C Programming- Pointers

Question

I came across the following code, the output for which is 0.

Is the statement vptr = &i correct? Can we assign a void pointer, address of an integer variable?

#include<stdio.h>

void fun(void *p);
int i;

int main()
{
    void *vptr;
    vptr = &i;
    fun(vptr);
    return 0;
}
void fun(void *p)
{
    int **q;
    q = (int**)&p;
    printf("%d\n", **q);
}

Define "correct". Syntax? Semantics? Usage? Behavior? Practice? — Sourav Ghosh
– Sourav Ghosh, Commented Sep 9, 2016 at 17:29
A void pointer can be assigned a memory address of anything. It is sometimes used in C to program "generics" — element11
– element11, Commented Sep 9, 2016 at 17:29
@SouravGhosh correct means its behaviour, can we assign an integer type memory to void using the above statement? — Shubham Bhatia
– Shubham Bhatia, Commented Sep 10, 2016 at 18:42

aschepler · Accepted Answer · 2016-09-09 17:32:11Z

8

The statement vptr = &i; is fine.

However, the statement q = (int**)&p; is incorrect. &p does not point at an int*, it points at a void*. It is not guaranteed that int* and void* have compatible layouts.

A correct implementation of fun would be

void fun(void *p)
{
    printf("%d\n", *(int*)p);
}

answered Sep 9, 2016 at 17:32

aschepler

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Comments

Eugene Sh. · Accepted Answer · 2016-09-09 17:32:57Z

0

Any pointer type can be converted into void*. From the C standard 6.3.2.3p1:

A pointer to void may be converted to or from a pointer to any object type. A pointer to any object type may be converted to a pointer to void and back again; the result shall compare equal to the original pointer.

answered Sep 9, 2016 at 17:32

Eugene Sh.

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