In computing, a magic cookie, or just cookie for short, is a token or short packet of data passed between communicating programs. The cookie is often used to identify a particular event or as "handle, transaction ID, or other token of agreement between cooperating programs".

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 operating systems with a non-byte-stream model of files, the result of "ftell" may be a magic cookie rather than a byte offset; it can be passed to "fseek", 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. 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. [Jargon File] (1995-01-25)

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