How to dereference a double void pointer to an int pointer

Question

I have a code that looks somewhat like this:

int num = 5;
int *ptr = #
void **xptr = &ptr;
printf ("values:%d\n",**(int *)xptr); 

Why can't i de-reference a void double pointer,which points to an int pointer ? The below two examples work.

Snippet:1

int *ptr = #
int **xptr = &ptr;
printf ("values:%d\n",**xptr);

Snippet 2:

void *ptr = #
printf ("values:%d\n",*(int *)ptr);

Answer 1

void ** is not a generic pointer unlike void *. Any void ** value you play with must be the address of an actual void * value somewhere. Compiler should raise a warning for void **xptr = &ptr;:

[Warning] initialization from incompatible pointer type [enabled by default]

You can do it as follows

int num = 5;
void *ptr = #
void **xptr = &ptr;
printf ("values:%d\n", *((int *)*xptr));  

For more detailed explanation, read comp.lang.c FAQ list · Question 4.9.