     private(set) var prefix: String?
 
     // These fields are present in gnu and star formats.
-    // - magic
+    // - magic ("ustar  \0" for [old] gnu)
     private(set) var atime: Date?
     private(set) var ctime: Date?
 

    let linkName: String

    // Ustar only
    // - magic
    private(set) var uname: String?
    private(set) var gname: String?
    private(set) var deviceMajorNumber: Int?

                out.append(Data(count: 155)) // Empty prefix
            }
        } else if format == .gnu {
            out.append(contentsOf: [0x75, 0x73, 0x74, 0x61, 0x72, 0x20, 0x20, 0x00]) // "ustar  \0"
            if let atime = self.atime?.timeIntervalSince1970 {
                out.append(tarInt: Int(atime), maxLength: 12)
            } else {
                out.append(tarInt: nil, maxLength: 12)
            }
            if let ctime = self.ctime?.timeIntervalSince1970 {
                out.append(tarInt: Int(ctime), maxLength: 12)
            } else {
                out.append(tarInt: nil, maxLength: 12)
            }
        }

        // Checksum calculation.
