

          fcntl handle attribute [value]
               This command either sets or clears a file option or
               returns its current value.  If value are not specified,
               then the current value of attribute is returned. 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.

               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 written, 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.
