Run an example component

This guide shows you how to build Fuchsia to include an example package from Fuchsia's source //examples directory and run a component on your Fuchsia target.

Prerequisites

Before you can run an example component, you must:

Set up the Fuchsia development environment

Exploring the example

This example component prints Hello, world! to the system log. The example has three main elements:

An executable binary written in a supported language.
A component manifest (.cml) file to declare the component and its capabilities.
A BUILD.gn file to define the component build target and include it in a Fuchsia package.

Executable program

Fuchsia components can execute programs written in any language with a supported runtime. The most common runtime used for C++ and Rust programs is the ELF runner.

C++

#include <iostream>

int main() {
  std::cout << "Hello, World!\n";
  return 0;
}

Rust

fn main() {
    println!("{}, world!", greeting());
    eprintln!("{}, world!", greeting());
}

fn greeting() -> String {
    return String::from("Hello");
}

Component manifest

A component manifest describes how to run a Fuchsia program as a component . This includes declaring program binary, runtime information, and any capabilities the component requires to execute, such as logging support.

C++

{
    // Enable system logging capabilities
    include: [ "syslog/client.shard.cml" ],

    // Information about the program to run
    program: {
        // Use the built-in ELF runner for the executable
        runner: "elf",
        binary: "bin/hello_world_cpp",

        // Enable stdout logging
        forward_stderr_to: "log",
        forward_stdout_to: "log",
    },
}

Rust

{
    // Enable system logging capabilities
    include: [ "syslog/client.shard.cml" ],

    // Information about the program to run
    program: {
        // Use the built-in ELF runner for the executable
        runner: "elf",
        binary: "bin/hello_world_rust",

        // Enable stdout logging
        forward_stderr_to: "log",
        forward_stdout_to: "log",
    },
}

For more details on component manifests and their declaration syntax, see component manifests.

BUILD.gn

Fuchsia uses the Generate Ninja (GN) meta-build system to generate inputs for Ninja, which executes the actual build. The BUILD.gn file declares the build targets for a fuchsia_component() and fuchsia_package().

C++

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

Rust

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

To learn more about how Fuchsia uses GN to define components and packages, see: Building components.

Include the example package in your Fuchsia image

To include the example package in your build configuration, use the --with flag when setting your product and board environment:

fx set product.board --with //examples/hello_world

For a Fuchsia emulator with the minimum build configuration, the command is:

fx set core.qemu-x64 --with //examples/hello_world

In this example, core is a product with a minimal feature set, which includes common network capabilities, and x64 refers to the x64 architecture.

For a Fuchsia device with the minimum build configuration, the command is:

fx set core.x64 --with //examples/hello_world

See Configure a build for more options.

Once you have set your build configuration, build Fuchsia with the following command:

fx build

You now have a build that includes the example package that can be fetched and launched on demand.

Explore your product configuration

You can explore the contents of your product configuration using the list-packages command.

List all:

fx list-packages

There may be many entries, so add the name to find the one you're looking for:

fx list-packages hello-world
hello-world-cpp-unittests
hello-world-rust-tests
hello-world-cpp
hello-world-rust

Run the example component

To run a Fuchsia component, use its Fuchsia package URL as an argument to the run command:

Open a terminal and run fx serve-updates:
```
fx serve-updates
```

Open another terminal and run the example component:

C++

ffx component run fuchsia-pkg://fuchsia.com/hello-world#meta/hello-world-cpp.cm

Rust

ffx component run fuchsia-pkg://fuchsia.com/hello-world#meta/hello-world-rust.cm

Open another terminal and view the system log:
```
fx log --only hello-world
```
The component prints the following output to the log:
```
[ffx-laboratory:hello-world] INFO: Hello, World!
```

Troubleshooting

If fx serve-updates is not running, the command prints an error message from the device or emulator.

If fx serve-updates is running, but the package is not found, repeat these steps and rebuild your Fuchsia image to include the appropriate packages.