assigning pointers vs memcpy/memmove

Question

Hi there I have a question pertaining to C pointers (especially void *)

I'm working with void * pointers that point to arbitrary blobs of memory that act as cells for a Vector implementation. These blobs are allocated on the heap.

My question is why does assigning

    void *dest = CVectorNth(cv, i);
    void *src = CVectorNth(cv, i-1);
    *(void **)dest = *(void **)src;

not work while

    memmove(dest, src, elementSize);

does work.

Why do I need to use memmove? In my head I am changing the value of the pointer to the address the src is pointing to.

I know memmove is the correct way to go, but now I can't even think of a reason why

    dest = src;

wouldn't work

Answer 1

because *(void **)dest = *(void **)src; != memmove(dest, src, elementSize); first is just assignment operation where as memmove() copy memory content from src to dest (deep-copy)

Edit

Suppose, your dest and src are something like this ?

  src           5   6  7  8    
  +-----+      +--+--+--+---+
  | 5   +----->| A|B |C | D |
  +-----+      +--+--+--+---+

  dest          18  19 20 21  
  +-----+      +--+--+--+---+
  | 18  +----->|  |  |  |   |
  +-----+      +--+--+--+---+

Now, what *(void **)dest = *(void **)src; ?

it like

  src           5   6  7  8    
  +-----+      +--+--+--+---+
  | 5   +----->| A|B |C | D |
  +-----+      +--+--+--+---+

  dest          18  19 20 21  
  +-----+      +--+--+--+---+
  | 18  +----->| A|  |  |   |
  +-----+      +--+--+--+---+

because by assignment you copy content (because using *) of location 5 into location 18.

_{Its rough diagram because *(void **)dest = *(void **)src; someting wrong in my understating}
consider larsmans's answer here

Whereas, by doing memmove(dest, src, elementSize);:

it like

  src           5   6  7  8    
  +-----+      +--+--+--+---+
  | 5   +----->| A|B |C | D |
  +-----+      +--+--+--+---+

  dest          18  19 20 21  
  +-----+      +--+--+--+---+
  | 18  +----->| A|B |C |   |
  +-----+      +--+--+--+---+

Suppose elementSize = 3 ,

memmove copy elementSize elements from src to dest pointed memory area (deepcopy)

by dest = src do like:

  src           5   6  7  8    
  +-----+      +--+--+--+---+
  | 5   +----->| A|B |C | D |
  +-----+  --->+--+--+--+---+
           | 
  dest     |    18  19 20 21  
  +-----+  |   +--+--+--+---+
  | 5   +---   |  |  |  |   |
  +-----+      +--+--+--+---+

Shadow copy:

From: Thomas (thanks!)

More specifically, the latter technique is known as "pointer swapping" and does have its uses, but is not directly compatible with deep memory copy (it requires a different design, in particular the two memory areas being "swapped" need to be persistent). For a vector implementation it's unlikely to be of any us