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Monitor FIDL messages on a device

The ffx debug fidl starts fidlcat (a Fuchsia program) to monitor the FIDL traffic on a Fuchsia device.

Concepts

fidlcat is a tool that allows you to monitor and debug FIDL messages on a Fuchsia device. fidlcat establishes a connection to the device and prints all FIDL messages that a Fuchsia component is sending and receiving in real time.

Similar to other Fuchsia debugging tools, for fidlcat to understand and unpack FIDL messages of a Fuchsia component, the component's debug symbols must be available in your development environment. (For more information on debug symbols, see Register debug symbols.)

However, in addition to the debug symbols,fidlcat needs to obtain a list of FIDL intermediate representation (IR) files for the component. This list is usually stored as a global file labeled all_fidl_json.txt in your Fuchsia project. If you're using Fuchsia's Bazel rules, this list file gets generated automatically as part of the build.

Start fidlcat

To monitor the FIDL traffic of a Fuchsia component on a device, run the following command:

ffx debug fidl -f <COMPONENT> --fidl-ir-path <LIST_OF_IR_FILES>

Replace the following:

  • COMPONENT - The name of a Fuchsia component you want to monitor.
  • LIST_OF_IR_FILES - The path to a file that contains the list of IR files, with @ as a prefix (for example, @/usr/home/alice/my-fuchsia-project/all_fidl_json.txt).

The example command below starts fidlcat that monitors the memory_monitor component:

$ ffx debug fidl -f memory_monitor.cm --fidl-ir-path @/usr/home/alice/my-fuchsia-project/all_fidl_json.txt

When successfully connected to the device, this command prints output similar to the following:

$ ffx debug fidl -f memory_monitor.cm --fidl-ir-path @/usr/home/alice/my-fuchsia-project/all_fidl_json.txt
INFO: [main.cc(242)] Connected to symbol server gs://our-fuchsia-artifacts/debug
INFO: [main.cc(125)] Connecting to /tmp/debug_agent_lpaMdL.socket...
INFO: [main.cc(92)] Connected!

From this point, the command prints the target component's FIDL messages in real time, for example:

2701.44 Monitoring memory_monitor.cm koid=47467

2703.286866 memory_monitor.cm 47467:47469 zx_channel_call_etc(handle: handle = 7dd2c40b, options: uint32 = ZX_CHANNEL_WRITE_USE_IOVEC, deadline: zx.time = ZX_TIME_INFINITE, rd_num_bytes: uint32 = 65536, rd_num_handles: uint32 = 64)
  sent request fuchsia.kernel/Stats.GetMemoryStats = {}
2703.286866   -> ZX_OK
    received response fuchsia.kernel/Stats.GetMemoryStats = {
      stats: fuchsia.kernel/MemoryStats = {
        total_bytes: uint64 = 8455393280
        free_bytes: uint64 = 6727020544
        wired_bytes: uint64 = 102539264
        total_heap_bytes: uint64 = 32464896
        free_heap_bytes: uint64 = 1672120
        vmo_bytes: uint64 = 1540259840
        mmu_overhead_bytes: uint64 = 50339840
        ipc_bytes: uint64 = 278528
        other_bytes: uint64 = 0
      }
    }

2703.815113 memory_monitor.cm 47467:47469 zx_channel_write_etc(handle: handle = 7df30f83, options: uint32 = 0)
  sent request fuchsia.memory/Watcher.OnChange = {
    stats: fuchsia.memory/Stats = {
      total_bytes: uint64 = 8455393280
      free_bytes: uint64 = 6727020544
      wired_bytes: uint64 = 102539264
      total_heap_bytes: uint64 = 32464896
      free_heap_bytes: uint64 = 1672120
      vmo_bytes: uint64 = 1540259840
      mmu_overhead_bytes: uint64 = 50339840
      ipc_bytes: uint64 = 278528
      other_bytes: uint64 = 0
    }
  }
2703.815113   -> ZX_OK

...

To exit fidlcat, press Ctrl-C.