VMS Help TCPIP Services, Programming Interfaces, Sockets API, sendmsg(), Arguments  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 s
    A socket descriptor created with the socket() function.
 msg
    A pointer to a msghdr structure containing the message to be
    sent.
    The msg_iov field of the msghdr structure is used as a series of
    buffers from which data is read in order until msg_iovlen bytes
    have been obtained.
 flags
    Can be either 0 or MSG_OOB. If it is equal to MSG_OOB, the data
    is sent out of band. Data can be received before other pending
    data on the receiving socket if the receiver specifies a flag of
    MSG_OOB.