How sendmsg works?

Question

As you know sendmsg has this declaration:

int sendmsg(int s, const struct msghdr *msg, int flags);

and msghdr

Answer 1

According to http://opengroup.org/onlinepubs/007908799/xns/sendmsg.html ...

The data from each storage area indicated by msg_iov is sent in turn.

My interpretation is that sendmsg() will not concatenate the message data stored in the iovec's; each will be sent as a separate message.

[Edit: My interpretation was not correct; see the other answers for a better explanation.]