ssize_t send (int sockfd, const void *buf, size_t len, int flags);
ssize_t sendto (int sockfd, const void *buf, size_t len, int flags,
                const struct sockaddr *to, socklen_t tolen);
ssize_t sendmsg (int sockfd, const struct msghdr *msg, int flags);

struct msghdr
{
  void         *msg_name;       // optional address
  socklen_t    msg_namelen;     // optional address length
  struct iovec *msg_iov;        // array for scatter gather
  size_t       msg_iovlen;      // array for scatter gather length
  void         *msg_control;    // additional control data
  socklen_t    msg_controllen;  // additional control data length
  int          msg_flags;
};