Fuchsia

FIDL attributes

The following FIDL attributes are supported:

Scope

An attribute precedes a FIDL element, for example:

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

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

Note: 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 = "Channel"]

Illustrates both aspects:

  • there are two attributes, Layout and Transport, and
  • the Transport attribute takes a value from the list enumerated below.

[Deprecated]

USAGE: [Deprecated]

MEANING: See FTP-013.

Note: Not 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. The text of the comment is emitted into the generated code, in a manner compatible with the syntax of the target language.

Note: To be identical, the [Doc] version should be [Doc = " Foo\n"]. Note the space before the “Foo” and the line-feed “\n”.

[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: [Selector = "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 union 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 = "tranportList"]

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

  • Channel — use a Zircon channel.
  • Syscall — transport used to specify that the protocol is used to define Zircon syscalls, rather than typical IPC.

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="Foo"] disables Channel and uses only Foo).