How exactly pointer subtraction works in case of integer array?

Question

#include<stdio.h>
int main()
{
    int arr[] = {10, 20, 30, 40, 50, 60};
    int *ptr1 = arr;
    int *ptr2 = arr + 5;
    printf("Number of elements between two pointer are: %d.", 
                                (ptr2 - ptr1));
    printf("Number of bytes between two pointers are: %d",  
                              (char*)ptr2 - (char*) ptr1);
    return 0;
}

For the first printf() statement the output will be 5 according to Pointer subtraction confusion

What about the second printf() statement, what will be the output?

Have you read the answer to the question you referred to? Think about it that way: when you add +5 to a pointer to an int, it advances 5 ints in size in memory. How many chars worth of memory is that? — Marcus Müller, Jul 08 '15 at 12:19

Sourav Ghosh · Accepted Answer · 2015-07-08T12:27:18.617

9

To quote C11, chapter §6.5.6, Additive operators

When two pointers are subtracted, both shall point to elements of the same array object, or one past the last element of the array object; the result is the difference of the subscripts of the two array elements.

So, when you're doing

printf("Number of elements between two pointer are: %d.", 
                            (ptr2 - ptr1));

both ptr1 and ptr2 are pointers to int, hence they are giving the difference in subscript, 5. In other words, the difference of the address is counted in reference to the sizeof(<type>).

OTOH,

 printf("Number of bytes between two pointers are: %d",  
                          (char*)ptr2 - (char*) ptr1);

both ptr1 and ptr2 are casted to a pointer to char, which has a size of 1 byte. The calculation takes place accordingly. Result: 20.

FWIW, please note, the subtraction of two pointers produces the result as type of ptrdiff_t and you should be using %td format specifier to print the result.

edited Jul 08 '15 at 12:27

answered Jul 08 '15 at 12:22

Sourav Ghosh

133,132
16
183
261

1

wow, Joachim and you answering the same question. There's an unholy aggregation of points here :) – Marcus Müller Jul 08 '15 at 12:24
1

Didn't know about `%td`. Neat ! – Quentin Jul 08 '15 at 12:33
1

@SouravGhosh: It's just cool to see the both of you answer within the same minute with quite a complex answer -- gives you the feeling there's a very solid base of competence on SO :) – Marcus Müller Jul 08 '15 at 13:23

Vlad from Moscow · Answer 2 · 2015-07-08T12:47:53.767

If you have two pointers of type T that point to elements of the same array then the difference of the pointers yields the number of elements of type T between these pointers

So the first output statement

printf("Number of elements between two pointer are: %d.", 
                            (ptr2 - ptr1));

outputs 5 - the number of elements of type int between pointers ptr1 and ptr2.

It is the so-called pointer arithmetic.

Pointers (char*)ptr1 and (char*)ptr2 have the same values as the original pointers ptr1 and ptr2 but they consider (reinterpret) the memory extent as an array of type char each element of which has size equal to sizeof( char ). In C sizeof( char ) is always equal to 1. Thus the difference ( char * )ptr2 - ( char * ) ptr1 gives the number of elements of type char that can fit the memory extent. It is evident that sizeof( char ) is not greater than sizeof( int ). So the same memory extent can accomodate more elements of type char than of type int. If for example sizeof( int ) is equal to 4 then the memory extent can accomodate 5 * sizeof( int ) elements of type char that is 20.

score 4 · Answer 3 · answered Jul 08 '15 at 12:22

Pointer arithmetics is always in the units of the base type.

In your case you have

int *ptr1 = ...

Then doing ptr1 + 5 will add sizeof(*ptr1) * 5 bytes to the pointer ptr1. Considering that sizeof(*ptr1) (which is the same as sizeof(int)) is 4, then you get 4 * 5 which equals 20 bytes.

score 4 · Answer 4 · answered Jul 08 '15 at 12:23

4

Each array element is an int and there are 5 elements between both the pointers. Thus there will be 5*sizeof(int) amount of bytes in between both the pointers.

answered Jul 08 '15 at 12:23

vineeth suhas

86
1
9

How exactly pointer subtraction works in case of integer array?

4 Answers4

Linked

Related