magic number n.
[Unix/C; common] 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.