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

 nfds
    Specifies the number of open objects that may be ready for
    reading or writing or that have exceptions pending. The nfds
    argument is normally limited to FD_SETSIZE. Note that a single
    process can have a maximum of 65535 simultaneous channels
    (including sockets) on OpenVMS Alpha systems, and a maximum of
    2047 on OpenVMS VAX systems.
 readfds
    A pointer to an array of bits, organized as integers, that should
    be examined for read readiness. If bit n of the longword is set,
    socket descriptor n is checked to see whether it is ready to be
    read. All bits set in the bit mask must correspond to the file
    descriptors of sockets. The select() function cannot be used on
    normal files.
    On return, the array to which readfds points contains a bit mask
    of the sockets that are ready for reading. Only bits that were
    set on entry to the select() function can be set on exit.
 writefds
    A pointer to an array of bits, organized as integers, that should
    be examined for write readiness. If bit n of the longword is
    set, socket descriptor n is checked to see whether it is ready
    to be written to. All bits set in the bit mask must correspond to
    socket descriptors.
    On return, the array to which writefds points contains a bit mask
    of the sockets that are ready for writing. Only bits that were
    set on entry to the select() function are set on exit.
 exceptfds
    A pointer to an array of bits, organized as integers, that
    is examined for exceptions. If bit n of the longword is set,
    socket descriptor n is checked to see whether it has any pending
    exceptions. All bits set in the bit mask must correspond to the
    file descriptors of sockets.
    On return, the array exceptfds pointer contains a bit mask of the
    sockets that have exceptions pending. Only bits that were set on
    entry to the select() function can be set on exit.
 timeout
    The length of time that the select() function should examine the
    sockets before returning. If one of the sockets specified in the
    readfds, writefds, and exceptfds bit masks is ready for I/O, the
    select() function returns before the timeout period expires.
    The timeout argument points to a timeval structure.