On Wed, Aug 26, 1998 at 07:54:24PM +0800, htyj wrote:
> I've seen the term "magic number" in many documents, I wonder what it is,
> and how to get it(calculate it?)? TIA.
Courtesy of "dict":
: magic number /n./ [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 ({hardcoded}),
: 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 algorithm in some opaque way. The
: classic examples of these are the numbers used in hash or CRC
: functions, or the coefficients in a linear congruential generator for
: pseudo-random numbers. This sense actually predates and was ancestral
: to the more commonsense 1. 3. Special data located at the beginning
: of a binary data file to indicate its type to a utility. Under Unix,
: the system and various applications programs (especially the linker)
: distinguish between types of executable file by looking for a magic
: number. Once upon a time, these magic numbers were PDP-11 branch
: instructions that skipped over header data to the start of executable
: code; 0407, for example, was octal for `branch 16 bytes relative'.
: Many other kinds of files now have magic numbers somewhere; some magic
: numbers are, in fact, strings, like the `!<arch>' at the beginning of
: a Unix archive file or the `%!' leading PostScript files. Nowadays
: only a {wizard} 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.
Sense #3 is likely to be the one you're looking for. See also file(1) and
magic(5).
HTH,
Ray
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