Question

Does memory allocation for pointer is same in both scenarios

struct_datatype * p=NULL;

struct_datatype * p=malloc;

apart from memory allocation from stack and heap ??

Answer 1

The question is not clear what you are asking about.

struct_datatype * p=NULL;
No memory has been assigned to p. It points to no allocated memory.

struct_datatype * p=malloc();
Some memory might have been allocated and the address of the memory allocated has been assigned to p. If malloc failed NULL would have been assigned to p.

NOTE: sizeof p will be sizeof struct_datatype* for both the cases.

NOTE: The pointer variable p will have been assigned memory in the stack which is sizeof struct_datatype* in both the cases during compile time. The address stored in pointer p might be pointing to any valid memory address accessible in user address space. In the NULL case no address is assigned( and NULL is not user accessible address) but when you do malloc(if successful) the address of the memory allocated will be assigned to the pointer p from the heap space else again it will be assigned to NULL.