Demonstration of file seek handle in Go.

go_seek_handle.go

package main

import (
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"strings"
)

func GetFile() *os.File {
	// Create a temporary file.
	file, _ := ioutil.TempFile("", "my_temp_file_*")

	// Write something,
	file.WriteString("Hello World")

	// Sync/flush in-memory content to disk.
	file.Sync()

	return file
}

func main() {
	f := GetFile()
	defer os.Remove(f.Name())

	// If you try to read anything from this file now,
	// you'll get empty result!
	buf := new(strings.Builder)
	io.Copy(buf, f)
	fmt.Println(buf.String()) // output: ""

	// Reason: the seek handle (pointer to the current
	// cursor position) has changed and now pointing
	// to the end of the file after writing the
	// previous content. (imagine the cursor position
	// when you type on a keyboard).

	// Fix: simply move the seek handle back to the
	// starting position.
	f.Seek(0, 0)

	buf = new(strings.Builder)
	io.Copy(buf, f)
	fmt.Println(buf.String()) // output: "Hello World"
}