CODEWITHSUNDEEP
cpointersmemory-address
user1471
user1471

Reputation: 469

Pointer arithmetic and address

I have come across a code which goes like :

#include <stdio.h>

int main(void)
{
 int a[5] = { 1, 2, 3, 4, 5};
 int *ptr = (int*)(&a + 1);
 int *ptr2 = (int*) &a;
 ptr2 +=1 ;
 printf("%d %d %d \n", *(a + 1),*(ptr - 1) ,*ptr2 );
 return 0;
}

The pointer arithmetic does it for me except this line :

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

Is it undefined behaviour ? Why do we get 5 on dereferencing *(ptr - 1) ?

Upvotes: 0

Views: 563

Answers (3)

Nguyen Thanh Ho
Nguyen Thanh Ho

Reputation: 59

For an array of n elements of type T, then the address of the first element has type ‘pointer to T’; the address of the whole array has type ‘pointer to array of n elements of type T’;

int *ptr = (int*)(&a + 1); //&a-> address whole array, size=20 bytes, 
         //ptr=&a+1: next element =adress of a +20 bytes.
         //ptr - 1 = address of a +16 = address of last a's element = address of 5

Upvotes: 0

Mike
Mike

Reputation: 49373

Try it out!

int a[5] = {1, 2, 3, 4, 5};
printf("%#x, %#x, %#x, %#x\n", a, &a, a+1, &a+1);

0xbfa4038c, 0xbfa4038c, 0xbfa40390, 0xbfa403a0

So what does that tell us?

0xbfa4038c == 0xbfa4038c which means a == &a. This is the address of the first element in the array or a[0].

We know that the size of an int is 4, and you know that *(a+1) == a[1] (the second element in the array) and this is proven by:

0xbfa4038c + 0x4 = 0xbfa40390 which means a + one int = address of the next element

Thus if we see &a+1 == 0xbfa403a0, that means we're ((0xa0-0x8c)/4) = 5 elements into the array. You know that a[5] is invalid, so that means we're one passed the end of the array.

so if you take:

int *ptr = (int*)(&a + 1); //one passed last element in the array
printf("%d",*(ptr - 1));//back up one, or last element in the array and deference

That's why you get 5

Upvotes: 2

unwind
unwind

Reputation: 399723

The size of a is "5 ints". So &a + 1 refers to the first memory location past all of a, since the pointer arithmetic is done in units of the size of a.

Upvotes: 3

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