Golang get struct tag without pointer

i was creating a simple marshaler for fixed with text into struct in golang, as detailed here http://ift.tt/2uwcqod

The marshaler functions as I expected now, albeit still lacking some features. What I got stuck in is in the marshal function.

The relevant code is as follow

func Marshal(obj interface{}) (str string, err error) {
...
    elemsType := reflect.TypeOf(obj).Elem()

As you can see, i tried to mimic the json package's marshal signature. Then only problem is when i tried to pass by value to the marshal function, the reflect.TypeOf returns a different type than the one i pass into it. The function can only executed if I'm passing pointer to the marshal function.

This works user := User{"johnjohnjohn", "the", "doe", "smart", 26} res, err := Marshal(&user)

This doesn't user := User{"johnjohnjohn", "the", "doe", "smart", 26} res, err := Marshal(user)

Is there any way to just pass by value, and then get the struct tag inside the marshal function?