int err = do_flush (file, flush);
int ZEXPORT gzflush (file, flush)     gzFile file;     int flush;{    gz_stream *s = (gz_stream*)file;    int err = do_flush (file, flush);    if (err) return err;    fflush(s->file);    return  s->z_err == Z_STREAM_END ? Z_OK : s->z_err;}