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.
The Jargon File
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