If we were starting from scratch, we'd use 64-bit types unconditionally. But given that we already have off_t that is sometimes 32 bits, we should be consistent with the other systems that have this distinction. That means F_GETLK64 should be a different value from F_GETLK in the ABI, and F_GETLK gets that value under __USE_FILE_OFFSET64 (where 'struct flock' also gets the 'struct flock64' layout).