LLCPP bindings


Given the library declaration:

library fuchsia.examples;

Bindings code for this library is generated in the llcpp::fuchsia::examples namespace.


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

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

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:

boolbool, (requires sizeof(bool) == 1)
array<T>:Nfidl::Array<T, N>
request<P>, Pzx::channel
handle:SThe 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 memory guide for information about tracking_ptr.

Type definitions


Given the bits definition:

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

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 READ
  • const static FileMode WRITE
  • const static FileMode EXECUTE
  • 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 fit::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 fit::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::EXECUTE.
  • 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.

Note: When applying bitwise negation to bits values that contain unknown members, the resulting bits value is only defined for the known bits.

Example usage:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/unittests/" region_tag="bits" adjust_indentation="auto" exclude_regexp="^TEST|^}" %}


Given the enum definition:

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

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 {
    MUSEUM = 1u;
    AIRPORT = 2u;
    RESTAURANT = 3u;

Example usage:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/unittests/" region_tag="enums" adjust_indentation="auto" exclude_regexp="^TEST|^}" %}

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 MUSEUM
  • const static LocationType AIRPORT
  • const static LocationType RESTAURANT


Given the struct declaration:

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

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:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/unittests/" region_tag="structs" adjust_indentation="auto" exclude_regexp="^TEST|^}" %}


Given the union definition:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="unions" %}

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.

Example usage:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/unittests/" region_tag="unions" adjust_indentation="auto" exclude_regexp="^TEST|^}" %}

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:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="tables" %}

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)

Example usage:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/unittests/" region_tag="tables" adjust_indentation="auto" exclude_regexp="^TEST|^}" %}

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

Note: Tables with unknown fields will decode successfully but will fail to encode.


Given the protocol:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="protocols" %}

Note: The MakeMove method above returns a bool representing success, and a nullable response value. This is considered un-idiomatic, you should use an error type instead.

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.


fidl::Client is thread-safe and supports both synchronous and asynchronous calls as well as asynchronous event handling. It also supports use with a multi-threaded dispatcher.


A client is created with a client-end zx::channel, an async_dispatcher_t* and an optional shared pointer on an AsyncEventHandler which defines the methods to be called when a FIDL event is received or when the client is unbound. If the virtual for a particular event is not overriden, the event is ignored.

class EventHandler : public TicTacToe::AsyncEventHandler {
  EventHandler() = default;

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

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

Client<TicTacToe> client;
zx_status_t status = client.Bind(
    std::move(client_end), dispatcher, std::make_shared<EventHandler>());

The channel may be unbound automatically in case of the server-end being closed or due to an invalid message being received from the server. You may also actively unbind the channel through client.Unbind().


Unbinding is thread-safe. In any of these cases, ongoing and future operations will not cause a fatal failure, only returning ZX_ERR_CANCELED where appropriate.

If you provided an unbound hook, it is executed as task on the dispatcher, providing a reason and error status for the unbinding. You may also recover ownership of the client end of the channel through the hook. The unbound hook is guaranteed to be run.

Interaction with dispatcher

All asynchronous responses, event handling, and error handling are done through the async_dispatcher_t* provided on creation of a client. With the exception of the dispatcher being shutdown, you can expect that all hooks provided to the client APIs will be executed on a dispatcher thread (and not nested within other user code).

Note: If you shutdown the dispatcher while there are any active bindings, the unbound hook may be executed on the thread executing shutdown. As such, you must not take any locks which could be taken by hooks provided to fidl::Client APIs while executing async::Loop::Shutdown()/async_loop_shutdown(). (You should probably ensure that no locks are held around shutdown anyway since it joins all dispatcher threads, which may take locks in user code).

Outgoing FIDL methods

You can invoke outgoing FIDL APIs through the fidl::Client instance. Dereferencing a fidl::Client 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(TicTacToeResponse* 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::StatusAndError 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 SyncClient.
  • UnownedResultOf::MakeMove_sync(fidl::BufferSpan _request_bufffer, uint8_t row, uint8_t col, fidl::BufferSpan _response_buffer): Synchronous, caller-allocated variant of a two way method. The same method exists on SyncClient.

Note: One-way and synchronous two-way FIDL methods have a similar API to the SyncClient versions. Aside from one-way methods directly returning fidl::StatusAndError and the added _Sync on the synchronous methods, the behavior is identical.

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.

Note: If the client is destroyed with outstanding asynchronous transactions, OnError() will be invoked for all of the associated ResponseContexts


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. 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.


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::BufferSpan _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::BufferSpan _request_buffer, uint8_t row, uint8_t col, fidl::BufferSpan _response_buffer);:

Result, ResultOf and UnownedResultOf [#resultof]

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 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.
  • 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;

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 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_incoming_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_incoming_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:

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

Note: Each Completer object must only be accessed by one thread at a time. Simultaneous access from multiple threads will result in a crash.

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.

Note: This use-case is currently possible only using the lib/fidl bindings.

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
fidl::Buffer<StartGameRequest> request_buffer;
auto result = client.StartGame(request_buffer.view(), true);

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

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

Note: Buffers passed to the bindings must be aligned to 8 bytes. The fidl::Buffer helper class does this automatically. Failure to align would result in a run-time error.


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

Async client

When using a fidl::Client, events can be handled asynchronously by passing the class a std::shared_ptr<TicTacToe::AsyncEventHandler>. The AsyncEventHandler class has the following members:

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

  • virtual Unbound(::fidl::UnbindInfo info) {}: method called when the client has been unbound.

To be able to handle events and unbound, a class which 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 TicTacToe::SyncEventHandler.

SyncEventHandler 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 overriden only if a specific status is needed.

To be able to handle events, a class which 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(zx::unowned_channel client_end): An unbound version that uses an unowned_channel 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 bare-metal channel

Note: Sending events using a server binding object should be preferred whenever possible. Using the methods listed below may introduce a race condition between unbinding the server connection and sending some final events on the same 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::BufferSpan _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::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(::llcpp::fuchsia::examples::User& user) {
  ::fidl::OwnedEncodedMessage<::llcpp::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()
  • ::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<::llcpp::fuchsia::examples::User> decoded(, static_cast<uint32_t>(buffer.size()));
  if (!decoded.ok()) {
    // Display an error.
  ::llcpp::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()
  • FidlType* decoded.PrimaryObject()
  • void decoded.ReleasePrimaryObject()

The FIDL type is the type used by the templated class (in the example above: ::llcpp::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 which 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.