package main

import (
  "fmt"
  "strings"
)

func main() {
  reader := strings.NewReader("abcdefgh")
  p := make([]byte, 6)
  _, err := reader.ReadAt(p, 2)
  if err != nil {
    panic(err)
  }
  fmt.Printf("%s\n", p)

}

输出：

cdefgh