#define _GNU_SOURCE
#include <wchar.h>
size_t wcsnrtombs(char *dest, const wchar_t **src, size_t nwc,
size_t len, mbstate_t *ps);
If dest is not a NULL pointer, the wcsnrtombs() function converts at most nwc wide characters from the wide-character string *src to a multibyte string starting at dest. At most len bytes are written to dest. The shift state *ps is updated. The conversion is effectively performed by repeatedly calling wcrtomb(dest, *src, ps), as long as this call succeeds, and then incrementing dest by the number of bytes written and *src by one. The conversion can stop for three reasons:
1. A wide character has been encountered that can not be represented as a multibyte sequence (according to the current locale). In this case *src is left pointing to the invalid wide character, (size_t) -1 is returned, and errno is set to EILSEQ.
2. nwc wide characters have been converted without encountering a Laq\0aq, or the length limit forces a stop. In this case *src is left pointing to the next wide character to be converted, and the number of bytes written to dest is returned.
3. The wide-character string has been completely converted, including the terminating Laq\0aq (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 bytes written to dest, excluding the terminating aq\0aq byte, is returned.
If dest is NULL, len is ignored, and the conversion proceeds as above, except that the converted bytes 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 the wcsnrtombs function is used instead.
The programmer must ensure that there is room for at least len bytes at dest.
Passing NULL as ps is not multithread safe.