The Standard specifies that a pointer can be converted to an
integral type, but the size of the integer required and the result
are implementation-defined. A pointer occupies the same amount of
storage as objects of type int or long (or
their unsigned equivalents). Therefore, a pointer can
be converted to any of these integer types and back again without
changing its value. No scaling takes place, and the representation
of the value does not change.
Converting between a pointer and a shorter integer type, such as
char , is similar to the conversion between an object
of unsigned long type and a shorted integer type. The
high-order bits of the pointer are discarded. Converting between a
shorter integer and a pointer is similar to the conversion between
the shorter integer type and unsigned long . The high-
order bits of the pointer are filled with copies of the sign bit if
the shorter integer type was signed. Messages are issued for cast
operations of these types under the error-checking compiler option.