Is your device using Apple File System (APFS)?

APFS.swift

// the original code by apple and its license
// https://developer.apple.com/library/prerelease/content/samplecode/APFSCloning/Listings/clonefile_main_c.html#//apple_ref/doc/uid/TP40017341-clonefile_main_c-DontLinkElementID_5

        NSLog("%@", "\(ProcessInfo().operatingSystemVersion)")

        /* Initailize state */
        let s = copyfile_state_alloc();

        let srcPath = NSHomeDirectory() + "/tmp/test.txt"
        let copyPath = NSHomeDirectory() + "/tmp/test-copy.txt"

        _ = try? FileManager.default.removeItem(atPath: srcPath)
        _ = try? FileManager.default.removeItem(atPath: copyPath)

        NSLog("--------------------------------\n")
        NSLog("%@", "srcPath: \(try? String(contentsOf: URL(fileURLWithPath: srcPath)))")
        NSLog("%@", "copyPath: \(try? String(contentsOf: URL(fileURLWithPath: copyPath)))")

        try! "1234567890".write(to: URL(fileURLWithPath: srcPath), atomically: true, encoding: .utf8)

        NSLog("--------------------------------\n")
        NSLog("%@", "srcPath: \(try? String(contentsOf: URL(fileURLWithPath: srcPath)))")
        NSLog("%@", "copyPath: \(try? String(contentsOf: URL(fileURLWithPath: copyPath)))")

        /* Copy the file 
         (but clone if available)  */
        srcPath.withCString { src in
            copyPath.withCString { dst in
                if (copyfile(src, dst, s, UInt32(COPYFILE_CLONE)) < 0) {
                    perror("Error in copyfile()");
                    exit(1);
                }
            }
        }

        /* Was the file cloned ? */
        var was_cloned: Bool = false
        if ((copyfile_state_get(s, UInt32(COPYFILE_STATE_WAS_CLONED), &was_cloned) == 0) && was_cloned) {
            NSLog("File was cloned\n");
        } else {
            NSLog("File was copied and not cloned\n");
        }

        NSLog("--------------------------------\n")
        NSLog("%@", "srcPath: \(try? String(contentsOf: URL(fileURLWithPath: srcPath)))")
        NSLog("%@", "copyPath: \(try? String(contentsOf: URL(fileURLWithPath: copyPath)))")

        /* Release the state variable */
        copyfile_state_free(s);