Pointer Arithmetic In C

Question

Consider the following code fragment:

int (*p)[3];
int (*q)[3];

q = p;
q++;
printf("%d, %d\n", q, p);
printf("%d\n", q-p);

I know that pointer arithmetic is intelligent, meaning that the operation q++ advances q enough bytes ahead to point to a next 3-integers-array, so it does not surprises me that the first print is '12, 0' which means that incrementing q made it larger in 12.

But the second print does surprises me. It prints 1!
So why would it print 1 instead of 12? it just puzzles me.

Greg Hewgill · Accepted Answer · 2009-04-17 09:07:54Z

27

Like the ++ increment operator, the - subtraction operator with pointers also takes into account the size of the objects being pointed to. Specifically, the result returned is the number of bytes difference in the pointer values divided by the size of the pointed-to object (12, in your example). So the difference is 12 bytes, divided by size 12, or 1.

answered Apr 17, 2009 at 9:07

Greg Hewgill

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3 Comments

Ori Popowski Over a year ago

So there's no way to take two pointers and get their difference in bytes?

Michael Burr Over a year ago

Cast the pointers to (char *) then you'll get the difference in bytes.

j_random_hacker Over a year ago

@Leif: Or just multiply the difference by sizeof(your_type).

AnnaR · Accepted Answer · 2009-04-20 07:04:47Z

4

If you really want to know the difference cast each pointers to a (char*) and then to (int) and then subtract. That should give you the answer.

This code gives you the absolute value:

printf("%d\n", abs((int)((char*)q) - (int)((char*)p)));

Remember to include math.h.

Edit: As pointed out in a comment we don't need a double cast. Casting each pointerpointer to an int and then subtracting gives the same answer as the (unecessary) double casting above.

printf("%d\n", abs((int)(q) - (int)(p)));

edited Apr 20, 2009 at 7:04

answered Apr 17, 2009 at 9:34

AnnaR

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1 Comment

Idelic Over a year ago

That won't work reliably if sizeof(int) < sizeof(pointer). Either cast to intptr_t or just cast to char*.

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