How does realloc know how much to copy?

how does realloc know the size of original data?

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

So, if the implementation is like this:

 temp = malloc(size);
 memcpy(.. // How much to copy?
 free(ptr);
 return temp;

I realize this is not the original implementation, and realloc doesn't always do free, but when it does, how much does it copy?

Edit: Thanks for the answers. But how can I then implement realloc in my code with malloc/free/..?