iovec_bytes
iov_bytes = iovec_bytes(iov, iovnum);
size_t iovec_bytes(const struct iovec *iov, size_t iovnum);
if (sendmsg_res != iovec_bytes(iovec, test_data->vecs_cnt)) {
iovec_bytes(iovec, test_data->vecs_cnt),
data_len = iovec_bytes(test_data->vecs, test_data->vecs_cnt);
data_len = iovec_bytes(test_data->vecs, test_data->vecs_cnt);