       fcntl fileId attribute ?value?
              This command either sets or clears a file option or
              returns its current value.  If value is 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.  Thefconfigure
              command is preferred method of getting and  setting
              these attributes.

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