if err != nil {
				Exitf("cannot reopen %s: %v", h.pn, err)
			}
			defer f.Close()
			if _, err := f.Seek(h.off, 0); err != nil {
				Exitf("cannot seek %s: %v", h.pn, err)
			}

			w, err := os.Create(dst)
			if err != nil {
				Exitf("cannot create %s: %v", dst, err)
			}
			if _, err := io.CopyN(w, f, h.length); err != nil {
				Exitf("cannot write %s: %v", dst, err)
			}

	if hex.DecodedLen(len(val)) > maxLen {
		Exitf("-B option too long (max %d digits): %s", maxLen, ov)
	}

	b, err := hex.DecodeString(val)
	if err != nil {
		if err == hex.ErrLength {
			Exitf("-B argument must have even number of digits: %s", ov)
		}
		if inv, ok := err.(hex.InvalidByteError); ok {
			Exitf("-B argument contains invalid hex digit %c: %s", byte(inv), ov)
		}
		Exitf("-B argument contains invalid hex: %s", ov)