Using off_t, at least, in DiskAdaptor layer is problematic because
torrent can contain under 2GiB files but total sum of those files may
exceed 2GiB limit, which makes off_t overflow in 32 bit system without
large file support. So we use int64_t in API. We'll check the file
length before download so that it does not exceed max off_t.
Tatsuhiro Tsujikawa