This document is a specification of the Fuchsia Interface Definition Language (FIDL) syntax.
For more information about FIDL's overall purpose, goals, and requirements, see Overview.
Also, see a modified EBNF description of the FIDL grammar.
FIDL provides a syntax for declaring named bits, constants, enums, structs, tables, unions, and protocols. These declarations are collected into libraries for distribution.
FIDL declarations are stored in plain text UTF-8 files. Each file consists of a sequence of semicolon-delimited declarations. The order of declarations within a FIDL file, or among FIDL files within a library, is irrelevant. FIDL does not require (or support) forward declarations of any kind.
FIDL comments start with two (//
) or three (///
) forward slashes, continue to the end of the line, and can contain UTF-8 content (which is, of course, ignored). The three-forward-slash variant is a “documentation comment”, and causes the comment text to be emitted into the generated code (as a comment, escaped correctly for the target language).
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="comments" %}
Note that documentation comments can also be provided via the @doc
attribute.
The following are keywords in FIDL.
alias, as, bits, compose, const, enum, error, flexible, library, optional, protocol, reserved, resource, service, strict, struct, table, type, union, using.
FIDL identifiers label declarations and their members. FIDL identifiers must match the regex [a-zA-Z]([a-zA-Z0-9_]*[a-zA-Z0-9])?
. In words: identifiers must start with a letter, can contain letters, numbers, and underscores, but cannot end with an underscore.
// a struct named "Foo" type Foo = struct {}; // an enum named "enum", containing a single member type enum = enum { WITH_A_MEMBER = 1; };
Note: While using keywords as identifiers is supported, it can lead to confusion, and should therefore be considered on a case-by-case basis. See the Names
section of the Style Rubric.
FIDL library names label FIDL libraries. FIDL library names consist of one or more elements each matching the regex [a-z][a-z0-9]*
. In words: library name elements must start with a lowercase letter, can contain lowercase letters, and numbers (they cannot contain uppercase letters, nor underscores). Library names are used in Qualified Identifiers.
// a library named "foo" library foo;
Identifiers and library names are case-sensitive.
FIDL always looks for unqualified symbols within the scope of the current library. To reference symbols in other libraries, they must be qualified by prefixing the identifier with the library name or alias thereof.
objects.fidl:
library objects; using textures as tex; protocol Frob { // "Thing" refers to "Thing" in the "objects" library // "tex.Color" refers to "Color" in the "textures" library Paint(struct { thing Thing; color tex.Color; }); }; type Thing = struct { name string; };
textures.fidl:
library textures; type Color = struct { rgba uint32; };
FIDL supports integer, floating point, boolean, string, and enumeration literals, using a simplified syntax familiar to C programmers (see below for examples).
FIDL supports the following constant types: bits, booleans, signed and unsigned integers, floating point values, strings, and enumerations. The syntax is similar to C:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="consts" %}
These declarations introduce a name within their scope. The constant's type must be either a primitive or an enum.
Constant expressions are either literals or the names of other constant expressions.
For greater clarity, there is no expression processing in FIDL; that is, you cannot declare a constant as having the value
6 + 5
, for example.
FIDL uses the semi-colon ‘;’ to separate adjacent declarations within the file, much like C.
Libraries are named containers of FIDL declarations.
// library identifier separated by dots library fuchsia.composition; // "using" to import library "fuchsia.buffers" using fuchsia.buffers; // "using" to import library "fuchsia.geometry" and create a shortform called "geo" using fuchsia.geometry as geo;
Libraries may declare that they use other libraries with a “using” declaration. This allows the library to refer to symbols defined in other libraries upon which they depend. Symbols imported this way may be accessed by:
In the source tree, each library consists of a directory with some number of .fidl files. The name of the directory is irrelevant to the FIDL compiler but by convention it should resemble the library name itself. A directory should not contain FIDL files for more than one library.
The scope of library
and using
declarations is limited to a single file. Each individual file within a FIDL library must restate the library
declaration together with any using
declarations needed by that file.
The library's name may be used by certain language bindings to provide scoping for symbols emitted by the code generator.
For example, the C++ bindings generator places declarations for the FIDL library fuchsia.ui
within the C++ namespace fuchsia::ui
. Similarly, for languages such as Dart and Rust, which have their own module system, each FIDL library is compiled as a module for that language.
FIDL supports a number of builtin types as well as declarations of new types (e.g. structs, unions, type aliases) and protocols.
The following primitive types are supported:
bool
int8 int16 int32 int64
uint8 uint16 uint32 uint64
float32 float64
Numbers are suffixed with their size in bits, bool
is 1 byte.
We also alias byte
to mean uint8
as a built-in alias.
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="primitives" %}
strict
or flexible
.flexible
.strict
bits must have at least one member (flexible
bits can be memberless).|
is the bitwise OR operator for bits.
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="bits" %}
strict
or flexible
.flexible
.strict
enums must have at least one member (flexible
enums can be memberless).The ordinal index is required for each enum element. The underlying type of an enum must be one of: int8, uint8, int16, uint16, int32, uint32, int64, uint64. If omitted, the underlying type is assumed to be uint32.
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="enums" %}
Enum types are denoted by their identifier, which may be qualified if needed.
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="enum-use" %}
<<../../../development/languages/fidl/widgets/_enum.md>>
Arrays are denoted array<T, N>
where T can be any FIDL type (including an array) and N is a positive integer constant expression that specifies the number of elements in the array.
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="arrays" %}
Note that N appears as a layout parameter, which means that it affects the ABI of the type. In other words, changing the parameter _N_
is an ABI-breaking change.
string:40
for a maximum 40 byte string.\\
, \"
, \n
, \r
, \t
, and \u{X}
where the X
is 1 to 6 hex digits for a Unicode code point.NUL
bytes, unlike traditional C strings.Strings are denoted as follows:
string
: required string (validation error occurs if absent)string:optional
: optional stringstring:N, string:<N, optional>
: string, and optional string, respectively, with maximum length of N bytes{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="strings" %}
Note that N appears as a constraint (it appears after the :
), which means that it does not affect the ABI of the type. In other words, changing the parameter _N_
is not an ABI-breaking change.
Strings should not be used to pass arbitrary binary data since bindings enforce valid UTF-8. Instead, consider
bytes
for small data orfuchsia.mem.Buffer
for blobs. See Should I use string or vector? for details.
vector<T>:40
for a maximum 40 element vector.Vectors are denoted as follows:
vector<T>
: required vector of element type T (validation error occurs if absent)vector<T>:optional
: optional vector of element type Tvector<T>:N, vector<T>:<N, optional>?
: vector, and optional vector, respectively, with maximum length of N elementsT can be any FIDL type.
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="vectors" %}
Handles are denoted:
zx.handle
: required Zircon handle of unspecified typezx.handle?
: optional Zircon handle of unspecified typezx.handle:H
: required Zircon handle of type Hzx.handle:<H, optional>
: optional Zircon handle of type Hzx.handle:<H, R>
: required Zircon handle of type H with rights Rzx.handle:<H, R, optional>
: optional Zircon handle of type H with rights RH can be any object supported by Zircon, e.g. channel
, thread
, vmo
. Please refer to the grammar for a full list.
R can be any right supported by Zircon. Rights are bits-typed values, defined in the zx
FIDL library, e.g. zx.rights.READ
. In both the incoming and outgoing directions, handles are validated to have the correct Zircon object type and at least as many rights as are specified in FIDL. If the handle has more rights than is specified in FIDL, then its rights will be reduced by a call to zx_handle_replace
. See Life of a handle for an example and RFC-0028: Handle rights for further details.
Structs, tables, and unions containing handles must be marked with the resource
modifier.
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="handles" %}
resource
modifier.box
ed.{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="structs" %}
Structs are denoted by their declared name (e.g. Circle):
Circle
: required Circlebox<Circle>
: optional Circle, stored out-of-line.{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="structs-use" %}
resource
modifier.type Profile = table { 1: locales vector<string>; 2: calendars vector<string>; 3: time_zones vector<string>; };
Tables are denoted by their declared name (e.g. Profile):
Profile
: required ProfileHere, we show how Profile
evolves to also carry temperature units. A client aware of the previous definition of Profile
(without temperature units) can still send its profile to a server that has been updated to handle the larger set of fields.
type TemperatureUnit = enum { CELSIUS = 1; FAHRENHEIT = 2; }; type Profile = table { 1: locales vector<string>; 2: calendars vector<string>; 3: time_zones vector<string>; 4: temperature_unit TemperatureUnit; };
resource
modifier.strict
or flexible
.flexible
.strict
unions must contain one or more non-reserved members. A union with no members would have no inhabitants and thus would make little sense in a wire format. However, memberless flexible
unions are allowed, as it is still possible to decode an memberless union (the contained data is always “unknown”).{% includecode gerrit_repo="fuchsia/samples" gerrit_path="src/calculator/fidl/calculator.fidl" region_tag="union" %}
Unions are denoted by their declared name (e.g. Result) and optionality:
Result
: required ResultResult:optional
: optional Result{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="unions-use" %}
FIDL type declarations can either have strict or flexible behavior:
strict
modifier.For strict types only, serializing or deserializing a value that contains data not described in the declaration is a validation error.
In this example:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="strict-vs-flexible" %}
By virtue of being flexible, it is simpler for FlexibleEither
to evolve to carry a third variant. A client aware of the previous definition of FlexibleEither
without the third variant can still receive a union from a server that has been updated to contain the larger set of variants. If the union is of the unknown variant, bindings may expose it as unknown data (i.e. as raw bytes and handles) to the user and allow re-encoding the unknown union (e.g. to support proxy-like use cases). The methods provided for interacting with unknown data for flexible types are described in detail in the bindings reference.
More details are discussed in RFC-0033: Handling of Unknown Fields and Strictness.
Note: A type that is both flexible and a value type will not allow deserializing unknown data that contains handles.
Every FIDL type is either a value type or a resource type. Resource types include:
resource
modifierAll other types are value types.
Value types must not contain resource types. For example, this is incorrect:
type Foo = struct { // ERROR: must be "resource struct Foo" h zx.handle; };
Types can be marked with the resource
modifier even if they do not contain handles. You should do this if you intend to add handles to the type in the future, since adding or removing the resource
modifier requires source-compatibility considerations. For example:
// No handles now, but we will add some in the future. type Record = resource table { 1: str string; }; // "Foo" must be a resource because it contains "Record", which is a resource. type Foo = resource struct { record Record; };
More details are discussed in RFC-0057: Default No Handles.
Describe methods that can be invoked by sending messages over a channel.
Methods are identified by their ordinal index. The compiler calculates the ordinal by
@selector
attribute. The value of the @selector
attribute will be used in the place of the method name above.Each method declaration states its arguments and results.
int32
, uint32
, or an enum
thereof.When a server of a protocol is about to close its side of the channel, it may elect to send an epitaph message to the client to indicate the disposition of the connection. The epitaph must be the last message delivered through the channel. An epitaph message includes a 32-bit int value of type zx_status_t. Negative values are reserved for system error codes. The value ZX_OK
(0) indicates an operation was successful. Application-defined error codes (previously defined as all positive zx_status_t
values) are deprecated. For more details about epitaphs, see rejection of RFC-0031: Typed Epitaphs. For more details about zx_status_t
see RFC-0085: Reducing the zx_status_t space.
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="calculator" %}
Protocols are denoted by their name, directionality of the channel, and optionality:
client_end:Protocol
: client endpoint of channel communicating over the FIDL protocolclient_end:<Protocol, optional>
: optional version of the aboveserver_end:Protocol
: server endpoint of a channel communicating over the FIDL protocolserver_end:<Protocol, optional>
: optional version of the above{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="endpoints" %}
A protocol can include methods from other protocols. This is called composition: you compose one protocol from other protocols.
Composition is used in the following cases:
In the first case, there might be behavior that's shared across multiple protocols. For example, in a graphics system, several different protocols might all share a common need to set a background and foreground color. Rather than have each protocol define their own color setting methods, a common protocol can be defined:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="composition-base" %}
It can then be shared by other protocols:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="composition-inherit" %}
In the above, there are three protocols, SceneryController
, Drawer
, and Writer
. Drawer
is used to draw graphical objects, like circles and squares at given locations with given sizes. It composes the methods SetBackground() and SetForeground() from the SceneryController
protocol because it includes the SceneryController
protocol (by way of the compose
keyword).
The Writer
protocol, used to write text on the display, includes the SceneryController
protocol in the same way.
Now both Drawer
and Writer
include SetBackground() and SetForeground().
This offers several advantages over having Drawer
and Writer
specify their own color setting methods:
Drawer
and Writer
without having to change their definitions, simply by adding them to the SceneryController
protocol.The last point is particularly important, because it allows us to add functionality to existing protocols. For example, we might introduce an alpha-blending (or “transparency”) feature to our graphics system. By extending the SceneryController
protocol to deal with it, perhaps like so:
protocol SceneryController { SetBackground(struct { color Color; }); SetForeground(struct { color Color; }); SetAlphaChannel(struct { a int; }); };
we've now extended both Drawer
and Writer
to be able to support alpha blending.
Composition is not a one-to-one relationship — we can include multiple compositions into a given protocol, and not all protocols need be composed of the same mix of included protocols.
For example, we might have the ability to set font characteristics. Fonts don't make sense for our Drawer
protocol, but they do make sense for our Writer
protocol, and perhaps other protocols.
So, we define our FontController
protocol:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="composition-multiple-1" %}
and then invite Writer
to include it, by using the compose
keyword:
protocol Writer { compose SceneryController; compose FontController; Text(struct { x int; y int; message string; }); };
Here, we‘ve extended the Writer
protocol with the FontController
protocol’s methods, without disturbing the Drawer
protocol (which doesn't need to know anything about fonts).
Protocol composition is similar to mixin. More details are discussed in RFC-0023: Compositional Model.
At the beginning of this section, we mentioned a second use for composition, namely exposing various levels of functionality to different audiences.
In this example, we have two protocols that are independently useful, a Clock
protocol to get the current time and timezone:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="layering-clock" %}
And an Horologist
protocol that sets the time and timezone:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="layering-horologist" %}
We may not necessarily wish to expose the more privileged Horologist
protocol to just any client, but we do want to expose it to the system clock component. So, we create a protocol (SystemClock
) that composes both:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="layering-systemclock" %}
Note: Unknown interaction handling is an experimental feature. Most libraries currently cannot use it. This notice will be updated when it is available for general use. Please contact fidl-dev@fuchsia.dev if you have questions about using it.
Protocols can define how they react when they receive a method call or event which has an ordinal which isn't recognized. Unrecognized ordinals occur primarily when a client and server were built using different versions of a protocol which may have more or fewer methods, though it can also occur if client and server are mistakenly using different protocols on the same channel.
To control the behavior of the protocol when these unknown interactions occur, methods can be marked as either strict
or flexible
, and protocols can be marked as closed
, ajar
, or open
.
The method strictness modifiers, strict
and flexible
, specify how the sending end would like the receiving end to react to the interaction if it does not recognize the ordinal. For a one-way or two-way method, the sending end is the client, and for an event the sending end is the server.
strict
means that it should be an error for the receiving end not to know the interaction. If a strict
unknown interaction is received, the receiver should close the channel.flexible
means that the unknown interaction should be handled by the application. If the protocol allows for that type of unknown interaction, the ordinal is passed to an unknown interaction handler which can then decide how to react to it. What types of unknown interactions a protocol allows is determined by the protocol modifier. flexible
is the default value if no strictness is specified.The protocol openness modifiers, closed
, ajar
and open
control how the receiving end reacts to flexible
interactions if it does not recognize the ordinal. For a one-way or two-way method, the receiving end is the server, and for an event, the receiving end is the client.
closed
means the protocol does not accept any unknown interactions. If any unknown interaction is received, the bindings report an error and end communication, regardless of whether the interaction is strict
or flexible
.strict
.ajar
means that the protocol allows unknown flexible
one-way methods and events. Any unknown two-way methods and strict
one-way methods or events still cause an error and result in the bindings closing the channel.strict
or flexible
.strict
.open
means that the protocol allows any unknown flexible
interactions. Any unknown strict
interactions still cause an error and result in the bindings closing the channel. Open is the default value if no openness is specified.strict
or flexible
.Here is a summary of which strictness modifiers are allowed for different kinds of methods in each protocol type. The default value of openness, open
, marked in italics. The default value of strictness, flexible
, is marked in bold.
strict M(); | flexible M(); | strict -> M(); | flexible -> M(); | strict M() -> (); | flexible M() -> (); | |
---|---|---|---|---|---|---|
open P | compiles | compiles | compiles | compiles | compiles | compiles |
ajar P | compiles | compiles | compiles | compiles | compiles | fails to compile |
closed P | compiles | fails to compile | compiles | fails to compile | compiles | fails to compile |
Example usage of the modifiers on a protocol.
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="unknown-interactions" %}
Keep in mind that unknown interaction handling applies only when the receivng end doesn‘t recognize the ordinal and doesn’t know what the interaction is. This means that the receiving end does not know whether the interaction is supposed to be strict or flexible. To allow the receiver to know how to handle an unknown interaction, the sender includes a bit in the message header which tells the receiver whether to treat the interaction as strict or flexible. Therefore, the strictness used in an interaction is based on what strictness the sender has for the method it tried to call, but the protocol's openness for that interaction depends on what openness the receiver has.
Here‘s how a method or event with each strictness is handled by a protocol with each openness when the receiving side doesn’t recognize the method.
strict M(); | flexible M(); | strict -> M(); | flexible -> M(); | strict M() -> (); | flexible M() -> (); | |
---|---|---|---|---|---|---|
open P | auto-closed | handleable | auto-closed | handleable | auto-closed | handleable |
ajar P | auto-closed | handleable | auto-closed | handleable | auto-closed | auto-closed |
closed P | auto-closed | auto-closed | auto-closed | auto-closed | auto-closed | auto-closed |
flexible
methods and events cannot be declared in closed
protocols and flexible
two-way methods cannot be declared in ajar
protocols. To ensure these rules are enforced across protocol composition, a protocol may only compose other protocols that are at least as closed as it is:
open
: Can compose any protocol.ajar
: Can compose ajar
and closed
protocols.closed
: Can only compose other closed
protocols.Before unknown interactions support was added to FIDL, all protocols behaved as if they were closed
and all methods behaved as if they were strict
. The default values for protocols and methods with the unknown_interactions
experiment enabled are open
and flexible
. This means that to avoid changing from closed
and strict
to open
and flexible
when you enable unknown_interactions
, you need to add explicit closed
and strict
modifiers to any existing protocols and methods.
See the compatibility guide for more information about migrating unknown interactions modifiers.
Type aliasing is supported. For example:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="aliasing" %}
In the above, the identifier StoryID
is an alias for the declaration of a string
with a maximum size of MAX_SIZE
. The identifier Chapters
is an alias for a vector declaration of five StoryId
elements.
The identifiers StoryID
and Chapters
can be used wherever their aliased definitions can be used. Consider:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="aliasing-usage" %}
Here, the Message
struct contains a string of MAX_SIZE
bytes called baseline
, and a vector of up to 5
strings of MAX_SIZE
called chapters
.
Note that byte
is a built-in aliases, see below.
<<../../../development/languages/fidl/widgets/_alias.md>>
FIDL provides several built-ins:
byte
)zx library
see belowThe byte
type is built-in, and is conceptually equivalent to:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference_builtin.test.fidl" region_tag="builtin" %}
When you refer to a name without specific scope, e.g.:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="builtin-aliases" %}
we treat this as builtin.byte
automatically (so long as there isn't a more-specific name in scope).
The fidlc
compiler automatically generates an internal ZX library for you that contains commonly used Zircon definitions.
Layouts can also be specified inline, rather than in a type
introduction declaration. This is useful when a specific layout is only used once. For example, the following FIDL:
type Options = table { 1: reticulate_splines bool; }; protocol Launcher { GenerateTerrain(struct { options Options; }); };
can be rewritten using an inline layout:
protocol Launcher { GenerateTerrain(struct { options table { 1: reticulate_splines bool; }; }); };
When an inline layout is used, fidlc
will reserve a name for it that is guaranteed to be unique, based on the naming context that the layout is used in. This results in the following reserved names:
Options
is reserved for the inlined table
.fidlc
concatenates the protocol name, the method name, and then either "Request"
or "Response"
depending on where the type is used.LauncherGenerateTerrainRequest
is reserved for the struct used as the request of the GenerateTerrain
method."Request"
suffix denotes that the type is used to initiate communication; for this reason, event types will have the "Request"
suffix reserved instead of the "Response"
suffix.The name that is actually used in the generated code depends on the binding, and is described in the individual bindings references.
For inline layouts used as the type of a layout member, there are two ways to obtain a different reserved name:
@generated_name
attribute.