The getdtablesize function returns the total number of file
    descriptors that a process can have open simultaneously. Each
    process is limited to a fixed number of open file descriptors.

    The number of file descriptors that a process can have open is
    the minumum of the following:

    o  C RTL open file limit-65535 on OpenVMS Alpha and
       Integrity servers.

    o  SYSGEN CHANNELCNT parameter-permanent I/O channel count.

    o  Process open file quota FILLM parameter-number of open files
       that can be opened by a process at one time.