magic number

magic number
    n 1: the atomic number of an extra stable strongly bound atomic
         nucleus: 2, 8, 20, 28, 50, 82 or 126
    

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 common sense

   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!

   4. An input that leads to a computational boundary condition, where
   algorithm behavior becomes discontinuous. Numeric overflows
   (particularly with signed data types) and run-time errors (divide by
   zero, stack overflows) are indications of magic numbers. The Y2K scare
   was probably the most notorious magic number non-incident.

   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.
    

magic number

   <jargon, programming> 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
   ({hard-coded}), 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
   {pseudorandom} 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 {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".  Nowadays
   only a {wizard} knows the spells to create magic numbers.  {MS
   DOS} executables begin with the magic string "MZ".

   *The* magic number, on the other hand, is 7+/-2.  The paper
   cited below 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.

   ["The magical number seven, plus or minus two: some limits on
   our capacity for processing information", George Miller, in
   the "Psychological Review" 63:81-97, 1956].

   [{Jargon File}]

   (2003-07-02)