# FIDL attributes

The following FIDL attributes are supported:

* [`[Deprecated]`](#deprecated)
* [`[Discoverable]`](#discoverable)
* [`[Doc]`](#doc)
* [`[Internal]`](#internal)
* [`[Layout]`](#layout)
* [`[MaxBytes]`](#maxbytes)
* [`[MaxHandles]`](#maxhandles)
* [`[Selector]`](#selector)
* [`[Transitional]`](#transitional)
* [`[Transport]`](#transport)

## Scope

An attribute precedes a FIDL element, for example:

```fidl
[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:

```fidl
[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]` {#deprecated}

**USAGE**: `[Deprecated]`

**MEANING**:
See [FTP-013].

Note: Not implemented.

## `[Discoverable]` {#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]` {#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:

```fidl
/// Foo
struct MyFooStruct { ...
```

and

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

have the same effect &mdash; 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]` {#internal}

**USAGE**: `[Internal]`

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

## `[Layout]` {#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:

```fidl
[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]` {#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]` {#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]` {#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:

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

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

## `[Transitional]` {#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]` {#transport}

**USAGE**: `[Transport = "`_tranportList_`"]`

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

* `Channel` &mdash; use a [Zircon channel][channel].
* `Syscall` &mdash; 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`).

<!-- xrefs -->
[channel]: /docs/reference/kernel_objects/channel.md
[FTP-013]: /docs/contribute/governance/fidl/ftp/ftp-013.md
[FTP-020]: /docs/contribute/governance/fidl/ftp/ftp-020.md
[FTP-021]: /docs/contribute/governance/fidl/ftp/ftp-021.md