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

    This function allows a named, unconnected socket to receive
    data. The data is placed in the buffer pointed to by buf, and
    the address of the sender of the data is placed in the buffer
    pointed to by from if from is non-null. The structure that from
    points to is assumed to be as large as the sockaddr structure.
    To receive bytes from any source, the socket does not need to be
    connected.
    You can use the select() function to determine if data is
    available.
    If no data is available at the socket, the recvfrom() call
    waits for data to arrive, unless the socket is nonblocking. If
    the socket is nonblocking, a -1 is returned with the external
    variable errno set to EWOULDBLOCK.
    Related Functions
    See also read(), send(),  sendmsg(), sendto(), and socket().