You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: archive_unix.go
    path of file: ./repos/podman/vendor/github.com/containers/storage/pkg/chrootarchive
    
    the code of the file until where you have to start completion: //go:build !windows && !darwin
// +build !windows,!darwin

package chrootarchive

import (
	"bytes"
	"errors"
	"flag"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"runtime"
	"strings"

	"github.com/containers/storage/pkg/archive"
	"github.com/containers/storage/pkg/reexec"
)

// untar is the entry-point for storage-untar on re-exec. This is not used on
// Windows as it does not support chroot, hence no point sandboxin
    
func untar() {
	runtime.LockOSThread()
	flag.Parse()

	var options archive.TarOptions

	// read the options from the pipe "ExtraFiles"
	if err := json.NewDecoder(os.NewFile(3, "options")).Decode(&options); err != nil {
		fatal(err)
	}

	dst := flag.Arg(0)
	var root string
	if len(flag.Args()) > 1 {
		root = flag.Arg(1)
	}

	if root == "" {
		root = dst
	}

	if err := chroot(root); err != nil {
		fatal(err)
	}

	if err := archive.Unpack(os.Stdin, dst, &options); err != nil {
		fatal(err)
	}
	// fully consume stdin in case it is zero padded
	if _, err := flush(os.Stdin); err != nil {
		fatal(err)
	}

	os.Exit(0)
}