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FIDL bindings specification

This document is a specification of Fuchsia Interface Definition Language (FIDL) bindings. It is meant to provide guidance and best practices for bindings authors, and recommend specific approaches for their ergonomic use.

In this document, the following keywords are to be interpreted as described in RFC2119: MAY, MUST, MUST NOT, OPTIONAL, RECOMMENDED, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT.

Generated code indication

A comment must be placed at the top of machine-generated code to indicate it is machine generated. For languages with a standard on how to indicate generated sources (as opposed to human-written code), that standard must be followed.

In Go for instance, generated sources must be marked with a comment following the pattern

// Code generated by <tool>; DO NOT EDIT.


It is RECOMMENDED to namespace machine-generated code to avoid clashing with user-defined symbols. This can be implement using scoping constructs provided by the language, like namespaces in C++, modules in Rust, or packages in Go and Dart. If the generated scope can have a name, it SHOULD be named using components of the FIDL library name which contains the definitions for the generated code, which allows each FIDL library to exist in a unique scope. In cases where scoping is not possible and the namespace is shared, some processing of the generated names (see Naming) may be necessary.


In general, the names used in the generated code SHOULD match the names used in the FIDL definition. Possible exceptions are listed in the following sections.


Casing changes SHOULD be made to fit the idiomatic style of the language (e.g. using snake_case or CamelCase). fidlc will ensure that identifier uniqueness is enforced taking into account potential casing differences (see FTP 40).

Reserved keywords and name clashes

The generated code MUST take into account the reserved keywords in the target language to avoid unexpected when a keyword from the target language is used in the FIDL definition. An example scheme would be to prefix conflicting names with an underscore _ (assuming no keywords begin with an underscore).

The generated code MUST avoid generating code that causes naming conflicts. For example, in a function whose parameters are generated based on a FIDL definition, it MUST be impossible for the names of the local variables in the generated to clash with possible generated names.


Method ordinals

Ordinals used for methods are large 64-bit numbers. Bindings SHOULD emit these ordinals in hexadecimal, i.e. 0x60e700e002995ef8, not 6982550709377523448.

Union, and table ordinals

Ordinals used for union and table start at 1, and must form a dense space. Therefore, these numbers are typically small, and bindings SHOULD emit these ordinals in decimal notation.

Native types

It is RECOMMENDED that bindings use the most specific and ergonomic native types where possible when converting built-in FIDL types to native types in the target language. For example, the Dart bindings use Int32List to represent a vector<int32>:N and array<int32>:N rather than the more generic List<int>.

Generated types and values

Constant support

Generated code MUST generate variables containing matching values for each const definition in the corresponding FIDL. These variables SHOULD be marked as immutable in languages that support this (e.g. const in C++, Rust, and Go, or final in Dart).

Bits support

Bindings MUST provide generated values for each bits member. They MAY also generate values representing the bits with no flags set, as well as the bits with every flag set (the "bits mask"). These values SHOULD be scoped to each set of bits.

It is RECOMMENDED to support the following operators over generated values:

  • bitwise and, i.e &
  • bitwise or, i.e |
  • bitwise exclusive-or, i.e ^
  • bitwise not, i.e ~

To provide bitwise operations which always result in valid bits values, implementations of bitwise not should further mask the resulting value with the mask of all values. In pseudo code:

~value1   means   mask & ~bits_of(value1)

This mask value is provided in the JSON IR for convenience.

Bindings SHOULD NOT support other operators since they could result in invalid bits value (or risk a non-obvious translation of their meaning), e.g.:

  • bitwise shifts, i.e << or >>
  • bitwise unsigned shift, i.e >>>

For cases where the generated code includes a type wrapping the underlying numeric bits value, it SHOULD be possible to convert between the raw value and the wrapper type. It is RECOMMENDED for this conversion to be explicit.

Enum support

Bindings MUST provide generated values for each enum member. These values SHOULD be scoped to each enum.

For cases where the generated code includes a type wrapping the underlying numeric enum value, it SHOULD be possible to convert between the raw value and the wrapper type. It is RECOMMENDED for this conversion to be explicit.

Struct support

Bindings MUST provide a type for each struct that supports the following operations:

  • Construction with explicit values for each member.
  • Reading and writing members.

Bindings MAY support default values for structs. The default values are specified in the JSON IR.

Union support

Bindings MUST provide a type for each union that supports the following operations:

  • Construction with an explicit variant set. It is NOT RECOMMENDED for bindings to offer construction without a variant. This should be considered only for performance reasons or due to limitations of the target language.
  • Reading/writing the variant of the union and the data associated with that variant.

For languages without union types or union value literals, it is RECOMMENDED to support factory methods for constructing new unions given a value for one of the possible variants. For example, in a C like language, this would allow replacing code like:

my_union_t foo;

with something like:


These factory methods SHOULD be named as "[Type]-with-[Variant]", cased properly for the target language.

Examples of this exist for the HLCPP and Go bindings.

Flexible unions

The bindings MUST succeed when decoding a flexible union with an unknown variant. The behavior of such a union can vary. Bindings MAY provide ways for the user to read the underlying raw bytes and handles of the payload, as well as the unknown ordinal. Bindings SHOULD either provide access to both bytes and handles, or neither.

For bindings that support storing accessing the unknown bytes, handles, and ordinals:

  • Bindings MAY provide a constructor to create a union with an unknown variant with specified ordinal, bytes, and handles.
    • Such a constructor is useful not just for testing the bindings, but also for end-developer testing needs (e.g. to check that unknown data is handled correctly in a proxy).
    • Having a constructor also prevents end-developers from constructing unions with unknown variants in roundabout ways, such as by manually decoding raw bytes.
    • Usage of this constructor is discouraged in production code.
  • Bindings SHOULD support re-encoding the union, writing the unknown ordinal, bytes, and handles back onto the wire.

For bindings that do not store the unknown bytes, handles, and ordinal:

  • Bindings SHOULD fail to encode rather than send a message with missing data.

Table support

Bindings MUST provide a type for each table that supports the following operations:

  • Construction where specifying values for each member is optional.
  • Reading and writing each member, including checking whether a given member is set. These SHOULD follow the naming scheme: get_[member], set_[member], and has_[member], cased properly for the target language.

Bindings MAY support default values for tables. The default values are specified in the JSON IR.

Bindings MAY provide constructors for tables that only require specifying values for fields that have a value. For example, in Rust this can be accomplished using the ::empty() constructor along with struct update syntax. Supporting construction this ways allows users to write code that is robust against addition of new fields to the table.

Protocol support

Error types

It is OPTIONAL that bindings provide some form of special support for protocol methods with an error type matching the idiomatic way errors are handled in the target language.

For example, languages that provide some form of a "result" type (i.e. a union type that contains a "success" variant and an "error" variant), such as Rust's result::Result, or fit::result in C++ MAY provide automatic conversions to and from these types when receiving or sending method responses with an error type.

Languages with exceptions can have the generated protocol method code optionally raise an exception corresponding to the error type.

In cases where this is not possible, the generated code MAY provide convenience functions for responding directly with a successful response or error value, or for receiving an error type response, in order avoid boilerplate user code for initializing result unions.

Error handling

Protocols MAY surface transport errors back to the user. Transport errors can be categorized as errors encountered when converting between the native type and the wire format data, or as errors from the underlying transport mechanism (for example, an error obtained from calling zx_channel_write). These errors MAY consist of the error status, as well as any other diagnostics information.


Bindings MUST support the following attributes:

  • [Transitional]

Best practices

Alternative output

It is OPTIONAL for bindings to provide alternative output methods to the FIDL wire format.

One type of output could be user-friendly debug printing for the generated types. For example, printing a value of the bits:

bits Mode {
  Read = 1;
  Write = 2;

could print the string "Mode.Read | Mode.Write" rather than the raw value "0b11".

Similar user-friendly printing can be implemented for each of the generated FIDL types.

Another example of alternative output would be serializing FIDL values to JSON. Users SHOULD have the option to opt-in or out to this functionality in order to follow the principle of "only pay for what you use". An example of this is dart_fidl_json, which is implemented using fidlmerge.

Message memory allocation

Bindings MAY provide the option for users to provide their own memory to use when sending or receiving messages, which allows the user to control memory allocation.

Wire format memory layout

Bindings MAY have the in memory layout of the generated FIDL types match the wire format of the type. Doing this can in theory avoid extra copies, as the data can be used directly as the transactional message, or vice versa. In practice, sending a FIDL message may still involve a copying step where the components of a message are assembled into a contiguous chunk of memory (called "linearization"). The downside of such an approach is that it makes the bindings more rigid: changes to the FIDL wire format become more complex to implement.

The LLCPP bindings are the only binding which take this approach.

Equality comparison

For aggregate types such as structs, tables, and unions, bindings MAY provide equality operators that perform a deep comparison on two instances of the same type. These operators SHOULD NOT be provided for resource types (see FTP-057) as comparison of handles is not possible. Avoiding exposing equality operators for resource types prevents source breakages caused by an equality operation 'disappearing' when a handle is added to the type.


For aggregate types such as structs, tables, and unions, bindings MAY provide functionality for copying instances of these types. Copying SHOULD NOT be provided for resource types (see FTP-057) as making copies of handles is not guaranteed to succeed. Avoiding exposing copy operators for resource types prevents source breakages caused by a copy operation 'disappearing' or having its signature change when a handle is added to the type.

Test utilities

It is OPTIONAL for bindings to generate additional code specifically to be used during testing. For example, the bindings can generate stub implementations of each protocol so that users only need too verride specific methods that are going to be exercised in a test.


Bindings SHOULD provide support for epitaphs, i.e. generated code that allows servers to send epitaphs and clients to receive and handle epitaphs.

Setters and Getters

Bindings MAY provide setters and getters for fields on aggregate types (structs, unions, and tables). Even in languages where getter/setter methods are un-idiomatic, using these methods will allow renaming internal field names without breaking usages of that field.

Request "responders"

When implementing a FIDL protocol using the FIDL bindings in a target language, the bindings provide an API to read the request parameters, and a way to write the response parameters, if any. For example, the request parameters could be provided as the arguments to a function, and the response parameters could be provided as the return type of the function.

For a FIDL protocol:

protocol Hasher {
  Hash(string value) -> (array<uint8>:10 result);

A binding might generate:

// Users would implement this interface to provide an implementation of the
// Hasher FIDL protocol
interface Hasher {
  // Respond to the request by returning the desired response
  hash: (value: string): Uint8Array;

Bindings MAY provide a responder object that is used to write responses. In the example above, this would mean passing an additional responder object in the function arguments, and having the function return void:

interface Hasher {
  hash: (value: string, responder: HashResponder): void;

interface HashResponder {
  sendResponse(value: Uint8Array);

The use of a responder object has the following benefits:

  • Improved ergonomics: responders can be used to provide any type of interaction with the client. For example, responders can have methods that close the channel with an epitaph, or provide APIs for sending events. For two-way methods, the responder could provide the mechanism to send a response.
  • Increased flexibility: encapsulating all these behaviors in a single type makes it possible to add or remove behavior from the bindings without making breaking changes to bindings users, by only changing the responder object, and not the protocol object.

When providing a responder object, bindings should be careful about responders being invoked on a different thread than the one the request was processed on. Responders may also be invoked much later than the request was processed, for instance when implement a handing get pattern. In practice this could be implemented by allowing users to move ownership of the responder out of the request handler class, e.g. into a callback for an asynchronous function.

The object MAY NOT necessarily be called responder. For example, it could have a different name depending on whether the method is fire and forget or two way:

interface Hasher {
  // the Hash method is a two-way method, so the object is called a responder
  hash: (value: string, responder: HashResponder): void;
  // the SetSeed method is a fire and forget method, so it gets a different name
  setSeed: (seed: number, control: HasherControlHandle): void;