mbsnrtowcs() is like mbsrtowcs(), except that the number of bytes to be converted, starting at src, is limited to nms.
If dest is not a NULL pointer, mbsnrtowcs() converts at most nms bytes from the multibyte string src to a wide-character string starting at dest. At most, len wide characters are written to dest. The state ps is updated.
The conversion is effectively performed by repeatedly calling:
mbrtowc(dest, *src, n, ps)
The conversion can stop for three reasons:
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
set to EILSEQ.
The nms limit forces a stop, or 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 returned.
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, is returned.
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 destination length limit exists.
In both of the above cases, if ps is a NULL pointer, a static anonymous state only known to mbsnrtowcs() is used instead.
The programmer shall ensure that there is room for at least len wide characters at dest.
mbsnrtowcs() 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 the global
errno is set to EILSEQ.
The behavior of mbsnrtowcs() depends on the
LC_CTYPE category of the current locale.
Passing NULL as ps is not multi-thread safe.