magic cookie n.
[Unix; common] 1. Something passed between routines or
programs that enables the receiver to perform some operation; a
capability ticket or opaque identifier. Especially used of small
data objects that contain data encoded in a strange or
intrinsically machine-dependent way. E.g., on non-Unix OSes with
a non-byte-stream model of files, the result of
ftell(3) may be a magic cookie rather than a byte
offset; it can be passed to fseek(3), but not
operated on in any meaningful way. The phrase `it hands you a
magic cookie' means it returns a result whose contents are not
defined but which can be passed back to the same or some other
program later. 2. An in-band code for changing graphic rendition
(e.g., inverse video or underlining) or performing other control
functions (see also cookie). Some
older terminals would leave a blank on the screen corresponding
to mode-change magic cookies; this was also called a glitch (or occasionally a `turd'; compare mouse droppings). See also
cookie.