FIDL lexicon

This document defines general terms that have a specific meaning in a FIDL context. To learn more about specific FIDL topics, refer to the FIDL traihead.

Type, layout, constraint

A type classifies a value in the FIDL type system. Types appear in the type-constructor position defined in the FIDL grammar. For example, uint32, string:10, and vector<bool> are types.

A layout is a parametrizable description of a type. It does not refer to a single type, but describes a family of types obtained by instantiating the layout with zero or more layout parameters. For example, vector is a layout which takes one layout parameter, a type such as vector<bool> itself a type. As another example, array is a layout which takes two layout parameters, producing types such as array<bool, 3>.

The difference between layout and type can be subtle. For example, the statement above that uint32 is a type is not strictly correct. Rather, it is a layout that takes zero parameters, distinct from the type obtained by instantiating it. The FIDL syntax does not capture this distinction, so when uint32 is used in type position (e.g. alias uint = uint32;), it is really referencing the layout uint32 and implicitly instantiating it with zero layout parameters.

A constraint restricts a type to only allow values satisfying a predicate. For example, the type string:10 has the constraint length <= 10 abbreviated as 10, meaning the string length cannot exceed 10 bytes.

Layouts and layout parameters affect how bytes are laid out in the FIDL wire format, while constraints affect validation that restricts what can be represented (see RFC-050 for more detail). Syntactically, layout parameters can only be applied to layouts, and constraints can only be applied to types. For example:

alias Bools = vector<bool>;       // ok: layout parameter applied to layout
alias MaxTenBools = Bools:10;     // ok: constraint applied to type
alias MaxTenBytes = Bools<byte>;  // INVALID: layout parameter applied to type
alias MaxTen = vector:10;         // INVALID: constraint applied to layout

The general form of a type instantiation is

L<L_1, L_2, ..., L_n>:<C_1, C_2, ..., C_n>

where L is a layout, L_1 through L_n are layout parameters, and C_1 through C_n are constraints.

Member, field, variant

A member of a declaration is an individual element belonging to a declaration, i.e. a declaration is comprised of zero, one, or many members.

For instance, consider the Mode bits declaration:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/" region_tag="mode" %}

Both READ and WRITE are members.

When referring to members of structs or tables, we can more specifically refer to these members as fields.

When referring to members of a union, we can more specifically refer to these members as variants.

For example, consider the Command union declaration:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/" region_tag="command" %}

The two variants are create_resource and release_resource.

Furthermore, the selected variant of an instance of a union is the current value held by the union at that moment.

Tag, and ordinal

The tag is the target language variant discriminator, i.e. the specific construct in a target language that is used to indicate the selected variant of a union. For example, consider the following TypeScript representation of the Command union:

enum CommandTag {

interface Command {
    tag: CommandTag,
    data: CreateResource | ReleaseResource,

The tag of Command is Command.tag and has type CommandTag. The actual values and type representing each variant of Command are up to the implementation.

Note that some languages will not require a tag. For example, some languages use pattern matching to branch on the variant of a union instead of having an explicit tag value.

The ordinal is the on the wire variant discriminator, i.e. the value used to indicate the variant of a union in the FIDL wire format. The ordinals are explicitly specified in the FIDL definition (in this example, 1 for create_resource and 2 for release_resource).


Encoding refers to the process of serializing values from a target language into the FIDL wire format.

For the C family of bindings (HLCPP, LLCPP), encode can have a more specific meaning of taking bytes matching the layout of the FIDL wire format and patching pointers and handles by replacing them with FIDL_ALLOC_PRESENT/FIDL_ALLOC_ABSENT or FIDL_HANDLE_PRESENT/FIDL_HANDLE_ABSENT in-place, moving handles into an out-of-band handle table.


Decoding refers to the process of deserializing values from raw bytes in the FIDL wire format into a value in a target language.

For the C family of bindings (HLCPP, LLCPP), decode can have a more specific meaning of taking bytes matching the layout of the FIDL wire format and patching pointers and handles by replacing FIDL_ALLOC_PRESENT/FIDL_ALLOC_ABSENT or FIDL_HANDLE_PRESENT/FIDL_HANDLE_ABSENT with the “real” pointer/handle values in-place, moving handles out of an out-of-band handle table.


Validation is the process of checking if constraints from the FIDL definition are satisfied for a given value. Validation occurs both when encoding a value before being sent, or when decoding a value after receiving it. Example constraints are vector bounds, handle constraints, and the valid encoding of a string as UTF-8.

When validation fails, the bindings surface the error to user code, either by returning it directly or via an error callback.

Result/error type

For methods with error types specified:

DoWork() -> (struct { result Data; }) error uint32

The result type refers to the entire message that would be received by a server for this method, i.e. the union that consists of either a result of Data or an error of uint32. The error type in this case is uint32, whereas Data can be referred to as either the response type or the success type.