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

The preprocessor does only do text-substitutions, so that code would look like

typedef struct Stack_T *Stack_T;

So every T in your code is first replaced to Stack_T, and after that your compiler kicks in, sees the typedef and uses struct Stack_T*.

It might be good to know that struct Type and Type are only the same in C++, not in C.