Recursive pointers

Question

I was talking to someone who was new to low level languages, and manual memory management today and doing my best to explain pointers.

He then came up with:

#include <stdio.h>

int main()
{
    int v = 0;
    void* vp = &v;
    int i = 0;
    for(; i < 10; ++i)
    {
        printf("vp: %p, &vp: %p\n", vp, &vp);
        vp = &vp;
    }
}

The output of this is:

vp: 0xbfd29bf8, &vp: 0xbfd29bf4
vp: 0xbfd29bf4, &vp: 0xbfd29bf4
vp: 0xbfd29bf4, &vp: 0xbfd29bf4
vp: 0xbfd29bf4, &vp: 0xbfd29bf4
vp: 0xbfd29bf4, &vp: 0xbfd29bf4
vp: 0xbfd29bf4, &vp: 0xbfd29bf4
vp: 0xbfd29bf4, &vp: 0xbfd29bf4
vp: 0xbfd29bf4, &vp: 0xbfd29bf4
vp: 0xbfd29bf4, &vp: 0xbfd29bf4
vp: 0xbfd29bf4, &vp: 0xbfd29bf4

Which makes no sense to him as vp should != &vp. I'm ashamed to say that I don't know what's going on here.. so, what's going on here?

jpalecek · Accepted Answer · 2012-01-20 10:55:21Z

7

When you have this line in code

    vp = &vp;

it's no surprise that vp == &vp...

In the first iteration, it is as you (and he) expect.

answered Jan 20, 2012 at 10:55

jpalecek

47.9k7 gold badges105 silver badges147 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Kim Sun-wu Over a year ago

Hmn, I didn't realize &vp decayed to vp? Why doesn't it do so in the printf?

Oliver Charlesworth Over a year ago

@KimSun-wu: It doesn't decay.

michael_wu Over a year ago

&vp does not decay to vp. What that assignment does is it stores the address of vp in vp (the RAM cell belonging to vp). So, at address 0xbfd29bf4 you have the value 0xbfd29bf4.

Kim Sun-wu Over a year ago

Ah. Doh. Obviously, this isn't what he was asking, he just wrote the code wrong, and I was too stupid to see it.

sharptooth · Accepted Answer · 2012-01-20 10:56:03Z

1

Once this code has run:

vp = &vp;

the pointer variable now indeed stores address of itself. Note that this only happens after the initial loop iteration - the first line output is

vp: 0xbfd29bf8, &vp: 0xbfd29bf4

and values are not the same yet because the assignment above has not yet been done.

answered Jan 20, 2012 at 10:56

sharptooth

171k110 gold badges545 silver badges1k bronze badges

Comments

Daniel Earwicker · Accepted Answer · 2012-01-20 11:09:53Z

1

Which makes no sense to him as vp should != &vp.

Why shouldn't it, after you've made that assignment?

If it helps, try thinking of every variable as occupying a numbered location in memory, and a pointer as holding one of those numbered memory location. Why shouldn't a void pointer be able to store a value that happens to identify its own location?

answered Jan 20, 2012 at 11:09

Daniel Earwicker

117k38 gold badges209 silver badges289 bronze badges

1 Comment

Kim Sun-wu Over a year ago

What he was actually asking was if he could recursively get pointers to the original pointer (i.e, void****..). I had a brain fart and assumed that's what the above code was doing, my bad.

Collectives™ on Stack Overflow

Recursive pointers

3 Answers 3

4 Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related