How do realloc and memcpy work?

Question

I have two questions.

1. Do realloc() and memcpy() copy the entries in an array to another in a way faster than just iterating on eac

Answer 1

Presuming you are talking about glibc, and since your questions are implementation dependent, it's probably best just to check the source:

malloc.c

memcpy.c

The way I read it, the answers would be:

1. O(N) --- there is no way to copy items in better than linear time.
2. Occasionally large items will be copied when realloc() is used to shrink them.

Answer 2

1. There is absolutely no way to copy N items faster than O(N). However, it might be able to copy multiple items at once, or use special processor instructions - so it still might be faster than you could do it yourself.
2. I don't know for sure, but I'd assume that the memory is completely reallocated. That's the safest assumption, and it's probably implementation dependent anyway.