Assign and dereference void * to an array pointer in C

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

Note that your pBuff is pointing at memory on the stack which is no longer valid. — Jesse Good
– Jesse Good, Commented Nov 15, 2012 at 6:23
It's bad enough to have globals, but it's a really really really bad idea to have globals and parameters with the same names. Also, you need to get a basic grasp on the notion of types. pBuff has type void* but &pBuff has type void**, which should not be passed to a function that takes an argument of type void*. — Jim Balter
– Jim Balter, Commented Nov 15, 2012 at 6:41
@JesseGood: actually, it is not (in an even worse way). The local variable pBuff is set but assigning to a local variable has no effect on anything outside the function. The global variable pBuff is never actually modified. — user102008
– user102008, Commented Nov 15, 2012 at 9:48

Alok Save · Accepted Answer · 2012-11-15 06:25:26Z

3

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.

answered Nov 15, 2012 at 6:25

Alok Save

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1 Comment

user102008 Over a year ago

actually, (confusinglly) the pBuff = pPB; line is fine, since that pBuff is a local variable.

paxdiablo · Accepted Answer · 2012-11-15 06:26:55Z

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 ints, 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.

edited Nov 15, 2012 at 6:26

answered Nov 15, 2012 at 6:20

paxdiablo

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Comments

Luchian Grigore · Accepted Answer · 2012-11-15 06:20:10Z

1

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.

answered Nov 15, 2012 at 6:20

Luchian Grigore

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2 Comments

Anirudha Over a year ago

there are other issues in his program that you are not addressing..specially the local scope problem

Luchian Grigore Over a year ago

@Fake.It.Til.U.Make.It ah, missed that. Pointed out in the other answers now.

user1825817 · Accepted Answer · 2012-11-15 06:24:02Z

0

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;
}

answered Nov 15, 2012 at 6:24

user1825817

4 Comments

Jim Balter Over a year ago

No it shouldn't. set_data takes an argument of type void*, but &pBuff is of type void**. And setting pBuff at the end of set_data sets the parameter, which is never used, not the global. There are other problems with this code as well.

Sam Over a year ago

It says unhandled exception at ((int*)pBuff)[i] as an access voilation with 0xC0000005.

Sam Over a year ago

@JimBalter : what are those? It would be great if you correct them.

Jim Balter Over a year ago

@Sam Those are that set_data pointlessly returns a value, malloc should not be cast, the argument to set_data should be void** pBuff, the final statement should be *pBuff = phy_bn, phy_bn can't be on the stack ... maybe there are more as well. A basic question: why use void* when int* is intended?

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