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

 s
    A socket descriptor created with the socket() function and
    bound to a name using the bind() function or as a result of the
    accept() function.
 buf
    A pointer to a buffer into which received data is placed.
 len
    The size of the buffer pointed to by buf.
 flags
    A bit mask that can contain one or more of the following
    flags. The mask is built by using a logical OR operation on the
    appropriate values.
    Flag               Description
    MSG_OOB            Allows you to receive out-of-band data. If
                       out-of-band data is available, it is read
                       first.
                       If no out-of-band data is available, the
                       MSG_OOB flag is ignored. To send out-of-band
                       data, use the send(), sendmsg(),  and sendto()
                       functions.
    MSG_PEEK           Allows you to examine the data that is next in
                       line to be received without actually removing
                       it from the system's buffers.
 from
    A buffer that the recvfrom() function uses to place the address
    of the sender who sent the data.
    If from is non-null, the address is returned. If from is null,
    the address is not returned.
 fromlen
    Points to an integer containing the size of the buffer pointed to
    by from. On return, the integer is modified to contain the actual
    length of the socket address structure returned.