Confused by #define and typedef

Question

#define T Stack_T
typedef struct T *T;

Then what does T in struct T mean,the one defined by #define or

Answer 1

#define directives are substituted early on in the compilation process (translation phase 4, compilation doesn't actually occur until phase 7, these phases and what happens during them are detailed in the standard, section 5.1.1.2).

That #define will simply change T pre-processing tokens into Stack_T.

The effect of that on the typedef will be to turn it into:

typedef struct Stack_T *Stack_T;

Following that, Stack_T is defined as a type, a pointer to another type of struct Stack_T. The Stack_T and struct Stack_T are two separate things.