       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
                     written, 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.  Thef-
                     configure  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.
