Data after dst+n is unchanged.

Sure but that means the part before that is garbage because you have a null terminated string without terminator.

Or at least that’s how I see it. If your intention isn’t to start and end with a null-terminated string you should be using memcpy. Let us not talk about situations where CHAR_BIT != 8 that’s not POSIX anyway.

Even better, just avoid doing string manipulation in C.