Storing an address or NULL at an address relative to a pointer

Question

I was wondering if there is a way I can store a NULL value or an address at an address relative to a pointer by using pointer arithmetic in C.

int *p;
p = NULL;  // is possible

int *p, *q;
p + 1 = NULL;  // ERROR: lvalue required as left operand of assignment
p + 1 = q;     // ERROR: lvalue required as left operand of assignment

Answer 1

You would need to do something like this, assuming p points to a valid piece of memory that you have rights to writing to:

int **p = something;
p[1] = NULL;
p[1] = q;

Answer 2

It isn't exactly clear to me what you are asking. You can set a pointer to a NULL value and you can point to a value that is NULL. You cannot set a pointer to NULL then attempt to use it as a target for a value. Here is an example of using pointer arithmetic

 int *p = (int*) malloc(3 * sizeof(int));
 *p = 0; // dereference p and set it's value to 0
 *(p + 1) = 1; // add 1 sizeof(int) to p and set it's value to 1

NULL is just an alias for 0.

Answer 3

int *p, *q;

*(unsigned int *)(p + 1) =  (intptr_t)NULL;
// or
*(unsigned int *)(p + 1) =  (intptr_t)q;

This seemed to work for me.