fidlcat: Monitor and debug your fidl calls


fidlcat is a tool that allows users to monitor FIDL connections. Currently, it can attach to or launch a process on a Fuchsia device, and will report its FIDL traffic.

Enabling it

To run fidlcat in-tree, you first build it, which you can do the following way:

fx set <whatever> --with //bundles:tools
fx build

If you want to add it to your existing gn args, you can do so by adding this stanza to the bottom of your <build_dir>/

universe_package_labels += [ "//bundles:tools" ]

To run fidlcat, you must boot with networking enabled. If you're running an emulator:

fx emu -N

In a separate console, you need to ensure your target is able to fetch updates:

fx serve

Running it

When your environment is properly set up, and fidlcat is built, you should be able to use it to monitor FIDL messages from processes on the target. There are several ways to do this.

Attaching to a running process

If you run the ps command in the shell, you can get a pid you want to monitor, and run:

fx fidlcat --remote-pid=<pid>

If your code is executed by a runner, you are likely to want to attach to the runner. For Dart JIT-executed code, run ps on the target, and look for the process named dart_jit_runner:

host$ fx shell ps
        j:21102           17.6M   17.6M
          p:21107         17.6M   17.6M     32k         dart_jit_runner.cmx

You can then attach directly to that process, and view all FIDL messages sent by Dart programs:

host$ fx fidlcat --remote-pid=21107

You can use the --remote-pid flag multiple times to connect to multiple processes:

fx fidlcat --remote-pid=<pid1> --remote-pid=<pid2>

Launching a component with fidlcat

Alternatively, you can launch a component directly using its URL:

fx fidlcat run fuchsia-pkg://

You can also specify the URL with a bash regex that matches a unique URL known to the build:

fx fidlcat run "echo_client_cpp_synchronous.*"
fx fidlcat run echo_client_cpp.cmx

Attaching to a program on startup

You can also attach to programs with their names by passing a regex to match their names. Fidlcat will attach to all currently running and subsequently started programs that satisfy the regex. If you issue the following command, fidlcat will connect to the system, and attach to all programs with the substring “echo_client”.

fx fidlcat --remote-name=echo_client

Mixed use

All three options --remote-pid, --remote-name and run can be used together. However, run must always be the last one.

When --remote-name and run are used together, only processes which match --remote-name are monitored.

Examples (echo_server is launched by echo_client):

Run and monitor echo_client.

fx fidlcat run echo_client_cpp.cmx

Run and monitor echo_client.

fx fidlcat --remote-name=echo_client run echo_client_cpp.cmx

Run echo_client and monitor echo_server.

fx fidlcat --remote-name=echo_server run echo_client_cpp.cmx

Run echo_client and monitor echo_client and echo_server.

fx fidlcat --remote-name=echo run echo_client_cpp.cmx

Run echo_client and monitor echo_client and echo_server.

fx fidlcat --remote-name=echo_client --remote-name=echo_server run echo_client_cpp.cmx

Monitoring a service

If you want to monitor a service, you should use --extra-name instead of --remote-name. The option --extra-name also monitors some processes. However, for these processes, monitoring starts only when one of of the “--remote-name” process is launched. Also, fidlcat stops when the last “--remote-name” process stops (even if some “--extra-name” processes are still monitored).

fx fidlcat --remote-name=echo --extra-name=appmgr run echo_client_cpp.cmx

Input options

You have two input options:

  • --from=device This is the default option which monitors in real time a device.

  • --from=<path> With this option, fidlcat replay a previously saved session.

Session save

The option --to=<path> The session is saved to the specified file (binary protobuf format). When a session is saved, you can replay it using “--from=<path>”. The raw data is saved. That means that the data saved is independent from what is displayed.

Format (output) options

You have the following output options:

  • --format=pretty This is the default output. The session is pretty printed (with colors).

  • --format=json The session is printed using a json format.

  • --format=textproto The session is printed using a text protobuf format.

  • --format=summary At the end of the session, a summary of the session is displayed.

  • --format= Nothing is displayed on the standard output (this option only makes sense when used with --to=<path>). When there is no output, fidlcat is much faster (this is better when you want to monitor real time components).

Running without the fx tool

Note that fidlcat needs two sources of information to work:

  • First, it needs the symbols for the executable. In practice, if you are running in-tree, the symbols should be provided to fidlcat automatically. Otherwise, you can provide fidlcat either a) a .build-id directory using --build-id-dir flag, b) an ids.txt file using --ids-txt flag, or c) an arbitrary ELF file or a directory of ELF files using --symbol-path flag. These flags can be combined and specified multiple times.

  • Second, it needs the intermediate representation for the FIDL it ingests, so it can produce readable output. If you are running in-tree, the IR should be provided to fidlcat automatically. Otherwise, you can provide fidlcat an IR path, which can be an explicit IR file path, a directory it will scan for IR files, or an argument file containing explicit paths. This can be provided to fidlcat with the --fidl-ir-path flag. The argument files need to be prepended with a @ character: --fidl-ir-path @argfile.

  • Third, regex URL matching does not work outside of the fx tool. You must specify the entire package URL.

Finally, if you are running fidlcat without the fx tool, the debug agent needs to be running on the target. Connect to the target and run:

run fuchsia-pkg:// --port=8080

And, when you run fidlcat on the host, make sure you connect to that agent:

tools/fidlcat --connect [$(fx get-device-addr)]:8080 <other args>

Read the guide

The fidlcat guide describes all the flags which modify the output. It also gives some examples of display interpretation.

Where is the code?

The code is located in //tools/fidlcat.