CareerCup

why not

if src < dst
copy from end to beginning;
else
copy from beginning to end

void* memcpy(void *src, void *dst,int len){
if (src == NULL || dst == NULL || len <= 0) return NULL ;
if (src - dst < len) { //Overlap
src = src + len ;
dst = dst + len ;
while(len--)
*dst-- = *src-- ;
}
else { // No overlap
while(len--)
*dst++ = *src++ ;
}
return dst ;
}

How to sovle this problem?

The simplest solution is ...
void memcpy(void *src, void *dst, int len)
{
while(len--)
*dst++ = *src++;
}

If you want to check errors such as NULL pointer or overlapping of dst and src, then it would be complicated than this.

The overlap thing isn't correct, because adding len to both src and dst, it still preserves the condition (dst - src < len), which will result in overlap again. Get rid of the src = src + len line, and just make the while loop like in the case of no overlap.

One more question: can we copy void* type memory? I guess we have to convert to char* type. Please comment.

http://www.vik.cc/daniel/portfolio/memcpy.htm

I believe memcpy does do the overlap check, memmove does. And memmov basically stores the overlapped ones in a temp location.

The link provided by trojanhorse is interesting. Thanks !

void my_memcpy(void*  dst, void* src,size_t n ) {

  char* d =  (char *)dst;
  char* s =  (char *)src;

  bool not_overlap = ((d + n) <= src) || ((s + n) <=d);

  if(! not_overlap) {
    std::cerr << "Overlapped dst and src\n";
    return;
  }

  for(int  i = 0 ; i < n ; i ++) {
    *d = *s;
    d++;
    s++;
  }

}