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LLCPP bindings


Given the library declaration:

library games.tictactoe;

Bindings code for this library is generated in the llcpp::games::tictactoe namespace.


Constants are generated as a constexpr. For example, the following constants:

const uint8 BOARD_SIZE = 9;
const string NAME = "Tic-Tac-Toe";

Are generated in the header file as:

constexpr uint8_t BOARD_SIZE = 9u;
extern const char[] NAME;

The correspondence between FIDL primitive types and C++ types is outlined in built-in types. Instead of constexpr, strings are declared as an extern const char[] in the header file, and defined in a .cc file.


This section describes how the FIDL toolchain converts FIDL types to native types in LLCPP. These types can appear as members in an aggregate type or as parameters to a protocol method.

Built-in types

The FIDL types are converted to C++ types based on the following table:

bool bool, (requires sizeof(bool) == 1)
int8 int8_t
int16 int16_t
int32 int32_t
int64 int64_t
uint8 uint8_t
uint16 uint16_t
uint32 uint32_t
uint64 uint64_t
float32 float
float64 double
array<T>:N fidl::Array<T, N>
vector<T>:N fidl::VectorView<T>
string fidl::StringView
request<P>, P zx::channel
handle zx::handle
handle<S> The corresponding zx type is used whenever possible. For example, zx::vmo or zx::channel.

Nullable built-in types do not have different generated types than their non-nullable counterparts in LLCPP, and are omitted from the table above.

User defined types

In LLCPP, a user defined type (bits, enum, constant, struct, union, or table) is referred to using the generated class or variable (see Type Definitions ). The nullable version of a user defined type T is referred to using a fidl::tracking_ptr of the generated type except for unions, which simply use the generated type itself. Refer to the LLCPP Tutorial for information about tracking_ptr.

Type definitions


Given the bits definition:

bits FileMode : uint16 {
    READ = 0b001;
    WRITE = 0b010;
    EXECUTE = 0b100;

The FIDL toolchain generates a FileMode class with a static member for each flag, as well as a mask member that contains a mask of all bits members (in this example 0b111):

  • const static FileMode READ
  • const static FileMode WRITE
  • const static FileMode EXECUTE
  • const static FileMode mask

FileMode provides the following methods:

  • explicit constexpr FileMode(uint16_t): Constructor for FileMode, which uses the specified underlying type (in this example uint16_t).
  • Bitwise operators: Implementations for the |, |=, &, &=, ^, ^=, and ~ operators are provided, allowing bitwise operations on the bits like mode |= FileMode::EXECUTE.
  • Comparison operators == and !=.
  • Explicit conversion functions for uint16_t and bool.


Given the enum definition:

enum Color {
    RED = 1;
    GREEN = 2;
    BLUE = 3;

The FIDL toolchain generates an equivalent C++ enum class using the specified underlying type, or uint32_t if none is specified:

enum class Color : uint32_t {
    RED = 1u;
    GREEN = 2u;
    BLUE = 3u;


Given the struct declaration:

struct Person {
    uint32 id;
    string name = "john";

The FIDL toolchain generates an equivalent struct:

struct Person {
    uint32_t id = {};
    fidl::StringView name = {};

LLCPP does not currently support default values, and instead zero-initializes all fields of the struct.


Given the union definition:

union JsonValue {
    1: reserved;
    2: int32 int_value;
    3: string string_value;

FIDL will generate a JsonValue class. JsonValue contains a public tag enum class representing the possible variants:

enum class Tag : fidl_xunion_tag_t {
  kIntValue = 2,
  kStringValue = 3,

Each member of Tag has a value matching its ordinal specified in the union definition. Reserved fields do not have any generated code

JsonValue provides the following methods:

  • JsonValue(): Default constructor. The constructed union is initially in an "invalid" state until a variant is set. The WithFoo constructors should be preferred whenever possible.
  • ~JsonValue(): Destructor that clears the underlying union data.
  • JsonValue(JsonValue&&): Default move constructor.
  • JsonValue& operator=(JsonValue&&): Default move assignment
  • static JsonValue WithIntValue(fidl::tracking_ptr<int32>&&) and static JsonValue WithStringValue(fidl::tracking_ptr<fidl::StringView>&&): Static constructors that directly construct a specific variant of the union.
  • bool has_invalid_tag(): Returns true if the instance of JsonValue does not yet have a variant set. Calling this method without first setting the variant leads to an assertion error.
  • bool is_int_value() const and bool is_string_value() const: Each variant has an associated method to check whether an instance of JsonValue is of that variant
  • const int32_t& int_value() const and const fidl::StringView& string_value() const: Read-only accessor methods for each variant. Calling these methods without first setting the variant leads to an assertion error.
  • int32_t& int_value() and fidl::StringView& string_value(): Mutable accessor methods for each variant. These methods will fail if JsonValue does not have the specified variant set
  • void set_int_value(fidl::tracking_ptr<int32_t>&& value) and void set_string_value(fidl::tracking_ptr<fidl::StringView>&& value): Setter methods for each variant. These setters will overwrite the previously selected member, if any.
  • Tag Which() const: returns the current tag of the JsonValue. Calling this method without first setting the variant leads to an assertion error.

Flexible unions and unknown variants

Flexible unions (that is, unions that are prefixed with the flexible keyword in their FIDL definition) have an extra variant in the generated Tag:

  enum class Tag : fidl_xunion_tag_t {
    ... // other fields omitted
    kUnknown = ::std::numeric_limits<::fidl_union_tag_t>::max(),

When a FIDL message containing a union with an unknown variant is decoded into JsonValue, JsonValue::Which() will return JsonValue::Tag::kUnknown.

A flexible JsonValue also has the following extra methods:

  • void* unknownData() const: Returns the raw bytes of the union variant. This method fails with an assertion error if the variant is not unknown.

Encoding a union with an unknown variant will write the unknown data and the original ordinal back onto the wire.

The decoding operation will fail when encountering an unknown variant at a non-flexible union type.


Given the table definition:

table User {
    1: reserved;
    2: uint8 age;
    3: string name;

The FIDL toolchain User class with the following methods:

  • User(): Default constructor, initializes with all fields unset.
  • User(User&&): Default move constructor.
  • ~User(): Default destructor.
  • User& operator=(User&&): Default move assignment.
  • bool IsEmpty() const: Returns true if no fields are set.
  • bool has_age() const and bool has_name() const: Returns whether a field is set.
  • const uint8_t& age() const and const fidl::StringView& name() const: Read-only field accessor methods. Calling these methods without first setting the field leads to an assertion error.
  • uint8_t& age() and fidl::StringView& mutable_age(): Mutable field accessor methods. Calling these methods without first setting the variant leads to an assertion error.
  • User& set_age(uint8_t _value) and User& set_name(std::string _value): Field setters.

In order to build a table, three additional classes are generated: User::Frame, User::Builder, and User::UnownedBuilder.

User::Frame is a container for the table's internal storage, and is allocated separately from the builder because LLCPP maintains the object layout of the underlying wire format. It only needs to be used in conjunction with User::Builder. User::Frame has the following methods:

  • Frame(): Default constructor.

User::Builder and User::UnownedBuilder both provide the following methods for constructing a new User:

  • Builder&& set_age(fidl::tracking_ptr<uint8_t> elem) and Builder&& set_name(fidl::tracking_ptr<fidl::StringView> elem): Sets the specified field and returns the updated Builder.
  • User build(): Returns a User based on the Builder's data.

However, they differ in that User::UnownedBuilder directly owns the underlying Frame, which simplifies working with unowned data. The unowned builder is constructed using the default constructor, whereas User::Builder explicitly takes in a Frame:

Builder(fidl::tracking_ptr<User::Frame>&& frame_ptr)

An example of using all of these pieces together would be:

User user = User::Builder(std::make_unique<User::Frame>())
                 .set_name(std::make_unique<fidl::StringView>("john doe"))

In addition to assigning fields with std::unique_ptr, any of the allocation strategies described in the tutorial can also be used.


Given the protocol:

protocol TicTacToe {
    StartGame(bool start_first);
    MakeMove(uint8 row, uint8 col) -> (bool success, GameState? new_state);
    -> OnOpponentMove(GameState new_state);

FIDL will generate a TicTacToe class, which acts as an entry point for types and classes that both clients and servers will use to interact with this service. The members of this class are described in individual subsections in the rest of this section.

Request and response structs

FIDL generates a type for each request, response, and event in the protocol by treating the parameters as struct fields. For example, the MakeMoveRequest is generated as if it were a struct with two fields: uint8 row, and uint8 col, providing the same generated code API as regular structs:

struct MakeMoveRequest final {
    uint8_t row;
    uint8_t col;

For this example, the following types are generated:

  • TicTacToe::StartGameRequest
  • TicTacToe::MakeMoveRequest
  • TicTacToe::MakeMoveResponse
  • TicTacToe::OnOpponentMoveResponse

The naming scheme for requests is [Method]Request, and the naming scheme for both responses and events is [Method]Response.

Any empty request, response, or event is aliased to fidl::AnyZeroArgMessage, which is a type representing an empty message, instead of having a new type generated.


The LLCPP bindings provides multiple ways to interact with a FIDL protocol as a client:

  • fidl::Client<TicTacToe>: This class exposes thread-safe APIs for outgoing asynchronous and synchronous calls as well as asynchronous event handling. It owns the client end of the channel. An async_dispatcher_t* is required to support the asynchronous APIs as well as event and error handling. This is the recommended variant for most use-cases, except for those where an async_dispatcher_t cannot be used.
  • TicTacToe::SyncClient: This class exposes purely synchronous APIs for outgoing calls as well as for event handling. It owns the client end of the channel.
  • TicTacToe::Call: This class is identical to SyncClient except that it does not have ownership of the client end of the channel. Call may be preferable to SyncClient when migrating code from the C bindings to the LLCPP bindings, or when implementing C APIs that take raw zx_handle_ts.


Dereferencing a fidl::Client provides access to the following methods:

  • fidl::StatusAndError StartGame(bool start_first): Managed variant of a fire and forget method.
  • fidl::StatusAndError StartGame(::fidl::BytePart _request_buffer, bool start_first): Caller-allocated variant of a fire and forget method.
  • fidl::StatusAndError MakeMove(uint8_t row, uint8_t col, fit::callback<void(bool success, fidl::tracking_ptr<GameState> new_state)> _cb): Managed variant of an asynchronous two way method. It takes a callback to handle responses as the last argument.
  • fidl::StatusAndError MakeMove(fidl::BytePart _request_buffer, uint8_t row, uint8_t col, MakeMoveResponseContext* _context): Asynchronous, caller-allocated variant of a two way method. The final argument is a response context, which is explained below.
  • ResultOf::MakeMove MakeMove_Sync(uint8_t row, uint8_t col): Synchronous, managed variant of a two way method. The same method exists on SyncClient.
  • UnownedResultOf::MakeMove_sync(fidl::BytePart _request_bufffer, uint8_t row, uint8_t col, fidl::BytePart _response_buffer): Synchronous, caller-allocated variant of a two way method. The same method exists on SyncClient.

Each two way method has a response context that is used in the caller-allocated, asynchronous case. TicTacToe has only one response context, TicTacToe::MakeMoveResponseContext, which has pure virtual methods that should be overriden to handle responses:

virtual void OnReply(fidl::DecodedMessage<MakeMoveResponse> msg)
virtual void OnError()

Only one of the two methods is called for a single response: OnReply is called with a successfully decoded response, whereas OnError is called on any error that would cause the response context to be discarded without OnReply being called. You are responsible for ensuring that the response context object outlives the duration of the entire async call, since the fidl::Client borrows the context object by address to avoid implicit allocation.


TicTacToe::SyncClient provides the following methods:

  • explicit SyncClient(zx::channel): Constructor.
  • ~SyncClient(): Default destructor.
  • SyncClient(&&): Default move constructor.
  • SyncClient& operator=(SyncClient&&): Default move assignment.
  • const zx::channel& channel() const: Returns the underlying channel as a const.
  • zx::channel* mutable_channel(): Returns the underlying channel as mutable.
  • TicTacToe::ResultOf::StartGame StartGame(bool start_first): Owned variant of a fire and forget method call, which takes the parameters as arguments and returns the ResultOf class.
  • TicTacToe::UnownedResultOf::StartGame StartGame(fidl::BytePart, bool start_first): Caller-allocated variant of a fire and forget call, which takes in backing storage for the request buffer, as well as request parameters, and returns an UnownedResultOf.
  • ResultOf::MakeMove MakeMove(uint8_t row, uint8_t col): Owned variant of a two way method call, which takes the parameters as arguments and returns the ResultOf class.
  • UnownedResultOf::MakeMove(fidl::BytePart _request_buffer, uint8_t row, uint8_t col, fidl::BytePart _response_buffer): Caller-allocated variant of a two way method, which takes in backing storage for the request buffer, followed by the request parameters, and finally backing storage for the response buffer, and returns an UnownedResultOf.
  • zx_status_t HandleEvents(EventHandlers handlers): Blocks to consume exactly one event from the channel. See Events

Note that each method has both an owned and caller-allocated variant. In brief, the owned variant of each method handles memory allocation for requests and responses, whereas the caller-allocated variant allows the user to pass in the buffers themselves. The owned variant is easier to use, but may result in extra allocation. Details as well as examples for each variant are provided in the LLCPP Tutorial.


TicTacToe::Call provides similar methods to those found in SyncClient, with the only difference being that they are all static and take an unowned_channel as the first parameter:

  • static ResultOf::StartGame StartGame(zx::unowned_channel _client_end, bool start_first):
  • static UnownedResultOf::StartGame StartGame(zx::unowned_channel _client_end, fidl::BytePart _request_buffer, bool start_first):
  • static ResultOf::MakeMove MakeMove(zx::unowned_channel _client_end, uint8_t row, uint8_t col):
  • static UnownedResultOf::MakeMove MakeMove(zx::unowned_channel _client_end, fidl::BytePart _request_buffer, uint8_t row, uint8_t col, fidl::BytePart _response_buffer);:
  • static zx_status_t HandleEvents(zx::unowned_channel client_end, EventHandlers handlers):

StatusAndError, ResultOf and UnownedResultOf

The managed variants of each method of SyncClient and Call all return a ResultOf:: type, whereas the caller-allocating variants all return an UnownedResultOf::. Fire and forget methods on fidl::Client return a StatusAndError. These types define the same set of methods:

  • zx_status status() const returns the transport status. it returns the first error encountered during (if applicable) linearizing, encoding, making a call on the underlying channel, and decoding the result. If the status is ZX_OK, the call has succeeded, and vice versa.
  • const char* error() const contains a brief error message when status is not ZX_OK. Otherwise, returns nullptr.
  • (only for ResultOf and UnownedResultOf for two-way calls) T* Unwrap() returns a pointer to the response struct. For ResultOf::, the pointer points to memory owned by the result object. For UnownedResultOf::, the pointer points to the caller-provided buffer. Unwrap() should only be called when the status is ZX_OK.

Additionally, ResultOf and UnownedResultOf for two-way calls will implement dereference operators that return the response struct itself. This allows code such as:

auto result = client->MakeMove_Sync(0, 0);
auto response = result->Unwrap();
bool success = response.success;

To be simplified to:

auto result = client->MakeMove_Sync(0, 0);
bool success = result->success;


Implementing a server for a FIDL protocol involves providing a concrete implementation of TicTacToe.

The generated TicTacToe::Interface class has pure virtual methods corresponding to the method calls defined in the FIDL protocol. Users implement a TicTacToe server by providing a concerete implementation of TicTacToe::Interface, which has the following pure virtual methods:

  • virtual void StartGame(bool start_first, StartGameCompleter::Sync _completer)
  • virtual void MakeMove(uint8_t row, uint8_t col, MakeMoveCompleter::Sync _completer)

Refer to the example LLCPP server for how to bind and set up a server implementation.

The LLCPP bindings also provide functions for manually dispatching a message given an implementation, TicTacToe::TryDispatch and TicTacToe::Dispatch:

  • static bool TryDispatch(Interface* impl, fidl_msg_t* msg, ::fidl::Transaction* txn): Attempts to dispatch the incoming message. If there is no matching handler, it returns false, leaving the message and transaction intact. In all other cases, it consumes the message and returns true.
  • static bool Dispatch(Interface* impl, fidl_msg_t* msg, ::fidl::Transaction* txn): Dispatches the incoming message. If there is no matching handler, it closes all handles in the message and closes the channel with a ZX_ERR_NOT_SUPPORTED epitaph, and returns false.


A completer is provied as the last argument of each generated FIDL method handler, after all the FIDL request parameters for that method. The completer classes capture the various ways one can complete a FIDL transaction, e.g. by sending a reply, closing the channel with an epitaph, etc, and come in both synchronous and asynchronous versions (though the ::Sync class is provided as an argument by default). In this example, this completers are:

  • Interface::TicTacToe::StartGameCompleter::Sync
  • Interface::TicTacToe::StartGameCompleter::Async
  • Interface::TicTacToe::MakeMoveCompleter::Sync
  • Interface::TicTacToe::MakeMoveCompleter::Async

All completer classes provide the following methods:

  • void Close(zx_status_t status): Close the channel and send status as the epitaph.

In addition, two way methods will provide two versions of a Reply method for replying to a response: a managed variant and a caller-allocating variant. These correspond to the variants present in the client API. For example, both MakeMoveCompleter::Sync and MakeMoveCompleter::Async provide the following Reply methods:

  • void Reply(bool success, fidl::tracking_ptr<GameState> new_state)
  • void Reply(fidl::BytePart _buffer, bool success, fidl::tracking_ptr<GameState> new_state)

Finally, sync completers for two way methods can be coverted to an async completer using the ToAsync() method. Async completers can out-live the scope of the handler by e.g. moving it into a lambda capture (see LLCPP tutorial for example usage), allowing the server to respond to requests asynchronously. The async completer has the same methods for responding to the client as the sync completer.



In LLCPP, events can be handled asynchronously or synchronously, depending on the type of client being used.

When using a fidl::Client, events can be handled asynchronously by passing the class a TicTacToe::AsyncEventHandlers object. This class has the following members:

  • std::variant<fit::function<void(GameState new_state)>, fit::function<void(fidl::DecodedMessage<OnOpponentMoveResponse> msg)>> on_opponent_move: Handler for an event. Each handler can be in the managed variant or the caller-allocated variant.

For SyncClient and Call clients, events are handled synchronously by calling a HandleEvents function and passing it a TicTacToe::EventHandlers. EventHandlers is a struct that contains handlers for each type of event. In this example, it consists of the following members:

  • fit::callback<zx_status_t(GameState new_state)> on_opponent_move:
  • fit::callback<zx_status_t()> unknown:

There are two variants of the HandleEvents function available:

  • TicTacToe::SyncClient::HandleEvents(EventHandlers handlers): A bound version for sync clients.
  • TicTacToe::Call::HandleEvents(zx::unowned_channel client_end, EventHandlers handlers): An unbound version that also takes in an unowned_channel.


Sending events using a server binding object

When binding a server implementation to a channel, calling fidl::BindServer will return a fidl::ServerBindingRef<Protocol> which is the means by which you may interact safely with a server binding. This class allows access to an event sender interface through the following operators:

typename Protocol::EventSender* get() const;
typename Protocol::EventSender* operator->() const;
typename Protocol::EventSender& operator*() const;

where Protocol is a template parameter.

The EventSender class contains managed and caller-allocated methods for sending each event. As a concrete example, TicTacToe::EventSender provides the following methods:

  • zx_status_t OnOpponentMove(GameState new_state): Managed flavor.
  • zx_status_t OnOpponentMove(fidl::BytePart _buffer, GameState new_state): Caller allocated flavor.
Sending events with a bare-metal channel

The TicTacToe class provides static methods for sending events on a channel. Like the client Call APIs, these methods take an unowned_channel as the first argument, sending the event over this channel. Each event has managed and caller-allocating sender events, analogous to the client API as well as the server completers.

The event sender methods are:

  • static zx_status_t SendOnOpponentMoveEvent(zx::unowned_channel _chan, GameState new_state)
  • static zx_status_t SendOnOpponentMoveEvent(zx::unowned_channel _chan, fidl::BytePart _buffer, GameState new_state)


Given a method:

protocol TicTacToe {
    MakeMove(uint8 row, uint8 col) -> (GameState new_state) error MoveError;

FIDL will generate convenience methods on the completers corresponding to methods with an error type. Depending on the Reply "variant", the completer will have ReplySuccess, ReplyError, or both methods to respond directly with the success or error data, without having to construct a union.

For the managed flavor, both methods are available:

  • void ReplySuccess(GameState new_state)
  • void ReplyError(MoveError error)

Since ReplyError doesn't require heap allocation, only ReplySuccess exists for the caller-allocated flavor:

  • void ReplySuccess(fidl::BytePart _buffer, GameState new_state)

Note that the passed in buffer is used to hold the entire response, not just the data corresponding to the success variant.

The regularly generated Reply methods are available as well:

  • void Reply(TicTacToe_MakeMove_Result result): Owned variant.
  • void Reply(fidl::BytePart _buffer, TicTacToe_MakeMove_Result result): Caller-allocated variant.

The owned and caller-allocated variant use a parameter of TicTacToe_MakeMove_Result, which is generated as a union with two variants: Response, which is a TicTacToe_MakeMove_Response, and Err, which is a MoveError. TicTacToe_MakeMove_Response is generated as a struct with the response parameters as its fields. In this case, it has a single field new_state, which is a GameState.

Protocol composition

FIDL does not have a concept of inheritance, and generates full code as described above for all composed protocols. In other words, the code generated for

protocol A {

protocol B {
    compose A;

Provides the same API as the code generated for:

protocol A {

protocol B {

The generated code is identical except for the method ordinals.

Protocol and method attributes


For protocol methods annotated with the [Transitional] attribute, the virtual methods on the protocol class come with a default Close(ZX_NOT_SUPPORTED) implementation. This allows implementations of the protocol class with missing method overrides to compile successfully.


A protocol annotated with the [Discoverable] attribute causes the FIDL toolchain to generate an additional static const char Name[] field on the protocol class, containing the full protocol name.