       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. The following attributes  may  be  speci-
              fied:

              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.

              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.  Note
              that SIGPIPE is  set  to  be  ignored  by  the  Tcl
              library  to  support pipes to processes in the exec
              and open commands.  If SIGPIPE is ignored, an error
              is  returned on the write.  This attribute is valid
              only on sockets.

              The APPEND, NONBLOCK, and CLOEXEC attributes may be
              set or cleared by specifying the attribute name and
              a value 1 to set the attribute and 0 to clear it.
              This command is provided by Extended Tcl.
