How does realloc know how much to copy?

Question

how does realloc know the size of original data?

 void *realloc(void *ptr, size_t size);

So, if the implementation is like this:

Answer 1

It knows because malloc recorded that information when you called it. After all, the system has to keep track of the sizes of allocated blocks anyway so that it doesn't allocate a particular region of memory twice.

If you mean, "how does it know how much of the array I've written in so far", it doesn't need to. It can just copy any uninitialised garbage as well.