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| magic number (Unix, C) 1. In source code, some non-obvious constant whose value is significant to the operation of a program and that is inserted inconspicuously in-line (), rather than expanded in by a symbol set by a commented "#define". Magic numbers in this sense are bad style. 2. A number that encodes critical information used in an in some opaque way. The classic examples of these are the numbers used in or functions or the coefficients in a for numbers. This sense actually predates, and was ancestral to, the more common sense 1. 3. Special data located at the beginning of a binary data file to indicate its type to a utility. Under , the system and various s (especially the ) distinguish between types of executable file by looking for a magic number. Once upon a time, these magic numbers were branch instructions that skipped over header data to the start of executable code; 0407, for example, was for "branch 16 bytes relative". Nowadays only a knows the spells to create magic numbers. How do you choose a fresh magic number of your own? Simple - you pick one at random. See? It's magic! *The* magic number, on the other hand, is 7+/-2. See "The magical number seven, plus or minus two: some limits on our capacity for processing information" by George Miller, in the "Psychological Review" 63:81-97 (1956). This classic paper established the number of distinct items (such as numeric digits) that humans can hold in short-term memory. Among other things, this strongly influenced the interface design of the phone system. [] (1995-01-25) |
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