wchar_t *wcsncat(wchar_t *s1, const wchar_t *s2, size_t n);
wcsncat appends not more than n wude characters from the array pointed to by s2 to the end of the wide string pointed to by s1. A null wide character in s1 and wide characters that follow it are not appended. The initial wide character of s2 overwrites the null wide character at the end of s1. A terminating wide null character is always appended to the result. The behavior of wcsncat is undefined if copying takes place between objects that overlap.
wcsncat returns the value of s1.