Question about userdata interface{} argument to callback function

[ikandaswamy]

Firstly, thank you very much for the go-curl package!
I have a question :
In my implementation, when I print out the user data field from the callback function that gets passed to WRITEFUNCTION, i get
{[] 0 [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] 0}

What is it supposed to contain ?

The following is the code I use
// Callback function to save data instead of redirecting it into stdout.
	writeToBufferFunc := func(buf []byte, userdata interface{}) bool {
		if silent == false {
			b.Write([]byte(buf))
		}
		logging.Infof("\n\n %v \n\n ", userdata)
		return true
	}

	this.myCurl.Setopt(curl.OPT_WRITEFUNCTION, writeToBufferFunc)

	this.myCurl.Setopt(curl.OPT_WRITEDATA, b)

	if err := this.myCurl.Perform(); err != nil {
		if show_error == true {
			return nil, err
		} else {
			return nil, nil
		}
	}

[andelf]

use (userdata).([]byte) ?