DECC$POSIX_SEEK_STREAM_FILE

   With DECC$POSIX_SEEK_STREAM_FILE enabled, positioning beyond
   end-of-file on STREAM files does not write to the file until the
   next write. If the write is beyond the current end-of-file, this
   positions beyond the old end-of-file, and the start position for
   the write is filled with zeros.

   With DECC$POSIX_SEEK_STREAM_FILE disabled, positioning beyond
   end-of-file will immediately write zeros to the file from the
   current end-of-file to the new position.