A.6.6 Pointers and Integers
An expression of integral type may be added to or subtracted from a pointer; in such a case the integral expression is converted as specified in the discussion of the addition operator (Par.A.7.7).
Two pointers to objects of the same type, in the same array, may be subtracted; the result is converted to an integer as specified in the discussion of the subtraction operator (Par.A.7.7).
An integral constant expression with value 0, or such an expression cast to type void *, may be converted, by a cast, by assignment, or by comparison, to a pointer of any type. This produces a null pointer that is equal to another null pointer of the same type, but unequal to any pointer to a function or object.
Certain other conversions involving pointers are permitted, but have implementation-defined aspects. They must be specified by an explicit type-conversion operator, or cast (Pars.A.7.5 and A.8.8).
A pointer may be converted to an integral type large enough to hold it; the required size is implementation-dependent. The mapping function is also implementation-dependent.
A pointer to one type may be converted to a pointer to another type. The resulting pointer may cause addressing exceptions if the subject pointer does not refer to an object suitably aligned in storage. It is guaranteed that a pointer to an object may be converted to a pointer to an object whose type requires less or equally strict storage alignment and back again without change; the notion of alignment is implementation-dependent, but objects of the char types have least strict alignment requirements. As described in Par.A.6.8, a pointer may also be converted to type void * and back again without change. A pointer may be converted to another pointer whose type is the same except for the addition or removal of qualifiers (Pars.A.4.4, A.8.2) of the object type to which the pointer refers. If qualifiers are added, the new pointer is equivalent to the old except for restrictions implied by the new qualifiers. If qualifiers are removed, operations on the underlying object remain subject to the qualifiers in its actual declaration. Finally, a pointer to a function may be converted to a pointer to another function type. Calling the function specified by the converted pointer is implementation-dependent; however, if the converted pointer is reconverted to its original type, the result is identical to the original pointer.