Thread: Pointer assignment using array name

Could someone please explain why the pointer ptr assignment causes ptr to point at one memory location past the array last element?

I have output the memory locations just for verification. I thought that &a could point to the first element of the array.

0022FF60 0022FF64 0022FF68 0022FF6C
0022FF70
4 3

Code:

#include <stdio.h>

int main(void)
{
  int  a[4] = {1,2,3,4};
  int *ptr = (int*)(&a+1); // Why does this assignement put the ptr pointer
						   // to point at one memory location after the array???
  int *p = a;

  printf("%p %p %p %p\n",p,(p+1),(p+2),(p+3));
  printf("%p \n",ptr);
  printf("%d %d", *(a+3), *(ptr-2) );
  
  return 0;
}

Thanks for your answer but I still do not understand.

What is the difference of

Code:

int *ptr = (int*)(&a+1);

and that of

Code:

int *ptr = (int*)(&a)+1;

since the & operator has a higher priority of the + operator?

If taking the &a, we take tha address of the array (actuallly the address of the first element), then this address plus one should point to the second element, right?

Why does it point to the next memory location past the array?

Code:

int *ptr = (int*)(&a+1);

The above statement will take the size of an array which is 16 bytes and when it is incremented by 1,16 bytes will be added to it.

In the case of second statement,

Code:

int *ptr = (int*)(&a)+1;

The above statement will take the size of an integer pointer which is 4 bytes not the size of an array which is 16 bytes.When it is incremented by 1,4 bytes will be added to it.So,it will point to the second element of the array because expressions within () will be executed first.

Ok, I understand.

I had not seen up to now such a definition where you can take the whole size of an array (besides "sizeof" operator) and add a multiple integral of such size to it.

This is what confused me.

This was an exercise I saw and tried to solve. I wonder if someone can actually take advantage of such assignments in a real program.

Why should someone want to add multiple times the size of an array??

Laserlight you are great.

Now I really understand that. "&a" and "a" are pointing to the same element (first of array) but they are different types of pointers.

Multidimensional arrays could certainly use this technique...you are right.

Thanks