 executable (compiled
     program) a.out file, whose format is defined in &lt;elf.h&gt;, &lt;a.out.h&gt; and
     possibly &lt;exec.h&gt; in the standard include directory.  These files have a
     ‘magic number’ stored in a particular place near the beginning of the
     file that tells the UNIX operating system that the file is a binary exe‐
     cutable, and which of several types thereof.  The concept of a ‘magic’
     has been applied by extension to data files.  Any file with some invari‐