LLCPP bindings


Given the library declaration:

library fuchsia.examples;

Protocol types are generated in the fuchsia_examples namespace. Domain objects for this library are generated in the fuchsia_examples::wire namespace, and test scaffolding is generated in the fuchsia::examples::testing namespace.

Generated type names are transformed to follow the Google C++ style guide.


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

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

Are generated in the header file as:

constexpr uint8_t kBoardSize = 9u;
extern const char[] kName;

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
client_end:P fidl::ClientEnd<P>
server_end:P fidl::ServerEnd<P>
zx.handle zx::handle
zx.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::ObjectView of the generated type except for unions, which simply use the generated type itself. Refer to the LLCPP memory guide for information about ObjectView.

Type definitions


Given the bits definition:

type FileMode = strict bits : 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 kMask member that contains a mask of all bits members (in this example 0b111):

  • const static FileMode kRead
  • const static FileMode kWrite
  • const static FileMode kExecute
  • const static FileMode kMask

FileMode provides the following methods:

  • explicit constexpr FileMode(uint16_t): Constructs a value from an underlying primitive value, preserving any unknown bit members.
  • constexpr static cpp17::optional<FileMode> TryFrom(uint16_t value): Constructs an instance of the bits from an underlying primitive value if the value does not contain any unknown members, and returns cpp17::nullopt otherwise.
  • constexpr static FileMode TruncatingUnknown(uint16_t value): Constructs an instance of the bits from an underlying primitive value, clearing any unknown members.
  • Bitwise operators: Implementations for the |, |=, &, &=, ^, ^=, and ~ operators are provided, allowing bitwise operations on the bits like mode |= FileMode::kExecute.
  • Comparison operators == and !=.
  • Explicit conversion functions for uint16_t and bool.

If FileMode is flexible, it will have the following additional methods:

  • constexpr FileMode unknown_bits() const: Returns a bits value that contains only the unknown members from this bits value.
  • constexpr bool has_unknown_bits() const: Returns whether this value contains any unknown bits.

Example usage:

auto flags = FileMode::kRead | FileMode::kWrite | FileMode::kExecute;
ASSERT_EQ(flags, FileMode::kMask);


Given the enum definition:

type LocationType = strict enum {
    MUSEUM = 1;
    AIRPORT = 2;

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

enum class LocationType : uint32_t {
    kMuseum = 1u;
    kAirport = 2u;
    kRestaurant = 3u;

Example usage:

ASSERT_EQ(static_cast<uint32_t>(fuchsia_examples::wire::LocationType::kMuseum), 1u);

Flexible enums

Flexible enums are implemented as a class instead of an enum class, with the following methods:

  • constexpr LocationType(): Default constructor, which initializes the enum to an unspecified unknown value.
  • constexpr LocationType(uint32_t value): Explicit constructor that takes in a value of the underlying type of the enum.
  • constexpr bool IsUnknown(): Returns whether the enum value is unknown.
  • constexpr static LocationType Unknown(): Returns an enum value that is guaranteed to be treated as unknown. If the enum has a member annotated with [Unknown], then the value of that member is returned. If there is no such member, then the underlying value of the returned enum member is unspecified.
  • explicit constexpr operator int32_t() const: Converts the enum back to its underlying value.

The generated class contains a static member for each enum member, which are guaranteed to match the members of the enum class in the equivalent strict enum:

  • const static LocationType kMuseum
  • const static LocationType kAirport
  • const static LocationType kRestaurant


Given the struct declaration:

type Color = struct {
    id uint32;
    name string:MAX_STRING_LENGTH = "red";

The FIDL toolchain generates an equivalent struct:

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

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

Example usage:

fuchsia_examples::wire::Color default_color;
ASSERT_EQ(, 0u);
// Default values are currently not supported.

fuchsia_examples::wire::Color blue = {1, "blue"};
ASSERT_EQ(, 1u);


Given the union definition:

type JsonValue = strict union {
    1: reserved;
    2: int_value int32;
    3: string_value string:MAX_STRING_LENGTH;

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::ObjectView<int32>) and static JsonValue WithStringValue(fidl::ObjectView<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::ObjectView<int32_t> value) and void set_string_value(fidl::ObjectView<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.

Example usage:

fidl::Arena allocator;
auto int_val = fuchsia_examples::wire::JsonValue::WithIntValue(1);
auto str_val = fuchsia_examples::wire::JsonValue::WithStringValue(allocator, "1");

Flexible unions and unknown variants

Flexible unions have an extra variant in the generated Tag class:

  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.

The LLCPP bindings do not store the raw bytes and handles of unknown variants.

Encoding a union with an unknown variant is not supported and will cause an encoding failure.


Given the table definition:

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

The FIDL toolchain User class with the following methods:

  • User(): Default constructor, initializes with all fields unset.
  • User(::fidl::AnyArena& allocator): Constructor which allocates the frame but 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& mutable_age() and fidl::StringView& mutable_name(): Mutable field accessor methods. Calling these methods without first setting the variant leads to an assertion error.
  • void set_age(::fidl::ObjectView<uint8>): set age an already allocated value.
  • void set_age(::fidl::AnyArena&, uint8_t): set age with the given value. The allocator is used to allocate the storage.
  • void set_age(std::nullptr_t): unset age.
  • void set_name(::fidl::ObjectView<::fidl::StringView>): set name with an already allocated value.
  • void set_name(::fidl::AnyArena&, ::fidl::AnyArena&, std::string_view): set name with the given value. The storage for the storage of the value (StringView) and the storage of the string are allocated using the two allocators. The same allocator should be given to the two allocator arguments.
  • void set_name(std::nullptr_t): unset name.

In order to build a table, one additional class is generated: User::Frame.

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 is only use internally by User(::fidl::AnyArena&).

User::Frame has the following methods:

  • Frame(): Default constructor.

Example usage:

fidl::Arena allocator;
fuchsia_examples::wire::User user(allocator);
user.set_name(allocator, allocator, "jdoe");
ASSERT_EQ(user.age(), 30);

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

Inline layouts

The generated C++ code uses the the name reserved by fidlc for inline layouts.

LLCPP also generates scoped names to refer to any inline layouts that were defined directly within a parent layout in FIDL. For example, for the FIDL:

type Outer = struct {
  inner struct {};

The inner struct can be referred to using its globally unique name Inner as well as the scoped name Outer::Inner. This can be useful when the top level name is overridden using the @generated_name attribute, for example in:

type Outer = struct {
  @generated_name("SomeCustomName") struct {};

the inner struct can be referred to as SomeCustomName or Outer::Inner.

Another example of this is the protocol result types: the success and error variants of a type such as TicTacToe_MakeMove_Result can be referenced as TicTacToe_MakeMove_Result::Response and TicTacToe_MakeMove_Result::Err, respectively.


Given the protocol:

protocol TicTacToe {
    StartGame(struct {
        start_first bool;
    MakeMove(struct {
        row uint8;
        col uint8;
    }) -> (struct {
        success bool;
        new_state box<GameState>;
    -> OnOpponentMove(struct {
        new_state GameState;

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.

Typed channel endpoints

LLCPP sends and receives FIDL protocol messages over the Zircon channel transport, which carry arbitrary blobs of bytes and handles. Rather than exposing raw endpoints, for instance zx::channel, the API exposes three templated endpoint classes:

  • fidl::ClientEnd<TicTacToe>: the client endpoint of the TicTacToe protocol; it owns a zx::channel. Client bindings that require exclusive ownership of the channel would consume this type. For example, a fidl::WireClient<TicTacToe> may be constructed from a fidl::ClientEnd<TicTacToe>, also known as "binding the channel to the message dispatcher".
  • fidl::UnownedClientEnd<TicTacToe>: an unowned value borrowing some client endpoint of the TicTacToe protocol. Client APIs that do not require exclusive ownership of the channel would take this type. An UnownedClientEnd may be derived from a ClientEnd of the same protocol type by calling borrow(). The borrowed-from endpoint may be std::move-ed within the same process, but cannot be dropped or transferred out-of-process, while there are unowned borrows to it.
  • fidl::ServerEnd<TicTacToe>: the server endpoint of the TicTacToe protocol; it owns a zx::channel. Server bindings that require exclusive ownership of the channel would consume this type. For example, a fidl::ServerEnd<TicTacToe> may be provided to fidl::BindServer<TicTacToe> among other parameters to create a server binding.

There is no UnownedServerEnd as it is not yet needed to safely implement the current set of features.

A pair of client and server endpoint may be created using the ::fidl::CreateEndpoints<TicTacToe> library call. In a protocol request pipelining scenario, one can immediately start performing operations on the client endpoint after std::move()-ing the server endpoint to the remote server.

See the class documentation on these types for more details.

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::WireClient<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. It must be used with a single-threaded dispatcher. Objects of this class must be bound to the client endpoint and destroyed on the same thread that is running the dispatcher. This is the recommended variant for most use cases, except for those where an async_dispatcher_t cannot be used or when the client needs to be moved between threads.
  • fidl::WireSharedClient<TicTacToe>: This class has less opinions on threading models compared to WireClient, but requires a two-phase shutdown pattern to prevent use-after-frees. Objects of this class may be destroyed on an arbitrary thread. It also supports use with a multi-threaded dispatcher. For more details, see LLCPP threading guide.
  • fidl::WireSyncClient<TicTacToe>: This class exposes purely synchronous APIs for outgoing calls as well as for event handling. It owns the client end of the channel.
  • fidl::WireCall<TicTacToe>: This class is identical to WireSyncClient except that it does not have ownership of the client end of the channel. WireCall may be preferable to WireSyncClient when migrating code from the C bindings to the LLCPP bindings, or when implementing C APIs that take raw zx_handle_ts.


fidl::WireClient is thread-safe and supports both synchronous and asynchronous calls as well as asynchronous event handling.


A client is created with a client-end fidl::ClientEnd<P> to the protocol P, an async_dispatcher_t*, and an optional pointer to an AsyncEventHandler that defines the methods to be called when a FIDL event is received or when the client is unbound. If the virtual method for a particular event is not overridden, the event is ignored.

class EventHandler : public fidl::WireAsyncEventHandler<TicTacToe> {
  EventHandler() = default;

  void OnOpponentMove(OnOpponentMoveResponse* event) override { /* ... */ }

  void on_fidl_error(fidl::UnbindInfo unbind_info) override { /* ... */ }

fidl::ClientEnd<TicTacToe> client_end = /* logic to connect to the protocol */;
EventHandler event_handler;
WireClient<TicTacToe> client;
zx_status_t status = client.Bind(
    std::move(client_end), dispatcher, &event_handler);

The binding may be torn down automatically in case of the server-end being closed or due to an invalid message being received from the server. You may also actively tear down the bindings by destroying the client object.

Outgoing FIDL methods

You can invoke outgoing FIDL APIs through the fidl::WireClient instance. Dereferencing a fidl::WireClient provides access to the following methods:

  • fidl::Result StartGame(bool start_first): Managed variant of a fire and forget method.
  • fidl::Result StartGame(::fidl::BufferSpan _request_buffer, bool start_first): Caller-allocated variant of a fire and forget method.
  • fidl::Result MakeMove(uint8_t row, uint8_t col, fit::callback<void(fidl::WireResponse<TicTacToe::MakeMove>* response)> _cb): Managed variant of an asynchronous two way method. It takes a callback to handle responses as the last argument. The callback is executed on response in a dispatcher thread. The returned fidl::Result refers just to the status of the outgoing call.
  • fidl::Result MakeMove(fidl::BufferSpan _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 WireSyncClient.
  • UnownedResultOf::MakeMove_Sync(fidl::BufferSpan _request_buffer, uint8_t row, uint8_t col, fidl::BufferSpan _response_buffer): Synchronous, caller-allocated variant of a two way method. The same method exists on WireSyncClient.

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 overridden to handle responses:

virtual void OnResult(fidl::WireUnownedResult<MakeMove>& result) = 0;
virtual void OnCanceled() = 0;

Exactly one of the two methods is called for a single response: OnResult is called with a result object either representing a successfully decoded response or an error that is specific to this call. OnCanceled is called when the two-way call is canceled due to unrelated errors or when the user destroys the client object. You are responsible for ensuring that the response context object outlives the duration of the entire async call, since the fidl::WireClient borrows the context object by address to avoid implicit allocation.

Centralized error handler

When the binding is torn down due to an error, fidl::AsyncEventHandler<TicTacToe>::on_fidl_error will be invoked from the dispatcher thread with the detailed reason. When the error is dispatcher shutdown, on_fidl_error will be invoked from the thread that is calling dispatcher shutdown. It is recommended to put any central logic for logging or releasing resources in that handler.


fidl::WireSyncClient<TicTacToe> is a synchronous client which provides the following methods:

  • explicit WireSyncClient(fidl::ClientEnd<TicTacToe>): Constructor.
  • ~WireSyncClient(): Default destructor.
  • WireSyncClient(&&): Default move constructor.
  • WireSyncClient& operator=(WireSyncClient&&): Default move assignment.
  • fidl::ClientEnd<TicTacToe>& client_end(): Returns a mutable reference to the underlying client endpoint.
  • const fidl::ClientEnd<TicTacToe>& client_end() const: Returns the underlying client endpoint as a const.
  • const zx::channel& channel() const: Returns the underlying channel as a const. Prefer using the client_end() accessors for improved type-safety.
  • 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. Buffer allocation for requests and responses are entirely handled within this function, as is the case in simple C bindings. The bindings calculate a safe buffer size specific to this call at compile time based on FIDL wire-format and maximum length constraints. The buffers are allocated on the stack if they fit under 512 bytes, or else on the heap. In general, the managed flavor is easier to use, but may result in extra allocation. See ResultOf for details on buffer management.
  • TicTacToe::UnownedResultOf::StartGame StartGame(fidl::BufferSpan, 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::BufferSpan _request_buffer, uint8_t row, uint8_t col, fidl::BufferSpan _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.
  • fidl::Result HandleOneEvent(SyncEventHandler& event_handler): 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.


fidl::WireCall<TicTacToe> provides similar methods to those found in WireSyncClient, with the only difference being that WireCall can be constructed with a fidl::UnownedClientEnd<TicTacToe> i.e. it borrows the client endpoint:

  • ResultOf::StartGame StartGame(bool start_first): Owned variant of StartGame.
  • UnownedResultOf::StartGame StartGame(fidl::BufferSpan _request_buffer, bool start_first): Caller-allocated variant of StartGame.
  • ResultOf::MakeMove MakeMove(uint8_t row, uint8_t col): Owned variant of MakeMove.
  • UnownedResultOf::MakeMove MakeMove(fidl::BufferSpan _request_buffer, uint8_t row, uint8_t col, fidl::BufferSpan _response_buffer);: Caller-allocated variant of MakeMove.

Result, ResultOf and UnownedResultOf

The managed variants of each method of WireSyncClient and WireCall all return a ResultOf:: type, whereas the caller-allocating variants all return an UnownedResultOf::. Fire and forget methods on fidl::WireClient return a Result. 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.
  • fidl::Reason reason() const returns details about which operation failed, when status() is not ZX_OK. For example, if encoding failed, reason() will return fidl::Reason::kEncodeError. reason() should not be called when status is ZX_OK.
  • const char* error_message() 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;

ResultOf manages ownership of all buffer and handles, while ::Unwrap() returns a view over it. Therefore, this object must outlive any references to the unwrapped response.

Allocation strategy And move semantics

ResultOf:: stores the response buffer inline if the message is guaranteed to fit under 512 bytes. Since the result object is usually instantiated on the caller's stack, this effectively means the response is stack-allocated when it is reasonably small. If the maximal response size exceeds 512 bytes, ResultOf:: instead contains a std::unique_ptr to a heap-allocated buffer.

Therefore, a std::move() on ResultOf::Foo may be costly if the response buffer is inline: the content has to be copied, and pointers to out-of-line objects have to be updated to locations within the destination object. Consider the following snippet:

int CountPlanets(ResultOf::ScanForPlanets result) { /* ... */ }

auto result = client->ScanForPlanets();
SpaceShip::ScanForPlanetsResponse* response = result.Unwrap();
Planet* planet = &response->planets[0];
int count = CountPlanets(std::move(result));    // Costly
// In addition, |response| and |planet| are invalidated due to the move

It may be written more efficiently as:

int CountPlanets(fidl::VectorView<SpaceShip::Planet> planets) { /* ... */ }

auto result = client.ScanForPlanets();
int count = CountPlanets(result.Unwrap()->planets);

If the result object need to be passed around multiple function calls, consider pre-allocating a buffer in the outer-most function and use the caller-allocating flavor.


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

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

  • virtual void StartGame(StartGameRequestView request, StartGameCompleter::Sync _completer)
  • virtual void MakeMove(MakeMoveRequestView request, 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, fidl::WireDispatch<TicTacToe>:

  • void fidl::WireDispatch<TicTacToe>(fidl::WireServer<TicTacToe>* impl, fidl::IncomingMessage&& msg, ::fidl::Transaction* txn): Dispatches the incoming message. If there is no matching handler, it closes all handles in the message and notifies txn of an error.


The request is provided as the first argument of each generated FIDL method handler. This a view of the request (a pointer). All the request arguments are accessed using the arrow operator and the argument name.

For example: * request->start_first * request->row

See LLCPP memory guide for notes on request lifetime.


A completer is provided 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:

  • fidl::WireServer<TicTacToe>::StartGameCompleter::Sync
  • fidl::WireServer<TicTacToe>::StartGameCompleter::Async
  • fidl::WireServer<TicTacToe>::MakeMoveCompleter::Sync
  • fidl::WireServer<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:

  • ::fidl::Result Reply(bool success, fidl::ObjectView<GameState> new_state)
  • ::fidl::Result Reply(fidl::BufferSpan _buffer, bool success, fidl::ObjectView<GameState> new_state)

Because the status returned by Reply is identical to the unbinding status, it can be safely ignored.

Finally, sync completers for two way methods can be converted 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.

Parallel message handling

By default, messages from a single binding are handled sequentially, i.e. a single thread attached to the dispatcher (run loop) is woken up if necessary, reads the message, executes the handler, and returns back to the dispatcher. The ::Sync completer provides an additional API, EnableNextDispatch(), which may be used to selectively break this restriction. Specifically, a call to this API will enable another thread waiting on the dispatcher to handle the next message on the binding while the first thread is still in the handler. Note that repeated calls to EnableNextDispatch() on the same Completer are idempotent.

void DirectedScan(int16_t heading, ScanForPlanetsCompleter::Sync& completer) override {
  // Suppose directed scans can be done in parallel. It would be suboptimal to block one scan until
  // another has completed.
  fidl::VectorView<Planet> discovered_planets = /* perform a directed planet scan */;

Caller-allocated methods

A number of the APIs above provide owned and caller-allocated variants of generated methods.

The caller-allocated variant defers all memory allocation responsibilities to the caller. The type fidl::BufferSpan references a buffer address and size. It will be used by the bindings library to construct the FIDL request, hence it must be sufficiently large. The method parameters (e.g. heading) are linearized to appropriate locations within the buffer. There are a number of ways to create the buffer:

// 1. On the stack
using StartGame = TicTacToe::StartGame;
fidl::SyncClientBuffer<StartGame> buffer;
auto result = client.StartGame(buffer.view(), true);

// 2. On the heap
auto buffer = std::make_unique<fidl::SyncClientBuffer<StartGame>>();
auto result = client.StartGame(buffer->view(), true);

// 3. Some other means, e.g. thread-local storage
constexpr uint32_t buffer_size = fidl::SyncClientMethodBufferSizeInChannel<StartGame>();
uint8_t* buffer = allocate_buffer_of_size(buffer_size);
fidl::BufferSpan buffer_span(/* data = */buffer, /* capacity = */request_size);
auto result = client.StartGame(buffer_span, true);

// Check the transport status (encoding error, channel writing error, etc.)
if (result.status() != ZX_OK) {
  // Handle error...

// Don't forget to free the buffer at the end if approach #3 was used...

When the caller-allocating flavor is used, the result object borrows the request and response buffers (hence its type is under UnownedResultOf). Make sure the buffers outlive the result object. See UnownedResultOf.


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

Async client

When using a fidl::WireClient, events can be handled asynchronously by passing the class a TicTacToe::AsyncEventHandler*. The AsyncEventHandler class has the following members:

  • virtual void OnOpponentMove(OnOpponentMoveResponse* message) {}: handler for the OnOpponentMove event (one method per event).

  • virtual on_fidl_error(::fidl::UnbindInfo info) {}: method called when the client encounters a terminal error.

To be able to handle events and errors, a class that inherits from AsyncEventHandler must be defined.

Sync client

For SyncClient and Call clients, events are handled synchronously by calling a HandleOneEvent function and passing it a fidl::WireSyncEventHandler<TicTacToe>.

WireSyncEventHandler is a class that contains a pure virtual method for each event. In this example, it consists of the following member:

  • virtual void OnOpponentMove(TicTacToe::OnOpponentMoveResponse* event) = 0: The handle for the OnOpponentMove event.
  • virtual zx_status_t Unknown() { return ZX_ERR_NOT_SUPPORTED; }: The status to be returned by HandleOneEvent if an unknown event is found. This method should be overridden only if a specific status is needed.

To be able to handle events, a class that inherits from SyncEventHandler must be defined. This class must define the virtual methods for the events it wants to handle. All the other events are ignored. Then an instance of this class must be allocated.

There are two ways to handle one event. Each one use an instance of the user defined event handler class:

  • ::fidl::Result TicTacToe::SyncClient::HandleOneEvent( SyncEventHandler& event_handler): A bound version for sync clients.
  • ::fidl::Result TicTacToe::SyncEventHandler::HandleOneEvent( fidl::UnownedClientEnd<TicTacToe> client_end): An unbound version that uses an fidl::UnownedClientEnd<TicTacToe> to handle one event for a specific handler.

For each call to HandleOneEvent, the method waits on the channel for exactly one incoming message. Then the message is decoded. If the result is ZX_OK then exactly one virtual method has been called. If not no virtual method has been called and the status indicates the error.

If the handlers are always the same (from one call to HandleOneEvent to the other), the SyncEventHandler object should be constructed once and used each time you need to call HandleOneEvent.

If an event is marked as transitional, then a default implementation is provided (instead of the pure virtual).


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::BufferSpan _buffer, GameState new_state): Caller allocated flavor.
Sending events with a ServerEnd object

Sending events using a server binding object is the standard approach to sending events while the server endpoint is bound to an implementation. However, there may be occasions which call for sending events on a fidl::ServerEnd<TicTacToe> object directly, without setting up a server binding.

The TicTacToe class contains an EventSender which provides methods for sending events on a channel. The EventSender may be constructed from a fidl::ServerEnd<TicTacToe>. Each event sending method has managed and caller-allocating variants, analogous to the client API as well as the server completers.

The event sender methods are:

class TicTacToe::EventSender {
  EventSender(fidl::ServerEnd<TicTacToe> server_end);

  zx_status_t SendOnOpponentMoveEvent(
      GameState new_state);
  zx_status_t SendOnOpponentMoveEvent(
      fidl::BufferSpan _buffer, GameState new_state);

  const fidl::ServerEnd<TicTacToe>& server_end() const;
  fidl::ServerEnd<TicTacToe>& server_end();

EventSender consumes the server endpoint upon construction, to ensure that the channel stays alive while events are being written. After sending events, the user may deconstruct the EventSender by extracting the server endpoint via server_end().


Given a method:

protocol TicTacToe {
    MakeMove(struct {
      row uint8;
      col uint8;
    }) -> (struct {
      new_state GameState;
    }) 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::BufferSpan _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::BufferSpan _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.

Explicit encoding and decoding

FIDL messages are automatically encoded when they are sent and decoded when they are received.

However, some use cases like persistence need to explicitly encode or decode a table or struct.

This section describes how to explicitly use the encoding and the decoding.


When an object is allocated and initialized, fidl::OwnedEncodedMessage<FidlType> can be used to encode it. For example:

void Encode(::fuchsia_examples::User& user) {
  ::fidl::OwnedEncodedMessage<::fuchsia_examples::User> encoded(&user);
  if (!encoded.ok()) {
    // Do something about the error.
  fidl_outgoing_msg_t* message = encoded.GetOutgoingMessage().message();
  // Do something with the data referenced by message.

At this point, the table user is encoded within encoded. The following methods are available on an encoded FIDL type:

  • bool encoded.ok()
  • zx_status_t encoded.status()
  • const char* encoded.error_message()
  • ::fidl::OutgoingMessage& encoded.GetOutgoingMessage()

::fidl::OutgoingMessage is defined in /zircon/system/ulib/fidl/include/lib/fidl/llcpp/message.h.


Once an object has been encoded (and eventually stored somewhere), fidl::DecodedMessage<FidlType> can be used to decode it. For example:

void UseEncodedUser(std::vector<uint8_t> buffer) {
  fidl::DecodedMessage<::fuchsia_examples::User> decoded(, static_cast<uint32_t>(buffer.size()));
  if (!decoded.ok()) {
    // Display an error.
  ::fuchsia_examples::User* user = decoded.PrimaryObject();
  // Do something with the table (user).

When an object is decoded, the following methods are available on a decoded FIDL type:

  • bool decoded.ok()
  • zx_status_t decoded.status()
  • const char* decoded.error_message()
  • FidlType* decoded.PrimaryObject()
  • void decoded.ReleasePrimaryObject()

The FIDL type is the type used by the templated class (in the example above: ::fuchsia_examples::User).

The primary object is decoded in place within the provided buffer. This is also the case of all the secondary objects. That means that the provided buffer must be kept alive while the decoded value is used.

For FIDL types that allow handles, the handles can be specified during construction after the bytes (the same way bytes are specified).


Persistence is not officially supported by LLCPP. However, explicit encoding and decoding can be used to store FIDL values by encoding a value and then writing it and by reading a value and then decoding it. In that case, the values can't use any handle.

Test scaffolding

The FIDL toolchain also generates a file suffixed with _test_base.h that contains convenience code for testing FIDL server implementations. This file contains a class for each protocol that provides stub implementations for each of the class’s methods, making it possible to implement only the methods that are used during testing. These classes are generated into a testing namespace that is inside of the generated library’s namespace (e.g. for library games.tictactoe, these classes are generated into games::tictactoe::testing).

For the same TicTacToe protocol listed above, the FIDL toolchain generates a TicTacToe_TestBase class that subclasses TicTacToe (see Protocols), offering the following methods:

  • virtual ~TicTacToe_TestBase() {}: Destructor.
  • virtual void NotImplemented_(const std::string& name, ::fidl::CompleterBase& completer) = 0: Pure virtual method that is overridden to define behavior for unimplemented methods.

TicTacToe_TestBase provides an implementation for the virtual protocol methods StartGame and MakeMove, which are implemented to just call NotImplemented_("StartGame", completer) and NotImplemented_("MakeMove", completer), respectively.