sendmsg 中 定义如下：
struct msghdr
{
void *msg_name; //Address to send to /receive from .
socklen_t msg_namelen; //Length of addres data
strcut iovec * msg_iov; //Vector of data to send/receive into
size_t msg_iovlen; //Number of elements in the vector
void * msg_control; //Ancillary dat
size_t msg_controllen; //Ancillary data buffer length
int msg_flags; //Flags on received message};
对于 msg_iov 结构，如果 msg_iovlen >1, 如何构造msghdr，正确的将消息发送出去。
