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

If these two strings overlap, then, while copying you'll run over the original a or b pointers.

Assuming that strcpy( a, b ) roughly means a <- b, i.e., the first parameter is the destination of the copy, then you only check whether the copy pointer reaches b's position.

You only need to save the b original position, and while copying, check you haven't reached it. Also, don't write the trailing zero if you have reached that position.

 char* my_strcpy(char *a, const char *b)
 {

    if ( a == NULL
      || b == NULL )
    {
        return NULL;
    }

    char *n = a;
    const char * oldB = b;

    while( *b != '\0'
       &&  a != oldB )
    {
        *a = *b;
        a++;
        b++;
    }

    if ( a != oldB ) {
        *a = '\0';
    }

    return n;
 }

This algorithm just stops copying. Maybe you want to do something else, such as marking the error condition, or add an end-of-the string mark to the previous position (though failing silently (as the algorithm does at the moment) isn't the best option).

Hope this helps.