Question

Using memcpy() when source and destination overlap can lead to undefined behaviour – in those cases only memmove() can be used.

But what if I know for sure buffers don’t overlap – is there a reason to use specifically memcpy() or specifically memmove()? Which should I use and why?

Answer 1

Assuming a sane library implementor, memcpy will always be at least as fast as memmove. However, on most platforms the difference will be minimal, and on many platforms memcpy is just an alias for memmove to support legacy code that (incorrectly) calls memcpy on overlapping buffers.

Both memcpy and memmove should be written to take advantage of the fastest loads and stores available on the platform.

To answer your question: you should use the one that is semantically correct. If you can guarantee that the buffers do not overlap, you should use memcpy. If you cannot guarantee that the buffers don’t overlap, you should use memmove.