How to read multiple times from same io.Reader

Question

I want to use request.Body(type io.ReadCloser) which is containing a image.

I dont want to use ioutil.ReadAll() as i want to write this body directly to the file as well as want to decode it, so i only want to use the reference to the content to pass to further function calls,

I tried creating multiple instances of reader for example shown below

package main  import (     "io/ioutil"     "log"     "strings" )   func main() {     r := strings.NewReader("some io.Reader stream to be read\n")     a := &r     b := &r     log.Println(ioutil.ReadAll(*a))     log.Println(ioutil.ReadAll(*b))  } 

but in second call it always results into nil.

Please help me how can i pass multiple separate reference for the same reader?

Answer 1

io.Reader is treated like a stream. Because of this you cannot read it twice. Imagine an incoming TCP connection - you cannot rewind what's coming in.

But you can use the io.TeeReader to duplicate the stream:

package main  import (     "bytes"     "io"     "io/ioutil"     "log"     "strings" )  func main() {     r := strings.NewReader("some io.Reader stream to be read\n")     var buf bytes.Buffer     tee := io.TeeReader(r, &buf)      log.Println(ioutil.ReadAll(tee))     log.Println(ioutil.ReadAll(&buf))  } 

Example on Go Playground

Edit: As @mrclx pointed out: You need to read from the TeeReader first, otherwise the buffer will be empty.