main:

example application to use the RawBytes() of 'archive/tar',
and show the diff of the bytes read from the file itself.

main.go

@@ -0,0 +1,62 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+
+	"./archive/tar"
+)
+
+func main() {
+	flag.Parse()
+	log.SetOutput(os.Stderr)
+	for _, arg := range flag.Args() {
+		func() {
+			// Open the tar archive
+			fh, err := os.Open(arg)
+			if err != nil {
+				log.Fatal(err)
+			}
+			defer fh.Close()
+
+			fi, err := fh.Stat()
+			if err != nil {
+				log.Fatal(err)
+			}
+			size := fi.Size()
+			var sum int64
+			tr := tar.NewReader(fh)
+			tr.RawAccounting = true
+			for {
+				hdr, err := tr.Next()
+				if err != nil {
+					if err != io.EOF {
+						log.Println(err)
+					}
+					break
+				}
+				pre := tr.RawBytes()
+				var i int64
+				if i, err = io.Copy(ioutil.Discard, tr); err != nil {
+					log.Println(err)
+					break
+				}
+				post := tr.RawBytes()
+				fmt.Println(hdr.Name, "pre:", len(pre), "read:", i, "post:", len(post))
+				sum += int64(len(pre))
+				sum += i
+				sum += int64(len(post))
+			}
+
+			if size != sum {
+				fmt.Printf("Size: %d; Sum: %d; Diff: %d\n", size, sum, size-sum)
+			} else {
+				fmt.Printf("Size: %d; Sum: %d\n", size, sum)
+			}
+		}()
+	}
+}