2014-04-26 10:52:28 -04:00
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magic number
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2014-04-26 11:54:15 -04:00
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n. [Unix/C; common] 1. In source code, some non-obvious constant whose value
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is significant to the operation of a program and that is inserted
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inconspicuously in-line ( hardcoded ), rather than expanded in by a symbol
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set by a commented #define. Magic numbers in this sense are bad style. 2. A
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number that encodes critical information used in an algorithm in some opaque
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way. The classic examples of these are the numbers used in hash or CRC
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functions, or the coefficients in a linear congruential generator for
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pseudo-random numbers. This sense actually predates and was ancestral to the
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more common sense 3. Special data located at the beginning of a binary data
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file to indicate its type to a utility. Under Unix, the system and various
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applications programs (especially the linker) distinguish between types of
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executable file by looking for a magic number. Once upon a time, these magic
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numbers were PDP-11 branch instructions that skipped over header data to the
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start of executable code; 0407, for example, was octal for branch 16 bytes
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relative. Many other kinds of files now have magic numbers somewhere; some
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magic numbers are, in fact, strings, like the ! arch at the beginning of a
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Unix archive file or the %! leading PostScript files. Nowadays only a wizard
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knows the spells to create magic numbers. How do you choose a fresh magic
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number of your own? Simple you pick one at random. See? It's magic! 4. An
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input that leads to a computational boundary condition, where algorithm
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behavior becomes discontinuous. Numeric overflows (particularly with signed
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data types) and run-time errors (divide by zero, stack overflows) are
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indications of magic numbers. The Y2K scare was probably the most notorious
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magic number non-incident. The magic number, on the other hand, is 72. See
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The magical number seven, plus or minus two: some limits on our capacity for
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processing information by George Miller, in the Psychological Review
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63:81-97 (1956). This classic paper established the number of distinct items
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(such as numeric digits) that humans can hold in short-term memory. Among
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other things, this strongly influenced the interface design of the phone
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system.
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