|
|
|
|
For a more thorough look at generated code, the testsuite includes
|
|
tests/qapi-schema/qapi-schema-tests.json that covers more examples of
|
|
what the generator will accept, and compiles the resulting C code as
|
|
part of 'make check-unit'.
|
|
|
|
=== scripts/qapi-types.py ===
|
|
|
|
Used to generate the C types defined by a schema, along with
|
|
supporting code. The following files are created:
|
|
|
|
$(prefix)qapi-types.h - C types corresponding to types defined in
|
|
the schema you pass in
|
|
$(prefix)qapi-types.c - Cleanup functions for the above C types
|
|
|
|
The $(prefix) is an optional parameter used as a namespace to keep the
|
|
generated code from one schema/code-generation separated from others so code
|
|
can be generated/used from multiple schemas without clobbering previously
|
|
created code.
|
|
|
|
Example:
|
|
|
|
$ python scripts/qapi-types.py --output-dir="qapi-generated" \
|
|
--prefix="example-" example-schema.json
|
|
$ cat qapi-generated/example-qapi-types.h
|
|
[Uninteresting stuff omitted...]
|
|
|
|
#ifndef EXAMPLE_QAPI_TYPES_H
|
|
#define EXAMPLE_QAPI_TYPES_H
|
|
|
|
[Built-in types omitted...]
|
|
|
|
typedef struct UserDefOne UserDefOne;
|
|
|
|
typedef struct UserDefOneList UserDefOneList;
|
|
|
|
struct UserDefOne {
|
|
int64_t integer;
|
|
bool has_string;
|
|
char *string;
|
|
};
|
|
|
|
void qapi_free_UserDefOne(UserDefOne *obj);
|
|
|
|
struct UserDefOneList {
|
|
UserDefOneList *next;
|
|
UserDefOne *value;
|
|
};
|
|
|
|
void qapi_free_UserDefOneList(UserDefOneList *obj);
|
|
|
|
#endif
|
|
$ cat qapi-generated/example-qapi-types.c
|
|
[Uninteresting stuff omitted...]
|
|
|
|
void qapi_free_UserDefOne(UserDefOne *obj)
|
|
{
|
|
Visitor *v;
|
|
|
|
if (!obj) {
|
|
return;
|
|
}
|
|
|
|
v = qapi_dealloc_visitor_new();
|
|
visit_type_UserDefOne(v, NULL, &obj, NULL);
|
|
visit_free(v);
|
|
}
|
|
|
|
void qapi_free_UserDefOneList(UserDefOneList *obj)
|
|
{
|
|
Visitor *v;
|
|
|
|
if (!obj) {
|
|
return;
|
|
}
|
|
|
|
v = qapi_dealloc_visitor_new();
|
|
visit_type_UserDefOneList(v, NULL, &obj, NULL);
|
|
visit_free(v);
|
|
}
|
|
|
|
=== scripts/qapi-visit.py ===
|
|
|
|
Used to generate the visitor functions used to walk through and
|
|
convert between a native QAPI C data structure and some other format
|
|
(such as QObject); the generated functions are named visit_type_FOO()
|
|
and visit_type_FOO_members().
|
|
|
|
The following files are generated:
|
|
|
|
$(prefix)qapi-visit.c: visitor function for a particular C type, used
|
|
to automagically convert QObjects into the
|
|
corresponding C type and vice-versa, as well
|
|
as for deallocating memory for an existing C
|
|
type
|
|
|
|
$(prefix)qapi-visit.h: declarations for previously mentioned visitor
|
|
functions
|
|
|
|
Example:
|
|
|
|
$ python scripts/qapi-visit.py --output-dir="qapi-generated"
|
|
--prefix="example-" example-schema.json
|
|
$ cat qapi-generated/example-qapi-visit.h
|
|
[Uninteresting stuff omitted...]
|
|
|
|
#ifndef EXAMPLE_QAPI_VISIT_H
|
|
#define EXAMPLE_QAPI_VISIT_H
|
|
|
|
[Visitors for built-in types omitted...]
|
|
|
|
void visit_type_UserDefOne_members(Visitor *v, UserDefOne *obj, Error **errp);
|
|
void visit_type_UserDefOne(Visitor *v, const char *name, UserDefOne **obj, Error **errp);
|
|
void visit_type_UserDefOneList(Visitor *v, const char *name, UserDefOneList **obj, Error **errp);
|
|
|
|
#endif
|
|
$ cat qapi-generated/example-qapi-visit.c
|
|
[Uninteresting stuff omitted...]
|
|
|
|
void visit_type_UserDefOne_members(Visitor *v, UserDefOne *obj, Error **errp)
|
|
{
|
|
Error *err = NULL;
|
|
|
|
visit_type_int(v, "integer", &obj->integer, &err);
|
|
if (err) {
|
|
goto out;
|
|
}
|
|
if (visit_optional(v, "string", &obj->has_string)) {
|
|
visit_type_str(v, "string", &obj->string, &err);
|
|
if (err) {
|
|
goto out;
|
|
}
|
|
}
|
|
|
|
out:
|
|
error_propagate(errp, err);
|
|
}
|
|
|
|
void visit_type_UserDefOne(Visitor *v, const char *name, UserDefOne **obj, Error **errp)
|
|
{
|
|
Error *err = NULL;
|
|
|
|
visit_start_struct(v, name, (void **)obj, sizeof(UserDefOne), &err);
|
|
if (err) {
|
|
goto out;
|
|
}
|
|
if (!*obj) {
|
|
goto out_obj;
|
|
}
|
|
visit_type_UserDefOne_members(v, *obj, &err);
|
|
if (err) {
|
|
goto out_obj;
|
|
}
|
|
visit_check_struct(v, &err);
|
|
out_obj:
|
|
visit_end_struct(v, (void **)obj);
|
|
if (err && visit_is_input(v)) {
|
|
qapi_free_UserDefOne(*obj);
|
|
*obj = NULL;
|
|
}
|
|
out:
|
|
error_propagate(errp, err);
|
|
}
|
|
|
|
void visit_type_UserDefOneList(Visitor *v, const char *name, UserDefOneList **obj, Error **errp)
|
|
{
|
|
Error *err = NULL;
|
|
UserDefOneList *tail;
|
|
size_t size = sizeof(**obj);
|
|
|
|
visit_start_list(v, name, (GenericList **)obj, size, &err);
|
|
if (err) {
|
|
goto out;
|
|
}
|
|
|
|
for (tail = *obj; tail;
|
|
tail = (UserDefOneList *)visit_next_list(v, (GenericList *)tail, size)) {
|
|
visit_type_UserDefOne(v, NULL, &tail->value, &err);
|
|
if (err) {
|
|
break;
|
|
}
|
|
}
|
|
|
|
visit_end_list(v, (void **)obj);
|
|
if (err && visit_is_input(v)) {
|
|
qapi_free_UserDefOneList(*obj);
|
|
*obj = NULL;
|
|
}
|
|
out:
|
|
error_propagate(errp, err);
|
|
}
|
|
|
|
=== scripts/qapi-commands.py ===
|
|
|
|
Used to generate the marshaling/dispatch functions for the commands
|
|
defined in the schema. The generated code implements
|
|
qmp_marshal_COMMAND() (registered automatically), and declares
|
|
qmp_COMMAND() that the user must implement. The following files are
|
|
generated:
|
|
|
|
$(prefix)qmp-marshal.c: command marshal/dispatch functions for each
|
|
QMP command defined in the schema. Functions
|
|
generated by qapi-visit.py are used to
|
|
convert QObjects received from the wire into
|
|
function parameters, and uses the same
|
|
visitor functions to convert native C return
|
|
values to QObjects from transmission back
|
|
over the wire.
|
|
|
|
$(prefix)qmp-commands.h: Function prototypes for the QMP commands
|
|
specified in the schema.
|
|
|
|
Example:
|
|
|
|
$ python scripts/qapi-commands.py --output-dir="qapi-generated"
|
|
--prefix="example-" example-schema.json
|
|
$ cat qapi-generated/example-qmp-commands.h
|
|
[Uninteresting stuff omitted...]
|
|
|
|
#ifndef EXAMPLE_QMP_COMMANDS_H
|
|
#define EXAMPLE_QMP_COMMANDS_H
|
|
|
|
#include "example-qapi-types.h"
|
|
#include "qapi/qmp/qdict.h"
|
|
#include "qapi/error.h"
|
|
|
|
UserDefOne *qmp_my_command(UserDefOneList *arg1, Error **errp);
|
|
|
|
#endif
|
|
$ cat qapi-generated/example-qmp-marshal.c
|
|
[Uninteresting stuff omitted...]
|
|
|
|
static void qmp_marshal_output_UserDefOne(UserDefOne *ret_in, QObject **ret_out, Error **errp)
|
|
{
|
|
Error *err = NULL;
|
|
Visitor *v;
|
|
|
|
v = qobject_output_visitor_new(ret_out);
|
|
visit_type_UserDefOne(v, "unused", &ret_in, &err);
|
|
if (!err) {
|
|
visit_complete(v, ret_out);
|
|
}
|
|
error_propagate(errp, err);
|
|
visit_free(v);
|
|
v = qapi_dealloc_visitor_new();
|
|
visit_type_UserDefOne(v, "unused", &ret_in, NULL);
|
|
visit_free(v);
|
|
}
|
|
|
|
static void qmp_marshal_my_command(QDict *args, QObject **ret, Error **errp)
|
|
{
|
|
Error *err = NULL;
|
|
UserDefOne *retval;
|
|
Visitor *v;
|
|
UserDefOneList *arg1 = NULL;
|
|
|
|
v = qobject_input_visitor_new(QOBJECT(args), true);
|
|
visit_start_struct(v, NULL, NULL, 0, &err);
|
|
if (err) {
|
|
goto out;
|
|
}
|
|
visit_type_UserDefOneList(v, "arg1", &arg1, &err);
|
|
if (!err) {
|
|
visit_check_struct(v, &err);
|
|
}
|
|
visit_end_struct(v, NULL);
|
|
if (err) {
|
|
goto out;
|
|
}
|
|
|
|
retval = qmp_my_command(arg1, &err);
|
|
if (err) {
|
|
goto out;
|
|
}
|
|
|
|
qmp_marshal_output_UserDefOne(retval, ret, &err);
|
|
|
|
out:
|
|
error_propagate(errp, err);
|
|
visit_free(v);
|
|
v = qapi_dealloc_visitor_new();
|
|
visit_start_struct(v, NULL, NULL, 0, NULL);
|
|
visit_type_UserDefOneList(v, "arg1", &arg1, NULL);
|
|
visit_end_struct(v, NULL);
|
|
visit_free(v);
|
|
}
|
|
|
|
static void qmp_init_marshal(void)
|
|
{
|
|
qmp_register_command("my-command", qmp_marshal_my_command, QCO_NO_OPTIONS);
|
|
}
|
|
|
|
qapi_init(qmp_init_marshal);
|
|
|
|
=== scripts/qapi-event.py ===
|
|
|
|
Used to generate the event-related C code defined by a schema, with
|
|
implementations for qapi_event_send_FOO(). The following files are
|
|
created:
|
|
|
|
$(prefix)qapi-event.h - Function prototypes for each event type, plus an
|
|
enumeration of all event names
|
|
$(prefix)qapi-event.c - Implementation of functions to send an event
|
|
|
|
Example:
|
|
|
|
$ python scripts/qapi-event.py --output-dir="qapi-generated"
|
|
--prefix="example-" example-schema.json
|
|
$ cat qapi-generated/example-qapi-event.h
|
|
[Uninteresting stuff omitted...]
|
|
|
|
#ifndef EXAMPLE_QAPI_EVENT_H
|
|
#define EXAMPLE_QAPI_EVENT_H
|
|
|
|
#include "qapi/error.h"
|
|
#include "qapi/qmp/qdict.h"
|
|
#include "example-qapi-types.h"
|
|
|
|
|
|
void qapi_event_send_my_event(Error **errp);
|
|
|
|
typedef enum example_QAPIEvent {
|
|
EXAMPLE_QAPI_EVENT_MY_EVENT = 0,
|
|
EXAMPLE_QAPI_EVENT__MAX = 1,
|
|
} example_QAPIEvent;
|
|
|
|
extern const char *const example_QAPIEvent_lookup[];
|
|
|
|
#endif
|
|
$ cat qapi-generated/example-qapi-event.c
|
|
[Uninteresting stuff omitted...]
|
|
|
|
void qapi_event_send_my_event(Error **errp)
|
|
{
|
|
QDict *qmp;
|
|
Error *err = NULL;
|
|
QMPEventFuncEmit emit;
|
|
emit = qmp_event_get_func_emit();
|
|
if (!emit) {
|
|
return;
|
|
}
|
|
|
|
qmp = qmp_event_build_dict("MY_EVENT");
|
|
|
|
emit(EXAMPLE_QAPI_EVENT_MY_EVENT, qmp, &err);
|
|
|
|
error_propagate(errp, err);
|
|
QDECREF(qmp);
|
|
}
|
|
|
|
const char *const example_QAPIEvent_lookup[] = {
|
|
[EXAMPLE_QAPI_EVENT_MY_EVENT] = "MY_EVENT",
|
|
[EXAMPLE_QAPI_EVENT__MAX] = NULL,
|
|
};
|
|
|
|
=== scripts/qapi-introspect.py ===
|
|
|
|
Used to generate the introspection C code for a schema. The following
|
|
files are created:
|
|
|
|
$(prefix)qmp-introspect.c - Defines a string holding a JSON
|
|
description of the schema.
|
|
$(prefix)qmp-introspect.h - Declares the above string.
|
|
|
|
Example:
|
|
|
|
$ python scripts/qapi-introspect.py --output-dir="qapi-generated"
|
|
--prefix="example-" example-schema.json
|
|
$ cat qapi-generated/example-qmp-introspect.h
|
|
[Uninteresting stuff omitted...]
|
|
|
|
#ifndef EXAMPLE_QMP_INTROSPECT_H
|
|
#define EXAMPLE_QMP_INTROSPECT_H
|
|
|
|
extern const char example_qmp_schema_json[];
|
|
|
|
#endif
|
|
$ cat qapi-generated/example-qmp-introspect.c
|
|
[Uninteresting stuff omitted...]
|
|
|
|
const char example_qmp_schema_json[] = "["
|
|
"{\"arg-type\": \"0\", \"meta-type\": \"event\", \"name\": \"MY_EVENT\"}, "
|
|
"{\"arg-type\": \"1\", \"meta-type\": \"command\", \"name\": \"my-command\", \"ret-type\": \"2\"}, "
|
|
"{\"members\": [], \"meta-type\": \"object\", \"name\": \"0\"}, "
|
|
"{\"members\": [{\"name\": \"arg1\", \"type\": \"[2]\"}], \"meta-type\": \"object\", \"name\": \"1\"}, "
|
|
"{\"members\": [{\"name\": \"integer\", \"type\": \"int\"}, {\"default\": null, \"name\": \"string\", \"type\": \"str\"}], \"meta-type\": \"object\", \"name\": \"2\"}, "
|
|
"{\"element-type\": \"2\", \"meta-type\": \"array\", \"name\": \"[2]\"}, "
|
|
"{\"json-type\": \"int\", \"meta-type\": \"builtin\", \"name\": \"int\"}, "
|
|
"{\"json-type\": \"string\", \"meta-type\": \"builtin\", \"name\": \"str\"}]";
|