运行示例

While exploring the source code, you may have noticed the examples directory. This guide will show you how to build Fuchsia to include these examples and then run them on the device.

Explore the GN file

Go ahead and open the BUILD.gn file in the examples directory.

If you aren’t familiar with GN, take a look at the introductory presentation or docs. In short, GN is a meta build system. Its output files serve as inputs to Ninja, the actual build system.

In this file, you can see that the examples are designated as a named group. This group names three targets as dependencies:

group("examples") {  testonly = true  deps = [    "cowsay",    "fortune",    "hello_world",  ]}

You can find a directory for each of these dependencies. Each directory has its own BUILD.gn file that defines a package for the specific example.Note: You can look at the build/package.gni file to learn more about how Fuchsia packages are defined by GN.

Let’s take a look at each example.

Cowsay

This sample takes a word and repeats it back from an ASCII-art cow.

To access a shell on the device, use the fx shell command:

fx shell

Now run the binary directly:

cowsay hi
 _____
| hi  |
 -----
     \  ^__^
      \ (oo)\_____
        (__)\     )\/\
           ||----w |
           ||     ||

You may notice that you can run this program without any changes to the build you already served to the device. Why? This example is pulled in as part of the command line utilities in garnet/packages/prod/BUILD.gn. But this isn’t typically how you run a Fuchsia package; you will see that soon.

Fortune

This sample prints a pithy observation. Let’s try to run it using the extended format for fx shell that accepts command arguments:

fx shell fortune # won't work, no package in build

This won’t work since this package wasn’t pulled into the last build. You can confirm it isn’t in the build by listing the contents of the bin directory:

fx shell ls bin

cowsay is there, but not fortune. Don’t worry, you will add it soon.

Hello world

This sample outputs Hello, world! and is written in both C++ and Rust. You may notice something different about this sample: each language-dependent directory also contains a meta subdirectory with .cmx files.

This type of file is known as a component manifest and describes how to run the application on Fuchsia as a component. This is the proper way to create a Fuchsia package.

You run a Fuchsia component by referencing its Fuchsia package URL.

Set the build to include examples

You can include the examples in the build, but you need to determine where they will be included:

  • Base: Packages that are included in paving images produced by the build. They are included in over-the-air updates and are always updated as a single unit.
  • Cache: Packages that are included in paving images, but are not included in over-the-air system updates. These packages can be updated at any time that updates are available.
  • Universe: Packages that are additional optional packages that can be fetched and run on-demand, but are not pre-baked into any paving images.

(For more information, see fx workflows.)

To include this package in Universe so it can be fetched on-demand, use the --with flag when setting the product and board:

fx set ... --with //examplesfx build

You now have a build that includes the examples. Now you need to serve this new build to the device. You probably already have fx serve running, but if not, go ahead and start it:

fx serve

Try the examples

Open a shell on the device to try the examples:

fx shell

Cowsay

This should work the same as before. It is still pulled into the base image.

Fortune

Try to run the fortune example again:

fortune

It should now return a pithy quote.

Hello world

You can’t run this sample as the other two (try to run hello_world_cpp or hello_world_rust). To run a Fuchsia component, you need to run it’s Fuchsia package URL. Luckily, there are some built-in conveniences to help you find this. You can use the run command:

run fuchsia-pkg://fuchsia.com/hello_world_cpp#meta/hello_world_cpp.cmx

And it should output the following:

Hello, World!

Important: You must have fx serve running in order to serve components from your repository to the device. If it is not running, you may get an error from the device (for example, fuchsia-pkg://fuchsia.com/hello_world_cpp#meta/hello_world_cpp.cmx: not found).

The run command can expand a string to a URL if the string only matches one component in your product configuration:

run hello_world_cpp

If there are multiple matches, the command will list them for you to choose from:

run hello
fuchsia-pkg://fuchsia.com/hello_world_cpp#meta/hello_world_cpp.cmx
fuchsia-pkg://fuchsia.com/hello_world_rust#meta/hello_world_rust.cmx
Error: "hello" matched multiple components.

You can explore what components are in your product configuration using the locate command.

  • Find your favorite component.locate hello_world_cpp
  • Find all runnable components in your universe.locate --list cmx
  • Find multiple test components.locate --list test

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