magic cookie(Noun)

A token or short packet of data passed between communicating programs, used to identify a particular event or transaction; the data is typically not meaningful to the recipient program and not usually interpreted until the recipient passes the data back to the sender or another program at a later time.

magic cookie

[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.