Implement an LLCPP FIDL server

Prerequisites

This tutorial builds on the Compiling FIDL tutorial. For the full set of FIDL tutorials, refer to the overview.

Overview

This tutorial shows you how to implement a FIDL protocol (fuchsia.examples.Echo) and run it on Fuchsia. This protocol has one method of each kind: a fire and forget method, a two-way method, and an event:

@discoverable
protocol Echo {
    EchoString(struct {
        value string:MAX_STRING_LENGTH;
    }) -> (struct {
        response string:MAX_STRING_LENGTH;
    });
    SendString(struct {
        value string:MAX_STRING_LENGTH;
    });
    -> OnString(struct {
        response string:MAX_STRING_LENGTH;
    });
};

For more on FIDL methods and messaging models, refer to the FIDL concepts page.

This document covers how to complete the following tasks:

Implement a FIDL protocol.
Build and run a package on Fuchsia.
Serve a FIDL protocol.

The tutorial starts by creating a component that is served to a Fuchsia device and run. Then, it gradually adds functionality to get the server up and running.

If you want to write the code yourself, delete the following directories:

rm -r examples/fidl/llcpp/server/*

Create the component

To create a component:

Add a main() function to examples/fidl/llcpp/server/main.cc:

#include <iostream>

int main(int argc, const char** argv) {
  std::cout << "Hello, world!" << std::endl;
  return 0;
}

Declare a target for the server in examples/fidl/llcpp/server/BUILD.gn:

import("//build/components.gni")

# Declare an executable for the server. This produces a binary with the
# specified output name that can run on Fuchsia.
executable("bin") {
  output_name = "fidl_echo_llcpp_server"
  sources = [ "main.cc" ]
}

# Declare a component for the server, which consists of the manifest and the
# binary that the component will run.
fuchsia_component("echo-server") {
  component_name = "echo_server"
  manifest = "meta/server.cml"
  deps = [ ":bin" ]
}

# Declare a package that contains a single component, our server.
fuchsia_package("echo-llcpp-server") {
  package_name = "echo-llcpp-server"
  deps = [ ":echo-server" ]
}

To get the server component up and running, there are three targets that are defined:

The raw executable file for the server that is built to run on Fuchsia.
A component that is set up to simply run the server executable, which is described using the component's manifest file.
The component is then put into a package, which is the unit of software distribution on Fuchsia. In this case, the package just contains a single component.

For more details on packages, components, and how to build them, refer to the Building components page.

Add a component manifest in examples/fidl/llcpp/server/meta/server.cml:

{
    include: [
        "syslog/client.shard.cml",
        "syslog/elf_stdio.shard.cml",
    ],

    // Information about the program to run.
    program: {
        // Use the built-in ELF runner.
        runner: "elf",

        // The binary to run for this component.
        binary: "bin/fidl_echo_llcpp_server",
    },

    // Capabilities provided by this component.
    capabilities: [
        { protocol: "fuchsia.examples.Echo" },
    ],
    expose: [
        {
            protocol: "fuchsia.examples.Echo",
            from: "self",
        },
    ],
}

Add the server to your build configuration:
```
fx set core.qemu-x64 --with //examples/fidl/llcpp/server:echo-llcpp-server
```
Note: This build configuration assumes your device target is the emulator. To run the example on a physical device, select the appropriate product configuration for your hardware.
Build the Fuchsia image:
```
fx build
```

Implement the server

Add a dependency on the FIDL library

Add the fuchsia.examples FIDL library target as a dependency of your executable in examples/fidl/llcpp/server/BUILD.gn:

executable("bin") {
  output_name = "fidl_echo_llcpp_server"
  sources = [ "main.cc" ]
  deps = [ "//examples/fidl/fuchsia.examples:fuchsia.examples_llcpp" ]
}

Import the LLCPP bindings at the top of examples/fidl/llcpp/server/main.cc:
```
#include <fidl/fuchsia.examples/cpp/wire.h>
```

Add an implementation for the protocol

Add the following to main.cc, above the main() function:

// An implementation of the Echo protocol. Protocols are implemented in LLCPP by
// creating a subclass of the ::Interface class for the protocol.
class EchoImpl final : public fidl::WireServer<fuchsia_examples::Echo> {
 public:
  // Bind this implementation to a channel.
  EchoImpl(async_dispatcher_t* dispatcher, fidl::ServerEnd<fuchsia_examples::Echo> request)
      : binding_(fidl::BindServer(dispatcher, std::move(request), this,
                                  // This is a fidl::OnUnboundFn<EchoImpl>.
                                  [this](EchoImpl* impl, fidl::UnbindInfo info,
                                         fidl::ServerEnd<fuchsia_examples::Echo> server_end) {
                                    if (info.is_peer_closed()) {
                                      std::cout << "Client disconnected" << std::endl;
                                    } else if (!info.is_user_initiated()) {
                                      std::cerr << "server error: " << info << std::endl;
                                    }
                                    delete this;
                                  })) {}

  // Handle a SendString request by sending on OnString "event" (an unsolicited server-to-client
  // message) back on the same channel.
  //
  // For fire-and-forget methods like this one, the completer is normally not used but its
  // Close(zx_status_t) method can be used to close the channel (either if the connection is "done"
  // or it encountered an unrecoverable error).
  void SendString(SendStringRequestView request, SendStringCompleter::Sync& completer) override {
    fidl::Status status = fidl::WireSendEvent(binding_)->OnString(request->value);
    ZX_ASSERT(status.ok());
  }

  // Handle an EchoString request by responding with the request value. For two-way methods (those
  // with a response) like this one, the completer is used to send the response.
  void EchoString(EchoStringRequestView request, EchoStringCompleter::Sync& completer) override {
    completer.Reply(request->value);
  }

 private:
  // A reference back to the Binding that this class is bound to, which is used
  // to send events to the client.
  fidl::ServerBindingRef<fuchsia_examples::Echo> binding_;
};

The implementation contains the following elements:

The class subclasses the generated protocol class and overrides its pure virtual methods corresponding to the protocol methods.
It contains a ServerBindingRef in order to be able to send events to the client.
The constructor method binds the implementation to a given request.
The method for EchoString replies synchronously with the request value by using the completer (for asynchronous replies, see responding to requests asynchronously in LLCPP).
The method for SendString uses the binding_ member (if defined) to send an OnString event containing the request value.

You can verify that the implementation builds by running:

fx build

Serve the protocol

When running a component that implements a FIDL protocol, you must make a request to the component manager to expose that FIDL protocol to other components. The component manager then routes any requests for the echo protocol to our server.

To fulfill these requests, the component manager requires the name of the protocol as well as a handler that it should call when it has any incoming requests to connect to a protocol matching the specified name.

The handler passed to it is a function that takes a channel (whose remote end is owned by the client), and binds it to our server implementation. The resulting fidl::ServerBindingRef is reference to a server binding that takes a FIDL protocol implementation and a channel, and then listens on the channel for incoming requests. The binding then decodes the requests, dispatches them to the correct method on our server class, and writes any response back to the client. Our main method will keep listening for incoming requests on an async loop.

This complete process is described in further detail in the Life of a protocol open.

Add new dependencies

This new code requires the following additional dependencies:

"//zircon/system/ulib/async-loop:async-loop-cpp" and "//zircon/system/ulib/async-loop:async-loop-default": These libraries contain the async loop code.
"//sdk/lib/fdio" and "//zircon/system/ulib/svc": These libraries are used to interact with the components environment (e.g. for serving protocols).

Add the library targets as dependencies of your executable in examples/fidl/llcpp/server/BUILD.gn:

executable("bin") {
  output_name = "fidl_echo_llcpp_server"
  sources = [ "main.cc" ]
  deps = [
    "//examples/fidl/fuchsia.examples:fuchsia.examples_llcpp",
    "//sdk/lib/fdio",
    "//sdk/lib/stdcompat",
    "//zircon/system/ulib/async-loop:async-loop-cpp",
    "//zircon/system/ulib/async-loop:async-loop-default",
    "//zircon/system/ulib/svc",
  ]
}

Import these dependencies at the top of examples/fidl/llcpp/server/main.cc:

#include <lib/async-loop/cpp/loop.h>
#include <lib/async-loop/default.h>
#include <lib/fdio/directory.h>
#include <lib/fidl/llcpp/server.h>
#include <lib/stdcompat/optional.h>
#include <lib/svc/outgoing.h>
#include <zircon/process.h>
#include <zircon/processargs.h>
#include <zircon/status.h>

Initialize the event loop

int main(int argc, char** argv) {
  // Initialize the async loop. The Echo server will use the dispatcher of this
  // loop to listen for incoming requests.
  async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);
  async_dispatcher_t* dispatcher = loop.dispatcher();

  // Create an Outgoing class which will serve requests from the /svc/ directory.
  svc::Outgoing outgoing(loop.dispatcher());
  zx_status_t status = outgoing.ServeFromStartupInfo();
  if (status != ZX_OK) {
    std::cerr << "error: ServeFromStartupInfo returned: " << status << " ("
              << zx_status_get_string(status) << ")" << std::endl;
    return -1;
  }

  // Register a handler for components trying to connect to fuchsia.examples.Echo.
  status = outgoing.svc_dir()->AddEntry(
      fidl::DiscoverableProtocolName<fuchsia_examples::Echo>,
      fbl::MakeRefCounted<fs::Service>(
          [dispatcher](fidl::ServerEnd<fuchsia_examples::Echo> request) mutable {
            std::cout << "Incoming connection for "
                      << fidl::DiscoverableProtocolName<fuchsia_examples::Echo> << std::endl;
            // Create an instance of our EchoImpl that destroys itself when the connection closes.
            new EchoImpl(dispatcher, std::move(request));
            return ZX_OK;
          }));

  std::cout << "Running echo server" << std::endl;
  loop.Run();
  return 0;
}

The event loop is used to asynchronously listen for incoming connections and requests from the client. This code initializes the loop, and obtains the dispatcher, which will be used when binding the server implementation to a channel.

At the end of the main function, the code runs the loop to completion.

Serve component's service directory

The svc::Outgoing class serves the service directory ("/svc") for a given component. This directory is where the outgoing FIDL protocols are installed so that they can be provided to other components. The ServeFromStartupInfo() function sets up the service directory with the startup handle. The startup handle is a handle provided to every component by the system, so that they can serve capabilities (e.g. FIDL protocols) to other components.

int main(int argc, char** argv) {
  // Initialize the async loop. The Echo server will use the dispatcher of this
  // loop to listen for incoming requests.
  async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);
  async_dispatcher_t* dispatcher = loop.dispatcher();

  // Create an Outgoing class which will serve requests from the /svc/ directory.
  svc::Outgoing outgoing(loop.dispatcher());
  zx_status_t status = outgoing.ServeFromStartupInfo();
  if (status != ZX_OK) {
    std::cerr << "error: ServeFromStartupInfo returned: " << status << " ("
              << zx_status_get_string(status) << ")" << std::endl;
    return -1;
  }

  // Register a handler for components trying to connect to fuchsia.examples.Echo.
  status = outgoing.svc_dir()->AddEntry(
      fidl::DiscoverableProtocolName<fuchsia_examples::Echo>,
      fbl::MakeRefCounted<fs::Service>(
          [dispatcher](fidl::ServerEnd<fuchsia_examples::Echo> request) mutable {
            std::cout << "Incoming connection for "
                      << fidl::DiscoverableProtocolName<fuchsia_examples::Echo> << std::endl;
            // Create an instance of our EchoImpl that destroys itself when the connection closes.
            new EchoImpl(dispatcher, std::move(request));
            return ZX_OK;
          }));

  std::cout << "Running echo server" << std::endl;
  loop.Run();
  return 0;
}

Serve the protocol

The server then registers the Echo protocol using ougoing.svc_dir()->AddEntry().

int main(int argc, char** argv) {
  // Initialize the async loop. The Echo server will use the dispatcher of this
  // loop to listen for incoming requests.
  async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);
  async_dispatcher_t* dispatcher = loop.dispatcher();

  // Create an Outgoing class which will serve requests from the /svc/ directory.
  svc::Outgoing outgoing(loop.dispatcher());
  zx_status_t status = outgoing.ServeFromStartupInfo();
  if (status != ZX_OK) {
    std::cerr << "error: ServeFromStartupInfo returned: " << status << " ("
              << zx_status_get_string(status) << ")" << std::endl;
    return -1;
  }

  // Register a handler for components trying to connect to fuchsia.examples.Echo.
  status = outgoing.svc_dir()->AddEntry(
      fidl::DiscoverableProtocolName<fuchsia_examples::Echo>,
      fbl::MakeRefCounted<fs::Service>(
          [dispatcher](fidl::ServerEnd<fuchsia_examples::Echo> request) mutable {
            std::cout << "Incoming connection for "
                      << fidl::DiscoverableProtocolName<fuchsia_examples::Echo> << std::endl;
            // Create an instance of our EchoImpl that destroys itself when the connection closes.
            new EchoImpl(dispatcher, std::move(request));
            return ZX_OK;
          }));

  std::cout << "Running echo server" << std::endl;
  loop.Run();
  return 0;
}

The call to AddEntry installs a handler for the name of the FIDL protocol (fuchsia_examples::Echo::Name, which is the string "fuchsia.examples.Echo"). The handler will call the lambda function that we created, and this lambda function will construct an EchoImpl with the fidl::ServerEnd<fuchsia_examples::Echo>, which internally wraps a zx::channel, that represents a request from a client. The EchoImpl stays alive until it is unbound, at which point it deletes itself.

When the handler is called (i.e. when a client has requested to connect to the /svc/fuchsia.examples.Echo protocol), it binds the incoming channel to our Echo implementation, which will start listening for Echo requests on that channel and dispatch them to the EchoImpl instance. EchoImpl's constructor creates a fidl::ServerBindingRef which is used to send events back to the client.

Test the server

Rebuild:

fx build

Then run the server component:

ffx component run fuchsia-pkg://fuchsia.com/echo-llcpp-server#meta/echo_server.cm

Note: Components are resolved using their component URL , which is determined with the `fuchsia-pkg://` scheme.

You should see output similar to the following in the device logs (ffx log):

[ffx-laboratory:echo_server][][I] Running echo server

The server is now running and waiting for incoming requests. The next step will be to write a client that sends Echo protocol requests. For now, you can simply terminate the server component:

ffx component destroy /core/ffx-laboratory:echo_server

Note: Component instances are referenced by their component moniker , which is determined by their location in the component instance tree