void * -> int

[abraham2119]

hello,

i am having the following problem: in my code, i have an array of void*, and to that void* i am currently adding int data-types, but after i cast them as an int and print them out, the output is: 2293596

i would give you my whole code but it is very, very long. i'll write up an example to illustrate my problem:

#include <stdio.h>

void **v;
int i = 0;

void set(void *vv)
{
    v[i++] = vv;
}

void *get(int index)
{
    return v[index];
}

int main()
{
    int a = 5;
    set((void *)&a);
    printf("%d\n", (int)get(0));
    getchar();
    return 0;
}

my expected output would be 5, but it is 2293596.

any help would be greatly appreciated!

[tabstop]

You need to dereference the pointer, not print the memory address it contains.

[MK27]

I couldn't even get this to compile, and when I did it seg faulted. But I made a couple changes and now the answer is 5:

#include <stdio.h>

void **v;
int i = 0;

void set(void *vv)
{
    v = vv;
}

void *get(int index)
{
    return v[index];
}

int main()
{
    int a = 5;
    set((void *)&a);
    printf("%d\n", (int*)get(0));
    getchar();
    return 0;
}

[laserlight]

i am having the following problem: in my code, i have an array of void*, and to that void* i am currently adding int data-types, but after i cast them as an int and print them out, the output is: 2293596

The first problem is that there is no array in the example. The next problem is that you cast the return value of get(), which is a pointer, directly to an int, and hence you are merely converting an address to int. MK27's example is on the right track, but that converts a void* to an int* and then prints the address. What you want to do is take it a step further and dereference the converted pointer:

#include <stdio.h>

void *v[10]; /* Some arbitrarily sized array of void pointers. */
int i = 0;

void set(void *vv)
{
    v[i++] = vv;
}

void *get(int index)
{
    return v[index];
}

int main()
{
    int a = 5;
    set(&a);
    printf("%d\n", *(int*)get(0));
    getchar();
    return 0;
}

[itCbitC]

Part of the problem stems from the fact that you're not dereferencing the pointer after casting it to an int, as others have pointed out, and part is equating global void **v with local void *v. After function set() returns its stack frame is popped; linkage to vv is broken; and it becomes the case of the dangling pointer. Changes to original are shown in red:

#include <stdio.h>

void **v;
void *vv;
int i = 0;

void set(void *x)
{
    vv = x;
    v = &vv;
}

void *get(int index)
{
    return v[index];
}

int main()
{
    int a = 5;
    set((void *)&a);
    printf("%d\n", *(int *) get(0));
    getchar();
    return 0;
}

[matsp]

Part of the problem stems from the fact that you're not dereferencing the pointer after casting it to an int, as others have pointed out, and part is equating global void **v with local void *v. After function set() returns its stack frame is popped; linkage to vv is broken; and it becomes the case of the dangling pointer. Changes to original are shown in red:

#include <stdio.h>

void **v;
void *vv;
int i = 0;

void set(void *x)
{
    vv = x;
    v = &vv;
}

void *get(int index)
{
    return v[index];
}

int main()
{
    int a = 5;
    set((void *)&a);
    printf("%d\n", *(int *) get(0));
    getchar();
    return 0;
}

And where is **v pointing?

Note also that storing the address of a local variable is possibly dangerous - if you leave the function that has the local variable, then the stack-space that holds the variable will be freed up and reused by other functions, so you will get random changes to the value (yes, I know, you will not leave main before the program finishes - but in case someone thinks that this pattern can be used without restriction).

--
Mats

[abraham2119]

thanks for all your help!

but i try doing this in my actual program and it's still printing out the same value. i managed to get some of my program's code onto here.

#include <stdlib.h>

typedef struct {
    int index;
    int max;
    void **a;
} list;

list arraylist()
{
    list alist;

    alist.a = malloc(10);
    alist.max = 10;
    alist.index = 0;

    return alist;
}

void add(list *l, void *obj)
{
    if (l->index > l->max)
        realloc(&(l->a), l->max*2);
 
    l->max *= 2;
    l->a[l->index++] = obj;
}

void delete(list *l, int index)
{
    l->a[index] = NULL;
}

void *get(list *l, int index)
{
    return l->a[index];
}

void clear(list *l)
{
    free(l->a);
}

int main()
{
    list l = arraylist();
    int i = 0;
    for (; i < 100; i++) 
        add(&l, &i);

    printf("COMPONENTS ADDED\n");

    for (i = 0; i < 100; i++)
        printf("%d\n", (int *) get(&l, i));

    clear(&l);

    getchar();
        
    return 0;
}

[matsp]

Yes, you are not dereferencing the memory address stored in add when you get it out with get().

--
Mats

[itCbitC]

And where is **v pointing?

global "v" points to local too, workaround to the dangling pointer issue as main() gets popped only when the process ends.
alternatively the local variable "a" can be pushed to the data segment but figure I will leave that to the OP to decide.

Note also that storing the address of a local variable is possibly dangerous - if you leave the function that has the local variable, then the stack-space that holds the variable will be freed up and reused by other functions, so you will get random changes to the value (yes, I know, you will not leave main before the program finishes - but in case someone thinks that this pattern can be used without restriction).

--
Mats

I agree

[laserlight]

global "v" points to local too, workaround to the dangling pointer issue as main() gets popped only when the process ends.
alternatively the local variable "a" can be pushed to the data segment but figure I will leave that to the OP to decide.

If you read my response in post #4 and abraham2119's own post #7, you will see that the real problem is that there is no array to begin with, despite the introductory phrase "i have an array of void*". abraham2119 corrected this in the actual code, but did not add tabstop's suggestion (which I unknowingly echoed) to dereference the pointer.

[abraham2119]

what a stupid mistake of mine not to dereference the pointer! thank you all!

fixed like this:
printf("%d\n", *((int *) get(&l, i)));