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/hlcpp/client_sync/*
Create a new component project at examples/fidl/hlcpp/client_sync
:
Add a main()
function to examples/fidl/hlcpp/client_sync/main.cc
:
int main(int argc, const char** argv) { printf("Hello, world!\n"); return 0; }
Declare a target for the client in examples/fidl/hlcpp/client_sync/BUILD.gn
:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client_sync/BUILD.gn" region_tag="imports" %} # Declare an executable for the client. executable("bin") { output_name = "fidl_echo_hlcpp_client_sync" sources = [ "main.cc" ] } {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client_sync/BUILD.gn" region_tag="rest" %}
Add a component manifest in examples/fidl/hlcpp/client_sync/meta/client.cml
:
Note: The binary name in the manifest must match the output name of the executable
defined in the previous step.
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client_sync/meta/client.cml" region_tag="example_snippet" %}
Once you have created your component, ensure that you can add it to the build configuration:
fx set core.x64 --with //examples/fidl/hlcpp/client_sync:echo-client
Build the Fuchsia image:
fx build
Add the following dependencies:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client_sync/BUILD.gn" region_tag="deps" %}
Then, include them in main.cc
:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client_sync/main.cc" region_tag="includes" %}
The reason for including these dependencies is explained in the server tutorial.
This section adds code the main()
function that connects to the server and makes requests to it.
The code then creates a proxy class for the Echo
protocol, and connects it to the server. In the context of FIDL, proxy designates the code generated by the FIDL bindings that enables users to make remote procedure calls to the server. In HLCPP, the proxy takes the form of a class with methods corresponding to each FIDL protocol method.
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client_sync/main.cc" region_tag="main" highlight="2,3,4" %}
fuchsia::examples::EchoSyncPtr
is an alias for fidl::SynchronousInterfaceRequest<fuchsia::examples::Echo>
generated by the bindings. This class will proxy requests for the Echo
protocol over the channel that it is bound to.EchoSyncPtr::NewRequest()
, which will create a channel, bind the class to one end, and return the other endsys::ServiceDirectory::Connect()
.context->out()->AddPublicService()
on the server side, Connect
has an implicit second parameter here, which is the protocol name ("fuchsia.examples.Echo"
). This is where the input to the handler defined in the previous tutorial comes from: the client passes it in to Connect
, which then passes it to the handler.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.
The code makes two requests to the server:
EchoString
requestSendString
request{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client_sync/main.cc" region_tag="main" highlight="6,7,8" %}
For EchoString
the code passes in a pointer for each response parameter (in this case, the EchoString
method only has one response parameter), which is written with the response from the server, whereas this does not apply to SendString
since it is a fire and forget method. The call to EchoString
will block until it receives a message from the server. Both methods will return a zx_status_t
indicating the result of the method call.
Though the server implementation sends an OnString
event in response to the SendString
request, the sync bindings do not provide a way to handle this event.
In order for the client and server to communicate using the Echo
protocol, component framework must route the fuchsia.examples.Echo
capability from the server to the client. For this tutorial, a realm component is provided to declare the appropriate capabilities and routes.
Note: You can explore the full source for the realm component at //examples/fidl/echo-realm
Configure your build to include the provided package that includes the echo realm, server, and client:
fx set core.x64 --with //examples/fidl/hlcpp:echo-hlcpp-client-sync
Build the Fuchsia image:
fx build
Run the echo_realm
component. This creates the client and server component instances and routes the capabilities:
ffx component run /core/ffx-laboratory:echo_realm fuchsia-pkg://fuchsia.com/echo-hlcpp-client-sync#meta/echo_realm.cm
Start the echo_client
instance:
ffx component start /core/ffx-laboratory:echo_realm/echo_client
The server component starts when the client attempts to connect to the Echo
protocol. You should see output similar to the following in the device logs (ffx log
):
[echo_server][][I] Running echo server [echo_client][][I] Got response: hello
Terminate the realm component to stop execution and clean up the component instances:
ffx component destroy /core/ffx-laboratory:echo_realm