Using extern in a function declaration we can make a function such that
it can used
outside the file in which it is defined.
An extern variable, function definition, or declaration also makes the
variable or function usable by the succeeding part of the current source file.
declaration does not replace the definition. The declaration is used to
variable that is externally defined.
If a declaration for an identifier already exists at file scope, any extern
the same identifier found within a block refers to that same object. If no
declaration for the identifier exists at file scope, the identifier has