Displaying Output

By default, Inspektor Gadget allows you to display the output of each data source in all the output modes supported by the CLI operator. Unless you have specific requirements, we recommend using the CLI operator's default configuration. However, for Map Iterators with a map value type of gadget_histogram_slot__u32 or gadget_histogram_slot__u64 (TODO: Add links), the output should be displayed as a chart, which the CLI operator doesn't support. In such cases, you can use the Otel Metrics operator to render the output as a histogram by annotating the data source with metrics.print: "true". The profile_blockio gadget is an example of this.

Fields

The fields to be displayed in the output when using the columns, json, jsonpretty or yaml output modes depend on the data source type:

• Tracers: The fields are all the elements of the event struct specified when defining the data source.
• Map Iterators: The fields are all the elements of the key and value structs used to define the eBPF map to be iterated. As mentioned above, there is a special case for the map iterators with a map value type of gadget_histogram_slot__u32 or gadget_histogram_slot__u64. In such cases, the data source created by the Otel Metrics operator will contain just one field called text which will carry the rendered histogram as a plain text.
• Snapshotters: The fields are all the elements of the snapshot entry struct specified when defining the data source.

Custom Text Output

There are cases when you want to print custom output. This can be implemented by using a WASM module and some annotations on the metadata file.

First of all, annotate the datasource with the name of the output mode:

datasources:
  mydatasource:
    annotations:
      cli.supported-output-modes: myoutputmode,myoutputmode1,myoutputmode2
      cli.default-output-mode: myoutputmode

Then, in the WASM module create the data source and a string field called text that will be used to emit the data:

warning

The field needs to be called text to be recognized by the CLI operator.

var (
	textds    api.DataSource
	textField api.Field
)

//go:wasmexport gadgetInit
func gadgetInit() int32 {
	var err error
	textds, err = api.NewDataSource("mydatasource", api.DataSourceTypeSingle)
	if err != nil {
		api.Errorf("creating datasource: %s", err)
		return 1
	}

	textField, err = textds.AddField("text", api.Kind_String)
	if err != nil {
		api.Errorf("adding field: %s", err)
		return 1
	}

	return 0
}

Then, format your output as a string and emit it:

//go:wasmexport gadgetStart
func gadgetStart() int32 {
	nd, err := textds.NewPacketSingle()
	if err != nil {
		api.Errorf("creating packet: %s", err)
		return 1
	}

	if err := textField.SetString(api.Data(nd), "hi there!!"); err != nil {
		api.Errorf("setting field: %s", err)
		return 1
	}

	if err := textds.EmitAndRelease(api.Packet(nd)); err != nil {
		api.Errorf("emitting packet: %s", err)
		return 1
	}

	return 0
}

$ sudo ig image build . -t hello-world
$ sudo ig run hello-world --verify-image=false
hi there!!