Void pointer to struct and accessing members

Question

I am fiddling around with and learning about void pointers and linked lists and I think I may have to concept and application a little wrong.

Currently, I have a void pointer which points to the address of a struct in memory. I am then trying to access this struct via casting the void pointer to that struct. However, I am not getting the expected value only 0.

Code is as follows.

void *data = ListGetItemAtIndex(freeList, i); // returns void *
memoryBlock *block = (memoryBlock *) data;
printf("%ld\n\n", block->startAddress);

The struct:

typedef struct
{
    size_t startAddress;
    size_t memory;
} memoryBlock;

Address when struct was added:

0x5593c4812720

Address of void *:

0x5593c4812720

The question at heart is, how do I use this void pointer to access the data in the struct.

Requested example :

Block creation

static memoryBlock* CreateMemoryBlockPointer(size_t startAdress, size_t size)
{
    memoryBlock block;

    block.startAddress = startAdress;
    block.memory = size;

    return malloc(sizeof(block));
}

Assignment:

memoryBlock *ptr = CreateMemoryBlockPointer(StartAddress, size);

// Add initial 1st element
ListAddTail(freeList, ptr);

Prototype:

int ListAddTail(linkedList *list, void* data)

Add to list

   el->data = data;
   el->next = NULL;

Answer 1

Solved, see :

Allocate the memory first, then write into the memory, e.g.:

memoryBlock *block = malloc(sizeof(*block));
block->startAddress = startAddress;

etc. then return block; at the end of the function –

• @UnholySheep