memmove implementation in C

Question

Can some one help me to understand how memmove is implemented in C. I have only one special condition right ?

if((src<dst)&&((src+sz) > dst))

copy from the back

Also does it depend on the way stack grows ?

Answer 1

Mathematically, you don't have to worry about whether they overlap at all. If src is less than dst, just copy from the end. If src is greater than dst, just copy from the beginning.

If src and dst are equal, just exit straight away.

That's because your cases are one of:

1) <-----s----->                start at end of s
                 <-----d----->

2) <-----s----->                start at end of s
            <-----d----->

3) <-----s----->                no action
   <-----d----->

4)          <-----s----->       start at beginning of s
   <-----d----->

5)               <-----s----->  start at beginning of s
   <-----d----->

Even if there's no overlap, that will still work fine, and simplify your conditions.

If you have a more efficient way to copy forwards than backwards then, yes, you should check for overlap to ensure you're using the more efficient method if possible. In other words, change option 1 above to copy from the beginning.