[This is] another of the differences between typedef and #define.

Type qualifiers like const can affect a pointer variable in two (or more) different ways: either the pointer can be qualified, or the value pointed to. As question 11.9 in the C FAQ list explains, when you write

	int * const p;

the pointer p is qualified, but when you write

	const int * p;

the value pointed to is qualified.

Now, if you say

	#define x int*

and then

	const x y;

the result is exactly as if you had written

	const int* y;

so it is the pointed-to value that is qualified. The preprocessor always does simple textual substitutions, and these take place before the compiler performs its parsing phase.

When you say

	typedef int * x;

on the other hand, x is a new type, encapsulating the notion “pointer to int”. Now, when you say

	const x y;

it is the variable y that is qualified, just as if you'd said

	const int y;