This tutorial builds on the FIDL server tutorial. For the full set of FIDL tutorials, refer to the overview.
This tutorial implements a client for a FIDL protocol and runs it against the server created in the previous tutorial. The client in this tutorial is synchronous. There is an alternate tutorial for asynchronous clients.
If you want to write the code yourself, delete the following directories:
rm -r examples/fidl/llcpp/client_sync/*
Set up a hello world component in examples/fidl/llcpp/client_sync. You can name the component echo-client, and give the package a name of echo-llcpp-client-sync.
Note: If necessary, refer back to the previous tutorial.
Once you have created your component, ensure that the following works:
fx set core.x64 --with //examples/fidl/llcpp/client_sync
Build the Fuchsia image:
fx build
In a separate terminal, run:
fx serve
In a separate terminal, run:
fx shell run fuchsia-pkg://fuchsia.com/echo-llcpp-client-sync#meta/echo-client.cmx
Add the following dependencies:
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/BUILD.gn" region_tag="deps" %}
Then, include them in main.cc:
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/main.cc" region_tag="includes" %}
These dependencies are explained in the server tutorial. The client requires far fewer dependencies because it does not need to run any asynchronous code.
Include the Echo protocol in the client component's sandbox by editing the component manifest in client.cmx.
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/client.cmx" %}
The steps in this section explain how to add code to the main() function that connects the client to the server and makes requests to it.
The client then connects to the service directory /svc, and uses it to connect to the server.
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/main.cc" region_tag="main" highlight="2,3,4,5,6,7,8,9" %}
In practice, this is done by calling fdio_service_connect_at, passing it the service directory, the name of the service to connect to, as well as the channel that should get passed to the server. In parallel, the component manager will route the requested service name and channel to the server component, where the [connect function][server-handler] is called with these arguments, binding the channel to the server implementation. The other end of this channel is then used to initialized the client object.
An important point to note here is that this code assumes that /svc already contains an instance of the Echo protocol. This is not the case by default because of the sandboxing provided by the component framework. A workaround will be when running the example at the end of the tutorial.
The get_svc_directory() function is defined as follows:
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/main.cc" region_tag="connect" %}
Similar to the fdio_service_connect_at call above, this function initializes a channel, and passes the server end to fdio_service_connect to connect to the /svc directory, returning the client end.
Note: This pattern of making a request to connect one end of the channel to a service, then immediately using the client end to communicate with the service is known as request pipelining. This topic is covered further in the request pipelining tutorial.
The code makes two requests to the server:
EchoString requestSendString request{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/main.cc" region_tag="main" highlight="11,12,13,14,15,16,17,18,19,20,22,23" %}
The protocol methods on the client object (EchoString and SendString) return a result object, which will contain either an error or the contents of the response (if any). When a response is expected, the client will block until the response is received.
A client object is generated for each protocol, which is described further in the LLCPP bindings reference
The client object allows handling events by specifying an event handlers struct, where each member corresponds to the handler for one of the events of the protocol.
The code defines a handler which prints the contents of an OnString event, then calls client.HandleEvents() to block until an event is received. The return value of the callback becomes the return value of the HandleEvents call:
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/main.cc" region_tag="main" highlight="25,26,27,28,29,30,31,32,33,34,35" %}
If you run the client directly, it will not connect to the server correctly because the client does not automatically get the Echo protocol provided in its sandbox (in /svc). To get this to work, a launcher tool is provided that launches the server, creates a new Environment for the client that provides the server's protocol, then launches the client in it.
Configure your GN build:
fx set core.x64 --with //examples/fidl/llcpp/server --with //examples/fidl/client/client_sync --with //examples/fidl/test:echo-launcher
Build the Fuchsia image:
fx build
Run the launcher by passing it the client URL, the server URL, and the protocol that the server provides to the client:
fx shell run fuchsia-pkg://fuchsia.com/echo-launcher#meta/launcher.cmx fuchsia-pkg://fuchsia.com/echo-llcpp-client-sync#meta/echo-client.cmx fuchsia-pkg://fuchsia.com/echo-llcpp-server#meta/echo-server.cmx fuchsia.examples.Echo
You should see the print output in the QEMU console (or using fx log).
[189209.659] 859216:859218> Running echo server [189209.778] 859216:859218> echo_server_llcpp: Incoming connection for fuchsia.examples.Echo [189209.803] 859554:859556> Got response: hello [189209.804] 859554:859556> Got event: hi