Can memcpy or memmove return a different pointer than dest?

memmove will never return anything other than dest.

Returning dest, as opposed to making memmove void, is useful when the first argument is a computed expression, because it lets you avoid computing the same value upfront, and storing it in a variable. This lets you do in a single line

void *dest = memmove(&buf[offset] + copiedSoFar, src + offset, sizeof(buf)-offset-copiedSoFar);

what you would otherwise need to do on two lines:

void *dest = &buf[offset] + copiedSoFar;
memmove(dest, src + offset, sizeof(buf)-offset-copiedSoFar);