a problem about XLogReader callback system

Hi, hackers

I have a problem about XLogReader callback system

In xlog.c, function StartupXLOG

    xlogreader =
        XLogReaderAllocate(wal_segment_size, NULL,
                           XL_ROUTINE(.page_read = &XLogPageRead,
                                      .segment_open = NULL,
                                      .segment_close = wal_segment_close),
                           &private);

XLogPageReader uses readFile to store the fd, and I can't find any location to set the value of seg.ws_file.
Is it necessary to set a segment_close callback?

Best regards
Wang