The wcsncat function is the wide-character equivalent of the strncat function. It copies at most n wide characters from the wide-character string pointed to by src to the end of the wide-character string pointed to by dest, and adds a terminating L\0 character.
The strings may not overlap.
The programmer must ensure that there is room for at least wcslen(dest)+n+1 wide characters at dest.
wcsncat returns dest.
ISO/ANSI C, UNIX98