If dest is not a NULL pointer, the mbsrtowcs function converts the
multibyte string *src to a wide-character string starting at
dest. At most len wide characters are written to dest.
The shift state *ps is updated. The conversion is effectively performed
by repeatedly calling mbrtowc( dest,*src,n,ps)
where n is some positive number, as long as this call succeeds, and
then incrementing dest by one and *src by the number of bytes
consumed. The conversion can stop for three reasons:
1. An invalid multibyte sequence has been encountered. In this case *src
is left pointing to the invalid multibyte sequence, (size_t)(-1) is returned,
and errno is set to EILSEQ.
2. len non-L'\0' wide characters have been stored at dest. In this
case *src is left pointing to the next multibyte sequence to be
converted, and the number of wide characters written to dest is
3. The multibyte string has been completely converted, including the terminating
'\0' (which has the side effect of bringing back *ps to the initial
state). In this case *src is set to NULL, and the number of wide
characters written to dest, excluding the terminating L'\0' character,
If dest is NULL, len 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 both of the above cases, if ps is a NULL pointer, a static anonymous
state only known to the mbsrtowcs function is used instead.
The programmer must ensure that there is room for at least len wide
characters at dest.
The mbsrtowcs 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, and errno set to EILSEQ.