if (chunk > kPosixCopyFileBufferSize) chunk = kPosixCopyFileBufferSize;

    rc = read(src_fd, buffer.get(), chunk);
    if (rc < 0) return -1;

    int total_write = 0;
    int total_read = rc;
    while (total_write < total_read && rc > 0) {
      rc = write(dst_fd, buffer.get() + total_write, chunk - total_write);
      if (rc < 0) return -1;

      total_write += rc;
      bytes_transferred += rc;
      offset += rc;