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))
         


        

Answer 1

memmove can be turned into a memcpy if the two memory regions don't overlap. Obviously memcpy is extremely optimised on most systems (one of the ones I use makes use of almost every trick in the book from unrolled loops to SSE operations where supported for maximum throughput).

If the two memory regions do overlap, for all intents and purposes the region to be copied is moved into a temporary buffer and the temporary buffer is copied (all with memcpy, most likely) back on top of the original buffer. You can't work from the start or work from the back with an overlapping region, because you'll always end up with at least some data being corrupted in the process.

That being said, it's been a long time since I've looked at libc code, so there may be an optimisation for memmove and overlapping regions that I haven't thought of yet.

memmove doesn't depend on the way the stack grows at all - it merely copies one region of memory to another location - exactly like memcpy, except that it handles overlapping regions and memcpy doesn't.

EDIT: Actually, thinking about it some more... Working from the back can work if you go from the right "source" (so to speak), depending on the move itself (eg, is source < dest or not?). You can read newlib's implementation here, and tt's fairly well-commented too.