The Jargon File (version 4.4.7, 29 Dec 2003):

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.


The Free On-line Dictionary of Computing (30 December 2018):

magic cookie

   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)