fidlcat: Monitor and debug your fidl calls

Overview

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 /args.gn.

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

To run fidlcat, you must boot with networking enabled.

For QEMU networking support, you need to setup your system with a TUN/TAP interface. Then, run:

fx run -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://fuchsia.com/echo_client_rust#meta/echo_client_rust.cmx

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 that have not started yet by passing a regex to match their names. If you issue the following command, fidlcat will connect to the system, wait for a program with the substring "echo_client" to start, and automatically attach to it.

fx fidlcat --remote-name echo_client

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 a symbol path, which can be a text file that maps build ids to debug symbols, an explicit ELF file path, or a directory it will scan for ELF files and index. This can be provided to fidlcat with the -s flag.

  • 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://fuchsia.com/debug_agent#meta/debug_agent.cmx --port=8080

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

tools/fidlcat --connect [$(fx netaddr --fuchsia)]: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.