The wcsncat function appends wide characters from the wide-
    character string wstr_2 to the end of wstr_1, up to a maximum of
    maxchar characters. A terminating null wide character is always
    appended to the result of the wcsncat function. Therefore, the
    maximum number of wide characters that can end up in wstr_1 is
    wcslen(wstr_1) + maxchar + 1).

    See also wcscat.