Fuchsia

FIDL language specification

This document is a specification of the Fuchsia Interface Definition Language (FIDL).

For more information about FIDL's overall purpose, goals, and requirements, see Overview.

Also, see a modified EBNF description of the FIDL grammar.

Syntax

FIDL provides a syntax for declaring data types and protocols. These declarations are collected into libraries for distribution.

FIDL declarations are stored in UTF-8 text files. Each file contains a sequence of semicolon-delimited declarations. The order of declarations within a FIDL file, or among FIDL files within a library, is irrelevant.

Comments

FIDL comments start with two forward slashes (//) and continue to the end of the line. Comments that start with three forward slashes (///) are called documentation comments, and get emitted as comments in the generated bindings.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="comments" %}

Keywords

The following words have special meaning in FIDL:

ajar, alias, as, bits, closed, compose, const, enum, error, false, flexible,
library, open, optional, protocol, resource, service, strict, struct, table,
true, type, union, using.

However, FIDL has no reserved keywords. For example:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="keywords" %}

Identifiers

Library names

FIDL library names label FIDL libraries. They consist of one or more components separated by dots (.). Each component must match the regex [a-z][a-z0-9]*. In words: library name components must start with a lowercase letter, can contain lowercase letters, and numbers.

// A library named "foo".
library foo;

Identifiers

FIDL identifiers label declarations and their members. They 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 {};

FIDL identifiers are case sensitive. However, identifiers must have unique canonical forms, otherwise the FIDL compiler will fail with fi-0035: Canonical name collision. The canonical form of an identifier is obtained by converting it to snake_case.

Qualified identifiers

FIDL always looks for unqualified symbols within the scope of the current library. To reference symbols in other libraries, you must qualify them with the library name or alias.

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;
};

Resolution algorithm

FIDL uses the following algorithm to resolve identifiers. When a “try resolving” step fails, it proceeds to the next step below. When a “resolve” step fails, the compiler produces an error.

  • If it is unqualified:
    1. Try resolving as a declaration in the current library.
    2. Try resoving as a builtin declaration, e.g. bool refers to fidl.bool.
    3. Resolve as a contextual bits/enum member, e.g. the CHANNEL in zx.handle:CHANNEL refers to zx.ObjType.Channel.
  • If it is qualified as X.Y:
    1. Try resolving X as a declaration within the current library:
      1. Resolve Y as a member of X.
    2. Resolve X as a library name or alias.
      1. Resolve Y as a declaration in X.
  • If it is qualified as x.Y.Z where x represents one or more components:
    1. Try resolving x.Y as a library name or alias:
      1. Resolve Z as a declaration in x.Y.
    2. Resolve x as a library name or alias:
      1. Resolve Y as a declaration in x:
        1. Resolve Z as a member of x.Y.

Note: If you import libraries X and X.Y, and library X defines an enum named Y, you cannot refer to a member of that enum since X.Y.Z would be interpreted as the declaration Z from library X.Y, even if no such declaration exists. To refer to the enum member, you would have to remove or alias one of the imports. This should not come up in practice when following the FIDL Style Guide, which mandates lowercase library names and UpperCamel declaration names.

Fully qualified names

FIDL uses fully qualified names (abbreviated “FQN”) to refer unambiguously to declarations and members. An FQN consts of a library name, a slash /, a declaration identifier, and optionally a dot . and member identifier. For example:

  • fuchsia.io/MAX_BUF refers to the MAX_BUF constant in library fuchsia.io.
  • fuchsia.process/Launcher.Launch refers to the Launch method in the Launcher protocol of library fuchsia.process.

FQNs are used in error messages, the FIDL JSON intermediate representation, method selectors, and documentation comment cross references.

Literals

FIDL supports the following kinds of literals:

  • Boolean: true, false
  • Integer: decimal (123), hex (0xA1B2), octal (0755), binary (0b101).
    • Only decimal literals can be negative.
  • Floating point: 1.23, -0.01, 1e5, 2.0e-3
    • Only e and e- are allowed, not e+.
  • String: "hello", "\\ \" \n \r \t \u{1f642}"

All letters in numeric literals (e.g. hex digits) are case insensitive.

Constants

FIDL allows defining constants for all types that support literals (boolean, integer, floating point, and string), and bits and enums. For example:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="consts" %}

Constant expressions are either literals, references to other constants, references to bits or enum members, or a combination of bits members separated by the pipe character (|). FIDL does not support any other arithmetic expressions such as 1 + 2.

Declaration separator

FIDL uses the semicolon (;) to separate adjacent declarations within the file, much like C.

Libraries

Libraries are named containers of FIDL declarations.

// library identifier separated by dots
library fuchsia.composition;

// "using" to import library "fuchsia.mem"
using fuchsia.mem;

// "using" to import library "fuchsia.geometry" under the alias "geo"
using fuchsia.geometry as geo;

Libraries import other libraries with using declarations. You can refer to symbols in an imported library by qualifying them with the library name, as in fuchsia.mem.Range. With the using ... as syntax you must qualify symbols with the alias, as in geo.Rect (fuchsia.geometry.Rect would not work).

The scope of library and using declarations is limited to a single file. Each file in a FIDL library must restate the library declaration and any using declarations that the file needs.

The FIDL compiler does not require any particular directory structure, but each FIDL library is usually organized in its own directory named after the library. Libraries with more than one file conventionally have an overview.fidl file containing only a library declaration along with attributes and a documentation comment.

The library's name may be used in language bindings as a namespace. For example, the C++ bindings generator places declarations for the FIDL library fuchsia.ui within the C++ namespace fuchsia_ui. Similarly, the Rust bindings generator would generate a crate named fidl_fuchsia_ui.

Types and type declarations

FIDL supports a number of builtin types as well as declarations of new types (e.g. structs, unions, type aliases) and protocols.

Primitives

  • Simple value types.
  • Cannot be optional.

FIDL supports the following primitive types:

  • Boolean bool
  • Signed integer int8 int16 int32 int64
  • Unsigned integer uint8 uint16 uint32 uint64
  • IEEE 754 Floating-point float32 float64

Numbers are suffixed with their size in bits. For example, int8 is 1 byte.

Use

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="primitives" %}

Bits

  • Named bit types.
  • Discrete subset of bit values chosen from an underlying integer type.
  • Cannot be optional.
  • Bits can either be strict or flexible.
  • Bits default to flexible.
  • strict bits must have at least one member, flexible bits can be empty.

Operators

| is the bitwise OR operator for bits.

Use

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="bits" %}

Enums

  • Proper enumerated types.
  • Discrete subset of named values chosen from an underlying integer type.
  • Cannot be optional.
  • Enums can be strict or flexible.
  • Enums default to flexible.
  • strict enums must have at least one member, flexible enums can be empty.

Declaration

The ordinal 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 defaults to uint32.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="enums" %}

Use

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

  • Fixed-length sequences of homogeneous elements.
  • Elements can be of any type.
  • Cannot be optional themselves; may contain optional types.

Use

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.

Strings

  • Variable-length sequence of bytes representing text in UTF-8 encoding.
  • Can be optional; absent strings and empty strings are distinct.
  • Can specify a maximum size, e.g. string:40 for a maximum 40 byte string. By default, string means string:MAX, i.e unbounded.
  • String literals support the escape sequences \\, \", \n, \r, \t, and \u{X} where the X is 1 to 6 hex digits for a Unicode code point.
  • May contain embedded NUL bytes, unlike traditional C strings.

Use

Strings are denoted as follows:

  • string : required string (validation error occurs if absent)
  • string:optional : optional string
  • string: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 or fuchsia.mem.Buffer for blobs. See Should I use string or vector? for details.

Vectors

  • Variable-length sequence of homogeneous elements.
  • Can be optional; absent vectors and empty vectors are distinct.
  • Can specify a maximum size, e.g. vector<T>:40 for a maximum 40 element vector. By default, vector<T> means vector<T>:MAX, i.e. unbounded.
  • There is no special case for vectors of bools. Each bool element takes one byte as usual.

Use

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 T
  • vector<T>:N and vector<T>:<N, optional> : vector, and optional vector, respectively, with maximum length of N elements

T 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

  • Transfers a Zircon capability by handle value.
  • Stored as a 32-bit unsigned integer.
  • Can be optional; absent handles are encoded as a zero-valued handle.
  • Handles may optionally be associated with a type and set of required Zircon rights.

Use

Handles are denoted:

  • zx.Handle : required Zircon handle of unspecified type
  • zx.Handle:optional : optional Zircon handle of unspecified type
  • zx.Handle:H : required Zircon handle of type H
  • zx.Handle:<H, optional> : optional Zircon handle of type H
  • zx.Handle:<H, R> : required Zircon handle of type H with rights R
  • zx.Handle:<H, R, optional> : optional Zircon handle of type H with rights R

H 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" %}

Structs

  • Record type consisting of a sequence of typed fields.
  • Adding or removing fields or changing their types is generally not ABI compatible.
  • Declaration can have the resource modifier.
  • References may be boxed.
  • Structs contain zero or more members.

Declaration

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="structs" %}

Use

Structs are denoted by their declared name (e.g. Circle):

  • Circle : required Circle
  • box<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" %}

Tables

  • Record type consisting of a sequence of typed fields with ordinals.
  • Declaration is intended for forward and backward compatibility in the face of schema changes.
  • Declaration can have the resource modifier.
  • Tables cannot be optional. The semantics of “missing value” is expressed by an empty table i.e. where all members are absent, to avoid dealing with double optionality.
  • Tables contain zero or more members.

Declaration

type Profile = table {
    1: locales vector<string>;
    2: calendars vector<string>;
    3: time_zones vector<string>;
};

Use

Tables are denoted by their declared name (e.g. Profile):

  • Profile : required Profile

Here, 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;
};

Unions

  • Record type consisting of an ordinal and an envelope.
  • Ordinal indicates member selection, envelope holds contents.
  • Declaration can be modified after deployment, while maintaining ABI compatibility. See the Compatibility Guide for source-compatibility considerations.
  • Declaration can have the resource modifier.
  • Reference may be optional.
  • Unions can either be strict or flexible.
  • Unions default to flexible.
  • strict unions must contain one or more 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”).

Declaration

{% includecode gerrit_repo="fuchsia/samples" gerrit_path="src/calculator/fidl/calculator.fidl" region_tag="union" %}

Use

Unions are denoted by their declared name (e.g. Result) and optionality:

  • Result : required Result
  • Result:optional : optional Result
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="unions-use" %}

Strict vs. flexible

FIDL type declarations can either have strict or flexible behavior:

  • Bits, enums, and unions are flexible unless declared with the strict modifier.
  • Structs always have strict behavior.
  • Tables always have flexible behavior.

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.

Value vs. resource

Every FIDL type is either a value type or a resource type. Resource types include:

  • handles
  • protocol endpoints
  • aliases of resource types
  • arrays and vectors of resource types
  • structs, tables, and unions marked with the resource modifier
  • optional (or boxed) references to any of the above types

All 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.

Protocols

  • Describe methods that can be invoked by sending messages over a channel.

  • Methods are identified by their ordinal. The compiler calculates it by:

    • Taking the SHA-256 hash of the method's fully qualified name.
    • Extracting the first 8 bytes of the hash digest,
    • Interpreting those bytes as a little endian integer,
    • Setting the upper bit (i.e. last bit) of that value to 0.
    • To override the ordinal, methods can have a @selector attribute. If the attribute's argument is a valid FQN, it will be used in place of the FQN above. Otherwise, it must be a valid identifier, and will be used in place of the method name when constructing the FQN.

  • Each method declaration states its arguments and results.

    • If no results are declared, then the method is one-way: no response will be generated by the server.
    • If results are declared (even if empty), then the method is two-way: each invocation of the method generates a response from the server.
    • If only results are declared, the method is referred to as an event. It then defines an unsolicited message from the server.
    • Two-way methods may declare an error type that a server can send instead of the response. This type must be an 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.

Declaration

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/language_reference.test.fidl" region_tag="calculator" %}

Use

Protocols are denoted by their name, directionality of the channel, and optionality:

  • client_end:Protocol : client endpoint of channel communicating over the FIDL protocol
  • client_end:<Protocol, optional> : optional version of the above
  • server_end:Protocol : server endpoint of a channel communicating over the FIDL protocol
  • server_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" %}

Protocol composition

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:

  1. you have multiple protocols that all share some common behavior(s)
  2. you have varying levels of functionality you want to expose to different audiences

Common behavior

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:

  • the way to set background and foreground colors is the same, whether it's used to draw a circle, square, or put text on the display.
  • new methods can be added to 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.

Multiple compositions

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.

Layering

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" %}

Unknown interactions

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.
    • All methods and events must be declared as 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.
    • One-way methods and events may be declared as either strict or flexible.
    • Two-way methods must be declared as 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.
    • All methods and events may be declared as either 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 Pcompilescompilescompilescompilescompilescompiles
ajar Pcompilescompilescompilescompilescompilesfails to compile
closed Pcompilesfails to compilecompilesfails to compilecompilesfails 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 receiving 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 Pauto-closedhandleableauto-closedhandleableauto-closedhandleable
ajar Pauto-closedhandleableauto-closedhandleableauto-closedauto-closed
closed Pauto-closedauto-closedauto-closedauto-closedauto-closedauto-closed

Interaction with composition

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.

Aliasing

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.

<<../../../development/languages/fidl/widgets/_alias.md>>

Builtins

FIDL provides the following builtins:

  • Primitive types: bool, int8, int16, int32, int64, uint8, uint16, uint32, uint64, float32, float64.
  • Other types: string, client_end, server_end.
  • Type templates: array, vector, box.
  • Aliases: byte.
  • Constraints: optional, MAX.

All builtins below to the fidl library. This library is always available and does not need to be imported with using. For example, if you declare a struct named string, you can refer to the original string type as fidl.string.

Library zx

Library zx is not built in, but it is treated specially by the compiler. It is defined in //zircon/vdso/zx. Libraries import it with using zx and most commonly use the zx.Handle type.

Inline layouts

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, FIDL 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:

  • For inline layouts used as the type of an outer layout member, the reserved name is simply the name of the corresponding member.
    • In the example above, the name Options is reserved for the inline table.
  • For top level request/response types, FIDL concatenates the protocol name, the method name, and then either "Request" or "Response" depending on where the type is used.
    • In the example above, the name LauncherGenerateTerrainRequest is reserved for the struct used as the request of the GenerateTerrain method.
    • Note that the "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:

  • Rename the layout member.
  • Override the reserved name using the @generated_name attribute.