[commandlinefan]

That was my first thought. No strncpy? Ok,

  while (*s1 && n--)  {
    *s2++ = *s1++;
  }
  *s2 = '\0';

it is, then.

[fwsgonzo]

I don't think strncpy is the function you are looking for, either. Its for zeroing fixed-size character buffers. And it doesn't indicate problems.

https://blog.liw.fi/posts/strncpy/