posix_info_sid_size
owner_len = posix_info_sid_size(sidsbuf, sidsbuf_end);
group_len = posix_info_sid_size(sidsbuf + owner_len, sidsbuf_end);
sid_len = posix_info_sid_size(sid, end);
sid_len = posix_info_sid_size(sid, end);
owner_len = posix_info_sid_size(owner_sid, end);
group_len = posix_info_sid_size(group_sid, end);
int posix_info_sid_size(const void *beg, const void *end);