Dereference void pointer

Question

Even after casting a void pointer, I am getting compilation error while dereferencing it. Could anyone please let me know the reason of this.

int lVNum = 2;
         


        

Answer 1

It doesn't make sense to dereference a void pointer. How will the compiler interpret the memory that the pointer is pointing to? You need to cast the pointer to a proper type first:

int x = *(int*)lVptr;

Answer 2

Example of what you might be trying to do:

#include <stdio.h>

int main () {
    void *v;
    unsigned long int *i = (unsigned long int *)v;

    *i = 5933016743776703571;

    size_t j = sizeof(i);
    printf("There are %ld bytes in v\n", j);

    size_t k;
    for (k = 0; k < j; k++) {
        printf("Byte %ld of v: %c\n", k, ((char *)v)[k]);
    }
}

Output:

There are 8 bytes in v
Byte 0 of v: S
Byte 1 of v: T
Byte 2 of v: A
Byte 3 of v: C
Byte 4 of v: K
Byte 5 of v: O
Byte 6 of v: V
Byte 7 of v: R

Answer 3

@ Code-Guru I tried to compile it in visual studio. It gives error - expression must be a pointer to complete object.

Thanks teppic, As you suggested, the following compiles and yields right result.

#include<stdio.h>

void main(){

printf("study void pointers \n");

int lvnum = 2;
void *lvptr;
lvptr = &lvnum;
printf("\n lvptr is %d\n",((int *)lvptr)[0]);


}

However if I try printf("\n lvptr is %d\n",((int *)lVptr)[60]); It compiles and runs but gives random number.

Thanks a lot, friends for all the suggestions. Apologies that I assigned a void pointer to unnecessarily casted int pointer and expected it to get dereferenced. However I should have casted it when I want to dereference it.

Purpose of the snippet: In my sources I found klocwork error which was caused by similar situation. On the contrary the program not only compiled but also gave correct results. Reason- it is a low level code (no OS) where the memory assigned to the void pointer is already reserved till the count of like 60. But the klocwork tool was unable to parse the files having that limit resulting in error. I did a lot of brain storming and ended up in something silly.

Saurabh

Answer 4

You can not dereference a void pointer because it doesn't have a type, first you need to cast it(int *)lVptr, then dereference it *(int *)lVptr.

int lVNum = 2;
void *lVptr;
lVptr = &lVNum;

printf("\nlVptr[60 ] is  %d \n",*(int *)lVptr);

Answer 5

It's still a void* because that's what you declared it as. Any pointer may be implicitly converted to a void*, so that cast does nothing and you are left with a pointer to void just as you began with.

You'll need to declare it as an int*.

void *some_ptr = /* whatever */;
int *p = (int*)some_ptr;
// now you have a pointer to int cast from a pointer to void

Note that the cast to an int* is also unnecessary, for the same reason you don't have to (and should not) cast the return value of malloc in C.

void*'s can be implicitly converted to and from any other pointer type. I added the cast here only for clarity, in your code you would simply write;

int *p = some_void_ptr;

---

Also, this:

lVptr[1]

Is wrong. You have a pointer to a single int, not two. That dereference causes undefined behavior.

Answer 6

printf("\nlVptr[60 ] is %d \n", *(int*)lVptr);

This will cast the void pointer to a pointer to an int and then dereference it correctly.

If you want to treat it as an array (of one), you could do a slightly ugly ((int *)lVptr)[0]. Using [1] is out of bounds, and therefore not a good idea (as for lVptr[60]...)