strncpy doesn't do what you think it does (it is not analogous to strncat). strncpy does not terminate strings on overflow. In C terms, it is not actually a string function and shouldn't be named with `str`.

snprintf or nul-plus-strncat do what you want, but snprintf has portability problems on overflow. Most projects I've been on rely on strlcpy (with a polyfill implementation where not available).