How to fix strcpy so that it detects overlapping strings

Question

In an interview, I was asked to write an implementation of strcpy and then fix it so that it properly handles overlapping strings. My implementation is below and it is very naiv

Answer 1

This SO entry is already older, but I am currently working on a old piece of code that copies overlapping strings with strcpy(). Characters were missing in the log output. I decided to use the following compact solution, which copies char by char.

static char *overlapped_strcpy(char *dest, const char *src)
{
  char *dst = dest;

  if (dest == NULL || src == NULL || dest == src)
    return dest;

  do {
    *dst++ = *src;
  } while (*src++);

  return dest;
}

---

EDIT:

As @Gerhardh pointed out, the code above only works if dest <= src (I just had to solve this case). For the case dest > src it is more complicated. However, copying from behind, as other answers have already mentioned, leads to success. For example:

if (dest <= src) {
  /* do the above */
} else {
  int i = (int)strlen(src);
  while (i >= 0) {
    dst[i] = src[i];
    i--;
  }
}