Writing a "Hello World" session

Sessions are regular components that the session_manager can launch at startup. This means that creating a session component follows all of the same steps as creating any other component. This document discusses creating an example session that launches at startup and prints "Hello World!" to the system log.

Create the directory structure

Components require a specific directory structure. The fx tool provides a generator that creates this structure for you. It takes the name of the component and the language you want to use as arguments. For example, this component is called hello-world-session and is written in Rust.

Run the following command to create the directory structure for this example:

fx create component --path hello-world-session --lang rust

This command creates the following directory structure with a template for a component offering a service:

  |- meta
  |   |- hello-world-session.cml
  |- src
  |   |- main.rs
  |- BUILD.gn

Create a component manifest

The component manifest file (hello-world-session.cml) declares the component to Fuchsia. For this example, the default manifest is sufficient but take a moment to explore the following lines of code from hello-world-session.cml:

  1. The file starts by including other cml files if needed.

    include: [

    This include key lets the session component use the fuchsia.logger.LogSink capability so that it can print to the system log.

  2. Next is the program block.

    // Information about the program to run.
    program: {
        // Use the built-in ELF runner to run native binaries.
        runner: "elf",
        // The binary to run for this component.
        binary: "bin/hello-world-session",

    The program block tells the component_manager where the binary for the session component can be found. The runner key tells the component_manager that is should run the component binary using the ELF runner.

  3. Finally the component manifest describes additional capabilities that the component can use, offer, or expose.

    // Capabilities used by this component.
    use: [
        // List your component's dependencies here, ex:
        // { protocol: "fuchsia.net.name.Lookup" }
    expose: [
        // session_manager uses this protocol to start the session.
            protocol: "fuchsia.component.Binder",
            from: "framework",

Writing a session in Rust

Now you can write the implementation for the session component. Inside the src/main.rs file that was generated there is a lot of code that isn't needed for this example.

Replace the contents of src/main.rs with the following code:

use anyhow::Error;

/// Creates a simple session that just prints "Hello World" to the syslog.
#[fuchsia::main(logging = true)]
async fn main() -> Result<(), Error> {
    tracing::info!("Hello World!");


This code initializes the system log and then prints "Hello World!". tracing::info! is a macro that prints to the log with a level of info. There are similar macros for error and warn.

Writing the BUILD.gn

The last file to modify is the BUILD.gn. This tells the compiler how to build the session component.

Rust binary

The next section describes the actual Rust binary. It tells the compiler what the name of the binary should be, that it includes unit tests, what dependencies it has, and where its source is located. For this example, the default set of dependencies are sufficient:

rustc_binary("bin") {
  edition = "2021"
  output_name = "hello-world-session"

  # Generates a GN target for unit-tests with the label `bin_test`, and
  # a binary named `hello_world_session_bin_test`.
  with_unit_tests = true

  deps = [

  sources = [ "src/main.rs" ]

The fuchsia_component() and fuchsia_package() templates tell Fuchsia more about the component including what it is called, where to find the manifest, and what dependencies the package and component have:

fuchsia_component("component") {
  component_name = "hello-world-session"
  manifest = "meta/hello-world-session.cml"
  deps = [ ":bin" ]

fuchsia_package("hello-world-session") {
  deps = [ ":component" ]

Identify the session URL

session_manager needs to know to which session component to launch at startup, and is configured by providing the component URL of the session.

Component URLs follow the format:


Notice that the path points to a .cm file. .cm files are compiled versions of .cml files that are generated when fx build is run. So, in this case, the component URL is:


Building the session

To build the session fx set must first be used to configure the build so that session_manager, your session component, and the session config are included in the base package set. This is done using the --with-base flag. The session URL must also be configured, which is done using the --args flag.

fx set core.x64 \
    --with-base //src/session/bin/session_manager \
    --with-base //path/to/your/session \
    --args=product_config.session_url = "fuchsia-pkg://fuchsia.com/<var>package_name</var>#meta/<var>component_name</var>.cm"

If you are using the example project from the //src/session/examples directory, the fx set command would be:

fx set core.x64 \
    --with-base //src/session/bin/session_manager \
    --with-base //src/session/examples/hello-world-session \
    --args=product_config.session_url = "fuchsia-pkg://fuchsia.com/hello-world-session#meta/hello-world-session.cm"

Once that's done and built session_manager should automatically start your session on boot. You should see the "Hello" message in the system log.

$ ffx log --filter hello
[session_manager] INFO: Launching session: fuchsia-pkg://fuchsia.com/hello-world-session#meta/hello-world-session.cm
[hello_world_session] INFO: Hello World!