Fuchsia

FIDL Attributes

The following FIDL attributes are supported:

Scope

An attribute preceeds a FIDL element, for example:

[Layout = "Simple"]
protocol MyProtocol {...

It's used to either modify the characteristics of the element, or provide documentation.

Note that the attribute applies only to the next element, not all subsequent ones. Elements after the current one revert to having no attributes.

Syntax

Attributes may include multiple values, and multiple attributes may be specified in the same element, for example:

[Layout = "Simple", Transport = "SocketControl, OvernetEmbedded"]

Illustrates both aspects:

  • there are two attributes, Layout and Transport, and
  • the Transport attribute has two values, SocketControl and OvernetEmbedded.

[Deprecated]

USAGE: [Deprecated]

MEANING: See FTP-013.

Note: not yet implemented.

[Discoverable]

USAGE: [Discoverable]

MEANING: Causes the service's name to be made available for lookup. A service with a [Discoverable] attribute can be found at run-time. That is to say, you can “request” this service, and zircon will locate it and provide access to it.

[Doc]

USAGE: [Doc = "string"]

MEANING: In FIDL, comments can start with two (“//”) or three slashes (“///”), or they can be embodied within a [Doc] attribute. The two-slash variant does not propagate the comments to the generated target, whereas both the three-slash and [Doc] variants do.

That is:

/// Foo
struct MyFooStruct { ...

and

[Doc = "Foo"]
struct MyFooStruct { ...

have the same effect — one (“///”) is syntactic sugar for the other.

Both have the same effect, namely that the text of the comment is emitted into the generated code, in a manner compatible with the syntax of the target language.

Technically, to be identical, the [Doc] version would be [Doc = " Foo\n"]; note the space before the “Foo” and the line-feed “\n”.

[FragileBase]

USAGE: [FragileBase]

MEANING: Denotes that the interface can be composed, otherwise it cannot. See also Protocol Composition.

[Internal]

USAGE: [Internal]

MEANING: This marks internal libraries, such as library zx. It should be used only by Fuchsia developers.

[Layout]

USAGE: [Layout = "layout"]

MEANING: This attribute currently has one valid value, Simple, and is meaningful only on protocols.

It's used to indicate that all arguments and returns must contain objects that are of a fixed size. The arguments and returns themselves, however, can be dynamically sized strings or vectors of primitives.

To clarify with an example, the following is valid:

[Layout = "Simple"]
protocol MyProtocol {
    DynamicCountOfFixedArguments(vector<uint8>:1024 inputs);
};

Here, the argument is a dynamically sized vector of unsigned 8-bit integers called inputs, with a maximum bound of 1024 elements.

The benefit of [Layout = "Simple"] is that the data can be directly mapped without having to be copied and assembled.

[MaxBytes]

USAGE: [MaxBytes = "number"]

MEANING: This attribute is used to limit the number of bytes that can be transferred in a message. The compiler will issue an error if the number of bytes exceeds this limit.

[MaxHandles]

USAGE: [MaxHandles = "number"]

MEANING: This attribute is used to limit the number of handles that can be transferred in a message. The compiler will issue an error if the number of handles exceeds this limit.

[Selector]

USAGE: [Select = "selector"]

MEANING: Allows you to change the hashing basis for the method ordinal, see FTP-020.

It can be used to rename a method without breaking ABI compatibility. For example, if we wish to rename the Investigate method to Experiment in the Science interface, we can write:

interface Science {
    [Selector="Investigate"] Experiment();
};

It can also be used for xunion variants to keep ABI compatibility in the same way.

[Transitional]

USAGE: [Transitional = "description"]

MEANING: Instructs bindings to generate code that will successfully build, regardless of whether the method is implemented or not. FTP-021 contains more details.

[Transport]

USAGE: [Transport = "tranport_list"]

MEANING: Allows you to select a transport. Provide a comma-separated list of values, selected from:

  • Channel — use a Zircon channel.
  • SocketControl — use zx_socket_write() (note: no handles allowed for such protocols).
  • OvernetEmbedded — uses a transport that makes Overnet available on a variety of operating systems by embedding the Overnet runtime as a C++ library and providing a C++ API for remotable handles.
  • OvernetInternal — transport that is used by Overnet internally to communicate between peers. Each end of the transport implements both client and server parts of the protocol, and no handles can be transferred.

The default is Channel if none specified. If you do specify a value or values, then only those values are used (e.g., specifying [Transport="SocketControl"] disables Channel and uses only SocketControl).