       fcntl fileId attribute ?value?
              This  command either sets or clears a file option or returns its
              current value.  If value is  not  specified,  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.  Thefconfigure command is  preferred  method
              of getting and setting these attributes.

              The  APPEND  and  CLOEXEC  options  are not available on Windows
              95/NT.
