如何将双重void指针解引用到int指针

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

The solution is to use both asterics on both sides.

printf ("values:%d\n",**(int **)xptr); 

Why? Because otherwise you give the compiler false information about what you are really doing. C-style casts can be forced to compile everything, but you must aware yourself: if you force the compiler to do something, then don't blame him to produce probably crash-code.

I'd wonder if the compiler didn't generate a warning on that. Did it and you ignored the warning?

Keep in mind that your initialization is probably wrong for the xptr . Please see post from haccks here .

Answer 2

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 .