magic number n. [Unix/C; common] 1. In source code,

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.