How to store int pointer in void*?

Question

Scenario is as follows:

typedef struct a {
    void* val;
} a_t;

void fun (void** val)
{
    int a = 5;
    *val = &a;
}

a_t *x;
x = malloc (sizeof *x);
fun (&x->val);

printf ("%d", *((int*)(x->val)));

I would expect, that the x->val is of type void* (when used in printf()). How can I get back that int value I stored into it?

https://en.wikipedia.org/wiki/Dangling_pointer – Hans Passant Nov 15 '14 at 16:56 — Hans Passant, Nov 15 '14 at 16:56

score 2 · Accepted Answer · edited May 23 '17 at 10:33

2

The problem is in fun If you expect 5 on STDOUT than this function should look like this:

void fun (void** val)
{
    int *a = malloc(sizeof(int));
    *a = 5;
    *val = a;
}

You should not return pointer to automatic variable because it's allocated on stack and deferred after function executions. To get more info look at this answer

edited May 23 '17 at 10:33

Community

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answered Nov 15 '14 at 16:55

janisz

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If you are OK with the answer, then you should accept it, so your question appears as answered in the feed. :) – gsamaras Nov 15 '14 at 17:09

gsamaras · Answer 2 · 2014-11-15T17:14:19.690

The problem is that the pointer is set to a local variable. So when the function terminates, it's memory will be de-allocated, thus the pointer will be set to garbage (it will become a dangling pointer as Hans suggested).

So if you are lucky, the printf() will print garbage, if not, then it will print 5.

A quick fix would be to add the static keyword to int a, which make it not to be de-allocated when the function gets terminated.

void fun(void** val) {
  static int a = 5;
  *val = &a;
}

However this is not very flexible, because all the instances of fun() will have the same variable a.

Or you could dynamically allocate memory like this:

void fun(void** val) {
  int *a = malloc(sizeof(int));
  *a = 5;
  *val = a;
}

Don't forget to free your memory like this:

  a_t *x;
  x = malloc (sizeof *x);
  fun (&x->val);

  printf ("%d", *((int*)(x->val)));
  free(x->val);  // this has to go first
  free(x);
  return 0;
}

Why a -1? I can't see why. I mean my answer even shows how to de-allocate the memory. — gsamaras, Nov 15 '14 at 17:07

How to store int pointer in void*?

2 Answers2