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.