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 stdio 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.

              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.

              The NOBUF and LINEBUF attributes may only be set (a
              value of 1) and only one  of  the  options  may  be
              selected.   Once set, it may not be changed.  These
              options should be set  before  any  I/O  operations
              have been done on the file or data may be lost.

              This command is provided by Extended Tcl.