How to read arbitrary amounts of data directly from a file in Go?

Without reading the contents of a file into memory, how can I read "x" bytes from the file so that I can specify what x is for every separate read operation?

I see that the Read method of various Readers takes a byte slice of a certain length and I can read from a file into that slice. But in that case the size of the slice is fixed, whereas what I would like to do, ideally, is something like:

func main() {
	f, err := os.Open("./file.txt")
	if err != nil {
		panic(err)
	}
	someBytes := f.Read(2)
	someMoreBytes := f.Read(4)
}

bytes.Buffer has a Next method which behaves very closely to what I would want, but it requires an existing buffer to work, whereas I'm hoping to read an arbitrary amount of bytes from a file without needing to read the whole thing into memory.

What is the best way to accomplish this?

Thank you for your time.

Use this function:

// readN reads and returns n bytes from the reader.
// On error, readN returns the partial bytes read and
// a non-nil error.
func readN(r io.Reader, n int) ([]byte, error) {
	// Allocate buffer for result
	b := make([]byte, n)
    // ReadFull ensures buffer is filled or error is returned.
	n, err := io.ReadFull(r, b) 
	return b[:n], err
}

Call like this:

someBytes, err := readN(f, 2)
if err != nil  { /* handle error here */
someMoreBytes := readN(f, 4)
if err != nil  { /* handle error here */

you can do something like this:

f, err := os.Open("/tmp/dat")
check(err)
b1 := make([]byte, 5)
n1, err := f.Read(b1)
check(err)
fmt.Printf("%d bytes: %s\n", n1, string(b1[:n1]))

for more reading please check site.