向C中的数组指针分配和取消引用void *

Question

I have a pointer to array of fixed size integer elements. After populating that array, I assigned it to void *pBuff. Later on, I need to access array elements through void pointer which I failed in doing so.

Here is the code using C:

void * pBuff = NULL;

int
set_data(void *pBuff)
{
    int ptr = 10, i;
    int phy_bn[8] = {0};
    int (*pPB)[8];

    for(i=0; i<8; i++){
        phy_bn[i] = ptr;
    }

    pPB = &phy_bn;
    pBuff = pPB;

    return 0;
}


int main()
{
    int i;

    set_data(&pBuff);

    for(i =0 ; i <8; i++){
         printf("\ndata : %d\n", *(int *)pBuff[i]);
    }
    return 0;
}

It prompts an error cast of 'void' term to non-'void' against *(int *)pBuff[i].

Any help will be really appreciated.

Thanks,

-Sam

Answer 1

Apart from the fact that you need to use:

((int*)pBuff)[i]

What you have in your code is Undefined Behavior .

pBuff = pPB;

pBuff points to a array which is local to the function and its lifetime does not exist beyond the function scope. So you have a pointer pointing to something that does not need to exist but may seemingly exist sometimes.

Answer 2

You should probably dereference with * or [] , not both at the same time :-)

If your intent is to get the integer at that i position of a void pointer which points to int s, use:

((int*)pBuff)[i]

The ((int*)pBuff) turns pBuff into a pointer to an integer and the [i] following that grabs the i 'th integer at that location.

So your loop would be:

for (i = 0 ; i < 8; i++)
     printf ("\ndata : %d\n", ((int*)pBuff)[i]);

---

Another thing you should probably watch out for is returning pointer to stack-based variables. Those variables disappear when the function exits at which point dereferencing pointers to them is undefined behaviour.

Answer 3

pBuff[i] is illegal, since pBuff is a void* . It's a matter of operator precedence:

((int *)pBuff)[i]

You don't need to dereference pBuff again with the first * because [i] already does that.

Answer 4

It should be

void * pBuff = NULL;

int
set_data(void *pBuff)
{
    int ptr = 10, i;
    int *phy_bn = (int*)malloc(sizeof(int)*8);
    //this is needed so that it is valid for pBuff to point phy_bn even if it is getting out of scope

    for(i=0; i<8; i++){
        phy_bn[i] = ptr;
    }

    pBuff = phy_bn;

    return 0;
}


int main()
{
    int i;

    set_data(&pBuff);

    for(i =0 ; i <8; i++){
         printf("\ndata : %d\n", ((int*)pBuff)[i]);
    }
    return 0;
}