Problem Statement

Pragmatic directives cause the language to operate in a specific manner,   allowing support for operational variances within the program code   (possibly by the loading of specific or alternative modules).

List any pragmatic directives supported by the language,   and demonstrate how to activate and deactivate the pragmatic directives and to describe or demonstrate the scope of effect that the pragmatic directives have within a program.

Let's start with the solution:

ffi = require("ffi")
ffi.cdef[[
  #pragma pack(1)
  typedef struct { char c; int i; } foo;
  #pragma pack(4)
  typedef struct { char c; int i; } bar;
]]
print(ffi.sizeof(ffi.new("foo")))
print(ffi.sizeof(ffi.new("bar")))