Casting from const void to char?

Question

Alright, I'm hashing an image. And as you all know, hashing an image takes FOREVER. So I'm taking 100 samples of the image, evenly spaced out. Here's the code.

#define NUM_HASH_SAMPLES 100

@implementation UIImage(Powow)

-(NSString *)md5Hash
{
    NSData *data = UIImagePNGRepresentation(self);

    char *bytes = (char*)malloc(NUM_HASH_SAMPLES*sizeof(char));
    for(int i = 0; i < NUM_HASH_SAMPLES; i++)
    {
        int index = i*data.length/NUM_HASH_SAMPLES;

        bytes[i] = (char)(data.bytes[index]); //Operand of type 'const void' where arithmetic or pointer type is required
    }

    unsigned char result[CC_MD5_DIGEST_LENGTH];
    CC_MD5( bytes, NUM_HASH_SAMPLES, result );
    return [NSString stringWithFormat:
            @"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
            result[0], result[1], result[2], result[3],
            result[4], result[5], result[6], result[7],
            result[8], result[9], result[10], result[11],
            result[12], result[13], result[14], result[15]
            ];
}

The error is on the commented line.

What am I doing wrong?

Answer 1

data.bytes is a void *, so it makes no sense to dereference it (or even to perform the necessary pointer arithmetic on it).

So, if you meant to take a byte out of the data, then obtain a pointer to const unsigned char and dereference that:

const unsigned char *src = data.bytes;
/* ..then, in your loop.. */
bytes[i] = src[index];

Oh, and do not cast the return value of malloc()!

Answer 2

According to the documentation for NSData, data.bytes returns a type of const void *. Basically, you're trying to access a pointer to void which makes no sense since void doesn't have a size.

Cast it to a char pointer and dereference it.

((const char *)data.bytes)[index]

or

*((const char *)data.bytes + index)

Edit: What I'd normally do is assign the pointer to a known data type straight away and use that instead.

I.e.

const char *src = data.bytes;
bytes[i] = src[index];

Edit2: You might also want to leave the const qualifier in as suggested by H2CO3. That way you won't accidentally write to a location you're not supposed to.