If dest is not a NULL pointer, the mbstowcs function converts the
multibyte string src to a wide-character string starting at
dest. At most n wide characters are written to dest. The
conversion starts in the initial state. The conversion can stop for three
1. An invalid multibyte sequence has been encountered. In this case (size_t)(-1)
2. n non-L'\0' wide characters have been stored at dest. In this
case the number of wide characters written to dest is returned, but the
shift state at this point is lost.
3. The multibyte string has been completely converted, including the terminating
'\0'. In this case the number of wide characters written to dest,
excluding the terminating L'\0' character, is returned.
The programmer must ensure that there is room for at least n wide
characters at dest.
If dest is NULL, n is ignored, and the conversion proceeds as
above, except that the converted wide characters are not written out to
memory, and that no length limit exists.
In order to avoid the case 2 above, the programmer should make sure n is
greater or equal to mbstowcs(NULL,src,0)+1.
The mbstowcs function returns the number of wide characters that make up
the converted part of the wide character string, not including the terminating
null wide character. If an invalid multibyte sequence was encountered,
(size_t)(-1) is returned.