Encode a base64 request body into a binary

I'm fairly new to the Go language and having a hard time achieving the following: I'm receiving a base64 string (basically, an encoded image) and need to transform it to the binary form on the server.

func addOrUpdateUserBase64(w http.ResponseWriter, r *http.Request, params martini.Params) {
	c := appengine.NewContext(r)
	sDec, _ := b64.StdEncoding.DecodeString(r.Body)
...

This is not working, because DecodeString expects a string... how do I transform request.Body into a string? Any tips are very much appreciated!

Do not use base64.StdEncoding.DecodeString, instead set up a decoder directly from the r.Body

dec := base64.NewDecoder(base64.StdEncoding, r.Body)`  // dec is an io.Reader

now use dec, e.g. dump to a bytes.Buffer like

buf := &bytes.Buffer{}
n, err := io.copy(buf, dec)

which will decode r.Body into buf or copy directly to a http.Response or a file.

Or use Peter's method below if keeping all in memory is okay.

> func (*Encoding) Decode
>
> func (enc *Encoding) Decode(dst, src []byte) (n int, err error)
>
> Decode decodes src using the encoding enc. It writes at most
> DecodedLen(len(src)) bytes to dst and returns the number of bytes
> written. If src contains invalid base64 data, it will return the
> number of bytes successfully written and CorruptInputError. New line
> characters (\r and \n) are ignored.

And one more option would be just casting r.Body to a string :

//Edit, fixed the code to work with an io.Reader

import "io/ioutil"
..........
if body, err := ioutil.ReadAll(r.Body); err == nil {
       sDec, _ := b64.StdEncoding.DecodeString(string(body))
}