       fcntl fileId attribute ?value?
              This command either sets or clears a file option or
              returns its current value.  If value are not speci-
              fied,  then  the  current  value  of  attribute  is
              returned.  All values are boolean. Some  attributes
              maybe  only be gotten, not modified.  The following
              attributes may be specified:

              RDONLY - The file is opened for reading only.  (Get
              only)

              WRONLY - The file is opened for writing only.  (Get
              only)

              RDWR - The file is opened for reading and  writing.
              (Get only)

              READ - If the file is readable. (Get only).

              WRITE - If the file is writable. (Get only).

              APPEND - The file is opened for append-only writes.
              All writes will be forced to the end of  the  file.
              (Get or set).

              NONBLOCK  -  The  file  is to be accessed with non-
              blocking I/O.  See  the  read  system  call  for  a
              description  of how it affects the behavior of file
              reads.

              CLOEXEC - Close the file on an  process  exec.   If
              the  execl  command  or some other mechanism causes
              the process to do an exec, the file will be  closed
              if this option is set.

              NOBUF  -  The  file  is  not buffered. If set, then
              there no buffering for the file.

              LINEBUF - Output the file will  be  line  buffered.
              The  buffer will be flushed when a newline is writ-
              ten, when the buffer is  full,  or  when  input  is
              requested.

              KEEPALIVE  -  Keep  a  socket connection alive.  If
              SIGPIPE is enabled, then it is sent  if  connection
              is  broken  and  data is written to the socket.  If
              SIGPIPE is ignored, an error  is  returned  on  the
              write.   This  attribute  is valid only on sockets.
              By default, SIGPIPE is ignored in Tcl.

              The NONBLOCK, NOBUF and LINEBUF  are  provided  for
              compatibility  with  older  scripts.  Thefconfigure
              command is preferred method of getting and  setting
              these attributes.
              The APPEND and CLOEXEC options are not available on
              Windows 95/NT.

              This command is provided by Extended Tcl.
