os.Open

a.go

package main

import (
  "fmt"
  "os"
  "log"
)

func main() {
  file, err := os.Open("sample.txt")
  if err != nil {
    log.Fatal(err)
  }
  data := make([]byte, 100)
  count, err := file.Read(data)
  if err != nil {
    log.Fatal(err)
  }
  fmt.Printf("read %d bytes: %q\n", count, data[:count])
  err = file.Close()
  if err != nil {
    log.Fatal(err)
  }
}

sample.txt

Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!

 $ go run a.go
read 100 bytes: "Hello, world!\nHello, world!\nHello, world!\nHello, world!\nHello, world!\nHello, world!\nHello, world!\nHe"