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

    This function can be used on sockets to send data to named
    sockets. The data in the msg buffer is sent to the socket whose
    address is specified in the to argument, and the address of
    socket s is provided to the receiving socket. The receiving
    socket gets the data using the read(), recv(),  recvfrom(), or
    recvmsg() function.
    If there is no space available to buffer the data being sent
    on the receiving end of the connection, the sendto() function
    blocks until buffer space becomes available. If the socket is
    defined as nonblocking, the sendto() 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 sendto() function fails with an
    errno indication of EMSGSIZE.
    No indication of failure to deliver is implicit in a sendto().
    All errors (except EWOULDBLOCK) are detected locally. You can use
    the select() function to determine when it is possible to send
    more data.
    If the address specified is a INADDR_BROADCAST address, then the
    SO_BROADCAST socket option must have been set and the process
    must have SYSPRV or BYPASS privilege for the I/O operation to
    succeed.
    Related Functions
    See also read(), recv(),  recvfrom(), recvmsg(), socket(),  and
    getsockopt().