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:

  1. {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/echo.test.fidl" region_tag="echo" %}

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:

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

Create and run a component {#component}

Create the component

To create a component:

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

    1. #include <stdio.h>
    3. int main(int argc, const char** argv) {
    4.   printf("Hello, world!\n");
    5.   return 0;
    6. }

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

    1. import("//build/components.gni")
    3. # Declare an executable for the server. This produces a binary with the
    4. # specified output name that can run on Fuchsia.
    5. executable("bin") {
    6.   output_name = "fidl_echo_llcpp_server"
    7.   sources = [ "main.cc" ]
    8. }
    10. {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/server/BUILD.gn" region_tag="rest" %}

    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.

  3. Add a component manifest in examples/fidl/llcpp/server/server.cmx:

    Note: The binary name in the manifest must match the output name of the executable defined in the previous step.

    ```cmx {%includecode gerrit_repo=”fuchsia/fuchsia” gerrit_path=”examples/fidl/llcpp/server/server.cmx” %}

Run the component

Note: The instructions in this section are geared towards running the component on QEMU, as this is the simplest way to get started with running Fuchsia, but it is also possible to pick a different product configuration and run on actual hardware if you are familiar with running components on other product configurations.

  1. Add the server to your configuration and build:

    1. fx set core.x64 --with //examples/fidl/llcpp/server && fx build

  2. Ensure fx serve is running in a separate tab and connected to an instance of Fuchsia (e.g. running in QEMU using fx qemu), then run the server:

    Note: The component should be referenced by its URL, which is determined with the fuchsia-pkg:// scheme. The package name in the URL matches the package_name field in the fuchsia_package declaration, and the manifest path in meta/ matches the target name of the fuchsia_component.

    1. fx shell run fuchsia-pkg://fuchsia.com/echo-llcpp-server#meta/echo-server.cmx

Implement the server

Add a dependency on the FIDL library

  1. Add "//examples/fidl/fuchsia.examples:fuchsia.examples_llcpp" to the deps of the executable
  2. Include the bindings into the main file with #include <fuchsia/examples/llcpp/fidl.h>

The full bin target declaration should now look like this:

  1. executable("bin") {
  2.   output_name = "fidl_echo_llcpp_server"
  3.   sources = [ "main.cc" ]
  4.   deps = [ "//examples/fidl/fuchsia.examples:fuchsia.examples_llcpp" ]
  5. }

Add an implementation for the protocol {#impl}

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

  1. {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/server/main.cc" region_tag="impl" %}

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 an optional ServerBindingRef in order to be able to send events to the client. It gets set later in the class’s Bind() function.
  • The Bind method binds the implementation to a given request.
  • The method for EchoString replies with the request value by using the completer.
  • 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:

  1. fx build

Serve the protocol {#main}

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.

Initialize the event loop

  1. {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/server/main.cc" region_tag="main" highlight="2,3,4,5,30" %}

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.

  1. {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/server/main.cc" region_tag="main" highlight="7,8,9,10,11,12,13,14,15,16,17" %}

Serve the protocol {#server-handler}

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

  1. {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/server/main.cc" region_tag="main" highlight="19,20,21,22,23,24,25,26,27,28,29,30,31" %}

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 call server.Bind() with the fidl::ServerEnd<fuchsia_examples::Echo>, which internally wraps a zx::channel, that represents a request from a client.

When a client requests access to /svc/fuchsia.examples.Echo, this function will be called with a channel that represents the request. This channel is bound to our server via the Bind() function, and future requests from this client will call.

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 call to fidl::BindServer returns a fidl::ServerBindingRef, which is then stored so the instance can be able to send events back to the client.

Add new dependencies {#deps}

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", which contain the async loop code.
  • "//sdk/lib/fdio" and "//zircon/system/ulib/svc": These are libraries used to interact with the components environment (e.g. for serving protocols).
  • "//zircon/public/lib/fidl": The LLCPP runtime, which contains utility code for using the FIDL bindings, such as the BindServer function.

The full bin target declaration should now look like this:

  1. {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/server/BUILD.gn" region_tag="bin" %}

Import the dependencies by including them at the top of examples/fidl/llcpp/server/main.cc:

  1. {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/server/main.cc" region_tag="includes" %}

Run the server

Rebuild:

  1. fx build

Then run the server:

  1. fx shell run fuchsia-pkg://fuchsia.com/echo-llcpp-server#meta/echo-server.cmx

You should see the std::cout output from the main() function followed by the server hanging. This is expected. Instead of exiting right away, the server keeps waiting for incoming requests. The next step will be to write a client for the server.