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

    You can use this function on any socket to send data to any named
    socket. The data in the msg_iov field of the msghdr structure
    is sent to the socket whose address is specified in the msg_name
    field of the structure. The receiving socket gets the data using
    the read(), recv(),  recvfrom(), or recvmsg() function. When the
    iovec array specifies more than one buffer, the data is gathered
    from all specified buffers before being sent.
    If no space is available to buffer the data on the receiving end
    of the connection, the sendmsg() function blocks until buffer
    space becomes available. If the socket is defined as nonblocking,
    the sendmsg() function fails with an errno indication of
    EWOULDBLOCK. If the message is too large to be sent in one piece
    and the socket type requires that messages be sent atomically
    (SOCK_DGRAM), the sendmsg() fails with an errno indication of
    EMSGSIZE.
    If the address specified is an INADDR_BROADCAST address, the
    SO_BROADCAST socket option must be set and the process must
    have a system UIC, OPER, SYSPRV, or BYPASS privilege for the
    I/O operation to succeed.
    No indication of failure to deliver is implicit in the sendmsg()
    function. All errors (except EWOULDBLOCK) are detected locally.
    You can use the select() function to determine when it is
    possible to send more data.
    Related Functions
    See also read(), recv(),  recvfrom(), recvmsg(), socket(),  and
    getsockopt().